Last time, I mentioned that reproduction in most animals is highly seasonal -- only certain times during the year that reproduction occurs --humans have deviated dramatically from this pattern.
Males: no cycles -- sexually mature males are reproductive and capable of fathering offspring at any time during the year.
At puberty, the hypothalmus (an endocrine gland) releases a hormone called GnRH which stimulates the anterior pituitary gland to secrete 2 hormones called gonadotropins: LH (luteinizing hormone) and FSH (follicle stimulating hormone).
1. LH acts on Leydig cells of testes -- stimulates them to begin producing testosterone (male sex hormone).
2. FSH and testosterone together act on seminiferous tubules to stimulate sperm production (spermatogenesis).
3. hormonal feedback loop regulates sperm production -- if testosterone level drops, hypothalmus secretes more GnRH which causes pituitary to secrete LH and FSH and testosterone/sperm production is stimulated -- if testosterone exceeds a certain level, GnRH secretion goes down and so does LH and FSH secretion -- by this mechanism, levels of LH, FSH and testosterone remain nearly constant from puberty onward in males.
Females: human females have retained some aspects of cyclic reproduction -- cycle of 28 days rather than seasonal cycle of other mammals -- actually two concurrent cycles -- both under hormonal control -- cycles occur from puberty until menopause (age 45-50), at which time the ovaries are no longer capable of being stimulated by pituitary hormones -- cycles are also interrupted by pregnancy.
ovarian cycle -- production of mature egg by follicle -- followed by ovulation -- then secretion of estrogen and progesterone by old follicle.
menstrual cycle -- the hormones changes in uterine lining -- loss of endometrium (menstrual phase; days 1-5)) -- rebuilding of endometrium (proliferative phase; days 6-13) -- thickening of endometrium and glycogen accumulation (secretory phase; days 15-28).
Let's look at these cycles.
These cycles involve changes at 4 different levels during the 28 day cycle -- 1) changes in levels of gonadotropins secreted by pituitary (LH, FSH); 2) changes in follicle leading to ovulation and the subsequent transformation of follicle into corpus luteum; 3) changes in levels of hormones secreted by the ovary (estrogen and progesterone); 4) changes in uterine lining (endometrium).
1. menstruation: days 1-5 -- loss of endometrium because of low estrogen and progesterone level.
2. follicular phase: days 1-13 -- pituitary begins secreting LH and FSH -- FSH causes follicle to mature and to begin secreting estrogen -- estrogen facilitates rebuilding of endometrium.
3. ovulation: day 14 -- preceded by peaks in LH, FSH and estrogen secretion -- follicle ruptures and egg is released.
4.. luteal phase: days 15-28 -- LH stimulates follicle to become corpus luteum and to being secreting estrogen and progesterone -- uterine lining thickens and becomes highly vascularized; glycogen accumulates in the endometrium.
5. high estrogen and progesterone causes decrease in LH -- causes corpus luteum to degenerate and levels of progesterone and estrogen to drop -- thick, blood-rich endometrium cannot be maintained w/o high levels of estrogen and progesterone -- begins to disintegrate leading to menstruation.
Figure 38.21.
This cycle will continue over and over again unless it is interrupted by fertilization which leads to pregnancy -- fertilization is preceded by sexual arousal, intercourse and orgasm.
There are two basic physiological processes that occur during arousal:
1. vasocongestion -- increase in amount of blood in vessels -- occurs primarily in the pelvic blood vessels, but also in other areas of the body as well.
2. myotonia -- increase in muscular tension culminating in strong muscular contractions during orgasm-- not just in genitals, but throughout the body.
Physiologists recognize 4 phases during sexual arousal and orgasm:
1. Excitement Phase -- beginning of arousal -- changes due to vasocongestion.
A. males -- most obvious characteristic is erection of the penis-- corpora cavernosa becomes engorged with blood due to vasocongestion -- usually occurs within seconds of stimulation -- time varies with age, fatigue and the influence of drugs,.
B. females -- lubrication of vagina by vaginal sweating -- vasocongestion forces water out of cells lining vagina -- vasocongestion also causes breasts to swell and clitoris to become erect.
C. both sexes -- myotonia may cause nipples to become erect -- blood pressure and pulse rates increase.
2. Plateau Phase -- maximal vasocongestion -- further increases in blood pressure and pulse rate.
A. females -- formation of orgasmic platform -- swelling of tissues surrounding lower 1/3 of vagina causing vagina to actually decrease in size -- labia minora change color: bright red to deep wine (woman that have had children) or pink to bright red (woman w/o children).
B. males -- penis reaches maximal erection -- testes become engorged with blood and may increase in size 50% -- few drops of Cowper's gland secretion appear on tip of penis.
3. Orgasmic Phase --
A. male -- rhythmic contractions of pelvic organs at 0.8 second intervals -- contractions of vas deferens, seminal vesicles, and prostate force semen into a bulb at base of urethra -- penis and urethra contract rhythmically forcing semen out with considerable force = ejaculation.
B. female -- rhythmic contractions (0.8 seconds) of orgasmic platform, uterus and other pelvic organs -- number and intensity of contractions highly variable -- several mild ones or a dozen intense ones.
C. both sexes -- myotonia culminates with contractions of muscles throughout the body -- facial, buttocks, legs, arms, etc. -- may be covered with a thin layer of perspiration.
4. Resolution Phase -- return to pre-arousal state by male and female.
A. female -- 15-30 minutes for woman reaching orgasm -- >1 hour in those not reaching orgasm -- may enter directly into another cycle of arousal and orgasm -- no refractory period -- females are capable of multiple orgasms.
B. males -- penis becomes flacid -- enter a refractory period during which they are incapable of arousal -- time varies from few minutes to 24 hours.
Fertilization and pregnancy:
1. during intercourse, about 300,000,000 sperm are usually introduced into the female reproductive tract by ejaculation -- several thousand may get to upper 1/3 of fallopian tube.
2. sperm must be introduced 2 days before or 15 hours after ovulation -- a single sperm fuses with an ovum to form a zygote.
3. 5-6 days after fertilization, zygote burrows into the endometrium of uterus -- the process is called implantation -- zygote produces a structure called the placenta.
4. placenta regulates interchange of nutrients between fetus and mother -- also secretes large quantities of progesterone and estrogen throughout pregnancy -- maintain thick, highly vascularized endometrium -- high estrogen levels decrease LH and FSH levels so that ovulation doesn't occur during pregnancy.
5. end of pregnancy signaled when placenta begins to digress and its secretions drop.
Birth Control: procedures for avoiding pregnancy and the birth of children --many reasons why birth control may be desirable -- health considerations, economic reasons, etc. -- let's look at some of the various methods, their advantages, disadvantages, and their effectiveness.
Birth control procedures can be placed into two general categories:
1. contraception -- techniques that prevent conception (= fertilization).
2. abortion -- techniques that terminate pregnancies -- used after conception.
Intercourse w/o any contraception is about 10% effective.
I. Rhythm -- "natural birth control" -- abstaining from intercourse during fertile period of ovarian cycle when ovulation occurs -- most effective for woman having regular 28 day cycle -- assumes ovulation occurs on day 14 -- could happen on 13 or 15 -- 3 days added to front and 2 to end -- no intercourse from days 10-17.
Advantages: accepted by Catholic Church; no side effects; inexpensive; easily reversible.
Disadvantages: high failure rate = 75% effective
II. Coitus interruptus -- "withdrawal" -- penis removed from vagina before ejaculation occurs -- most ancient form of birth control -- Onan in Old Testament chastised by God for "spilling his seed on the ground" --
Advantages: cheap, easily reversible.
Disadvantages: psychologically stressful; not very effective (74% effective) due to poor timing and the fact that bulbourethral gland fluid may contain live sperm -- fertilization possible even without ejaculation.
III. Douching -- flushing the vagina with a fluid after intercourse -- may fail to reach sperm in uterus -- may even wash sperm up -- not very effective as a birth control procedure (40% effective).
IV. Condom -- "rubber" -- thin, elastic sheath that fits over the penis.
Advantages: fairly effective (85%); only birth control device available to men (except sterilization); relatively inexpensive; also protects against sexually transmitted diseases.
Disadvantages: loss of sensitivity; loss of spontaneity; must be used properly (e.g. removed immediately after ejactulation).
V. Spermacides -- foams, creams, etc. that kill sperm on contact -- applied in vagina near cervix -- left in 6-8 hours after intercourse.
Advantages: fairly inexpensive; no prescription needed.
Disadvantages: may irritate vagina or penis; loss of spontaneity; may interfere with oral sex; moderately effective by themselves (80%).
VI. Diaphragm -- circular, dome-shaped piece that fits snugly over the cervix -- mechanical contraceptive -- left in at least 6 hours after intercourse -- blocks passage of sperm into uterus.
Advantage: moderately effective if used with a spermacide (84%); allows intercourse during menstruation.
Disadvantages: requires fitting by medical person; requires forethought or loss of spontaneity.
VII. Intrauterine Device (IUD) -- plastic or metal piece surgically inserted into the uterus -- not exactly sure how this works but does prevent zygote from implanting in uterine lining -- NOTE: this is NOT contraception!
Advantages: effective (98%); simple to use (check to make sure its there); no forethought required.
Disadvantages: side effects include uterine and tubal infections from careless insertion; may perforate uterus (1/10,000) and be fatal; 10%-20% women report cramping, irregular bleeding, increased menstrual flow.
VIII. The Pill -- pill contains estrogen and progestin (synthetic progesterone) -- doses higher than body levels for 20-21 days -- no pill for 7 days -- high estrogen/progestin levels prevent pituitary from secreting FSH -- follicle does not mature and ovulation is prevented -- 7 days without pill allows menstruation to occur -- "artificial pregnancy."
Advantages: effective (99%).
Disadvantages: must be used properly; may be expensive over long period of time; myriad of side effects including nausea, weight gain, irritablity, vaginal discharge, vaginitis, headaches, increase blood pressure and blood clotting that can lead to stroke -- 3/100,000 per year die.
IX. Vasectomy -- male sterilization -- section removed from each vas deferens -- ejaculate doesn't contain sperm -- simple 20 minute surgical procedure -- does not effect secondary sex characteristics -- not same as castration.
Advantages: nearly 100% effective; allows men to assume b.c. responsibility; minimal health risk; no further planning needed.
Disadvantages: not easily reversible; psychological effects?
X. Tubal Ligation -- section removed from each oviduct so that egg and sperm cannot meet.
Advantages: nearly 100% effective.
Disadvantages: not easily reversible but becoming more so; more serious surgical procedure that may have complications.
XI. Abortion -- termination of pregnancies -- controversial and emotional issue -- may be "spontaneous" or "induced".
1. dilation and evacuation (D and E)-- up to 20 weeks into pregnancy -- cervix dilated and suction used to remove uterine contents -- takes about 10 minutes -- most common technique.
2. dilation and curettage (D and C) -- up to 20 weeks -- dilation of cervix and uterine contents scraped out with surgical instrument -- more discomfort, more expensive.
Later in pregnancy, miscarriage induced to abort fetus -- induced labor by chemical means -- expensive and hazardous.
[If time permits!!]
Now, let's consider sexually transmitted diseases (STD's) -- infectious diseases primarily transmitted by sexual contact between individuals -- also called venereal diseases -- with the exception of herpes and acquired immune deficiency syndrome (AIDS), STD's are treatable.
Gonorrhea -- bacterial disease -- best-known STD -- in males, pus is discharged from the penis and urination burns a few days after infection -- few symptoms in females -- may lead to extensive tubal infections that can cause sterility -- usually treated with penicillin, although penicillin-resistent strains are becoming common.
Syphilis -- also a bacterial disease -- painful ulcer called a chancre develops at site of infection -- if untreated chancre goes away -- no disease for 2-4 months -- secondary stage is a generalized skin rash -- can go into latent phase lasting rest of life -- may enter stage where it does severe nervous and circulatory damage leading to death -- can be treated with penicillin.
Herpes -- viral infection that is an epidemic today -- estimated that about 20 million Americans have genital herpes -- Herpes simplex virus I causes cold sores and fever blisters on lips -- HSV type II causes genital lesions -- 2-20 days after infection, blisters at site of infection develop -- these rupture leaving painful ulcers that may take 5 days to 3 weeks to heal -- disease goes dormant and blisters may reappear at variable intervals -- dormant virus resides in nerve cells in spinal cord -- HSV II has been linked to a form of cervical cancer -- while drugs are available to alleviate symptoms and reduce discomfort, there is no known cure for herpes infection.
Acquired Immune Deficiency Syndrome (AIDS) -- viral infection that disables the body's immune system -- makes victim susceptible to infections and even some cancers -- caused by human immunodeficiency virus (HIV) -- a retrovirus -- transmitted by blood and body fluids during sexual intercourse -- may also be transmitted by transfusion, sharing of contaminated needles and from mothers to children during pregnancy or birth.
No known cure and is almost always fatal -- researchers do not yet have a vaccine -- once infected, individuals may remain symptom-free for years -- may unknowingly transmit disease-- estimated that 25%-50% of those infected will develop full-blown AIDS within 5 years --
World Health Organization estimates that 14 million people have died from AIDS world-wide -- a global problem -- infection rates in some African cities is 100X infection rate in U.S.
Who are AIDS victims in the U.S.? 73% are homosexual or bisexual males; 17% are intraveneous drug users; 3% are hemophiliacs or others requiring blood products; 1% are woman who are sexual partners of infected men and the children of these woman.
Next time: Exam III