Human Development - Weeks 4-8

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

  • epidermis
  • anterior pituitary
  • enamel
  • internal ear
  • lens of eye

Neuroectoderm

 

Endoderm - epithelia of

  • respiratory system
  • gut, liver, pancreas
  • bladder
  • pharynx, thyroid, other

Mesoderm

  • bones
  • muscles
  • dermis and connective tissue
  • urogenital system
  • serous membranes
  • primordial heart
  • blood and lymphatic cells and vessels

 

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.