Gastrulation and Early Embryonic Development

Flashcards covering the topic of gastrulation and early embryonic development, focusing on key events, structures, and cell fates during the third week of development.

Gastrulation

A major differentiation event that forms the 3 primary germ layers: ectoderm, mesoderm, and endoderm.

Ectoderm, Mesoderm, Endoderm

The three primary germ layers formed during gastrulation.

Invagination

An epithelial sheet of cells cups inward to form a pocket.

Ingression/Delamination

Individual cells leave an epithelial sheet and migrate.

Involution

A continuous sheet of cells rolls inward to form an additional layer (not used in human gastrulation).

Epiblast and Hypoblast

Two layers of the bilaminar germ disc present by day 15.

Amniotic cavity, yolk sac, and chorionic cavity

Three fluid-filled cavities present by day 15.

Primitive Streak

A midline thickening of cells on the surface of the epiblast that appears around day 15.

Primitive Node

The round depression at the cranial end of the primitive streak.

Primitive Pit

The opening within the primitive node.

Primitive Groove

The opening in the center of the primitive streak.

Cranial-caudal, medial-lateral, dorsal-ventral

The major body axes defined by the formation of the primitive streak.

Definitive Endoderm

The first cells to migrate through the primitive streak replace the hypoblast and become this.

Intraembryonic Mesoderm

The next wave of epiblast cells to migrate through the primitive streak fills the middle space and forms this.

Ectoderm

Cells left behind in the epiblast after migration through the primitive streak will become this.

Epithelial-to-Mesenchymal Transition (EMT)

The process where cells convert from epithelial to mesenchymal cells as they migrate through the primitive streak.

Epithelial Cells

A highly organized, connected sheet of cells with apical and basal surfaces.

Mesenchymal Cells

Loosely organized individual cells embedded in an extracellular matrix.

Cell Identity/Fate Determination Factors

The time and location the cells migrate, and what signals they receive.

Trilaminar Germ Disc

The structure formed when the endoderm and mesoderm have moved through the primitive streak, and the remaining epiblast cells are now ectoderm.

Caudal Eminence (Tail Bud)

The caudal structure of pluripotent cells formed by the regressed primitive streak.

Oropharyngeal and Cloacal Membranes

Regions where ectoderm adheres directly to endoderm and mesoderm is absent.

Oropharyngeal and Cloacal Membranes

The future ends of the gut tube (mouth/anus).

Primitive Streak

Forms along caudal midline of bilaminar embryonic disc.

Epiblast cells

Displace hypoblast during gastrulation, and form definitive endoderm.

Ingressing cells

Migrates between epiblast and endoderm to form mesoderm

Mesoderm

Extends cranially from primitive node to form notochordal process

Mesoderm lateral to midline

Paraxial, intermediate, and lateral plate mesoderm

Paraxial Mesoderm

Forms head mesoderm in the future head

Paraxial mesoderm

Forms somites in the future trunk

Layers of lateral plate mesoderm

Somatic mesoderm and splanchnic mesoderm

Notochordal process

Transiently with endoderm to form notochordal plate

Paraxial mesoderm

Coalescing to eventually form in craniocaudal succession a total of 42-44 somites

Notochordal plate

Separates from endoderm to form solid notochord

Neural plate

Expanded cranial part represents future brain, whereas narrow caudal part represents future spinal cord