Human Development Week 3

Following embryonic bilaminar disc formation in week two, the embryo begins rapidly developing in week three. This begins with the formation of a trilaminar disc, gastrulation, as the beginning of morphogenesis.

 

Primitive Streak

Occurring at the beginning of the third week, the primitive streak is the first sign of the third dimension in the laminar disc. Epiblast cells proliferate and migrate to the middle of the disc, forming a thickened ridge that grows on either side of the "primitive groove".

 

Cells soon leave the epiblast surface and migrate towards the hypoblast, and in doing so form the first mesoderm and endoderm. Loose connective tissue soon follows, producing mesenchyme.

 

Notochord

Some mesenchymal cells migrate from the primitive groove to form the notochord, which defines the axis of the embryo and serves as the basis of the axial skeleton. It develops cranially until it reaches the prechordal plate, the site of the future mouth.

 

Neurulation

Neurulation, or development of the neural tube, represents the beginning of the central nervous system from the ectoderm of the neural plate (neuroectoderm). It is induced by the lengthening notocord.

 

Neural Crest Cells

As neural folds fuse to form the neural tube, some neuroectodermal cells lose their epithelial differentiation and cell attachments. They form an irregular cell mass between the neural tube and the overlying surface ectoderm, and from there migrate to various and wodespread locations in the mesenchyme.

Neural crest cells differentiate into spinal ganglia and ganglia of the autonomic nervous system, meningeal coverings of the CNS, melanocytes, the adrenal medulla, and several skeletal and muscular components of the head.

 

BMP, Wnt, Notch, and FGF are involved in neural crest formation and migration and differentiation of neural crest cells.

 

Somites

As the notochord and neural tube develop, the mesoderm on eaither side proliferates to form a thick, longitudinal column of paraxial mesoderm. By the end of the third week, these thickenings divide into paired cuboidal bodies called somites. Somites are prominent in the 4th and 5th weeks and asre used to determine the embryo's age.

Somites give rise to most of the axial skeleton, the associated musculature, and the adjacent dermis.

There are 38 or 39 pairs of somites, though are not all visible at the same time.

 

Intraembryonic Coelom

The intraembryonic coelom, or body cavity, first appear as spaces in the lateral mesoderm and soon join together to form a single horseshoe-shaped cavity that divides the lateral mesoderm into two layers:

At first, the coelom is connected to the extraembryonic coelom.

 

During the second month, the intraembryonic coelom divides into three body cavities - the pericardial, pleural, and peritoneal cavities.

 

The Early Cardiovascular System

At the beginning of the third week, angiogenesis begins in the extraembryonic mesoderm of the yolk sac, chorion, and connecting stalk. Embryonic blood vessels soon follow surrounding the neural plate, and by the end of the third week a primitive cardiovascular system is formed.

 

 

The Allantois

The allantois appears on day 16 from the yolk sac as a diverticulum extending into the connecting stalk. It is involved in early blood formation and is associated with the development of the urinary bladder. As the bladder develops, the allantois becomes the urachus, or the median umbilical ligament in adults.

 

During the fourth to eighth weeks, most of the embryonic development occurs, including the bulk of organogenesis.