2011 (SBTE) 25th Annual Meeting Proceedings - International ...

R.C. Cheb
hebel.
el. 2011. Use of Applied Reproductive Technologies (FTAI, FTET) to Improve the Reproductive Efficiency in
Dairy Cattle. Acta Scientiae Veterinariae. 39(Suppl 1): s203 - s221.
process of separating into two distinct cell
populations. Hence, the most “ventral” cell layer
towards the blastocyst cavity flattens and, finally,
delaminates forming the hypoblast. The “dorsal” cell
population establishes the epiblast. The hypoblast
subsequently extends along the inside of the
trophectoderm forming a complete inner epithelial
lining of the embryo. The polar trophectoderm
covering the epiblast (known as the Rauber´s layer)
becomes very thin around Day 9 of gestation and
gradually disintegrates exposing the epiblast to the
uterine environment. Before the shedding of Rauber’s
layer, tight junctions are formed between the
outermost epiblast cells to maintain the epithelial
sealing the embryo despite the loss of the polar
trophectoderm. Apparently, the porcine epiblast forms
a small cavity, which finally opens dorsally followed
by an “unfolding” of the complete epiblast upon the
disintegration of Rauber’s layer (Figure 3; [12]). After
the loss of this component about Day 10 of gestation,
the epiblast is discernable in the stereo microscope
as a circular lucent structure known as the embryonic
disc (Figure 4; [43]). Along with the formation of the
embryonic disc, the blastocyst enlarges, and by Day
10 it reaches a diameter of more than half a
centimetre. In parallel, with the formation of the
embryonic disc, the porcine epiblast starts to express
not only OCT4, but also NANOG (Figure 5; [46]).
At this stage of development, the first sign of
anterior-posterior polarization develops in the
embryonic disc: As mentioned earlier, the epiblast is
underlaid by the hypoblast, and an area of increased
density of closely apposed hypoblast cells develops.
This area is approximately the same size as the em-
Figure 3. Light micrograph of sections of the same porcine Day 9 blastocyst. (A) Rauber’ layer (RL), continuous with the remaining portion of
the trophectoderm (Te), covers the epiblast (Ep), in which a cavity (C) has developed. The epiblast is underlaid by the epiblast-related taller
hypoblast (Ep-Hy) and the trophectoderm by the trophectoderm-related lower hypoblast (Te-Hy). (B) Another section from the same epiblast
showing the opening of the cavity towards the external environment and the “unfolding” (arrows) of the epiblast to form the embryonic disc.
Figure 4. Porcine Day 10 blastocyst. (A) Stereo micrograph showing the blastocyst presenting the embryonic disc (arrow). (B) Light
micrograph of section of the embryonic disc showing the dome-shaped epiblast (Ep) underlaid by the hypoblast (Hy). The epiblast is continuous
with the trophectoderm (Te) indicated by the arrows.
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