HUMAN REPRODUCTIVE DEVELOPMENT

• Human life cycle

  Starts with fertilization, followed by a series of growth and developmental processes that allow the formation of a fully functional individual
• Fertilization

  Male parent's sperm cell unites with female parent's egg cell
• Zygote


  Fertilized egg that will continue to develop and become an embryo through a series of active cell division and differentiation
• Stages of human development


  • Embryo
  • Fetus
  • Infant
  • Childhood
  • Adolescence
  • Adulthood
• Embryonic stage

  • Developmental phase inside the female's womb before becoming a fetus
• Fetal stage

  • Developmental phase between the embryonic state and birth, where the fetus receives oxygen and nourishment through the placenta
• Infancy

  • Stage from birth to 1 year old, where babies continue their development to attain full functionality of their organs
• Childhood

  • Stage from 1 to 12 years old, where children develop locomotor and cognitive skills
• Adolescence


  • Stage from 13 to 19 years old, where teenagers undergo puberty and develop secondary sex characteristics
• Adulthood


  • Stage where humans become sexually mature physically and behaviorally, categorised as young adults (20-35), middle-aged adults (36-54), and elderly (55-65)
• Reproduction in humans is exclusively sexual, involving the production of sex cells or gametes and their fusion during fertilization
• Male reproductive system


  Consists of the gonads, accessory glands, ducts, and the penis
• Testes


  • Primary reproductive glands that produce sperm cells, contained outside the abdomen in the scrotum
• Ejaculation


  Forceful expulsion of the semen, where sperm cells from the testes exit into the vas deferens
• Glands contributing to semen

  • Seminal vesicle
  • Prostate gland
  • Bulbourethral gland
• Penis


  Male intromittent organ, consists of nerves that aid in erection and forceful expulsion of semen
• Female reproductive system


  Provides the proper condition for the production of egg cells and for fertilization to occur
• Ovaries


  • Primary reproductive structures that produce eggs
• Fallopian tubes/Oviducts


  • Catch the eggs during ovulation and are the site of fertilization
• Uterus/Womb


  • Chamber with a highly vascularized epithelial layer called the endometrium to house and nourish the developing embryo, has a muscular layer to help expel the baby during labor
• Vagina/Birth canal


  • Elastic channel that accommodates the penis during sexual intercourse and expels the baby during labor
• Cervix

  • Narrow region between the uterus and the vagina that helps prevent the entry of other foreign materials
• Ovaries


  • Primary reproductive structures that produce eggs
• Fallopian tubes/oviducts


  Catch the eggs during ovulation through their finger-like appendages called fimbriae, site of fertilization
• Uterus/womb


  Highly vascularized epithelial layer called the endometrium to house and nourish the developing embryo, muscular layer helps expel the baby during labor
• Vagina


  Elastic channel that accommodates the penis during sexual intercourse to receive the sperm cells
• Cervix


  Narrow region between the uterus and vagina that helps prevent the entry of other foreign materials
• External genitalia of females

  • Labia minora and labia majora, clitoris functions for sexual stimulation, Bartholin's gland secretes mucus for lubrication
• Gonads


  Gamete-producing organs (testes and ovaries)
• Gametogenesis

  Process of gamete formation, wherein the diploid chromosome set of precursor cells is reduced into a haploid set
• Spermatogenesis


  1. Sperm cell production and development in males, occurs in the seminiferous tubules of the testes

  2. Proliferation: Mitotic division of spermatogonia

  3. Growth: Spermatogonia transform into primary spermatocytes

  4. Maturation: Meiotic division to reduce chromosome number, secondary spermatocytes produce spermatids

  5. Differentiation: Spermatids mature into spermatozoa with distinct head and tail regions
• Oogenesis

  1. Egg cell production and nourishment, occurs in the ovaries

  2. Proliferation: Mitotic division of oogonia during pregnancy

  3. Growth: Oogonia transform into primary oocytes

  4. Maturation: Meiotic division of primary oocytes, secondary oocyte arrested at metaphase II

  5. Differentiation: Secondary oocyte becomes the mature ovum
• Fertilization


  Fusion of sperm and egg cells to form a diploid zygote
• Fertilization process


  1. Sperm cells travel to the fallopian tube to meet the egg cell

  2. Sperm releases acrosomal enzymes to degrade the zona pellucida layer

  3. Plasma membranes of sperm and egg fuse, paternal genetic material combines with egg

  4. Mechanisms to prevent polyspermy: fast-block (electrical charge change) and slow-block (cortical granule reaction)
• Pre-implantation embryonic development

  1. Zygote travels to the uterus, undergoes mitotic cell divisions (2-cell, 4-cell, 8-cell, 16-cell morula, blastocyst)

  2. Inner cell mass becomes the embryo, outer trophoblast develops into the chorionic sac and placenta
• Transition periods


  1. Zygote divides

  2. Two-cell stage

  3. Four-cell stage

  4. Eight-cell stage
• Morula stage


  16-cell phase arrives in the uterus
• Blastocyst


  Ball of cells formed as the zygote continues to divide in the uterus
• Inner cell mass


  Becomes the embryo
• Trophoblast

  Outer shell that develops into the chorionic sac and the fetal portion of the placenta