Chondrogenesis and cell proliferation in limb bud cell cultures treated with cytosine arabinoside and vitamin A

Targeted stimulation of retinoic acid receptor-γ mitigates the formation of heterotopic ossification in an established blast-related traumatic injury model

Heterotopic ossification (HO) involves formation of endochondral bone at non-skeletal sites, is prevalent in severely wounded service members, and causes significant complications and delayed rehabilitation. As common prophylactic treatments such as anti-inflammatory drugs and irradiation cannot be used after multi-system combat trauma, there is an urgent need for new remedies. Previously, we showed that the retinoic acid receptor γ agonist Palovarotene inhibited subcutaneous and intramuscular HO in mice, but those models do not mimic complex combat injury. Thus, we tested Palovarotene in our validated rat trauma-induced HO model that involves blast-related limb injury, femoral fracture, quadriceps crush injury, amputation and infection with methicillin-resistant Staphylococcus aureus from combat wound infections. Palovarotene was given orally for 14days at 1mg/kg/day starting on post-operative day (POD) 1 or POD-5, and HO amount, wound dehiscence and related processes were monitored for up to 84days post injury. Compared to vehicle-control animals, Palovarotene significantly decreased HO by 50 to 60% regardless of when the treatment started and if infection was present. Histological analyses showed that Palovarotene reduced ectopic chondrogenesis, osteogenesis and angiogenesis forming at the injury site over time, while fibrotic tissue was often present in place of ectopic bone. Custom gene array data verified that while expression of key chondrogenic and osteogenic genes was decreased within soft tissues of residual limb in Palovarotene-treated rats, expression of cartilage catabolic genes was increased, including matrix metalloproteinase-9. Importantly, Palovarotene seemed to exert moderate inhibitory effects on wound healing, raising potential safety concerns related to dosing and timing. Our data show for the first time that Palovarotene significantly inhibits HO triggered by blast injury and associated complications, strongly indicating that it may prevent HO in patients at high risk such as those sustaining combat injuries and other forms of blast trauma.

Inhibition of cell differentiation by manganese chloride in micromass cultures of mouse embryonic limb bud cells

Although manganese is an essential element, in excess it has proven to be toxic to a number of organ systems. In several studies, exposure to manganese compounds during embryological development has been linked to skeletal abnormalities. Micromass cultures of dissociated mouse embryonic limb bud cells were used to investigate whether manganese could interfere with the process of chondrogenesis. Cell viability was significantly reduced in cultures grown in the presence of 1 and 10 mM MnCl2, whereas as little as 10 microM MnCl2 inhibited differentiation of limb cells into proteoglycan-producing nodules. Because the concentration of MnCl2 needed to interfere with differentiation is so much lower than the cytotoxic concentration, it is suggested that manganese has the potential to be teratogenic, and to specifically interfere with the development of a cartilage matrix, although by an unknown mechanism.

In vitro retinoid-induced growth inhibition and morphologic differentiation of canine osteosarcoma cells

OBJECTIVE

To determine differentiation and growth inhibition effects of retinoids on canine osteosarcoma cells.

SAMPLE POPULATION

3 osteosarcoma cell lines established from osteosarcomas in dogs.

PROCEDURE

Osteosarcoma cells were incubated with various concentrations of all-trans-retinoic acid and 9-cis-retinoic acid or control medium, counted daily for 10 days, and evaluated for morphologic changes. Synthesis of DNA was measured by use of a cell proliferation ELISA. To analyze effect of retinoids on colony formation on plastic dishes, cells were cultured for 14 days, fixed, and stained; number of colonies was counted.

RESULTS

In a dose-dependent manner, both retinoids induced morphologic differentiation and growth inhibition in the 3 osteosarcoma cell lines and inhibited each cell's ability to form anchorage-dependent colonies.

CONCLUSION AND CLINICAL RELEVANCE

Retinoids induced differentiation of osteosarcoma cells of dogs, resulting in altered expression of their malignant phenotype. Induction of differentiation by retinoids may have potential as an adjunctive treatment for osteosarcoma in dogs.

Retinoids and their receptors in skeletal development

The embryonic vertebrate limb serves as an excellent experimental model system in which to study mechanisms that regulate morphogenesis of the skeleton. The appendicular skeleton arises through the process of endochondral ossification, whereby a cartilage template is initially formed and subsequently replaced by bone. One molecule that has a dramatic effect on these processes is the vitamin-A metabolite, retinoic acid (RA). RA functions through a class of nuclear hormone receptors, the retinoic acid receptors (RARs) and retinoid-X-receptors (RXRs), to regulate gene transcription. Experimental evidence from RA teratogenesis suggests that the presence of ligand-activated RARs and/or inappropriate expression of RARs inhibits chondrogenesis. Conversely, genetic analysis has shown that the absence of the receptors can lead to deficiencies in cartilage formation while also promoting chondrogenesis at ectopic sites. Taken together, these studies suggest that the RARs play a fundamental role in the early stages of skeletal development, specifically those involved in the formation of prechondrogenic condensations and their subsequent differentiation into chondroblasts.

Three-dimensional culture of bovine chondrocytes in rotating-wall vessels

The Rotating-Wall Vessel (RWV) was used to culture chondrocytes for 36 d to observe the influence of low-shear and quiescent culture conditions allowing three-dimensional freedom on growth, differentiation, and extracellular matrix formation. Chondrocytes were freshly isolated from bovine cartilage and placed into the RWV with Cytodex-3 microcarriers. Nonadherent petri dishes were initiated with microcarriers as representative of standard culture conditions. In the RWV, large three-dimensional aggregates (5-7 mm) were formed in suspension. In addition, a large sheet of matrix adhered to the oxygenator core and vessel endcaps. Petri dish culture resulted in the formation of sheets of chondrocytes with no matrix production. Immunocytochemical analyses on histologic sections of tissue obtained from the RWV and the petri dish controls were performed with antibodies against fibronectin, collagen II, chondroitin-4-sulfate, chondroitin-6-sulfate, and vimentin. Results demonstrated increased signal in the RWV material while the petri dishes demonstrated a slight decrease in signal. In addition, differentiated chondrocytes were observed in sections of RWV material through 36 d, while few were observed in the sections of petri dish material. These results indicate that the unique conditions provided by the RWV afford access to cellular processes that signify the initiation of differentiation as well as production of normal matrix material.

The Comparative Testing of Eight Coded Chemicals in the Rat Limb Bud Micromass and Rat Embryo Culture Systems

When cultured at high density, mesenchymal cells from rat limb buds proliferate and differentiate into chondrocytes. Inhibition of this in vitro chondrogenic process has been used for the preliminary evaluation of teratogenic potential. Alternatively, intact post-implantation rat embryos, maintained in short-term culture, provide a system for the in vitro study of abnormal development not limited to the skeletal system. Both systems isolate the test agent from maternal metabolism and pharmacokinetic restraints. In this study, drug-associated selective inhibition of alcian blue uptake by cartilage proteoglycans, in micromass cultures of limb bud cells prepared from 13-day-old rat embryos, was used to assess teratogenic potential in vitro following exposure for 48 hours to eight coded compounds (acetylsalicylic acid, isoniazid. penicillin G, saccharine, vincristine sulphate, 6-aminonicotinamide, retinoic acid, and amaranth). Following drug exposure, cultures were incubated for another 96 hours, and the cells were then fixed and stained with 0.5% alcian blue. Bound dye was then extracted and quantitated. In parallel cultures, cell viability was measured by neutral red uptake, and protein content was assayed by using the bicinchoninic acid method. Except for retinoic acid and vincristine sulphate, the maximum test concentration was 1000μg/ml. Inhibition of alcian blue uptake (> 50%) was noted at 0.001μg/ml vincristine sulphate, 0.5/μg/ml retinoic acid and 5μg/ml 6-aminonicotinamide, demonstrating that strong teratogens inhibit differentiation in micromass cultures at lower concentrations than those which affect limb cell viability. When the same eight compounds were tested in a 24-hour embryo culture model, dysmorphogenesis was evident at 0.005μg/ml vincristine sulphate, 0.1μg/ml retinoic acid and 0.3μg/ml 6-aminonicotinamide. For the other five chemicals, little or no toxicity was noted up to the maximum test concentration in either model. We conclude that the two test systems, both based on the developing rat embryo, are consistent with each other, and that either of them would be useful for the preliminary screening of potential teratogens.

The effects of natural and synthetic retinoids on the differentiation of RCJ C5.18 chondrogenic cells

RCJ C 5.18 (C 5.18) is a chondrogenic clonal cell line which, under standard culture conditions, develops chondroblastic features including the production of a cartilagenous matrix. Retinoic acid (RA) is known to inhibit the chondrogenic differentiation of C 5.18 cells and this may parallel the teratogenic effects of retinoids in vivo; however, the question as to which of the 3 retinoic acid receptors (RAR alpha, beta, gamma) or the 3 retinoid X receptors (RXR alpha, beta, gamma) mediate this RA-induced inhibition remains unanswered. We tested several retinoids with different receptor binding characteristics. Cartilage formation in C 5.18 cultures was evaluated by counting the number of cartilage nodules formed, and by quantitating the glycosaminoglycan content of the cultures using alcian blue staining. All of the retinoids prevented cartilage formation in a dose-dependent manner. Treatment with the retinoids did not affect cell number, thereby ruling out any toxic effects. RA, which binds to all 3 RARs with similar affinity, produced a 50% inhibition (IC50) of cartilage formation at 4 x 10(-10) M. We also tested Ch55, which also binds to all 3 RARs, but with higher affinity than RA. This compound was approximately 10 times more potent than RA (IC50 2 x 10(-11) M). 9-cis RA, which binds to the 3 RARs with affinities similar to RA and also binds to the 3 RXRs, was less active (IC50 8 x 10(-9) M), suggesting that RXR binding interferes with the inhibitory effect of ligand-activated RARs. 9-cis retinal, for which the binding characteristics are unknown, had the same effect as 9-cis RA.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of retinoic acid on cartilage differentiation in a chondrogenic cell line

Previously we have isolated the monopotential chondrogenic cell line RCJ 3.1 C5.18 from the multipotential mesenchymal cell line RCJ 3.1 [Grigoriadis et al.: Endocrinology, 125:2103-2110, 1989]. When cultured for approximately 20 days under appropriate conditions, these cells from cartilage nodules. In the present investigation, we have used this cell line to study the effects of all-trans retinoic acid (RA) on chondroblast differentiation, cartilage formation, and cartilage degradation. Continuous exposure of cultures to RA (0.01-100 nM) inhibited chondroblast differentiation and glycosaminoglycan (GAG) accumulation in a dose-dependent manner, without comparable effects on cell growth. Pulse treatment with RA for various 4 day periods during a 17-24 day culture period established that RA inhibited differentiation of chondroprogenitors at all periods tested. These effects were reversible, except for part of the effect on early chondroprogenitors. Treatment with RA on days 13-17 in 17 day cultures not only resulted in cessation of cartilage formation, but also in disappearance of pre-existing cartilage nodules. We demonstrated that this was associated with RA-induced downregulation of GAG synthesis and increased degradation of cartilage proteoglycans. Hence, the inhibitory effects of RA on cartilage formation consist of inhibition of chondroblast differentiation, inhibition of GAG synthesis by differentiated chondroblasts, and stimulation of cartilage proteoglycan degradation by differentiated chondroblasts and/or chondrocytes. These results indicate that the clonal monopotential chondrogenic cell line RCJ 3.1 C5.18 forms a good model system to study the effects of retinoids on cartilage differentiation, formation, and degradation.

Hypertrophic chondrocytes undergo further differentiation in culture

Conditions have been defined for promoting growth and differentiation of hypertrophic chondrocytes obtained in culture starting from chick embryo tibiae. Hypertrophic chondrocytes, grown in suspension culture as described (Castagnola P., G. Moro, F. Descalzi Cancedda, and R. Cancedda. 1986. J. Cell Biol. 102:2310-2317), when they reached the stage of single cells, were transferred to substrate-dependent culture conditions in the presence of ascorbic acid. Cells showed a change in morphology, became more elongated and flattened, expressed alkaline phosphatase, and eventually mineralized. Type II and X collagen synthesis was halted and replaced by type I collagen synthesis. In addition the cells started to produce and to secrete in large amount a protein with an apparent molecular mass of 82 KD in reducing conditions and 63 KD in unreducing conditions. This protein is soluble in acidic solutions, does not contain collagenous domains, and is glycosylated. The Ch21 protein, a marker of hypertrophic chondrocytes and bone cells, was synthesized throughout the culture. We have defined this additional differentiation stage as an osteoblast-like stage. Calcium deposition in the extracellular matrix occurred regardless of the addition of beta glycerophosphate to the culture medium. Comparable results were obtained both when the cells were plated at low density and when they were already at confluence and maintained in culture without passaging up to 50 d. When retinoic acid was added to the hypertrophic chondrocyte culture between day 1 and day 5 the maturation of the cells to the osteoblast-like stage was highly accelerated. The switch in the collagen secretion was already observed after 2 d and the production of the 63-kD protein after 3 d. Mineralization was observed after 15-20 d.

Physiological and clinical aspects of vitamin A and its metabolites

Retinoids, including retinol and retinoic acid (RA), are a group of naturally occurring and synthetic compounds that exhibit vitamin A-like biological activity. They achieve their effects by binding to intracellular proteins. Important sites of action are the nuclear retinoic acid receptors (RAR). These receptors, namely, RAR alpha, RAR beta, and RAR gamma, function as transcription factors by binding to RA-responsive elements (RARE) of multiple genes. Retinoids play a role in vision, embryogenesis, immune modulation, growth and differentiation of normal, premalignant and malignant tissues, the suppression of carcinogenesis, and the inhibition of tumor growth in experimental systems and humans. Reports of the significant antitumor effect of all-trans-RA in acute promyelocytic leukemia and the synthesis of new, less toxic, and more potent retinoids has generated renewed interest in these compounds. Retinoids may have an important role to play in the chemoprevention and therapy of cancer.

Influence of retinol on human chondrocytes in agarose culture

Vitamin A and its congeners, collectively called retinoids, are known to have teratogenic potential and have induced craniofacial and limb malformations in numerous animal species. More importantly, retinoids are recognized as teratogenic to fetuses of pregnant women who have taken such preparations for dermatologic disorders. Information gathered from the study of animal models suggests that retinoids interfere with cartilage differentiation. If chondrogenesis in limb development is disturbed it may contribute to limb reductions and malformations. In vitro studies using various animal systems have shown that cartilage matrix macromolecules are altered to resemble those secreted by mesenchymal cells. The response of human chondrocytes to retinoids in vitro is not known. Culture of human chondrocytes in agarose maintains the cartilage phenotype and therefore serves as a model system to evaluate the influence of retinoids directly on human chondrogenesis. The studies presented in this paper were done to determine if the expression of specific matrix macromolecules of human chondrocytes in agarose culture is altered by retinol treatment. Immunocytochemistry demonstrated enhanced labeling of type I collagen while type II collagen labeling was reduced in cultures treated with retinol. In addition, morphometric analyses indicated a decrease in the size and number of chondrogenic clusters and that individual cells synthesized less alcian blue matrix when compared to parallel control cultures. The size of the proteoglycan monomers, glycosaminoglycan side chains as well as the disaccharide composition were not affected. However, there was a reduction in the quantity of proteoglycan monomers produced.

Retinoic acid treatment induces type X collagen gene expression in cultured chick chondrocytes

The vitamin A derivative retinoic acid (RA) is widely thought to be involved in cartilage development, but its precise roles and mechanisms of action in this complex process remain unclear. We have tested the hypothesis that RA is involved in chondrocyte maturation during endochondral ossification and, in particular, is an inducer of maturation-associated traits such as type X collagen and alkaline phosphatase. Immature chondrocytes isolated from the caudal region of Day 19 chick embryo sterna were seeded in secondary monolayer cultures and treated either with a high dose (100 nM) or with physiological doses (10-35 nM) of RA for up to 3 days. We found that after an initial lag of about 24 h, physiological doses of RA indeed induced type X collagen gene expression in the immature cells. This induction was not accompanied by obvious changes in expression of the type II collagen and large aggregating proteoglycan core protein genes. As revealed by immunocytochemistry, 30-35% of the cells in cultures treated with RA for 3 days were engaged in type X collagen production. Interestingly, these cells were relatively similar in size to chondrocytes in which no type X collagen was detected, suggesting that chondrocytes can initiate type X collagen production independent of cell hypertrophy. RA treatment also led to increased alkaline phosphatase activity occurring as early as 24 h after the start of treatment. The data in this study indicate that RA may have a role in endochondral ossification as an inducer/promoter of maturation-associated traits during chondrocyte maturation.

Chemically induced growth inhibition and cell cycle perturbations in cultures of differentiating rodent embryonic cells

Ethylnitrosourea (ENU) is a proven animal teratogen, although the mechanism of its developmental toxicity is unknown. The micromass rat embryo midbrain (CNS) and limb bud (LB) cultures were used in an effort to determine potential mechanisms by which ENU may exert its teratogenic effect. When cultured at high cell densities, both cell types undergo several rounds of replication while differentiating into discrete foci of neuronal cells and chondrocytes, respectively. Differentiation was monitored after 5 days by staining with hematoxylin (CNS) and alcian blue (LB). Our objectives were to (1) apply flow cytometry technology to the micromass cultures and (2) determine how ENU disrupts the normal growth, differentiation, and cell cycling of these cultures. Dose-dependent decreases in cell attachment and viability were observed in the first 24 hr after ENU exposure. Exposed cultures also exhibited dose-dependent growth inhibition over 5 days in culture as determined by cell counts. Flow cytometric cell cycle analysis of treated cultures revealed a dose-related accumulation of CNS cells in late G1/early S. Treated LB cells also displayed dose-related cell cycle changes with cells accumulating throughout the S phase. The concentration-dependent changes in both the CNS and the LB cell cycle profiles were observed in the attached cell populations which had greater than 94 +/- 3% viability at all ENU concentrations tested. This suggests that flow cytometric analysis allowed description of cellular alterations that would have been overlooked if only cell viability had been examined. Our examinations suggest that the effects of ENU on cell differentiation are related to its early effects on cell attachment, cell cycling, and cell proliferation.

Metabolism in vitro of cartilage proteoglycans in rat (pre)chondrocytes from different embryonic regions

Diabetes in the mother may cause disturbances in the chondrocyte development in the embryo. A rat model was used to investigate whether this was reflected in the production of proteoglycans by cells from two embryonic regions. One of these regions is resistant (limb bud) and the other susceptible (mandibular arch) to malformation in diabetic pregnancy. Chondroitin sulphate proteoglycans from cultures of day-12 rat embryo limb bud and mandibular arch chondrocytes were extracted with guanidine-HCl and analyzed by gel chromatography after in vitro 35S-sulphate-labeling. Two sizes of proteoglycans (Kav 0.26 and 0.66 on CL-2B Sepharose) were found in both types of chondrocytes and in all media. The polysaccharide chain length was the same (Kav 0.36 on CL-6B Sepharose) for both proteoglycans. Elevated levels of D-glucose or beta-hydroxybutyric acid had no effect on either proteoglycan size or proportion, nor on polysaccharide chain length. However, there were differences (in all culture conditions) between limb bud and mandibular arch cultures in that the larger proteoglycan accounted for 80% of total radioactivity in the limb bud cultures, 53% in the mandibular arch cultures, and only 25-29% in the media from both types of cultures. Furthermore, different ratios between radioactive proteoglycans in medium and matrix suggested markedly different efficiencies for matrix formation in the two cell types. These findings indicate differences in the metabolism of the proteoglycans in these two cell types which may be related to the induction of mandibular malformation in diabetic pregnancy.

Selective stimulation of in vitro limb-bud chondrogenesis by retinoic acid

Embryonic exposure to pharmacologic doses of vitamin A analogs (retinoids) is a well-known cause of limb-skeletal deletions, limb truncation and other skeletal malformations. The exclusively inhibitory effect of retinoic acid (RA) on chondrogenesis in standard serum-containing cultures of limb-bud mesenchymal cells is equally well known and has provided a means to explore the cellular basis for RA-mediated skeletal teratogenesis. Recent studies showing that lower RA concentrations can cause skeletal duplication when applied directly to the anterior border of a developing limb, suggest that RA may have a role in normal limb development as a diffusible morphogen capable of regulating skeletal pattern. While RA treatment causes both, skeletal deletions and duplications are clearly different (if not opposing) effects, the latter of which is difficult to reconcile with RA's heretofore exclusively inhibitory effect on in vitro chondrogenesis. In the present study. RA's effects on chondrogenesis and myogenesis were examined in serum-free cultures of chick limb-bud mesenchymal cells and compared with its effects on similar cultures grown in serum-containing medium. When added to serum-free medium, concentrations of RA known to cause skeletal duplication in vivo dramatically enhanced in vitro chondrogenesis (to over 200% of control values) as judged by both Alcian-blue staining and [35S]sulfate incorporation, while having little effect on myogenesis. Higher concentrations inhibited both chondrogenesis and myogenesis. The results indicate that at physiological concentrations. RA can selectively modulate chondrogenic expression and suggest that at higher concentrations, RA's inhibitory effects are less specific.(ABSTRACT TRUNCATED AT 250 WORDS)

Retinoic acid promotes proliferation and chondrogenesis in the distal mesodermal cells of chick limb bud

Distal and proximal mesoderm of chick limb bud was respectively dissociated and cultured in the medium containing various concentrations of retinoic acid (RA). At low concentrations (5-50 ng/ml), RA promoted proliferation and chondrogenesis in the distal mesodermal cells. The distal cells of stage 20-24 limb buds were responsive to RA, although those of stages 25-27 were unresponsive. Both the cells of anterior and posterior regions of the distal mesoderm were responsive to RA, while the cells of proximal mesoderm were unresponsive. At higher concentrations, the growth-promoting effect of RA was reduced and chondrogenesis in the distal cells was rather inhibited. These results were discussed in relation to the role of RA as the morphogen in normal limb development and experimental duplicate formation.

Inhibition of chondrogenesis by retinoic acid in limb mesenchymal cells in vitro: effects on PGE2 and cyclic AMP concentrations

Effects of retinoic acid (RA) on prostaglandin E2 (PGE2) and cyclic AMP (cAMP) concentrations were investigated in high density, micromass cultures of mesenchymal cells derived from chick limb buds. Exposure of cells during the initial 24 h of culture to RA concentrations between 0.05-1.0 micrograms/ml inhibited chondrogenesis in a dose-dependent manner with 1.0 micrograms/ml totally inhibiting cartilage formation. Concentrations of PGE2 and cAMP increased during the prechondrogenic period in control cells in a closely related way and remained elevated throughout the six-day period examined. Addition of RA (0.05 and 0.5 micrograms/ml) did not significantly alter cAMP concentrations at any time point, but significantly elevated PGE2 levels relative to control cells in six-day cultures in a concentration-dependent manner. Addition of dibutyryl cAMP enhanced chondrogenesis in control cells between days 3 and 4, but failed to alter the inhibitory effect of RA on chondrogenesis. The results indicate that while PGE2 and cAMP are important signals in cartilage differentiation, the inhibitory effects of RA on this process are mediated through some other mechanism.

Effects of molecular oxygen on chick limb bud chondrogenesis

Chondrogenesis is an important process in the development of the embryonic chick limb. If limb buds are dispersed just prior to the initiation of chondrogenic differentiation and their cells seeded densely in culture as three-dimensional "micromasses," some of the cells differentiate to form chondrogenic nodules. These nodules characteristically produce sulfated proteoglycans and type II collagen. Two conditions within the early avian limb core have been linked causatively to the initiation of chondrogenesis: a limitation in the availability of molecular oxygen and a low NAD content of the tissue. The O2 limitation is thought to be responsible for the low NAD level. We examined the effects of molecular oxygen on the NAD content of chick limb-bud cells in micromass culture, the formation of chondrocytic nodules, and the production of type II collagen and sulfated proteoglycans. The NAD content of the cells in the micromasses and the production of type II collagen did not vary greatly as a function of oxygen availability. The development of the nodules was modified, but not eliminated, by high oxygen partial pressure (0.95). It was eliminated by anoxia. Proteoglycan synthesis was decreased significantly by high oxygen tension and its sulfation was also decreased, more so in the wing-bud than the leg-bud cells. The results suggest that in culture, high oxygen tension is compatible with some, but not all, aspects of chondrogenic differentiation of cells from embryonic chick limbs.

The effects of local application of retinoids to different positions along the proximo-distal axis of embryonic chick wings

Two retinoids, all-trans-retinoic acid and a synthetic analog, TTNPB, were locally applied to different positions along the proximo-distal axis of embryonic chick wing buds using controlled release carriers. Truncations or limbs with duplicated structures across the antero-posterior axis develop after retinoid application to distal positions in buds from stage 20-24 embryos. Phocomelic limbs develop when the retinoids are applied more proximally to buds of stage 23-24 embryos. Duplications of the pattern of structures along the proximo-distal axis never occur.Using TTNPB that is relatively stable, the amount of retinoid in the wing tissue when phocomelia is induced was measured. There is twice as much retinoid per cell in the proximal half of the bud as in the distal half of the bud. The concentration of TTNPB in proximal tissue is estimated to be three times higher than in distal tissue in which pattern formation and cartilage morphogenesis are relatively normal.At early stages in the development of phocomelia, the shape of the bud changes and the indentation that marks the elbow does not arise. Neither retinoid-induced cell killing nor effects on the pattern of programmed cell death were detected.The induction of phocomelia by retinoids appears to be based on effects on proximal cells, whereas retinoids produce pattern changes by acting on distal cells. Furthermore, compared with pattern changes, higher concentrations of retinoid in the bud tissue are required to produce phocomelia.

Comparative effects of retinoic acid and jervine on chondrocyte differentiation

Jervine and retinoic acid are both teratogenic to structures which are initially modelled in cartilage. Differences in periods of maximal sensitivity, as well as in certain aspects of the morphological manifestations of exposure, indicate that these two teratogens act via different molecular mechanisms. Here we compare the effects of jervine and retinoic acid in three culture systems which represent sequential stages of the chondrocyte lineage. Proliferation of pluripotent C3H 10T 1/2 cells was decreased by exposure to jervine but was not affected by retinoic acid. Differentiation of high-density "spot" cultures of embryonic limb bud mesenchyme were sensitive to both compounds. Mature chondrocytes were resistant to jervine but "dedifferentiated" after 48-hour exposure to retinoic acid. We conclude that jervine compromises rapidly dividing chondrogenic precursors, whereas retinoic acid has little effect prior to the expression of cartilage-specific proteins.

The effects of retinoids on cartilage differentiation in micromass cultures of chick facial primordia and the relationship to a specific facial defect

Retinoids produce facial defects in chicken embryos. Outgrowth of the frontonasal mass with accompanying cartilage differentiation and pattern formation is inhibited. In contrast, the development of the mandibular primordia that give rise to the lower beak proceeds normally. To investigate whether the upper beak defect is based on the inhibition of cartilage differentiation specifically in the frontonasal mass, the effects of retinoids on chondrogenesis in micromass (high density) cultures of cells from facial primordia have been studied. When either 10(-6) M retinoic acid or 10(-8) M (E)-4-[2-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-napthalenyl-1- propenyl]benzoic acid (TTNPB; a stable retinoid) is added to the culture medium, cartilage differentiation is inhibited. Both frontonasal mass and mandible cultures are equally affected. The concentration of TTNPB found in both facial primordia in vivo, after a treatment that produces the defect, is also about 10(-8) M. This rules out preferential accumulation of the retinoid by the frontonasal mass as an explanation for the defect. In fact, the concentration of retinoid found in vivo, should, from the culture studies, be sufficient to markedly inhibit chondrogenesis in both the frontonasal mass and mandibles. The effects of exposure to retinoids in the intact face appear to be different to those in culture. Furthermore, when cells from retinoid-treated facial primordia are cultured in micromass, the extent and pattern of chondrogenesis in frontonasal mass cultures is identical to that of cells from untreated primordia. Cartilage differentiation in mandible cultures is slightly affected. These findings suggest that retinoids do not produce the specific facial defect by directly interfering with cartilage differentiation.

Craniofacial dysmorphogenesis following hypervitaminosis A in mice

Malformations of the temporomandibular joint (TMJ), zygomatic arch, middle ear ossicles, and mandibular musculature following hypervitaminosis A were described in fetal mice. Pregnant mice (Mus musculus) were each given a 0.2-ml solution of corn oil containing 10,000 IU of retinol palmitate by gavage on day 8.7. Thirty-eight fetal heads were collected and hemisected. The left hemiheads were serially sectioned, stained, and examined histologically. Right halves were cleared and double-stained with alizarin red and alcian blue. The craniofacial morphologies of the test mice were found to vary from normal to maximally involved (feature unidentifiable) even among littermates. Both inter- and intralitter variation were discussed. Errors in chondrogenesis were determined to have produced the variety of dysmorphologies observed. Changes in Meckel's cartilage affected both the TMJ and ossicles; the presence of ectopic cartilages affected the zygomatic arch; and the musculature was affected secondary to skeletal system changes. Several modes of vitamin A interference leading to craniofacial dysmorphogenesis have been proposed in the literature. It was determined that proposals implicating altered cellular differentiation were the most compatible with the in vivo data recorded here and elsewhere.

Inhibition of limb chondrogenesis by a Veratrum alkaloid: temporal specificity in vivo and in vitro

It has been demonstrated that jervine, a steroidal alkaloid derived from plants of the genus Veratrum, exerts teratogenic effects in several animal species. Defects were restricted to structures which depend upon normal chondrogenesis for their development. Here we report studies of the temporal specificity of cellular sensitivity using limb bud mesenchyme cells obtained from Day 10 mouse embryos. These cells, when grown as high-density "spot" cultures, undergo chondrogenesis in vitro. Prior to differentiation, exposure of limb cell cultures to jervine suppressed subsequent accumulation of cartilage proteoglycans. Treatment after differentiation had no significant effect. Additionally, there was a genetic component to jervine sensitivity: C57BL/6J mice were sensitive, whereas NIH Swiss-Webster mice were insensitive. This strain-dependent difference was observed both in vivo and in vitro, supporting the validity of limb mesenchyme spot cultures as a model for jervine-induced teratogenicity. Our studies indicate that jervine acts specifically during an early phase of the differentiation of mesenchyme into cartilage. It is likely that a specific stem cell population is the target tissue of this compound.

Effect of retinoids on collagen production by chondrocytes in culture

The effect of vitamin A and of some of its derivatives on chondrocytes in culture has been studied. In the presence of retinoids the proliferation of the cells decreased and they lost their characteristic polygonal shape and assumed a fibroblast-like morphology. All retinoids also caused dedifferentiation of chondrocytes as indicated by the induction of types I and III collagen. 13-cis retinoic acid (= isotretinoin) was the most active derivative in this aspect. Since appropriate control of the synthesis of extracellular matrix proteins is a prerequisite for their normal physiological function, alterations such as those observed here may be involved in the pathogenesis of side effects which are observed during the treatment of dermatological disorders with retinoic acid derivatives.

Retinoic acid enhances the displacement of newly synthesized hyaluronate from cell layer to culture medium during early phases of chondrogenesis

Chondrogenic differentiation in mouse limb bud mesenchymal cells cultured at high density was suppressed by supplementation of the medium with retinoic acid (1 microgram/ml or 3.3 X 10(-6) M). Since in control medium overt chondrogenesis begins on day 3, retinoic acid was introduced on day 2 so that the relationship between initial biosynthetic changes and inhibition of chondrogenesis could be studied. During the first 24 h of exposure the treated cells remained viable but suffered 10% inhibition in growth and synthesized [3H]glucosamine-labeled glycosaminoglycan at a level 24% below untreated cells. The amount of labeled hyaluronic acid released into the culture medium by the treated cells was, however, 2-fold greater, on a per cell basis, than that in the untreated cultures. It is suggested that the displacement of hyaluronate may play a role in the disruption of mesenchymal cell differentiation and of limb morphogenesis as observed in other systems.

A fraction from extracts of demineralized adult bone stimulates the conversion of mesenchymal cells into chondrocytes

Demineralized adult bone contains factors which stimulate nonskeletal mesenchymal cells to undergo a developmental progression resulting in de novo endochondral ossification. In this study, isolated embryonic stage 24 chick limb bud mesenchymal cells maintained in culture were utilized as an in vitro assay system for detection of specific bioactive components solubilized from adult chicken bone matrix. Guanidinium chloride extracts (4 M) of demineralized-defatted bone were fractionated and tested in limb mesenchymal cell cultures for possible effects upon growth and chondrogenesis. Two low-molecular-weight fractions were found to be active in these cultures. A cold water-insoluble, but warm Tris-buffered saline-soluble fraction provoked a dose-dependent increase in the amount of cartilage formed after 7 days of continuous exposure as evidenced by an increased number of chondrocytes observed in living cultures, elevated cell-layer-associated 35S incorporation per microgram DNA, and greater numbers of toluidine blue-staining foci (i.e., cartilage nodules). Growth inhibitory substances were detected in a low-molecular-weight, water-soluble fraction; 7 days of continuous exposure to this material resulted in less cartilage formation and reduced cell numbers (accumulated DNA) on each plate. These observations demonstrate the usefulness of stage 24 chick limb bud cell cultures for identifying bioactive factors extracted from adult bone matrix. In addition, the action of these factors on mesenchymal cells may now be studied in a cell culture system.

Differentation of rat embryo cells in culture: response following acute maternal exposure to teratogens and non-teratogens

An in vivo-in vitro test system with high sensitivity to teratogens has been developed and validated. A single acute intra-peritoneal injection of teratogens (18) and non-teratogens (13) was administered to pregnant rats on the 12th day after fertilisation, and uteri were removed after 16 h by laparotomy. 34-36 Embryos somites were selected, and mid-brain (CNS) and fore-limb buds (LB) were dissected free and dispersed as single-cell suspensions in Ham's F12 culture medium. The cells were cultured as micromass cell islands for 5 days, and discrete foci of neuronal cells differentiated in CNS cultures and chondrocytes in LB cultures. After 5 days, differentiation as determined by number of stainable foci of differentiated cells and 3H-GABA incorporation in CNS or 35SO4 incorporation in LB and growth (as determined by total protein) were measured. Both differentiation and growth of CNS and LB cultures were markedly reduced following exposure of the dam to teratogens, whereas no significant effect was observed with non-teratogens. One teratogen (amaranth) and one non-teratogen (nitrilotriacetic acid) were classified as false negative and positive, respectively; the sensitivity of the test (proportion of teratogens correct) was therefore 92% and the specificity (proportion of non-teratogens correct) was 94%. Inhibition of growth and differentiation in the rat embryo cell cultures following maternal exposure forms the basis of a short-term in vitro test for teratogens.

Development of a mouse embryo limb bud cell culture system for the estimation of chemical teratogenic potential

High-density cultures of mouse embryo limb bud cells differentiate and synthesize an extracellular matrix containing sulfated proteoglycans. Since these in vitro events are related to those that occur during fetal development, we have investigated the use of cultured limb bud cells for the analysis of the activities of chemical teratogens. We have established the conditions for the use of the radiochemicals 35SO=4 and 3H-thymidine for the assessment of chemical effects on sulfated proteoglycan and DNA synthesis, respectively, in mouse limb bud cells. By performing double-labeling experiments in the presence of test chemicals, the preferential inhibition of either proteoglycan synthesis or DNA synthesis could be demonstrated for 19 of 22 known mouse teratogens tested. The five nonteratogens tested did not cause significant differences in the inhibition of either macromolecular class. The overall predictive accuracy of the system was approximately 89%, and the false-negative rate was approximately 14.8%. No false positives were observed. These data showed that this cell culture system differentiated between the activities of a limited set of selected mouse teratogens and nonteratogens, suggesting that cultured mouse embryo limb bud cells may constitute the basis for the development of a powerful tool for analyses of chemical teratogenic potential.

Restoration by parathyroid hormone and dibutyryl cyclic AMP of expression of the differentiated phenotype of chondrocytes inhibited by a tumor promoter, 12-0-tetradecanoylphorbol-13-acetate

12-0-Tetradecanoylphorbol-13-acetate (TPA) inhibited expression of the differentiated phenotype of chondrocytes in rabbit costal chondrocytes in culture. TPA transformed typical polygonal chondrocytes into multilayered, fibroblastic cells and also inhibited the rate of [35S]sulfate incorporation into glycosaminoglycan (GAG), a differentiated phenotype of chondrocytes. These changes were apparent within 24 h and reached a plateau at 48 h after the addition of TPA. Phorbol didecanoate and phorbol dibenzoate also inhibited sulfation of GAG, even though the effect was weaker than that of TPA. Phorbol diacetate and 4-0-methyl TPA did not inhibit sulfation of GAG. Addition of parathyroid hormone (PTH) or dibutyryl cyclic AMP simultaneously with TPA overcame the inhibition caused by TPA. PTH and dibutyryl cyclic AMP also reversed the inhibition and stimulated expression of the differentiated phenotype of chondrocytes even in de-differentiated cells which had been pretreated for 3 days with TPA. These findings suggest that cyclic AMP plays an important role in the restoration of the differentiated phenotype of chondrocytes in TPA-treated chondrocytes, and that the TPA-treated cells retain some of the differentiated phenotype of the original cells, such as responsiveness to PTH.

Teratogenic effects of cholinergic insecticides in chick embryos--IV. The role of tryptophan in protecting against limb deformities

The mechanism by which organophosphate (OP) insecticides cause micromelia in embryonic chick limbs was examined using a tissue culture approach. Limb bud cells in micromass culture were assayed for their proliferative and chondrogenic activities, [3H]thymidine and 35SO4 = incorporation, respectively, into the trichloroacetic acid-insoluble constituents of the cell masses and/or the accumulation of 35S-labeled soluble macromolecular products in the culture medium. There was no obvious correlation between either the teratogenicity or toxicity of the insecticide in ovo and the inhibition of proliferation and chondrogenesis in vitro. In addition, nicotinamide, which prevents insecticide-induced micromelia in ovo, did not improve the proliferative and chondrogenic performance of insecticide-treated cells in culture. On the other hand, 2-pyridinealdoxime methochloride, which offers little or no protection against micromelia in ovo, did protect both the proliferative and chondrogenic activities of the limb bud cells in micromass culture. These observations suggest that the actions of the insecticides on the cells in culture are not the same as those that produce micromelia in ovo. L-Tryptophan antagonized OP insecticide-induced micromelia in the embryo. In micromass culture, a much greater concentration of tryptophan was needed to support the chondrogenic than the proliferative activities of the limb bud cells. Moreover, a greater concentration of tryptophan was needed to support the chondrogenic activities of the leg bud than the wing bud cells. These in vitro responses of the limb bud cells to tryptophan deprivation are analogous to the in ovo response of the limbs to the teratogenic OP insecticides. A possible explanation of the roles of tryptophan and nicotinamide in preventing the limb deformities is offered.

Proliferation of cells undergoing chondrogenesis in vitro

Continuous exposure of chicken embryo limb bud mesenchyme cells undergoing chondrogenesis in vitro to [3H] thymidine thymidine [(3H]TdR) revealed that more than 90% of the cells synthesized DNA at least once during 120 h of culture. When cells were exposed to [3H]TdR for 24 h beginning at various times throughout the culture period, the percentage of cells which incorporated [3H]TdR during each period was approximately 92%. However, when the period for incorporation of radioisotope was limited to two hours, the number of cells which incorporated [3H]TdR was found to decline during chondrogenesis in vitro. This decline was coincident with the appearance of extracellular matrix material and occurred in those cells which had, and had not, expressed the cartilage phenotype. We conclude from these studies that (1) practically all of the cells continue to proliferate while chondrogenesis is occurring in vitro, (2) there is an increase in the length of the cell cycle during chondrogenesis in vitro, and (3) withdrawal from the cell cycle is not required for differentiation of mesenchyme into cartilage.

Cyclic nucleotides during chondrogenesis: concentration and distribution in vivo and in vitro

This study correlates endogenous levels of cAMP and cGMP with their immunohistochemical localization during chondrogenic differentiation of C57B1/6J mouse limb mesenchyme in vivo and in vitro. A transient decrease in cGMP but not cAMP was found from days 12 to 13 in vivo correlating with early stages of chondrogenesis in the developing limb. Intracellular levels of both cAMP and cGMP in high density limb mesenchyme cultures increased 25% after 24 hr in culture when aggregate and nodule formation was detectable. When cells were seeded at different initial plating densities to delay the onset of aggregate and nodule formation, increased levels of intracellular cAMP correlated temporally with the appearance of nodules. Both cyclic AMP and cGMP were immunohistochemically localized in perichondrial cells and chondrocytes in vivo and in vitro. Therefore, (1) cAMP levels correlated temporally with the appearance of chondrogenic cells and (2) cAMP and cGMP were immunohistochemically localized to chondrogenic cells. These data indicate that fluctuations of both cAMP and cGMP levels may be involved in limb cartilage differentiation. Although increases in both nucleotides were found to correlate with the onset of chondrogenesis in vitro, in vivo data suggest that the amount of cAMP relative to cGMP rather than the absolute amount of an individual cyclic nucleotide may be more significant in modulating differentiation.

Histological changes during regression induced by retinoic acid in a transplantable rat chondrosarcoma

Daily oral treatment with retinoic acid (100 mg/kg bodyweight) induced regression of a transplantable rat chondrosarcoma. In a previous biochemical investigation we have shown that the tissue breakdown is preceded by the loss of proteoglycan. The present study describes the histological changes induced by retinoic acid. A decrease in the intensity of metachromatic staining with toluidine blue was noted already after 1 day and the discoloration was almost complete after 4 days correlating with the loss of proteoglycan. Especially in the perichondrium there was a rapid proliferation of fibroblasts and monocytes. Osteoclast-like cells were missing, but tumor nodules were eroded and split up by penetrating perichondrium. After 4 days of treatment larger necrotic areas were found, initially in the center of tumor nodules only. In other areas the majority of tumorous chondroblasts survived. Tumor nodules appeared partly mesenchyma-like with some fibroblast-like cells suggesting a dedifferentiation of chondroblasts by retinoic acid. we believe that tumor regression induced by retinoic acid involved proteoglycan degradation by chondroblasts themselves and chondroclast-like activity of monocytes and fibroblasts.

Restoration by cyclic AMP of the differentiated phenotype of chondrocytes from de-differentiated cells pretreated with retinoids

Parathyroid hormone (PTH) increases the cyclic AMP level in rabbit costal chondrocytes in culture. PTH, dibutyryl cyclic AMP (DBcAMP), and 8-bromo cyclic AMP (8-Br cAMP)induce ornithine decarboxylase (ODC) and expression of the differentiated phenotype of chondrocytes in this cell system. On the other hand, retinoids inhibit expression of the differentiated phenotype of chondrocytes. In the present study, the effects of PTH, DBcAMP, and 8-Br cAMP on rabbit costal chondrocytes pretreated with retinoids were examined. PTH did not increase the cellular cyclic AMP level in de-differentiated cells that had been pretreated with retinyl acetate or retinoic acid for three days, but it did increase the cyclic AMP level four days after removal of retinoids. PTH did not stimulate ODC activity or expression of the differentiated phenotype of chondrocytes in the de-differentiated state. On the other hand, DBcAMP or 8-Br cAMP stimulated expression of the differentiated phenotype of chondrocytes even in de-differentiated cells, as judged by morphological and histological changes of the cells and increase in glycosaminoglycan synthesis. Cyclic AMP analogues also induced ODC in these cells.

Detection of teratogens by differentiating embryonic neural crest cells in culture: evaluation as a screening system

We have tested the effects of teratogenic and nonteratogenic compounds on cultures of differentiating embryonic neural crest cells. Compounds were evaluated for their effects on the growth or differentiation of the cells based on morphologic criteria or on quantitative changes in biochemical markers for the differentiated cells. When metabolic activation may be required, compounds were incubated in the cultures in the presence of the postmitochondrial supernatant fraction of rat liver microsomes. Nine of 11 compounds with proven teratogenic effects in vivo induced readily detectable morphologic changes in differentiating cultures. In contrast, none of the nonteratogenic compounds tested had an effect on these cultures. The use of differentiating cell cultures as a screening system for teratogens is discussed.

Chondrogenesis: a model developmental system for measuring teratogenic potential of compounds

A simple test for determining the teratogenic potential of compounds is described using embryonic limb bud cells in culture. These mesenchyme cells multiply and differentiate into chondrocytes during a 6-day culture period. The extent of chondrogenesis is assessed by staining for the cartilage specific proteoglycan with alcian blue. The amount of stain is then measured spectrophotometrically. Compounds which interfere with growth or differentiation reduce the amount of proteoglycan and as a consequence, reduce alcian blue staining. Compounds can be added directly to the media or be activated using several different metabolizing systems. The dose of a compound needed to reduce alcian blue staining by 50% is designated the teratogenic potential (TP50) of that compound. TP50's of proven teratogens compare favorably with in vivo teratogenic doses of the teratogens.

Teratogenesis of retinoic acid in rats: susceptible stages and suppression of retinoic acid-induced limb malformations by cycloheximide

The susceptible stages and the malformation pattern produced by excess retinoic acid were investigated in rat fetuses. Retinoic acid (120 mg/kg body weight, suspended in rape oil) was administered orally to pregnant females on one of the first 20 days of gestation. Fetuses were examined for external and, after visualization of the skeleton with alizarin red, for skeletal malformations on the 21st day of gestation. Retinoic acid was highly embryolethal when administered on days 9 and 10 of gestation (96.2 and 100% resorptions). The earliest teratogenic effect of retinoic acid was noted on the 9th day of gestation. Severe multiple defects were produced by retinoic acid administration on days 9 and 11 of gestation, but more specific malformations involving the axial skeleton, the fore- and hindlimbs, and cleft palate resulted from treatment on days 12-18 of gestation. Cycloheximide, and inhibitor of protein synthesis, reduced the incidence of limb defects induced by retinoic acid. This result indicates that the teratogenic effect of retinoic acid on limb morphogenesis may be dependent upon continuous protein synthesis and is compatible with the view that vitamin A may act like a hormone.

Enhancement of melanotic expression in cultured mouse melanoma cells by retinoids

Retinoic acid (RA), which reduces the rate of cell proliferation in S91 mouse melanoma clone C2 cells, was found to stimulate the expression of their melanotic phenotype. RA treatment also induced the extension of long cellular processes. The RA effects on melanogenesis included stimulation of tyrosinase activity and augmentation of cellular melanin content to levels 3- to 4-fold higher than in untreated cultures at similar cell densities. These effects became apparent after 48 hours of exposure to 10(-5) M RA and increased thereafter. Half-maximal stimulation in cells treated for 6 days occurred at 5 X 10(-7) M RA. Although the degrees of melanogenesis enhancement by RA (10(-5) M) and by alpha-melanocyte stimulatory hormone (2 X 10(-7) M) were similar, the former did not alter the intracellular cAMP level, whereas the latter induced a transient 4-fold increase. In high-passage (p28) cells, as well as in low-passage cells (less than p10) treated with tyrosinase inhibitor phenylthiocarbamate, melanin synthesis was suppressed in the absence and presence of RA, yet the ability of RA to inhibit cell proliferation was not compromised. In the presence of the tumor promotor phorbol myristate acetate (greater than 5 X 10(-9) M) melanin synthesis in control as well as in cells exposed to RA was dramatically inhibited. Phorbol which is not active in tumor promotion had no effect on melanogenesis. In addition to RA, other retinoids, such as 13-cis-retinoic acid, retinyl acetate, the TMMP analog of RA and the phenyl analog of RA, but not the pyridyl analog of RA or retinyl palmitate, also inhibited cell growth and enhanced melanin synthesis.

Change in the expression of collagen genes in dividing and nondividing chondrocytes

Chondrocytes were isolated from the sterna of 17-day-old chick embryos by enzyme digestion. Rapid proliferation of chondrocytes was achieved in the presence of chick serum (10%, v/v). Addition of either hydroxyurea (10(-3) M) or cytosine arabinoside (10 microgram/ml) to the culture medium was used to arrest growth of cells. Under both conditions, however, a rather fast switch of synthesis from type II to type I collagen was observed. This suggests that the loss of the differentiated state of chondrocytes in culture is not necessarily bound to mitosis.

Estimation of creatine phosphokinase and hydroxyproline in the developing limb: its use in evaluating the effect of teratogens on myogenesis and chondrogenesis

Forelimbs of mouse fetuses were examined for tissue-specific, drug-induced alterations in their biochemical composition. The activity of the enzyme creatine phospholinase (CPK; to estimate myogenesis) and the content of hydroxyproline (HP; to estimate chondrogenesis) were compared in homogenates of control and treated mouse-fetus forelimbs on day 14 of gestation. In addition, the content of DNA, RNA, and protein was also measured. Injection of 6-aminonicotinamide (6-AN) (15 mg/kg) on day 10 resulted in an overall growth retardation in day 14 fetuses and all biochemical parameters tested were reduced. The ratio of PH:CPK was unaffected by 6-AN treatment. Retinoic acid (vitamin A acid; 100 mg/kg), administered to pregnant female mice on day 10, produced severe forelimb defects and resulted in a signific 10 resulted in an overall growth retardation in day 14 fetuses and all biochemical parameters tested were reduced. The ratio of PH:CPK was unaffected by 6-AN treatment. Retinoic acid (vitamin A acid; 100 mg/kg), administered to pregnant female mice on day 10, produced severe forelimb defects and resulted in a signific 10 resulted in an overall growth retardation in day 14 fetuses and all biochemical parameters tested were reduced. The ratio of PH:CPK was unaffected by 6-AN treatment. Retinoic acid (vitamin A acid; 100 mg/kg), administered to pregnant female mice on day 10, produced severe forelimb defects and resulted in a significant reduction in day 14 forelimb HP and RNA content, without altering CPK, DNA, or protein; thus, the HP:CPK ratio was decreased. These results indicated that 1) 6-AN nonspecifically retards growth and cyto-differentiation in limbs; 2) retinoic acid inhibits synthesis of collagen and RNA; 3) retinoic acid has a differential effect upon chondrogenic and myogenic tissues of the limb, and 4) the comparison of HP content and CPK activity in tissue homogenates is an acceptable method of evaluating teratogenic compounds for selective effects.