880
UNIT SIX
The blastocyst attaches to the uterine lining, aided by
its secretion of proteolytic enzymes that digest part of the
endometrium (F
g. 23.6
b, c
). The blastocyst sinks slowly into
the resulting depression, becoming completely buried in the
uterine lining. At the same time, the uterine lining is stimu-
lated to thicken below the implanting blastocyst, and cells
of the trophoblast begin to produce tiny, F ngerlike extensions
By the end of the F rst week of development, the blasto-
cyst superF
cially implants in the endometrium
(f
g. 23.6
a
)
.
Up until this point, the cells that will become developing off-
spring are pluripotent stem cells, which means they can give
rise to several specialized types of cells, as well as yield addi-
tional stem cells.
Within the blastocyst, cells in one region group to form an
inner cell mass
that eventually gives rise to the
embryo proper
(em
bre-o prop
er)—the body of the developing offspring. The
cells that form the wall of the blastocyst make up the
tropho-
blast,
which develops into structures that assist the embryo.
Inner cell mass cells can be cultured in a laboratory dish and, in the pres-
ence of a speciF
c “cocktail” of growth factors and other biochemicals,
become human embryonic stem cells. These cells are useful in research
to study normal development as well as the beginnings of diseases.
FIGURE 23.3
Steps in fertilization: (
1
) The sperm cell reaches the corona radiata surrounding the oocyte. (
2
) and (
3
) The acrosome of the sperm
cell releases a protein-digesting enzyme. (
4
) The sperm cell penetrates the zona pellucida surrounding the oocyte. (
5
) The sperm cell’s membrane
fuses with the oocyte’s membrane.
First polar body
Corona radiata
Second meiotic
spindle
Zona pellucida
Cell membrane
of secondary
oocyte
Cytoplasm of secondary
oocyte
Nucleus
containing
chromosomes
Acrosome
containing
enzymes
1
3
2
4
5
If two ovarian follicles release secondary oocytes simultaneously, and
both are fertilized, the resulting zygotes develop into fraternal (dizy-
gotic) twins, which are no more alike genetically than any nontwin
siblings. Twins may develop from a single fertilized oocyte (monozy-
gotic twins) if two inner cell masses form within a blastocyst and
each produces an embryo. Twins of this type usually share a single
placenta, and they are genetically identical. They are always the same
sex and are similar in appearance.
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