Endoderm cells of the equine yolk sac from Day 7 until formation of the definitive yolk sac placenta

Abstract

Summary On Day 7 of pregnancy, when the inner cell mass coalesces to one side of the blastocyst, a discontinuous layer of cells is present underlying the inner cell mass and isolated cells were found around the mural trophoblast. As early as Day 8, these cells that were scattered around the blastocyst cavity unite to form a continuous endodermal layer, resulting in a bilaminar blastocyst. The endodermal cells have slightly bulging apical surfaces towards the cavity of the blastocyst, but in many areas have flanges and projections that form complex basolateral compartments within the endoderm and especially between the endoderm and trophoblast. These cells also develop an extensive series of unusual apical ridges. With the development of the primitive streak and the trilaminar blastocyst, angiogenesis proceeds from the forming embryo into the yolk sac. At first the anastomotic vessels form a simple mesh, but by Day 20, erythroblasts are present in the newly formed sinus terminalis. The loss of the capsule (Day 21) and the development of the embryonic circulation establish the relationships of the definitive yolk sac. There is a bilaminar region with endodermal cells showing basolateral compartmentalisation and a specialised sinus terminalis region bordering the choriovitelline placenta. The largest endodermal cells are associated with the choriovitelline placenta, but a substantial cuboidal lining also covers the splanchnopleuric portion of the vascular yolk sac. The allantochorion rapidly assumes predominance in respiratory exchanges, and the choriovitelline placenta not only decreases in relative area but increases in thickness. However, the 3 major regions of the yolk sac persist for several weeks after their formation, and both the vasculature and especially the endodermal cells remain robust despite the decrease in size of the yolk sac. It is concluded that the pleomorphic endodermal cells of the equine yolk sac differentiate in response to local signals. By Day 25 the allantochorion is an increasingly significant exchange unit, and the continued robustness of the vascular portions of the yolk sac and especially its endodermal layer suggests that synthetic and metabolic activities of the endodermal cells will be the dominant function of the yolk sac subsequently.

Uterine influences on embryogenesis and early placentation in the horse revealed by transfer of day 10 embryos to day 3 recipient mares

“…Although the exact method of embryonic expansion remains unclear in the horse (Enders et al 1993, Waelchli et al 1997, Crews et al 2007, Budik et al 2008, the day 10 embryo certainly has the capacity to expand. Thus, its initial reduced expansion, compared with the controls, when placed in the day 3 uterus, could be a reflection of the uterine milieu.…”

The uterine environment modulates trophectodermal POU5F1 levels in equine blastocysts

“…This differs from the asymmetric cavitation process observed in mouse blastocysts (Wiley & Eglitis 1980). Enders et al (1993) reported that in equine embryos, some of these inner network cells migrate directly to the inner aspect of the TE and proliferate to form the endodermal cell layer; although not described, it appears that other inner cells coalesce to form the ICM.…”

Cryopreservation of equine embryos

“…Thus, they achieve diameters beyond 300µm usually by Day 7 after ovulation (Colchen et al 2000, Carnevale 2006. By Day 8 after ovulation, a continuous endodermal layer is formed representing the bilaminar blastocyst (Enders et al 1993).…”

“…As the embryo grows, two distinct cell types develop: trophoblast cells lining the outer surface of the embryo, and the inner cell mass (ICM) (Betteridge et al 1982, Enders et al 1993. The capsule, a glycoprotein layer, forms between zona pellucida and trophoblast at the early blastocyst stage (Betteridge et al 1982, Betteridge 1989.…”

“…However, the role of the equine capsule concerning the poor ability of large equine embryos for freezing could not be clarified by now. It has also to be taken into account that the trophoblast of Day 8 blastocysts becomes bilaminar (Enders et al 1993), which might represent an additional barrier for the permeation of cryoprotectants.…”