2. fertilisation and embryo development

Question 1

how is ovulation induced

Answer 1

spike of plasma LH stimulates ovulation but there are two types

Question 2

outline the journey of the oocyte from the follicle to the infundibulum

Answer 2

• follicular wall bursts
• oocyte and GC released from GC anchor point
• oocyte carried with follicular fluid into the peritoneal cavity
• captured by fimbria of the infundibulum

Question 3

when can oocytes be fertilised

Answer 3

• the fertilisation period is the time when oocytes are availiuble to be fertilised by sperm. in most species this is immediately after ovulation for up to hours or days
• the fertile period is the time when mating could result in pregnancy

Question 4

how long is the fertilisation period in the bitch

Answer 4

day 2-5 after ovulation

Question 5

how long is the fertile period in the bitch

Answer 5

5 days before and 5 days after ovulation

Question 6

what is embryonic diapause

Answer 6

a temporary arrest of embryo development characterised by delayed implantation in the uterus

Question 7

what is a zygote

Answer 7

following the fusion of the pronuclei: single cell embryo

Question 8

mitotic divisions in the zygote are different from normal mitotic divisions because:

Answer 8

there is no duplication of cytoplasm as space is limited. genetic material is dublicated, but cells continue to get smaller

Question 9

after the fist division of the zygote, the cells are called:

Answer 9

blastomeres

Question 10

identical twins can occur at which stage of development and how

Answer 10

• early stages of blastomere development
• derived from blastomeres of a two-cell embryo that divide independently to form 2 separate embryos

Question 11

once the blastomeres divide into 16 cells, it is called:

Answer 11

morula

Question 12

what is meant by totipotent

Answer 12

a stage wherein all cells have the ability to form all cells necessary for normal development
- not demonstrated beyond the 16 cell stage (morula), once this stage is reached, cells begin to differentiate and specialise

Question 13

how is the blastocyst formed

Answer 13

• when a distinct cavity is formed
• outer cells within the morula are more squashed than inner cells
• outer cells form cell-cell tight junctions
• inner cells have looser lines of communication (gap junction)
• outer cells pump sodium into the morula, ionic concentration therefore rises and water then diffuses through the zona pellucida (= fluid accumulation)
• the gap junctions connecting the inner cells allow these cells to polarise as a group
• the outer cells become the trophoblast
• inner cells become the inner cell mass

Question 14

the trophoblast will give rise to

Answer 14

• chorion
• placenta

Question 15

the inner cell mass will give rise to

Answer 15

embryo

Question 16

how does the blastocyst hatch

Answer 16

• the blastocyst continues to undergo mitosis and fluid continues to be produced (increasing pressure)
• proteolytic enzymes are produced by trophoblast
• zona pellucida weakens
• zona pellucida splits and the blastocyst emerges (hatch)

Question 17

outline the different types of progression for the blastocyst in the female tract

Answer 17

• slow continuous (rabbit, sow)
• discontinuous (cow, ewe) = prolonged stay in the uterine tube. antiperistaltic contractions maintain egg for 2-3 days, stronger peristaltic contractions then transport the egg onward (doesnt matter if egg is fertilised or not)
• fertilised progression (mare) = only fertilised eggs pass into uterus, embryo secretes prostaglandins E and F that are hypothesized to promote embryo migration through oviduct as well as in the uterus

Question 18

what occurs to the blastocyst after hatching

Answer 18

• becomes a free gloating embryo (no placenta yet)
• secretions derived from endometrial glands provide nutritional support before the placenta is established (uterine milk)
• most species have prolonged pre-attachment phase
• conceptus undergoes massive growth

Question 19

outline the stages prior to gastrulation

Answer 19

1. ICm gives rise to epiblast and hypoblast
2. epiblast forms amniotic ectoderm
3. hypoblast form extraembryonic or primitive endoderm

Question 20

what occurs at gastrulation

Answer 20

• begins at the posterior end of the embryo where the node arises
• cells from the epiblast migrate through the primitive streak and differentiate into ectoderm, mesoderm and endoderm

Question 21

how do the foetal membranes develop

Answer 21

• primitive endoderm essentiallt form a lining and when complete results in cavity called the yolk sac
• it does not contain yolk but is analogous to that in birds
• it enlarges to form a reserve for nutrient transfer
• extraembryonic mesoderm forms a sac that surrounds the yolk sac
• extraembryonic mesoderm continues to grow and forms a sac that surrounds the yolk sac
• it also folds dorsally to form the amniotic folds
• a third fluid filled sac forms from an out-pouching of the hind gut (allantois)
• the allantois makes contact with the chorion and eventially fuses to form the chorioallantoic membrane
• the amniotic folds from the amnion
• yolk sac regresses