Biomedical Engineering Reference
In-Depth Information
Table 2.1. Biological signposts during human development
Development
time*
Stage
Remarks**
0 hours
Conception
Sperm penetrates the egg cell membrane;
sperm nucleus with paternal chromosomes
enters the egg cytoplasm.
24 hours
Zygote
Sperm nucleus and egg nucleus fuse, mark-
ing fertilization or
syngamy
; maternal and
paternal chromosomes intermingle; the cell
is
totipotent,
i.e., it can give rise to an entire
individual plus placental tissues.
30 - 48 hours
Cleavage
Zygote divides to produce 2 cells, 4 cells, 8
cells, etc. Cells are called
blastomeres
and are
totipotent to about 8-cell stage, i.e., each
blastomere can produce an entire individual if
separated from the others.
48-72 hours
Morula
Fewer than 100 cells; individual cells are no
longer totipotent but most are
pluripotent,
i.e.,
able to produce all tissues in a fully formed
individual.
4 -7 days
Blastocyst
Embryo one-fifth the size of a pinhead;
normal time of implantation in uterine wall;
produced in IVF clinics; cells of the inner cell
mass (ICM) of surplus IVF blastocysts are po-
tential sources of new embryonic stem cell
lines; ICM contains 150 to a few hundred cells
that can produce a fetus, whereas cells sur-
rounding ICM produce placental tissue; all
ICM cells are pluripotent.
14 days
Primitive
streak
Embryo the size of a poppy seed; rudiments of
three major classes of embryonic tissue (
ecto-
derm, mesoderm, endoderm
) formed; embryo
can no longer split to form twins, cells are no
longer pluripotent;
individuation.
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