Gametes and Gamete Formation |
| Histology of the Testis |
- Testes in the scrotal sac (→extension of abdominal cavity)
- Produce sperm and male sex hormones
- Have a constant temp "body temp - 5°C" maintained by)
- Heat exchange system in arteries and veins
- Semi-external position
- Muscles in scrotal sac → move testes up/down against warmer abdomen
- Sterility if testes do not descend during development
- Spermatogenesis occurs in walls of seminiferous tubules (→compartment of testes)
- Sperm are passively carried to the epididymis
- Network of seminiferous tubules → leads into vas deferens
- Sperm acquire ability to swim and are stored
- Urethra (joining of vas deferens) carries fluids from the urinary and reproductive system
- During interaction / pressure in erectile tissue rises / arteries supplying it dilate and veins draining it constrict
- Not discharged sperm are degenerated, absorbed, lost via urine
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Histology of the Ovary |
- Ovaries in the abdominal cavity produce gametes + sex hormones
- Stroma within the ovary contains blood vessels
- Supply of oxygen and nutrients
- Removal of waste products → CO2
- Transports hormones which control process of reproduction
- Follicle development occurs within the ovary
- Each follicle consists of follicle cells surrounding an oocyte (→developing egg)
- At birth ≈1 million primary follicles in each ovary
- Remain in suspension until puberty → at puberty, only ≈400k are present
- ≈400 will be released into the oviduct during reproductive life of a female
- At month intervals ≈20-25 follicles begin to develop further, from these only a single oocyte is released
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Gametogenesis (→Formation of Gametes) |
| SAME in both sexes a) spermatogenesis, b) oogenesis |
- Multiplication of diploid cells by
MITOSIS
- Epithelium of seminiferous tubules multiplies Daughter cells are pushed towards lumen of tubule
- Epithelial cell inside ovary of female fetus multiplies
- GROWTH of daughter cells from mitotic divisions
- Products of the growth phase divide by
MEIOSIS producing haploid cells (46→23)
- MATURATION of haploid daughter cells into gametes (eggs, sperm)
- Heads are embedded in Sertoil cells
- Prevent destruction of sperm by immune system
- Sperm and body cells are genetically different
- Provide nutrients Tails are projected into fluid-filled lumen
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How gametogenesis differs in females |
- "b) Unequal cell division in meiosis / 1 ovum and tiny polar bodies produced
- b) Primary oocytes form before birth / growth phase before birth
- b) Pause in meiosis at prophase I / further development suspended until puberty
- b) Pause in meiosis at metaphase II / meiosis not complete until fertilisation occurs"1
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Follicle Development/Ovarian Cycle |
| Follicular stage [days1-13] |
- Anterior pituitary gland is secreting follicle-stimulating hormone FSH
- FSH travels in bloodstream to ovary
- Stimulates development (division) of follicle cells surrounding oocyte
- Developing follicle cells secrete oestrogen
- Stimulates proliferation of endometrium and its blood supply
- Inhibits further secretion of FSH by negative feedback
- Stimulates anterior pituitary gland to secrete luteinising hormone LH
- LH brings about ovulation
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Ovulation [day14] |
| // Mature ovarian follicle ≈15mm in diameter |
- Mature ovarian follicle bursts and releases secondary oocyte (now called ovum!)
- Corpus luteum forms from remaining follicle cells in ovary
- Ovum passes down fallopian tube/oviduct towards uterus; fertilization now possible
- Happens once a month, menstrual cycle ≈28days
- Each ovary alternatively releases an ovum every ≈56days (→56/2)
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Luteal phase [days15-28] |
- Corpus luteum secretes sex hormones
- Progesterone continues to stimulate
- Profileration of endometrium and its blood supply
- Development of nutrient fluid glands in uterus lining
- High levels of sex hormones inhibit secretion of LSH and LH
IF FERTILISATION DOES NOT OCCUR
- Corpus luteum degenerates \ levels of sex hormones fall
- Uterus lining breaks down; FSH is not inhibited anymore → cycle starts again
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Uterine Cycle |
- Menstruation [days1-5] → Endometrium breaks down
- Low levels of female sex hormones causes
- uterus lining/endometrium to disintegrate
- its blood vessels to rupture
- Flow of blood (menses) passes out of the vagina
- Proliferative phase [days6-13] → Endometrium rebuilds
- Increased production of oestrogen by ovarian follicle
- Secretory phase [days15-28] → Endometrium thickens and glands are secretory
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Structure of a Mature Sperm Cell |
- Head: acrosome (enlarged lysosome → digestive enzymes → penetrate egg), nucleus (n)
- Middle piece: mitochondria, ATP needed for tail movement
- Tail: flagellum, movement
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Movement of Sperm in the Female Reproductive Tract |
- Sperm are ejaculated into the vagina / deposited outside the cervix
- Alkalinity of semen neutralises acidic pH in vagina
- Mucus allows sperm to swim through cervix / mucus is thin and watery during ovulation / glycoprotein chains run parallel
- Wall of uterus has two distinct layers
- Bulk of uterus wall consists of myometrium (→smooth muscle) / expels fetus at birth
- Active muscular contractions during intercourse support sperm
- Travel to oviduct in ≈5hours / survival rate of sperm ≈48hours
- Endometrium is concerned with anchorage and nourishment of embryo
- Sperm undergo capacitation while travelling
- Acquire ability to fertilise 2° oocyte by removal of acrosome membrane proteins
- Takes ≈6hours
- Fertilisation occurs in the fallopian tube
- Walls are lined with ciliated epithelia and contain smooth muscles
- Egg moves to uterus via cilia movement and peristaltic muscle contraction
- Contact between sperm and oocyte is by chance
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Structure of the Egg |
- 0.1mm (100um) in diameter. Sperm's head is only 2.5um across
- Haploid nucleus is surrounded by cytoplasm / with enzymes and organelles
- Yolk droplets contain proteins and lipids
- Outside plasma membrane is a glycoprotein coat called a jelly coat
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Acrosome Reaction and Penetration of the Oocyte Membrane |
- Acrosome reaction
- Contact of jelly coat and sperm
- Triggers Ca2+ to enter membrane of sperm
- Causes acrosome to burst / releases enzymes / digest jelly coat
- // enzymes: hyluronidase + acrosin
- Sperm filament attaches to receptor on vitelline membrane
- Sperm and egg plasma membranes fuse
- Sperm nucleus enters egg, tail and middle piece remain outside
- Depolarisation of membrane / blocks entry of more sperm
- 2° oocyte undergoes 2nd division of meiosis / produces ovum + second polar body
- Nuclei (ovum + sperm) fuse forming a zygote
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Female Infertility |
- Pituitary gland fails to produce FSH and prevents ovulation
- TREATMENT: Injections of FSH
- SIDE EFFECTS: May produce several eggs at the same time (→twins)
- Too much oestrogen is secreted which inhibits FSH secretion
- TREATMENT: non-steroidal drugs (e.g. clomiphene) which oppose action of oestrogen
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Male Infertility |
- Semen contain too few sperm
- TREATMENT: natural/synthetic androgens such as testosterone
- // Viagra (sildenafil) is an enzyme inhibitor → causes smooth muscle surrounding erectile tissue to relax → more blood can be pumped into them during erection process
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In Vitro Fertilisation (IVF) |
- FSH is injected in woman
- Just before ovulation eggs are collected and matured
- Fertilisation → collected sperm sample is added to collected mature egg
- Nucleus of sperm is micro-injected into the egg
- Fertilisation outside the body in a plastic disc
- Advantage: possible to screen embryos for genetic defects
- Maximum of three are transferred into the uterus
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