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: Normal Germ Layer Origin of Structures in Some or all Vertebrates
(Harrison, 1969)

Normal Germ Layer Origin of Structures in Some or All Vertebrates

Ectoderm Mesoderm Endoderm Skin epidermis
Hair Feathers Scales Beaks Nails Claws Sebaceous, sweat, and
mammary glands Oral and anal lining tooth enamel Nasal epithelium Lens of
the eye Inner earBrainSpinal cordRetina and other eye partsNerve cells and
gangliaPigment cellsCanal of external earmedulla of the adrenal glandPituitary
gland Dermis of the skinConnective tissueMusclesSkeletal componentsOuter
coverings of the eyeCardiovascular system Heart Blood cells Blood
vesselsKidneys and excretory ductsGonads and reproductive ductsCortex of the
adrenal glandSpleenLining of coelomic cavitiesMesenteries LiverGall
bladderPancreasThyroid glandThymus glandParathyroid glandsPalatine tonsilsMiddle
earEustachian tubeUrinary bladderPrimordial germ cellsLining of all organs of
digestive tract and respiratory tract

During the third week of development, the cephalic (head) and caudal
(tail) end of the embryo become distinguishable. Most of the substance of the
early embryo will enter into the formation of the head. Blood vessels begin to
develop in the mesoderm and a primitive heart may also be observed (Harrison,
1969). Cells rapidly spread away from the primitive streak to eventually form
the neural groove, which will form a tube to the gut. When the neural folds
develop on either side of the groove, the underlying mesoderm forms segmentally
arranged blocks of mesoderm called somite. These give rise to the dermis of the
skin, most skeletal muscles, and precursors of vertebral bodies. the otocyst,
which later becomes the inner ear, and the lens placodes, which later form the
lenses of the adult eyes, are derived from the ectoderm.
The strand of cardiovascular functioning is apparent during the fourth
week. The heart shows early signs of different chambers and begins to pump
blood through the embryo which simultaneously has well developed its kidneys,
thyroid gland, stomach, pancreas, lungs, esophagus, gall bladder, larynx, nd
trachea (Carlson, 1981).
Several new structures are observed, organs continue developing, and
some previously formed structures reorganize during the fifth week of
embryogenesis. The cranial and spinal nerves begin to form and the cerebral
hemispheres and the cerebellum are visible. The spleen, parathyroid glands,
thymus gland, retina, and gonads, all new structures, also begin to form. The
gastrointestimer tract undergoes considerable development as the middle part of
the primitive intestine becomes a loop larger than the abdominal cavity. Next,
it must then project into the umbilical cord until there is room for the entire
bowel. Finally, the heart develops