Flashcards in Embryology 2 Deck (49):
Where does fertilisation normally occur?
Ampullary region of Fallopian tube
How long are sperm and secondary oocytes viable?
Sperm viable for 5 days
2 oocytes viable for 12-24hrs
How many sperm enter the vagina during intercourse?
Which obstacles prevent sperm fertilising an ovum?
1. Millions of sperm flow out of vagina or die in its acidic environment.
2. Many sperm are trapped in cervical mucus.
3. Resident immune cells of uterus destroy 1000s of sperm.
4. Sperm must swim against current of ciliary beating propelling egg down Fallopian tube - some get trapped in cilia.
Which factors facilitate sperm fertilising an ovum?
1. Cervix opens during the few days of ovulation and the cervical mucus is thinned to a more watery consistency.
2. Muscular uterine contractions propel sperm towards egg.
What must happen for sperm to be capable of fertilisation?
Capacitation: Chemicals (progesterone) in reproductive tract cause membranes covering sperm heads to change... sperm become hyperactive, swimming harder and faster towards egg.
How many sperm reach the egg?
A few dozen
What are the layers of the egg sperm must pass through?
- Corona radiata
- Zona pellucida
What happens when sperm reach the zona pellucida?
Attach to specialised sperm receptors... trigger release of digestive enzymes from acrosome... enables sperm to burrow into layer.
Which sperm fertilises an egg?
The first sperm to reach the fluid-filled perivitelline space.
What happens once a sperm reaches the perivitelline space?
Outer membranes of sperm and egg fuse, egg pulls sperm inside.
What processes prevent polyspermy?
1. Fusion of egg and sperm membrane causes changes in egg membrane - prevents other sperm from attaching to it.
2. Egg releases chemicals that push sperm away and creates impenetrable fertilisation membrane.
3. Zona pellucida hardens, trapping any other sperm.
What happens to the male and female genetic material in a fertilised egg?
Male genetic material spreads out and a new membrane forms around it - male pronucleus. The DNA reforms into 23 chromosomes.
Female genetic material is activated by sperm and egg fusion and completes meiosis II - female pronucleus.
Microtubules pull pronuclei together and these fuse - forms a single cell: zygote.
How many cells does a zygote have?
2 due to DNA replication upon fertilisation
What prevents immune reaction against sperm in males?
Sertoli cells in the testes form a barrier of tight junctions to prevent contact between sperm cells and blood cells - antibodies against sperm surface antigen cannot be produced.
What is sympathetic autoimmune orchitis?
Production of antibodies against sperm causing male infertility. E.g. As a result of injury breaking the Sertoli cell barrier.
Why does the female immune system not destroy all sperm?
Seminal fluid contains immunosuppressants. Alters genetic expression in cervix (and alters female behaviour: more aggressive/happy).
What is the difference between embryonic age and gestational age?
Embryonic age = time since fertilisation
Gestational age = time since last menstruation (embryonic age - 2 weeks)
What are the germinal stage, embryonic period and foetal period?
Germinal stage = from fertilisation to end of 2nd week.
Embryonic period = from 3rd to end of 8th week.
Foetal period = from 9th to 38th week (birth).
Describe the stages of development from a zygote to an embryo.
1. Zygote (fertilised ovum)
2. Mitosis... Early cleavage (4 cell stage)
3. Mitosis... Morula (solid ball of cells) - 3-4 days post fertilisation
4. Development... Blastocyst (hollow ball of cells) -4-5 days post fertilisation.
Why is the blastocyst the point of embryo formation?
Cells start differentiating.
When does implantation occur?
6 days post fertilisation
What must happen before the blastocyst can implant?
Hatching: loss of zona pellucida
Why is the zona pellucida maintained until implantation?
1. Prevents morula/blastocyst (use aerobic and anaerobic pathways) from adhering to oviduct - movement to where there is nutrient supply and protection
2. Stops morula enlarging
What does the trophoblast develop into?
Yolk sac and placenta.
What is the difference between histiotrophic and haemotrophic nutrition?
Histiotrophic = nutrition provided to embryo not from the maternal blood. Important up to 12th wk.
Haemotrophic = nutrition provided by maternal blood. Important from 12 wks on.
What does histiotrophic nutrition consist of?
Uterine glands remain active until at least wk 10 - secrete glycoproteins mucin and glycodelin = source of nutrients.
Where does the blastocyst implant and what does this site determine?
Determines site of placenta formation.
Normally occurs in the superior, posterior uterine wall.
What is the most common site of ectopic pregnancy?
What is the result of blastocyst implantation in lower uterus segment?
Later placenta praevia
What are the 2 components of the blastocyst once it has burrowed into the endometrium (day 8)?
Trophoblast (outer cell mass)
Embryoblast (inner cell mass)
What happens to the trophoblast when it makes contact with the endometrium?
Differentiates into 2 layers:
- inner cytotrophoblast
- outer syncitiotrophoblast
How does the embryoblast differentiate?
Into a bilaminar embryonic disc: hypoblast (outer) and epiblast (inner).
What happens after bilaminar embryonic disc formation?
Cavity forms between epiblast and cytotrophoblast: amniotic cavity.
What happens to the hypoblast? What structure does this form?
Cells originating from hypoblast migrate, forming a thin membrane covering the inner surface of cytotrophoblast = exocoelomic membrane.
Cells of hypoblast and exocoelomic membrane form walls of primitive yolk sac.
On what day is the blastocyst completely embedded in the uterine wall?
What happens on day 9?
Rapid growth of cytotrophoblast and syncitiotrophoblast. Lacunae form in syncitiotrophoblast.
What happens by day 12?
Lacunae stop growing and fuse to form large interconnecting spaces - lacunar networks.
Endometrium capillaries dilate (=maternal sinusoids).
Syncitiotrophoblast continues to expand and enzymes degrade lining of sinusoids and uterine glands, allowing maternal blood and uterine secretions to flow into the lacunar networks - establishes uteroplacental circulation: allows exchange of gases and metabolites.
Which population of cells arises around day 12?
Extraembryonic mesoderm appears between cytotrophoblast and primitive yolk sac.
What happens to the extraembryonic mesoderm?
Large cavities appear that fuse to form the chorionic cavity.
What happens 13 days post fertilisation?
A large portion of the extracoelomic cavity is pinched off forming a secondary cavity - secondary yolk sac.
What happens by the end of the 2nd week of development?
The chorionic cavity enlarges, and the bilaminar embryonic disc is joined to the trophoblast by a band of extraembryonic mesoderm called the connecting stalk (future umbilical cord).
Where does the yolk sac attach?
Connected to umbilical cord by a yolk stalk.
What is the function of the yolk sac?
Acts as the preliminary circulatory system and is eventually absorbed into embryo gut.
1. provides nourishment during early stages of development
2. Initial circulation - delivers nutrients via a primitive aorta = vitelline circulation.
What is the yolk sac lined by?
Extraembryonic endoderm and mesoderm
When does an embryo develop a circulatory system?
By week 12 - yolk sac ceases own circulatory functions.
Why is the 2nd week of development known as the week of 2s?
Trophoblast differentiates into 2 layers: cytotrophoblast and syncitiotrophoblast.
Embryoblast forms 2 layers: epiblast and hypoblast - bilaminar embryonic disc.
Hypoblast contributes to formation of 2 cavities - yolk sac and chorionic cavity.
Why is the foetus not routinely rejected?
1. Placenta acts as an immune barrier (diffusion of components occurs from mother to baby blood).
2. Suppression of cell-mediated immunity, esp T cells.