Lecture Notes, Biology 203, Human Sexuality and Reproduction


  1. Fertilization and Early Embryonic and Fetal Development
    1. Genetics
      1. Egg (if human, 23 chromosomes) fertilized by sperm (23 chromosomes) results in zygote (46 chromosomes)
      2. If egg or sperm had whole extra set of chromosomes, zygote would have 69, or 92 if both had extra sets: polyploidy (lethal in humans)
      3. Gametes (egg and sperm) have 23 chromosomes instead of 46 due to special reductive cell division during gametogenesis: meiosis (chromosome number reduced by 1/2)
      4. Each body cell has 46 chromosomes in 23 pairs, 22 pairs of non-sex chromosomes, or autosomes, and 1 pair of sex chromosomes
      5. Each gamete has 23 single (unpaired) chromosomes, 22 autosomes and 1 sex chromosome (either an X or a Y)
      6. Mature sperm has head with nucleus (23 chr) and acrosome (enzymes important for fertilization), midpiece (has mitochondria that produce energy), and tail (make sperm motile)
    2. Fertilization
      1. Egg is fertilizable for 24 hrs. after ovulation; sperm survive in female reproductive tract about 72 hours
      2. Only about 0.01% (1 in 10,000) of sperm reach upper 1/3 of Fallopian tube where fertilization usually takes place
      3. As sperm travels up Fallopian tube (about 7 hours), become able to fertilize egg through activation of enzymes in acrosome (capacitation)
      4. One sperm penetrates corona radiata and zona pellucida (dissolved by enzymes temporarily, becomes impenetrable after one sperm's head enters egg)
      5. Sperm nucleus fuses with egg nucleus--46 single chr pair up to make 23 pairs in zygote
    3. Early cell divisions
      1. After floating downstream in the Fallopian tube for about 30 hours, the first cell division occurs to produce a two-cell stage
      2. At about 40 hours, 2 cells divide into 4
      3. By the time the pre-embryo reaches the uterus (day 4), it is at the morula stage, a ball of undifferentiated cells
      4. By day 5, it is an early blastocyst, still enclosed in the zona pellucida, but with some cells arranged in an outer ring and some in an inner cell mass
      5. By day 6, a late blastocyst has shed the zona pellucida and is ready to implant in the uterine lining
      6. By day 7, implantation has occurred, and embryonic cells (trophoblast) are invading the uterine lining
      7. By day 10, the pre-embryo is in the form of a bilaminar disk, with two layers of somewhat differentiated cells
      8. By day 12, a trilaminar disc has formed, with three layers of cells, each of which contains cells that are the ancestors of certain body systems
        1. Endoderm (the innermost layer, next to the amnion): digestive tract, respiratory tract
        2. Mesoderm (the middle layer): muscle, bone, connective tissue, reproductive tract, circulatory system
        3. Ectoderm (the outermost layer, next to the yolk sac): nervous system, skin and its derivatives (lens of eye)
  2. Later Embryonic and Fetal Development
    1. From 2 to 8 weeks, embryo undergoes rapid structural development
    2. By 3-4 weeks, head (with eye and ear pits) and tail ends are distinguishable, brain and spinal cord present
    3. By 4-8 weeks, embryo develops eyes, ears, arm buds develop into hands and then fingers, later leg bugs develop feet, and the toes
    4. Development progresses from head end (details develop earlier) to tail end (details develop later)
    5. By 7th week, gonads have differentiated, forms of liver, lungs, pancreas, kidney, and intestines are present, but with only limited function
    6. By 10th week, fingers have fingernails
    7. By 12th week, fetus has eyebrows, lashes, is 4" long, weighs 2/3 oz.
    8. Second and third trimesters (of mother's pregnancy)
      1. Major changes are differentiation (development of function) and growth (enlargement)
      2. By 14th week, fetus can move (not necessarily detectable by mother)
      3. By 20th week, opens eyes
      4. By 24th week, sensitive to light and sound, makes vigorous arm and leg movements, has "sleeping/waking" cycle, wrinkled skin covered with downy hair, weighs about 2 lbs.
      5. In 7th month, usually settles in a head-down position
      6. In 3rd trimester, hair lost, fats deposited under skin (so less wrinkled)
      7. By end of 8th month, weighs 5 lbs. 4 oz. (average)
      8. By end of 9th month, weighs 7 lbs. 8 oz., is 20" long (averages)
    9. Placental development after 11-12th weeks
      1. Produces hormones (estrogen and progesterone)
      2. Acts as barrier between mother and fetus
      3. Controls passage of molecules across barrier
      4. On one side of trophoblast layer in placenta is fetal circulation; on other side is maternal circulation
      5. Fetus needs to obtain nutrients (amino acids, carbohydrates, vitamins, water, lipids, etc.)and antibodies from mother's blood, and to discharge waste products (carbon dioxide, urea, etc) into mother's blood
      6. Most bacteria and most kinds of antibodies and other large molecules are prevented from crossing the placenta
      7. A few bacteria and protozoa, some viruses, and some harmful molecules (many drugs, carbon monoxide, alcohol) can cross the placenta
    10. Umbilical cord
      1. Contains 2 arteries and 1 vein embedded in Wharton's jelly to stiffen cord and prevent kinks that would cut off circulation to the fetus
      2. Usually about 20" long
      3. Extends from fetus' future navel to placenta (therefore fetal in origin)
    11. Amniotic sac
      1. Contains fetus and amniotic fluid enclosed in two membranes
      2. Chorion is outer membrane
      3. Amnion is inner membrane
      4. Amniotic fluid cushions fetus, maintains constant temperature, has molecules and cells from fetus
      5. Sac ruptures in 2nd stage of labor, if not before
  3. Multiple Gestation: Twins (and higher multiples)
    1. 30% are monozygotic twins (identical)--one ovum is fertilized by one sperm, producing one zygote, which separates into two individuals at some later time
    2. 70% are dizygotic twins (fraternal)--two separate ova are fertilized by two separate sperm, and develop separately sharing the same uterus (inherited through the female line)
    3. See handout for early differentiation of monozygotic and dizygotic twins
    4. The later monozygotic twins separate into two individuals, the more tissues/organs are likely to be shared
      1. If zygote separates early into two blastocysts, monozygotic twins may develop completely separately thereafter, with separate chorions, amnions, and placentas, unless they implant very close together and the placentas fuse (risky)
      2. If zygote separates a little later into two cell masses within one blastocyst, monozygotic twins may share chorion and placenta but have separate amnions
      3. If zygote separates even later, monozygotic twins may share chorion, placenta and amnion
      4. Later separation may result in conjoined twins ("Siamese" twins)
    5. Shared tissues between dizygotic twins depend on how close to each other they implant.
      1. If very close, placentas may fuse, so that both twins share one placenta and one chorion, but are in separate amnions (risky)
      2. If farther apart, may develop completely separately
    6. Twins may have various positions near time of delivery (both cephalic, one breech/one cephalic, one cephalic/one transverse)
    7. If cephalic twin is born first, transverse or breech position of second twin may be corrected, avoiding Caesarian delivery
  4. Abnormalities of Early Development
    1. Chromosomal Defects (Inherited or Accidental)
      1. Errors of fertilization
        1. Extra copy of chromosome or part of chromosome
        2. Deletion of chromosome or part of chromosome
        3. Translocation of part of chromosome
        4. Down's syndrome
          1. 95% of cases are trisomy-21 (extra copy of chr 21)
          2. Children have small round heads, protruding tongue, small mouth, slanting eyes, variable extent of retardation
          3. More common in older mothers: under 35 1/1000-2000, 35-40 1/300, over 45 1/30-35
        5. Cri du chat syndrome
          1. Deletion of part of chr 5
          2. Defects cause peculiar cry, early death
        6. Enzymatic defects (phenylketonuria)
        7. Turner's syndrome (genetic female)
          1. Only one X chromosome (XO)
          2. Immaturre gonads, sterile, short webbed neck, chest heart arm, facial defects, learning disabilities
        8. Klinefelter's syndrome (genetic male)
          1. Extra copy of X chromosome (XXy)
          2. Immature gonads, sterile, poor secondary sexual development, tall, retarded, behavioral problems
          3. More severe if two extra X chromosomes
        9. Triple X syndrome (genetic female)
          1. Extra copy of X chromosome (XXX)
          2. Extra X chromosome suppressed, only two used
          3. Usually normal, occasionally have fertility problems, some retardation
    2. Environmentally Induced Defects
      1. Biological agents
        1. Rubella virus (and others)
        2. Syphilis (bacterial)
        3. Toxoplasmosis (protozoan)
      2. Chemical agents
        1. Often teratogenic
        2. Drugs (over-the-counter, prescription, abused)
        3. Environmental chemicals (dioxin, etc.)
        4. Accutane (used to treat severe acne)
        5. Thalidomide (originally a sedative)
        6. Dilantin (anticonvulsant)
      3. Radiation (penetrating radiation such as Xrays or gamma rays)
      4. Folic acid and spina bifida
        1. Occurs in some women who have a defective gene for an enzyme
        2. Normal gene for enzyme: homocysteine (HC, toxic) converted by normal enzyme to methionine (MET, nontoxic)
        3. Defective gene for enzyme requires folic acid to function
          1. If woman's diet is rich in folic acid, defective enzyme still works, so there is no toxic accumulation of HC
          2. If woman's diet is poor in folic acid, defective enzyme doesn't work, and HC accumulates to toxic levels, causing central nervous system damage that results in spina bifida
      5. Refer to handout
        1. If exposure to agent is in first two weeks postconception, pre-embryo either dies, or normal cells replace damaged cells because they are still quite undifferentiated
        2. If exposure to agent occurs after first two weeks, type and extent of damage depend on the agent, length of exposure, time of exposure, etc.
        3. Major morphological defects occur mostly in weeks 3-8, when lots of structural development occurs
        4. After week 8, defects may be minor morphological defects or physiological defects, when refinements of structure and functional development occur