Lecture Notes, Biology 203, Human Sexuality and Reproduction
Conception
- Fertilization and Early Embryonic and Fetal Development
- Genetics
- Egg (if human, 23 chromosomes) fertilized by sperm (23 chromosomes) results in zygote (46 chromosomes)
- 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)
- 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)
- Each body cell has 46 chromosomes in 23 pairs, 22 pairs of non-sex chromosomes, or autosomes, and 1 pair of sex chromosomes
- Each gamete has 23 single (unpaired) chromosomes, 22 autosomes and 1 sex chromosome (either an X or a Y)
- 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)
- Fertilization
- Egg is fertilizable for 24 hrs. after ovulation; sperm survive in female reproductive tract about 72 hours
- Only about 0.01% (1 in 10,000) of sperm reach upper 1/3 of Fallopian tube where fertilization usually takes place
- As sperm travels up Fallopian tube (about 7 hours), become able to fertilize egg through activation of enzymes in acrosome (capacitation)
- One sperm penetrates corona radiata and zona pellucida (dissolved by enzymes temporarily, becomes impenetrable after one sperm's head enters egg)
- Sperm nucleus fuses with egg nucleus--46 single chr pair up to make 23 pairs in zygote
- Early cell divisions
- After floating downstream in the Fallopian tube for about 30 hours, the first cell division occurs to produce a two-cell stage
- At about 40 hours, 2 cells divide into 4
- By the time the pre-embryo reaches the uterus (day 4), it is at the morula stage, a ball of undifferentiated cells
- 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
- By day 6, a late blastocyst has shed the zona pellucida and is ready to implant in the uterine lining
- By day 7, implantation has occurred, and embryonic cells (trophoblast) are invading the uterine lining
- By day 10, the pre-embryo is in the form of a bilaminar disk, with two layers of somewhat differentiated cells
- 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
- Endoderm (the innermost layer, next to the amnion): digestive tract, respiratory tract
- Mesoderm (the middle layer): muscle, bone, connective tissue, reproductive tract, circulatory system
- Ectoderm (the outermost layer, next to the yolk sac): nervous system, skin and its derivatives (lens of eye)
- Later Embryonic and Fetal Development
- From 2 to 8 weeks, embryo undergoes rapid structural development
- By 3-4 weeks, head (with eye and ear pits) and tail ends are distinguishable, brain and spinal cord present
- By 4-8 weeks, embryo develops eyes, ears, arm buds develop into hands and then fingers, later leg bugs develop feet, and the toes
- Development progresses from head end (details develop earlier) to tail end (details develop later)
- By 7th week, gonads have differentiated, forms of liver, lungs, pancreas, kidney, and intestines are present, but with only limited function
- By 10th week, fingers have fingernails
- By 12th week, fetus has eyebrows, lashes, is 4" long, weighs 2/3 oz.
- Second and third trimesters (of mother's pregnancy)
- Major changes are differentiation (development of function) and growth (enlargement)
- By 14th week, fetus can move (not necessarily detectable by mother)
- By 20th week, opens eyes
- 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.
- In 7th month, usually settles in a head-down position
- In 3rd trimester, hair lost, fats deposited under skin (so less wrinkled)
- By end of 8th month, weighs 5 lbs. 4 oz. (average)
- By end of 9th month, weighs 7 lbs. 8 oz., is 20" long (averages)
- Placental development after 11-12th weeks
- Produces hormones (estrogen and progesterone)
- Acts as barrier between mother and fetus
- Controls passage of molecules across barrier
- On one side of trophoblast layer in placenta is fetal circulation; on other side is maternal circulation
- 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
- Most bacteria and most kinds of antibodies and other large molecules are prevented from crossing the placenta
- A few bacteria and protozoa, some viruses, and some harmful molecules (many drugs, carbon monoxide, alcohol) can cross the placenta
- Umbilical cord
- 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
- Usually about 20" long
- Extends from fetus' future navel to placenta (therefore fetal in origin)
- Amniotic sac
- Contains fetus and amniotic fluid enclosed in two membranes
- Chorion is outer membrane
- Amnion is inner membrane
- Amniotic fluid cushions fetus, maintains constant temperature, has molecules and cells from fetus
- Sac ruptures in 2nd stage of labor, if not before
- Multiple Gestation: Twins (and higher multiples)
- 30% are monozygotic twins (identical)--one ovum is fertilized by one sperm, producing one zygote, which separates into two individuals at some later time
- 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)
- See handout for early differentiation of monozygotic and dizygotic twins
- The later monozygotic twins separate into two individuals, the more tissues/organs are likely to be shared
- 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)
- If zygote separates a little later into two cell masses within one blastocyst, monozygotic twins may share chorion and placenta but have separate amnions
- If zygote separates even later, monozygotic twins may share chorion, placenta and amnion
- Later separation may result in conjoined twins ("Siamese" twins)
- Shared tissues between dizygotic twins depend on how close to each other they implant.
- If very close, placentas may fuse, so that both twins share one placenta and one chorion, but are in separate amnions (risky)
- If farther apart, may develop completely separately
- Twins may have various positions near time of delivery (both cephalic, one breech/one cephalic, one cephalic/one transverse)
- If cephalic twin is born first, transverse or breech position of second twin may be corrected, avoiding Caesarian delivery
- Abnormalities of Early Development
- Chromosomal Defects (Inherited or Accidental)
- Errors of fertilization
- Extra copy of chromosome or part of chromosome
- Deletion of chromosome or part of chromosome
- Translocation of part of chromosome
- Down's syndrome
- 95% of cases are trisomy-21 (extra copy of chr 21)
- Children have small round heads, protruding tongue, small mouth, slanting eyes, variable extent of retardation
- More common in older mothers: under 35 1/1000-2000, 35-40 1/300, over 45 1/30-35
- Cri du chat syndrome
- Deletion of part of chr 5
- Defects cause peculiar cry, early death
- Enzymatic defects (phenylketonuria)
- Turner's syndrome (genetic female)
- Only one X chromosome (XO)
- Immaturre gonads, sterile, short webbed neck, chest heart arm, facial defects, learning disabilities
- Klinefelter's syndrome (genetic male)
- Extra copy of X chromosome (XXy)
- Immature gonads, sterile, poor secondary sexual development, tall, retarded, behavioral problems
- More severe if two extra X chromosomes
- Triple X syndrome (genetic female)
- Extra copy of X chromosome (XXX)
- Extra X chromosome suppressed, only two used
- Usually normal, occasionally have fertility problems, some retardation
- Environmentally Induced Defects
- Biological agents
- Rubella virus (and others)
- Syphilis (bacterial)
- Toxoplasmosis (protozoan)
- Chemical agents
- Often teratogenic
- Drugs (over-the-counter, prescription, abused)
- Environmental chemicals (dioxin, etc.)
- Accutane (used to treat severe acne)
- Thalidomide (originally a sedative)
- Dilantin (anticonvulsant)
- Radiation (penetrating radiation such as Xrays or gamma rays)
- Folic acid and spina bifida
- Occurs in some women who have a defective gene for an enzyme
- Normal gene for enzyme: homocysteine (HC, toxic) converted by normal enzyme to methionine (MET, nontoxic)
- Defective gene for enzyme requires folic acid to function
- If woman's diet is rich in folic acid, defective enzyme still works, so there is no toxic accumulation of HC
- 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
- Refer to handout
- 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
- 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.
- Major morphological defects occur mostly in weeks 3-8, when lots of structural development occurs
- After week 8, defects may be minor morphological defects or physiological defects, when refinements of structure and functional development occur