Advances in Anatomy Embryology and Cell Biology最新文献

In the domestic dog, placentation arises from central implantation, passing through a transitional, yet important stage of choriovitelline placenta (yolk sac placenta), on the way to the formation of the definite, deciduate, zonary (girdle) allantochorionic endotheliochorial placenta.Sharing some similarities with other invasive types of placentation, e.g., by revealing decidualization, it is characterized by restricted (shallow) invasion of trophoblast not affecting maternal capillaries and maternal decidual cells. Thus, being structurally and functionally placed between noninvasive epitheliochorial placentation and the more invasive hemochorial type, it presents an interesting and important model for understanding the evolutionarily determined aspects of mammalian placentation. More profound insights into the biological mechanisms underlying the restricted invasion of the fetal trophoblast into maternal uterine structures and the role of decidual cells in that process could provide better understanding of some adverse conditions occurring in humans, like preeclampsia or placenta accreta. As an important endocrine organ actively responding to ovarian steroids and producing its own hormones, e.g., serving as the source of gestational relaxin or prepartum prostaglandins, the canine placenta has become an attractive research target, both in basic and clinical research. In particular, the placental feto-maternal communication between maternal stroma-derived decidual cells and fetal trophoblast cells (i.e., an interplay between placenta materna and placenta fetalis) during the maintenance and termination of canine pregnancy serves as an interesting model for induction of parturition in mammals and is an attractive subject for translational and comparative research. Here, an updated view on morpho-functional aspects associated with canine placentation is presented.

Establishment of viviparity in mammals evolved through not only the long-term retainment of the fetus within the maternal uterus but differentiation and expansion of cell layers to form functional membranes to exchange O₂/CO₂ and nutrients between the placenta and maternal circulations. Development of a fetal placental vascular circulation to interact with the maternal uterus is critical to the survival of all species. However, the fascination with the mammalian placenta is the robust variation in types, form, attachment, invasiveness, structure, cell differentiation, endocrine function, and regulation of the maternal immune system. Despite the obvious role of the placenta to support fetal development, mammals have evolved multiple strategies to give live birth at term. The placenta and the maternal-fetal interface during pregnancy can be quite simple to very complex. Professor E.C. Amoroso contributed greatly to the study of comparative placentation in animals. His paper "Placentation" in Marshall's Physiology of Reproduction published in 1952 remains the standard for comparative placental anatomy today. The present volume on "Mammalian Placentation" brings together current reviews for leading experts to diversity of placentation in a number of mammalian species. Chapters will discuss viviparity, blastocyst formation, and placentation in the cow, pig, horse, mouse, dog, primate, human, elephant, and marsupials.

The female elephant shows a 3-week "follicular phase" to commence her 16-week estrous cycle at the end of which a second surge in pituitary luteinizing hormone (LH) release matures and ovulates an ovarian follicle in association with estrous behavior and mating, whereas the first LH surge at the start of the follicular phase causes luteinization of 3-5 partially developed follicles. The prolonged pregnancy of 22 months is supported by a zonary endotheliochorial placenta which secretes placental lactogen (ePL) from around 40 days of gestation in association with replacement of the lumenal epithelium of the endometrium by trophoblast and the development of large corpora lutea (CLs) in the maternal ovaries from the previously formed luteinized follicles in response to the first LH peak early in the follicular phase. The zonary placenta develops above, rather than within, the endometrium. The elephant placenta secretes neither estrogens nor progestagens throughout gestation, as pregnancy maintenance relies on 5α-dihyroprogesterone and other 5α reduced progestagens secreted by secondary CLs stimulated by ePL and the stromal tissue of the fetal gonads, which become extremely enlarged during the second half of the 22-month pregnancy. In female fetuses, this ovarian enlargement includes the development and subsequent regression of multiple primary and secondary follicles with a consequent substantial decline in primary follicle numbers at birth. During the next 8-9 years of pre-pubertal life, however, oocyte and primary follicle numbers recover to levels near those found in late gestation, which may be evidence of postnatal oogenesis occurring in the elephant.

Placentation in humans is precocious and highly invasive compared to other mammals. Implantation is interstitial, with the conceptus becoming completely embedded within the endometrium towards the end of the second week post-fertilization. Villi initially form over the entire surface of the chorionic sac, stimulated by histotrophic secretions from the endometrial glands. The secondary yolk sac never makes contact with the chorion, and a choriovitelline placenta is never established. However, recent morphological and transcriptomic analyses suggest that the yolk sac plays an important role in the uptake of nutrients from the coelomic fluid. Measurements performed in vivo demonstrate that early development takes place in a physiological, low-oxygen environment that protects against teratogenic free radicals and maintains stem cells in a multipotent state. The maternal arterial circulation to the placenta is only fully established around 10-12 weeks of gestation. By then, villi have regressed over the superficial, abembryonic pole, leaving the definitive discoid placenta, which is of the villous, hemochorial type. Remodeling of the maternal spiral arteries is essential to ensure a high-volume but low-velocity inflow into the mature placenta. Extravillous trophoblast cells migrate from anchoring villi and surround the arteries. Their interactions with maternal immune cells release cytokines and proteases that are key to remodeling, and a successful pregnancy.

The preimplantation mammalian embryo is a simplistic, self-contained, and a superior model for investigating the inherent complexities of cell fate decision mechanisms. All mammals begin their humble journey from a single-cell fertilized zygote contained within a proteinaceous coat called the zona pellucida. The zygote embarks on a series of well-orchestrated events, beginning with the activation of embryonic genome, transition from meiotic to mitotic divisions, spatial organization of the cells, timely differentiation into committed trophectoderm (TE) and primitive endoderm (PrE), and ultimately escape from zona pellucida for implantation into the uterus. The entire development of preimplantation embryo can be studied in vitro using a minimalistic and defined culture system. The ease of culture along with the ability to manipulate gene expression and image the embryos makes them an ideal model system for investigation into the first two of several cell fate decisions made by the embryo that result in a pluripotent epiblast (EPI) and differentiated TE and PrE lineages. This chapter reviews our latest knowledge of preimplantation embryo development, setting the stage for understanding placental development in subsequent chapters in this Book.

Placenta forms as a momentary organ inside the uterus with a slew of activities only when the woman is pregnant. It is a discoid-shaped hybrid structure consisting of maternal and embryonic components. It develops in the mesometrial side of the uterus following blastocyst implantation to keep the two genetically different entities, the mother and embryo, separated but connected. The beginning and progression of placental formation and development following blastocyst implantation coincides with the chronological developmental stages of the embryo. It gradually acquires the ability to perform the vascular, respiratory, hepatic, renal, endocrine, gastrointestinal, immune, and physical barrier functions synchronously that are vital for fetal development, growth, and safety inside the maternal environment. The uterus ejects the placenta when its embryonic growth and survival supportive roles are finished; that is usually the birth of the baby. Despite its irreplaceable role in fetal development and survival over the post-implantation progression of pregnancy, it still remains unclear how it forms, matures, performs all of its activities, and starts to fail functioning. Thus, a detailed understanding about normal developmental, structural, and functional aspects of the placenta may lead to avoid pregnancy problems that arise with the placenta.

In comparison to many other mammalian species, ruminant ungulates have a unique form of placentation. Ruminants initially display an epitheliochorial type of placentation; however, during the period of placental attachment, trophoblast giant binucleate cells (BNC) develop within the chorion to migrate and fuse with the uterine surface epithelium to form syncytial plaques. Binucleate cell migration and fusion continues throughout pregnancy but never appears to breach the basal lamina, beneath the uterine surface or luminal epithelium. Therefore, the semi-invasive type of placentation in ruminants is classified as synepitheliochorial. The endometrium of ruminant species also contains unique specialized aglandular structures termed "caruncles" in which the chorioallantois (cotyledons) interdigitates and forms highly vascularized fetal-maternal "placentomes." This chapter will discuss the current knowledge of early conceptus development during the peri-attachment period, establishment of pregnancy, conceptus attachment, and placentation in ruminant ungulates. The features of placentomes, BNCs, fetomaternal hybrid cells, and multinucleated syncytial plaques of the cotyledonary placenta of ruminant species will be reviewed to highlight the unique form of placentation compared to the placentae of other artiodactyls.

Endometriosis, the presence and growth of uterine endometrial glandular epithelial and stroma cells outside the uterine cavity, causes pain and infertility in women and girls of reproductive age. As randomized, double-blinded, controlled studies of endometriosis in women are impractical and at times ethically prohibitive, animal models for endometriosis arose as an important adjunct to gain mechanistic insights into the etiology and pathophysiological mechanisms of this perplexing disorder. A more thorough understanding of endometriosis in women may help develop novel noninvasive diagnostics, classification systems, therapeutic regimes, and even preventative methods for the management of endometriosis. This chapter is intended to introduce a brief historical background, biological and epidemiological aspects, the major symptoms, the effects of endocrine-disrupting chemicals, and an example of an epigenetic factor of endometriosis in women.