EMBRYONIC DIAPAUSE IN VERTEBRATES

Embryonic diapause occurs in many species of vertebrates, but the phys iological mechanisms which control this fascinating process are exceed ingly different in the diverse groups which employ this reproductive s trategy. In nonmammalian species and some bats, reduction in rate of e mbryonic development is temperature dependent, but this is not the cas e in most mammals. Development becomes arrested at the blastocyst stag e of embryogenesis in mammals which exhibit delayed implantation, wher eas postimplantation development is continuous but retarded in species exhibiting delayed development. The hormonal control of diapause is r emarkably different in the various species. Pituitary secretion of pro lactin prevents implantation in the tammar wallaby but hastens renewed development and implantation in the mink and spotted skunk. Ovariecto my results in the eventual death of blastocysts in mustelids but induc es renewed development and implantation in the armadillo. Luteal funct ion, as evidenced by elevated progesterone secretion, is essentially c onstant in the roe deer and armadillo, whereas the luteal cells fail t o complete their differentiation and secrete low levels of progesteron e in carnivores. Progesterone will induce implantation in the tammar w allaby, but estrogen is required to induce renewed development and imp lantation in rodents. Neither progesterone and/or estrogens appear to be capable of stimulating implantation in carnivores. The uterus plays an important role in maintaining the embryos in a viable state throug hout the period of diapause. In many species the uterus undergoes hist ological changes and secretes increased amounts of protein, yet we sti ll do not understand the role, if any, these proteins play in initiati ng renewed embryonic development. Thus the phenomenon of embryonic dia pause still holds many mysteries for scientists to solve. (C) 1993 Wil ey-Liss, Inc.