Amphibians further study

Until now, no experimental results support the hypothesis of a carbohydrate-mediated species-speeifie sperm binding for amphibians, similar to that demonstrated for mammalians. Further studies will probably indicate if the pattern of glycan structures found in each amphibian species represent the support for a specifie recognition of the ovule by the spermatozoon. 

We can point out to the reader areas of research that seem to be particularly worthy of further study. Thus, the mechanisms responsible for kin recognition have been well studied in amphibians, especially anuran amphibians however. 

This suggests that cyclin A2 is not essential for the early embryonic cell cycles. Also D-type cyclins seem to be dispensable for the early mouse embryo cell cycle progression since embryonic stem (ES) cells do not express them at all before differentiation (Savatier et al 1996). We do not know, however, whether the D-type cyclins are also absent in the early embryo. These observations suggest that not only could the first cell cycles of the mouse embryo have specific modifications, but also further embryonic cell cycles are specifically modified as well. Mammalian embryonic cell cycles are probably modified often during development. Such studies could allow us to determine a profile of a minimal cell cycle in mammals which must, however, be much more complex than a simple S M phase embryonic cell cycle of amphibians or insects. 

Virtually all studies on early development of amphibians have used this technique because of its speed and simplicity. One experiment, however, involved the injection of RA suspended in corn oil (32). Stage 10-11 embryos had their fertilization envelope manually removed, and were injected with a 1-nL droplet between the surface and deep ectodermal layers by means of an air pressure injection system. The droplet also contained a fluorescent dye, Dil, so its presence could be traced throughout further development of the embryo. The additional unique feature of this system was that the effects of RA were asymmetric because only one side of the embryo was injected. 

To date, MHCs have been rather clearly defined in at least ten species (Table 3.1). These are all mammalian species, except the chicken. The finding of MHCs in a number of orders of mammals suggests that probably all mammals have such a complex. The occurrence of an apparently comparable system in the chicken and preliminary evidence for similar systems in amphibians suggest that the MHC had its origins at a rather early point in vertebrate evolution. Further phylogenetic studies promise to yield useful information about the evolution of the MHC. 

Earlier, some of the same considerations had led me in 1957 to initiate a study of amphibian venoms of the steroidal bufadienolide (6) type as potential sources of new antineoplastic substances. Eventually we found that some of the toad venom bufadienolides such as marinobufagin significantly inhibit growth of the National Cancer Institute s KB cell line derived from a human nasopharynx carcinoma and lead to a curative response with the murine Ehrlich ascites system (7). However, the therapeutic indices were unattractive for further development. So the effort was extended in 1965-66, as Just noted, to encompass a geographically far-reaching area (Asia, Africa, Australia etc.) and an extensive research program to evaluate marine invertebrates and arthropods for structurally unique and useful anticancer constituents. Subsequently we isolated the first such invertebrate antineoplastic constituents (8-12). Meanwhile, our early expectations have been abundantly realized and the discovery of the bryostatins provides a splendid illustration. 

In one experiment, toads were trained in simple T-mazes to move to the right in response to cedarwood oil and a square and to the left in response to citral and a circle. In another experiment, toads were trained to move toward a specific sequence of odors. In both experiments, toads oriented with respect to olfactory cues and the spatial configuration of the maze but not to visual cues. Thus, this study further supports the view that amphibians, as represented by B. valliceps in this study, possess the olfactory mechanisms necessary to orient with reference to an odoriferous landscape in nature (Grubb 1976). 

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