2. fertilisation and embryo development Flashcards
how is ovulation induced
spike of plasma LH stimulates ovulation but there are two types
outline the journey of the oocyte from the follicle to the infundibulum
- 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
when can oocytes be fertilised
- 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
how long is the fertilisation period in the bitch
day 2-5 after ovulation
how long is the fertile period in the bitch
5 days before and 5 days after ovulation
what is embryonic diapause
a temporary arrest of embryo development characterised by delayed implantation in the uterus
what is a zygote
following the fusion of the pronuclei: single cell embryo
mitotic divisions in the zygote are different from normal mitotic divisions because:
there is no duplication of cytoplasm as space is limited. genetic material is dublicated, but cells continue to get smaller
after the fist division of the zygote, the cells are called:
blastomeres
identical twins can occur at which stage of development and how
- early stages of blastomere development
- derived from blastomeres of a two-cell embryo that divide independently to form 2 separate embryos
once the blastomeres divide into 16 cells, it is called:
morula
what is meant by totipotent
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
how is the blastocyst formed
- 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
the trophoblast will give rise to
- chorion
- placenta
the inner cell mass will give rise to
embryo
how does the blastocyst hatch
- 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)
outline the different types of progression for the blastocyst in the female tract
- 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
what occurs to the blastocyst after hatching
- 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
outline the stages prior to gastrulation
- ICm gives rise to epiblast and hypoblast
- epiblast forms amniotic ectoderm
- hypoblast form extraembryonic or primitive endoderm
what occurs at gastrulation
- 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
how do the foetal membranes develop
- 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
