Chapter: Human Reproduction

• Introduction

  • Viviparous sexual reproduction
  • Major biological events in humans

• The Male Reproductive System

  • Primary Sex Organ
    • Testis (Pair)
      • Located in scrotum (low temperature for spermatogenesis)
      • Composed of testicular lobules containing seminiferous tubules
      • Sertoli cells (nutritional support)
      • Leydig cells (interstitial cells secreting androgens/testosterone)
  • Accessory Ducts
    • Rete testis
    • Vasa efferentia
    • Epididymis
    • Vas deferens
  • Accessory Glands
    • Seminal vesicles
    • Prostate gland
    • Bulbourethral glands (Cowper's glands)
    • Secretions form seminal plasma (fructose, calcium, enzymes)
  • External Genitalia
    • Penis (copulatory organ)
    • Glans penis covered by foreskin

• The Female Reproductive System

  • Primary Sex Organ
    • Ovary (Pair)
      • Produces ovum and ovarian hormones (estrogen, progesterone)
  • Accessory Ducts
    • Fallopian tubes (Oviducts)
      • Parts: Infundibulum (with fimbriae), Ampulla, Isthmus
    • Uterus (Womb)
      • Layers: Perimetrium, Myometrium, Endometrium
    • Cervix and Cervical canal (forms birth canal with vagina)
    • Vagina
  • External Genitalia (Vulva)
    • Mons pubis
    • Labia majora & Labia minora
    • Hymen
    • Clitoris
  • Mammary Glands
    • Glandular tissue with alveoli
    • Milk storage in lactiferous ducts
    • Controlled by prolactin and oxytocin

• Gametogenesis

  • Spermatogenesis (Formation of sperms)
    • Spermatogonia ($2n$) $\rightarrow$ Primary Spermatocyte $\rightarrow$ Meiosis I $\rightarrow$ Secondary Spermatocyte $\rightarrow$ Meiosis II $\rightarrow$ Spermatids $\rightarrow$ Spermatozoa (Sperms)
    • Hormonal control: GnRH $\rightarrow$ LH & FSH
    • Structure of sperm: Head (acrosome, nucleus), Middle piece (mitochondria), Tail
    • Spermiation: Release of sperms from Sertoli cells
  • Oogenesis (Formation of ovum)
    • Oogonia ($2n$) $\rightarrow$ Primary Oocyte ($2n$, arrested in Prophase I)
    • First meiotic division completes just before ovulation $\rightarrow$ Secondary Oocyte ($n$) + First polar body
    • Second meiotic division completes upon fertilization $\rightarrow$ Ovum ($n$) + Second polar body

• Menstrual Cycle

  • Cyclic changes in female primates (reproductive cycle)
  • Menstrual Phase (Days 1-5): Breakdown of endometrium due to low progesterone/estrogen
  • Follicular / Proliferative Phase (Days 6-14):
    • Growth of primary follicles to Graafian follicle
    • Regeneration of endometrium via estrogen
    • Surge of LH and FSH on day 14
  • Ovulatory Phase (Day 14): LH surge triggers rupture of Graafian follicle (ovulation)
  • Luteal / Secretory Phase (Days 15-28):
    • Graafian follicle transforms into Corpus luteum
    • Secretes high progesterone (maintains endometrium)

• Fertilization and Implantation

  • Fertilization: Fusion of sperm and ovum in the ampullary-isthmic junction
    • Acrosomal reaction facilitates sperm entry
    • Prevents polyspermy (cortical reaction)
    • Completes meiosis II of secondary oocyte
  • Cleavage: Mitotic divisions of zygote forming blastomeres (2, 4, 8, 16 cells)
  • Morula: 8-16 celled stage
  • Blastocyst:
    • Outer layer: Trophoblast (attaches to endometrium)
    • Inner group: Inner cell mass (forms embryo)
  • Implantation: Embedding of blastocyst into the endometrium (leads to pregnancy)

• Pregnancy and Embryonic Development

  • Placenta Formation:
    • Interdigitation of fetal tissue (chorionic villi) and maternal tissue
    • Functions: Nutritive, respiratory, excretory, endocrine (hCG, hPL, estrogen, progesterone)
  • Embryonic Development:
    • Trimesters (3 months each)
    • First Month: Heart formation
    • Second Month: Limbs and digits
    • Third Month: Most organ systems formed (major organogenesis)
    • Fifth Month: First movements and head hair
    • Sixth Month: Eyelids separate, eyelashes formed
    • Ninth Month: Fully developed, ready for delivery

• Parturition and Lactation

  • Parturition (Childbirth):
    • Induced by neuroendocrine mechanism involving fetal ejection reflex
    • Release of oxytocin from maternal posterior pituitary
    • Uterine contractions become stronger, expelling the fetus and placenta
  • Lactation:
    • Production of milk at the end of pregnancy
    • First milk is colostrum (contains rich IgA antibodies for infant immunity)