ZOO 3713C Lecture Notes - Lecture 7: Primitive Streak, Primitive Knot, Notochord
Lecture 7- Gastrulation
Gastrulation
- Epiblast -> Primary Germ Layers
- Ectoderm – outer layer – Skin, Nervous System, etc.
- Mesoderm – middle layer – Muscle, Bones, etc.
- Endoderm – Inner layer- Digestive Tract, Lungs, etc.
- Process – Morphogenetic movements organized cell migration
Primitive Streak
- Embryonic Day 15
- Primitive groove – initiates gastrulation
- Primitive streak – includes groove, node and pit
- The primitive streak defines:
o Anterior – cranial
o Posterior – caudal
o Right and left – lateral
- Streak extends cranially then regresses caudally – depositing the notochordal process
during regression
- The tip of the regressing streak is the primitive pit and the primitive node ( also called
Hensen’s Node)
Endoderm
- First cells to go through the Streak form the endodermal
- These cells integrate and displace hypoblast cells
Mesoderm
- Complex pattern of movements
- Streak formation – lateral migration – cardiac mesoderm
- Streak regression – lateral and cranial migration
o Lateral Plate Mesoderm
o Somitic Mesoderm
- Streak Regression – Central and Cranial Migration
o Notochord – cellular rod, central long axis of embryo
Ectoderm
- Ectodermal cells don’t enter the streak
- Cell layer expands as endodermal and mesodermal cells enter the streak
- Cranial to the notochord – ectoderm and endoderm are in direct contact
o Oropharyngeal membrane
- Between the oropharyngeal membrane and the notochord is the pre-chordal plate –
important for inducing the brain
Embryonic Induction
- Definition: signal from one group of cells influences the development of an adjacent
group of cells
- Inducing Tissue or Inducer
- Inductive Signal – Morphogen
- Responding Tissue
