In week 4, the embryo undergoes major morphological changes as it changes from a trilaminar disk-shaped embryo to a cylindrical embryo. This is also an important week in terms of determining placement of future organs. Following median and horizontal folding, many organs and body cavities will begin to form or will be repositioned.
At the beginning of week 4, the embryo is 2.0-3.5 mm long, straight, has 4-12 somites, and a neural tube that has begun to close at the cranial end (rostropore). Somites will continue to develop, increasing in number to 20-30 during this week, the somite period of development. They can be visualized on the surface of the embryo and are used to estimate the age of the embryo. They will eventually give rise to the vertebrae, ribs, and musculature of the axial skeleton, as well as the dermis.
Meanwhile, a horseshoe shaped intraembryonic coelom formed as coelomic spaces in the lateral plate mesoderm and cardiogenic mesenchyme came together at the end of the third week. This coelom will provide a space for the development and necessary movement of organs. It will form the future pericardial, pleural and peritoneal cavities. It also provides the framework for the future body wall, somatopleure, composed of surface ectoderm, lateral plate mesenchyme and a mesothelial lining, which lines the amnion, as well as the future gut wall, splanchnopleure, of endoderm, lateral plate mesenchyme and mesothelium around the yolk sac. The mesothelial linings of these walls will eventually form the visceral and parietal layers of the various cavities.
Rapid growth of the brain and amniotic cavity, as well as the
development of the somites cause the length of the embryo to increase
until horizontal growth cannot keep up and it begins to fold at
the cranial and caudal ends. This head-tail
folding results in a repositioning of the heart, pericardial
cavity, septum transversum, and bucco-pharyngeal membrane from
the cranial end to a ventral, more caudal position. Simultaneously,
rapid growth causes the embryo to fold
laterally, resulting in the somatopleure layers folding in
laterally to enclose the gut. These two folding processes will
reduce the size of the extra-embryonic coelom and the yolk sac.
After it has occurred, the primitive body cavities, gut and respiratory
system will all be in their proper place. The primitive gut will
be divided into foregut, midgut and hindgut segments, and the
future sites of the mouth and anus will be decided. The umbilical
cord will form as amniotic epithelium encloses extra-embryonic
mesenchyme, part of the olk sac, allantois, and a remnant of the
extra-embryonic coelom. The ventral thoracic region will begin
to develop in its proper location and all body cavities will be
properly positioned.
During this last week, the neural tube will complete the closing
process, with the caudal neuropore fusing towards the end of the
week. Pharyngeal (branchial) arches appear, as does the upper,
then lower limb bud, the otic pit and the lens placode.