Stimulation of embryonic mouse limb bud mesenchymal cell growth by peptide growth factors

From Skeletal Development to Tissue Engineering: Lessons from the Micromass Assay

Damage and degeneration of the skeletal elements due to disease, trauma, and aging lead to a significant health and economical burden. To reduce this burden, skeletal tissue engineering strategies aim to regenerate functional bone and cartilage in the adult body. However, challenges still exist. Such challenges involve the identification of the external cues that determine differentiation, how to control chondrocyte hypertrophy, and how to achieve specific tissue patterns and boundaries. To address these issues, it could be insightful to look at skeletal development, a robust morphogenetic process that takes place during embryonic development and is commonly modeled in vitro by the micromass assay. In this review, we investigate what the tissue engineering field can learn from this assay. By comparing embryonic skeletal precursor cells from different anatomic locations and developmental stages in micromass, the external cues that guide lineage commitment can be identified. The signaling pathways regulating chondrocyte hypertrophy, and the cues required for tissue patterning, can be elucidated by combining the micromass assay with genetic, molecular, and engineering tools. The lessons from the micromass assay are limited by two major differences between developmental and regenerative skeletogenesis: cell type and scale. We highlight an important difference between embryonic and adult skeletal progenitor cells, in that adult progenitors are not able to form mesenchymal condensations spontaneously. Also, the mechanisms of tissue patterning need to be adjusted to the larger tissue engineering constructs. In conclusion, mechanistic insights of skeletal tissue generation gained from the micromass model could lead to improved tissue engineering strategies and constructs.

Insulin-like growth factors in embryonic and fetal growth and skeletal development (Review)

The insulin-like growth factors (IGF)-I and -II have a predominant role in fetal growth and development. IGFs are involved in the proliferation, differentiation and apoptosis of fetal cells in vitro and the IGF serum concentration has been shown to be closely correlated with fetal growth and length. IGF transcripts and peptides have been detected in almost every fetal tissue from as early in development as pre‑implantation to the final maturation stage. Furthermore, IGFs have been demonstrated to be involved in limb morphogenesis. However, although ablation of Igf genes in mice resulted in growth retardation and delay in skeletal maturation, no impact on outgrowth and patterning of embryonic limbs was observed. Additionally, various molecular defects in the Igf1 and Igf1r genes in humans have been associated with severe intrauterine growth retardation and impaired skeletal maturation, but not with truncated limbs or severe skeletal dysplasia. The conflicting data between in vitro and in vivo observations with regard to bone morphogenesis suggests that IGFs may not be the sole trophic factors involved in fetal skeletal growth and that redundant mechanisms may exist in chondro- and osteogenesis. Further investigation is required in order to elucidate the functions of IGFs in skeletal development.

Disparate Igf1 expression and growth in the fore- and hind limbs of a marsupial mammal (Monodelphis domestica)

Proper regulation of growth is essential to all stages of life, from development of the egg into an embryo to the maintenance of normal cell cycle progression in adults. However, despite growth's importance to basic biology and health, little is known about how mammalian growth is regulated. In this study, we investigated the molecular basis of the highly disparate growth of opossum fore- and hind limbs in utero. We first used a novel, opossum-specific microarray to identify several growth-related genes that are differentially expressed in opossum fore- and hind limbs of comparable developmental stages. These genes included Igf1. Given Igf1's role in the growth of other systems, we further investigated the role of Igf1 in opossum limb growth. Supporting the microarray results, RT-PCR indicated that Igf1 levels are approximately two times higher in opossum fore- than hind limbs. Consistent with this, while Igf1 transcripts were readily detectable in opossum forelimbs using whole-mount in situ hybridization, they were not detectable in opossum hind limbs. Furthermore, opossum limbs treated with exogenous Igf1 protein experienced significantly greater cellular proliferation and growth than control limbs in vitro. Taken together, results suggest that the differential expression of Igf1 in developing opossum limbs contributes to their divergent rate of growth, and the unique limb phenotype of opossum newborns. This study establishes the opossum limb as a new mammalian model system for study of organ growth.

Differentiation and mineralization of murine mesenchymal C3H10T1/2 cells in micromass culture

The murine mesenchymal cell line, C3H10T1/2 in micromass culture undergoes chondrogenic differentiation with the addition of BMP-2. This study compares the use of BMP-2 vs. insulin, transferrin, and sodium selenite (ITS) to create a chondrogenic micromass cell culture system that models cartilage calcification in the presence of 4mM inorganic phosphate. BMP-2 treated cultures showed more intense alcian blue staining for proteoglycans than ITS treated cultures at early time points. Both ITS and BMP-2 treated cultures showed similar mineral deposition in cultures treated with 4mM phosphate via von Kossa staining, however FTIR spectroscopy of cultures showed different matrix properties. ITS treated cultures produced matrix that more closely resembled mouse calcified cartilage by FTIR analysis. (45)Ca uptake curves showed delayed onset of mineralization in cultures treated with BMP-2, however they had an increased rate of mineralization (initial slope of (45)Ca uptake curve) when compared to the cultures treated with ITS. Immunohistochemistry showed the presence of both collagens type I and type II in BMP-2 and ITS treated control (1mM inorganic phosphate) and mineralizing cultures. BMP-2 treated mineralizing cultures displayed more intense staining for collagen type II than all other cultures. Collagen type X staining was detected at Day 9 only in mineralizing cultures treated with ITS. Western blotting of Day 9 cultures confirmed the presence of collagen type X in the mineralizing ITS cultures, and also showed very small amounts of collagen type X in BMP-2 treated cultures and control ITS cultures. By Day 16 all cultures stained positive for collagen type X. These data suggest that BMP-2 induces a more chondrogenic phenotype, while ITS treatment favors maturation and hypertrophy of the chondrocytes in the murine micromass cultures.

Molecular profiling of the developing mouse axial skeleton: a role for Tgfbr2 in the development of the intervertebral disc

Background

Very little is known about how intervertebral disc (IVD) is formed or maintained. Members of the TGF-β superfamily are secreted signaling proteins that regulate many aspects of development including cellular differentiation. We recently showed that deletion of Tgfbr2 in Col2a expressing mouse tissue results in alterations in development of IVD annulus fibrosus. The results suggested TGF-β has an important role in regulating development of the axial skeleton, however, the mechanistic basis of TGF-β action in these specialized joints is not known. One of the hurdles to understanding development of IVD is a lack of known markers. To identify genes that are enriched in the developing mouse IVD and to begin to understand the mechanism of TGF-β action in IVD development, we undertook a global analysis of gene expression comparing gene expression profiles in developing mouse vertebrae and IVD. We also compared expression profiles in tissues from wild type and Tgfbr2 mutant mice as well as in sclerotome cultures treated with TGF-β or BMP4.

Results

Lists of IVD and vertebrae enriched genes were generated. Expression patterns for several genes were verified either through in situ hybridization or literature/database searches resulting in a list of genes that can be used as markers of IVD. Cluster analysis using genes listed under the Gene Ontology terms multicellular organism development and pattern specification indicated that mutant IVD more closely resembled vertebrae than wild type IVD. We also generated lists of genes regulated by TGF-β or BMP4 in cultured sclerotome. As expected, treatment with BMP4 resulted in up-regulation of cartilage marker genes including Acan, Sox 5, Sox6, and Sox9. In contrast, treatment with TGF-β1 did not regulate expression of cartilage markers but instead resulted in up-regulation of many IVD markers including Fmod and Adamtsl2.

Conclusions

We propose TGF-β has two functions in IVD development: 1) to prevent chondrocyte differentiation in the presumptive IVD and 2) to promote differentiation of annulus fibrosus from sclerotome. We have identified genes that are enriched in the IVD and regulated by TGF-β that warrant further investigation as regulators of IVD development.

A predominantly articular cartilage-associated gene, SCRG1, is induced by glucocorticoid and stimulates chondrogenesis in vitro

OBJECTIVE

Cartilage tissue engineering using multipotential human mesenchymal stem cells (hMSCs) is a promising approach to develop treatment for degenerative joint diseases. A key requirement is that the engineered tissues maintain their hyaline articular cartilage phenotype and not proceed towards hypertrophy. It is noteworthy that osteoarthritic articular cartilage frequently contains limited regions of reparative cartilage, suggesting the presence of bioactive factors with regenerative activity. Based on this idea, we recently performed cDNA microarray analysis to identify genes that are strongly expressed only in articular cartilage and encode secreted gene products. One of the genes that met our criteria was SCRG1. This study aims to analyze SCRG1 function in cartilage development using an in vitro mesenchymal chondrogenesis system.

METHODS

Full-length SCRG1 cDNA was subcloned into pcDNA5 vector, and transfected into hMSCs and murine C3H10T1/2 mesenchymal cells, placed in pellet cultures and micromass cultures, respectively. The cultures were analyzed by reverse transcription-polymerase chain reaction for the expression of SCRG1 and cartilage marker genes, and by histological staining for cartilage phenotype.

RESULTS

Induction of SCRG1 expression was seen during in vitro chondrogenesis, and was dependent on dexamethasone (DEX) known to promote chondrogenesis. Immunohistochemistry revealed that SCRG1 protein was localized to the extracellular matrix. Forced expression of SCRG1 in hMSCs suppressed their proliferation, and stimulated chondrogenesis in C3H10T1/2 cells, confirmed by reduced collagen type I and elevated collagen type IIB expression.

CONCLUSION

These results suggest that SCRG1 is involved in cell growth suppression and differentiation during DEX-dependent chondrogenesis. SCRG1 may be of utility in gene-mediated cartilage tissue engineering.

Fibroblast Growth Factor (FGF) 18 Signals through FGF Receptor 3 to Promote Chondrogenesis

Signaling by fibroblast growth factor (FGF) 18 and FGF receptor 3 (FGFR3) have been shown to regulate proliferation, differentiation, and matrix production of articular and growth plate chondrocytes in vivo and in vitro. Notably, the congenital absence of either FGF18 or FGFR3 resulted in similar expansion of the growth plates of fetal mice and the addition of FGF18 to human articular chondrocytes in culture enhanced proliferation and matrix production. Based on these and other experiments it has been proposed that FGF18 signals through FGFR3 to promote cartilage production by chondrocytes. Its role in chondrogenesis remains to be defined. In the current work we used the limb buds of FGFR3(+/+) and FGFR3(-/-) embryonic mice as a source of mesenchymal cells to determine how FGF18 signaling affects chondrogenesis. Confocal laser-scanning microscopy demonstrated impaired cartilage nodule formation in the FGFR3(-/-) cultures. Potential contributing factors to the phenotype were identified as impaired mitogenic response to FGF18, decreased production of type II collagen and proteoglycan in response to FGF18 stimulation, impaired interactions with the extracellular matrix resulting from altered integrin receptor expression, and altered expression of FGFR1 and FGFR2. The data identified FGF18 as a selective ligand for FGFR3 in limb bud mesenchymal cells, which suppressed proliferation and promoted their differentiation and production of cartilage matrix. This work, thus, identifies FGF18 and FGFR3 as potential molecular targets for intervention in tissue engineering aimed at cartilage repair and regeneration of damaged cartilage.

Distinct functions of BMP4 and GDF5 in the regulation of chondrogenesis

Bone morphogenetic protein 4 (BMP4) and growth/differentiation factor 5 (GDF5) are closely related protein family members and regulate early cartilage patterning and differentiation. In this study, we compared the functional outcome of their actions systematically at various stages of chondrogenesis in mouse embryonic limb bud mesenchyme grown in micromass cultures. Overall, both growth factors enhanced cartilage growth and differentiation in these cultures. Uniquely, BMP4 not only accelerated the formation and maturation of cartilaginous nodules, but also induced internodular mesenchymal cells to express cartilage differentiation markers. On the other hand, GDF5 increased the number of prechondrogenic mesenchymal cell condensation and cartilaginous nodules, without altering the overall pattern of differentiation. In addition, GDF5 caused a more sustained elevated expression level of Sox9 relative to that associated with BMP4. BMP4 accelerated chondrocyte maturation throughout the cultures and sustained an elevated level of Col10 expression, whereas GDF5 caused a transient increase in Col10 expression. Taken together, we conclude that BMP4 is instructive to chondrogenesis and induces mesenchymal cells toward the chondrogenic lineage. Furthermore, BMP4 accelerates the progression of cartilage differentiation to maturation. GDF5 enhances cartilage formation by promoting chondroprogenitor cell aggregation, and amplifying the responses of cartilage differentiation markers. These differences may serve to fine-tune the normal cartilage differentiation program, and can be exploited for the molecular manipulation in biomimetics.

Fibroblast growth factor signaling regulates Dach1 expression during skeletal development

Dach1 is a mouse homologue of the Drosophila dachshund gene, which is a key regulator of cell fate determination during eye, leg, and brain development in the fly. We have investigated the expression and growth factor regulation of Dach1 during pre- and postnatal skeletal development in the mouse limb to understand better the function of Dach1. Dach1 was expressed in the distal mesenchyme of the early embryonic mouse limb bud and subsequently became restricted to the tips of digital cartilages. Dach1 protein was localized to postmitotic, prehypertrophic, and early hypertrophic chondrocytes during the initiation of ossification centers, but Dach1 was not expressed in growth plates that exhibited extensive ossification. Dach1 colocalized with Runx2/Cbfa1 in chondrocytes but not in the forming bone collar or primary spongiosa. Dach1 also colocalized with cyclin-dependent kinase inhibitors p27 (Kip1) and p57 (Kip2) in chondrocytes of the growth plate and in the epiphysis before the formation of the secondary ossification center. Because fibroblast growth factors (FGF), bone morphogenetic proteins (BMP), and hedgehog molecules (Hh) regulate skeletal patterning of the limb bud and chondrocyte maturation in developing endochondral bones, we investigated the regulation of Dach1 by these growth and differentiation factors. Expression of Dach1 in 11 days postcoitus mouse limb buds in organ culture was up-regulated by implanting beads soaked in FGF1, 2, 8, or 9 but not FGF10. BMP4-soaked beads down-regulated Dach1 expression, whereas Shh and bovine serum albumin had no effect. Furthermore, FGF4 or 8 could substitute for the apical ectodermal ridge in maintaining Dach1 expression in the limb buds. Immunolocalization of FGFR2 and FGFR3 revealed overlap with Dach1 expression during skeletal patterning and chondrocyte maturation. We conclude that Dach1 is a target gene of FGF signaling during limb skeletal development, and Dach1 may function as an intermediary in the FGF signaling pathway regulating cell proliferation or differentiation.

Studies on insulin-like growth factor-I and insulin in chick limb morphogenesis

The apical ectodermal ridge (AER) promotes the proliferation and directed outgrowth of the subridge mesodermal cells of the developing limb bud, while suppressing their differentiation. Insulin-like growth factor-I (IGF-I) and its receptor are expressed by the subridge mesodermal cells of the chick limb bud growing out in response to the AER, and specific insulin receptors are present in the limb bud during its outgrowth. To study the possible roles of IGF-I and insulin in limb outgrowth, we have examined their effects on the morphogenesis of posterior and anterior portions of the distal tip of stage 25 embryonic chick wing buds subjected to organ culture in serum-free medium in the presence or absence of the AER and limb ectoderm. The distal mesoderm of control posterior explants lacking an AER or all limb ectoderm ceases expressing IGF-I mRNA, exhibits little or no proliferation, fails to undergo outgrowth, and rapidly differentiates. Exogenous IGF-I and insulin promote the outgrowth and proliferation and suppress the differentiation of distal mesodermal cells in posterior explants lacking an AER or limb ectoderm, thus mimicking at least to some extent the outgrowth promoting and anti-differentiative effects normally elicited on the subridge mesoderm by the AER. Furthermore, IGF-I and insulin-treated posterior explants exhibit high IGF-I mRNA expression, indicating that IGF-I and insulin maintain the expression of endogenous IGF-I by the subridge mesoderm. We have also found IGF-I and insulin can affect the morphology and activity of the AER. When the posterior portion of the wing bud tip is cultured with the AER intact in control medium, on day 4-5 the AER flattens, ceases expressing high amounts of the AER-characteristic homeobox-containing gene Msx2, and concomitantly an elongated cartilaginous element differentiates in the subridge mesoderm. In contrast, in the presence of exogenous IGF-I or insulin the AER of such explants does not flatten, continues expressing high amounts of Msx2, and the subridge mesoderm remains undifferentiated and proliferative. Thus, exogenous IGF-I and insulin maintain the thickness of the AER and sustain its expression of Msx2, while sustaining the anti-differentiative effect normally elicited on the subridge mesoderm by a thickened functional AER. Notably, we have also found that exogenous IGF-I and insulin induce the formation of a thickened ridge-like structure that expresses high amounts of Msx2 from the normally thin distal anterior ectoderm of the limb bud, while promoting dramatic outgrowth and proliferation of the anterior mesoderm, which normally undergoes little outgrowth or proliferation.(ABSTRACT TRUNCATED AT 400 WORDS)

Maintenance of ZPA signaling in cultured mouse limb bud cells

The positional signal localized to the posterior (zone of polarizing activity or ZPA) region of the vertebrate limb is transiently expressed during development and a decline in ZPA signaling is accelerated when posterior cells are dissociated and cultured in vitro. The evidence that cultured posterior cells display a precocious decline in ZPA signaling when compared to in vivo studies suggests that a factor present in the limb bud maintains or stabilizes ZPA signaling during limb outgrowth and that this maintenance factor is lost and/or exhausted in in vitro studies. We have developed a new culture technique, ‘microdissociation’, which preserves extracellular components that we have found to be necessary for ZPA signal maintenance. Our data suggest that the limb bud ectoderm produces a maintenance activity that becomes stored in the extracellular matrix where it acts on limb bud cells to stabilize the activity of the ZPA signal. Using our initial characterization of this maintenance activity, we have identified a growth factor, FGF-2 (bFGF), that can replace all of the ZPA signaling maintenance activity observed in microdissociate cultures. The existence of various members of the FGF family in the developing limb strongly argues a role for FGF in stabilizing ZPA signaling in vivo.

Growth regulatory properties of endothelins

Endothelins are produced by endothelial and epithelial cells, macrophages, fibroblasts, and many other types of cells. Their receptors are present in numerous cells, including smooth muscle cells, myocytes, and fibroblasts. Evidence now suggests that the three isoforms of endothelins (ET-1 and the other two related isopeptides, ET-2 and ET-3) regulate growth in several of these cells. Endothelin-1 influences DNA synthesis, the expression of protooncogenes, cell proliferation, and hypertrophy. The participation of ET in mitogenesis involves activation of multiple transduction pathways, such as the production of second messengers, the release of intracellular pools of calcium, and influx of extracellular calcium. Moreover, ET-1 acts in synergism with various factors, such as EGF, PDGF, bFGF, TGFs, insulin, etc., to potentiate cellular transformation or replication. Several of these factors may in turn stimulate the synthesis and/or the release of endothelins. The production and release of endothelins are also increased in acute and chronic pathological processes, e.g., atherosclerosis, postangioplastic restenosis, hypertension, and carcinogenesis. It is postulated that endothelins act in a paracrine/autocrine manner in growth regulation and play an important role mediating vascular remodeling in some cardiovascular diseases. The present review analyses the implication of endothelins (ET-1, -2, and -3) in physiopathology related to their growth regulatory properties.

Insulin-like growth factor I and II and insulin-like growth factor binding protein-2 RNAs are expressed in adjacent tissues within rat embryonic and fetal limbs

Since the rapid proliferation of cells in a directed manner is a necessary component of limb formation, the distribution of locally produced mitogenic molecules within the developing limb is of considerable interest. We have used in situ hybridization to localize transcripts for both insulin-like growth factor binding protein-2 (IGFBP-2) and its ligands, the insulin-like growth factors I and II (IGF-I and IGF-II), within limb buds of rat embryos 10-16 days after conception (equivalent to stages 1-12 of mouse limb morphogenesis, Wanek et al, 1989. J. Exp. Zool. 249, 41-49). The mRNA for IGFBP-2 is very abundant in an anterior-posterior strip of ectoderm along the distal edge of the limb bud (the progenitor of the apical ectodermal ridge or AER) from as early as limb stage 1 (Embryonic Day 10) and is much less abundant in the rest of the limb ectoderm. A high level of IGFBP-2 expression continues to characterize the AER following its definitive appearance (stage 3) and throughout its existence (until stage 7). This is a period of rapid outgrowth during which the rate of mesodermal cell division is highest in cells nearest to the AER. The AER is known to have mitogenic activity in vitro and to direct limb outgrowth in vivo, but, until recently, few putative molecular correlates of these activities have been detected. The transcripts for IGF-I and IGF-II are also present at high abundance in developing limbs, especially in mesodermally derived cells. IGF-I mRNA is abundant in presumptive limb mesoderm from the beginning of limb outgrowth (just before stage 1), but is very low or undetectable in much of the rest of the embryo, while IGF-II mRNA becomes very abundant in limb mesoderm at stage 2. The distribution in limbs of both IGF-I and IGF-II mRNA changes dramatically during outgrowth and differentiation, so that their expression characterizes complementary populations of cells by stage 11. Taken together, these data suggest that IGFs and the IGF binding proteins, which may modulate IGF action, contribute to limb outgrowth and patterning.

Position-specific growth of mouse limb bud cells in vitro

The relationship between cellular position and growth control has been studied in cultures of dissociated fragments of mouse limb bud cells. Using cells derived from various positions along the anterior-posterior axis of the limb bud we have developed culture conditions that optimize growth of positionally isolated cells. Under these conditions limb bud cells display an inherent, position-specific growth response; proliferation of cells derived from anterior and central regions of the limb is enhanced over that of posterior derived cells. Thus, within the total population of limb bud cells the in vitro growth of posterior cells is unique and correlates with the positional activity associated with the zone of polarizing activity. Anterior and posterior cells were cocultured to determine whether interactions between these two groups of positionally distinct cells lead to the stimulation of growth that has been observed in vivo. We observe a slight but consistent position-dependent stimulation of growth that is indicative of a mitogenic signal passing between these positionally disparate cells. Similarities between position-related growth dynamics in vivo and in vitro suggest that positional interactions that are important for limb formation can occur between dissociated cells cultured under standard conditions.

Development and growth of mouse embryonic kidney in organ culture and modulation of development by soluble growth factor

Differentiation of the metanephrogenic mesenchyme is triggered by an inductive tissue interaction between an inducer tissue and the mesenchyme. It is generally believed that the epithelial ureter bud acts as an inducer during in vivo development. In response to the inductive stimulus most of the mesenchymal cells convert into epithelial cells, while a small fraction differentiates into stromal cells. In vitro, differentiation of isolated mesenchyme to epithelium can be induced by a variety of embryonic tissues, but nothing is known about the molecular nature of the inducing stimulus. In recent years, large numbers of polypeptide growth factors have been described, which in addition to proliferative effects were shown to exert effects on a variety of biological phenomena such as chemotaxis, inflammation, tissue repair, or induction of embryonic development. We therefore analyzed whether growth factors in the absence of inducer tissue can induce isolated kidney mesenchyme to differentiate into epithelium or interstitium. As expected, both growth and differentiation into epithelium were stimulated by an inducer tissue, the spinal cord. We found that none of the various growth factors tested (including epidermal growth factor, transforming growth factors alpha and beta, insulin-like growth factors I and II, fibroblast growth factor, platelet-derived growth factor, and retinoic acid) could mimick the effect of an inducer tissue, although we tested the factors over a wide concentration range. One of the tested factors, epidermal growth factor (EGF) stimulated the mesenchymal cells to become stromal cells, although it could not stimulate development into epithelium. EGF could stimulate stromal development both when the mesenchyme was cultured in isolation and when the mesenchyme was stimulated by an inducer tissue to become epithelium. The expansion of the stromal compartment in response to EGF treatment occurred at the expense of the epithelial cells, but EGF could not completely suppress the formation of epithelium. These data suggest the presence of EGF receptors in the developing kidney, but since application of soluble EGF leads to abnormal development, soluble EGF cannot be the natural ligand. We suggest that locally produced mitogens with an EGF-like structure may regulate the relative amounts of stroma (interstitium) and epithelium in the developing kidney.

A gradient of responsiveness to the growth-promoting activity of ZPA (zone of polarizing activity) in the chick limb bud

Limb bud mesoderm of stage 22-23 embryos was dissected into four pieces along the anteroposterior axis and dissociated cells of each region were separately cultured under various conditions. When the cells were cultured in medium containing 0.1% fetal calf serum (serum-poor medium) only a slight increase in cell number occurred in the cultures of all four regions. However, when the cells were cultured in medium containing 10% FCS, only cells of two central regions proliferated rapidly, and no growth promotion was observed in cells in the most anterior and posterior regions. Using the serum-poor medium, we examined the growth-promoting effects of cocultured limb bud fragments and of some growth factors on the cells of four regions. Anterior, distal, and proximal fragments promoted cell proliferation and their promotive effect on the cells of each region was equal. On the other hand, posterior fragments (containing ZPA) showed stronger promotive effects on preaxial cells than on postaxial cells. For comparison with the growth-promotive effect of the posterior fragment, fibroblast growth factor (FGF), epidermal growth factor (EGF), insulin, and retinoic acid were tested in cell culture. FGF showed position-dependent growth promotion, while EGF and insulin promoted growth in the cells of all four regions to a similar degree. Retinoic acid showed no effect on cell growth at low concentrations, and was rather toxic at high concentrations. These results suggest that the cells of the posterior region secrete an FGF-like growth factor(s), which controls normal limb development and experimental duplicate formation.

Localization and quantitation of 125I-epidermal growth factor binding in mouse embryonic tooth and other embryonic tissues at different developmental stages

We have shown earlier that epidermal growth factor (EGF) inhibits morphogenesis and cell differentiation in mouse embryonic teeth in organ culture. This inhibition depends on the stage of tooth development so that only teeth at early developmental stages respond to EGF (A-M. Partanen, P. Ekblom, and I. Thesleff (1985) Dev. Biol. 111, 84-94). We have now studied the quantity and pattern of EGF binding in teeth at various stages of development by incubating the dissected tooth germs with 125I-labeled EGF. Although the quantity of 125I-EGF binding per microgram DNA stays at the same level, localization of 125I-EGF binding by autoradiography reveals that the distribution of binding sites changes dramatically. In bud stage the epithelial tooth bud that is intruding into the underlying mesenchyme has binding sites for EGF, but the condensation of dental mesenchymal cells around the bud does not bind EGF. At the cap stage of development the dental mesenchyme binds EGF, but the dental epithelium shows no binding. This indicates that the dental mesenchyme is the primary target tissue for the inhibitory effect of EGF on tooth morphogenesis during early cap stage. During advanced morphogenesis the binding sites of EGF disappear also from the dental papilla mesenchyme, but the dental follicle which consists of condensed mesenchymal cells surrounding the tooth germ, binds EGF abundantly. We have also studied EGF binding during the development of other embryonic organs, kidney, salivary gland, lung, and skin, which are all formed by mesenchymal and epithelial components. The patterns of EGF binding in various tissues suggest that EGF may have a role in the organogenesis of epitheliomesenchymal organs as a stimulator of epithelial proliferation during initial epithelial bud formation and branching morphogenesis. The results of this study indicate that EGF stimulates or maintains proliferation of undifferentiated cells during embryonic development and that the expression of EGF receptors in different organs is not related to the age of the embryo, but is specific to the developmental stage of each organ.

Paracrine functions of somatomedins

Evidence is growing that the somatomedins act by a paracrine and/or autocrine mechanism. The importance of these mechanisms relative to the traditional endocrine actions is not clear, and it is possible that these growth factors act through all three mechanisms. Supporting the possible paracrine/autocrine mechanisms are reports that production of somatomedins or somatomedin-like peptides is widespread throughout the body. Additionally, the somatomedins have biological actions on remarkably diverse cell types, and these responsive cells are found in close proximity to cells known to produce somatomedin. Finally, factors that alter the growth rate of cultured cells produce parallel changes in somatomedin secretion, suggesting that these phenomena are closely linked.

Synthesis of somatomedin C/insulin-like growth factor I by human placenta

We have reported the presence of insulin-related poly A+ RNA sequences in human placenta by RNA to DNA hybridization. In this study we have used a monoclonal antibody to somatomedin C/insulin-like growth factor I (Sm-C/IGF-I) to identify somatomedin-like proteins whose synthesis is directed by placental mRNA. Poly A+ RNA from first trimester and term placenta was translated in a cell-free system using micrococcal nuclease-treated reticulocyte-lysate and [35S]methionine as a label. From 2.0 X 10(6) cpm of specifically incorporated [35S]methionine labeled protein, an immunoprecipitate with an apparent molecular weight of 14,000 represented about 0.1% of total radioactivity in the translational products of poly A+ RNA of first trimester placenta. A less prominent band (0.006%) of the same apparent molecular weight was also evident from translational products of term placental mRNAs. This protein could be competed with either acromegalic serum or synthetic Sm-C/IGF-I when added prior to immunoprecipitation. Translational products synthesized from mRNA of term placenta showed a second labeled band of 24,000 daltons. This band was less effectively competed by acromegalic serum and not competed with either Sm-C/IGF-I or IGF-II and therefore its identity is uncertain. A protein similar to Sm-C/IGF-I is, therefore synthesized in first trimester placenta and to a lesser extent at term, suggesting developmental changes in Sm-C/IGF-I synthesis. Because Sm-C/IGF-I may act in a paracrine fashion, our findings suggest a role for Sm-C/IGF-I in growth of the placenta during early gestation.

Somatomedin-C as a fetal growth promoting factor and amino acid composition of cord blood in Japanese neonates

A study was undertaken to determine the relationship of somatomedin-C as a growth promoting factor to amino acid composition of umbilical venous blood in relationship to fetal growth. In full term pregnancy somatomedin-C from cases with appropriate fetal growth was greater than those with fetal growth retardation. Within the physical parameters of the neonate only birth weight of appropriate for date babies had significant positive relationship to somatomedin-C in cord blood. Of the 40 amino acids and their analogues, significant positive correlations were found for glutamine, prolin, asparagin, methionin, isoleucin and ethanolamine in relation to the level of somatomedin-C, while a significant negative correlation was found between tryptophane and somatomedin-C of appropriate for date babies. In cases with fetal growth retardation, the evaluation of amino acid-somatomedin-C relationship revealed that glutamine demonstrated a significant positive correlation and tryptophane demonstrated a significant negative correlation. While many details of these relationships are not clear, the results of this report lead us to speculate that somatomedin-C as a growth promoting factor in the fetus has some relationship to metabolites of tryptophane, for example serotonine and or melatonine, in regulating fetal growth in utero.

Partial purification of a low-molecular-weight growth factor from chicken brain

The regenerating amphibian limb serves as a useful model for studying factors influencing cell proliferation and differentiation. In particular, peripheral nerves are thought to provide a stimulus for growth of the blastema, presumably via the elaboration of an as yet unidentified neurotrophic factor. In the present study, pressure ultrafiltration coupled with chromatofocusing have proven to be effective methods of partially purifying a neurotrophic factor from adult chicken brains. This chick brain growth factor (CBGF) appears to be a heat-stable, basic peptide of low molecular weight (less than 6,000). It is a potent mitogen in vitro, at nanomolar concentrations, for both blastema cells and Swiss mouse 3T3 fibroblasts. CBGF is apparently distinct from other peptide mitogens and/or neuromodulators that have been reported to stimulate blastema growth in vivo and in vitro. These include substance P, FGF from bovine brain and pituitary, EGF, transferrin (sciatin), and spinal cord growth factor (SCGF). The possible relationship of CBGF to other neural regulatory molecules is discussed.

Control of kidney differentiation by soluble factors secreted by the embryonic liver and the yolk sac

Since transferrin is necessary for the differentiation of the embryonic kidney in organ culture, we have suggested that the component is a growth factor for in vivo development as well. In the present study we demonstrate that transferrin is present in the serum of 11-day-old mouse embryos, at the time when kidney differentiation starts. We have also tested whether various embryonic tissues can replace transferrin as stimulators of the differentiation and proliferation of the metanephric mesenchyme. We used a transfilter model system where nephrogenic mesenchymes are cultured with spinal cord, a known inductor of kidney tubules. The embryonic liver could not replace the spinal cord as an inducer of tubular differentiation. However, when the kidney mesenchymes were cultured together with both the spinal cord and the liver, the mesenchymes proliferated and differentiated also in the absence of exogenous transferrin. In such cocultures the spinal cord had to be in close contact with the mesenchyme while the embryonic liver could be located several cell layers apart. The liver-mediated stimulation of proliferation of the induced mesenchyme could be inhibited by anti-transferrin antibodies. Immunoprecipitation and immunoblotting with these antibodies of the liver-conditioned medium demonstrated that the 11-day mouse liver produces transferrin. Other potential mitogens produced by liver cells, alpha-fetoprotein, or multiplication stimulating activity, did not in any way stimulate the proliferation of induced mesenchymes. These studies suggest that the mitogen in the liver medium is transferrin. This is supported by data which show that another embryonic transferring producer, the visceral yolk sac, can replace the effect of the liver, whereas a tissue not producing transferrin, the salivary mesenchyme, cannot. In conclusion, an essential function of the inducer is to make the mesenchyme responsive to transferrin. The liver and the yolk sac stimulate early kidney differentiation by producing the soluble factor, transferrin, but they are ineffective as inductors of the transferrin responsiveness.

Transferrin is required for early tooth morphogenesis

The role of circulating molecules during early tooth morphogenesis was studied in organ cultures of mouse embryonic molar-tooth germs. Special attention was focused on the effect of transferrin and insulin, which are necessary for the growth of most cells in culture. The requirement of serum factors for tooth morphogenesis was shown to diminish as the developmental stage advances from the bud stage in day-13 embryos to the cap stage at day 15. The day-15 teeth underwent morphogenesis and cell differentiation in unsupplemented basal culture medium, but the addition of transferrin (50 micrograms/ml) was necessary for the morphogenesis of day-14 tooth germs. We demonstrated, by using transferrin-depleted serum, that transferrin is also necessary for the morphogenesis of day-13 tooth germs. However, some still-unidentified serum components are also required for the morphogenesis of the bud-stage day-13 teeth. These factors apparently do not include insulin, since it was shown to inhibit tooth development. Analysis of the DNA content of tooth germs cultured in various culture media showed that the ability of transferrin to support tooth morphogenesis correlated with a stimulation of growth. The results support our earlier suggestions that transferrin functions as a fetal growth factor. The availability of the transferrin-containing chemically defined medium facilitates studies on the roles of other growth factors during tooth development.

Fetal growth control: the role of insulin and related peptides

Both insulin and the related peptides, the insulin-like growth factors/somatomedins, may function as anabolic factors in the regulation of fetal body size. Infants born to women suffering from diabetes mellitus may show increased deposition of subcutaneous fat and enhanced lean body mass, findings reproduced in experimental animal fetuses with induced hyperinsulinaemia. Fetal adiposity may be associated with a life-time tendency to obesity and its associated diseases. Insulin-like growth factors I (IGFI) and II are present in the circulation of the newborn infant and animal fetus and correlate positively with birth size. The fetal tissues are biologically responsive to IGFs in vitro and are rich in specific cell membrane receptors, those predominantly recognizing IGFI being structurally and functionally similar to the insulin receptor. Insulin could theoretically influence fetal tissues by an interaction with either the insulin or IGF receptor. IGF release is a property of multiple fetal tissues in vitro, but, in contrast to postnatal life, is not dependent on growth hormone. Fetal IGF production may be influenced by placental lactogen, especially IGFII which rapidly declines in the circulation following parturition in the rat and sheep. A positive association also exists between circulating levels of insulin and IGFs when the former is experimentally manipulated in the animal fetus. Similarly the infant born with transient diabetes mellitus has low cord blood levels of insulin and IGFI. Insulin has a dual role in prenatal life. In the last trimester insulin functions as a glucoregulatory hormone, but from much earlier in gestation creates an anabolic environment in the fetus supplied with optimal nutrients. This latter mechanism of action is unclear and probably heterogeneous, but in overview is permissive rather than obligatory. In contrast the growth-promoting role of the IGFs is direct but their interaction with fetal tissues, and thus the overall emphasis of fetal growth, may be paracrine.

Receptors for multiplication-stimulating activity (MSA) on rabbit chondrocytes

Multiplication-stimulating activity (125I-MSA) has been shown to bind to isolated rabbit chondrocytes, the binding being dependent on time, temperature, and cell density. Nonspecific binding was approximately 15%. Unlabelled MSA at 100 ng/ml inhibited 125I-MSA binding by 50%. Porcine insulin (0.5-10 micrograms/ml) did not compete with MSA but resulted in a 10-15% increase in 125I-MSA binding. The data suggest that normal chondrocytes carry IGF2-type receptors as well as the IGF1 type previously described.

Cultured fetal rat myoblasts release peptide growth factors which are immunologically and biologically similar to somatomedin

The production of immunologically and biologically active somatomedin activity from isolated myoblasts and fibroblasts from fetal rats of 21 days gestational age was investigated. Myoblast-rich cell populations were derived from primary cultures of dispersed muscle cells by the tendency of myoblasts to become detached from the culture dish in the presence of cytochalasin B. Fibroblasts were obtained from fetal muscle. Culture medium conditioned by exposure to myoblasts for 48 hours produced an increased incorporation of both [35S]sulphate and [3H]thymidine by explants of fetal rat costal cartilage in vitro compared to fresh medium. Myoblast-conditioned medium also contained somatomedin-C-like immunoreactivity which diluted in parallel with partially purified human somatomedin-C (3,271 +/- 446 mU/mg cell protein; mean +/- SEM, seven experiments). Medium conditioned by exposure to fetal rat fibroblasts did not promote isotope uptake by fetal rat cartilage above control values, and contained only low levels of somatomedin-C-like immunoreactivity (343 +/- 89 mU/mg cell protein, three experiments). The release of both somatomedin bioactivity and immunoreactivity into conditioned medium was significantly reduced by the incubation of myoblasts in the presence of rat growth hormone (100 ng/ml and 500 ng/ml). We conclude that fetal rat myoblasts released growth factor activity during culture which exhibited biological and immunologic characteristics of somatomedin. Since the bioactivity was demonstrated on skeletal tissues from rat fetuses of the same gestational age as those that yielded myoblasts such growth factor release may be physiological.

Insulin and insulin-like growth factors/somatomedins in fetal and neonatal development

In this presentation we have attempted to review the status of knowledge of the hormonal regulation of fetal and neonatal growth, and have placed particular emphasis on the roles of insulin and somatomedin in fetal growth. The inaccessibility of the fetus and the physical and ethical constraints on study of its growth have resulted in slow progress. Furthermore, the opportunity for the fetus to benefit from homologous hormones from a variety of sources has made it difficult to assess the effect of withdrawal of individual hormones. Many of the advances made have resulted from attempts to determine hormone concentrations in fetal blood, identify hormone binding by fetal tissues, and examine the effects of natural or experimentally induced deviations in hormone availability. In the future, attention should be focused on developing study models which better isolate the fetus from the influence of multiple, homologous hormones. Progress should also come as the result of more detailed study of the influence of individual growth factors on in vitro growth of fetal cells and tissues, assessment of control mechanisms for growth factors in the fetus, and experiments directed at recognizing the complex interactions between individual growth factors and between growth factors and hormones.