Effect of 3-acetylpyridine on tissue differentiation of the embryonic chick limb

Test of six chemicals for embryotoxicity using fetal mouse salivary glands in culture

Many new chemicals come into use each year, and the need for rapid and cost-effective methods for testing developmental toxicity is apparent. Establishing reliable in vitro techniques is important to a tier approach to testing for developmental toxicity. The fetal mouse salivary gland was selected as a possible test system because several interacting developmental processes occur in gland growth, and development is quantifiable by counting lobes. For each chemical tested, 20 glands from 13-day embryos were treated in a control media and in three concentrations of the test chemicals. The number of lobes present after 48 hours is dependent on the number of lobes at explantation. Glands with different numbers of lobes at explantation were compared by dividing the number of lobes present after 48 hours by the number present at explantation to determine a growth ratio. Mean growth ratios were used to construct dose-response curves, and from these curves the concentration that reduced growth by 50% (TP50) was determined. Comparisons with in vivo data were made by calculating three ratios; the TP50 was divided into the lowest teratogenic dose, the lowest maternal toxic dose, and the dose that was lethal to 50%. Four in vivo teratogens, 6-aminonicotinamide, cytochalasin B, hydroxyurea, and 3-acetylpyridine, all had ratios much higher than 1, indicating a very sensitive response by the glands. One in vivo teratogen, dexamethasone, had much lower ratios, indicating less sensitivity. Acetaminophen, a nonteratogen in vivo, actually stimulated growth of the glands at 10(-5) M and had very low ratios indicating a minimal response by the glands.

The relationship of nicotinamide adenine dinucleotide to the chondrogenic differentiation of limb mesenchymal cells

The nicotinamide adenine dinucleotide (NAD) content of mesenchymal cells from the embryonic chick limb has been hypothesized to control the differentiation of these cells by modulation of ADP-ribosylations. To test this hypothesis, [35S]sulfate incorporation into proteoglycans was monitored as an estimate of the chondrogenic expression of cultured limb mesenchymal cells treated with nicotinamide and nicotinic acid to elevate cellular NAD levels or with nicotinamide and benzamide compounds to inhibit ADP-ribosylations. The results of this study indicated that serum component(s) modulate the interactions between these chemical agents and limb mesenchymal cells and, thus, complicate the interpretations of experiments performed in the presence of serum. With a chemically defined medium that promotes limb mesenchymal cell proliferation and differentiation in vitro, it was demonstrated that: (1) no clear correlation exists between cellular NAD content and the chondrogenic expression of cultured limb mesenchymal cells, (2) nicotinamide and benzamide compounds reduce cell proliferation and, at the higher doses tested, considerably reduce chondrogenesis in limb mesenchymal cell cultures, and (3) limb mesenchymal cells exhibit an enhanced susceptibility to benzamide compounds at a time very early in the culture period which temporally coincides with a transient increase in cellular ADP-ribosylation activity and initial chondrogenic differentiation. These results suggest that NAD does not control the differentiation of limb mesenchymal cells and that ADP-ribosylations are an integral, though not controlling, component of limb mesenchyme cytodifferentiation. A model is presented which proposes a role for ADP-ribosylations during the differentiation of limb mesenchymal cells.

ADP-ribosyl transferase, rearrangement of DNA, and cell differentiation

Cell differentiation is the process by which genetic information is selectively expressed to produce cells with various morphologies and functions. The integrated changes necessary for this fundamentally important process have recently been the subject of intense study. This review will summarize data from several laboratories correlating differentiation with the activity of the enzyme ADP-ribosyl transferase and with changes in single-strand DNA breaks in various diverse eukaryotic systems. We will then discuss the implications of these observations for differentiation in general, including the possibility that rearrangement of genetic material is a widespread mechanism for controlling gene expression.

Histological study of muscular hypoplasia in the crooked neck dwarf mutant (cn/cn) chick embryo

Tibiotarsal segments of 12-day chick embryos homozygous for the crooked neck dwarf gene (cn/cn) were examined histologically following routine methods of preparation. The myogenic mass fails to divide into separate muscle bundles during the early stages of differentiation. Myoblasts and myotubes are observed, although the proportion favors the mononucleate cell population. Multinucleate myotubes are often wavy in appearance and many contain eosinophilic cytoplasmic inclusions. The entire tissue mass of mutants appears more compacted than in control limbs. Poor organization of muscle appears related to the lack of a suitable connective tissue system. Epimysia, perimysia, and subcutaneous connective tissue fail to develop properly. Tendons are poorly developed or absent. Comparisons between mutant and control embryos show no differences in peripheral innervation. Nerve fascicles penetrate deeply into the developing muscle of both species. The distribution of vascular elements is seemingly normal also. Skeletal muscle of cn/cn embryos is capable of differentiating to the myotube stage, after which it undergoes cellular degeneration without achieving a functional state. Comparisons of this mutant with alleged chemical phenocopies show important differences.