Current TopicA Review of Implantation and Early Placentation in the Mare
Introduction
The equine embryo does not enter the uterus until 6.0–6.5 days after ovulation [1] and it moves frequently throughout the uterine domain over the next 10 days, driven by peristaltic contractions of the myometrium stimulated by rhythmical releases of prostaglandin F2α (PGF2α) and PGE2 from the embryo [2]. Within only 12 h after entering the uterus the outer palisade layer of trophectoderm cells of the as yet unexpanded blastocyst secrete appreciable quantities of high molecular weight, mucin-like glycoproteins rich in threonine and serine residues [3] which, trapped by the still-persisting zona pellucida, are moulded to form a tough, elastic covering, the blastocyst capsule [4], that encapsulates the embryo for the next 20–25 days.
These two features of early equine embryogenesis, continued movement of the embryo throughout the uterine lumen and the development and persistence of the blastocyst capsule, have at least three, and possibly many more, vital functions. First, the internal turgour pressure exerted by the capsule prevents elongation of the trophoblast and ensures the embryo's maintenance of a spherical configuration to enable its propulsion around the uterus [5]. Second, the capsule provides tensile strength to enable the embryo to withstand the considerable myometrial forces that move it throughout the uterus; day 7 capsule-denuded embryos are unable to survive when transferred back to the uterus [6]. Third, the high concentration of negatively charged sialic acid residues within its constituent glycoproteins [3] may function to both regulate the embryo's intrauterine movement and assist in the accumulation and uptake of endometrial gland exocrine secretions (histotroph or ‘uterine milk’). These secretions, which contain the transport proteins uterocalin [7] and uteroglobulin [8] among other constituents, are the principal source of nutrients for the still unattached and rapidly expanding conceptus during its first 40 days of life.
The intrauterine mobility of the embryo also enables it to either secrete an anti-luteolytic hormone onto the majority of the surface of the endometrium or, more likely but as yet unproven, physically interact with the endometrium in some way [9] to prevent the latter from secreting its cyclical, spike-like, luteolytic pulses of PGF2α [10] and so achieve luteostasis to maintain the pregnancy state.
Embryonic movement ceases abruptly on days 16–17 after ovulation when the increasing diameter of the conceptus physically prevents it from further passage through the narrow uterine lumen [11]. It now becomes “fixed” at the base of one or other of the uterine horns where it is held firmly in place by an increase in myometrial tone [12]. Coincidentally, the vesicle rotates within the tightening uterine lumen, probably in response to a combination of the increased thickness of the endometrium in the mesometrial (dorsal) segment of the uterus and the difference in thickness between the bi- and tri-laminar portions of the embryonic membranes, so that the embryo proper comes to lie in the ventral (antimesometrial) quadrant of the uterine horn [13]. Then, due to the subsequent expansion of the allantois, the yolk sac, and eventually the attachment of the umbilical cord, comes to reside in the dorsal quadrant of the uterus [14]. Having stopped moving, the conceptus now undergoes, during the next 60–70 days, the processes of embryogenesis and organogenesis, differentiation and growth of the embryonic/fetal membranes, endometrial cup formation, microvillous attachment of the trophoblast to the endometrial epithelium to achieve implantation and, finally, increasing interdigitation between fetal and maternal epithelial layers to create the diffuse, microcotyledonary, epitheliochorial equine placenta that provides sustenance for the fetus to term at around 340 days of gestation.
This paper describes the gross and microscopic anatomical changes associated with what may be termed the placentation period of equine pregnancy; namely, between 20 and 90 days after ovulation. Material used includes conceptuses and biopsies of endometrium and placenta recovered opportunistically over many years from pregnant mares undergoing surgical hysterotomy or post mortem and from 11 experimental mares at known stages of early gestation from which the uterus was recovered intact at post mortem and perfused-fixed with the conceptus in situ. In all these mares ovulation had been determined accurately by daily or alternate day palpation and/or ultrasound scanning of the mare's ovaries combined with daily measurements of peripheral serum progesterone concentrations. Day 0 of pregnancy was taken as the day on which ovulation was found to have occurred.
Section snippets
Days 20–30 of gestation
At day 20 after ovulation the conceptus is still spherical and the embryo proper, with its primitive beating heart, is discernible in the ventral (antimesometrial) quadrant of the conceptus. The vitelline artery, highlighting the leading edge of the vascularised mesoderm which is developing between the outermost chorion and inner yolk sac membranes, is now situated about one third of the way up the conceptus (Fig. 1a). The allantois is just appearing as an outpouching of the embryonic hindgut.
The endometrial cup reaction
Around day 35 the apical or maternal surface of at least the central region of the chorionic girdle, due to continued growth of the folds of specialised trophoblast cells and the downward compression of uterine tonicity, now constitutes a solid wall of elongated columnar trophoblast cells which appears to be conjoined with the lumenal epithelium of the endometrium by the alcian blue-positive exocrine material secreted by the girdle cells (Figs. 5d and 6a) [20]. Then, between days 35 and 37, the
Days 40–60
As myometrial tone declines from day 40 onwards the conceptus begins to lose its spherical shape (Fig. 7a). Continuing expansion of the allantois forces the allantochorion further up the gravid uterine horn in one direction, and towards the uterine body in the other. The now vestigial yolk sac is completely surrounded by allantois and it becomes incorporated into the base of the umbilical cord (Fig. 9a). The conceptus occupies the whole of the gravid horn and the uterine body by days 55–60 (
Days 60–80
By day 60 the fetus has become clearly horse-like in appearance and it lies on the floor of the uterus in the anterior body close to the bifurcation of the two horns (Fig. 9a). Over the next 80–100 days, its lengthening umbilical cord allows it a considerable degree of movement within the proportionally large volume of allantoic fluid to the extent that excessive twisting of the umbilical cord leading to torsion and vascular occlusion is the commonest cause of later abortion in Thoroughbred
General discussion
Amongst the large domestic animal species, the equine fetus is probably the slowest in its achievement of implantation and the development of a stable placenta to sustain intrauterine growth to term. In addition, the equine embryo appears to have to inform its mother, not once but twice, of its presence in the uterus and the need for her to establish a progesterone-dominated pregnancy state of non-cyclicity, myometrial quiescence and nutritive placentation. The first such embryonic message is
Acknowledgements
The authors are grateful to Mrs Sue Gower for expert technical assistance with histology.
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Formerly The Equine Fertility Unit, Mertoun Paddocks, Newmarket.