Chapter 20. Week 3 of Development: Intraembryonic Mesoderm, Somite Development, and The Intraembryonic Coelom - Review of Medical Embryology Book

AS THE NOTOCHORD AND NEURAL TUBE FORM, the intraembryonic mesoderm on each side forms longitudinal columns, the paraxial mesoderm, each in turn being continuous laterally with the intermediate mesoderm, and the latter gradually thinning out further laterally into the lateral mesoderm
PARAXIAL MESODERM AND SOMITE FORMATION: somite development begins about day 20 and is the result of segmentation of the paraxial mesoderm

The paraxial mesoderm thickens and fragments metamerically, dividing into paired cuboid bodies called somites which give rise to most of the axial skeleton and associated musculature as well as much of the dermis of the skin
The first pair of somites develops just caudal to the cranial end of the notochord (future occipital area), and subsequent pairs form in a craniocaudal sequence after the appearance of the first somites
About 38 somite pairs form during days 20-30, the so-called somite period. Eventually about 42-44 somite pairs develop by the end of week 5

The somites form distinct surface elevations and are triangular in shape when seen in a transverse section
Each somite develops a slitlike cavity, the myocele, which eventually is occluded
Somites give origin to the sclerotome, whose cells condense around the notochord and give rise to the vertebral primordia and the myotome, which gives rise to the vertebral muscles

The myotome with the somatopleure gives origin to the muscles of the limbs and the anterior lateral body wall

Intermediate mesoderm gives rise to the nephrogenic cord from which the excretory apparatus originates

This mesodermal cell aggregation undergoes metameric segmentation parallel to the somites and forms nephrotomes. Segmentation, however, is never completed and never reaches the extreme caudal end of the nephrogenic cord

The intraembryonic splanchnopleure, which gives rise to muscle and connective tissue layers of the trunk, lines the entoderm and continues in the extraembryonic splanchnopleure
The intraembryonic somatopleure or outer layer which lines the ectoderm and helps form the lateral and ventral trunk walls

The intraembryonic coelom first appears as many small isolated coelomic spaces in the lateral mesoderm and cardiogenic mesoderm (between the 2 layers of the lateral plate mesoderm) which coalesce to form a horseshoe-shaped cavity, the intraembryonic coelom, which is lined by flattened epithelial (mesothelial) cells. It will become the pleuropericardial-peritoneal cavity

THE COELOM DIVIDES the lateral mesoderm into 2 layers: a somatic (parietal) layer continuous with the extraembryonic mesoderm over the amnion and a splanchnic (visceral) layer, which is continuous with the extraembryonic mesoderm over the yolk sac

Somatic mesoderm plus overlying embryonic ectoderm forms the body wall or somatopleure
Splanchnic mesoderm plus embryonic entoderm forms the wall of the primitive gut and is called the splanchnopleure

DURING THE SECOND MONTH, the intraembryonic coelom is divided into the body cavities, namely, the pericardial cavity, the pleural cavities, and the peritoneal cavity