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Flashcards in Exam 2 Deck (122)
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1

Zones of the mesoderm

Paraxial: thick column of mesoderm closest to and parallel with the notochord. Becomes segmented into somites
Intermediate: narrow column of mesoderm that is lateral to the paraxial mesoderm and gives rise to urogenital system
Lateral plate: thin plate of mesoderm lateral to intermediate mesoderm and splits to form lining of body cavities and mesoderm of most internal organs as well as limbs

2

Somitomeres

initial pairs of segments of mesenchyme that develop in the paraxial mesoderm
new ones are added caudally as the primitive streak regresses
11 pairs are kept at the caudal end

3

Somites

after 20 pairs of somitomeres have formed, the first somite form caudal to the 7th pair of somitomeres at the expense of the 8th.

4

Wavefront mechanism in somite differentiation

FGF-8 is increased in posterior primitive streak
RA is increased anteriorly and opposes FGF-8
this balance results in the cessation of somitogenesis
Mesp-2 is expressed because of this balance

5

Segmental clock and somite differentiation

At critical locations that will divide adjacent somites, lunatic fringe becomes concentrated at the future anterior border
C-hairy becomes concentrated at the future posterior border
Cells at the anterior border of an established somite express Eph A (receptor) and posterior cells express Eph B (ligand) creating a fissure.

6

Steps in somite differentiation

Formation of somitocoel
Formation of sclerotome
formation of dermomyotome and then separation into dorsal and ventral

7

Dermomyotome

Dorso-lateral part of a somite
Shh from notochord and Wnt from dorsal neural tube create balance that makes myotome commit to myogenetic lineage
Noggin inhibits BMP-4 which would normally inhibit myogenesis
BMP-4 suppresses myogenesis in verntrolateral dermomyotome and stimulates cells to migrate to limb bud
FGF from myotome signals production of scleraxis which causes formation of syndetome, the precursor of tendons

8

Intermediate mesoderm

Associated with the formation of the pronephros and mesonephros
Responds to BMP and activin -> becomes intermediate mesoderm -> expresses Pax2
Cranial and caudal extent is dependent on expression of Hox4 - Hox11

9

Which cells form the outflow tract of the heart?

Cells migrating through the anterior primitive streak

10

Which cells form the ventricles of the heart?

Cells migrating through the middle of the primitive streak

11

Which cells form the atria of the heart?

Cells migrating through the posterior primitive streak

12

Early heart formation

BMPs and FGFs act on the anterior visceral endoderm and cause the commitment to the heart-forming pathway.
Cells of the cardiac crescent (cells that migrated through the primitive streak) then express Nkx2-5, MEF2 and GATA4 which are necessary for heart formation.

13

What does the cardiogenic plate arise from?

splanchnic mesoderm

14

Which layer of a cardiac tube forms the myocardium?

Outer layer

15

Which layer of a cardiac tube forms the endocardium?

Inner layer

16

What is the source of pericardium and myocardial fibroblasts?

Proepicardial primordium

17

What circulatory arcs are present in a 4 week embryo?

Vitelline arc - vitelline vessels
Allantoic arc - allantoic vessels
Embryonic arc - dorsal aorta, aortic arches, anterior cardinal vein, common cardinal veins, posterior cardinal veins, atrium, ventricle and ventral aorta

18

Extraembryonic tissues

Amnion (inner cell mass: epiblast derivative)
Yolk sac (inner cell mass: hypoblast derivative)
Chorion (part of fetal maternal interface)
Allantois (inner cell mass: interfaces with placenta via umbilical cord)

19

What makes up the fetal-maternal interface?

Placenta (trophoblast derivative)
Chorion (trophoblast derivative)

20

Previllous embryo (placenta)

No villi have been formed on the trophoblast

21

Primary villous stage (placenta)

Solid, cytotrophoblastic, ectodermal primary villi appear

22

Secondary villous stage (placenta)

Mesodermal cores appear within the primary villi

23

Tertiary villous stage (placenta)

Characterized by the appearance of blood vessels within the mesenchymal core of the secondary villi

24

Final development of the placenta

Cytotrophoblastic columns
Cytotrophoblastic shells formed by the expansion of the columns over maternal decidual cells
Anchoring villi

25

Maternal-Fetal blood flow pattern

Maternal blood enters intervillous space from spiral arteries
Exchange of materials between maternal blood in lacunae and fetal blood in the capillaries in the villi
Maternal blood retruns to maternal veins in the decidua basalis
Fetal blood travels to capillary beds within the placental villi via umbilical arteries and returns via umbilical vein

26

Functions of the placenta

Diffusion of oxygen and carbon dioxide
Diffusion of foodstuffs
Excretion of waste products

27

Early vs late placenta

Early: thick, low permeability, small surface area, minimal diffusion
Late: thin, high permeability, large surface area, increase in diffusion

28

Why can oxygenation occur despite the low pressure gradient between mother and fetus?

Fetal Hb has a higher affinity for oxygen
Fetal blood Hb concentration is 50% greater
Bohr effect (pH)

29

Human Chorionic Gonasotropin

Secreted by syncytial trophoblasts into maternal fluids
starts 8-9 days after ovulation
max secretion 10-12 wks of pregnancy

30

Functions of HCG

Prevents involution of corpus luteum
causes CL to increase secretion of progesterone and estrogen
causes increased growth in CL
exerts interstitial cell-stimulating effect on testes of male fetuses resulting in a production of testosterone