Gastrulation Flashcards
When is each term used: Blastocyst, blastodisc, blastula
Blastocyst- mammals, blastodisc- birds, blastula- amphibians
Gastrulation
- The process of formation of germ layers (from a blastocyst to a gastrula)
- A single layered blastula is converted into a trilaminar structure containing the outer ectoderm, middle mesoderm, and inner endoderm
- Depends on species, but forms within ~12-15 days post-coitum
Mammals vs avian embryonic disc
- Mammals= embryonic disc
- Avian= blastoderm
Keys to Gastrulation/ embryonic development
- Cell division
- Cell migration
Gastrulation in Chickens
- Blastoderm has two layers: area opaca and area pellucida
- Area pellucida forms upper epiblast and lower hypoblast - Epiblast cells divide into mes-endoderm cells and migrate similar to that in mammals forming the primitive streak, primitive node, and primitive groove. This results in right and left sides and the cranial and caudal regions. These mes-endoderm cells also migrate to form the mesoderm (or mesenchymal cells) and the endoderm (which replace the hypoblast region within the intra-embryonic mesoderm region)
- Area Opaca forms the ectoderm layer
How is the cranial-caudal axis and right/left sides formed?
Through the formation of the primitive node and primitive streak
Formation of Primitive streak
Forms when cells converge and thicken on the surface of the blastoderm towards the midline during gastrulation
Formation of Primitive node or Hensen’s node
An increased concentration of cells accumulate at the cranial end during gastrulation
Formation of primitive groove
Cells of the epiblast converge on the primitive streak and enter the space between the epiblast and the hypoblast
Intra-embryonic mesoderm vs. Extra-embryonic mesoderm
- Intra-embryonic mesoderm- mesenchymal cells that stay between the endoderm and ectoderm
- Extra-embryonic mesoderm- mesenchymal cells that migrate laterally out between the hypoblast and trophectoderm
Bottle Cells
Elongated, migratory epiblast cells give rise to the ectoderm which are also called bottle cells
Gastrulation in mammals
- Start with single embryonic disc layer that gives rise to epiblast and hypoblast
Blue cells= epiblast
green cells= hypoblast - Epiblast cells divide and newly divided cells called mes-endoderm cells (orange) begin to move in all directions. Will result in left and right sides, and cranial and caudal regions from formation of primitive streak and primitive node. These mes-endoderm cells (orange) also move to the sides and produce other cells (yellow, red)
Yellow= endoderm cells which will replace the green hypoblast cells in the middle region
Red= mesoderm cells and fill up middle region (also called mesenchymal cells) - After a few hours and previous steps, the epiblast cells will form ectoderm
Other structures outside 3 germ layer region will be covered later
Notochord
- Forms when the primitive streak and primitive node recede
- Composed of epiblast cells which have migrated through the cranial most portion of the primitive streak
- Eventually spans the entire length of the embryo. Cartilaginous structure that secretes key proteins that is important in the formation of the nervous system (spinal cord)
The formation of left-right symmetry in vertebrates
- Developed during gastrulation.
- Gene transcription factors and proteins involved (Most important= situs inversus viscerum= lv, lefty, nodal)
–>Defects or absence of some of these elements cause developmental abnormalities. For Ex. mutations in lv results in random placement of organs on either side of the left-right axis. This is lethal.
Genes needed for left and right formation
- Lefty-1: needed on left side for leftward flow
–> Triggers release of fibroblast growth factor-8 (Fgf-8) which stimulates both Nodal and Lefty-2. Lefty-2 shuts off Snail. Nodal stimulates Pitx-2. - Snail: needed on right side
