Reproductive System and Development

Reproductive System

  • Male and female reproductive structures and their functions
    • gonads
      • male: testes
        • makes sperm in the seminiferous tubules.
        • makes testosterone.
        • external.
      • female: ovaries
        • houses immature egg, which matures monthly after puberty.
        • makes estrogen.
        • internal.
    • genitalia
      • male: testes, penis, and various ducts and glands.
        • sperm made in the seminiferous tubules.
        • stored in the epididymis.
        • travels through vas deferens → ejaculatory duct → urethra → penis
        • mnemonic: seven up = Seminiferous tubules, Epididymis, Vas deferens, Ejaculatory duct, nothing, Urethra, Penis.
      • female: ovaries, fallopian tubes, uterus, vagina
        • Monthly cycle: primary oocyte matures into secondary oocyte every month. To prepare for it, the endometrium thickens. If fertilization doesn't occur, menses occur, and the cycle begins anew.
        • GnRH = stimulates release of FSH and LH.
        • FSH = folicle stimulating hormone = stimulates growth and maturation of follicle.
        • Follicle = houses oocyte and produces estrogen.
        • Estrogen = normally inhibits LH and FSH, but causes LH surge when it reaches a certain threshold.
        • Estrogen reaches this threshold → surge of LH occurs.
        • LH = leutinizing hormone = luteinizing hormone = stimulates the outer cells of the follicle = turns it into corpus luteum + maintains it.
        • LH surge triggers primary oocyte → secondary oocyte → rupture of follicle.
        • Corpus luteum = makes estrogen and progesterone = maintains endometrium.
        • No fertilization → LH falls → corpus luteum dies → estrogen and progesterone fall → endometrium dies (menses) → cycle begins anew with FSH and LH re-rising.
        • Fertilization occurs → implanted embryo releases hCG → hCG mimics LH to maintain corpus luteum → estrogen and progesterone maintained by corpus luteum → placenta takes over the responsibility of making estrogen and progesterone later on.
        • menstrual cycle hormones
    • differences between male and female structures
      • male: mostly external. Shared passage with urinary tract.
        male genitalia structure and spermatogenesis
      • female: mostly internal. Separate passage from urinary tract.
        female genitalia structure and oogenesis
  • Gametogenesis by meiosis
    gametogenesis and embryogenesis
    • Male = spermatogenesis = occurs in the seminiferous tubules.
      1. Spermatogonium (2n) = stem cell. Mitosis of spermatogonium can either create more spermatogonium or create primary spermatocyte.
      2. Spermatogonium (2n) → mitosis → primary spermatocyte (2n). Occurs after puberty.
      3. Primary spermatocyte (2n) → meiosis I → Secondary spermatocyte (n).
      4. Secondary spermatocyte (n) → meiosos II → spermatid (n).
      5. Spermatid (n) → mature → sperm (n). The fancy name for sperm is spermatozoa.
    • Female = oogenesis = occurs in the ovaries, then fallopian tubes.
      1. Oogonium (2n) = stem cell.
      2. Oogonium (2n) → mitosis → primary oocyte.
      3. Primary oocyte (2n) arrests at prophase I (occurs before birth). One comes out of arrest every month (between puberty and menopause).
      4. Primary oocyte (2n) → meiosis I → secondary oocyte (n). Ruptures from ovary follicle into the fallopian tube.
      5. Secondary oocyte (n) arrests at metaphase II. Comes out of arrest if fertilization occurs.
      6. Secondary oocyte (n) → meiosis II → ovum (n).
  • Ovum and sperm
    • differences in formation
      Male and female gametogenesis side by side
      Male Female Difference
      Spermatogonium (2n) Oogonium (2n) Spermatogonium renews its population by mitosis throughout life. Oogonium stops renewing its population sometime before birth
      Primary spermatocyte Primary oocyte Primary oocye arrests at prophase I
      Secondary spermatocyte Secondary oocyte Secondary oocyte arrests at metaphase II
      Sperm Ovum Between the secondary spermatocyte and the sperm, there's the spermatid
    • differences in morphology
      • Sperm = motile = flagella.
      • Egg = non-motile = round.
    • relative contribution to next generation
      • Sperm contributes DNA only (the egg actively destroys sperm mitochondria).
      • Egg contributes DNA + everything else (mitochondria, organelles, epigenetics).
  • Reproductive sequence (fertilization, implantation, development, birth)
    1. fertilization: sperm + egg → zygote
    2. implantation:
      1. zygote
      2. morula (solid ball)
      3. blastula (sea urchins) or blastocyst (mammals)
      4. the blastocyst is the one that implants in the endometrium
    3. development:
      1. zygote
      2. blastocyst
      3. implantation
      4. gastrulation
      5. organogenesis
    4. Birth:
      • Switch from getting oxygen from mom's blood → breathing.
      • Switch from getting nutrients from mom's blood → suckling.
      • Fetal circulation (which bypasses lungs and liver) → normal circulation (by closing off ducts and opennings).

Embryogenesis

  • Stages of early development (order and general features of each)
    • fertilization
      1. Sperm meets egg
      2. Acrosomal reaction causes sperm to penetrate egg
      3. Cortical reaction causes egg to prevent additional sperm from penetrating
      4. Egg completes meiosis II
      5. Sperm and egg nuclei fuse
    • cleavage
      • Normal mitotic cell divisions: cell grows then divides, grows again, then divides.
      • Cleavage = mitotic divisions without cell growth.
    • blastula formation
      1. fertilization produces zygote
      2. cleavage produces a solid ball called the morula
      3. morula hollows out into the blastula or blastocyst
        • blastula occurs in non-mammals
        • blastocyst occurs in mammals
      4. blastocyst implants
    • gastrulation
      • first cell movements
        • Cells from the surface migrate inwards.
        • gastrulation occurs slightly different for different animals. Some by invagination, some by migration, some by splitting.
        • In mammals, the cells start migrating inward at the primitive streak.
      • formation of primary germlayers (endoderm, mesoderm, ectoderm)
        • The cells that migrate inwards form the endoderm.
        • The cells that remain outside is the ectoderm.
        • The cells in the middle are the mesoderm.
    • neurulation
      • ectoderm → brain and spinal cord
      • the ectoderm does so by folding into a tube
  • Major structures arising out of primary germ layers
    • endoderm = innermost layer = guts, lungs, and digestive internal organs (liver, pancreas).
    • mesoderm = middle layer = muscle, blood and bone tissues, and interal organs (kidney and gonads).
    • ectoderm = outermost layer = skin and nerves (including the brain).

Developmental Mechanisms

  • Cell specialization = commitment followed by differentiation
    • commitment = specification followed by determination
    • specification = cell is just beginning to be commited to develope into a certain cell type. The commitment can be reversed at this stage.
    • determination = irreversible commitment to become a certain cell type.
    • differentiation = becoming a cell type and adopting its specialized functions.
      • epidermal cells produce keratin to protect skin against abrasion.
      • myocyte produce actin and myosin to make muscles contract.
      • neurons make neurotransmitters to transmit electrochemical impulses.
    • tissue types
      • Epithelial: skin, lining of organs
      • Connective: blood, bone, tendons, ligaments, cartilage
      • Nervous: brain, spinal cord, nerves
      • Muscle: skeletal, smooth, and cardiac muscle
  • Cell communication in development
    • Induction: one group of cells changing the behavior of an adjacent group of cells.
    • inducer = the one that sends the signal for the other to change.
    • responder = the one that gets the signal and changes.
    • For example, the optic vesicle is able to induce the ectoderm to develope into lens.
    • Another example is the induction of wing feathers in the chick by the dermal mesenchyme.
    • Induction mechanisms: physical touching of cells (juxtracine) or by releasing chemicals (paracrine).
  • Gene regulation in development
    • Differential gene transcription:
      • modification of DNA (methylations) can shut off or turn on genes.
      • modification on histones (methylations, acetylations) that wrap the DNA can shut off or turn on genes.
      • to make or not to make transcription factors can regulate what genes get transcribed.
    • Differential RNA processing:
      • selecting what RNA make it outside the nucleus to be translated.
      • alternative splicing of RNA.
    • Translation regulation
      • some mRNA are made to last longer than others (more proteins translated off of it), and some are made to be rapidly degraded (less proteins translated off of it).
      • selective inhibition of translation of stored RNA in the oocyte. Get translated only when needed after fertilization.
    • Post-translational regulation
      • some proteins are inactive until modified by certain enzymes.
      • active proteins can be selectively marked for degradation by ubiquitin.
  • Programmed cell death
    • apoptosis = programmed cell death.
    • During apoptosis, strong proteases are activated and they digest the cell from within. In mammals, the proteases are called caspases.
    • The spaces between our fingers are created by apoptosis.
    • The tail of a tadpole undergoes apoptosis when it morphs into a frog.