11. Embryology 2 Flashcards
What happens in the embryonic period?
All major structures (limbs) and systems are formed
When and where does the primitive streak appear?
In week 3
Appears on dorsal surface of epiblast
What are the 2 main steps in gastrulation?
Migration and invagination
Displacement of hypoblast and creation of third layer
What happens in migration and invagination?
Epiblast cells push through epithelial layer
What happens in displacement of hypoblast?
3rd layer is formed
Becomes trilaminar disc
Holes from in mesoderm which will layer become mouth and anus
What are the 3 layers that are created in gastrulation?
Ectoderm
Mesoderm
Endoderm
What does the ectoderm form into?
Organs and structures that maintain contact with outside world
E.g. nervous system, epidermis
What does the mesoderm form into?
Supporting tissues
E.g. muscle, cartilage, bone, vascular system
What does the endoderm form into?
Internal structures
E.g. epithelial lining of GI tract, respiratory tract, parenchyma of glands
When does gastrulation occur?
Week 3 of development
Start of embryonic period
What happens in gastrulation?
Bilaminar disc converted to trilaminar disc
3 germ layers: ectoderm, mesoderm, endoderm
How does gastrulation occur?
Primitive streak forms in caudal epiblast, leads to migration and invagination of epiblast cells
Why does gastrulation occur?
Ensure correct placement of precursor tissues to allow subsequent morphogenesis
What is the notochord and when is it formed?
Solid rod of cells running in midline with important signalling role
Formed in neurulation
What does the notochord do?
Releases chemicals so ectoderm can respond, responds by thickening
What are somites?
Organisation of parasail mesoderm into segments
First pair appear at day 20 in occipital region
Appear 3 pairs/day until 42-44 pairs present
How many somites are there in total at the end?
31, therefore 31 pairs of spinal nerves
What happens in organised degeneration of somites?
Ventral wall of somite breaks down leaving to formation of sclerotome
Further organisation of dorsal portion forms combined dermomyotome
What happens to the myotome?
Proliferates and migrates
Dermatologist disperses
What does the dermatome later form?
Skin section, dermis
What does the myotome late form?
Muscle section, muscles
What does the sclerotome later form?
Hard tissue section, bones
What does the organisation of mesoderm into somites give rise to?
Repeating structures
E.g. vertebrae,ribs, intercostal muscles, spinal cord segments
What is the order of folding?
Head fold
Tail fold
Lateral folding
What does folding achieve?
Creates ventral body
Pulls amniotic membrane around disc
Embryo becomes suspended in amniotic sac
Pulls connecting stalk ventrally
What has happened by the end of week 4?
Nervous system started to form
Segments appeared, assigning specific tasks to specific cells
Embryo folded, putting everything in right place
