Microanatomy 3

Question 1

what is the placenta

Answer 1

• this is a temporary structure which arises from both fetal and maternal cells

Question 2

what is the function of the placenta

Answer 2

• nutrient trasnrpot
• waste removal
• gases transport
• site of synthesis for a variety of molecules including hCG and relaxin and steroid hormones particular oestrogen’s and progesterone

Question 3

what week does the placenta assume the role of steroid hormone synthesis

Answer 3

12 week (between the 2nd and third months of pregnancy) -
- takes over form the corpus luteum which degenerates and becomes the corpus albicans

Question 4

what are the keys stages in placentation

Answer 4

• implantation and invasion into uterine tissues
• differentiation of the trophoblast
• develop of the villous structure
• remodelling of the spiral arteries
• maturation

Question 5

describe the development of the blastocyst

Answer 5

• fertilising occurs in the ampulla of the uterine tube
• 3-4 days after fertilisation the morula arrives in the uterine body where implantation occurs
• by day 5 the blastocyst has formed and has hatched through the zone pelluida
• at the blastocyst stage the blastomeres of the inner cell mass are also called the embryo blast as they give rise to the embryo proper
• peripheral blastomeres constitute the trophoblast which gives rise to the fetal component of the placenta

Question 6

when does implantation occur

Answer 6

• when the endometrium mucosa is sufficiently prepared for implantation in the secretory phase of the central cycle - days 22-24
• it can provide a suitable cellular and nutrient environment for the embryo, the trophoblast cells over the inner cell mass adhere to its surface

Question 7

what is the stromal reaction

Answer 7

• this is also called the decidual reaction
• involves the thickening of the endometrium at the site of implantation
• local endometrial glands enlarge
• the area become more vascularised and oedematous
• inflammatory cells also invade
• secretions of the decidual cells and the endometrial glands are through to support the growth of implanting embryo

Question 8

what is the stormal reaction also called

Answer 8

the decidual reaction

Question 9

what do the stromal cells differentiate into

Answer 9

• the stromal cells of the endometrium differentiated into metabolically active secretory cells called decidual cells
• by the end of two weeks, storm cell changes and increased vascularisation spreads throughout the endometrium which is now known as the decidua

Question 10

describe the differentiation of the trophoblast

Answer 10

• happens around day 7
• the trophoblast at the embryonic pole proliferates when it is contact with the endometrium
• some of the cell loose there plasma membranes and form a syncytium called the syncytiotrophoblast
• the cells lining the blastocyst are now called the cytotrophoblast
• the synctitrophoblast starts to invade and erode the underlying endometrium
• the rapidly proliferating cut-trophoblast cells will continue to add to the non dividing mass of the syncytiotrophoblast

Question 11

what does the trophoblast divide into

Answer 11

• syncytiotrophoblast

- cytotrophoblast

Question 12

when does trophoblast differentiation happen

Answer 12

day 7

Question 13

what does the embryoblsat differntatie into

Answer 13

epiblast

hypoblast

Question 14

what does the epiblast and hypoblast form

Answer 14

• bilayered embryonic disc

- and form two extraembyronic membranes that enclose the amnionic and yolk sac cavities

Question 15

what draws the blastocyst into the endometrial mucosa

Answer 15

• the syncytiotrophoblast continues to erode into the endometrium and its fingerlike projects draw the blastocyst in to the endometrial mucosa
• by day 9 only a small fibrin clot marks the post the blastocyst implanted

Question 16

by what day does the syntriotrophoblast cover the entire blastocyst

Answer 16

• around day 11

Question 17

what happens once the synctiotrophoblasts cover the entire blastocyst

Answer 17

• one it covers the entire blastocyst it begins to develop vacuoles called trophoblastic lacunae
• the maternal capillaries widen to form sinusoids and are eroded by the syncytiotrophoblast causing the trophoblastic lacunae to fill with blood
• at the same time the extra embryonic mesoderm begins to form under the cytotrophoblast

Question 18

how does the formation of chorionic villi take place

Answer 18

• the extra embryonic mesoderm induces the overlying cut-trophoblast to proliferate and form projects that extend into the trophoblastic lacunae
• the trophoblast layers with the underlying extra embryonic mesoderm forms the chorion
• these projects of the trophoblast are therefore called the primary chorionic stem vili

Question 19

why do we form chorionic vili

Answer 19

• embryo is rapidly growing in week 2 therefore there is need for a more efficient nutritional and gas exchange
• hypoxia stimulates placental villus invasion into the endometrium

Question 20

where and when does the chorionic cavity develop

Answer 20

• by 16 days

- develops in the middle of the extra embryonic mesoderm

Question 21

what forms the three primary germ layers of the embryo

Answer 21

• gastrulation

Question 22

what suspends the embryo in the chorionic cavity

Answer 22

• the embryo is now suspended in the chorionic cavity by a connecting stalk of extra embryonic mesoderm

Question 23

what covers primary chorionic stem vili into secondary chorionic stem vili

Answer 23

• the extra embryonic mesoderm penetrates the core of the primary chorionic stem vili transforming them into secondary chorionic stem vili

Question 24

how does the cytotrophoblast shell form

Answer 24

• the columns of cytotrophoblast invading into the synctiotrophoblast extend out to the periphery forming the interface between the trophoblast and the endometrium
• this is the cytotrophoblast shell

Question 25

what does the secondary chorionic villus differentiate into

Answer 25

- by day 21 the extra embryonic mesenchymal core of the secondary chorionic villus differentiates into connective tissue and blood vessels - this vili is called tertiary stem vili

Question 26

what do the tertiary stem vili develop into

Answer 26

- these connect with vessels forming the embryo proper linked to the primitive heart

Question 27

when does the primitive heart start beating

Answer 27

day 22

Question 28

what are the four tissues materials must pass

Answer 28

- the endothelium of the villus capillaries - the loose connective tissue in the core - a layer of cytotrophoblast - a layer of syncytiotrophoblast

Question 29

how does remodelling of spiral arteries happen

Answer 29

- the extravillus trophophoblasts invade the spiral arteries and replace the smooth muscle and endothelium there with trophoblasts - this creates high flow low resistance vessels which supply the lacunae with maternal blood

Question 30

what do trophoblasts do in spiral arteries

Answer 30

- this creates high flow low resistance vessels which supply the lacunae with maternal blood

Question 31

where the embryo is implanted what is the maternal endometrium called

Answer 31

the decidua basalis

Question 32

on the opposite pole to the embryo what is the endometrium called

Answer 32

decidua capsularis

Question 33

what happens to the placenta at 4 and 1/2 weeks

Answer 33

- the chorionic vili extend completely around the growing embryo into both decidua

Question 34

what happens with further maturation to the decidua capsularis

Answer 34

- it looses its chorionic vili and vascular role of rate fetus

Question 35

what does the decidua basalis do in further maturation

Answer 35

- with its chorionic vili it form the mature disc shaped placenta

Question 36

when is the placenta fully developed

Answer 36

from the fourth month onwards

Question 37

what happens to the placenta after it is fully developed

Answer 37

- it will only increase in diameter as uterine growth proceeds - the cytotrophoblast layer within the villi becomes smaller - the barrier between the maternal blood in the lacunae and capillaries can be reduced to a layer of synctiotrophoblasts and the endothelium of the foetal capillary is separated only by a shared basement membrane

Question 38

describe the umbilical arteries

Answer 38

- have a smaller diameter and thicker muscular wall than the veins

Question 39

what are the umbilical arteries and veins supported by

Answer 39

- they are supported by a mucoid connective tissue called Wharton's jelly

Question 40

describe the structure of Wharton's jelly

Answer 40

it is mainly made of ground substance with fibroblasts - like precursors "mesenchymal cells"