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

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.

Incudomalleal joint formation: the roles of apoptosis, migration and downregulation

Background

The middle ear of mammals is composed of three endochondrial ossicles, the stapes, incus and malleus. Joints link the malleus to the incus and the incus to the stapes. In the mouse the first arch derived malleus and incus are formed from a single Sox9 and Type II collagen expressing condensation that later subdivides to give rise to two separate ossicles. In contrast the stapes forms from a separate condensation derived from the second branchial arch. Fusion of the malleus and incus is observed in a number of human syndromes and results in conductive hearing loss. Understanding how this joint forms during normal development is thus an important step in furthering our understanding of such defects.

Results

We show that the developing incudomalleal joint is characterised by a lack of proliferation and discrete areas of apoptosis. Apoptosis has been suggested to aid in the removal of pre-cartilaginous cells from the joint region, allowing for the physical separation of the cartilaginous elements, however, we show that joint initiation is unaffected by blocking apoptosis. There is also no evidence of cell migration out of the presumptive joint region, as observed by labelling of joint and ossicle cells in culture. Using Type II collagen lacZ reporter mice, however, it is evident that cells in the presumptive joint region remain in place and downregulate cartilage markers.

Conclusion

The malleus and incus first appear as a single united condensation expressing early cartilage markers. The incudomalleal joint region forms by cells in the presumptive joint region switching off cartilage markers and turning on joint markers. Failure in this process may result in fusion of this joint, as observed in human syndromes such as Branchio-Oto-Renal Syndrome or Treacher Collins Syndrome.

Promotion of skeletal muscle differentiation by K252a with tyrosine phosphorylation of focal adhesion: a possible involvement of small GTPase Rho

K252a, a protein kinase inhibitor, acts as a neurotrophic factor in several neuronal cells. In this study we show that K252a enhanced the differentiation of C2C12 myoblasts as well as tyrosine phosphorylation of several focal adhesion-associated proteins including p130(Cas), focal adhesion kinase, and paxillin. The tyrosine phosphorylation of these proteins, reaching a maximum at 30 min after K252a treatment, closely correlated with the colocalization of these proteins in focal adhesion complexes and the coimmunoprecipitation of these proteins with p130(Cas). In addition, K252a stimulated longitudinal development of stress fiber-like structures and cell-matrix interaction in postmitotic myoblasts and eventually formation of well-developed myofibrils in multinucleated myotubes. Herbimycin A, a potent inhibitor of Src family kinases, and cytochalasin D, a selective disrupting-agent of actin filament, completely inhibited K252a-induced tyrosine phosphorylation as well as myoblast differentiation. Similar inhibitory effect was observed in the cells scrape loaded with a Rho inhibitor, C3 transferase, and the treatment of K252a induced a rapid translocation of Rho. These results are consistent with the model that Rho-dependent tyrosine phosphorylation of focal adhesion-associated proteins plays an important role in skeletal muscle differentiation.

Morphological foundations of precartilage development in mesenchyme

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

Roles of transforming growth factor-alpha and epidermal growth factor in chick limb development

We have examined the distribution of transforming growth factor-alpha (TGF-alpha), epidermal growth factor (EGF), and the chicken EGF receptor (c-erbB), in embryonic chick limbs. Prior to limb budding, TGF-alpha is present in prospective limb-forming mesoderm and in prospective apical ectodermal ridge (AER)-forming ectoderm, but is not detected in non-limb-forming flank mesoderm or ectoderm, nor in presumptive non-AER-forming limb ectoderm, suggesting possible roles in initial limb formation and AER induction. Consistent with this possibility, TGF-alpha is present in the mesoderm of the wing buds of the amelic chick mutants limbless and wingless, which form and bud normally, but is absent from limbless and wingless ectoderm, which fails to form an AER. TGF-alpha and EGF are present in the AER of the developing limb, and TGF-alpha, EGF, and c-erbB are present in the underlying subridge mesoderm, suggesting possible roles in reciprocal AER/subridge mesoderm interactions required for limb outgrowth. We found that exogenous TGF-alpha and EGF can promote the outgrowth of limb mesoderm in the absence of the AER in vitro and can also promote the outgrowth of limbless and wingless wing bud explants. EGF is present in ventral but not dorsal limb ectoderm, suggesting a role for EGF in specification of ventral ectoderm. TGF-alpha and EGF are not detected in the differentiating cartilaginous elements or muscle primordia of the limb, suggesting that cessation of TGF-alpha and EGF expression may be required for cartilage and muscle formation. We have found that exogenous TGF-alpha and EGF inhibit chondrogenesis and myogenesis of limb mesenchyme in vitro. Together these results indicate that signaling through the EGF receptor via endogenous TGF-alpha and EGF may be important for initial limb formation, AER induction, outgrowth of limb mesoderm, and regulation of limb chondrogenic and myogenic differentiation.

Integrin alpha subunit ratios, cytoplasmic domains, and growth factor synergy regulate muscle proliferation and differentiation

The role of integrins in muscle differentiation was addressed by ectopic expression of integrin alpha subunits in primary quail skeletal muscle, a culture system particularly amenable to efficient transfection and expression of exogenous genes. Ectopic expression of either the human alpha5 subunit or the chicken alpha6 subunit produced contrasting phenotypes. The alpha5-transfected myoblasts remain in the proliferative phase and are differentiation inhibited even in confluent cultures. In contrast, myoblasts that overexpress the alpha6 subunit exhibit inhibited proliferation and substantial differentiation. Antisense suppression of endogenous quail alpha6 expression inhibits myoblast differentiation resulting in sustained proliferation. These effects of ectopic alpha subunit expression are mediated, to a large extent, by the cytoplasmic domains. Ectopic expression of chimeric alpha subunits, alpha5ex/6cyto and alpha6ex/5cyto, produced phenotypes opposite to those observed with ectopic alpha5 or alpha6 expression. Myoblasts that express alpha5ex/6cyto show decreased proliferation while differentiation is partially restored. In contrast, the alpha6ex/5cyto transfectants remain in the proliferative phase unless allowed to become confluent for at least 24 h. Furthermore, expression of human alpha5 subunit cytoplasmic domain truncations, before and after the conserved GFFKR motif, shows that this sequence is important in alpha5 regulation of differentiation. Ectopic alpha5 and alpha6 expression also results in contrasting responses to the mitogenic effects of serum growth factors. Myoblasts expressing the human alpha5 subunit differentiate only in the absence of serum while differentiation of untransfected and alpha6-transfected myoblasts is insensitive to serum concentration. Addition of individual, exogenous growth factors to alpha5-transfected myoblasts results in unique responses that differ from their effects on untransfected cells. Both bFGF or TGFbeta inhibit the serum-free differentiation of alpha5-transfected myoblasts, but differ in that bFGF stimulates proliferation whereas TGF-beta inhibits it. Insulin or TGF-alpha promote proliferation and differentiation of alpha5-transfected myoblasts; however, insulin alters myotube morphology. TGF-alpha or PDGF-BB enhance muscle alpha-actinin organization into myofibrils, which is impaired in differentiated alpha5 cultures. With the exception of TGF-alpha, these growth factor effects are not apparent in untransfected myoblasts. Finally, myoblast survival under serum-free conditions is enhanced by ectopic alpha5 expression only in the presence of bFGF and insulin while TGF-alpha and TGF-beta promote survival of untransfected myoblasts. Our observations demonstrate (1) a specificity for integrin alpha subunits in regulating myoblast proliferation and differentiation; (2) that the ratio of integrin expression can affect the decision to proliferate or differentiate; (3) a role for the alpha subunit cytoplasmic domain in mediating proliferative and differentiative signals; and (4) the regulation of proliferation, differentiation, cytoskeletal assembly, and cell survival depend critically on the expression levels of different integrins and the growth factor environment in which the cells reside.

Role of the brachial somites in the development of the appendicular musculature in rat embryos

DiI, a fluorescent lipophilic dye, was micro-injected into the brachial somites of 10.5 day rat embryos to determine whether these somites can contribute cells to the development of the fore-limb bud. The injected embryos were cultured and harvested at the 20-25-somite stage. The dye did not interfere with somitogenesis because, at the injection site, the DiI-labelled somites were able to differentiate into dermomyotome and sclerotome. We have analyzed cryo-sections of 20-21-somite stage embryos and were unable detect the presence of DiI-labelled cells in the fore-limb buds. However, at the 22-somite stage, a few DiI-positive cells were found in the proximal region of the limb bud. These labelled cells had migrated into the limb from the lateral border of the dermomyotome. From the 23-somite stage onwards, there were even more DiI-positive cells inside the limb. We have performed an additional set of experiments to confirm that the somitic cells do have the ability to invade and colonize the limb bud. This was achieved by first labelling newly formed somites isolated from the caudal region of 10.5 day embryos with DiI and then grafting them into corresponding regions in 8-11-somite stage hosts. The donor somites were not orientated when they were implanted into the host. However, this did not disrupt their ability to undergo normal somitogenesis. We have detected the presence of DiI-positive cells in the limb buds of approximately 71% of the 19-30-somite stage embryos that have been examined. This is similar to what we obtained for the injected embryos. Nevertheless, there is one slight difference and that is the stage the somitic cells begin their invasion of the limb. For the injected embryos, migration began at the 22-somite stage but in the transplanted embryos, it commenced as early as the 18-somite stage. We have also investigated the myogenic potential of the fore-limb bud at various stages of development to ascertain whether there is a correlation between the stage the somitic cells first appear in the limb bud and the stage the bud acquires the capacity to form skeletal muscles. This was realized by culturing fore-limb buds excised from 18-30-somite stage embryos conventionally and in the kidney capsules of adult rats. In both methods, bone and cartilage were present in all of the cultures whereas skeletal muscles were only present in cultured explants older than the 21-22-somite stage.(ABSTRACT TRUNCATED AT 400 WORDS)

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)

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)

Widespread distribution of type II collagen during embryonic chick development

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

Differentiation of myogenic cells in micromass cultures of cells from chick facial primordia

Antibodies to the myosin heavy chains of striated muscle were used to trace myogenic differentiation in the developing face and in cultures of cells from the facial primordia of chick embryos. In the intact face, myogenic cells differentiate first in the mandibular primordia and can be detected at stage 28. The early muscle blocks contain both fast and slow classes of myosin heavy chains. At stages 20 and 24, no myogenic cells are found in any of the facial primordia. However, when the cells are placed in micromass (high density) cultures, myogenic cells differentiate, revealing the presence of potentially myogenic cells in all the facial primordia. The number of myogenic cells bears no consistent relationship to the extent and pattern of chondrogenesis. Therefore the ability of the cell populations of the facial primordia to differentiate into cartilage when placed in culture is independent of the muscle cell lineage. The facial primordia represent a mixed cell population of neural crest and mesodermal cells from at least as early as stage 18.

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.

Chondrogenesis in chick limb mesenchyme in vitro derived from distal limb bud tips: changes in cyclic AMP and in prostaglandin responsiveness

Chondrogenesis was monitored in micromass cultures of mesenchymal cells derived from the distal tip of stage-25 chick limb buds over a 6-day period. Alcian green staining and immunofluorescent localization of cartilage-specific proteoglycans revealed the appearance of cartilage matrix by day 3 of cell culture. By day 6, cultures contained a uniform and homogeneous population of fully differentiated chondrocytes throughout the cell layer, with only a narrow rim of nonchondrogenic cells around the extreme periphery of the culture. Synthesis of sulfated glycosaminoglycans also progressively increased between days 3 and 6, being 8-fold higher at day 6 than at day 1 of culture. Both adenylate cyclase (AC) activity and cAMP concentrations increased dramatically during the first 2 days of culture, reaching maximal levels by day 2, which remained elevated and stable throughout the remaining chondrogenic period (days 3-6). Responsiveness of both AC and cAMP concentrations of the cells to PGE2 was maximal by day 1 of culture and was increased over control cells by 12-fold and 8-fold respectively. Both responses, however, were dramatically reduced by day 3, at which time the initiation of cartilage formation was apparent. Responsiveness of cells during the prechondrogenic period to PGE2 was relatively specific in that no effects could be demonstrated with equivalent concentrations of PGF2 alpha or 6-keto-PGF1 alpha, although PGl2 did produce increases in cAMP concentrations of about 50% of those of PGE2. These results indicate that previously reported changes in the cAMP system in heterogeneous cell cultures derived from whole limb buds reflect changes occurring in the chondrogenic cell type and indicate further that peak responsiveness of the cAMP system of these cells to prostaglandins is restricted to prechondrogenic developmental periods.

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.

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.