Fertilization and embryonic development Flashcards
Sperm cell structure
1) acrosome (vesicle with digestive enzymes inside, helps the cell digest its way to egg cell – three barriers, one zone pellucida)
2) haploid nucleus (head)
3) mitochondria (in the neck)
4) microtubules/microfibrils or protein fibers (tail, provide a mechanism for propulsion)
Egg cell (secondary oocyte) structure
1) haploid nucleus (nucleolus inside)
2) yolk (cytoplasm)
3) cortical granules (vesicles with glycoproteins)
4) plasma membrane
5) 1st polar body (no purpose, will degenerate)
6) zona pellucida (pretty thick glycoprotein)
7) corona radiata (follicular cells that surround the secondary oocyte)
explain the process of fertilization
1) Sperm cells attracted by chemical signals released from the ovum
2) Sperm cell penetrates corona radiata to reach zona pellucida were the acrosome reaction is triggered
3) Their membranes fuse and the sperm injects its genetic material inside of the egg cell – two sets of chromosomes fuse
4) Fusion of the membranes changes the membrane polarity of the oocyte so Ca2+ ions get released from the ER, causing the cortical granules to move to the plasma membrane, release their components and harden the zona pellucida – this is called the cortical reaction – by producing what is called the fertilization membrane produced, it prevents more sperm cells to enter and polyspermy (causes polyploidy)
5) Immediately after fertilization, the nuclear envelope diffuses so the first mitotic division can start (as mitosis happens the embryo is pushed into the uterus)
When is fertilization (in humans) successful and when not?
Only successful if fertilization happens in the first third of the oviduct, not later because in that case the zygote/embryo does not have enough time to grow enough to be able to implant itself in the endometrium
How does fertilization differ in aquatic organisms compared to terrestrial?
mostly have external fertilization where habitat is a determining factor here
Which molecular mechanism prevents hybridization in all species but is particularly important in aquatic organisms?
ova release species-specific chemical signals that attract sperms – receptors on zona pellucida have to correspond to receptors on the sperm cell
Early embryonic development steps
1) Second meiotic division completed after fertilization – secondary oocyte into ovum and secondary polar body
2) Ovum differentiates into zygote which divides by mitosis – cell division in embryonic development is called cleavage (occurs as the embryo moves down the oviduct)
3) Morula formed after 4 days – a solid ball completely filled with totipotent stem cells
4) Blastocyst forms on the 5th day – a hollow ball as a result of some cells migrating toward the ends of the sphere – pluripotent stem cells in the inner cell mass (ICM) and trophoblastic cells on the blastoderm (they will form the placenta)
5) The blastocyst implants itself in the endometrium on the 7th day after fertilization
6) At 8th week, the embryo starts to form bones and then it’s called a fetus
How does the blastocyst implant itself in the endometrium?
Trophoblastic cells (in the blastoderm) produce projections called trophoblastic villi and release enzymes that digest a part of the endometrium, they perform mitosis and “fill” this cavity in the endometrium and thus implant the blastocyte
Trophoblastic villi also allow the exchange of materials between maternal blood and the embryo (they make the baby’s portion of the placenta and increase its surface area). Blastocyst implants into endometrium on the 7th day after fertilization
What is an amnion?
an envelope that protects and supports the baby (amniotic fluid)
What happens upon implantation of the blastocyst? What is its role? When does it stop?
It starts the production of hCG (human chorionic gonadotropin)
It passes into the maternal blood (peak concentration at 8-10 weeks) and it stimulates the further progesterone and estrogen secretion by the Corpus luteum to continue the FSH and LH inhibition and prevent endometrium shedding
By the 12th week, the ovary stops hormone production and the placenta takes over, producing placental estrogen and progesterone
How long is gestation (pregnancy) in humans? How long is it generally? How long does a pregnancy last?
40 weeks – counting from the first day of the last period because the follicular phase is variable, so we don’t know exactly when the ovulation happened
varies considerably and is based mostly upon size: hamster<dog, lion, goat<human, horse<giraffe, elephant
What is a placenta? What is it made of?
A disk-shaped structure (diameter 20 cm, thickness 2 cm) with many projections (trophoblastic villi) embedded in the uterine wall. The human placenta is more deeply integrated into the maternal tissue than in any other animal
1. Umbilical cord – connects the fetus to the placenta
2. Two umbilical arteries – carry the deoxygenated blood to the placenta
3. Umbilical vein – returns the oxygenated blood to the fetal circulation
4. The mother’s blood vessels – supply the fetus with O2 and nutrients (glucose, lipids, minerals, vitamins, AB, hormones, a-a), removes waste from fetal blood and excretes them though maternal systems (CO2, urea, hormones (hCG))
5. Open-ended blood arterioles and capillaries – produce the inter-villous “blood pools”, filled with maternal blood that surround the placental villi and allow for efficient material exchange
6. Placental villi – provide large SA for material exchange
How do hormone concentrations change during pregnancy? What are their functions?
hCG reaches peak at around 2 months when it suddenly drops because placental estrogen and progesterone start to be produced (around the 3rd month)
estrogen and progesterone levels rise steadily (more progesterone) and steeply drop by the end – estrogen encourages myometrium growth, stimulates mammary glands growth and production of oxytocin receptors in myometrium later during the pregnancy (oxytocin from hypothalamus stimulates contractions) – progesterone maintains a highly vascular endometrium tissue and prevents premature myometrium contractions – together they inhibited FSH, LH, and oxytocin
How do contractions happen?
When the concentrations of estrogen and progesterone steeply drop at the end of pregnancy, oxytocin is produced
The placenta produces prostaglandins which stimulate myometrium to contract
Stretch receptors in the cervix detect the head engaging
The nerve impulse is sent to the maternal pituitary which in response releases more oxytocin and the uterine contraction is thus enhanced – the same cycle repeats until the baby is delivered (positive feedback mechanism)
What are the three stages of childbirth (labor)?
- Complete cervix dilation
- Delivery of the baby
- Delivery of the placenta
What is the cause of the recent decline in male fertility?
during the last 50 years the sperm count has fallen by 50% and continues to drop 2%/year, mostly due to endocrine disrupting chemicals – use of female contraceptive pills and chemicals with similar structure and effect as female sex hormones – underlines the importance of testing for harmful effects of chemicals before their introduction
What are the methods or getting over reproductive challenges in order to conceive?
- Low sperm count – solved by concentrating the male sperm before being placed in the uterus
- Blockage of the oviduct – solved by mechanical introduction of sperm
- More complex cases of infertility – solved by in vitro fertilization (IVF) – first case is Louise Brown by Sir Robert Winston in 1978 in England:
- Down-regulation (natural cycle shut down by gonadotropin agonist (nasal spray))
- Artificial doses of hormones which lead to superovulation (injection of artificial FSH and LH, stimulating multiple primary follicles)
- hCG administration (stimulates final maturation of the follicles)
- Egg retrieval and fertilization (don’t wait for natural ovulation, sterile aqueous solution with sperm, aiding with a needle)
- Establishing pregnancy (progesterone vaginal tablets to thicken the endometrium)
