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Human Biology > Reproduction
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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

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

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

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

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

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)

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

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

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

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

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

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

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

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

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