3. implantation and placentation

Question 1

what is a placenta

Answer 1

a placenta is formed when fetal membranes become closely attached to the uterine wall to facilitate physiological exchange of gases, nutrients and waste products

Question 2

briefly outline how the extraembryonic membranes are developed

Answer 2

Question 3

where is the chorion

Answer 3

where the mesoderm and the trophoblast come into contact
- outermost layer of placenta in all mammalian species

Question 4

the yolk sac is an extension of the

Answer 4

midgut

Question 5

the allantoic sac is an extension of the

Answer 5

hindgut

Question 6

what is the chorioallantoic membrane

Answer 6

where the chorion makes contact with the allantois

Question 7

what is the choriovitalline placenta

Answer 7

where the chorion and the vitalline (yolk sac) meet
- often transient

Question 8

what is the amnion and what is its function

Answer 8

innermost fluid filled membrane
- allows symmetrical growth and protection of foetus
- temperature control and foetal movement

Question 9

what is the yolk sac and what is its function

Answer 9

• reptile and bird eggs: srrounds the yolk
• marsupials: definitive placenta
• higher mammals: transient with nutritional role (doesnt last long)
• role in early pregnancy

Question 10

what is the allantois and what is the function

Answer 10

• sac from hindgut
• connection becomes umbilical cord

Question 11

what is the chorion and what is its function

Answer 11

outer membrane
- becomes foetal contribution to placenta
- all placentas chorionic at some stage of development

Question 12

what are the 3 stages of implantation

Answer 12

• apposition: blastocyst or foetal membranes become closely apposed to uterine lining (epithelium)
• adhesion: a complex biochemical interaction between molecules on the trophoblast and epithelium that alow membrane to attach
• firm attachment OR invasion of trophoblast into uterus: depends on species: non-invasive or invasive

Question 13

what is the difference between invasive and non-invasive implantation

Answer 13

• non invasive: blastocyst undergoes elongation, uterine epithelium alignment, trophoblast layer attaches to epithelial layer of uterue = placenta formation
• invasive: apposition and attachment (no elongation) attaches to uterine epithelium then invades into uterus iself

Question 14

how is the type of placenta classified

Answer 14

• foetal extraembryonic membranes (chorionic, yolk sac and allantoic)
• source of nutrition (hemotrophic: from circulating maternal blood, histotrophic: from endometrium)
• maternal tissue shed with fetal tissue at birth (deciduate vs non-deciduate)
• histological structure (number of tissue layers interposed between maternal and foetal circulations)
• macroscopic: nutrient exchange ares (distribution of chorionic villi or chorionic sac and relationship with endometrium (diffuse, cotyledonary, zonary)

Question 15

how many layers of tissue are possible between the maternal and foetal blood

Answer 15

Question 16

explain the layers between maternal and fetal blood in epitheliochorial, endothelialchorial and haemochorial placentas

Answer 16

• epitheliochorial = sow, mare
• endothelialchroial = bitch
• hemochorial = rodents, rabbit, primates

difference brought about by degree of invasion on fetal side

Question 17

what are the different types of placental villi

Answer 17

• diffuse = nutrient exchange all over
• cotyledonary = clusters all over membrane. between clusters no nutrient exchange. bits that attach to the uterus
• zonary = around centre of the chorionic membrane going around fetus. like a thick belt
• discoid = disk like amalgamation

discoid = primates and rodents

Question 18

describe the placenta in the sow

Answer 18

• true epitheliochorial placentation (6 layers between maternal and fetal blood)
• diffuse (villi distributed over entire membrane)
• non-deciduate( when delivered no maternal tissue delivered with it

Question 19

describe the placenta in the ewe/cow

Answer 19

• synepitheliochorial placentation
• cotyledonary
• non-deciduate

Question 20

what is a synepitheliochorial placenta

Answer 20

fusion of binucleate giant cells to maternal eptithelial cell leading to maternal fetal hybrid cell. granules from within then released into the maternal circulation (placental lactogen)

Question 21

what are placentomes

Answer 21

where the membrane makes contact with the caruncles - cotyledun. caruncle pluse cotyledun = placentom
cow = convex
sheep/goat = concave

Question 22

describe the placenta in the horse

Answer 22

• epitheliochorial placentation
• diffuse
• non-deciduate

Question 23

what is the chorionic girdle

Answer 23

• forms around day 30
• junction between chorionallontois and yolk sac
• visible in mare
• where developing embryo first makes contact with uterus
• moves as allantois overtakes yolk sac

Question 24

what are equine endometrial cups and what do they do

Answer 24

• around day 36-37 junction between 2 plcentsa (girdle) makes contact with uterus
• bunches of cells invade into uterus and form cups
• cups = glands that secrete eCG
• transient (after day 100, surrounded by leukocytes and destroyed) = ECG levels drop

Question 25

describe placentation in the dog

Answer 25

• endotheliochorial placenta (4 layers between maternal and fetal circ)
• zonary
• deciduate

Question 26

describe the placenta in the rodent/rabbit/primate

Answer 26

• haemochorial placentation
• invasive
• decidual

Question 27

what is decidualisation and what species does it occur in

Answer 27

• endometrial stromal cells proliferate and change to become decidual cells
• the decidua is the maternal placenta
• important in communication between mother and fetus and limits invasion of trophoblast
• seen in rodents, rabbits and primates (anything with a haemochorial placenta

Question 28

explain how the umbilical cord works

Answer 28

• umbilical vein carries oxygen rich blood to fetus, bypassing liver into heart the circ
• umbilical arteries carry blood back to placenta (deficient in oxygen back to mother)