Human reproduction

• Characteristics of Reproduction in Human

  • Organisms are unisexual and dimorphic
  • Fertilization is internal
  • Viviparous, i.e.; development is internal
  • Direct development, i.e.; no metamorphosis
  • Continuous breeder
• Events in Human Reproduction

  1. Gametogenesis: Production of male and female gametes

  2. Insemination: Sperms enter the female body

  3. Fertilization: Fusion of male and female gametes

  4. Implantation: Fertilized ovum gets attached to uterine wall

  5. Gestation: Period during which embryonic development occurs

  6. Parturition: Giving birth to young ones

  
• Primary sex organs


  Produce gametes and sex hormones
• Secondary sex organs


  Neither produce gametes nor secrete sex hormones but perform important function in reproduction
• Secondary Sex Characters


  Characters with the help of which an individual can be distinguished as male or female externally
• Testes


  • Extra-abdominal, situated outside the abdomen in a pouch called scrotum
  • Scrotum helps maintaining the low temperature of testes (2-2.5° C lower than normal internal body temperature) necessary for spermatogenesis
  • Covered by dense white fibrous capsule called tunica albuginea
  • Capsule extends inward forming septa that divide each testis into a series of compartments called lobules
  • Each lobule contains one to three highly coiled seminiferous tubules in which sperms are produced by the process called spermatogenesis
  • Seminiferous tubule is lined by two types of cells: male germ cells (spermatogonia) and Sertoli cells
  • Spermatogonia undergo meiotic division to form sperms
  • Sertoli cells provide nutrition to the germ cells
  • Region outside the seminiferous tubules called interstitial space which contains blood vessels and interstitial cells or Leydig cells
  • Leydig cells secrete male sex hormones called androgens

  
• Accessory Ducts


  1. Seminiferous tubules lead to a network of ducts called rete testis

  2. Rete testis opens into numerous fine ductules called vasa efferentia

  3. Vasa efferentia leave testis and open into epididymis located along posterior surface of each testis

  4. Epididymis is the site of maturation of sperms and also stores sperms temporarily

  5. Epididymis leads to vas deference that ascends to the abdomen and loops over the urinary bladder

  6. It receives or duct from seminal vesicle and forms ejaculatory duct which joins urethra

  7. These ducts store and transport the sperms from the testis to outside through urethra by an opening called urethral meatus

  
• Urethra in male


  Urino-genital duct as it carries urine as well as semen
• Seminal Vesicles


  • Secrete an alkaline viscous fluid that contains fructose and prostaglandins
  • Alkaline nature of fluid neutralizes acidity of urine which would inactivate and kill sperms
  • Fructose is used for energy production by sperms
  • Prostaglandins contribute to sperm motility

  
• Seminal plasma


  Secretions of accessory glands
• Semen


  Seminal plasma + sperms

  
• Penis


  Male external genitalia, made up of erectile tissue which facilitates insemination
• Spermatogenesis


  Immature male germ cells (spermatogonia) produce sperms, begins at puberty

  
• Spermiogenesis/ Spermioteliosis


  Transformation of non-motile, rounded and haploid spermid into a functional and motile spermatozoan

  
• Hormonal Regulation of Spermatogenesis

  • Growth, maintenance and function of secondary sex organs and secondary sex characters are under regulation of androgens
  • Growth, maintenance and function of seminiferous tubules and Leydig cells are under control of FSH and LH

  
• Acrosome


  • Small pointed structure present at the tip of nucleus, formed of Golgi body
  • During sperm entry it secretes lytic enzymes (sperm lysins): proteases and hyaluronidase which help penetrating ovum

  
• Nucleus


  • Basic proteins rich in amino acids arginine called protamines cause high degree condensation of genome
• Neck


  • Formed of 2 centrioles: proximal and distal
  • Proximal centriole is donated to ovum during fertilization which forms spindle for cleavage of zygote
  • Distal centriole acts as basal body and gives rise to axoneme of sperm tail

  
• Middle Piece


  • Formed of numerous mitochondria, which produce energy for the movement of tail that facilitate sperm motility essential for fertilization

  
• Tail


  • Axial filament of tail is formed of 11 proteinous microtubules arranged in 9+2 manner
• Ovaries


  • Primary female sex organs that produce female gametes, i.e.; ova and female sex hormones
  • Germinal epithelium is a layer of simple cuboidal epithelium which covers surface of ovary
  • Tunica albuginea is a capsule of dense connective tissue immediately beneath germinal epithelium
  • Stroma is a region of connective tissue beneath tunica albuginea and is composed of outer dense layer called cortex and inner loose region called medulla
  • Ovarian Follicles lie in cortex and consist of oocytes in various stages of development surrounded by follicular cells

  
• Oviducts/ Fallopian tubes


  • Each fallopian tube is about 10-12 cm long and extends from periphery of each ovary to the uterus
  • The part closer to ovary is funnel shaped infundibulum
  • The edges of infundibulum have finger like projections called fimbriae
  • Infundibulum leads to a wider part of oviduct called ampulla
  • Ampulla opens in abdominal cavity by an aperture called ostium
  • Ostium receives secondary oocyte released from ovary
  • The last part of oviduct is isthmus that joins uterus
  • Oviducts transport secondary oocytes and fertilized ova from ovaries to uterus
  • Fallopian tube is site of fertilization which occurs at the junction of ampulla and isthmus
  • Fallopian tube is lined by ciliated columnar epithelium

  
• Uterus/ Womb


  • Divided into: Dome shaped portion superior to oviducts, called fundus
  • Middle portion called body
  • Inferior narrow portion opening into vagina called cervix
  • Interior of body of uterus is called uterine cavity and that of cervix is called cervical canal
  • Cervical canal along with vagina forms birth canal
  • Histologically, uterus consists of 3 layers: Outermost: Perimetrium consists of simple squamous epithelium and areolar connective tissue
  • Middle: Myometrium consists of smooth muscles
  • Innermost: Endometrium consists of columnar epithelium

  
• Vagina


  • Lined by stratified non-keratinized squamous epithelium
  • Opening of vagina is covered partially by a membrane called hymen
  • Transverse folds in wall of vagina called rugae
• Mammary glands


  • Modified sweat glands that produce milk
  • Each mammary gland consists of 15-20 mammary lobes containing clusters of cells called alveoli
  • Alveoli secrete milk which flows through following duct system to feed the young one
  • Alveoli open into mammary tubules which join to form mammary duct
  • Several mammary ducts join to form mammary ampulla which is connected to lactiferous duct through which milk is sucked out
• Oogenesis


  1. The process of formation of mature gamete

  2. Oogenesis is initiated during embryonic development stage when millions of gamete mother cells (oogonia) are formed within each fetal ovary

  3. Oogonia enter prophase-1 of meiotic division and get temporarily arrested at that stage (till puberty) called primary oocytes

  4. Each primary oocyte is surrounded by a layer of granulosa cells and then is called primary follicle

  5. A large number of follicles degenerate under a process called atresia during a phase from birth to puberty

  6. After puberty, under influence of gonadotropins, primary follicles further develop to form secondary and then tertiary follicles

  7. Tertiary follicle is characterized by a fluid filled cavity called antrum

  8. Tertiary follicle changes into a mature follicle or Graafian follicle

  9. Each month the primary oocyte in the tertiary follicle completes first meiotic division

  10. Asymmetrical cytokinesis results in formation of larger haploid secondary oocyte and tiny first polar body

  11. Secondary oocyte retains bulk of nutrient rich cytoplasm of primary oocyte

  12. Secondary oocyte proceeds to metaphase II and halts at this stage

  13. Graafian follicle soon ruptures and releases secondary oocyte in the pelvic cavity. The process is called ovulation

  
• Graafian follicle


  • Has secondary oocyte surrounded by a few layers of follicular cells which are endocrine in nature and secrete estrogens
  • Protected by 2 layers: inner theca interna and outer theca externa
  • Antrum is a fluid filled cavity

  
• Ovum


  • Human ovum is alecithal (devoid of yolk) and is about 100µ
  • Nucleus is excentric so ovum is polar
  • Surrounded by 3 layers: Vitelline membrane (Primary membrane) is thin and transparent and is secreted by the ovum itself
  • Zona pellucida (Secondary membrane) is secreted by follicular cells (Corona radiata)
  • Corona radiata is formed of radially arranged follicular cells

  
• Menstrual Cycle


  • The reproductive cycle in female primates (monkey, ape and human)
  • Comprises of ovarian cycle which is a series of events associated with maturation of oocyte in ovary and uterine cycle which is a series of changes in endometrium of uterus

  
• Phases of menstrual cycle

  1. Menstrual phase (1st - 5th day)

  2. Pre-ovulatory phase (6th - 13th day)

  3. Ovulation/ Ovulatory phase (14th day)

  4. Post-ovulatory phase (15th - 28th day)

  
• Signs of Ovulation


  • Increase in body temperature by 0.4°C
  • Increase in cervical mucus secretion
  • Slight discomfort or pain in lower abdomen
• Species-specific barrier to sperm

  Corona radiata is formed of radially arranged follicular cells
• The reproductive cycle in female primates (monkey, ape and human) is called menstrual cycle
• The menstrual cycle comprises of ovarian cycle which is a series of events associated with maturation of oocyte in ovary and uterine cycle which is a series of changes in endometrium of uterus

  
• In human female, one menstrual cycle is completed in about 28-29 days starting from first day of menstruation till the next one
• Pre-ovulatory phase (6th - 13th day)


  It is more variable in length than other phases and accounts for most of differences when cycles are shorter or longer than 28 days

  
• FSH


  Follicle Stimulating Hormone
• LH


  Luteinizing Hormone

  
• Signs of Ovulation


  • Increase in body temperature by 0.4 to 0.6° F
  • Secretory cells of cervix produce large amount of thin cervical mucus which indicates time of greatest fertility

  
• Post ovulatory phase (15th to 28th day)


  It is the most constant in duration and lasts for 14 days