Third Week of Development Flashcards
The third week of development coincides with which week off the menstrual period
the 1st missed menstrual period (5th week)
What events occur in the third week of development
- Primitive streak is formed creating the 3 germ layers
- The notochord, neural tube and somites are formed
What is gastrulation
- This is when the bilaminar disc converts to a tri-laminar disc when the primitive streak is formed
Where is the primitive streak developed?
- On the dorsal aspect (in communication with the amniotic sac), caudally along the midline
- This includes the primitive streak → groove, node → pit
Explain the process of gastrulation
- Epiblast cells thicken to form the primitive streak on the caudal end with a node on the cranial end
- The primitive streak cells die, making this thickening hollow
- Primitive streak = primitive groove and primitive node = primitive pit meaning there’s a space where thickening was
- The cells near the edge of primitive streak secrete FGF8 laterally to bind unto epiblast cells causing the cells to migrate towards the streak downwards replacing the hypoblast. Hypoblast = endoderm and epiblast = ectoderm
- More epiblast cells move through primitive groove in a forward motion, filling in between creating the mesoderm
Explain the development of the notochord
- FGF8 is released and the ectodermal cells migrate through the primitive pit cranially towards the prechordal plate forming a tube which is called the notochord
What is the purpose of the notochord?
- Helps develop the axial skeleton
- Induces the formation of vertebral bodies
- later becomes the nucleus pulposus of the intervertebral discs
- Induces the ectoderm to form the neural plate
Where are the three places that no mesodermal cells are found
- Where the notochord is
- Where the prechordal plate is
- Where the cloacal plate is
What is neurulation
- The formulation of the neural tube by the thickening of the ectoderm which is complete by the end of the 4th week
Explain the process of neurulation
- The notochord releases growth factors for ectodermal cells to proliferate to form the neural plate day 17
- The longitudinal part of plate is pulled downwards = neural groove and transverse part of the plate is folded into itself along the primitive pit to the prechordal plate = neural fold
- Ectodermal cells around the edges of the neural fold → neural crest cells on day 18 cells fuse with each other = neural tube
- Tube goes underneath ectoderm to sit on top of notochord and neural crest cells hovers on top of tube. Day 21
- Tube has cranial and caudal ends still open (neuropores) that are closed off on day 25 (anterior) and day 27 (posterior)
What are Neural Tube Defects (NTD)
- This is when the anterior and/or posterior neuropores fail to close
- Cells use folate to synth. DNA, lack of folate = no DNA synth. = cells won’t close
- Anterior neuropore failure = anencephaly (cephal for head!)
- Posterior neuropore failure = Spina Bifida
- Pregnant women should take 400µg of folic acid per day to prevent NTDs
Describe the differentiation of the intra-embryonic mesoderm
- Mesoderm differentiates to the paraxial, intermediate and the lateral mesoderm
- Top part of lateral mesoderm = intra-embryonic somatic mesoderm
- Bottom part of lateral mesoderm = intra-embryonic splanchnic mesoderm
- The space in between merges on the far end to create ‘holes’ called the intraembryonic coelom
- Paraxial mesoderm = Somites (sclera, myo, and dermatomes)
Describe the development of somites
- Paraxial mesoderm develops and breaks into cubed shaped cells = somites that have a cavity in the center = somatocoeles
- Somites differentiates to the sclerotomes (vertebrae), myotomes (muscles of the neck, body wall and limbs), and dermatomes (dermis of the scalp, neck and trunk)
Purpose of the intraembryonic mesodermal layer
- Intraembryonic coelom: gives ride to pericardial, peritoneal and pleural layers
- Intraembryonic somatic mesoderm + ectoderm = body wall (somatopleure)
- Intraembryonic splanchnic mesoderm + endoderm = gut wall (splanchnopleure)
What happens in the early development of the cardiovascular system
- Consists of vasculogenesis and angiogenesis
- 1º chorionic villi are developed in the 2nd week and are converted into the 2º villi when the extra-embryonic mesodermal cells grow into the 1º villi.
- The extra-embryonic mesoderm differentiated to form blood vessels converting the villi to tertiary