Embryology Chapter 5: Gastrulation

Chapter 5: Gastrulation; establishment of embryonic germ layers and early patterning

• Key terms: Axis specification, body axes, cilia, pattern formation, blastocyst (blastula), hypoblast, inner cell mass, trophoblast (trophectoderm), embryonic germ layers (mesoderm, endoderm, ectoderm), gastrula, gastrulation, E-cadherin, mesenchyme, mesenchymal, epithelial cells, epithelium, epithelial-mesenchymal transition (EMT), notochord, anterior/posterior, dorsal/ventral, extra-embryonic tissues (membranes), prechordal plate, primitive node, primitive streak, cloacal membrane, buccopharyngeal membrane

Learning Objectives

• Name the three germ layers of the embryo established during gastrulation and give two examples of adult tissues that arise from each.
• Describe "axis specification" and give an example of axis specification occurring during gastrulation.
• Identify the embryonic germ layer that gives rise to the notochord and describe its role in early development.
• Explain the process of establishing right/left body axis patterning, including the role of cilia.
• Differentiate between mesoderm and mesenchyme.
• Describe the epithelial-mesenchymal transition (EMT) and name one important molecule in this transition.
• Explain the importance of "pattern formation" in embryogenesis and describe potential consequences of abnormal patterning.

Gastrulation

• Gastrulation is a stage of embryonic development where the embryo establishes the three germ layers (endoderm, ectoderm, and mesoderm) that will form all adult tissues.
• Body axes (cranial/caudal, left/right, dorsoventral) are patterned during gastrulation.
• Morphogenesis is the process of structure building, occurring during gastrulation.
• Initially, the embryo consists of the inner cell mass and trophoblast/trophectoderm.
• Inner cell mass cells migrate to the underside of the embryonic disc, detaching and forming the hypoblast (later endoderm).
• Hypoblast expands to line the blastocoel/yolk sac.
• The epiblast cells form the remaining embryo.
• Cadherins are cell adhesion molecules that help establish tissue structure and are important in gastrulation.

Gastrulation-Cells Migrate

• Cells of the inner cell mass migrate, detach from the disc, arrange into a sheet (hypoblast), expanding to form the interior of the blastocoel(yolk sac)
• Scattered cells migrate to the underside of the disc, detach, and arrange themselves into a sheet called the hypoblast.
• The hypoblast will later become the endoderm.
• The remaining cells of the inner cell mass are called the epiblast and these form the embryo.

Cell Migration into Primitive Streak

• Epiblast cells migrate towards the midline forming a groove called the primitive streak.
• Cells lose epithelial characteristics and become migratory through an epithelial-to-mesenchymal transition (EMT), driven by the loss of E-cadherin.

Formation of Notochord and Prechordal Plate

• The notochord is a mesodermal structure crucial for vertebrate development.
• Mesodermal cells from Hensen's node migrate cranially forming the notochord.
• Notochord runs along the midline, between the ectoderm and endoderm.
• Prechordal plate is mesoderm cranial to the notochord, and important for head development.
• It forms at the cranial end of the embryo.

Buccopharyngeal and Cloacal Membranes

• These membranes are formed by the fusion of ectoderm and endoderm.
• These membranes separate the digestive tube from the external environment at the mouth and anus. (later degenerate).

Completion of Gastrulation

• The embryonic disc, at this point, has three germ layers: ectoderm, mesoderm, and endoderm.
• Body axes have been established.