Placenta and fetal membranes of the four-toed elephant shrew, Petrodromus tetradactylus

Endogenous Retroviruses and Placental Evolution, Development, and Diversity

The main roles of placentas include physical protection, nutrient and oxygen import, export of gasses and fetal waste products, and endocrinological regulation. In addition to physical protection of the fetus, the placentas must provide immune protection throughout gestation. These basic functions are well-conserved; however, placentas are undoubtedly recent evolving organs with structural and cellular diversities. These differences have been explained for the last two decades through co-opting genes and gene control elements derived from transposable elements, including endogenous retroviruses (ERVs). However, the differences in placental structures have not been explained or characterized. This manuscript addresses the sorting of ERVs and their integration into the mammalian genomes and provides new ways to explain why placental structures have diverged.

Characteristics of the endometrium in menstruating species: lessons learned from the animal kingdom†

Here we have summarized what is currently known about menstruating animal species with special emphasis on non-primate species: length of their menstrual cycle, ovulation, implantation, placentation, decidualization, and endometrial characteristics. Having an overview of all the possible animal models that can be used to study menstruation and the menstrual cycle could be useful to select the one that better matches the needs of the individual research projects. The most promising species to study menstruation seems to be the spiny mouse Acomys cahirinus. It is a rodent that could be easily held in the existing laboratory facilities for rats and mice but with the great advantage of having spontaneous menstruation and several human-like menstrual cycle characteristics. Among the species of menstruating bats, the black mastiff bat Molossus ater and wild fulvous fruit bat Rousettus leschenaultii are the ones presenting the most human-like characteristics. The elephant shrew seems to be the less suitable species among the ones analyzed. The induced mouse model of menstruation is also presented as an adaptable alternative to study menstruation.

A missing piece: the spiny mouse and the puzzle of menstruating species

We recently discovered the first known menstruating rodent. With the exception of four bats and the elephant shrew, the common spiny mouse (Acomys cahirinus) is the only species outside the primate order to exhibit menses. There are few widely accepted theories on why menstruation developed as the preferred reproductive strategy of these select mammals, all of which reference the evolution of spontaneous decidualisation prior to menstrual shedding. Though menstruating species share several reproductive traits, there has been no identifiable feature unique to menstruating species. Such a feature might suggest why spontaneous decidualisation, and thus menstruation, evolved in these species. We propose that a ≥3-fold increase in progesterone during the luteal phase of the reproductive cycle is a unique characteristic linking menstruating species. We discuss spontaneous decidualisation as a consequence of high progesterone, and the potential role of prolactin in screening for defective embryos in these species to aid in minimising implantation of abnormal embryos. We further explore the possible impact of nutrition in selecting species to undergo spontaneous decidualisation and subsequent menstruation. We summarise the current knowledge of menstruation, discuss current pre-clinical models of menstruation and how the spiny mouse may benefit advancing our understanding of this rare biological phenomenon.

Classics revisited: C. J. van der Horst on pregnancy and menstruation in elephant shrews

BACKGROUND

Menstruation occurs only in higher primates, some bats, the spiny mouse and the elephant shrew. Our knowledge of the latter species is due to work by C. J. van der Horst.

FINDINGS

Changes in the uterine stroma are initially similar in fertile and infertile cycles and are confined to a small area. In pregnant animals, the presence of the conceptus causes further development to an implantation chamber. In infertile cycles an outgrowth of highly glandular stroma (a polyp) appears. With decline of the corpora lutea it is shed in a process equivalent to menstruation. Van der Horst described the further development of the placenta and a decidua pseudocapsularis in pregnant animals. In addition he built a unique collection that has thrown light on embryonic development and placentation in other South African mammals.

CONCLUSIONS

The changes in endometrial stromal cells during the menstrual cycle appear similar between primates and the elephant shrew and deserve to be studied at the molecular level.

RNA-seq reveals conservation of function among the yolk sacs of human, mouse, and chicken

The yolk sac is phylogenetically the oldest of the extraembryonic membranes. The human embryo retains a yolk sac, which goes through primary and secondary phases of development, but its importance is controversial. Although it is known to synthesize proteins, its transport functions are widely considered vestigial. Here, we report RNA-sequencing (RNA-seq) data for the human and murine yolk sacs and compare those data with data for the chicken. We also relate the human RNA-seq data to proteomic data for the coelomic fluid bathing the yolk sac. Conservation of transcriptomes across the species indicates that the human secondary yolk sac likely performs key functions early in development, particularly uptake and processing of macro- and micronutrients, many of which are found in coelomic fluid. More generally, our findings shed light on evolutionary mechanisms that give rise to complex structures such as the placenta. We identify genetic modules that are conserved across mammals and birds, suggesting these modules are part of the core amniote genetic repertoire and are the building blocks for both oviparous and viviparous reproductive modes. We propose that although a choriovitelline placenta is never established physically in the human, the placental villi, the exocoelomic cavity, and the secondary yolk sac function together as a physiological equivalent.

Phylogenetic evidence for early hemochorial placentation in eutheria

The eutherian placenta is remarkable for its structural and functional variability. In order to construct and test comparative hypotheses relating ecological, behavioral and physiological traits to placental characteristics it is first necessary to reconstruct the historical course of placental evolution. Previous attempts to do so have yielded inconsistent results, particularly with respect to the early evolution of structural relationships between fetal and maternal circulatory systems. Here, we bring a battery of phylogenetic methods - including parsimony, likelihood and Bayesian approaches - to bear on the question of placental evolution. All of these approaches are consistent in indicating that highly invasive hemochorial placentation, as found in human beings and numerous other taxa, was an early evolutionary innovation present in the most ancient ancestors of the living placental mammals.

Uterine natural killer cells: a specialized differentiation regulated by ovarian hormones

In adult females of many species, a transient population of natural killer (NK) cells appears in cycles within the uterine endometrium (lining). Appearance of these lymphocytes coincides with specific phases of the ovarian hormone cycle and/or early pregnancy. Studies in rodents, women, and pigs dominate the literature and suggest the uterine (u)NK cells are an activated subset sharing many but not all features with circulating or lymphoid organ-residing NK cells. During successful murine pregnancy, uNK cells appear to regulate initiation of structural changes in the feed arterial systems that support maternal endometrial tissue at sites of implantation and subsequent placental development. These changes, which reverse after pregnancy, create a higher volume arterial bed with flaccid vessels unresponsive to vasoactive compounds. These unique pregnancy-associated arterial changes elevate the volume of low-pressure, nutrient-rich, maternal arterial blood available to conceptuses. Regulation of the differentiation, activation, and functions of uNK cells is only partially known, and there is lively debate regarding whether and how uNK cells participate in infertility or spontaneous abortion. This review highlights the biology of uNK cells during successful pregnancy.

The Fetal Membranes of the Otter Shrews and a Synapomorphy for Afrotheria

The otter shrews of mainland Africa are the closest relatives of the Madagascar tenrecs. We sought for similarities in placentation between the two groups and, in a wider context, with other mammals of the Afrotheria clade. Specimens of the Nimba otter shrew (Micropotamogale lamottei) were obtained from the Ivory Coast and examples of the giant otter shrew (Potamogale velox) from the Hill Collection. The Nimba otter shrew has a central haemophagous organ similar to that in tenrecs. The labyrinth of the Nimba otter shrew, however, is endotheliochorial with syncytial trophoblast enclosing the maternal vessels. On the other hand tenrecs have cellular haemomonochorial placentae and an associated spongy zone, which is not present in the Nimba otter shrew. The placenta of the giant otter shrew is also endotheliochorial. The central region of its placenta is particularly interesting, since the juxtafetal portion is clearly a haemophagous region whereas the labyrinth feeding this region is endotheliochorial. Thus there is considerable variation in placental morphology within Tenrecidae. Importantly, however, both otter shrews have a large allantoic sac divided into four intercommunicating lobes by two pairs of septal folds. A similar arrangement has been described for representatives of each of the remaining five orders within Afrotheria. This is significant because previous anatomical studies have failed to establish a single synapomorphy in support of Afrotheria.

Development of the haemophagous region and labyrinth of the placenta of the tenrec, Echinops telfairi

Our purpose was to determine how the central haemophagous region and cellular haemomonochorial labyrinth of the tenrec placenta are formed. The haemophagous region is preceded by a region of invasion of the endometrium by trophoblast comprising a cytotrophoblast layer covered by syncytial trophoblast and contiguous with numerous masses of multinucleate trophoblast. The trophoblast intrudes into the endometrium, eliminating the stroma, although small vessels and clumps of glandular epithelium persist. This extensive central region is connected to the forming disk by a ring of chorioallantois covered by a single layer of columnar trophoblast. Later the multinucleate masses and syncytial trophoblast degenerate. The unilaminar cytotrophoblast remains, is elaborated into folds, and phagocytoses glandular secretion, cell debris and erythrocytes. As the central area is transforming, fetal capillaries move into the cytotrophoblast pads surrounding the central zone. Prior to this, the cytotrophoblast has formed a multilayered structure and interrupted maternal vessels to create an anastomotic network of blood spaces lined by cytotrophoblast. The invasion of fetal capillaries transforms this preplacental pad into a cellular haemomonochorial labyrinth with the uninvaded portion forming an underlying spongy zone. Thus interaction of the trophoblast with the endometrium is substantially different in the central zone compared to the area of the preplacental pad.