Development Of The Human Zygote

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Term Paper Title	Development Of The Human Zygote
# of Words	2290
# of Pages (250 words per page double spaced)	9.16

Development of the Human Zygote

Development of the Human Zygote

November 16, 1995

     Hundreds of thousands of times a year a single-celled zygote, smaller
than a grain of sand, transforms into an amazingly complex network of cells, a
newborn infant.  Through cellular differentiation and growth, this process is
completed with precision time and time again, but very rarely a mistake in the
"blueprint" of growth and development does occur.  Following is a description of
how the pathways of this intricate web are followed and the mistakes which
happen when they are not.
     The impressive process of differentiation changes a single-cell into a
complicated system of cells as distinct as bold and bone.  Although embryonic
development takes approximately nine months, the greatest amount of cellular
differentiation takes place during the first eight weeks of pregnancy.  This
period is called embryogenesis.
     During the first week after fertilization, which takes place in the
Fallopian tube, the embryo starts to cleave once every twenty-four hours (Fig.
1).  Until the eight or sixteen cell stage, the individual cells, or blastomeres,
are thought to have the potential to form any part of the fetus (Leese, Conaghan,
Martin, and Hardy, April 1993).  As the blastomeres continue to divide, a solid
ball of cells develops to form the morula (Fig. 1).  The accumulation of fluid
inside the morula, transforms it into a hollow sphere called a blastula, which
implants itself into the inner lining of the uterus, the endometrium (Fig. 1).
The inner mass of the blastula will produce the embryo, while the outer layer of
cells will form the trophoblast, which eventually will provide nourishment to
the ovum (Pritchard, MacDonald, and Gant, 1985).

Figure 1:Implantation process and development during
         embryogenesis (Pritchard, MacDonald and
         Gant, 1985)

     During the second week of development, gastrulation, the process by
which the germ layers are formed, begins to occur.  The inner cell mass, now
called the embryonic disc, differentiates into a thick plate of ectoderm and an
underlying layer of endoderm.  This cellular multiplication in the embryonic
disc marks the beginning of a thickening in the midline that is called the
primitive streak.  Cells spread out laterally from the primitive streak between
the ectoderm and the endoderm to form the mesoderm.  These three germ layers,
which are the origins of many structures as shown in Table 1, begin to develop.

Table 1: ...

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