Loss of polar trophoblast during differentiation of the blastocyst of the horse

Abstract

Twelve blastocysts, collected 7-12 days after ovulation (Day 0), were examined by light and electron microscopy to investigate the nature of the relationship of the polar trophoblast (Rauber's layer) to the inner cell mass. On Day 7, the polar trophoblast was intact and formed a flattened layer overlying the epiblast cells of the inner cell mass. As blastocysts enlarged to greater than 1 mm in diameter, small discontinuities appeared in the polar trophoblast, where epiblast cells intruded onto the surface. At this time, trophoblast cells adhered closely to adjacent and underlying epiblast cells, forming an irregular layer of cells capping the epiblast. With continued increase in blastocyst size, polar trophoblast cells became isolated but maintained their characteristic apical endocytic structures. By Days 10-12, the scattered trophoblast cells showed evidence of deterioration, and vacuoles containing cell debris were common within the epiblast. It is suggested that polar trophoblast cells become scattered, rather than withdrawing as a unit, because they become more adherent to subjacent epiblast cells than to adjacent trophoblast cells. It is further suggested that most of the isolated cells are eventually phagocytosed by epiblast cells.

Integrity of the preimplantation pig blastocyst during expansion and loss of polar trophectoderm (Rauber cells) and the morphology of the embryoblast as an indicator for developmental stage

“…This is in accordance with observations on the pig (Norberg, 1973), cow (Betteridge & Fléchon, 1988) and horse (Enders et al, 1988 (Betteridge & Fléchon, 1988).…”

“…The forma¬ tion of junctional complexes is likely to be a prerequisite for transport processes through the outer cell layer. In the pig, extreme expansion of the blastocyst takes place during, and proceeds after, hatching, as holds for other ungulates (Betteridge & Fléchon, 1988;Enders et al, 1988 Fig. 4.…”

“…In the pig, hatching takes place at Day 6 (Perry & Rowlands, 1962;Hunter, 1977), while attachment of trophectoderm to the uterine epithelium does not take place before Day 13 (Dantzer, 1985). In cow (Betteridge & Fléchon, 1988), horse (Enders et al, 1988) and pig (Geisert et al, 1982;Stroband et al, 1984) the trophectoderm cells covering the ICM ('polar trophectoderm' or 'Rauber cells') disappear some days after hatching: in the pig usually around Day 10 when the rapidly expanding blastocysts have reached a mean diameter of 3 mm. As a consequence, ICM cells form a part of the outer cell layer from that moment onwards.…”

Polar trophoblast (Rauber's layer) of the rabbit blastocyst

“…Species in which the polar trophoblast is lost include, in addition to the rabbit, the cat, dog, sheep, pig, shrew, deer, tree-shrew (Tupaza), mole (Talpa), and tarsier (Tarsius) (see Da Costa, 1920;Mossman, 1937;Goetz, 1938;Boyd and Hamilton, 1952, for reviews). Recently the loss of the polar trophoblast has been re-examined in the rabbit (Flechon, 1978;Mootz, 19791, pig (Geisart et al, 1982;Stroband et al, 19841, western spotted skunk (Enders et al, 19861, and horse (Enders et al, 1988). The polar trophoblast of the rabbit was initially referred to as the Deckschicht, but was then called Rauber's layer, named by Koelliker in honor of its discoverer (Rauber, 1875;Lieberkuhn, 1879;Koelliker, 1880).…”

Role of ultrastructural studies in the analysis of cell lineage in the mammalian pre‐implantation embryo

“…Rodent and primate embryos expand minimally, accumulating only small volumes of fluid (Benos and Biggers, 1981;Overstrom, 1987), while blastocysts of the horse, pig, rabbit, and domestic ungulates expand to several times their original volume (Benos and Biggers, 1981;Betteridge and Flechon, 1988;Denker and Gerdes, 1979;Enders, 1989;Enders et al, 1988;Flood et al, 1982;Overstrom, 1987;Papaioannou and Ebert, 1988;Stroband et al, 1984;Wintenberger-Torres and Flechon, 1974). Blastocyst expansion follows escape from the zona pellucida but precedes and apparently serves to appose the embryo to the uterine surface preparatory to implantation (Enders, 1971).…”