4.1 Sexual Reproduction in Humans

• Male reproductive system


  A system adapted for creation of the male gamete, spermatozoa and sexual reproduction
• Spermatogenesis


  1. Occurs in the seminiferous tubules of the testes

  2. Diploid (2n) germinal epithelium divides by mitosis to form spermatogonia (2n)

  3. Spermatogonia divide by mitosis resulting in cells that increase in size-forming primary spermatocytes (2n)

  4. Primary spermatocytes(2n) divide by meiosis to form haploid (n) secondary spermatocytes after the first division and eventually spermatids (n) after the second meiotic division

  5. Spermatazoa mature and differentiate into a head, middle piece and tail adjacent to the lumen of the tubule

  
• Vas deferens


  • Carries sperm towards the penis during ejaculation
• Testes


  • Produce gametes (spermatozoa) and the hormone testosterone
  • Surrounded in a skin sac called the scrotum
• Penis


  • Organ adapted to transfer the sperm into the vagina during sexual intercourse
• Seminal vesicle


  • Secretes an alkaline, nutrient rich fluid onto the sperm in the vas deferens forming semen
  • The alkaline nature serves to neutralise any remaining acidic urine in the urethra and acidic conditions in the vagina
• Prostate gland


  • Secretes an alkaline fluid containing zinc ions onto sperm in the vas deferens
• Urethra


  • Carries semen and urine out of the body
• Epididymis and seminiferous tubules

  • Spermatozoa are made in the seminiferous tubules and stored in the epididymis to mature and become mobile
• Sertoli cells


  • Provide nutrients to the developing Spermatazoa
• Interstitial cells (Leydig cells)


  • Secrete testosterone to stimulate spermatogenesis
• Female reproductive system


  Adapted for the creation of the female gamete, the ovum, and sexual reproduction
• Oogenesis


  1. Takes place in an ovary up to secondary oocyte stage prior to birth

  2. Diploid germinal epithelium cells (2n) divide by mitosis to make oogonia that will increase in size forming primary oocytes (2n)

  3. Primary oocytes (2n) present at birth are stopped in prophase of meiosis I

  4. Germinal epithelium cells divide to form diploid follicle cells making primary follicles

  5. Secondary oocytes (n) and a small polar body (n) are formed when the primary oocyte completes meiosis I

  6. The secondary follicle containing the haploid secondary oocyte matures into a graafian follicle

  7. One graafian follicle migrates to the surface of the ovary and bursts releasing the secondary oocyte

  8. Meiosis II begins but stops at metaphase II until fertilisation

  
• Fallopian tube/oviduct


  • Tube lined with ciliated epithelium which waft the secondary oocyte from the ovary to the uterus
• Uterus


  • A specialist organ where the embryo implants into the blood rich endometrial layer and grows to term
  • If there is no fertilisation, this layer is shed during menstruation
• Ovary


  • Produces gametes up to secondary oocyte stage
  • Also produces hormones progesterone and oestrogen
• Cervix


  • Ring of muscle at the neck of the uterus
• Vagina


  • A muscular walled structure
  • Sperm is deposited here during sexual reproduction and the foetus passes through here during birth

  
• Corpus luteum


  • Becomes the corpus luteum after ovulation
  • If fertilisation occurs it will produce hormones, otherwise it degenerates
• Fertilisation


  1. Sperm swim to the fallopian tube

  2. Capacitation increases the permeability of the membrane in front of the acrosome

  3. The acrosome releases proteases that digest the corona radiata

  4. The acrosome membrane ruptures releasing hydrolase enzymes which digest the zona pellucida on contact

  5. The membranes of the sperm and secondary oocyte fuse and the genetic material of the sperm enters the secondary oocyte

  6. Cortical granules fuse with the cell membrane and alter the zona pellucida to form the fertilisation membrane; this prevents polyspermy

  7. Meiosis II completes forming the ovum and the second polar body

  8. Sperm and ovum nuclei fuse. A diploid zygotic nucleus is formed
• Sperm


  • Mitochondria in mid-section produces ATP to power microtubules that move tail
  • Axial filament in tail
  • Nucleus in the head
  • Acrosome containing protease enzymes to digest the cells of the corona radiata
• Implantation


  1. Mitotic divisions of the newly formed zygote occur - this is called cleavage

  2. A hollow ball of cells called the blastocyst is produced and this implants into the endometrial lining of the uterus

  3. The blastocyst secretes HCG (human gonadotrophic hormone) which maintains the corpus luteum

  4. The corpus luteum produces progesterone which maintains the endometrium, inhibits the production of FSH and LH, and supresses contraction of the uterine wall

  5. When formed, the placenta takes over producing HCG and after 16 weeks it takes over the production of progesterone, it also produced oestrogen that stimulates the growth of the uterus and the mammary glands

  
• Placenta


  • Forms from the outer blastocyst cells (trophoblast cells) that extend into the endometrium linking the foetus to the mother's tissues to collect nutrients
  • Chorionic villi extend into the maternal blood and have microvilli to increase surface area, thin walls for easy diffusion and blood flows in a counter current direction to the maternal blood to maintain diffusion gradients along the whole blood vessel
  • Cells also fuse together preventing the mother's white blood cells from entering the foetal blood
  • Antibodies can pass through though conveying some immunity
  • Intervillous spaces contain maternal blood and bathe chorionic villi
  • Umbilical arteries carry deoxygenated blood and waste materials away from the foetus to the chorionic villi where gas exchange and excretion occurs
  • Umbilical veins carry oxygenated blood and nutrients to the foetus
  • Amniotic fluid acts as a shock absorber protecting the foetus from bumps
• Menstrual cycle


  1. A series of hormonal and physiological changes involving the brain, ovaries and uterus

  2. Controlled by the hormones: LH (lutenising hormone), FSH (Follicle stimulating hormone), Oestrogen, Progesterone

  3. FSH stimulates the maturation of a follicle and the production of oestrogen

  4. Oestrogen builds up and inhibits FSH secretion, this is negative feedback and results in a decrease in FSH levels

  5. The level of oestrogen, secreted by the developing follicle, increases in the blood which triggers the repair of the endometrium; this inhibits FSH production and stimulates LH production

  6. The corpus luteum produces progesterone that further develops the endometrium

  7. With no implantation falling FSH and LH levels cause the corpus luteum to degenerate

  8. This means progesterone levels fall, the endometrium breaks down and is lost during menstruation

  9. FSH is no longer inhibited and the cycle is initiated again

  10. A high level of LH from the anterior pituitary gland initiates ovulation and the graafian follicle becomes the corpus luteum
• Birth


  1. Just before birth oestrogen levels increase and progesterone levels decrease - the uterine wall can now contract

  2. The posterior pituitary gland secretes oxytocin that simulates uterine contractions

  3. Positive feedback occurs where the uterine contractions in turn stimulate more oxytocin to be produced

  4. The anterior pituitary gland secretes prolactin stimulating the production of milk by the mammary glands