Human Development - Weeks 4-8
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Introduction
Following the gastrulation and formation of the early nervous and cardiovascular systems in week three, the embryo is about 1 mm in length. The bulk of embryo organogenesis occurs during weeks 4-8.
Folding of the Embryo
Folding occurs at the cranial and caudal ends and sides simultaneously, incorporating parts of the yolk sac into the embryo as the foregut, lying between the brain and the heart, and the hindgut.
Lateral folds lead to closure of the intraembryonic coelom and reduces the region of attachment of the amnion to a relatively narrow umbilical region. This leads to formation of the umbilical cord, which is composed of the connecting stalk and covered with epithelial amnion.
Germ Layer Derivitives
The three germ layers formed during gastrulation - ectoderm, endoderm, and mesoderm - give rise to all tissues.
Ectoderm
Neuroectoderm
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Endoderm - epithelia of
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Mesoderm
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Fourth Week
The neural tube is widely open at the cranial and caudal neuropores at the beginning of the fourth week, but the forebrain soon becomes prominent.
Pharyngeal arches appear and begin to form the architecture of the head and neck.
Upper and lower limb buds appear, as do the beginnings of the eyes.
Fifth Week
Not so much happens during the fifth week, except the head grows a lot
Sixth Week
Upper limbs continue developing, with digital rays appearing.
The head prominence is much larger relative to the trunk.
Reflex responses to touch have been reported.
Seventh Week
The limbs undergo considerable change during the 7th week.
As the intestines develop, they enter the umbilical cord temporarily.
Eighth Week
Purposeful limb movements occur during the eighth week.
The head is distinctly human and represents half the body.
After the eighth week, the embryo becomes a fetus.
Resources and References
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