early embryonic development Flashcards
When do somites appear and what is the rate they appear at? how many somites do you end up with?
1st pair at 20 days, then grows 3pairs/day at craniocaudal sequence until 42-44 pairs (end of week 5)
some then disappear, leaving 31 pairs - spinal nerves
What does organisation of mesoderm to somites give rise to?
ribs, vertebrae, intercostal muscle
all guides the distribution of organs & nerves
What structures are there in the differentiation of mesoderm?
notochord, somites, intermediate mesoderm & lateral plate mesoderm (somatic, splanchnic)
What does lateral plate mesoderm give rise to?
somatic (top): skeletal muscles (contributes to body structures)
splanchnic (bottom): organs
What does intermediate mesoderm give rise to?
gonads & kidneys
What does paraxial mesoderm give rise to?
somite –> sclerotome, myotome, dermatome
What is the space between splanchnic & somatic mesoderm called?
intraembryonic coelom
How do somites appear?
as a block of mesoderm cells arranged around small cavity
How do somites differentiate into sclerotome?
through ‘organised degeneration’
ventral (front) wall of somite breaks down
How do somites differentiate into dermomyotomes?
further organisation of dorsal portion
forms combined dermomyotome - skin, muscle, tissue
How does dermomyotome separate?
myotome proliferates & migrates (spreads out around body)
dermatome disperses
What are somites?
segments in pairs, either side of axis, surrounding neural tube & notochord
What is segmentation and what does it achieve?
organise mesoderm into somites to give:
vertebrae (spinal cord segment), ribs, intercostal muscles
& guide innervation: distribution of nerves
What is epimere & hypomere?
epimere: supplied by dorsal branch of spinal nerves
hypomere: supplied y ventral branch of spinal nerves
Explain the use of the term ‘dermatome’ in describing the innervation of skeletal muscle in adults
Developmentally: part of somite which gives rise to dermis
Clinically: a strip of skin supplied by a single spinal nerve
Explain the use of the term ‘myotome’ in describing the innervation of skeletal muscle in adults
Developmentally: part of smite which gives rise to muscles
Clinically: muscle / group of muscles supplied by a single spinal nerve
List the different regions of mesoderm
- notochord
- intermediate mesoderm
- lateral plate mesoderm
- paraxial mesoderm
What is the derivative of paraxial mesoderm?
sclerotome, myotome, dermatome
axial skeleton (vertebral column & ribs)
dermis, muscles of A/L wall
some limb muscles
What is the derivative of intermediate mesoderm?
urinary system e.g. kidneys, ureters, gonads
What is the derivative of the lateral plate mesoderm?
Somatic (top): C.T. of limbs, contributes to anterior/lateral body walls (skeletal muscles)
Splanchnic (bottom): (organs) smooth musculature, C.T. & vasculature of gut
Describe the formation of the head, tail & body folds
cephalocaudal folding (head --> tail) so endoderm on inside: gives rise to lining of GI
Describe the formation of the lateral body folds
the sides (determined by size of developing somites) so amniotic cavity (top) folds over yolk sac
What is the purpose of folding? What doe it achieve?
draws together margins of disk
- create ventral body wall (active closure)
- pull amniotic membrane around everything (embryo suspended in sac)
- lining of GI
- heart move from head –> chest
- yolk sac forms connection with gut (umbilical out of belly)
Describe the formation of the coelom
when the embryo undergoes lateral folding & the lateral branches of mesoderm connect to form the coelom (inside embryo where the organs grow within) (intraembryonic coelom join)
