Reproduction

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