Embryology Third Week Quiz

Embryonic Layers and Development Overview

• The embryo begins as a bilaminar disc, consisting of two layers: the superior epiblast (high columnar cells) and the inferior hypoblast (small cuboidal cells).
• During the third week, the primitive streak develops in the bilaminar embryonic disc, marking the onset of gastrulation, which leads to the formation of three germ layers.

Gastrulation Process

• The primitive streak is characterized by a rod-shaped thickening, facilitating bilateral symmetry, dividing the disc into distinct right and left halves.
• Cell migration results in the formation of the mesoderm layer, transforming the bilaminar disc into a trilaminar embryonic disc.
• The epiblast layer becomes ectoderm, while the hypoblast layer is redefined as endoderm.

Cephalic End and Membrane Formation

• The trilaminar disc has a defined cephalic end where the oropharyngeal (buccopharyngeal) membrane forms, consisting solely of ectoderm and endoderm, without mesoderm.

CNS Development

• By the end of the third week, the central nervous system (CNS) begins to develop, starting from neuroectoderm, which differentiates from the ectoderm.
• The neural plate, forming from neuroectoderm, extends from the cephalic to caudal end of the embryo and undergoes invagination to create the neural groove, leading to the formation of the neural tube.

Neural Crest Cells

• Neural crest cells, considered the fourth embryonic layer, develop from the neuroectoderm and migrate from the crests of the neural folds, eventually contributing to specific oral structures.

Somite Formation

• As the mesoderm differentiates, it divides into 38 paired cuboidal segments called somites, located alongside the developing CNS, which will form the vertebral plate.

Embryonic Folding and Axis Establishment

• In the fourth week, embryonic folding occurs, establishing the axis of the embryo and organizing tissue types for further development, resulting in a somewhat tubular shape.

Developmental Physiological Processes

• Induction is the first physiological process, involving intercellular actions where one group of cells influences another, often initiated by chemical precursors.
• Following induction, proliferation occurs through appositional (adding layers to the outside) and interstitial growth (adding layers from within).
• Differentiation leads to the specialization of embryonic tissues into various types, including cytodifferentiation (cell types), histodifferentiation (tissue types), and morphodifferentiation (organ structure).

Maturation of Embryonic Structures

• Maturation encompasses proliferation, differentiation, and morphogenesis, exemplified by the development of ectodermal cells into the tooth germ and eventually into a tooth.