Human Embryology 3 Flashcards
when does gastrulation occur? What does it consist of?
third week of development
1) Epiblast movements
2) Primitive streak Formation
3) Segregation of the three germ layers
4) Node Development & Function
what is gastrulation
- coordinated process of cell & tissue migration
- blastula cells are rearranged
- transforms epiblast into 3-layered embryo
- establishment of 3 primary germ layers
what are the 3 primary germ layers?
- ectoderm, mesoderm and endoderm
- building blocks of the body
Ectoderm consist of
- the epidermis, central & peripheral nervous systems, eyes and internal ears, neural crest cells, and many connective tissues of the head.
Mesoderm consist of
Muscle, Bone, Kidneys, Blood
-skeletal muscles, blood cells, lining of blood vessels, all visceral smooth muscular coats
Endodem consist of
Gut, Thyroid, Lungs, Pancreas
- -epithelial linings of respiratory and alimentary (digestive) tracts
- glands opening into the GI tract
- glandular cells of associated organs (liver & pancreas)
Where is the ectoderm, mesoderm, and endoderm tissue in the early gastrula
- above the midline (opposite side of streak)= ecdoderm
- below midline (on side of streak): mesoderm and endoderm
Gastrulation initation
- hypoblasts and epiblasts move in circular fashion (polynation movements), allows accumulation in bottom of embryo
- converging circular motions of epiblast converge at posterior end, leads to primitive streak formation
Gastrulation: Primitive streak Initiation?
- cells converge to form valley/depression (primitive streak)
- streak grows (cordial to cranial); when hit max length develop Hensen’s node at cranial end of streak
Hensen’s node?
- only forms when streak reaches full length
- is circular depression at end cranial streak
- at most anterior end endowed w/ “organizing” capabilities
Gastrulation, EMT & Mesenchymal migration?
-epiblast cells move inward into streak, down the streak, then laterally to dispense between the hypoblast and previous epiblast layer
(invagination)
What happens to hypoblast cells as epiblasts invaginate into the primitive streak?
-they are displaced; and start to become the endoderm germ layer
what does the ectoderm get made out of?
-ectoderm= the previous layer of epiblasts before invagination happened
How make the mesoderm layer?
-the epiblasts cells that invaginated into the primitive streak and remained between the now ectoderm and endoderm layers differentiate to become mesoderm
When do epiblasts stop migrating, inward, down, and laterally into the primitive streak?
-once ectoderm, mesoderm, and endoderm layers are formed
What does the primitive streak represent?
-the first indication of asymmetric Embryonic development
-BUT is clear that asymmetry (axis specification) was
established prior to primitive streak formation
primitive streak formation molecular steps?
- Established in epiblast stages, defines the position of Primitive streak
1) VG1 + Wnt8C induce Nodal in nearby epiblast
2) Nodal inhibited by Cerberus(made by underlying hypoblast )
3) before primitive streak formation, the hypoblast are displaced by non-Cerberus-expressing endoblast
4) Nodal now signals to act Nodal + FGF induce ingression of cells from epiblast to form primitive streak
How is Hensen’s node an organizer? How prove it?
-transplanted the node from frog (A) into frog (B) embryo, put in opposite end (cordial, posterior)
-now frog B embryo has TWO nodes, made 2 weird embryos
-
Conclusion of the node transplant exp
- Tissue (node) they transplanted organized HOST cells around it, changed their function from making belly…to making neural tube, endorser etc for new embryo
- if original tissue cells were left unintterupted, would have made belly, but instead the node from diff tissue organized host cells, made new axis, new embryo
- ended up with 2 weird embryos from 2 different tissue samples
- NODE CAN FORM FULL SECONDARY AXIS
Inducer vs organizer molecules
Organizer: Cells/Tissue able to force the change of fate of other cells
Inducer:
act on organizer signals, are the molecules responsible for Organizer effect
What is the notochord? How made? Where positioned & functions?
1) Defining structure from chordate embryos
2) axial mesoderm, derived from the node, laid down as the node regresses
3) Medially positioned signaling center that helps form adjacent tissues (i.e. Neural Tube)
Role of notochord (x5)
1) structural support to the embryo
2) “Cartilage related” serves as axial skeleton of embryo until vertebrae forms
3) Defines the primordial longitudinal axis of the embryo
4) Contributes to nucleus pulposus of the intervertebral disc of vertebral column
5) helps form adjacent tissues (i.e. Neural Tube)
notochord present in adults?
- Nope
- Is transient, not present in adults
mesendoderm movement?
-Mesendoderm ingression starts with PS growth
-
mesendoderm?
- both mesoderm & endoederm cells
- the cells that go through primitive streak & invaginate to from the two germ layers
What happens to mesendoderm when primitive streak (PS) reaches max length?
maximum length (most anterior position):
1) mesendoderm starts to regress
2) Newly formed Node generate mesoderm cells that move anteriorly (toward cranial end) to generate notochordal process under the ectoderm
Notochord develop into?
- has organizational role in nervous system development
- becomes part of the vertebral column
- invovled with proper axis development of embryo
Mesoderm Derivatives?
1) Notochord- Axial Mesoderm
2) Somites - paraxial mesoderm
3) Intermediate mesoderm
4) Lateral mesoderm - somatic & splanchnic
5) Intraembryonic coelom
Contributions of mesoderm derivatives?
1) Head: Portions of cranium & head connective tissue
2) paraxial mesoderm: Muscles of Head, Dermis of Skin, Connective Tissue
3) intermediate mesoderm:Urogenital System (Gonads, Ducts, Glands)
4) lateral mesoderm: Connective Tissue, heart, steel
5) axial mesoderm: Embryonic cartilagenous structural support, and nucleus of intervertebral disc
