Gastrulation Flashcards
Week 3
“the week of threes”
Three cavities
Three “germ” layers
- rudimentary lineages from which all others will arise
Gastrulation
- the process establishing the three germ layers and hence the origin of all tissues of the body
- trilaminar disk
- sets the axes observed in the adult
Beginning of gastrulation
Primitive streak appears (in the third week) - primitive node with primitive pit Cellular rearrangement - migration - invagination
Primitive streak, pit & node
- streak is a narrow groove with bulging edges
- node is located on the cranial end of the streak
- pit is located at the centre of the node
Once the three layers are established the primitive streak regresses
Formation the trilaminar disk
Epiblast cells migrate and differentiate to the edges of the primitive streak - they displace the hypoblast and create a third layer
- fate of the invaginating epiblast depends on where in the steak or node they invaginate
Trilaminar disk
The ectoderm/endoderm can be seen in two places when looking at the mesoderm from below/above
The notochord
- defines the phylum Chordata
- basis for axial skeleton - defines the midline
- drives the formation of the nervous system
- is what herniates when a disk slips
- it regresses - vestigial remnant in the adult is the nucleus pulposus of intervertebral discs
Ectoderm
- organs and structures that maintain contact with the outside world
E.g. Nervous system, epidermis
Mesoderm
- supporting tissue
E.g. Muscle, cartilage, bone, heart and vessels
Endoderm
- internal structures
E.g. Epithelial lining of GI tract, respiratory tract
Side-specific signalling cascade
- action of ciliated cells at the node results in a leftward flow of signalling molecules
Situs Inversus
heart appears on RHS
- commonly results from immobile cilia
- problems arise if there is both normal and mirror-image disposition
Monozygotic Twins
- embryo splits after first cleavage – two embryos each with its own placenta
OR - inner cell mass duplicated – two embryos sharing a placenta
Splitting can occur very late (embryos can share the same amniotic sac)
Sometimes separation is not complete - conjoined twins
Teratogenesis
- normal embryonic development is disrupted
- week 3 to 8 is the most sensitive to teratogenic insult
- each organ system will have a particular sensitive window
Teratogenic Agents
Chemical and infectious agents are known to cause developmental defects
- thalidomide (limbs)
- rubella (head and neck)
- alcohol (CNS)
- certain therapeutic drugs