The stimulation of DNA synthesis by cytochalasin B in proliferative epidermal and dermal cells

Cytochalasin B influences a variety of cellular events that are associated with the contractile microfilament system and the formation of binucleate cells. Along with the formation of binucleate cells, cytochalasin B also causes an acceleration of cells from G1 to S in the cell cycle. By pulsing the cytochalasin B for 30 minutes and allowing for a previously established lag time (17.5 hours) a stimulation of thymidine incorporation into DNA of proliferative epidermal and dermal cells was found in both control and stripped epidermis. Autoradiographic analysis confirmed that the stimulation was due to an increased number of basal cells accelerated from G1 to S phase. A minimal number of binucleate basal cells, 1 in 300, was observed, which suggests that the stimulated synthesis is independent of binucleate cell formation. The amount of stimulation is maximum with cytochalasin B concentration pulse between 5gamma and 30gamma/ml. The results suggest a possible link in coupling cell membrane and surface events with subsequent increased cell nuclei synthetic activity.     

The cyclized C-terminal dipeptide of arginine vasopressin: Metabolic stability and antagonism of puromycin-induced amnesia

The cyclized derivative of the C-terminal dipeptide of arginine vasopressin (AVP), cyclo(Arg-Gly), can, when administered peripherally, block puromycininduced amnesia in mice. The potency and time course of action of this peptide are very similar to that of the parent hormone, AVP. Radioactively labelled cyclo(Arg-Gly), injected sc, was found to be degraded mainly by kidney and liver, and both intact peptide and degradation products reached the brain. However, cyclo(Arg-Gly) injected directly into brain was stable for up to 24 hr. The disappearance of cyclo(Arg-Gly) from plasma and brain was biphasic, with halflives longer than that reported for AVP. The results suggest that cyclo(Arg-Gly), formed in vivo from AVP, could contribute to the long-term effects of AVP in the CNS. However, not all of the actions of the hormone can be attributed to formation of longer-acting derivatives, and it is postulated that separate receptors may exist in brain for the neurohypophyseal hormones and their active fragments.     

Retinoic acid enhances and depresses in vitro development of cartilaginous bone anlagen in embryonic mouse limbs

Summary Forelimbs of Day 11 and Day 12 embryonic mice were excised and cultured for 3 d in the presence of either 0.25 μg (8×10⁻⁷ M), 0.5 μg(1.7×10⁻⁶ M), or 1.0 μg (3.3×10⁻⁶ M) of all-rans retinoic acid (RA) per milliliter of culture medium. Cultured limbs were fixed, stained, and mounted whole on glass slides and evaluated with computerized optical image analysis for RA-induced effects on the area and shape of the total limb and individual bone anlagen. Relative effects of RA on total bone, soft tissue, long bone, and paw regions were also examined. With Day 11 forelimbs total bone area was increased by 10.5% by the low dose of RA. The increase was mostly in long bones and at the expense of soft tissue. Total bone area was increased 9.3% with Day 12 forelimbs. This increase was primarily in the paw. The high dose of RA decreased Day 11 forelimb area, primarily affecting long bones. Day 12 forelimbs were not significantly affected by the high dose of RA. Effects of the imtermediate dose were primarily limited to reduction in soft tissue area. Long bone:paw and soft tissue: bone ratios reflected these effects. The high dose produced a consistent rounding or shortening of Day 11 forelimb bones. On Day 12 0.5 μg/ml RA produced an inconsistent pattern of rounding of bone anlagen. Treatment with the high dose on Day 12 produced angular rather than rounded contours in many cases, as indicated by shape factor values closer to zero than obtained with controls. These data show that direct exposure to RA can affect both the size and shape of bone anlagen of the developing limb; the low dose enhances and the high dose depresses development. The results support previous studies which suggest that RA may play a critical role in the control of cell activities such as cell migration, proliferation, and cytodifferentiation in the development of the cartilaginous bone anlagen.     

Localization of the human fibronectin (FN) gene on chromosome 8 by a specific enzyme immunoassay

The fibronectin produced by clonal murine-human hybrid cell lines containing various complements of human chromosomes was measured. Human and murine fibronectins were assayed by specific immunoassay, and the production of human fibronectin was correlated with karyology and isozyme markers for specific human chromosomes. The data show a 100% concordance between the expression of human fibronectin and glutathione reductase, a marker for human chromosome 8, indicating that chromosome 8 codes for the fibronectin polypeptide.     

The evolution of large‐scale body size clines in Plethodon salamanders: evidence of heat‐balance or species‐specific artifact?

A major goal in macroecology is to determine how body size varies geographically, and explain why such patterns exist. Recently, a grid‐cell assemblage analysis found significant body size trends with latitude and temperature in Plethodon salamanders, and support for the heat‐balance hypothesis as a possible explanation for these trends. Here we demonstrate that the heat‐balance hypothesis is unlikely to have generated this pattern, and that there is no overall body size trend with temperature in Plethodon. Using data from 3155 local Plethodon assemblages, we find no support for body size clines with latitude, and no relationship between body size and temperature. We also found that body size did not covary with elevation, in contrast to what was predicted by heat‐balance. We then examined the various scenarios under which body size clines across grid‐cell assemblages could evolve via heat‐balance, and found that none were tenable in light of the existing data. Instead, a single, widely distributed species was responsible for the pattern across grid‐cell assemblages. Finally, we examined why phylogenetic eigenvector regression does not account for phylogenetic non‐independence among taxa, and should not be used to account for shared evolutionary history in assembly‐level analyses. Assemblage‐level patterns are a useful means of assessing biogeographic trends, and are an important complement to within‐species and cross‐species patterns. However, while the use of grid‐cell assemblage approaches from digital databases is expedient, their results must be examined critically, and whenever possible, compared with data obtained from local species assemblages (particularly for ecological mechanisms that operate at the level of individuals). Finally, our results emphasize the importance of using corroborative data to evaluate alternative hypotheses, so that potential mechanisms that explain bioegeographic patterns are properly assigned.