Figure 6D: Early Blastocyst (www.visembryo.com by Mouseworks. Inc.| 
The cells of the inner cell mass can give rise to progeny differentiating 
into all the types of cells in the adult body, so they are called 
pluripotent. They have not usually been described as totipotent 
because, the inner cell mass having already differentiated from 
trophoblast, the cells of the inner cell mass were believed to be no 
longer able to give rise to the cells of the trophoblast. Recent work, 
however, describes culture conditions under which human embryonic 
stem cells can differentiate to trophoblast cells.® Although the new 
offspring itself develops only from the inner call mass, the trophoblast 
is not just passive padding. Its progeny are the essential and 
specialized connection between the embryonic and maternal 
systems. Embryonic stem cells can be isolated from the inner cell 
mass (see Chapter Four). 
IV. TROPHOBLAST TO PLACENTA 
After the embyo covering degenerates, the blastocyst, now in the 
uterus, enlarges and its trophoblast attaches to the endometrium 
(the uterine lining) at about six days after fertilization. This begins 
the process of implantation, during which the blastocyst becomes 
integrated with the endometrium through specialized membranes. 
The embryo is now beginning its second week of development. The 
process of implantation takes three to four days, but is generally 
completed by day twelve. The trophoblast area that binds to the 
endometrium first differentiates into an inner layer of cells and an 
exterior layer in which the membranes dividing the cells degenerate 
and the cells fuse. As the blastocyst become more deeply embedded 
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