Fertilisation and early embryo development Flashcards
What is the fertilisastion period?
The time when oocytes are available to be fertilised by sperm
how long is the fertile life span of sperm in the bitch?
5-11 days (prolonged)
How long is the fertile lifespan of sperm in the cow?
1-2 days
How long is the fertile life span of sperm in the mare?
4-5 days
What is different about the time of ovulation in dogs?
Ovulation occurs at metaphase of meiosis I (immature oocytes)
Completion of meiosis II and formation of second polar body occurs after fertilisation
What is embryonic diapause?
Temporary arrest of embryo development characterised by delayed implantation in the uterus to allow birth at time for optimal survival chance for offspring
What causes embryonic diapause?
Descreased daylight => increased melatonin => increase in prolactin => luteal suppressive effect => decreased progesterone => embryonic diapause
What ends embryonic diapause in carnivores?
increased daylight decrease melatonin => increased prolactin => luteotrophic effects => increased progesterone => embryo reactivated
Where does fertilisation usually occur?
first 1/3 of oviduct
What is an ootid?
When both pronuclei from the sperm and egg are visible
What is syngamy?
Egg and sperm nuclei fuse
What occurs to the sperm in fertilisation?
Sperm capacitation - removal of glycoprotein coat and alteration of sperm mitochondria => hypermotile
Sperm binding - ZP3 acts as a receptor
Acrosome reaction - loss of acrosomal contents, enzymes digest zona pellucida, exposed sperm surface proteins bind to ZP2
What is an embryo called a zygote?
When the pronuclei of then sperm and egg are fused
Describe the process of the first cleavage division
Zygote becomes a 2-cell embryo via mitotic division
DNA replicates but cytoplasm does not so the individual cells (blastomeres) have a lower volume
How do identical twins develop?
In the 2-cell embryo, each blastomere has the potential to develop into separate offspring by dividing independently to form 2 separate embryos
How does the morula form?
The 2-celled embryo undergoes more cleavage divisions within the zona pellucida eventually resulting in a 16 celled embryo = morula
What is totipotency and where is it found?
Blastomeres have the ability to form all cells necessary for normal development
Not seen beyond the 16-cell stage (morula)
Label the early cell divisions of the embry
a - zygote
b - 2 cell embryo
Describe the formation of the blastocyst
The outer cells within the morula are more squashed than the inner cells
- outer cells form tight junctions
- inner cells have looser communication lines cause gap junctions
- outer cells pump sodium into morula from extracellular space
- ionic conc rises and water diffuses into zona pellucida (fluid accumulation)
- fluid forms a distinct cavity => blastocyst
Describe the cell types within a blastocyst and what they give rise to
Outer cells = trophoblast => chorion and placenta
Inner cells = inner cell mass => embryo
Describe the process of hatching of the blastocyst
Blastocyst continues to undergo mitosis and fluid production => increased pressure
Proteolytic enzymes produced by trophoblast
Zona pellucida weakens and splits => hatching of blastocyst
Describe discontinuous progression of the egg in cows and ewes
Describe fertilised progression of the egg in mares
What happens to the blastocyst after hatching?
becomes free floating embryo
secretions derived from endometrial glands (histiotroph) provide nutritional support before placenta is established
What tissues arise from the inner cell mass of the blastocyst?
ICM gives rise to epiblast and hypoblast:
- epiblast gives rise to amnionic ectoderm (amnion) and the primary germ layers
- hypoblast gives rise to extraembyonic or primitive endoderm (yolk sac)
Describe the formation of the yolk sac, amnion and allantois
The primitive endoderm forms a lining which results in a cavity (yolk sac) when complete.
The extra embryonic mesoderm grows around the yolk sac and also fold dorsally to form the amniotic folds which forms the amnion.
A 3rd fluid filled sac forms from an outpouching of the hind gut (allantois)
The yolk sac regresses
How does the chorioallantoic membrane form?
The allantois makes contact with the chorion
What is the chorion?
membranes between the developing fetus and mother
Formed by extraembryonic mesoderm and layers of trophoblast
Describe the process of gastrulation
Begins at posterior end of embryo, where the node arises
Cells from the epiblast migrate through the primitive streak and differentiate in ectoderm, mesoderm and endoderm
