Gastrulation (Days 15 - 22)

Gastrulation is the name given to the process of the bilaminar embryonic disc being converted into the three germ layers of the embryo: the ectoderm, the mesoderm, and the endoderm. The ectoderm (formed from the epiblast) forms the epidermis, nervous system, and retina amongst other minor structures. The mesoderm forms the cardiovascular system, skeleton, smooth and striated muscle, connective tissue, blood cells, and the excretory and reproductive organs. The endoderm (formed from the hypoblast) forms the gastrointestinal tract and associated organs, bladder, and epithelial lining of the respiratory and gastrointestinal tracts.

This process commences at the beginning of the third week (Day 15) with the formation of the primitive streak. This midline streak starts appearing from the caudal (tail) end of the embryonic disc at the cloacal membrane (primitive anus) and slowly ascends up towards the cranial end. As the streak elongates it forms a primitive node at its expanding cranial end which contains a primitive pit, which stops growing halfway up the disc and is where the primitive streak ends. A narrow groove forms behind the extending streak called the primitive groove. The streak emerges due to the proliferation and migration of cells from the epiblast (wall of amnion/ectoderm) to the hypoblast (wall of umbilical vesicle/endoderm), forming a new layer in-between the two called intraembryonic mesoderm (mesoblast). The primitive streak forms this mesoderm until the beginning of the 4th week.

Gastrulation AGastrulation B

A = elongating primitive streak of a 16-day embryo, B = mesenchymal migration forming the germ layers

Some cells from the primitive node and pit migrate cranially towards the prechordal plate where it stops. These mesenchymal cells migrate within the new mesenchyme layer between the ectoderm and endoderm, forming a midline cord called the notochordal process. This cord soon develops a lumen and is then termed the notochordal canal. This cord extends halfway along the length of the disc, from the primitive node to the oropharyngeal membrane at the cranial end (primitive mouth). The notochord defines the axis of the embryo and provides a base for the formation of the axial skeleton, with vertebral bodies forming around the cord later on in development. Once the vertebral column has been formed the notochord degenerates, leaving a small part as the nucleus pulposus (centre) of each intervertebral disc.

Notochord formation

A/B = Day 16, C = Day 17, D = Day 18

The notochord also has another function; it induces the ectoderm covering it to increase in thickness and form the neural plate, the beginning of the central nervous system. Read the next page for more information of the development of the nervous system.

An animation of the events occurring during gastrulation