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known about these processes in other mammalian species, including humans.
Notably, compaction occurs much earlier in mouse embryos, at the eight-cell stage,
than in human embryos, where it begins at the 16-cell stage and, even later, at the
32-cell stage, in bovine embryos (Steptoe et al. 1971 ; Edwards et al. 1981 ; Nikas
et al. 1996 ; Van Soom et al. 1997 ). Almost immediately following compaction, the
formation of a fluid-filled cavity called the blastocoel is initiated by the assembly of
tight junctions, which includes occludin, cingulin, as well as other components, and
the establishment of high epithelial resistance in TE cells until the 32-cell stage
(Fleming et al. 1993 ; Sheth et al. 1997 , 2000 ). Once the blastocoel is formed, human
embryos are likely to undergo at least one additional round of cell division to form
a ~256-cell blastocyst, whereas mouse blastocysts typically comprise ~164 cells
(Niakan and Eggan 2013 ). Until recently, it was unknown how these differences in
the timing of compaction or number of cells may affect polarization and asymmetric
cell divisions in the human embryo. In contrast to the mouse, wherein TE and ICM
fates are established in a positional and cell polarization-dependent manner at the
morula stage as described above, human embryos appear to establish the TE as well
as the epiblast and primitive endoderm lineages concurrently at the blastocyst stage
(Petropoulos et al. 2016 ). This study also noted that human embryo compaction is
not as prominent as in the mouse, with only partial compaction occurring in a cer-
tain number of blastomeres on embryonic day 4. Consequently, it is not until embry-
onic day 5 and upon blastocyst formation that distinct inner and outer compartments
are observed in human embryos. Whether other mammals undergo compaction via
a cell polarization-dependent mechanism at the morula stage for lineage specifica-
tion or establish the first lineages concomitant with blastocyst formation is unclear,
but partial compaction has been observed in bovine, porcine, and rabbit embryos
(Reima et al. 1993 ; Koyama et al. 1994 ). Taken together, this suggests that although
mammalian embryos morphologically resemble each other during preimplantation
development, there are several notable species-specific differences that may limit
extrapolation between mammals.
4.5 Evolutionary Relationship Between Holoblastic
and Meroblastic Cleavage Types
The phylogenetic distribution of holoblastic and meroblastic cleavage indicates that
the latter has evolved independently five times in craniates (a phylogenetic group
containing the vertebrates and hagfish (Myxini)) (Collazo et al. 1994 ; Collazo 1996 ;
Fig. 4.13). Several closely related groups outside of craniates, such as ascidians,
tunicates, and echinoderms, exhibit holoblastic cleavage, suggesting that this type
of cleavage is the ancestral mode. Within craniates, meroblastic cleavage appears to
have evolved independently in Myxini (hagfish), Chondrichthyes (sharks, skates,
and rays), Teleostei (largest infraclass of ray-finned fishes), Actinistia (coelacanths),
and Amniota (certain non-eutherian mammals (e.g., egg-laying monotremes), birds,
A. Hasley et al.