Autumn.wmf (12088 bytes) Concepts of Biology (BIOL116) - Dr. S.G. Saupe; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; ssaupe@csbsju.edu; http://www.employees.csbsju.edu/ssaupe/

Human Reproduction

 I. Female Anatomy/physiology

A. Ovary - the key organ of reproduction. The ovary has two main functions:

1. Egg (oocyte) production.
    Ovum is another term for egg. The ovum develops in a follicle. The follicle is lined with special cells (granulosa cells). All of the ova that a female will ever possess are present at birth (they are as old as she is). In fact, the maximum number is achieved before birth and continues to decrease through her lifetime (Table 1).

Table 1. Human Oocyte Production – data from a seminar presented by Dr. Randle Korfman
Time Oocyte Number
8 weeks prior to birth 6-7 million
birth 600,000
puberty 300,000
menopause few

    Each month following puberty, one follicle enlarges and releases an egg (ovulation). The egg is swept into the oviduct by finger-like extensions. When the follicle collapses it becomes the corpus luteum.

2. Hormone production. 
    The ovary produces progesterone and estrogen.

B.  Oogenesis - egg production
    Diploid cells (oogonium) in the ovary begin meiosis. Oogonia divide mitotically to produce primary oocytes.  These begin meiosis but stop in prophase of the first meiotic division.  Once a month a primary oocyte in the follicle enlarges and continues the meiotic process.  During this process, the first meiotic division is completed, which produces a secondary oocyte and polar body (which disintegrates).  The secondary oocyte is released from the ovary on ovulation but doesn't complete the second meiotic division until stimulated by fertilization. 

C. Menstrual Cycle.
    On average the menstrual cycle lasts 28 days (some longer, some shorter). In a perfect 28 day cycle, Day 1 is the first day of bleeding, Day 14 is ovulation. The cycle can be divided into two phases:

D. Hormonal Control.
    There is a complex regulatory control over the cycle. Involves various hormones and body parts. The basic scheme is:

Brain (hypothalamus) � gonadotropal releasing hormone (GnRH) � anterior pituitary � follicle stimulating hormone (FSH) and luteinizing hormone (LH) � ovary � stimulates growth of a follicle � which produces estrogen � estrogen levels increase (days 1 –14) � estrogen on day 14 causes "feedback" to the anterior pituitary stimulating a large production of LH ("LH surge"; by the way, ovulation prediction kits test for LH) � LH surge stimulates ovulation � follicle collapses, becomes corpus luteum � corpus luteum produces progesterone � shuts off hypothalamus � no FSH or LH production by anterior pituitary � no developing follicles/ova during the luteal phase (waiting to see what happens to the 'bun in the oven') � if no pregnancy by day 28 corpus luteum disintegrates � no progesterone � hypothalamus turns on � FSH/LH production � cycle repeats.

E. Uterine Changes
    Estrogen stimulates growth of new cells in the endometrium (lining) during days 1 - 14 of the cycle.  As a result, the first half of the cycle can also be considered the Proliferative phase. Progesterone stimulates glands in lining to secrete mucous and other materials and is required for the maintenance of the endometrium. This is the Secretory Phase; Day 14 � 28. Thus, estrogen is like a "fertilizer" and progesterone is like a "stabilizer" to maintain the lining.

F. If No Pregnancy.
    If there is no pregnancy, by day 28 the corpus luteum disintegrates, progesterone production stops, the lining flows off (menstruation).

G. If Pregnancy Occurs.
    The egg is viable for about 24 hours. It moves into the oviduct where fertilization occurs (about day 14). From day 14 to day 21 the zygote/embryo travels to the uterus where it implants (day 21). The zygote cells divide during the travels and form a blastula. The chorion (part of the embryo) produces chorionic gonadotropin (hCG) that stimulates the corpus luteum to continue to produce hormones to maintain the uterine lining that must persist throughout pregnancy. Eventually the corpus luteum stops producing progesterone, etc. - but fortunately, the hormones are supplemented by those produced by the placenta.

Note: (1) continued progesterone production by the corpus luteum or placenta means that there will be no FSH or LH � no ovulation � no further eggs will be released until the "one in the oven if done"; and (2) hCG (human chorionic gonadotropin) is the marker used in pregnancy test kits.

H. Puberty.
    Sexual maturity is initiated when the level of melatonin, produced by the pineal gland, drops.  Melatonin inhibits the hypothalamus from producing GnRH. Thus, until the levels decline, there can be no ovulation.

I. Menopause.
    The ovaries stop responding to FSH and LH = no estrogen � no ovulation � no periods

II. Male Reproductive Anatomy/Physiology

A. Testis - key organ of reproduction in males. The testes have two main functions:

1. Sperm production. 
    Requires low temperature, therefore a scrotal sac. Warm temperatures results in decreased and abnormal sperm production (consider boxer vs. brief undies). Occurs in the seminiferous tubules (another example of the importance of S/V ratios). Sperm production is a continuous process once puberty is reached. FSH stimulates spermatogenesis. FSH acts on Sertoli cells – provide nourishment and chemical signals to the sperm - sperm nursemaids.  The Sertoli cells that produce androgen binding protein that binds testosterone to keep it in the testis.  

2. Hormone production
    Produces testosterone. Produced by interstitial cells (Leydig cells) in the testes. LH stimulates testosterone production.

B.  Spermatogenesis.
    Spermatogonial cell (2n) divides mitotically to produce lots of primary spermatocytes (2n) which can undergo meiosis I to produce secondary spermatocytes (n).  These  complete meiosis II to form spermatids which look nothing like mature sperm.  They require a maturation period during which they take on the final form of the sperm.  The production of sperm is something like a factory conveyor belt system.  As the cells proceed through meiosis and maturation they are ultimately released into the duct in the seminiferous tubules

B. Hormonal regulation.

hypothalamus � GnRH � pituitary gland � FSH and LH (also called Interstitial Cell Stimulating Hormone, ICSH) � the LH stimulates interstitial cells to produce testosterone � the testosterone and FSH (working via the sertoli cells) in combination stimulate spermatogonial cells to produce sperm. Sperm production is constant following puberty. It is not cyclic as in females.

C. Other structures of the male reproductive tract.

III. Fertilization.
    As stated above, fertilization occurs on about day 14. Of the many sperms released (in a typical male, there are 100 million sperm mL-1 x 3 mL semen = 300 million sperm per ejaculation.  As an aside, if there are less that 20 million sperm mL-1 , a male is considered to be functionally sterile).  Relatively few sperm ever make it to the egg. Reasons for the failure of sperm to make it to the egg include:

  1. defective sperm
  2. hostile environment in the vagina – sperm often swim in dense packs (like hungry dogs). This may be a way to protect the inner ones.
  3. female immune systems attacks some
  4. swim up the wrong oviduct (how many males do you know who ask for directions?)
  5. some get stuck in the oviduct
  6. don't find mucin – secreted by the cervix; liquid during ovulation; easy to swim in; provides a "channel" or perhaps a road into the uterus.

    The sperm require contact with the female reproductive tract to become "capacitated" – capable of fertilizing the egg. Once at the egg, the sperm race to be the first to enter. They release digestive enzymes from the acrosome (pointed head region) to begin digesting the outer covering. The entry of a sperm stimulates the formation of the zona pellucida, a shell-like structure, around the egg. This prevents any additional sperm from penetrating. As the fertilized egg moves toward the uterus for implantation, cilia along the way release enzymes to digest the zona pellucida. If the zona pellucida is still intact, the embryo can't implant in the uterus. As a side note – this can be a minor problem for in vitro fertilization – the zona must be removed before the fertilized egg is reimplanted in the mother.

IV. Preventing Fertilization

A. Natural Family Planning
    Determine time of ovulation and plan intercourse accordingly (remember that safe/fertile periods must account for egg (24 h) and sperm (24-48 h) viability and irregularities of the cycle. To determine the time of ovulation a woman can:

  1. monitor the consistency of the cervical mucous – it changes consistency at ovulation and becomes watery (vs. sticky);
  2. count days – in a regular cycle, ovulation typically occurs halfway;
  3. monitor her basal body temperature – during the follicular phase it is about 97.5 F but increases to about 98.3.

B. Barrier Methods.
    These methods provide a barrier that prevents the sperm from leaving the vagina and entering the uterus. These include condom, foam, diaphragm, cervical cap (like a smaller more permanent diaphragm), sponge, suppository; all can be used +/- spermicide. Each method has it’s advantages and disadvantages. One additional benefit of the condom is that is reduces the spread of sexually-transmitted diseases.

C. Hormonal Methods.
    These methods supply hormones, usually progesterone or analog that disrupts the normal menstrual by "tricking the body into thinking it is pregnant". These include:

  1. Oral contraceptives (the "Pill") - Contains progestin (progesterone, prevents FSH & LH release) and synthetic estrogens; body "thinks" it is in the luteal phase (pregnant), therefore no ovulation. On 20 days, off 5.
  2. Norplant. - Capsule implanted in the arm, releases progestin, inhibits ovulation up to 5 years.
  3. Depo-Provera – injected, progestin, lasts about 3 months.

D. Douche.
    cleanse sperm from vagina; very low success rate

E. Intra-uterine device.
    No longer popular because it can cause pelvic inflammatory disease. Camel herders knew that a stone in the uterus of a camel would prevent her from becoming pregnant (how did they figure that out?). Anyway, presumably these uterine implants irritate the lining of the uterus and prevent fertilized egg implantation.

F. Sterilization Methods
    Vasectomy and tubal ligation.

G. Withdrawal.
     Coitus interruptus; very low success rate

H. Ru486.
    The "morning after pill". Blocks progesterone receptors/production; uterine lining and implanted egg sloughs off .

V. Preventing Contraception – the future?

A. Anti-pregnancy vaccine.
    Tested in chimps/baboons. Stimulates production of antibodies against hCG, which is required to maintain the uterine lining. Therefore, no hCH, no corpus luteum, no lining maintained, no pregnancy.

B. Male Birth Control Pill.

C.  Immunotherapy against GnRH - this would prevent FSH & LH production and hence sperm formation.

D.  Note – there is currently interest in developing male contraceptives but, in general, there is comparatively little for males, possibly because: (1) the physiology of reproduction; female reproduction is designed to be turned on/off; (2) gender bias of scientists.


VI. Reproductive Technologies. (improving fertility)

VII. Miracle of Life.
     Check out the excellent video, Nova - Miracle of Life, available in the library. It includes remarkable footage of the entire reproductive process from start (intercourse) to finish (baby).


VIII. Why Reproduce Sexually?
    At first you might consider this a silly question but consider:

  1. Sex is not necessary for reproduction. 
        Asexual methods (producing new individual without involving sperm, egg and fertilization), work well, are simpler, and less risky. Asexual reproduction is particularly common in plants and a few animals. Examples including budding (hydra), fragmentation or regeneration (starfish), parthenogenesis ("virgin birth" – an unfertilized egg develops into an adult; dandelions, aphids, honeybees).

  2. Sex, in most species, is not for pleasure. 
        There is little foreplay in most species and orgasms are rare in other animal species. Also, consider fish and other species that simply shed gametes into the environment (external vs. internal fertilization).

  3. Sex is risky business. 
        With sex comes the increased, risk of disease, ectopic pregnancy, or getting eaten (sexual cannibalism - praying mantis & black widow stories).

  4. Sex is not selfish. 
        In other words, the result of sex is offspring that have different genes than their parents.  

    So, why sex? Because Sex is not selfish.  The fact that offspring are genetically different than their parents is a major advantage because it provides for genetic diversity that is the raw material for evolution.


IX. Evolutionary Strategies

A. Males vs. females.
    For most species, males and females have different strategies when it comes to sexual reproduction. First, we will make the assumption that both sexes are interested in producing the maximum number of genetically fit individuals. So how do males and females optimize this possibility?

    Consider gametes - females produce few, large gametes whereas males produce numerous small ones. Since eggs are expensive and only a few offspring can be raised, females evolutionary pressure is for "choosiness" in mate selection. In contrast, sperm are "cheap" and males evolutionary success is limited by their ability to deliver sperm to the egg. Males have evolved to be "salesman", attractive to females. They advertise good genes by holding territories, displays, courtship rituals, gifts, appearance.

    Do humans follow these evolutionary ideas? There is considerable debate. Whadayathink? Check out EO Wilson’s book On Human Nature. 1978. Harvard Univ. Press, Cambridge MA. (copy in file in botany lab).

B.  Female Gamete Choice.
    Courtship and mating are not the only places females can exert control over breeding.  The females of several species are able to control which sperm fertilize an egg AFTER copulation.  These females have the ability to discriminate between sperm of different species as well as individual males of their own species (Birkhead, 2000).  For example, female fruit flies that mate with another species will not produce fertile eggs even though her sperm stores are full.  If artificially inseminated with the same sperm she will lay fertilized eggs.  Similarly, if mated with a closely related male and a distantly related male of her species, she will preferentially produce offspring from the distantly related species. 

B. Reproductive Output.
    An individual man can theoretically have more children than an individual female. In fact, the maximum number of children for a female is about 20 whereas the most prolific father in history is presumed to be Ismail the Bloodthirsty, a Moroccan emperor from 1672 – 1727, who sired 888 children (see Natural History, Sept 1999, p 23).  This record is rather suspect even considering his harem of 500 women.  It's virtually impossible for him to have fathered all of the children attributed to him when you consider:

  1. the short period of fertility for a woman during her monthly cycle,
  2. the high degree of infertile women in a group (about 10%);
  3. miscarriages and other problems (10-20%),
  4. generally low incidence of conception (15-40%), and
  5. a man's sperm count decreases with sexual activity.

    To father all of these children in face of these statistics he would have had to mate with 4.8 women daily for four decades. This performance makes Wilt Chamberlain look like a slacker. As an aside, even though males have the potential to have more offspring, in practice there isn’t great differences in human reproductive rates between males and females.

C. Monogamy.
    However, if males and females have such differing evolutionary strategies, why isn't monogamy less common? Presumably because a monogamous coupling is paternity insurance for the male, since he can never be sure that the offspring is his.

    Are humans a monogamous species? Consider that for other monogamous species the females are usually more aggressive and larger than the males, are territorial, and show other features that we don't possess. As an aside, a study of blood types of parents and offspring done in the 1950's showed that about 10% of offspring born were fathered by someone other than the man presumed to be the father.

References

| Top| SGS Home | CSB/SJU Home | Biology Dept | Biol116  Section Home Page | Concepts Home Page | Disclaimer |

Last updated: January 03, 2004        � Copyright by SG Saupe

Last updated: January 03, 2004     Visitors to this site:  Hit Counter
� Copyright by SG Saupe / URL:http://www.employees.csbsju.edu/ssaupe/index.html