Third Week of Development Flashcards

1
Q

The third week of development coincides with which week off the menstrual period

A

the 1st missed menstrual period (5th week)

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2
Q

What events occur in the third week of development

A
  • Primitive streak is formed creating the 3 germ layers
  • The notochord, neural tube and somites are formed
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3
Q

What is gastrulation

A
  • This is when the bilaminar disc converts to a tri-laminar disc when the primitive streak is formed
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4
Q

Where is the primitive streak developed?

A
  • On the dorsal aspect (in communication with the amniotic sac), caudally along the midline
  • This includes the primitive streak → groove, node → pit
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5
Q

Explain the process of gastrulation

A
  • 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
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6
Q

Explain the development of the notochord

A
  • 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
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7
Q

What is the purpose of the notochord?

A
  • 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
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8
Q

Where are the three places that no mesodermal cells are found

A
  • Where the notochord is
  • Where the prechordal plate is
  • Where the cloacal plate is
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9
Q

What is neurulation

A
  • The formulation of the neural tube by the thickening of the ectoderm which is complete by the end of the 4th week
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10
Q

Explain the process of neurulation

A
  • 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)
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11
Q

What are Neural Tube Defects (NTD)

A
  • 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
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12
Q

Describe the differentiation of the intra-embryonic mesoderm

A
  • 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)
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13
Q

Describe the development of somites

A
  • 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)
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14
Q

Purpose of the intraembryonic mesodermal layer

A
  • Intraembryonic coelom: gives ride to pericardial, peritoneal and pleural layers
  • Intraembryonic somatic mesoderm + ectoderm = body wall (somatopleure)
  • Intraembryonic splanchnic mesoderm + endoderm = gut wall (splanchnopleure)
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15
Q

What happens in the early development of the cardiovascular system

A
  • 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
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16
Q

What is the difference between vasculogenesis and angiogenesis

A
  • Vasculogenesis is the formation of new vascular channels when hemangioblasts assemble
  • Angiogenesis is the formation of new vessels by budding and branding from pre-existing vessels
17
Q

Explain the different stages of the chorionic villi

A
  • Primary (1º): consists of the syncytiotrophoblast and the cytotrophoblast
  • Secondary (2º): consists of the syncytiotrophoblast, cytotrophoblast and the extraembryonic mesoderm
  • Tertiary (4º): consists of the syncytiotrophoblast, cytotrophoblast, and the mesoderm lined with blood vessels
18
Q

What is the first functional system in the embryo?

A

The cardiovascular system

19
Q

what week marks the circulation between the chorionic villi and the embryo

A

The third week DUH!

20
Q

what is the purpose of the midline on the dorsal surface?

A
  • It produces Shh and Lefty1 to prevent the left-sided genes from moving to the right side of the embryo
21
Q

What is the function of FGF8

A
  • This growth factor is required in left-asymmetric expression of the genes nodal, lefty2 and PITX2
  • Nodal is a gene remaining on the left side of the embryo to increase the expression of Lefty2 which in turn increases the expression of PITX2 on the ventral surface of the embryo