Quantitation of type II procollagen mRNA levels during chick limb cartilage differentiation

Prostaglandin F2α receptor (FP) signaling regulates Bmp signaling and promotes chondrocyte differentiation

Prostaglandins are a group of lipid signaling molecules involved in various physiological processes. In addition, prostaglandins have been implicated in the development and progression of diseases including cancer, cardiovascular disease, and arthritis. Prostaglandins exert their effects through the activation of specific G protein-coupled receptors (GPCRs). In this report, we examined the role of prostaglandin F2α receptor (FP) signaling as a regulator of chondrocyte differentiation. We found that FP expression was dramatically induced during the differentiation of chondrocytes and was up-regulated in cartilages. Forced expression of FP in ATDC5 chondrogenic cell line resulted in the increased expression of differentiation-related genes and increased synthesis of the extracellular matrix (ECM) regardless of the presence of insulin. Similarly, PGF2α treatment induced the expression of chondrogenic marker genes. In contrast, knockdown of endogenous FP expression suppressed the expression of chondrocyte marker genes and ECM synthesis. Organ culture of cartilage rudiments revealed that PGF2α induces chondrocyte hypertrophy. Additionally, FP overexpression increased the levels of Bmp-6, phospho-Smad1/5, and Bmpr1a, while knockdown of FP reduced expression of those genes. These results demonstrate that up-regulation of FP expression plays an important role in chondrocyte differentiation and modulates Bmp signaling.

Fibronectin matrix assembly is essential for cell condensation during chondrogenesis

Mesenchymal cell condensation is the initiating event in endochondral bone formation. Cell condensation is followed by differentiation into chondrocytes, which is accompanied by induction of chondrogenic gene expression. Gene mutations involved in chondrogenesis cause chondrodysplasias and other skeletal defects. Using mesenchymal stem cells (MSCs) in an in vitro chondrogenesis assay, we found that knockdown of the diastrophic dysplasia (DTD) sulfate transporter (DTDST, also known as SLC26A2), which is required for normal cartilage development, blocked cell condensation and caused a significant reduction in fibronectin matrix. Knockdown of fibronectin with small interfering RNAs (siRNAs) also blocked condensation. Fibrillar fibronectin matrix was detected prior to cell condensation, and its levels increased during and after condensation. Inhibition of fibronectin matrix assembly by use of the functional upstream domain (FUD) of adhesin F1 from Streptococcus pyogenes prevented cell condensation by MSCs and also by the chondrogenic cell line ATDC5. Our data show that cell condensation and induction of chondrogenesis depend on fibronectin matrix assembly and DTDST, and indicate that this transporter is required earlier in chondrogenesis than previously appreciated. They also raise the possibility that certain of the skeletal defects in DTD patients might derive from the link between DTDST, fibronectin matrix and condensation.

Chondrocyte-specific inhibition of β-catenin signaling leads to dysplasia of the caudal vertebrae in mice

STUDY DESIGN

To inhibit β-catenin specifically signaling in chondrocytes Col2-ICAT transgenic mice were generated. Anomalies in caudal vertebrae were detected during embryonic and postnatal stages of Col2-ICAT transgenic mice.

OBJECTIVE

To determine the role of canonical β-catenin signaling in caudal vertebral development.

SUMMARY OF BACKGROUND DATA

β-catenin signaling plays a critical role in skeletal development. Col2-ICAT transgenic mice were generated to selectively block β-catenin signaling by overexpression of the ICAT gene in chondrocytes.

METHODS

Tails of E16.5 transgenic embryos and adult Col2-ICAT transgenic mice and their wild-type littermates were collected and analyzed. Skeletal preparation, 3-dimensional micro-computed tomographic and histological analyses were performed to evaluate changes in the structure of caudal vertebrae. Bromodeoxyuridine labeling was performed to evaluate changes in chondrocyte proliferation in caudal vertebrae.

RESULTS

Skeletal preparation and 3-dimensional micro-computed tomographic analyses revealed bone deformation and angulated deformities in tail tissue in Col2-ICAT transgenic mice. Histological studies revealed abnormal bone development and dysplastic caudal vertebrae in Col2-ICAT transgenic mice. Inhibition of β-catenin signaling in cartilage resulted in vertebral dysplasia leading to aberrant resegmenting process. Thus, 2 poorly developed sclerotomes failed to fuse to form a complete vertebrae. BrdU labeling revealed a decreased chondrocyte proliferation in both cartilageous templates of transgenic embryos and the growth plate of adult Col2-ICAT transgenic mice.

CONCLUSION

Wnt/β-catenin signaling plays an important role in vertebral development. Inhibition of β-catenin signaling in chondrocytes results in caudal vertebra deformity in mice, which may occur as early as in the stage of sclerotome formation.

LEVEL OF EVIDENCE

N/A.

Fibronectin and stem cell differentiation - lessons from chondrogenesis

The extracellular matrix (ECM) is an intricate network of proteins that surrounds cells and has a central role in establishing an environment that is conducive to tissue-specific cell functions. In the case of stem cells, this environment is the stem cell niche, where ECM signals participate in cell fate decisions. In this Commentary, we describe how changes in ECM composition and mechanical properties can affect cell shape and stem cell differentiation. Using chondrogenic differentiation as a model, we examine the changes in the ECM that occur before and during mesenchymal stem cell differentiation. In particular, we focus on the main ECM protein fibronectin, its temporal expression pattern during chondrogenic differentiation, its potential effects on functions of differentiating chondrocytes, and how its interactions with other ECM components might affect cartilage development. Finally, we discuss data that support the possibility that the fibronectin matrix has an instructive role in directing cells through the condensation, proliferation and/or differentiation stages of cartilage formation.

Articular cartilage development: a molecular perspective

In this article, development of articular cartilage and endochondral ossification is reviewed, from the perspective of both morphologic aspects of histogenesis and molecular biology, particularly with respect to key signaling molecules and extracellular matrix components most active in cartilage development. The current understanding of the roles of transforming growth factor β and associated signaling molecules, bone morphogenic proteins, and molecules of the Wnt-β catenin system in chondrogenesis are described. Articular cartilage development is a highly conserved complex biological process that is dynamic and robust in nature, which proceeds well without incident or failure in all joints of most young growing individuals.

Recombinant human bone morphogenetic protein-2 promotes Indian hedgehog-mediated osteo-chondrogenic differentiation of a human chondrocytic cell line in vivo and in vitro

We examined osteo-chondrogenic differentiation of a human chondrocytic cell line (USAC) by rhBMP-2 in vivo and in vitro. USAC was established from a transplanted tumor to athymic mouse derived from an osteogenic sarcoma of the mandible. USAC usually shows chondrocytic phenotypes in vivo and in vitro. rhBMP-2 up-regulated not only the mRNA expression of types II and X collagen, but also the mRNA expression of osteocalcin and Cbfa1 in USAC cells in vitro. In vivo experimental cartilaginous tissue formation was prominent in the chamber with rhBMP-2 when compared with the chamber without rhBMP-2. USAC cells implanted with rhBMP-2 often formed osteoid-like tissues surrounded by osteoblastic cells positive for type I collagen. rhBMP up-regulated Ihh, and the expression of Ihh was well correlated with osteo-chondrogenic cell differentiation. These results suggest that rhBMP-2 promotes chondrogenesis and also induces osteogenic differentiation of USAC cells in vivo and in vitro through up-regulation of Ihh.

Bioreactors mediate the effectiveness of tissue engineering scaffolds

We hypothesized that the mechanically active environment present in rotating bioreactors mediates the effectiveness of three-dimensional (3D) scaffolds for cartilage tissue engineering. Cartilaginous constructs were engineered by using bovine calf chondrocytes in conjunction with two scaffold materials (SM) (benzylated hyaluronan and polyglycolic acid); three scaffold structures (SS) (sponge, non-woven mesh, and composite woven/non-woven mesh); and two culture systems (CS) (a bioreactor system and petri dishes). Construct size, composition [cells, glycosaminoglycans (GAG), total collagen, and type-specific collagen mRNA expression and protein levels], and mechanical function (compressive modulus) were assessed, and individual and interactive effects of model system parameters (SM, SS, CS, SM*CS and SS*CS) were demonstrated. The CS affected cell seeding (higher yields of more spatially uniform cells were obtained in bioreactor-grown than dish-grown 3-day constructs) and subsequently affected chondrogenesis (higher cell numbers, wet weights, wet weight GAG fractions, and collagen type II levels were obtained in bioreactor-grown than dish-grown 1-month constructs). In bioreactors, mesh-based scaffolds yielded 1-month constructs with lower type I collagen levels and four-fold higher compressive moduli than corresponding sponge-based scaffolds. The data imply that interactions between bioreactors and 3D tissue engineering scaffolds can be utilized to improve the structure, function, and molecular properties of in vitro-generated cartilage.

p107 and p130 Coordinately regulate proliferation, Cbfa1 expression, and hypertrophic differentiation during endochondral bone development

During endochondral bone development, both the chondrogenic differentiation of mesenchyme and the hypertrophic differentiation of chondrocytes coincide with the proliferative arrest of the differentiating cells. However, the mechanisms by which differentiation is coordinated with cell cycle withdrawal, and the importance of this coordination for skeletal development, have not been defined. Through analysis of mice lacking the pRB-related p107 and p130 proteins, we found that p107 was required in prechondrogenic condensations for cell cycle withdrawal and for quantitatively normal alpha1(II) collagen expression. Remarkably, the p107-dependent proliferative arrest of mesenchymal cells was not needed for qualitative changes that are associated with chondrogenic differentiation, including production of Alcian blue-staining matrix and expression of the collagen IIB isoform. In chondrocytes, both p107 and p130 contributed to cell cycle exit, and p107 and p130 loss was accompanied by deregulated proliferation, reduced expression of Cbfa1, and reduced expression of Cbfa1-dependent genes that are associated with hypertrophic differentiation. Moreover, Cbfa1 was detected, and hypertrophic differentiation occurred, only in chondrocytes that had undergone or were undergoing a proliferative arrest. The results suggest that Cbfa1 links a p107- and p130-mediated cell cycle arrest to chondrocyte terminal differentiation.

The bone morphogenetic protein antagonist Noggin is regulated by Sox9 during endochondral differentiation

Noggin has been described to be capable of binding bone morphogenetic proteins (BMP) and inhibiting BMP signaling by preventing the interactions of BMP with their receptors. Noggin expression during endochondral differentiation was analyzed to elucidate its potential role during chondrogenesis. Throughout mouse development, Noggin was expressed abundantly in the chondrocytic lineage as early as collagen type II RNA was detectable. In addition, a strong correlation was detected between Noggin expression and the expression profile of Sox9 during chondrogenesis. Sox9 (known to play an important role during chondrogenesis) and Noggin expression were investigated in the pluripotent mesenchymal cell line C3H10T1/2, stimulated by BMP-2. BMP-2 caused significant upregulation of Sox9 and Noggin expression in these cells. The upregulation of Noggin could be inhibited by introducing antisense oligonucleotides against Sox9 mRNA into the cells. Using mouse limb bud cultures, the role of Sox9 and Noggin during endochondral tissue differentiation was further studied. Treatment of cultures with Sox9 antisense oligonucleotides and/or Noggin protein caused significant alterations in limb morphogenesis and endochondral development. The data suggest that the transcriptional control of Noggin by Sox9 is a potent regulatory mechanism in chondrocyte differentiation.

Temporal expression of CD44 during embryonic chick limb development and modulation of its expression with retinoic acid

Hyaluronan-cell interactions are initiated co-ordinately with mesenchymal condensation during chondrogenic differentiation in the limb bud. Hyaluronan is responsible for the retention and organization of proteoglycan within the cartilage matrix. Hyaluronan-CD44 binding also retains proteoglycan aggregates to the chondrocyte plasma membrane. A sequence for CD44 protein in chick has recently been reported, but never evaluated in chick chondrocytes. Total RNA was isolated from embryonic chick limb buds, stages 18, 19, 24, 25 and 30. Using semi-quantitative RT-PCR, expression of aggrecan, this chick CD44 orthologue and GAPDH mRNA was analyzed. Aggrecan expression was detected at all stages, but was increased at stage 30. CD44 mRNA was detected at extremely low levels at stage 18 to higher levels in the latter stages. Thus, the temporal expression of CD44 mRNA correlated with the onset of pre-cartilage condensation. The full-length chick chondrocyte CD44 cDNA was obtained following RT-PCR using RNA derived from tibial chondrocytes from stage 37 chick embryos. The nucleotide sequence was used to generate an amino acid sequence and analyses revealed homologies of 44.4% with mouse, 47.8% with bovine and 46.3% with human CD44. Tibial chondrocytes were cultured in the presence or absence of retinoic acid for 36 or 72 h. By RT-PCR, expression of aggrecan and the CD44 mRNA by chick chondrocytes was decreased after retinoic acid treatment, while GAPDH expression showed no change. As expected, control chondrocytes exhibited a round morphology while retinoic acid-treated chondrocytes were elongated. The retinoic acid-treated chondrocytes also exhibited reduced hyaluronan binding. This functional assay indicates a role for a CD44 receptor in matrix retention by chick chondrocytes.

Cellular interactions and signaling in cartilage development

The long bones of the developing skeleton, such as those of the limb, arise from the process of endochondral ossification, where cartilage serves as the initial anlage element and is later replaced by bone. One of the earliest events of embryonic limb development is cellular condensation, whereby pre-cartilage mesenchymal cells aggregate as a result of specific cell-cell interactions, a requisite step in the chondrogenic pathway. In this review an extensive examination of historical and recent literature pertaining to limb development and mesenchymal condensation has been undertaken. Topics reviewed include limb initiation and axial induction, mesenchymal condensation and its regulation by various adhesion molecules, and regulation of chondrocyte differentiation and limb patterning. The complexity of limb development is exemplified by the involvement of multiple growth factors and morphogens such as Wnts, transforming growth factor-beta and fibroblast growth factors, as well as condensation events mediated by both cell-cell (neural cadherin and neural cell adhesion molecule) and cell-matrix adhesion (fibronectin, proteoglycans and collagens), as well as numerous intracellular signaling pathways transduced by integrins, mitogen activated protein kinases, protein kinase C, lipid metabolites and cyclic adenosine monophosphate. Furthermore, information pertaining to limb patterning and the functional importance of Hox genes and various other signaling molecules such as radical fringe, engrailed, Sox-9, and the Hedgehog family is reviewed. The exquisite three-dimensional structure of the vertebrate limb represents the culmination of these highly orchestrated and strictly regulated events. Understanding the development of cartilage should provide insights into mechanisms underlying the biology of both normal and pathologic (e.g. osteoarthritis) adult cartilage.

The transcription factor deltaEF1 is inversely expressed with type II collagen mRNA and can repress Col2a1 promoter activity in transfected chondrocytes

The regulation of Col2a1, which encodes type II collagen, likely results from a balance of both positive and negative proteins. Here we present evidence that the transcription factor deltaEF1 participates in the negative regulation of Col2a1 transcription. A deletion analysis suggested that a region between -100 and -307 of the rat Col2a1 gene was required for activity in differentiating chick limb bud mesenchymal cells; however, mutation of a conserved E2 box site in this region actually increased promoter activity. Supershift analysis demonstrated that deltaEF1, a known transcriptional repressor, bound to the E2 box in a sequence-dependent manner. Chick limb bud mesenchymal cells, which do not express type II collagen, expressed abundant deltaEF1 mRNA, but, following differentiation in micromass culture, deltaEF1 mRNA expression was lost. Primary embryonic chick sternal chondrocytes, which express abundant type II collagen, displayed minimal levels of deltaEF1 mRNA. The inhibition of Col2a1 transcription following treatment of chick sternal chondrocytes with growth factors was accompanied by increased deltaEF1 expression. Overexpression of deltaEF1 in differentiated chondrocytes resulted in decreased expression of a reporter construct containing a collagen II promoter/enhancer insert; however, this negative regulation was not dependent on the proximal E2 box. This is the first report of a specific transcription factor involved in the negative regulation of Col2a1.

Pax1 and Pax9 synergistically regulate vertebral column development

The paralogous genes Pax1 and Pax9 constitute one group within the vertebrate Pax gene family. They encode closely related transcription factors and are expressed in similar patterns during mouse embryogenesis, suggesting that Pax1 and Pax9 act in similar developmental pathways. We have recently shown that mice homozygous for a defined Pax1 null allele exhibit morphological abnormalities of the axial skeleton, which is not affected in homozygous Pax9 mutants. To investigate a potential interaction of the two genes, we analysed Pax1/Pax9 double mutant mice. These mutants completely lack the medial derivatives of the sclerotomes, the vertebral bodies, intervertebral discs and the proximal parts of the ribs. This phenotype is much more severe than that of Pax1 single homozygous mutants. In contrast, the neural arches, which are derived from the lateral regions of the sclerotomes, are formed. The analysis of Pax9 expression in compound mutants indicates that both spatial expansion and upregulation of Pax9 expression account for its compensatory function during sclerotome development in the absence of Pax1. In Pax1/Pax9 double homozygous mutants, formation and anteroposterior polarity of sclerotomes, as well as induction of a chondrocyte-specific cell lineage, appear normal. However, instead of a segmental arrangement of vertebrae and intervertebral disc anlagen, a loose mesenchyme surrounding the notochord is formed. The gradual loss of Sox9 and Collagen II expression in this mesenchyme indicates that the sclerotomes are prevented from undergoing chondrogenesis. The first detectable defect is a low rate of cell proliferation in the ventromedial regions of the sclerotomes after sclerotome formation but before mesenchymal condensation normally occurs. At later stages, an increased number of cells undergoing apoptosis further reduces the area normally forming vertebrae and intervertebral discs. Our results reveal functional redundancy between Pax1 and Pax9 during vertebral column development and identify an early role of Pax1 and Pax9 in the control of cell proliferation during early sclerotome development. In addition, our data indicate that the development of medial and lateral elements of vertebrae is regulated by distinct genetic pathways.

Preparation of a cDNA library and preliminary assessment of 1400 genes from mouse growth cartilage

Cartilage is an inconvenient tissue for the isolation of mRNA, and this has hampered studies of its component mRNAs conducted to date. Here, we describe the preparation of a good quality cDNA library from mouse growth cartilage (mGC). A total of 1.7 microg of poly(A)+ RNA was obtained from about 1200 pieces of the mGC zone of 60 young mice (BALB/c, 4 weeks old). Using this poly(A)+ RNA, we constructed a cDNA library using the pAP3neo vector by the linker-primer method. The complexity of the cDNA library was 2.6 x 106 colony-forming units (cfu), which signified that almost all of the mRNA components in the mGC were present in this cDNA library. From this library, 1401 clones were randomly selected and their insert sizes were examined. Of these clones, 166 (12%) had no inserts, 466 (33%) had inserts ranging in size from 0-0.9 kbp, 480 (34%) had inserts of 1. 0-1.9 kbp, 162 (12%) had inserts of 2.0-2.9 kbp, and 127 (9%) had sizes greater than 3.0 kbp. The average insert size was 1.45 kbp. The number of cfu and the insert size data qualified this library as of reasonably good quality. Clones with an insert size greater than 1 kbp (769 clones) were sequenced from their 5' ends. Among the 769 clones examined, 608 gave sequence data. Among these, 196 (32%) were unknown, 2 were only poly A, and 410 (67%) coded for known proteins. Of these, 55 clones coded for type II (pro)collagen, 54 for osteonectin, and 22 for other cartilage collagens (type IX, type X, and type XI). The rest included cartilage extracellular matrix genes, general cellular genes, and others. To judge further the quality of the library, 45 species coding for type II collagen chain were aligned based on their 5' end sequences. Three species (7%) contained almost the full-length insert, and the shortest one was 1. 5 kbp in length (full-length 5.6 kbp). These data show that this cDNA library is of reasonably good quality, making it likely that the large number of unknown inserts (32%) will provide a suitable pool for the identification and functional determination of new GC genes.

Morphological foundations of precartilage development in mesenchyme

Hyaline cartilage is archetypic for the appendicular skeleton and the vertebral column. It arises from pluirpotential mesenchymal ancestor cells that remain morphologically undifferentiated prior to a localized cell aggregation in specific regions destined to undergo chondrogenesis. The critical ultrastructural studies of limb bud mesenchymal differentiation prior to, during, and after aggregation were largely completed during the 1970s. These studies accurately and reproducibly described the changes in the cells and matrix with reference to the developmental stages of the embryonic chick and mouse. Collectively, the morphological literature concerning mouse and chick chondrogenesis is in fundamental agreement on the timing and sequence of cell and matrix changes. The morphological observations are foundational and are now extensively correlated with the molecular events of cartilage differentiation.

Different cis-regulatory DNA elements mediate developmental stage- and tissue-specific expression of the human COL2A1 gene in transgenic mice

Expression of the type II collagen gene (human COL2A1, mouse Col2a1) heralds the differentiation of chondrocytes. It is also expressed in progenitor cells of some nonchondrogenic tissues during embryogenesis. DNA sequences in the 5' flanking region and intron 1 are known to control tissue-specific expression in vitro, but the regulation of COL2A1 expression in vivo is not clearly understood. We have tested the regulatory activity of DNA sequences from COL2A1 on the expression of a lacZ reporter gene in transgenic mice. We have found that type II collagen characteristic expression of the transgene requires the enhancer activity of a 309-bp fragment (+2, 388 to +2,696) in intron 1 in conjunction with 6.1-kb 5' sequences. Different regulatory elements were found in the 1.6-kb region (+701 to +2,387) of intron 1 which only needs 90-bp 5' sequences for tissue-specific expression in different components of the developing cartilaginous skeleton. Distinct positive and negative regulatory elements act together to control tissue-specific transgene expression in the developing midbrain neuroepithelium. Positive elements affecting expression in the midbrain were found in the region from -90 to -1,500 and from +701 to +2,387, whereas negatively acting elements were detected in the regions from -1,500 to -6,100 and +2,388 to +2,855.

Trabecular bone formation in the healing of the rodent molar tooth extraction socket

The aim of this study was to investigate the nature of the template structure on which trabecular bone formation occurs during healing of the rodent tooth extraction socket, a well studied bone healing system. The presence of collagen type II mRNA has previously been described in the healing socket, although the formation of the protein or cartilage has not been observed. However, recent evidence from developmental and other bone healing studies indicates that collagen type III may be important in forming the preliminary scaffold on which bone trabeculae are formed. The maxillary right molar teeth were removed from rats under general anaesthesia and the animals killed at various times afterward. The tissues were examined using histological, in situ hybridization, and immunohistochemical staining techniques. It was concluded that collagen type IIA mRNA was produced by osteoblast cells of the socket, but that collagen type II, if present, would account for less than 0.01% of the total proteins extracted. During bone formation, Sharpey's fibers were seen radiating from the peripheral bone toward the center of the socket. These optically active collagen fibers were inserted into the forming bone trabeculae and were recognized by antibodies raised against collagen type III. The arrangement and composition of these fibers therefore suggest that they form a preliminary framework on which deposition of woven bone trabeculae occurs.

Mouse clavicular development: analysis of wild-type and cleidocranial dysplasia mutant mice

Cleidocranial dysplasia (CCD) is an autosomal dominant disease characterized by hypoplasia or aplasia of clavicles, open fontanelles, and other skeletal anomalies. A mouse mutant, shown by clinical and radiographic analysis to be strikingly similar to the human disorder and designated Ccd, was used as a model for the human disorder. Since malformation of the clavicle is the hallmark of CCD, we studied clavicular development in wild-type and Ccd mice. Histology and in situ hybridization experiments were performed to compare the temporal and spatial expression of several genes in wild-type and Ccd mutant mouse embryos. Bone and cartilage specific markers--type I, II, and X collagens, Sox9, aggrecan, and osteopontin were used as probes. The analyses covered the development of the clavicle from the initial mesenchymal condensation at embryonic day 13 (E13) to the late mineralization stage at embryonic day 15.5. At day 13.5, cells in the center of the condensation differentiate into characteristic precursor cells that were not observed in other bone anlagen. In the medial part of the anlage these cells express markers of the early cartilage lineage (type II collagen and Sox9), whereas cells of the lateral part express markers of the osteoblast lineage (type I collagen). With further development the medial cells differentiate into chondrocytes and start to express chondrocyte-specific markers such as aggrecan. Cells of the lateral part differentiate into osteoblasts as indicated by the production of bone matrix and the expression of osteopontin. At day 14.5 a regular growth plate has developed between the two parts where type X collagen expression can be demonstrated in hypertrophic chondrocytes. The data indicate that the medial part of the clavicle develops by endochondral bone formation while the lateral part ossifies as a membranous bone. The clavicle of Ccd mice showed a smaller band of mesenchymal cell condensation than in wild-type mice. Cells of the condensation failed to express type I and type II collagen at E13.5. In the lateral part of the clavicle type I collagen expression was not detected until E14.5 and osteopontin expression only appeared at E15.5. At E15.5, a small ossification center appears in the lateral part which is, in contrast to the wild-type clavicular bone, solid and without primary spongiosa as well as bone marrow. In the medial portion, type II collagen expression and endochondral ossification never occurs in Ccd mice; this portion of the clavicle is therefore missing in Ccd.

SOX9 binds DNA, activates transcription, and coexpresses with type II collagen during chondrogenesis in the mouse

Two lines of evidence suggest that the Sry-related gene Sox9 is important for chondrogenesis in mammalian embryos. Sox9 mRNA is expressed in chondrogenic condensations in mice, and mutations in human SOX9 are known to cause skeletal dysplasia. We show here that mouse SOX9 protein is able to bind to a SOX/SRY consensus motif in DNA and contains a modular transcriptional activation domain, consistent with a role for SOX9 as a transcription factor acting on genes involved in cartilage development. One such gene is Col2a1, which encodes type II collagen, the major structural component of cartilage. We have compared, in detail, the expression of Sox9 and Col2a1 during mouse development. In chondrogenic tissues the expression profiles of the two genes were remarkably similar. Coexpression was detected in some nonchondrogenic tissues such as the notochord, otic vesicle, and neural tube, but others such as heart and lung differed in their expression of the two genes. Immunohistochemistry using an antibody specific for SOX9 revealed that expression of SOX9 protein mirrored the distribution of Sox9 mRNA. Our results suggest that SOX9 protein is involved in the regulation of Col2a1 during chondrogenesis, but that this regulation is likely to depend on additional cofactors.

Alternative transcript of the chick alpha 2(I) collagen gene is transiently expressed during endochondral bone formation and during development of the central nervous system

Endochondral bone formation is characterized by several transitions in the pattern of collagen gene expression, the best characterized of which occurs during chondrogenesis. Prechondrogenic mesenchymal cells synthesize predominantly type I collagen; during chondrogenesis, type I collagen synthesis ceases and production of cartilage-characteristic collagens is initiated. We previously identified the molecular mechanism that mediates cessation of alpha 2(I) collagen synthesis in chondrocytes (Bennett and Adams [1990] J. Biol. Chem. 265:2223-2230). This mechanism involves a change in the transcription initiation site, resulting in an alternative transcript that cannot encode alpha 2(I) collagen. In this report we demonstrate that the alternative transcript appears only transiently in cartilage. Its initial appearance is coincident with the onset of high levels of type II collagen synthesis in differentiated chondrocytes. However, it disappears in hypertrophic cartilage, and production of the authentic alpha 2(I) collagen mRNA is reinitiated, contributing to synthesis of a high level of type I collagen in hypertrophic chondrocytes at the chondro-osseous junction. We also show that the alternative transcript is not restricted to cartilage during embryonic development, since it initially appears in presomite embryos, well before the appearance of cartilage. At early stages of embryo-genesis the alternative transcript is restricted to tissues derived from neuroectoderm; its appearance in those tissues is also transient. These data suggest that production of the alternative transcript of the alpha 2(I) collagen gene may be required for cessation of alpha 2(I) collagen synthesis during chondrogenesis, but the alternative transcript may be involved in other important developmental programs as well.

Differential regulation of COL2A1 expression in developing and mature chondrocytes

To investigate the regulation of type II collagen gene expression in cells undergoing chondrogenic differentiation, we have employed a 5-kbp genomic fragment of the human type II collagen gene which contains 1.8kbp of upstream sequences, the transcription start site, the first exon and 3 kbp of intronic sequences, fused to either lac Z or chloramphenicol acetyl transferase-reporter gene. Transient expression studies revealed a parallel increase in transgene activity and endogenous type II collagen mRNA levels during the onset of the cartilage differentiation of limb mesenchymal cells in high-density micromass cultures. At later periods in culture, however, the transgene activity declines, although steady-state levels of type II collagen mRNA are reported to continue to increase (Kosher et al.: J. Cell. Biol. 102: 1151-1156, 1986; Kravis and Upholt. Dev. Biol. 108: 164-172, 1985). In addition, the activity of the transgene is seven-fold higher at the onset of chondrogenic differentiation in micromass cultures that in well differentiated sternal chondrocytes, although similar levels of type II collagen transcripts are found in these cells. Furthermore, deletions of intronic segments resulted in greater drop in activity of the constructs in differentiating chondrocytes in micromass cultures than in mature sternal chondrocytes. The expression of the construct in transgenic mice is higher at the onset of chondrogenic differentiation and in newly differentiated chondrocytes than in more mature differentiated chondrocytes. Based on these observations, it appears that the mechanisms involved in the regulation of the type II collagen gene at the onset of chondrocyte differentiation are different from those resulting in the maintenance of its expression in fully differentiated chondrocytes.

Developmental expression of a type II collagen/beta-galactosidase fusion gene in transgenic mice

The correct temporal and spatial expression of the type II collagen gene is believed to be important for normal development and growth of the skeleton and the eye, i.e., tissues where the protein product is predominantly found. To study transcriptional activation of type II collagen gene in skeletal and nonskeletal tissues we produced transgenic mice carrying murine proalpha1(II) collagen/beta-galactosidase fusion gene constructs. The expression of the fusion gene was found to depend on the presence of intron 1 deleted failed to reveal any beta-galactosidase activity confirming the important role of regulatory sequences within intron 1 of the gene. High-level expression of the functional construct was clearly confined to cartilaginous tissues but transient low-level expression was also observed in extraskeletal locations, such as the developing brain and the notochord. The results demonstrate that the regulatory elements in the proalpha1(II) collagen/beta-galactosidase fusion gene construct confer both temporal and spatial specificity indistinguishable from that of the endogenous proalpha1(II) collagen gene as determined by the presence of the corresponding mRNA by in situ hybridization. Furthermore the beta-galactosidase activity correlated well with the progression of chondrogenesis as seen by staining of whole mouse embryos with Alizarin red S and Alcian blue in the hybrid mouse strain used for microinjections. The transgenic mouse line produced should prove useful for studies on various aspects of chondrogenesis. Furthermore, the data shows that the regulatory elements present in the construct are sufficient for targetting the expression of other genes in cartilage.

Tissue-specific and differential expression of alternatively spliced alpha 1(II) collagen mRNAs in early human embryos

Expression of the alpha 1(II) procollagen gene is not confined to chondrogenic tissues during vertebrate development. Transcripts of the human gene (COL2A1) are alternatively spliced to give mRNAs which either exclude (type IIB mRNA) or include (type IIA mRNA) an exon encoding a cysteine-rich domain in the amino-propeptide. The distribution of COL2A1 mRNAs in 27- to 44-day human embryos and 8- to 24-week fetuses was studied by in situ hybridization and RNase protection analyses. Type IIA mRNAs were expressed in prechondrogenic cells and were also preferentially expressed in chondrogenic tissues at regions of chondrocyte commitment and cartilage growth. During maturation of chondrocytes, there is a switch to expression of type IIB mRNAs. In non-chondrogenic tissues of early embryos, type IIA mRNA expression was associated with active tissue remodeling, epithelial organization, and sites of tissue interaction. Type IIA mRNAs were also expressed in some non-chondrogenic tissues where expression had previously been undetected, such as the tooth bud, liver, adrenal cortex, apical ectodermal ridge, and indifferent gonad. In older fetuses type IIA mRNAs were the sole or major transcript in most non-chondrogenic tissues except the choroid plexus and tendon. In the meninges there was a unique switch from type IIB to type IIA expression. The expression pattern of COL2A1 transcripts suggests that, in addition to contributing to the structural integrity of the cartilage extracellular matrix, type II procollagen may serve a morphogenetic role in embryonic development. Our findings clearly show that the pattern of expression of type II procollagen mRNAs is largely conserved between man and mouse. However, some differences exist, and these should be taken into consideration when animal models are used to study human diseases associated with COL2A1.

In situ expression of collagen and proteoglycan genes in notochord and during skeletal development and growth

Cartilage is an important tissue in skeletogenesis, in the growth of long bones, and as a flexible component of the mature skeleton. The extracellular matrix proteins type II collagen and aggrecan comprise 90% of the matrix and are characteristic of cartilage. Type II collagen provides structural integrity to the tissue, while aggrecan confers resiliency. The quantity of type II procollagen is controlled at the level of transcription of mRNA from the COL2A1 gene. In addition, type II procollagen can be expressed in two isoforms by differential splicing of the primary gene transcript, a post-transcriptional control mechanism. The two mRNAs either include exon 2 (type IIA) or exclude exon 2 (type IIB) which encodes the major portion of the amino (NH2)-propeptide [Ryan and Sandell (1990), J. Biol. Chem., 265:10334-10339]. The aggrecan gene also encodes alternative splice forms that may be developmentally expressed. The regulation of aggrecan splicing or transcription has not been studied in detail. To determine the spatial and temporal patterns of expression of extracellular matrix in the development of cartilage, we have examined the expression of type II collagen and aggrecan during chondrogenesis in the vertebral column and during elongation of a newborn growth plate. Our results indicate that there is a developmental sequence of type II collagen splice form expression during chondrogenesis with type IIA expressed in prechondrocytes and type IIB expressed in chondrocytes. During elongation of the growth plate, mature chondrocytes express type IIB procollagen and then differentiate into hypertrophic chondrocytes and initiate expression of type X collagen. In all cases, aggrecan was coordinately expressed with type IIB procollagen. As cartilage-like proteins have been observed in more primitive structures such as notochord, the expression of type II collagen mRNAs was also examined in the notochordal remnants of the vertebral column. In the notochord, the predominant collagen expressed was the type IIA collagen prechondrocyte isoform. Notochordal cells also expressed mRNAs more characteristic of fibroblasts such as versican and decorin: low expression of type I collagen, type IIB collagen, and aggrecan were observed.

Transient expression of type II collagen and tissue mobilization during development of the scleral ossicle, a membranous bone, in the chick embryo

Development of the chick scleral ossicle was studied with respect to expression of various collagen types, cartilage matrix molecules, and osteoblastic cell surface antigens. The extra-cellular matrix of the scleral ossicle primordium of stage 35.5 chick sclera and the mesenchyme beneath the conjunctival epithelium was immunoreactive with anti-type II collagen antibody, giving the impression that certain materials and/or cell clusters surrounded by reactive matrix were descending from the epithelial-mesenchymal interface to the scleral ossicle primordium. In stage 37 embryos, type II collagen immunoreactivity was restricted to the bone matrix of the scleral ossicles, and persisted through stage 39. However, at stage 41, virtually no type II collagen was detected. In contrast, strong immunostaining of type I collagen was first detected in the developing scleral ossicle at stage 37, coinciding with the formation of mineralized bone matrix. Following the extensive accumulation of type I collagen in bone matrix, type XII collagen was detected at the surface of the bone; both type I and type XII collagen immunostainings then remained. By stage 37, immunoreactivity with a pre-osteoblastic cell surface marker was detected on cells of the scleral ossicle, and typical osteocytes were subsequently identified by both morphological and specific immunostaining techniques. Antibodies other than for type II collagen, specific to chondrogenic mesenchyme or cartilage matrix, never reacted with the scleral ossicle and its primordium during development. Taken together, these observations indicate that the scleral ossicle is a membranous bone, whose development may not require overt chondrogenesis. Implications of type II collagen distribution during the positioning of scleral ossicles and their early bone matrix formation are discussed with respect to the origin and evolution of endoskeletons in vertebrate animals.

Pre-metatarsal skeletal development in tissue culture at unit- and microgravity

Explant organ culture was used to demonstrate that isolated embryonic mouse pre-metatarsal mesenchyme is capable of undergoing a series of differentiative and morphogenetic developmental events. Mesenchyme differentiation into chondrocytes, and concurrent morphogenetic patterning of the cartilage tissue, and terminal chondrocyte differentiation with subsequent matrix mineralization show that cultured tissue closely parallels in vivo development. Whole mount alizarin red staining of the cultured tissue demonstrates that the extracellular matrix around the hypertrophied chondrocytes is competent to support mineralization. Intensely stained mineralized bands are similar to those formed in pre-metatarsals developing in vivo. We have adapted the culture strategy for experimentation in a reduced gravity environment on the Space Shuttle. Spaceflight culture of pre-metatarsals, which have already initiated chondrogenesis and morphogenetic patterning, results in an increase in cartilage rod size and maintenance of rod shape, compared to controls. Older pre-metatarsal tissue, already terminally differentiated to hypertrophied cartilage, maintained rod structure and cartilage phenotype during spaceflight culture.

Enhancement of avian mandibular chondrogenesis in vitro in the absence of epithelium

The roles of mandibular epithelium in chondrogenesis and growth of mandibular mesenchyme were examined in organ cultures. Epithelium and mesenchyme were separated from the mandibular arches of chick embryos at stages before and after the onset of chondrogenesis in vivo (stages 18-28). Isochronic and heterochronic tissue recombinations were prepared. Removal of the mandibular epithelium resulted in reduced growth of the explants and enhanced chondrogenesis, resulting in increased levels of mRNAs for type II collagen and aggrecan. The presence of mandibular epithelium promoted cell division in loosely arranged undifferentiated tissue from the mandibular mesenchyme and resulted in increased levels of type I collagen mRNA. Enhanced chondrogenesis was also observed in the mesenchyme isolated with basement membrane and isolated mesenchyme grown within Matrigel. These findings suggest that mandibular epithelium has mitogenic and chondrogenic-inhibitory effects on the underlying mesenchyme that are stage independent. Furthermore, the chondrogenic-inhibitory effect of mandibular epithelium on the underlying mesenchymal cells is not mediated by basement membrane.

Intron-exon structure, alternative use of promoter and expression of the mouse collagen X gene, Col10a-1

The entire mouse collagen X gene (Col10a-1) has been isolated. The gene is composed of three exons and two introns spanning 7.0 kb of the DNA sequence. Exons 2 and 3 together encode 15-bp of 5' untranslated sequence, a 2040-bp open reading frame and an 895-nucleotide 3' non-coding region. In the 5' flanking region of the gene, two consensus TATA-box sequences were found. Identification of the first exon by ribonuclease-protection assays and the determination of the 5' end of Col10a-1 mRNA transcripts by primer-extension analyses show that the more 3' TATA box is probably predominantly used and that there are at least three transcription start sites in the exon 1 sequence 3' to this, resulting in 5' untranslated regions of 78, 77 and 55 nucleotides. By means of rapid amplification of cDNA ends by polymerase chain reaction, an additional mRNA species was detected which overlapped the other Col10a-1 transcripts, including the 3' TATA box sequence, giving a 5' untranslated sequence of approximately 235 bases. This latter transcript starts approximately 20 bp 3' to the more 5' TATA box. The data suggest alternative use of promoters and transcription starts for the Col10a-1 gene. Comparison of the combined nucleotide and deduced amino acid sequences of exons 2 and 3 with chicken, bovine and human collagen X genes, showed a high degree of similarity indicating conservation of this gene throughout evolution. Mouse Col10a-1 mRNA was shown to be approximately 3.0 kb and the pepsinized protein, as detected by SDS/PAGE, was approximately 45 kDa. The mRNA and protein sizes correlate with that predicted by the open reading frame. Reverse-transcription polymerase chain reaction assays indicate that the mouse collagen X gene is first expressed at 13.5 days post coitum, temporally preceding the onset of endochondral ossification. In agreement with the generally accepted association of type-X collagen with endochondral ossification, in situ hybridization analyses indicate that Col10a-1 mRNA are restricted to the hypertrophic regions of growth cartilage.

Embryonic mouse pre-metatarsal development in organ culture

Embryonic mouse pre-metatarsals were removed from embryos at 13 days of gestation and cultured in a defined, serum-free medium for up to 15 days. By histological analysis, we observe that the cultured pre-metatarsal tissue undergoes a similar developmental profile as pre-metatarsals growing normally in vivo. The initial mesenchyme condensation regions undergo differentiation and morphogenesis to form distinct rods made up of cartilage tissue. A marker of this differentiation step is the synthesis of type II collagen. Metabolic labelling, pepsin digestion, SDS-PAGE, and autoradiography were used to demonstrate this protein when cartilage tissue is present in the cultures. After additional culture time, terminal chondrocyte differentiation and morphogenesis take place in specific regions of the cartilage rods to form bands of hypertrophied chondrocytes. One marker of this differentiation step is the synthesis of the enzyme alkaline phosphatase. We have measured the activity of this enzyme throughout the culture period and see a substantial increase at the time of terminal chondrocyte differentiation. Another feature of hypertrophied chondrocytes is that the matrix around the cells becomes calcified. Calcified matrix in our cultured pre-metatarsals was visualized by staining with alizarin red. By supplementing the defined culture medium with ITS, we observed that terminal chondrocyte differentiation took place in a shorter culture time. Supplementation of the medium with serum results in a similar acceleration of terminal differentiation, and, with additional culture time, an osteoid-like matrix forms around the central region of the rods.

The membranous skeleton: the role of cell condensations in vertebrate skeletogenesis

Elements of the vertebrate skeleton are initiated as cell condensations, collectively termed the 'membranous skeleton' whether cartilages or bones by Grüneberg (1963). Condensations, which were identified as the basic cellular units in a recent model of morphological change in development and evolution (Atchley and Hall 1991) are reviewed in this paper. Condensations are initiated either by increased mitotic activity or by aggregation of cells towards a centre. Prechondrogenic (limb bud) and preosteogenic (scleral ossicle) condensations are discussed and contrasted. Both types of skeletogenic condensations arise following epithelial-mesenchymal interactions; condensations are identified as the first cellular product of such tissue interactions. Molecular characteristics of condensations are discussed, including peanut agglutinin lectin, which is used to visualize prechondrogenic condensations, and hyaluronan, hyaladherins, heparan sulphate proteoglycan, chondroitin sulphate proteoglycan, versican, tenascin, syndecan, N-CAM, alkaline phosphatase, retinoic acid and homeo-box-containing genes. The importance for the initiation of chondrogenesis or osteogenesis of upper and lower limits to condensation size and the numbers of cells in a condensation are discussed, as illustrated by in vitro studies and by mutant embryos, including Talpid3 in the chick and Brachypod, Congenital hydrocephalus and Phocomelia in the mouse. Evidence that genes specific to the skeletal type are selectively activated at condensation is discussed, as is a recent model involving TGF-beta and fibronectin in condensation formation. Condensations emerge as a pivotal stage in initiation of the vertebrate skeleton in embryonic development and in the modification of skeletal morphology during evolution.

Localization of type II collagen, long form alpha 1(IX) collagen, and short form alpha 1(IX) collagen transcripts in the developing chick notochord and axial skeleton

In this study we compare, by in situ hybridization, the spatial and temporal expression patterns of transcripts of avian type II collagen and the long and short forms of the (alpha 1) chain of type IX collagen during the development of the notochord and axial skeleton. We observed type II collagen and short form type IX collagen transcripts in the developing (stage 25-28) nonchondrogenic notochord. Conversely, long form type IX transcripts were not detectable in the notochord or perinotochordal sheath. Interestingly, all three transcripts colocalized in the developing chondrogenic vertebrae of the axial skeleton as well as in the chondrocranium and Meckel's cartilage. The expression of the short form of type IX collagen in these regions was more restricted than that of the long form. This report provides additional support for a complex regulatory pathway of cartilage marker gene expression in chondrogenic vs. nonchondrogenic tissues during avian embryogenesis.

Collagen synthesis in granuloma annulare

Previous research has demonstrated active collagen synthesis in granuloma annulare (GA), a mainly degenerative disease of the skin. The present investigation is aimed to characterize details of the collagen synthesis and its regulation. Northern and in situ hybridization techniques and immunohistochemical methods are used to identify type I and type III collagen synthesis, regulation-associated polypeptides TGF-beta, Il-1 alpha, and Il-1 beta and an extracellular matrix protein tenascin, as well as lymphohistiocytic cells present in GA lesions. High mRNA levels of both pro-alpha 1 (I) and pro-alpha 1 (III) collagens were detected in GA lesions. In situ hybridization with cDNA probes revealed active fibroblasts with signals for both type I and III collagen mRNA around GA lesions. Some TGF-beta expression was found within the areas of inflammatory cells. Immunohistochemically, most of the mononuclear/lymphatic cells were CD3+ T cells. The helper/inducer phenotype (CD4+) was common among them, but there were no T-suppressor (CD8) cells. CD1+ cells were few in number, as were cells with activation or proliferation markers (CD26, CD30, and Ki67 antigens). Il-1 alpha- and Il-1 beta-positive lymphocytes/monocytes as well as interleukin-2 receptor containing cells were detected around the lesions, i.e., in the same areas as collagen-synthesizing fibroblasts. Another possible association with the regulation of collagen synthesis was the finding of an accumulation of tenascin, a growth-promoting extracellular matrix protein, in the surroundings of the GA lesions. We suggest that the firmly established and seemingly well-regulated type I and type III collagen synthesis presents a reparative phenomenon in the cutaneous lesions of GA.

Studies of mineralization in tissue culture: optimal conditions for cartilage calcification

The optimal conditions for obtaining a calcified cartilage matrix approximating that which exists in situ were established in a differentiating chick limb bud mesenchymal cell culture system. Using cells from stage 21-24 embryos in a micro-mass culture, at an optimal density of 0.5 million cells/20 microliters spot, the deposition of small crystals of hydroxyapatite on a collagenous matrix and matrix vesicles was detected by day 21 using X-ray diffraction, FT-IR microscopy, and electron microscopy. Optimal media, containing 1.1 mM Ca, 4 mM P, 25 micrograms/ml vitamin C, 0.3 mg/ml glutamine, no Hepes buffer, and 10% fetal bovine serum, produced matrix resembling the calcifying cartilage matrix of fetal chick long bones. Interestingly, higher concentrations of fetal bovine serum had an inhibitory effect on calcification. The cartilage phenotype was confirmed based on the cellular expression of cartilage collagen and proteoglycan mRNAs, the presence of type II and type X collagen, and cartilage type proteoglycan at the light microscopic level, and the presence of chondrocytes and matrix vesicles at the EM level. The system is proposed as a model for evaluating the events in cell mediated cartilage calcification.

Peptide growth factors and their interactions during chondrogenesis

Peptide growth factors have been implicated in three aspects of cartilage growth and metabolism; the induction of mesoderm and differentiation of a cartilaginous skeleton in the early embryo, the growth and differentiation of chondrocytes within the epiphyseal growth plates leading to endochondral calcification, and the processes of articular cartilage damage and repair. Three peptide growth factor classes have been strongly implicated in these processes, the fibroblast growth factor family (FGF), the insulin-like growth factors (IGFs) including insulin, and transforming growth factor-beta (TGF-beta) and related molecules. Each of these peptide groups are expressed in the early embryo. Basic FGF, TGF-beta and the related activin have been shown to induce the appearance of mesoderm from primitive neuroectoderm. TGF-beta and related bone morphometric proteins can induce the differentiation of cartilage from primitive mesenchyme, and together with basic FGF and IGFs promote cartilage growth. Each class of growth factor is expressed within the epiphyseal growth plate where their autocrine/paracrine interactions regulate the rate of chondrocyte proliferation, matrix protein synthesis and terminal differentiation and mineralization. Basic FGF may prove useful in articular cartilage repair, while basic FGF, IGFs and TGF-beta are among a number of growth factors and cytokines that have been implicated in cartilage disease.

Alternatively spliced type II procollagen mRNAs define distinct populations of cells during vertebral development: differential expression of the amino-propeptide

Type II collagen is a major component of cartilage providing structural integrity to the tissue. Type II procollagen can be expressed in two forms by differential splicing of the primary gene transcript. The two mRNAs either include (type IIA) or exclude (type IIB) an exon (exon 2) encoding the major portion of the amino (NH2)-propeptide (Ryan, M. C., and L. J. Sandell. 1990. J. Biol. Chem. 265:10334-10339). The expression of the two procollagens was examined in order to establish a potential functional significance for the two type II procollagen mRNAs. First, to establish whether the two mRNAs are functional, we showed that both mRNAs can be translated and the proteins secreted into the extracellular environment. Both proteins were identified as type II procollagens. Secondly, to test the hypothesis that differential expression of type II procollagens may be a marker for a distinct population of cells, specific procollagen mRNAs were localized in tissue by in situ hybridization to oligonucleotides spanning the exon junctions. Embryonic vertebral column was chosen as a source of tissue undergoing rapid chondrogenesis, allowing the examination of a variety of cell types related to cartilage. In this issue, each procollagen mRNA had a distinct tissue distribution during chondrogenesis with type IIB expressed in chondrocytes and type IIA expressed in cells surrounding cartilage in prechondrocytes. The morphology of the cells expressing the two collagen types was distinct: the cells expressing type IIA are narrow, elongated, and "fibroblastic" in appearance while the cells expressing type IIB are large and round. The expression of type IIB appears to be correlated with abundant synthesis and accumulation of cartilagenous extracellular matrix. The expression of type IIB is spatially correlated with the high level expression of the cartilage proteoglycan, aggrecan, establishing type IIB procollagen and aggrecan as markers for the chondrocyte phenotype. Transcripts of type II collagen, primarily type IIA, are also expressed in embryonic spinal ganglion. While small amounts of type II collagen have been previously detected in noncartilagenous tissues, the detection of this new form of the collagen in relatively high abundance in embryonic nerve tissue is unique. Taken together, these findings imply a potential functional difference between type IIA and type IIB procollagens and indicate that the removal of exon 2 from the pre-mRNA, and consequently the NH2-propeptide from the collagen molecule, may be an important step in chondrogenesis. In addition, type II procollagen, specifically type IIA, may function in noncartilage tissues, particularly during development.

Fibronectin and collagen gene expression in healing experimental colonic anastomoses

The temporal and spatial expression of fibronectin and type I and III collagen genes were studied 1-14 days after surgery in the healing rat colonic anastomosis using recombinant deoxyribonucleic acid techniques. Messenger ribonucleic acids (mRNAs) coding for fibronectin and type III collagen synthesis increased from the first day after operation and type I collagen synthesis increased from the second day after operation, as demonstrated by Northern hybridizations. Maximal mRNA production for fibronectin and collagens was seen at 2 and 7 days, respectively, after anastomosis. Activation of type I and III collagen genes in the anastomotic area was confined to tissues developing in the anastomotic line, the serosal surface and the submucosal layer. Strong fibronectin expression was observed in the same areas. The results suggest that genetic events leading to collagen synthesis in the anastomotic area start immediately after surgery. Maximal gene expression is not reached until 1 week after surgery.

Temporal and spatial expression of genes for cartilage extracellular matrix proteins during avian mandibular arch development

We have examined the temporal expression of genes for extracellular matrix proteins (type I collagen, type II collagen, and the cartilage specific proteoglycan core protein) during the development of the avian mandibular arch. We detected low levels of type II collagen mRNA in the mandibular arch as early as stage 15. Type II collagen mRNA remained low but increased slightly as development progressed from stage 15 to stage 25. More dramatic increases occurred after stage 25 coincident with overt chondrogenesis. In contrast, mRNA for the core protein of cartilage specific proteoglycan was not detected prior to the onset of chondrogenesis, appeared at stage 25, and increased thereafter. Type I collagen mRNA was also present as early as stage 15 and dramatically increased after stage 28/29, coincident with initiation of osteogenesis. Using in situ hybridization, we found that type II collagen mRNA became detectable in the center of the mandible around stage 24/25 coincident with the initiation of chondrogenesis. At later stages (26-32) type II collagen mRNA was localized in the cartilaginous rudiment. The pattern of hybridization observed with the proteoglycan core protein probe at later stages of development was essentially identical to that observed with the type II collagen probe. In contrast, the probe for the alpha 1 (I) collagen mRNA was localized over the perichondrium, over differentiated bone, and in areas within the mandibular arch where bone formation had been initiated.

Regulation of alkaline phosphatase and alpha 2(I) procollagen synthesis during early intramembranous bone formation in the rat mandible

We have studied intramembranous bone formation in the developing rat mandible. In this system discrete developmental stages can be readily distinguished: mesenchymal condensation, osteoid deposition, and mineralization. In mandibles of 14-day rat embryos avascular condensed mesenchymal cells can be discerned in a region lateral to Meckel's cartilage and anterior to the first molar bud. In 18-day embryos primary bone structures with mineral deposition are evident, and at 2 days postnatally the mandible is extensively mineralized. In the developing mandible we investigated the pattern of bone/liver/kidney/placenta (BLKP) alkaline phosphatase (ALP) and alpha 2(I) procollagen expression in the differentiating osteoblasts. The level of ALP activity in loose mesenchymal tissue is close to background levels. In contrast, the condensed mesenchymal cells in 14-day embryos, which will subsequently form bone, display intense ALP activity prior to discernible osteoid or mineral deposition. ALP activity in the condensed mesenchymal cells can be inhibited by levamisole, indicating activity of the BLKP gene product. We could not detect a corresponding increase in transcript level for either ALP or alpha 2(I) in the condensed mesenchyme in 14-day embryo using in situ hybridization, probably due to low message abundance. At 18 days, cells throughout the developing mandible express ALP activity, and intense in situ hybridization to BLKP ALP probes is evident in cells lining the developing bone trabeculae. Alpha 2(I) procollagen transcripts have accumulated in cells of the developing mandibular bone, but are not specifically localized to osteoblastic cells. Our results demonstrate that ALP activity is a very early marker of differentiation of cells of the osteogenic lineage, since a marked increase in ALP enzyme activity is clearly detectable in condensed mesenchymal cells prior to osteoid or mineral deposition. In contrast, Wright and Leblond, using the same model system and immunohistochemistry, could not localize type I collagen to preosteoblastic cells surrounding the developing bone trabeculae, and demonstrated localization of type I collagen to osteoblasts bordering developing trabeculae, indicating a substantial increase in type I collagen expression (at least at the protein level) during preosteoblast to osteoblast differentiation. These results indicate a discrete pattern of regulation for both the ALP and alpha 2(I) genes during osteogenic differentiation, which may involve both transcriptional and posttranscriptional regulation.

Developmental expression of genes in chick growth cartilage detected by in situ hybridization

We have used in situ hybridization to examine expression of collagen type I, II, and X mRNA and osteonectin mRNA in the chick epiphysis. Tissue samples from the proximal tibial growth cartilage were fixed in modified Carnoy's solution, dehydrated in ethanol, and embedded in paraffin. Longitudinal and transverse sections were demineralized with HCl and digested with hyaluronidase and proteinase K. In situ hybridization was carried out using biotinylated cDNA probes; the hybridized probe was detected using a streptavidin-biotinylated alkaline phosphatase conjugate. This procedure permitted detection of the corresponding mRNAs in cartilage with high sensitivity and low background. Osteonectin mRNA was detected in proliferating cartilage; lower levels of osteonectin mRNA were seen in the mid-hypertrophic region. This mRNA species was also expressed in cells that border the vascular canals in the premineralized region of the epiphysis. Collagen type X mRNA was detected throughout the hypertrophic zone. As localization of collagen type X mRNA corresponded to the site of maximal synthesis of the protein, reported in other studies, our results would further support the suggestion that this protein is associated with mineralization of cartilage. Collagen type II mRNA was seen in both the proliferating and the hypertrophic regions of the cartilage. Highest levels of expression were observed in the proliferative region. The results suggest that the transcriptional control of collagen type II and X by cells of the proliferating and hypertrophic regions of the growth cartilage may be related.

Dependence of inessential late gene expression on early meiotic events in Saccharomyces cerevisiae

SPR3 is one of at least nine genes which are expressed in sporulating Saccharomyces cerevisiae cells at the time of meiosis I. We show below that strains homozygous for null alleles of SPR3 are capable of normal meiosis and the production of viable ascospores. We have also monitored SPR3 expression in a series of strains that are defective in meiotic development, using an SPR3:lacZ fusion carried on a single copy plasmid. beta-Galactosidase activity occurred at wild-type levels in diploid strains homozygous for mutations in spo13, rad50, rad57 and cdc9, but was greatly reduced in strains carrying cdc8 or spo7 defects. We conclude that SPR3 expression is a valid monitor of early meiotic development, even though the gene is inessential for the sporulation process.

Widespread distribution of type II collagen during embryonic chick development

Type II collagen is a major component of hyaline cartilage, and has been suggested to be causally involved in promoting chondrogenesis during embryonic development. In the present study we have performed an immunohistochemical analysis of the distribution of type II collagen during several early stages of embryonic chick development. Unexpectedly, we have found that type II collagen is widely distributed in a temporally and spatially regulated fashion in basement membranes throughout the trunk of the embryo at stages 14 through 19, including regions with no apparent relationship to chondrogenesis. Immunohistochemical staining with two different monoclonal antibodies against type II collagen, as well as with an affinity-purified polyclonal antibody, is detectable in the basement membranes of the neural tube, notochord, auditory vesicle, dorsal/lateral surface ectoderm, lateral/ventral gut endoderm, mesonephric duct, and basal surface of the splanchnic mesoderm subjacent to the dorsal aorta, and at the interface between the epimyocardium and endocardium of the developing heart. In contrast, immunoreactive type IX collagen is detectable only in the perinotochordal sheath in the trunk of the embryo at these stages of development. Thus type II collagen is much more widely distributed during early development than previously thought, and may be fulfilling some as yet undefined function, unrelated to chondrogenesis, during early embryogenesis.