Gastrulation Overview

Gastrulation

• The most important event during the third week of gestation
• Establishes the three germ layers: ectoderm, mesoderm, and endoderm
• Begins with the formation of the primitive streak on the epiblast
• Primitive streak has a slightly bulging region on either side
• Primitive node is the cephalic end of the streak (slightly elevated area around the primitive pit)
• Primitive pit is a depression in the center of the primitive node
• Cells of the epiblast migrate toward the primitive streak becoming flask-shaped, detaching and slipping beneath it (inward movement called invagination)
• Cells that displace the hypoblast create the embryonic endoderm
• Cells that lie between the epiblast and the endoderm form the mesoderm
• Cells that remain in the epiblast form the ectoderm
• Cell migration and specification are controlled by fibroblast growth factor 8 (FGF8)
• Cells spread laterally and cranially after migrating between the epiblast and hypoblast layers
• The epiblast is the source of all three germ layers
• In the cranial part of the embryonic disc, hypoblast cells become taller and columnar, forming the prochordal plate
• Prochordal plate is a thickening firmly attached to the overlying epiblastic disc on day 14
• The appearance of the prochordal plate determines the embryo's pole and its central axis which becomes the cranial end
• The tail end is on the opposite side

Notochord

• Develops from the upper end of the primitive streak to the lower end of the prochordal plate
• Notochordal process - cells multiply in the midline between the ectoderm and endoderm until they reach the lower end of the prochordal plate (day 17-18)
• Notochordal canal - the primitive pit extends into the notochordal process, converting it into a canal
• Floor of the notochordal canal breaks down
• Notochordal canal communicates with the yolk sac and amniotic cavity through the primitive pit
• Neurenteric canal - temporary connection between amniotic and yolk sac cavities
• Prenotochordal cells invaginate in the primitive node and move cranially in the midline until they reach the prechordal plate
• Notochord and prenotochordal cells extend cranially to the prechordal plate (area just caudal to the oropharyngeal membrane) and caudally to the primitive pit
• Notochordal plate - two-cell layers in the midline of the embryo where prenotochordal cells become intercalated in the hypoblast
• Definitive notochord - solid cord of cells formed by notochordal plate, lies under the neural tube
• Notochord is the basis for the axial skeleton
• Notochord induces the formation of the neural tube
• Notochord persists in the first decade of life as the nucleus pulposus of the intervertebral disc

Trophoblast Development

• By the start of the third week, the trophoblast is characterized by primary villi, which consist of a cytotrophoblastic core covered by a syncytial layer
• Mesodermal cells enter the core of primary villi and grow towards the decidua, forming a secondary villus
• Secondary villus mesodermal cells differentiate into blood cells and small blood vessels, forming the villous capillary system
• Tertiary villus (definitive placental villus) - the villus after developing a capillary system
• Villi capillaries connect with capillaries developing in the chorionic plate mesoderm and the connecting stalk - these vessels connect the placenta and the embryo
• Heart begins to beat in the fourth week of development, allowing the villous system to supply the embryo with nutrients and oxygen
• Cytotrophoblastic cells in the villi penetrate the syncytium until they meet similar extensions from neighboring villous stems (thin outer cytotrophoblast shell)
• The cytotrophoblast shell surrounds the trophoblast and anchors the chorionic sac to the maternal endometrial tissue
• Stem or anchoring villi - extend from the chorionic plate to the decidua basalis (decidual plate)
• Free (terminal) villi - extend from the sides of stem villi and are involved in nutrient exchange
• Chorionic cavity becomes larger by the 19th or 20th day
• The embryo is attached to the shell by a narrow connecting stalk (develops into the umbilical cord)

Intra-embryonic Mesoderm

• Spreads all over the disc except for the prochordal plate
• Prochordal plate is very thin with adherent ectoderm and endoderm, no mesoderm between the layers
• The prochordal plate will later form the buccopharyngeal membrane (oral cavity)
• The cloacal membrane is formed at the caudal end of the embryonic disc - similar structure to the oropharyngeal membrane, with tightly adherent ectoderm and endoderm with no intervening mesoderm.

Intraembryonic Coelom

• Caudal end of the connecting stalk forms a small diverticulum called the allantoenteric diverticulum (allantois) around day 16
• The allantois extends into the connecting stalk
• Mesoderm divides into:
  • Somatic mesoderm (parietal mesoderm) - forms lateral body wall.
  • Splanchnic mesoderm (visceral mesoderm) - forms wall of viscera.

This quiz focuses on the key events during gastrulation, particularly in the third week of embryonic development. It covers the formation of the three germ layers, the role of the primitive streak, and the process of cell migration and specification. Enhance your understanding of these fundamental developmental concepts.