week 2: bilaminar disk and embryo annexes

Question 1

factors affecting successful implantation

Answer 1

• 25% morphology
• 25% kinetics
• 50% genetics

Question 2

immunogenic dilemma

Answer 2

• systematic immunocompetence and protection vs. uterine immunotolerance
• symbiosis between semi-allogenic individuals
• how fetus avoids immune rejection remains paradox

Question 3

factors preventing host-versus-graft rejection

Answer 3

• fetal and maternal cells are physically separated from one another
• embryo is antigenically immature: poor MHC molecules expression
• maternal immune system is locally immunosuppressed
• extracellular vesicles and microRNAs

Question 4

chorionic gonadotropines

Answer 4

• major role in LOCAL suppression
• hCG produced by trophoblast suppress the immune reactions of maternal WBCs
• embryo produced immunisuppressors
• additional molecules

Question 5

proliferation of trophoblast

Answer 5

• stimulated by interaction with uterine mucosa
• decidua: endometrium in pregnant woman
• implantation occurs on the side of embryonic plate
• trophoblast differentiates while invading the endometrium into syncytio-trophoblast and cyto-trophoblast in implanting blastocyst of 6 days

Question 6

additional differentiation of ICM in blastocyst

Answer 6

• epiblast: cylindrical epithelium like tissue
• and hypoblast: flat epithelium like tissue
• bilaminar disk

Question 7

prechordal plate

Answer 7

• not all cells of hypoblast are equal
• a thickening of endoderm at the cranial end of primitive streak
• origin of rostral cranial mesoderm

Question 8

two cavities formed in the second week

Answer 8

• amniotic cavity
• yolk sac

Question 9

primitive amniotic cavity

Answer 9

• by 8 days the amniotic cavity has appeared within the epiblast
• epiblast cells differentiate into amnioblasts that line cavity and form amniotic membrane

Question 10

day 9 and primary yolk sac

Answer 10

• by 9 days the embryo is completely implanted
• hypoblast cells proliferate and migrate out over cytotrophoblast to form Heuser’s membrane around yolk sac and extraembryonic mesoderm
• trophoblastic lacunae appear in syncytiotrophoblast which now completely surrounds embryo
• point of implantation is marked by a transient coagulation plug in endometrial surface

Question 11

formation of extraembryonic mesoderm

Answer 11

• day 10-11: space between Heuser’s membrane and the cytotrophoblast fills with loosely associated extraembryonic mesodermal cells
• 11-12: mesoderm expands between amnion and cytotrophoblast also
• forms between embryo and yolc sac in CTB
• initially continuous layer then spaces appear causing separation into two
• splanchnopleuric (visceral) and somatopleuric (parietal) mesoderm
• splanchno around yolk sac and embryo
• somato lining the cytotrophoblast

Question 12

which tissue forms extra-embryonic mesoderm

Answer 12

• from hypoblast/contribution from trophoblast in humans

Question 13

extra-embryonic cavity

Answer 13

• squeezes the yolk sac
• chorionic cavity:
• ex-e coelom and medoserm

Question 14

secondary yolk sac

Answer 14

• primary yolk sac breaks up and is reduced to a collection of vesicles at the abemrbyionic end of the chorionic cavity
• secondary (definitive yolk sac) is formed

Question 15

trophoblast derivatives

Answer 15

• syncytiotrophoblast erodes the endometrium, forms lacunae
• cytotrophoblast provides additional cells to syncy

Question 16

lacunae

Answer 16

• fill with maternal blood
• within a few days, as syncy erodes uterine vessels
• finger like projections of syncy covering a core of cyto project into blood filled regions: primary villi: most form on the embryonic pole
• 12 days
• embryo now susceptible to infections

Question 17

membranes functions

Answer 17

• mechanical protection and hydration: amnion
• nutrition and vascularisation: yolk sac (umbilical cord + hemopoiesis + PGC)
• waste disposal: allantois: ambilical cord ligament
• delimitation and respiration: chorion: fetal part of placenta

Question 18

development of amnion

Answer 18

• amniotic fluid (water/liquor amnii) protects embryo
• 25 ml at 10wks to 400ml at 20wks
• contains electrolytes, proteins, carbs, phospholipids, urea…

Question 18

trophoblast and the embryo at the end of w2

Answer 18

• 2 germ layers
• 2 cavities
• 2 out of 3 embryo annexes: yolk sac, chorion (placenta) and yet to come the allantois
• the amnion
• the extra-embryonic cavity will be taken over by embryo and amnion

Question 19

yolk sac development

Answer 19

• ultimately yolk sac is incorporated into umbilical cord
• hosts primitive angiogenesis: primitive cardiovascular system
• yolk sac mesenchyme cells - blood islands - central and peripheral cells
• central to primitive blood cell - blood cell
• peri to endothelial cell - vessels
• endothelial tubes of intra and extraembryo are connected by connective body stalk
• yolk sac hosts information of the PGC

Question 20

development of allantois

Answer 20

• waste disposal
• allantoid is initially a canal that drains the urinary bladder of fetus that runs within umbilical cord
• contributes to the formation of umbilical cord vessels
• provides ligaments and vessels to UC
• support: postnatal remnant of allantois: converted to a fibrous structure: the umbilical ligament supporting the bladder

Question 21

early trophoblast (placental development)

Answer 21

• uteroplacental circulatory system begins 11-13 days: cytotrophoblast cells proliferate budding into overlaying syncy:
• formation of the primary chorionic stem villi occurs
• extraembryonic mesoderm penetrates core transforming them to secondary chorionic stem villi
• at end of W3 villous mesoderm forms blood vessels establishing uteroplacental circulaion
• villi w blood vessels are tertiary

Question 22

tertiary villi

Answer 22

• vessels greatly increase efficiency
• mean diffusivity is higher in uterine wall than placenta: high levels of perfusion in maternal vessels

Question 23

amniochorionic membrane

Answer 23

• at 12 weeks the chorion laeve (smooth chorion) fuses with the decidua parietalis compressed by the amnios and getting poorly vasculorised
• expansion of amnion: fusion with smooth chorion: amniochorionic membrane: ruptures before birth

Question 23

cotyledons

Answer 23

• The maternal cotyledons, or lobes, of the placenta are on the maternal side of the surface
• Each cotyledon is a perfusion chamber partly or wholly separated from adjacent chambers by a wall of connective tissue, which may be lined by trophoblast cells

Question 24

cotyledon development

Answer 24

• 4-5 months
• decidual septa outgrow into intervillous spaces without reaching the chorionic plate
• as a result placeneta is divided into compartments: cotyledons

Question 25

functions of the placenta

Answer 25

• gas exchange
• nutrient and electrolyte exchange
• transmission of maternal antibodies
• hormone production: progesterone, oestrogen, hCG, somatotropin

Question 26

placenta

Answer 26

• made up by embryonic chorion frondosum (villous chorion) and maternal decidua basilis
• maternal and fetal tissues form two closely bound units
• chorionic plate consists of the amnion, extra-embryonic mesoderm, cytotrophoblast and syncytiotrophoblast
• decidua basalis contains uterine vessels and glands

Question 27

cellular bases of placental barrier

Answer 27

• barrier formed by the endothelium of fetal capillaries, connective tissue, trophoblasts
• permeable to many viruses, bacteria and teratogens

Question 28

spontaneous twinning

Answer 28

• monozygotic (3-4/1000)
• dipolyzygotic (7-11/1000 increasing w maternal age
• only 29% women pregnant w twins give birth
• 10-20% die at birth
• 12% premature infants are twins