The development and phosphatase activity in vivo and in vitro of the mandibular skeletal tissue of the embryonic fowl

Precancerous changes induced by 20-methylcholanthrene in mouse prostates grown in vitro

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Conserved molecular program regulating cranial and appendicular skeletogenesis

The majority of in vivo studies on bone and cartilage differentiation are carried out using the appendicular skeleton as a model system, with the implicit assumption that skeletal formation is equivalent throughout the body. This assumption persists, despite differences in the cellular origins of the skeletogenic precursors. To test the hypothesis that a fundamental set of genes directs skeletal cell differentiation throughout the body, we analyzed cartilage and bone of the chick limb and head during mesenchymal condensation, and when the skeletal tissues had matured. First, we analyzed the expression patterns of transcription factors in early skeletogenic condensations, which revealed similarities among skeletal cell specification in the limb and head. For example, skeletogenic condensations that undergo endochondral ossification had equivalent expression patterns of skeletogenic transcription factors in both limb and head. In the head, many elements also differentiate through intramembranous ossification, or through persistent cartilage formation. Our analyses of these skeletogenic condensations revealed that a unique expression pattern of transcription factors distinguishes among three skeletal tissue fates. The vasculature was excluded from all three skeletogenic condensations, demonstrating that this is not a characteristic unique to endochondral ossification. Second, we compared three different types of more mature cartilage and bone tissue in both the limb and the head, by analyzing a variety of skeletal collagens and signaling molecules. Histological and molecular markers of cartilage and bone generally were conserved between the limb and head skeletons, although we uncovered subtle differences in signaling pathways that might influence cranial and appendicular skeletogenesis.

The effects of surgical section of the embryonic chick mandibular arch

The embryonic chick mandibular arch was surgically sectioned in ovo on day 7 of incubation and the subsequent wound healing of the arch, together with the response of Meckel's cartilage to fracture, was examined. The repair process observed (in contrast to that in adults) was characterised by minimal haematoma formation or cell death and the absence of formation of either cellular blastema or fracture callus. re-epithelialisation was complete within 48 h with no scar tissue formed. Continuity of Meckel's cartilage, together with restoration of its histological appearance and that of the surrounding soft tissues, was re-established within 24 h in 88% of cases. In the case of the cartilage this was due to fusion of the matrix followed apparently by chondrocytic and perichondrial proliferation. This differs from the repair of embryonic long bone cartilages. In 12% of cases, however, mal-union or non-union of the cartilage resulted in mandibular arch deviation. This observation suggests that mandibular arch growth and morphogenesis may parallel the development of Meckel's cartilage. Where cartilaginous non-union occurred, some irregularities in the pattern of the developing mandibular bones were evident, and it is argued that deformity in the cartilage may ultimately affect the length and shape of the adult mandible.

Teratogenic drugs inhibit the differentiation of fetal rat limb buds grafted in athymic (nude) mice

Forelimb buds of day-14 rat fetuses were cut into pieces and transplanted subcutaneously into athymic (nude) mice. On the 7th, 9th, and 11th days after grafting, the nude mice were treated with various drugs including rat teratogens. On the 20th day, the grafted tissue was examined macroscopically and histologically. While control grafts showed substantial growth and tissue differentiation similar to that observed in vivo, the differentiation of grafts was significantly inhibited by the treatment with 5-fluorouracil, cyclophosphamide, hydroxyurea, cycloheximide, mitomycin C, caffeine, aspirin, retinol palmitate, all-trans-retinoic acid, and ascorbic acid. Hydrocortisone, tetracycline, and thalidomide did not adversely affect the differentiation of grafts. Thus, the susceptibility of transplanted rat limb buds was generally close to the teratologic sensitivity of rat fetuses in vivo. The heterotransplantation method of embryonic tissues may be useful as a new experimental system in developmental toxicology.

Cartilage, bone and tooth induction during early embryonic mouse mandibular morphogenesis using serumless, chemically-defined medium

Studies were designed to test the hypothesis that plasma- and serum-deprived embryonic cells and tissues in vitro are capable of producing growth regulating factors which augment cartilage, bone and tooth induction during mouse mandibular process development. Embryonic mouse first branchial arch-derived mandibular processes (E11-E12, Theiler stages 18-19) or cap stage molar tooth (M1) organs (E15-E16, Theiler stage 23) expressed morphogenesis, histogenesis and cytodifferentiation (e.g., Meckel's cartilage and mandibular bone) when cultured as explants in permissive serumless and chemically-defined BGJB medium for periods up to 31 days in vitro. Organ cultures of early mandibular process explants in serumless conditions showed DNA synthesis comparable to the time- and position-restricted patterns characteristic for control in vivo development. As a paradigm for embryonic cell expression of putative growth factors, sense and antisense oligodeoxynucleotide probes corresponding to amino acids 1070-1081 for preproEGF, and antibodies directed against amino acids 348-691 of preproEGF, were used to identify and localize mRNA transcripts and translation products. Our preliminary evidence suggests that odontogenic epithelial and ectomesenchyme cells produce EGF-like products during instructive phases of tooth development. We suggest that plasma- and serum-deprived cells and tissues in vitro produce autocrine and/or paracrine growth factors which mediate embryonic mandibular morphogenesis, histogenesis and cytodifferentiation.

Hypertrophy and calcification of rabbit permanent chondrocytes in pelleted cultures: synthesis of alkaline phosphatase and 1,25-dihydroxycholecalciferol receptor

Cartilage calcification at specific sites is a key event that leads to skeletal development and growth. To obtain insights into the control of cartilage calcification, we examined whether cells distributed in permanent cartilage regions might have the ability to express the calcification-related phenotype in a permissive environment. Chondrocytes were isolated from the permanent and growth plate cartilages of 4-week-old rabbit ribs. They were seeded as a pelleted mass in a centrifuge tube and cultured in Eagle's minimum essential medium supplemented with 10% fetal bovine serum. These cells proliferated for several generations, and then synthesized large amounts of proteoglycans, yielding a cartilage-like tissue in 16 days. Cultures from the permanent and growth plate cartilages showed similar time courses for increases in DNA synthesis and proteoglycan production that reached similar maximal levels. Thereafter, they initiated the syntheses of alkaline phosphatase and 1,25-dihydroxycholecalciferol receptor and induced matrix calcification without additional phosphate. The increases in alkaline phosphatase, 1,25-dihydroxycholecalciferol receptor, and calcium contents in cultures from the permanent cartilage were consistently delayed for 4-7 days relative to the growth plate-derived cells, but caught up by Day 28. The maximal levels of alkaline phosphatase and 1,25-dihydroxycholecalciferol receptor in the cultures from the permanent cartilage were 40- to 100-fold higher than that of the in vivo permanent cartilage. These results provide evidence that permanent cartilage cells in postnatal young rabbit ribs have the capacity to express alkaline phosphatase and 1,25-dihydroxycholecalciferol receptor and induce calcification in a permissive environment, although they never express these calcification-related phenotypes in vivo.

Immunoelectron microscopic studies of type X collagen in endochondral ossification

Immunofluorescence and immunoelectron microscopy were used in conjunction with a monoclonal antibody to investigate the localization of type X collagen in the proximal tibial growth plate of 7-d-old chicks. This molecule was detected throughout the hypertrophic zone first appearing when chondrocytes exhibited hypertrophy: it was absent from the proliferative zone. Type X collagen was primarily associated with type II collagen fibrils as demonstrated by immunogold staining. Type X collagen was not concentrated in the focal calcification sites nor was it associated with matrix vesicles. These observations suggest that type X collagen may play a role other than that directly related to the nucleation of calcification.

Cartilage macromolecules and the calcification of cartilage matrix

The calcification of cartilage matrix in endochondral bone formation occurs in an extracellular matrix composed of fibrils of type II collagen with which type X collagen is closely associated. Also present within this matrix are the large proteoglycans containing chondroitin sulfate which aggregate with hyaluronic acid. In addition, the matrix contains matrix vesicles containing alkaline phosphatase. There is probably a concentration of calcium as a result of its binding to the many chondroitin sulfate chains. At the time of calcification, these proteoglycans become focally concentrated in sites where mineral is deposited. This would result in an even greater focal concentration of calcium. Release of inorganic phosphate, as a result of the activity of alkaline phosphatase, can lead to the displacement of proteoglycan bound calcium and its precipitation. The C-propeptide of type II collagen becomes concentrated in the mineralizing sites, prior to which it is mainly associated with type II collagen fibrils and is present in dilated cisternae of the enlarged hypertrophic chondrocytes. The synthesis of type II collagen and the C-propeptide, together with alkaline phosphatase, are regulated by the vitamin D metabolites 24,25(OH)2 cholecalciferol and 1,25 (OH)2 cholecalciferol. At the time of calcification, type X collagen remains associated with type II collagen fibrils. It may play a role in preventing the initial calcification of these fibrils focusing mineral formation in focal interfibrillar sites. This process of calcification is clearly very complex, and involves different interacting matrix molecules and is carefully regulated at the cellular level.

Development of osteogenic and chondrogenic potentials along the mediolateral axis of the embryonic chick mandible

Various regions of the mandibular process were tested for these potentials to determine whether regional differences exist and vary with embryonic age. Mandibular processes from HH stages 17-21.5 were cultured and grafted intact, or were subdivided into medial, mediolateral and lateral fragments and the separate regions cultured or grafted. The intact mandible from all these stages can form cartilage and membrane bone, but the 3 regions are not equally osteogenic and chondrogenic. The lateral region from all stages could form cartilage and membrane bone; the mediolateral region could form cartilage and membrane bone but, in mediolateral fragments from HH stage 17, membrane bone was formed only in scant amounts. The medial region from HH stages 17 and 18 formed cartilage in only 50 per cent of cases and never formed membrane bone. By HH stage 20, the medial region could form membrane bone, but only in scant amounts. Medial fragments from HH stage 21.5 formed extensive membrane bone and cartilage. The acquisition of these potentials, therefore, proceeds in a lateral-to-medial sequence, and the acquisition of an osteogenic potential lags slightly behind that of a chondrogenic potential. These findings do not indicate the mechanisms by which the two subpopulations of chondrogenic and osteogenic cells are distinguished from one another, but they give the temporal and spatial sequence in which this determination must occur.

Environmental regulation of type X collagen production by cultures of limb mesenchyme, mesectoderm, and sternal chondrocytes

We have examined whether the production of hypertrophic cartilage matrix reflecting a late stage in the development of chondrocytes which participate in endochondral bone formation, is the result of cell lineage, environmental influence, or both. We have compared the ability of cultured limb mesenchyme and mesectoderm to synthesize type X collagen, a marker highly selective for hypertrophic cartilage. High density cultures of limb mesenchyme from stage 23 and 24 chick embryos contain many cells that react positively for type II collagen by immunohistochemistry, but only a few of these initiate type X collagen synthesis. When limb mesenchyme cells are cultured in or on hydrated collagen gels or in agarose (conditions previously shown to promote chondrogenesis in low density cultures), almost all initiate synthesis of both collagen types. Similarly, collagen gel cultures of limb mesenchyme from stage 17 embryos synthesize type II collagen and with some additional delay type X collagen. However, cytochalasin D treatment of subconfluent cultures on plastic substrates, another treatment known to promote chondrogenesis, induces the production of type II collagen, but not type X collagen. These results demonstrate that the appearance of type X collagen in limb cartilage is environmentally regulated. Mesectodermal cells from the maxillary process of stages 24 and 28 chick embryos were cultured in or on hydrated collagen gels. Such cells initiate synthesis of type II collagen, and eventually type X collagen. Some cells contain only type II collagen and some contain both types II and X collagen. On the other hand, cultures of mandibular processes from stage 29 embryos contain chondrocytes with both collagen types and a larger overall number of chondrogenic foci than the maxillary process cultures. Since the maxillary process does not produce cartilage in situ and the mandibular process forms Meckel's cartilage which does not hypertrophy in situ, environmental influences, probably inhibitory in nature, must regulate chondrogenesis in mesectodermal derivatives. (ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of membrane bone formation by vitamin A in the embryonic chick mandible

Embryonic chick mandibles (Hamburger and Hamilton [HH] stages 17-25) were cultured in the presence of various concentrations of vitamin A to determine the effect of hypervitaminosis A on membrane bone formation. In normal development, the mandible differentiates a centrally located Meckel's cartilage surrounded by membrane bones. Mandibles cultured without added vitamin A differentiated normally, though the timing of differentiation was retarded from that in ovo. Treatment with vitamin A interfered with skeletogenesis to varying degrees depending upon the initial age of the explant and the concentration of vitamin A. At low concentrations of vitamin A (1 microgram/ml), neither cartilage nor membrane bone formed in young explants (HH stage 17), whereas cartilage formed in 78% and membrane bone in 11% of older explants (HH stage 25). Higher concentrations of vitamin A (2-4 micrograms/ml) inhibited membrane bone formation in all explants, and 4 micrograms/ml of vitamin A inhibited chondrogenesis in most (88%) of the older explants. To determine whether tissue interactions influence this effect of vitamin A on skeletogenesis, mandibular mesenchyme was separated from its epithelium and treated with vitamin A. Under normal culture conditions, isolated mesenchyme (HH stage 25) differentiated both cartilage and membrane bone. Hypervitaminosis A inhibited membrane bone formation in the isolated mesenchyme at all levels tested (1-4 micrograms/ml) and inhibited chondrogenesis at levels 2-4 micrograms/ml. Hence, vitamin A can act directly upon the mesenchyme to inhibit both membrane bone formation and chondrogenesis, but its action is mitigated by the presence of the epithelium.

Bone cell differentiation and growth factors

Bone morphogenetic protein and bone-derived growth factors are biochemical tools for research on induced cell differentiation and local mechanisms controlling cell proliferation. Bone morphogenetic protein irreversibly induces differentiation of perivascular mesenchymal-type cells into osteoprogenitor cells. Bone-derived growth factors are secreted by and for osteoprogenitor cells and stimulate DNA synthesis. Bone generation and regeneration are attributable to the co-efficiency of bone morphogenetic protein and bone-derived growth factors.

Intracellular and extracellular control of the differentiation of cartilage and bone

This paper provides an overview of one aspect of the differentiation of cartilage and bone, namely, the degree of control provided by the extracellular matrix and microenvironment. A brief review of the diagnostic features of cartilage and bone is followed by a discussion of stem cells, emphasizing how to identify them using cytochemical, ultrastructural or experimental procedures. The role of extracellular matrices in the initiation of differentiation is discussed with reference to the initiation of chondrogenesis in the vertebral skeleton of the embryonic chick and of osteogenesis in the mandibular skeletons of embryonic chick and mice. The role of extracellular matrices in the maintenance of the differentiated state is discussed with reference to the ability of chondrocytes to compensate for depletion of their extracellular matrices and to the maintenance of altered differentiated states in achondroplasia. Some emphasis is placed on the notion that skeletal cells can neither be considered nor studied in isolation. The epigenetic approach used in studies of growth and morphogenesis needs to be applied to studies on both the initiation and the maintenance of cytodifferentiation.

Observations on the effects of condensates from cigarette smoke on human foetal lung in vitro

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Autoradiographic Studies of the Utilization of Ca45 by the Chick Embryo

Calcium-45 was injected into the dense albumen of fertile hen's eggs, to the extent of 25 microc. per egg. The eggs were incubated under standard conditions and three or more embryos removed daily and fixed in 10 per cent neutral formalin. Stripping-film autoradiograms were prepared from paraffin sections of the tibiofibulae. Exposure varied with the isotope concentration. The tissue sections with their autoradiograms in place were stained with dilute Giemsa, while other sections were stained with hematoxylin-azure-eosin and by von Kossa to demonstrate bone salt. At about 9 days, Ca(45) is found in the cartilage template both intra- and extracellularly. Between 9 and 11 days, a primary diaphyseal lamella is deposited which is largely acellular. The lamella is eroded by capillaries from the periosteum and a resorption center is established in the cartilage. New lamellae of bone are deposited centrifugally in an imbricated pattern. Bone matrix formation precedes calcification by about 1 to (1/2) days, and calcification in a particular lamella is not uniform. Endochondral bone formation is described, as well as calcification of the epiphyseal/diaphyseal cartilage. Calcium-45 occurs intracellularly in the osteocyte during bone formation.

Comparative biochemical studies on normal and on poliomyelitis virus-infected tissue cultures. V. Profund alteration of acid and alkaline phosphatase activity in infected rhesus kidney cells

Experimental evidence is presented for drastic changes in phosphomonoesterase activities of tissue cultures, brought about by infection with poliomyelitis viruses. Acid phosphatase activity went through a maximum before decreasing almost to zero level. Alkaline phosphatase activity diminished progressively to zero, then with disruption of the cells attamed normal levels. Various aspects of the kinetics were investigated and illustrated. The initial increase of acid phosphatase, in contrast with the alkaline, may mean that the reactions catalyzed by this enzyme continue during the early phase. This period is the time of intense virus production and therefore it was supposed that this enzyme may play some role in virus synthesis. It was assumed that the virus acts as a particle of molecular size and becomes associated with the enzyme complex physicochemically or chemically. This association ends with the disintegration of the host cells. During the cell-virus interaction a toxin may develop which is a strong and general enzyme inhibitor. Various enzyme systems differ in sensitivity toward these virus effects; for instance, acid phosphatase is irreversibly inhibited or may be destroyed. The visible CPE of virus is preceded by a drastic reduction of enzyme activities in whole TC and in its various fractions, which may suggest causal relationship in the mechanism of cell destruction. In arrested or latent infection these processes are operative, but on a smaller scale. The drop in activities cannot be explained by the reduction of tissue mass, which is the consequence, rather than the cause, of enzyme changes. Besides the theoretical significance of these observations the following practical points can be summarized: 1. Changes in phosphatase activities are most strikingly demonstrated in whole tissue cultures inoculated with poliomyelitis virus. 2. There is causal relationship among infection, enzyme changes, and transformation of cell physiology. 3. The biochemical approach provides a quantitative measure of the extent of cell damage, before visible CPE is detectible. 4. Unapparent and active infections with poliomyelitis virus could be differentiated from normal controls by this method. 5. By various manipulations (freezing, long incubation) the difference between normal and infected TC can be enhanced. Suitable technical methods were proposed for various types of investigations.

Observations on the initial stages of ossification in vitro

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