Embryonic diapause, a condition of temporary suspension of development of the mammalian embryo, occurs due to suppression of cell proliferation at the blastocyst stage It is an evolutionary strategy to ensure the survival of neonates Obligate diapause occurs in every gestation of some species, while facultative diapause ensues in others, associated with metabolic stress, usually lactation The onset, maintenance and escape from diapause are regulated by cascades of environmental, hypophyseal, ovarian and uterine mechanisms that vary among species and between the obligate and facultative condition In the best-known models, the rodents, the uterine environment maintains the embryo in diapause, while estrogens, in combination with growth factors, reinitiate development Mitotic arrest in the mammalian embryo occurs at the G0 or G1 phase of the cell cycle, and may be due to expression of a specific cell cycle inhibitor Regulation of proliferation in non- mammalian models of diapause provide clues to orthologous genes whose expression may regulate the reprise of proliferation in the mammalian context

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Embryonic diapause and its regulation.

Embryonic diapause occurs in many species of vertebrates, but the physiological mechanisms which control this fascinating process are exceedingly different in the diverse groups which employ this reproductive strategy. In nonmammalian species and some bats, reduction in rate of embryonic development is temperature dependent, but this is not the case in most mammals. Development becomes arrested at the blastocyst stage of embryogenesis in mammals which exhibit delayed implantation, whereas postimplantation development is continuous but retarded in species exhibiting delayed development. The hormonal control of diapause is remarkably different in the various species. Pituitary secretion of prolactin prevents implantation in the tammar wallaby but hastens renewed development and implantation in the mink and spotted skunk. Ovariectomy results in the eventual death of blastocysts in mustelids but induces renewed development and implantation in the armadillo. Luteal function, as evidenced by elevated progesterone secretion, is essentially constant in the roe deer and armadillo, whereas the luteal cells fail to complete their differentiation and secrete low levels of progesterone in carnivores. Progesterone will induce implantation in the tammar wallaby, but estrogen is required to induce renewed development and implantation 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 throughout the period of diapause. In many species the uterus undergoes histological changes and secretes increased amounts of protein, yet we still do not understand the role, if any, these proteins play in initiating renewed embryonic development. Thus the phenomenon of embryonic diapause still holds many mysteries for scientists to solve. © 1993 Wiley-Liss, Inc.

Embryonic diapause in vertebrates

Marked impacts of climate change on biodiversity have frequently been demonstrated, including temperature-related shifts in phenology and life-history traits. One potential major impact of climate change is the modification of synchronization between the phenology of different trophic levels. High phenotypic plasticity in laying date has allowed many bird species to track the increasingly early springs resulting from recent environmental change, but although changes in the timing of reproduction have been well studied in birds, these questions have only recently been addressed in mammals. To track peak resource availability, large herbivores like roe deer, with a widespread distribution across Europe, should also modify their life-history schedule in response to changes in vegetation phenology over time. In this study, we analysed the influence of climate change on the timing of roe deer births and the consequences for population demography and individual fitness. Our study provides a rare quantification of the demographic costs associated with the failure of a species to modify its phenology in response to a changing world. Given these fitness costs, the lack of response of roe deer birth dates to match the increasingly earlier onset of spring is in stark contrast with the marked phenotypic responses to climate change reported in many other mammals. We suggest that the lack of phenotypic plasticity in birth timing in roe deer is linked to its inability to track environmental cues of variation in resource availability for the timing of parturition.

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Mismatch between birth date and vegetation phenology slows the demography of roe deer.

Conceptus attachment in the ewe: an ultrastructural study.

Background A better understanding of endometrial receptivity is crucial to the creation and optimization of tests to assess the window of implantation in a clinical setting. Testing endometrial receptivity via scanning electron microscopy of endometrial samples reveals that pinopodes are a very good marker of endometrial receptivity in the rat. There is still disagreement in the literature as to their usefulness as a receptivity marker in both mice and humans. Methods Publications related to the discovery, study and usefulness of pinopodes as a marker of endometrial preparation for implantation in both rodents and humans were identified through MEDLINE and other bibliographic databases. Results There is substantial evidence that pinopodes are good markers of endometrial receptivity in the rat. Pinopodes are not useful in the mouse or human as consistent markers of endometrial receptivity for implantation. In the human, pinopodes have a prolonged (>5 days) presence in the luteal phase and fail to delineate the brief (24-48 h) window of receptivity. Conclusions While there are many publications arising from one group supporting the use of pinopodes as a reliable marker of human endometrial receptivity, few independent groups have been able to confirm these results. The clinical usefulness of pinopodes to delineate a period of endometrial receptivity seems unlikely following recent findings that pinopodes are present throughout the luteal phase of the menstrual cycle.

Pinopodes: a questionable role in endometrial receptivity

Transmission electron microscopy of the trophoblast cells during diapause revealed an abundance of electron-dense, membrane-bound granular inclusions and a marked lack of cytoplasmic organelles. The cells also possessed a well-developed covering of branched microvilli, numerous caveolae, micropinocytotic vesicles and a lamina of fine fibrillae. The progressive enlargement of the blastocyst during diapause was correlated with a decline in the height of the trophoblast cells and a reduction in the density of microvilli and caveolae associated with the outer membrane. The granular inclusions also declined in number and electron density during the delay phase, suggesting the progressive utilisation of energy reserves. Embryonic elongation was associated with the disappearance of the granular inclusions, a reduction in the amount of fibrillar material and a dramatic increase in the development of cytoplasmic organelles. During diapause, clear vesicles, apparently derived from the Golgi apparatus, gradually accumulated in the supranuclear region of each gland and non-ciliated duct cell. Embryonic elongation was associated with the sudden release of these vesicles into the glandular lumen and thence into the uterine lumen. Numerous apical protrusions were also observed projecting from the luminal and ductal epithelia at this time, suggesting the formation of an apocrine secretion. Another type of secretion was produced during the early stages of placental attachment by the hypertrophied granular endoplasmic reticulum of the ductal epithelium.

R J Aitken

Papers

Ultrastructure of the blastocyst and endometrium of the roe deer (Capreolus capreolus) during delayed implantation.