Anatomy: 3rd Week of Pregnancy - Gastrulation

Gastrulation and Formation of Germ Layers

• Gastrulation is the process that establishes the three primary germ layers (ectoderm, mesoderm, and endoderm) in the embryo.
• It begins with the formation of the primitive streak on the surface of the epiblast.
• The primitive streak is a narrow groove in the epiblast with slightly bulging regions on either side.
• Cells of the epiblast migrate towards the primitive streak, causing an inward movement (invagination).
• This process displaces the hypoblast, creating the embryonic endoderm, and forms mesoderm between the epiblast and newly created endoderm.
• The remaining cells in the epiblast then form ectoderm.

Notochord Formation

• The notochord is a cylindrical rod-shaped structure induced by the primitive node.
• It lies in the midline between the ectodermal and endodermal layers of the embryonic disc.
• It extends from the primitive node to the prechordal plate.
• The notochord forms the basis for the vertebral column and induces the overlying ectoderm to form neuroectoderm and neural plate.
• Prenotochordal cells invaginate in the primitive pit, move forward, and eventually form the notochordal plate.
• The notochordal plate proliferates and detaches from the endoderm, forming a solid cord of cells, the definitive notochord.

Fate Map Established During Gastrulation

• The cells that ingress through the cranial region of the node become notochord.
• Cells migrating at the lateral edges of the node become paraxial mesoderm.
• Cells migrating through the midstreak region become intermediate mesoderm.
• Cells migrating through the more caudal part of the streak form lateral plate mesoderm.

Teratogenesis Associated With Gastrulation

• The beginning of the third week of development is a highly sensitive stage for teratogenic insult.
• Genetic abnormalities and toxic insults can disrupt gastrulation.
• Examples of teratogenesis include holoprosencephaly, caudal dysgenesis (sirenomelia), and situs inversus.

Tumors Associated With Gastrulation

• Sometimes, remnants of the primitive streak persist in the sacrococcygeal region and form tumors, known as sacrococcygeal teratomas.
• These tumors may also arise from primordial germ cells that fail to migrate to the gonadal ridge.

Development of the Trophoblast

• By the beginning of the third week, the trophoblast is characterized by primary villi.
• Mesodermal cells penetrate the core of primary villi and grow toward the decidua, forming secondary villi.
• By the end of the third week, mesodermal cells in the core of the villus begin to differentiate into blood cells and small blood vessels, forming the villous capillary system (tertiary villus or definitive placental villus).