Separation of precursor myogenic and chondrogenic cells in early limb bud mesenchyme by a monoclonal antibody

Identification and characterization of chondrogenic progenitor cells in the fascia of postnatal skeletal muscle

Intramuscular injection of bone morphogenetic proteins (BMPs) has been shown to induce ectopic bone formation. A chondrogenic phase is typically observed in this process, which suggests that there may exist a chondrogenic subpopulation of cells residing in skeletal muscle. Two prospective cell populations were isolated from rat skeletal muscle: fascia-derived cells (FDCs), extracted from gluteus maximus muscle fascia (epimysium) and muscle-derived cells (MDCs) isolated from the muscle body. Both populations were investigated for their cell surface marker profiles (flowcytometry analysis), proliferation rates as well as their myogenic and chondrogenic potentials. The majority of FDCs expressed mesenchymal stromal cell markers but not endothelial cell markers. FDCs underwent chondrogenic differentiation after BMP4 treatment in vitro, but not myogenic differentiation. Although MDCs showed chondrogenic potential, they expressed the myogenic cell marker desmin and readily underwent myogenic differentiation in vitro; however, the chondrogenic potential of the MDCs is confounded by the presence of FDC-like cells residing in the muscle perimysium and endomysium. To clarify the role of the muscle-derived myogenic cells in chondrogenesis, mixed pellets with varying ratios of FDCs and L6 myoblasts were formed and studied for chondrogenic potential. Our results indicated that the chondrogenic potential of the mixed pellets decreased with the increased ratio of myogenic cells to FDCs supporting the role of FDCs in chondrogenesis. Taken together, our results suggest that non-myogenic cells residing in the fascia of skeletal muscle have a strong chondrogenic potential and may represent a novel donor cell source for cartilage regeneration and repair.

Cellular origin of microfibrils explored by monensin-induced perturbation of secretory activity in embryonic primary cultures

Fibrillin is a primary component of elastin-associated microfibrils. Since microfibrils are distributed rather ubiquitously in embryonic tissues, attention has focused on the types of cells responsible for producing fibrillin. To clarify this issue, we employed monensin-induced perturbation of secretory activity in embryonic primary cultures, as this would allow examination of both the secreted protein and the formation of extracellular fibrils in the same culture. Micromasses of avian limb bud mesoderm, its ectodermal covering and several explants from other sources were cultured in the presence and absence of monensin, and evaluated immunohistochemically using antibodies against fibrillin and cell lineage markers. The results indicated that monensin perturbation induced intracellular accumulation of fibrillin and prevented the formation of microfibrils. It was shown specifically that not only mesodermally derived fibrogenic cells and myogenic cells of skeletal and smooth muscle cell lineage, but also epithelial-type cells such as endothelial and ectodermal cells, are producers of fibrillin. This dual cellular origin of fibrillin at the ectomesenchymal interface is considered significant for understanding the formation and remodeling of microfibrils originating from the basal lamina.

Msx1 antagonizes the myogenic activity of Pax3 in migrating limb muscle precursors

The migration of myogenic precursors to the vertebrate limb exemplifies a common problem in development - namely, how migratory cells that are committed to a specific lineage postpone terminal differentiation until they reach their destination. Here we show that in chicken embryos, expression of the Msx1 homeobox gene overlaps with Pax3 in migrating limb muscle precursors, which are committed myoblasts that do not express myogenic differentiation genes such as MyoD. We find that ectopic expression of Msx1 in the forelimb and somites of chicken embryos inhibits MyoD expression as well as muscle differentiation. Conversely, ectopic expression of Pax3 activates MyoD expression, while co-ectopic expression of Msx1 and Pax3 neutralizes their effects on MyoD. Moreover, we find that Msx1 represses and Pax3 activates MyoD regulatory elements in cell culture, while in combination, Msx1 and Pax3 oppose each other's trancriptional actions on MyoD. Finally, we show that the Msx1 protein interacts with Pax3 in vitro, thereby inhibiting DNA binding by Pax3. Thus, we propose that Msx1 antagonizes the myogenic activity of Pax3 in migrating limb muscle precursors via direct protein-protein interaction. Our results implicate functional antagonism through competitive protein-protein interactions as a mechanism for regulating the differentiation state of migrating cells.

Response of "naive" cutaneous and muscle afferents to potential targets in vitro

It is now well documented that motoneurons are specified to innervate particular target muscles prior to axon outgrowth. Here we investigate whether sensory neurons are similarly specified to innervate target skin or muscle, taking advantage of the avian trigeminal system where cutaneous and muscle afferents are anatomically separate. Using this system, we have previously shown that by embryonic day 10 (E10) (approximately 4-5 days after target innervation), regenerating cutaneous and muscle afferents differ in their response to various potential targets in vitro, in manners consistent with their normal innervation patterns in vivo. Thus, by E10 these two populations of sensory neurons have distinct identities as skin and muscle afferents. In contrast, we report here that the responses of younger, naive cutaneous and muscle afferents that have not yet, or only recently, innervated peripheral targets are indistinguishable, regardless of the target tissue tested. These findings suggest that at stages when innervation is being established, cutaneous and muscle afferents, unlike motoneurons, may not yet have acquired rigidly specified identities and/or the ability to recognize and respond selectively to their appropriate peripheral targets.

Developmental restriction of embryonic calvarial cell populations as characterized by their in vitro potential for chondrogenic differentiation

The mechanism(s) by which the cells within the calvaria tissue are restricted into the osteogenic versus the chondrogenic lineage during intramembranous bone formation were examined. Cells were obtained from 12-day chicken embryo calvariae after tissue condensation, but before extensive osteogenic differentiation, and from 17-day embryo calvariae when osteogenesis is well progressed. Only cell populations from the younger embryos showed chondrogenic differentiation as characterized by the expression of collagen type II. The chondrocytes underwent a temporal progression of maturation and endochondral development, demonstrated by the expression of collagen type II B transcript and expression of collagen type X mRNA. Cell populations from both ages of embryos showed progressive osteogenic differentiation, based on the expression of osteopontin, bone sialoprotein, and osteocalcin mRNAs. Analysis using lineage markers for either chondrocytes or osteoblasts demonstrated that when the younger embryonic cultures were grown in conditions that were permissive for chondrogenesis, the number of chondrogenic cells increased from approximately 15 to approximately 50% of the population, while the number of osteogenic cells remained almost constant at approximately 35-40%. Pulse labeling of the cultures with BrdU showed selective labeling of the chondrogenic cells in comparison with the osteogenic cells. These data indicate that the developmental restriction of skeletal cells of the calvaria is not a result of positive selection for osteogenic differentiation but a negative selection against the progressive growth of chondrogenic cells in the absence of a permissive or inductive environment. These results further demonstrate that while extrinsic environmental factors can modulate the lineage progression of skeletal cells within the calvariae, there is a progressive restriction during embryogenesis in the number of cells within the calvaria with a chondrogenic potential. Finally, these data suggest that the loss of cells with chondrogenic potential from the calvaria may be related to the progressive limitation of the reparative capacity of the cranial bones.

Monoclonal antibodies reactive with human osteogenic cell surface antigens

Monoclonal antibodies (McAbs) against the surface of osteoblastic cells have been used to characterize the osteogenic lineage. In view of the paucity of probes against the surface of normal human osteogenic cells, we sought to generate McAbs which could be used for both in vivo and in vitro studies. We raised a series of McAbs against early osteoblastic cell surface antigens by immunizing mice with human mesenchymal stem cells (MSCs) that had been directed into the osteogenic lineage in vitro. After screening against the surface of osteogenic cells at various stages of differentiation in vitro, as well as evaluating in situ reactivity with human fetal limbs, we isolated three hybridoma cell lines referred to as SB-10, SB-20, and SB-21. Immunocytochemical analyses during osteogenic differentiation demonstrate that SB-10 reacts with MSCs and osteoprogenitors, but no longer reacts with cells once alkaline phosphatase (APase) is expressed. Flow cytometry documents that SB-10 is expressed on the surface of all purified, culture-expanded human MSCs, thus providing further evidence that these cells are a homogeneous population. By contrast, SB-20 and SB-21 do not react with the progenitor cells in situ, but bind to a subset of the APase-positive osteoblasts. None of these antibodies stain terminally differentiated osteocytes in sections of developing bone. Furthermore, these McAbs were not observed to react in samples from chick, rat, rabbit, canine, or bovine bone, although selected extraskeletal human tissues were immunostained. In all cell and tissue specimens examined, SB-20 immunostaining is identical to that observed with SB-21. We have used these McAbs to refine our understanding of the discrete cellular transitions that constitute the osteogenic cell lineage. We suggest a refined model for understanding osteoblast differentiation that is based on the proposition that the sequential acquisition and loss of specific cell surface molecules can be used to define positions of individual cells within the osteogenic cell lineage.

BMP signaling during bone pattern determination in the developing limb

To examine the role of BMP signaling during limb pattern formation, we isolated chicken cDNAs encoding type I (BRK-1 and BRK-2) and type II (BRK-3) receptors for bone morphogenetic proteins. BRK-2 and BRK-3, which constitute dual-affinity signaling receptor complexes for BMPs, are co-expressed in condensing precartilaginous cells, while BRK-1 is weakly expressed in the limb mesenchyme. BRK-3 is also expressed in the apical ectodermal ridge and interdigital limb mesenchyme. BRK-2 is intensely expressed in the posterior-distal region of the limb bud. During digit duplication by implanting Sonic hedgehog-producing cells, BRK-2 expression is induced anteriorly in the new digit forming region as observed for BMP-2 and BMP-7 expression in the limb bud. Dominant-negative effects on BMP signaling were obtained by over-expressing kinase domain-deficient forms of the receptors. Chondrogenesis of limb mesenchymal cells is markedly inhibited by dominant-negative BRK-2 and BRK-3, but not by BRK-1. Although the bone pattern was not disturbed by expressing individual dominant-negative BRK independently, preferential distal and posterior limb truncations resulted from co-expressing the dominant-negative forms of BRK-2 and BRK-3 in the whole limb bud, thus providing evidence that BMPs are essential morphogenetic signals for limb bone patterning.

Monoclonal antibodies as tools for studying the osteoblast lineage

Knowledge of the number and kinds of differentiation steps characterizing cells of the osteoblast lineage is inadequate. To analyze further osteoblast differentiation, a number of labs have generated monoclonal antibodies to osteogenic cells, derived from both normal bone and osteosarcomas. A variety of immunolabelling patterns on primary cell cultures, cell lines, and tissue sections has been reported, including cell surface, cytoplasmic, and extracellular matrix-associated patterns. Most of the antibodies selected recognize predominantly the mature osteoblast and osteocyte; in addition, however, antibodies have been generated that recognize pre-osteoblasts. Some recognize cells of both the osteoblast and chondroblast lineages and may contribute to a better understanding of the lineage and phenotypic relationships between these two cell types. In addition to recognition in vivo of cell subpopulations of discrete maturational stages, changes in the immunolabelling patterns in vitro have also documented a differentiation sequence in cells undergoing osteogenesis in cell and tissue cultures. In at least two cases, the antibodies have been used to isolate subpopulations of cells from bone, including relatively pure populations of osteocytes. With the exception of several antibodies that are against alkaline phosphatase or known matrix proteins including osteocalcin, the nature of the macromolecular species recognized by most of the antibodies generated to date are unknown. Recently, however, one antibody was used to clone the cDNA for the beta-galactoside-binding lectin, galectin 3 or epsilon binding protein (epsilon BP; IgE-binding protein; Mac-2), from a lambda gt11 osteoblast expression library; another was used to clone from an ROS 17/2.8-COS cell expression library the cDNA for OTS-8, a putative target gene of early response genes stimulated in response to phorbol esters in MC3T3-E1 cells. Neither of these macromolecules had previously been identified in bone cells, but the recent molecular and cellular analyses have shown them to be developmentally and/or hormonally regulated in osteoblastic cells. These antibodies extend the available markers and support earlier observations that a variety of molecules are differentially expressed by cells at different stages of the osteoblast lineage. This chapter will not be an exhaustive survey of all immunocytochemical and immunohistochemical analyses of osteogenic cells and tissues but will focus on the approach of eliciting novel monoclonal antibodies by the injection of osteogenic cells or crude bone extracts and its potential for establishing new markers of the osteoblast lineage. We have not included a large number of studies documenting the use of antibodies raised against several known bone matrix proteins; while these have been crucial in developing our current understanding of osteogenic differentiation, we sought rather to highlight the potential of the "random" injection approach.

Ca2+ channel activities in the limb bud of early embryonic chick

Ca2+ channel activities were recorded in the limb bud of embryonic day 4 chick with Ca2+ sensitive fluorescence (Fura-2) measurements and patch clamp techniques. Rises in intracellular Ca2+ concentrations were evoked by depolarization with the application of 100 mM K+ and this Ca2+ response was abolished by removing extracellular Ca2+. The Ca2+ response was blocked by 10 microM nifedipine and enhanced by 5 microM Bay K 8644. Long-lasting inward currents were revealed by whole-cell patch clamp recordings from dissociated cells of the limb bud. The inward current was also blocked by 10 microM nifedipine. Our study suggested the presence of L-type Ca2+ channels in the limb bud cells.

Leg bud mesoderm retains morphogenetic potential to express limb-like characteristics ("limbness") in collagen gel culture

Recent in situ hybridization studies have correlated expression of potential regulatory genes with pattern formation in limb bud mesoderm (Tabin: Cell 66:199-217, 1991); however, the mechanism(s) controlling their expression in mesoderm and their relevance to the establishment of a limb morphogenetic pattern remain unknown. One likely candidate for regulating patterning events in limb mesoderm is the apical ectodermal ridge, as its removal in ovo results in a graded truncation of limb skeletal elements in the proximal-distal axis dependent upon the time of excision (Rowe and Fallon: J Embryol Exp Morph 68:1-7, 1982). In the present study, we investigate whether the hypothetical imprint of ridge ectoderm is retained in cultured mesoderm. Specifically, we sought to determine if a subpopulation of limb mesoderm that forms in collagen gel culture (Markwald et al: Anat Rec 226:91-107, 1990), retains any expression of "limbness" in the absence of limb ectoderm as characterized by the formation of a predictable number and distribution of limb-like chondrogenic elements in comparison to the temporal and spatial relationships of the in situ proximal, hindlimb skeletal structures. Accordingly, explants of undissociated mesoderm from stage 18-22 chicken leg buds were cultured without ectoderm on collagen gel lattices and the central subpopulation of mesoderm was examined morphologically. We show that this central subset of mesoderm will form chondrogenic cells which were not expressed uniformly throughout the subset, but rather distinct nodules or elements of cartilage were elaborated. Moreover, the number of elements expressed by the central subset increased with the age of the mesoderm at the time of explantation; spatially and temporally, the sequence of elements that formed always proceeded from the proximal, anterior margin of the subset to its distal, posterior border. The shapes of the initial elements (designated I and II) resembled the forms of in situ proximal skeletal structures (girdle and femur-like), whereas more distal elements (III-V) were often fused and without structural similarity to in situ skeletal structures. When cultures were established from the posterior mesoderm of stage 19/20 or 21 mesoblasts, the frequency of element I formation was reduced approximately one-half, whereas formation of more distal elements was unaffected. Conversely, element formation from the central subset established from isolated anterior mesoderm was virtually identical to intact mesoblasts, indicating a capacity to regulate for the loss of mesoderm as occurs in situ (Hampé: Archs Anat Microsc Morph Exp 48:345-378, 1959).(ABSTRACT TRUNCATED AT 400 WORDS)

Promotion of embryonic limb cartilage differentiation in vitro by staurosporine, a protein kinase C inhibitor

Phorbol 12-myristate 13-acetate (PMA), a protein kinase C-activating phorbol ester, is known to inhibit chondrogenic differentiation by embryonic limb mesenchyme cells in vitro. The present study demonstrates that staurosporine, a potent inhibitor of protein kinase C, conversely stimulates cartilage differentiation in cultures of limb mesenchyme cells isolated from whole wing buds of stage 23/24 chick embryos or from the distal subridge region of stage 25 wing buds. In high density micromass cultures, in which limb mesenchyme cells undergo extensive spontaneous cartilage differentiation, exposure to 5-20 nM staurosporine promotes an accelerated accumulation of type II collagen and cartilage proteoglycan mRNA transcripts and a 2- to 3-fold increase in matrix glycosaminoglycan deposition. Even in low density, monolayer cultures in which the mesenchymal cells do not normally form cartilage, treatment with 5 nM staurosporine induces extensive Alcian blue-positive matrix production, a striking 4- to 18-fold rise in sulfated glycosaminoglycan accumulation, and a dramatic elevation of cartilage-characteristic gene transcript expression. Moreover, concurrent treatment with staurosporine overcomes the inhibitory effects of PMA on in vitro limb cartilage differentiation. The results suggest the hypothesis that protein kinase C might function as a negative modulator of chondrogenic differentiation during embryonic limb development.

Effects of ascorbate on myogenesis in micromass culture

Micromass cultures from stage 23 and 24 chick wing mesenchyme were grown in serum-containing medium with or without additional ascorbic acid. It was found that ascorbic acid administered as a single pulse or present continuously throughout culture, in concentrations as low as 25 micrograms/ml, was sufficient to abolish 80% of myogenesis as assessed by immunolocalization using muscle-specific antibodies. This effect was not significantly altered when cultures were maintained in a serum-free medium that promotes myogenesis. In contrast to the above findings, spectrophotometric analysis of accumulated sulphated glycosaminoglycans, an indicator of chondrogenesis, was elevated by ascorbate treatment. Furthermore, a similar level of glycosaminoglycan stimulation was found in ascorbate treated stage 23 distal-tip limb cultures that were essentially free of myogenic cells. We conclude, therefore, that the presence of myoblasts in whole-limb cultures has no appreciable inhibitory effects on chondrogenesis.

Morphogenesis of precursor subpopulations of chicken limb mesenchyme in three dimensional collagen gel culture

Although homogeneous in appearance, several lines of evidence suggest early (stage 17-19) limb mesenchymal cells are committed to particular cell lineages, e.g., myogenic or chondrogenic. However, subsequent expression of cell or tissue phenotype in the developing limb does not occur in a randomized process but rather in a spatially specific pattern. The potential regulatory mechanisms controlling the "patterned" expression of tissue phenotype in the limb have not been resolved. The purpose of this study was to determine if, prior to the formation of an apical ectodermal ridge, nondissociated limb mesenchyme has inherent morphogenetic potential to form nonrandomized patterns of tissue organization. The hypotheses to be tested were that, if provided a spatially permissive culture environment, 1) mesenchymal cells committed to a particular lineage would segregate into precursor (sub)populations prior to overt expression of phenotype and 2) the ultimate expression of a tissue phenotype may be regulated, in part, by histogenic interactions between the precursor cell groups. For these studies, mesoblasts (intact mesenchyme minus ectoderm) from stage 17-19 hindlimb buds were explanted intact to the surface of a 1-3 mm thick hydrated lattice of repolymerized type I collagen and incubated for 2-11 days. Examination of cultures at variable intervals revealed three distinct temporal sequences (periods) which were arbitrarily termed early morphogenesis (0-3 days), cytodifferentiation (3-5.5 days), and primitive tissue formation (5.5-11 days) based on similarities to in situ limb development. By the end of the first period, the mesenchymal cells had sorted into three distinct precursor populations: 1) an epithelial-like outgrowth of premyogenic and prefibrogenic cells at the surface of the gel lattice (termed the "surface subset") which circumscribed, 2) a centrally positioned prechondrogenic condensate ("central subset"), and overlaid 3) a dispersed, population of free cells that invaded the collagen lattice ("seeded subset"). Subsequent cytodifferentiation led to the appearance of multinucleated myotubes within the surface subset and chondrification of the central subset. Cells of the seeded subset remained dispersed within the collagen lattice. Primitive histogenic events were initiated during the final period of development including 1) at sites where surface cells established boundaries with the central subset, collectives or "bundles" of variable sized myotubes were formed which became partially ensheathed by the attenuated processes of fibroblastlike cells; and 2) a secondary site of chondrogenic activity was initiated within the gel lattice at the boundary between the central and seeded cell populations. Transformation of seeded fibroblasts into chondroblasts accompanied expansion of the secondary chondrogenic element within the gel lattice.(ABSTRACT TRUNCATED AT 400 WORDS)

Posterior half amputation of the chick wing bud: the response of the developing vasculature, and subsequent wound healing

Experimental analyses examining pattern formation in the developing chick limb have concentrated on the skeleton, muscles and nerves, and have rarely considered blood vessels. To investigate the relationship between the vasculature and limb development, posterior amputations were performed on 3.5-4 day chick limb-buds. It has been shown that the removal of the posterior half alters the developmental fate of the anterior tissue: it becomes necrotic and fails to differentiate into the complement of skeletal parts predicted by fate maps. The possibility that this developmental failure results from interference with the future arterial supply was examined by Indian ink injection between 3-48 h after operation. Scanning electron microscopy (SEM) and resin histology were used to examine the wound repair at similar post-operative intervals. Results from the Indian ink injections showed that within 6 h of operation a collateral circulation was established by means of a branch from the truncated primary subclavian artery. The capillary density in the operated limbs appeared normal when compared to the contralateral limb. The results support the view that the poor developmental performance of the anterior half is due to removal of the zone of polarizing activity (ZPA) rather than to experimentally-induced alteration to the vascular supply. Histological and SEM examination of the wound healing process showed that epithelialization of the cut surface occurred within 24 h, and that the peridermal cells of the bilayered ectoderm appeared to initiate the regrowth. The wound site was not visible 48 h after operation, showing that wound healing at these developmental ages occurs quickly, with no scar tissue formation.(ABSTRACT TRUNCATED AT 250 WORDS)

Ontogeny of architectural complexity in embryonic quail visceral arch muscles

Understanding the mechanisms of muscle pattern formation requires that the complete sequence of ontogenetic events be defined, particularly in the emergence of architectural complexity and in the spatial relations between muscles and skeletal elements. This analysis of visceral arch myogenesis in quail (Coturnix coturnix japonica) embryos identifies the location of premuscle condensations and subsequent segregation of individual muscles, documents the initial orientation of myofibers and changes in alignment associated with maturation, and describes the spatial and temporal relations between muscle development and the formation of connective tissues. Premuscle condensations form within the visceral arches on embryonic days 2-4, before skeletal elements make their appearance. Discrete muscles may form from the subdivision of a muscle mass after fiber orientations have been established (e.g., jaw adductor and hyobranchial muscles) or by the segregation of a mesenchymal cluster from the condensation prior to the appearance of oriented fibers (e.g., protractor, muscle of the columella). The rate and pattern of subsequent muscle maturation are closely associated with the development of the hard tissues. Myogenesis in 4-9-day embryos centers around the quadrate cartilage, the retroarticular process of the mandibular (Meckel's) cartilage, and the epibranchial cartilage. Muscles form attachments on these elements and remain without additional attachments until the appropriate elements (e.g., otic capsule, pterygoid bone) develop. No single description of myogenic events applies to all visceral arch muscles, nor is there an arch-specific pattern of ontogeny. Rather, each muscle has distinctive characteristics based on its spatial relations within the developing head.

In situ hybridization analysis of the expression of the type II collagen gene in the developing chicken limb bud

In situ hybridization with [32P]- or [35S]-labeled double-stranded DNA or single-stranded RNA probes was used to investigate the temporal and spatial distribution of cartilage-characteristic type II collagen mRNA during embryonic chick limb development and cartilage differentiation in vivo. When the type II collagen probes were hybridized to sections through embryonic limb buds at the earliest stages of their development (stages 18-25), an accumulation of silver grains representing type II collagen mRNA first became detectable in the proximal central core of the limb coincident with the prechondrogenic condensation of mesenchymal cells that characterizes the onset of cartilage differentiation. At later stages of development (stage 32; 7 days) intense hybridization signals with the type II collagen probes were localized over the well differentiated cartilage rudiments, whereas few or no silver grains above background were observed over the non-chondrogenic tissues. In contrast, sections hybridized with a probe complementary to mRNA for the alpha 1 chain of type I collagen exhibited an intense hybridization signal over the perichondrium and little or no signal over the cartilage primordia. At all stages of development examined, [32P]-labeled double-stranded DNA probes or single-stranded RNA probes labeled with either [32P] or [35S] provided adequate hybridization signals. Several experimental protocols were employed to control for the potential cross-hybridization and non-specific hybridization of the type II collagen probes. These included the utilization of labeled noncomplementary "sense-strand" type II collagen RNA as a control probe for nonspecific background, and prehybridization with a large excess of appropriate unlabeled RNA to block sequences in heterologous collagen RNAs that might cross-hybridize to the specific labeled probe.

Spatial and temporal changes in the pattern of glycosylation of the developing chick limb tissue components as revealed by fluorescent conjugated lectin probes

The changing pattern of expression of glycoconjugates during the differentiation of the chick leg bud between stages 17 to 34 (days 3 to 8 of incubation) was studied using fluorochrome-labelled plant lectins. Limb buds were fixed in cold acetic-alcohol and wax-embedded. Agglutinins of peanut (PNA), soybean (SBA) and succinylated wheat germ (WGAs) revealed a specific binding pattern in the apical ectodermal ridge (AER) between Hamburger and Hamilton stages 19-32. These stages coincide with the period of elevation of the AER. This specific binding pattern was absent from the adjacent dorsal and ventral ectoderm. Prechondrogenic cells were positive for WGA and for PNA, and the PNA-binding capacity was intensified after neuraminidase treatment. Premyogenic cells at stage 23 can be identified as negative to PNA after neuraminidase, while the blood vessels became positive. PNA, SBA, WGA, WGAs and, in addition, Ricinus communis (RCA-I) lectins stained the basal membrane. Strands of extracellular matrix which connect with the basal membrane and cross the limb transversely between dorsal and ventral ectoderm were stained by RCA-I, SBA and PNA after neuraminidase.

Microtiter micromass cultures of limb-bud mesenchymal cells

A method is described for growing high-density micromass cultures of chick and mouse limb mesenchyme cells in 96-well microtiter plates (microT microM cultures). Rapid quantitative estimates of chondrogenic expression were obtained by automated spectrophotometric analysis of Alcian-blue-stained cartilage matrix extracts performed in the wells in which the cells had been grown. Quantitative estimates of myogenic expression were obtained similarly using anti-sarcomere myosin monoclonal antibody and modified ELISA techniques. This microT microM-ELISA method may be adapted for use with other antigens for which specific antibodies are available. These methods were used to compare cartilage and muscle differentiation in 1 to 4 d microT microM cultures grown in serum-containing (SCM) and defined (DM) media. The DM contains minimal additives (insulin, hydrocortisone, and in some cases, ascorbate or transferrin) and supports both chondrogenesis and myogenesis. The colorimetric analyses agree well with the morphologic appraisal of chondrogenesis and myogenesis. Similar numbers of cartilage nodules formed in all cultures, but in DM the nodules failed to enlarge; explaining the reduced matrix synthesis in DM as compared with SCM, and suggesting that nodule enlargement is a discrete, serum-dependent step. Studies of selected additives to DM show that transferrin enhances myogenesis, ascorbic acid enhances chondrogenesis, and retinoic acid inhibits chondrogenesis. Together, the microT microM system, in situ colorimetric assays of chondrogenesis and myogenesis, and DM will allow rapid prescreening of teratogens and screening of various bioactive compounds (e.g., hormones, growth factors, vitamins, adhesion factors) for effects on limb mesenchymal cell differentiation.

Combinations of monoclonal antibodies distinguish mesenchymal, myogenic, and chondrogenic precursors of the developing chick embryo

Monoclonal antibodies (MAbs) were used as probes for molecular differences in the surfaces of nonterminally differentiated cells of the developing chick limb. The specificity of the MAbs was determined by immunofluorescent localization performed on cultured breast muscle and limb bud cells and cryosections of a variety of embryonic (stages 15-37) and neonatal tissues. Subpopulations of MAb-positive and -negative cells were isolated by fluorescence-activated cell sorting and their developmental potential was assessed in vitro. Cells of the compacted somite, lateral plate mesoderm, and early limb bud were labeled with the CSAT MAb. Myogenic precursors of the dermatome and limb bud were labeled with the CSAT and L4 MAbs. Chondrogenic precursors of the sclerotome and limb bud were labeled with the CSAT, L4, and C5 MAbs. These precursors were distinguished from fibroblasts which were labeled with the CSAT and C1 MAbs. The differentiation and maturation of muscle and cartilage were accompanied by alterations in the labeling patterns of the MAbs. These results indicate that combinations of these MAbs can be used to distinguish mesenchymal, myogenic, and chondrogenic precursors, identify their site of origin during development, and isolate subpopulations of embryonic cells.

Expression of a single transfected cDNA converts fibroblasts to myoblasts

5-azacytidine treatment of mouse C3H10T1/2 embryonic fibroblasts converts them to myoblasts at a frequency suggesting alteration of one or only a few closely linked regulatory loci. Assuming such loci to be differentially expressed as poly(A)+ RNA in proliferating myoblasts, we prepared proliferating myoblast-specific, subtracted cDNA probes to screen a myocyte cDNA library. Based on a number of criteria, three cDNAs were selected and characterized. We show that expression of one of these cDNAs transfected into C3H10T1/2 fibroblasts, where it is not normally expressed, is sufficient to convert them to stable myoblasts. Myogenesis also occurs, but to a lesser extent, when this cDNA is expressed in a number of other cell lines. The major open reading frame encoded by this cDNA contains a short protein segment similar to a sequence present in the myc protein family.

Regression of blood vessels precedes cartilage differentiation during chick limb development

We have previously investigated distinct areas of vascular regression in the developing vascular system of the chick limb bud. Avascular areas appear in a characteristic spatial and temporal pattern, and are correlated with the position of developing cartilage. In the present study, we examined limb-bud sections which had been double labeled for endothelial cells and developing cartilage in order to determine the relationship between the appearance of cartilage and the disappearance of capillaries. Endothelial cells, which specifically take up acetylated low-density lipoprotein (acLDL), were labeled by intravenously injecting fluorescent acLDL (DiIacLDL) into chick embryos at Hamburger and Hamilton stages 26-30. Avascular zones, which correspond to the developing digits, were clearly visible within the fluorescently labeled distal vasculature. The same sections were labeled with monoclonal antibodies specific for cartilage. We found that progressing avascularity in the digital regions was followed by increased staining for cartilage antigens in the same areas. Zones of avascularity always developed earlier than morphologically and immunologically detectable cartilage in all planes of section and were always larger than the areas of cartilage. These results demonstrate that blood vessels disappear in predictable areas prior to the overt differentiation of cartilage.

Separation of the myogenic and chondrogenic progenitor cells of undifferentiated limb mesenchyme

Undifferentiated limb bud mesenchyme consists of at least two separate, possibly predetermined, populations of progenitor cells, one derived from somitic mesoderm that gives rise exclusively to skeletal muscle and one derived from somatopleural mesoderm that gives rise to the cartilage and connective tissue of the limb. In the present study, we demonstrate that the inherent migratory capacity of myogenic precursor cells can be used to physically separate the myogenic and chondrogenic progenitor cells of the undifferentiated limb mesenchyme at the earliest stages of limb development. When the undifferentiated mesenchyme of stage 18/19 chick embryo wing buds or from the distal subridge region of stage 22 wing buds is placed intact upon the surface of fibronectin (FN)-coated petri dishes, a large population of cells emigrates out of the explants onto the FN substrates and differentiates into an extensive interlacing network of bipolar spindle-shaped myoblasts and multinucleated myotubes that stain with monoclonal antibody against muscle-specific fast myosin light chain. In contrast, the cells of the explants that remain in place and do not migrate away undergo extensive cartilage differentiation. Significantly, there is no emigration of myogenic cells out of explants of stage 25 distal subridge mesenchyme, which lacks myogenic progenitor cells. Myogenic precursor cells stream out of mesenchyme explants in one or occasionally two discrete locations, suggesting they are spatially segregated in discrete regions of tissue at the time of its explantation. There are subtle overall differences in the morphologies of the myogenic cells that form in stage 18/19 and stage 22 distal subridge mesenchyme explants. Finally, groups of nonmyogenic nonfibroblastic cells which are fusiform-shaped and oriented in distinct parallel arrays characteristically are found along the periphery of stage 18/19 wing mesenchyme explants. Our observations provide support for the concept that undifferentiated limb mesenchyme consists of independent subpopulations of committed precursor cells and provides a system for studying the early determinative and regulatory events involved in myogenesis or chondrogenesis.

The differentiation of normal and muscle-free distal chick limb bud mesenchyme in micromass culture

Distal chick wing bud mesenchyme from stages 19 to 27 embryos has been grown in micromass culture. The behavior of cultures comprising mesenchyme located within 350 microns of the apical ectodermal ridge (distal zone mesenchyme) was compared to that of cultures of the immediately proximal mesenchyme (subdistal zone cultures). In cultures of the distal mesenchyme from stages 21-24 limbs, all of the cells stained immunocytochemically for type II collagen within 3 days, indicating ubiquitous chondrogenic differentiation. At stage 19 and 20, this behavior was only observed in cultures of the distal most 50-100 microns of the limb bud mesenchyme. Between stages 25 and 27, distal zone cultures failed to become entirely chondrogenic. At all stages, subdistal zone cultures always contained substantial areas of nonchondrogenic cells. The different behavior observed between distal zone and corresponding subdistal zone cultures appears to be a consequence of the presence of somite-derived presumptive muscle cells in the latter, since no such difference was observed in analagous cultures prepared from muscle-free wing buds. The high capacity of the distal zone for cartilage differentiation supports a view of pattern formation in which inhibition of cartilage is an important component. However, its consistent behavior in vitro indicates that micromass cultures do not reflect the in vivo differences between the distal zones at different stages. The subdistal region retains a high capacity of cartilage differentiation and the observed behavior in micromass reflects interactions with a different cell population.

The independence of myogenesis and chondrogenesis in micromass cultures of chick wing buds

Micromass cultures prepared from stage 23, 24, or 25 chick wing buds and cultured under identical conditions produce similar numbers of myoblasts. After treatment with the DNA synthesis inhibitor cytosine-1-beta-D-arabinofuranoside, [3H]thymidine labeling and autoradiography of the cultures show that the increase in myoblast number during the first 48 hr of culture is due primarily to cell division. Micromass cultures prepared from proximal and distal portions of stage 23 or 24 wing buds have very different chondrogenic potentials in vitro (B.J. Swalla, E.M. Owens, T.F. Linsenmayer, and M. Solursh (1983). Dev. Biol. 97, 59-69) but a similar myogenic potential under these culture conditions. Medium supplements that significantly enhance chondrogenesis by proximal cell cultures, such as low serum or 1 mM db cyclic AMP, do not affect the number of myoblasts per unit area of culture during the first 3 days. Muscle cells are eventually reduced in number in whole limb micromass cultures, yet persist as long as 6 days in proximal and distal cultures. These results suggest that myogenic cells are already committed in the early limbs but are inhibited from differentiation in situ until a later time. Myogenesis and chondrogenesis occur independently in culture, consistent with the idea that these two differentiated cells are derived from two separate cell populations. Furthermore, treatments which enhance chondrogenesis do not act indirectly by killing the myoblast population in these cultures.

Differential survival of cartilage and muscle cells in chick limb-bud cell cultures maintained in chemically defined and serum-containing media

Chick limb buds at stages 22-23 largely consist of replicating presumptive chondroblasts and presumptive myoblasts. To study the influence that different medium compositions may have on the survival, replication, and terminal differentiation of these dissociated cells in vitro, micromass cultures were reared in either standard Dulbecco's modified Eagle's medium containing fetal calf serum (SC-DMEM) or in serum-free DMEM. By day 4, approximately 80% and 50% of the original cell inoculum had been lost in DMEM and SC-DMEM cultures, respectively, as estimated from the recovery of incorporated 3H-thymidine. Between days 1 and 4, the total-DNA content remained virtually constant in DMEM cultures, while it increased five- to sixfold in SC-DMEM cultures. In both media, definitive myoblasts and chondroblasts first emerged on day 1 and day 2, respectively, as determined by immunofluorescence staining using antibodies against muscle light meromyosin (LMM) or the major cartilage proteoglycan. In both media, the chondroblasts increased in number and, by day 4, had formed sizable chondroblast nodules. The number of chondroblasts in SC-DMEM cultures exceeded that observed in DMEM cultures. In DMEM, the LMM-positive myoblasts had an atypical morphology and failed to fuse into elongated myotubes; these cells began to degenerate on about day 4, being undetectable by day 8. In SC-DMEM, the numerous LMM-positive myoblasts located in the center of the micromasses also had an atypical morphology, failed to form multinucleated myotubes, and were absent by day 8.(ABSTRACT TRUNCATED AT 250 WORDS)

A cartilage-derived growth factor enhances hyaluronate synthesis and diminishes sulfated glycosaminoglycan synthesis in chondrocytes

Cartilage-derived growth factor purified to homogeneity by affinity chromatography on columns of heparin-Sepharose was mitogenic for early passage bovine fetal chondrocytes. Hyaluronate and sulfated glycosaminoglycan synthesis in these cells was analyzed by differential enzymatic digestion of the glycosaminoglycans labeled with [14C] glucosamine or [35S]. It was found that chondrocyte proliferation was accompanied by about a four-fold increase in hyaluronate synthesis over a two-day period, while the synthesis of sulfated glycosaminoglycans decreased by about 2-fold. Chromatographic analysis of the sulfated glycosaminoglycans showed decreases in chondroitin 4 and 6 sulfates. It was concluded from these results that cartilage-derived growth factor was a proliferative factor for chondrocytes and differed from the somatomedins.

Development of avascularity during cartilage differentiation in the embryonic limb. An exclusion model

The differentiation of cartilage and muscle in limb-bud mesenchyme has been interpreted by some investigators in terms of a vascular pre-pattern model. It has been argued that a pre-pattern of the early limb vasculature compartmentalises the mesenchyme into specific microenvironmental areas in which, depending on the oxygen tension and nutrient supply, cartilage or muscle will differentiate. However, recent analyses of the development and differentiation of blood vessels in limbs have shown that regional variations in vascularization develop co-incidentally with the earliest indication of cartilage formation or mesenchymal condensation. The simple model described in the present study suggests that the mechanical compression/tension forces generated by the condensing mesenchyme are sufficient to constrict and eventually close off the thin-walled undifferentiated vessels caught in the condensation foci, thus leading to the avascularity of cartilage rudiments. This view suggests that the vasculature has no major function in governing the pattern of cartilage differentiation.

Protein extracts from early embryonic hearts initiate cardiac endothelial cytodifferentiation

Prior to the formation of multiple chambers, the embryonic heart consists of two epithelial tubes, one within the other. As development proceeds, portions of the inner epithelium, i.e., the endothelium, undergo a morphological transformation into a migrating mesenchymal cell population. Our results show that this transformation is affected by proteins secreted by the outer epithelium, i.e., the myocardium, into the extracellular matrix between these two tissues. This conclusion is based on tissue autoradiographic studies of whole embryo cultures with 3H-amino acids. Continuous labeling conditions generated an apparent gradient of proteins extending away from the myocardium and contacting the endothelium just prior to the formation of mesenchyme, i.e., activation of the transformation sequence. Pulse/chase studies confirmed this directional movement of matrix protein. By performing sequential extractions of preactivation staged embryonic hearts with EDTA and testicular hyaluronidase followed by ammonium sulfate precipitation we obtained an enriched preparation of cardiac extracellular matrix. This fraction was capable of eliciting several of the events characteristic of endothelial activation in vitro. These events included: (i) cell-cell separation, (ii) lateral cell mobility, and (iii) hypertrophy and polarization of intracellular PAS staining (Golgi apparati). The biological activity of the extract was sensitive to heat denaturation: a homogenate of the remaining extracted tissue would not substitute for the matrix extract. Morphologically the extracted hearts appeared intact, however, the extracellular matrix space was significantly diminished. No more than 6% of the total lactic dehydrogenase activity, a cytosolic enzyme, was found in the extract. Preliminary electrophoretic characterization of the extract (metabolically labeled with 14C-amino acids) indicated that it may contain as many as 35 proteins or subunits. The relationship of ECM to endothelial differentiation in cardiac morphogenesis is discussed as a model for other developmental systems.

Immunological analysis of chick notochord and cartilage matrix development with antisera to cartilage matrix macromolecules

Transverse frozen sections from the postcephalic region of stage 9-16 chick embryos and from the wing bud region of stage 17-31 embryos were stained with antibodies to the major extracellular matrix components of cartilage. These probes included unfractionated A1 and A2 antisera to the major cartilage proteoglycan, affinity-purified purified antibodies to the proteoglycan core protein and to Type II collagen, and a monoclonal antibody to keratan sulfate. In embryos as early as stage 10, notochord stained specifically with the keratan sulfate monoclonal antibody. At this stage the notochord, as well as surrounding tissues, were negative to cartilage proteoglycan and collagen antibodies. Positive staining with the latter probes was coordinately acquired by notochord cells and their accompanying sheath around stage 15, while surrounding tissues remained negative. At this stage, the ventral region of the perispinal cord sheath exhibited light staining with the proteoglycan and keratan sulfate antibodies though failing to react to Type II collagen antibodies. Positive staining of notochord and ventral spinal cord persisted through later developmental stages. As revealed by immunofluorescence, definitive vertebral chondroblasts first emerged at approximately stage 23 and definitive limb chondroblasts at stage 25. The results are discussed in terms of the possible multiple roles of notochord in early embryogenesis.