Sexual reproduction Flashcards
1
Q
What is gametogenesis?
A
- Production of female and male haploid gametes cells
- The germinal epithelium undergoes cells division to differentiate into gamete cells
2
Q
What happens in oogenesis during pregnancy?
A
- The production of ova (female gametes) in the ovaries before birth
- The outer layer of ovaries called germinal epithelium undergoes mitosis forming diploid cells called oogonia in the first 7 months of pregnancy
- The oogonia in the fetus’ ovaries grow in size and enter meiosis 1, forming a layer of follicle cells. Together known as primary follicles
- Next the oogenesis process pauses until puberty starts
- Most primary follicles never reach maturity
3
Q
What happens in oogenesis during puberty?
A
- Hormone FSH stimulates the continued development of several primary follicles
- Only one of them will reach maturity
- Meiosis 1 occurs, dividing the primary follicle into two cells
- One of them is a secondary oocyte and a small cell called polar body (unequal division)
- Now the secondary oocyte enters meiosis 2 and leaves the ovary with is layer of follicle cells: ovulation
- Remains of the follicle in the ovary develop into corpus luteum
- If the secondary oocyte is fertilised by a sperm, chemical changes trigger the completion of meiosis 2.
- When meiosis 2 is complete, the s.o. becomes an ovum between the end of meiosis 2 and fusion of the two nuclei
- The second polar body is produced at completion of meiosis 2
4
Q
What is spermogenesis?
A
- Production of sperm (male gamete) within the testes from puberty onward
- Testes contain tubules, called seminiferous tubules (where sperm cells develop)
- Leydig cells in the interstitial space of the testis produce the hormone testosterone, allows spermatocyte to complete meiotic division form spermatozoa
- The outer layer of seminiferous tubules is made of germinal epithelium. The cells in it divide by mitosis producing 2 diploid spermatogonia
- One spermatogonia will become a sperm, the other remains germinal epithelium
5
Q
What happens to the spermatogonia that becomes a sperm?
A
- The spermatogonia grows into a primary spermatocyte (2n), carries out meiosis 1 to produce 2 secondary spermatocyte (n)
- Each secondary spermatocyte carry out meiosis 2 to form two spermatids (4 in total)
- Spermatids become associated with sertoli cells (compose lining of seminiferous tubules)
- This helps spermatids to differentiate into spermatozoa (immature sperm)
- Sertoli cells provide support and nutrition to develop the sperm cells
- Speratozoa matures fully and move along seminiferous tubule lumen to sperm duct and pass through epididymis
6
Q
What are the similarities between oogenesis and spermatogenesis?
A
Similarities
- Both gametes are specialised haploid cells
- Both processes involve mitosis and two rounds of meiosis, cell growth and differentiation
- Mitosis results on many new cells, which have the potential to grow into gametes
- Meiosis results in haploid gametes
7
Q
What are the differences between oogenesis and spermatogenesis?
A
- Structure of gametes: ova is large, immobile, large cytoplasm, surrounding layer of follicle cells. Sperm is small, mobile, little cytoplasm, acrosome contains enzymes
- Daughter cell per meiotic division: 1 ovum produced after meiosis 2, polar bodies do not from ova, 4 sperm cells at end of meiosis 2
- Frequency production: 1 per menstrual cycle, men continuously puberty onwards
8
Q
How does the timing of the process in spermatogenesis and oogenesis differ?
A
- Spermatogenesis, production is continuous, from puberty onward
- Oogenesis, finite process, starts before brith, forms 40,000 primary oocytes, happens with puberty in menstrual cycle, ends with menopause, hormonal changes
9
Q
What is the role of testosterone in spermatogenesis?
A
- Stimulates meiotic divisions of spermatognia into spermatozoa
10
Q
Be able to draw seminiferous tubules and an ovary
A
11
Q
Be able to draw a mature sperm and egg
A
12
Q
Explain the structure and function of a sperm.
A
- Haploid nucleus
- Acrosome contains digestive enzymes to aid entry into the ovum
- Many mitochondria, release energy to aid movement
- Tail made of protein microtubules to aid movement
13
Q
Explain the structure of an egg.
A
- Haploid nucleus
- Jelly layer called zona pellucida prevents polysperm (more than one sperm to enter)
- Vesicles and cortical granules contain digestive enzymes, release into the zona pellucida to prevent polysperm
- Cytoplasm rich in nutrients for the developing embryo after fertilisation
14
Q
Explain the structure of an egg.
A
- Haploid nucleus
- Jelly layer called zona pellucida prevents polysperm (more than one sperm to enter)
- Vesicles and cortical granules contain digestive enzymes, release into the zona pellucida to prevent polysperm
- Cytoplasm rich in nutrients for the developing embryo after fertilisation
15
Q
What is fertilisation?
A
- The fusion of a sperm cell and an ovum to form a diploid zygote
- At the point when sperm cells are attracted towards the secondary oocyte, meiosis 2 is not complete (is not yet an ovum)
- As soon as the sperm reaches the secondary oocyte, mechanism started that prevents more than one sperm from passing through
16
Q
How does fertilisation prevent polyspermy?
A
- Polyspermy: entry of more than one sperm into a single oocyte
- When the sperm cell digests through the zona pellucida it reaches the oocyte membrane
- Complementary receptors on the head of the sperm bind with proteins on the oocyte membrane, enabling membranes of the two gametes to fuse together
- Once the sperm nucleus enters the oocyte, meiosis 2 continues, which releases the second polar body and formation of mature ovum
- When the membranes fuse, the movement of cortical granules is triggered
- Granules move from cytoplasm to membrane, releasing enzymes that digest receptor proteins, causes zona pellucida to harden
- Enzymes released by exocytosis. When zona pellucida hardened, no more sperm can enter
17
Q
What are the features of external fertilisation?
A
- In aquatic animals, sperm are surrounded by fluid medium, sperm released into the environment, fertilisation can occur externally
- External fertilisation: fish and amphibians
- Very large quantities of ova and sperm produced, due to risk of predators eating then or being washed away, increase chances
18
Q
What are the features of internal fertilisation?
A
- Sperm need to be released into a fluid medium (semen) into internal reproductive tract of the female animal
- Animals: birds and mammals
- Gametes are close in proximity to each other, protected from predators, gamete numbers smaller
19
Q
How is the blastocyst implanted in the uterus?
A
- The fertilised ovum divides by mitosis to form 2 diploid nuclei (two-cell embryo)
- Another round of mitosis forms a four-cell embryo, this repeats until the embryo forms a hollow ball called blastocyst
- The blastocyst contains group of cells: blastomeres, will eventually develop into fetus
- The embryo called blastocyst found in oviduct and is surrounded by protective extracellular coat (zona pellucida)
- After 7 days: blastocyst consists of 125 cells, reaches the uterus and z.p. breaks down
- The blastocyst now requires external nutrition supply which it obtains by implanting into endometrium (uterus lining)
- Outer layer of blastocyst forms finger-like projections, allows to penetrate endometrium
- Exchange of nutrients and O2 with mothers blood can occur
20
Q
What is the implantation of blastocyst in the endometrium essential for continuation of pregnancy?
A
- The uterine lining provides the developing embryo with necessary nutrients and oxygen
- The blastocyst used up the nutrient supplied by the egg cell
21
Q
What are the two categories of gestational duration in mammals?
A
- Gestation period 38 weeks
- Altricial species: infants that are immobile, unable to obtain food after birth, have shorter gestation periods e.g. humans
- Precocial species: species born in advanced state, able to move around, good vision, feed independently, longer gestation period e.g. ducks, horses
22
Q
How do you compare the animal size and development of young at birth?
A
- Compare gestation periods with adult body masses according to development of the young at birth (altricial or precocial)
- For both categories, the length of gestation period is positively correlation with adult body mass
- Not always true, some gestation periods similar, but different body masses
23
Q
What role does hCG play in early pregnancy?
A
- After the blastocyst is implanted into the endometrium, the hormone hCG is secreted
- Secreted in first 8-10 weeks
- Stimulates corpus luteum in the ovary to maintain secretion of oestrogen and progesterone (develop uterus more)
- Stimulate growth of placenta and uterine enlargement
- Inhibits menstruation
- After 12 weeks hCG declines, placenta takes over role of stimulating secretion of the hormones
- Pregnancy kits detect presence of hCG in urine
24
Q
What role does progesterone play during pregnancy?
A
- Secreted by the placenta, inhibits the production of oxytocin by the pituitary gland, in brain (negative feedback)
- Inhibits contractions of muscles of uterus wall (avoid giving birth)
- End of pregnancy, fetus produces oestrogen, signals to stop progesterone production (negative feedback)
- This in turn initiates oxytocin production at start of labour (positive feedback)
25
What role does oestrogen play during pregnancy?
- At the end of pregnancy, it is produced by fetus and the placenta
- Makes the uterine wall more sensitive to oxytocin
- It inhibits progesterone production, in turn stimulates oxytocin production (positive feedback)
26
What hormones are involved during labour?
- Labour: oxytocin stimulates contractions of muscles in myometrium
- Stretch receptors in cervix detect contractions, sends signal to increase oxytocin secretion (positive feedback loop)
- Slowly increases contractions rhythmically
27
What happens during birth?
- Cervix muscles cause it to dialte and widen, the contractions cause amniotic sac to burst
- Amniotic fluid passes through cervix, baby pushed out
- Baby pushed out, umbilical cord cut
28
What happens after birth?
- Uterine contractions continue, causing placenta to separate from uterine wall
- Mother births placenta and remains of umbilical cord
29
What is the placenta and what does it do?
- Contains blood vessels that maximise exchange of substances between mother and fetus without direct contact between them
- Produces hormones
- Fetus connected with an umbilical cord and found inside the amniotic sac, filled with amniotic fluid
30
Explain the structure of the placenta.
- Complex arrangement of blood vessels arranged inside placental villi, maternal blood flow is found around villi (number of villi increases)
- Maternal and fetal blood never directly mix, yet very short distance between them, exchange nutrients and gases
- Maternal blood flows out of the blood vessels and pools in spaces surrounding the placental villi, the spaces are called inter-villous spaces
- Placental membrane is selectively permeable, restricts exchange of substances
31
What are substances that can move across the barrier from mother to fetus?
- O2 (diffusion), antibodies and hormones (endocytosis), water (osmosis), glucose (facilitated diffusion), harmful substances (alcohol, pathogens)
32
What substances can move across the barrier from fetus to mother?
- CO2, water, urea, hormones (exocytosis)
33
How does the placenta regulate hormones?
- The degenerating corpus luteum releases hormones that support the initial stages of pregnancy up to 9 weeks
- The placenta then releases oestrogen to stimulate growth of uterus and mammary glands
- The placenta releases progesterone to support lining of the uterus to maintain pregnancy
- If placenta fails to initiate hormones production, miscarriage occurs
34
What is oestrogen pollution? What are its consequences?
- Synthetic oestrogen introduced as contraceptive, increased usage, found in environment
- Accumulation of hormones from human sewage
- Can lower average sperm count in males, reduced fertility
- Cause certain fish to have female characteristics
- Correlation, casual link, not enough evidence
- Possible procedure to reduce levels in waterways are expensive, require more research
