Unit 25 - Reproductive System
The reproductive system shows a more obvious sexual dichotomy than any of the other organ systems. In males it produces the sex cells only. In females it not only produces sex cells, but it also provides for the care and nourishment of the developing fetus. Because of the significant differences in the two sexes, it is necessary to examine each sex separately.
A. Anatomy of the male - The male tract consists of the following structures which produce, nourish, and deliver the spermatozoa.
1. Testes - These are the organs which produce the spermatozoa. They are paired and lie in a sac termed the scrotum. Each testes consists of the following.
a. Tunica albuginea - This is the connective tissue capsule which surrounds each testes.
b. Septa - These are inward extensions of the tunica which divide each testes into compartments.
c. Seminiferous tubules - These are highly convoluted tubules which lie in the septal compartments. They are the sites of spermatogenesis (sperm production).
d. Rete testis - This is a network of tubes formed by the joining of the seminiferous tubules from the various compartments. The tubules of the rete testis empty into the efferent ductules which leave the testes and join the epididymis.
2. Epididymis - A structure which sits on each testes much like the cap of an acorn. It is composed of highly convoluted tubules which receive the secretions from the testes via the efferent ductules. There is some smooth muscle surrounding the tubules. The function of this structure is the storage of sperm.
3. Vas deferens (sperm duct) - This tube is a continuation of the duct of the epididymis. It is line by ciliated columnar epithelium and passes into the abdominopelvic cavity with the spermatic cord. Each tube swings over the urinary bladder and joins with the duct of a seminal vesicle to form a short ejaculatory duct that passes through the prostate gland to join the urethra.
4. Accessory glands - The male has three major accessory glands. Two are paired but one, the prostate, is single.
a. Seminal vesicles - The produce a thick fluid which makes up the largest part of the semen. It is very rich in fructose and serves to nourish the sperm.
b. Prostate - This produces an alkaline fluid which is rich in enzymes and other agents, most of which have an unknown function. The alkalinity of the prostate fluid will neutralize any excess acidity in the female tract. It is apparently prostate fluid which activates sperm into swimming form.
c. Bulbourethral glands - These are small glands which lie at the base of the penis. They produce a fluid which lubricates the urethra and neutralizes any uric acid which may be present.
5. Urethra - This is the common urogenital tract of the male. It receives the secretions of the testes and accessory glands and transports them through the penis to the outside of the body.
6. Penis - This is the copulatory organ of the males. It consists of an elongated shaft and an enlarged end termed the glans. This skin extends over the glans to form the prepuce which may be surgically removed by circumcision.
a. Internally the penis contains highly vascularized connective tissue termed erectile tissue. There are three cylindrical bodies of this tissue.
(1) Corpora cavernosa - There are a pair of these located dorsally.
(2) Corpus spongiosum - This is the single ventral column that surrounds the urethra.
B. Sexual response in males - Males deposit semen in the female reproductive tract. In order for this to occur the penis must become enlarged and firm, a process called erection.
1. Erection - This process may be initiated by both psychic and tactile stimuli. The following mechanisms are involved.
a. Parasympathetic stimulation causes dilation of the arterioles which supply the erectile tissue. This increases blood flow into this tissue which now begins to swell.
b. The increasing volume of the erectile tissue compresses the veins which drain the penis. This causes the blood to remain in the penis. As a result it becomes firm and erect.
2. Ejaculation - Tactile stimulation of the erect penis initiates a sympathetic reflex which causes rhythmic contractions of the epididymis, vas deferens, seminal vesicles, and the prostate gland. These contractions propel the components into the urethra where they become semen. Forceful contractions of the skeletal muscles at the base of the penis now forces the semen of the urethra.
a. Normally about 2 ml of semen are ejaculated. These 2 ml contain about 300 million sperm.
b. Following ejaculation the arterioles to the erectile tissue constrict and pressure is taken off of the veins. As a result blood leaves the penis and it returns to the flaccid state.
C. Hormonal regulation in males - FSH, LH, and testosterone are responsible for the hormonal regulation of reproductive processes.
1. FSH stimulates the production of sperm by acting on the Sustentacular (Sertoli) cells of the seminiferous tubules. Sustentacular cells function as nurse cells to the sperm producing cells. FSH causes the sustentacular cells to release androgen binding protein (ABP) which causes the spermatogenic cells to bind testosterone. Testosterone then stimulates spermatogenesis. FSH release is controlled by Gn-RH from the hypothalamus. This releasing hormone also regulates LH release. FSH is inhibited by a hormone called inhibin which is released by the sustentacular cells.
2. LH stimulates the Leydig cells to produce testosterone. The Leydig cells are also referred to as the interstitial cells and are found between the seminiferous tubules. Testosterone is also required for sperm production and therefore LH also indirectly stimulates sperm production. LH secretion is controlled by Gn-RH. LH is inhibited by negative feedback from testosterone.
3. Testosterone - Testosterone is a steroid hormone which is produced under the influence of LH. It has the following effects.
a. It initiates sexual differentiation in the fetus at about five to six weeks of gestation. Until this time the fetus is sexually indifferent. In the absence of testosterone the fetus becomes a female (default mode).
b. Testosterone initiates the descent of the testes from the body cavity into the scrotum at about the eighth to ninth month of gestation. The testes form inside of the body cavity but later drop into the scrotum. If this descent does not occur then sterility will result.
c. It is responsible for initiating the changes that occur at puberty. During the period ranging from ten to fifteen years of age there is a dramatic increase in Gn-RH. This causes an increase in the gonadotropins which in turn results in a substantial increase in testosterone. This increase has the following effects.
(1) Maturation and enlargement of the primary sex organs.
(2) Initiation of the growth of body hair in the male pattern.
(3) It initiates the lowering of the voice by causing a thickening of the vocal cords.
(4) It stimulates the development of male musculature.
(5) It aids FSH in stimulating spermatogenesis.
(6) It regulates sex drive (libido) in both males and females. It is also associated to some extent with aggressive behavior.
d. Under secretion during puberty results in failure to develop mature primary and secondary sexual characteristics. Over secretion apparently does not cause clinical symptoms.
D. Anatomy of the female - The female reproductive tract provides the same general functions as that of the male, but in addition, provides for the development of the fetus. The principal organs of the female tract are as follows.
1. Ovaries - These are the primary female sex organs in which the female sex cells (ova) are produced. Unlike the testes the ovaries remain in the body cavity, supported by several ligaments. The key parts of the ovaries are as follows.
a. Germinal epithelium - This is the outer covering of the ovary and is composed of a simple columnar epithelium that is continuous with the peritoneum.
b. Tunica albuginea - This is a fibrous connective tissue layer which connects the germinal epithelium.
c. Cortex - This is the outer layer of the ovary in which the primary follicles, containing immature ova, are found.
d. Medulla - This is the inner part of the ovary that contains the major blood vessels.
2. Uterine (Fallopian) tubes - These are the tubes that receive the mature ovum from the ovaries and conduct it to the uterus. The tube is connected to each ovary by ciliated finger-like projections called fimbriae. These are thought to create currents in the peritoneal fluid which transport the ovum from the ovary into the tubes. The tubes are line by a mucosa which is composed of a ciliated columnar epithelium. It is the cilia which move the ovum through the tube to the uterus.
3. Uterus - This is a muscular hollow organ, shaped like a pear, which connects to the uterine tubes. The enlarged upper portion is referred to as the body and this tapers out into the isthmus which terminates with the cervix where the vagina joins. The uterus consists of three layers.
a. Serosa - This is the outer layer which is formed by a folding of the peritoneum .
b. Myometrium - This is the thick muscular layer beneath the serosa.
c. Endometrium - This is the lining of the uterus and is composed of a simple columnar epithelium and a thick, highly cellular, connective tissue. This is where the embryo embeds itself and the placenta forms.
It is also the layer that undergoes marked changes during the menstrual cycle.
4. Vagina - This is a muscular tube that leads from the cervix of the uterus to the outside. It serves as the female copulatory organ as well as the birth canal. The vagina has a mucosal lining that has a surface composed of stratified squamous epithelium.
5. External genitalia - The collective name for all of the external genitalia is the vulva. It is composed of the following structures.
a. Mons pubis - This is a fatty mound of tissue that forms over the pubic symphysis.
b. Labia majora - These are two rounded folds that extend back from the mons pubis. The outside surfaces are covered with pubic hair while the inner surface is smooth and moist.
c. Labia minora - These are two smaller folds that lie medially to the labia majora. They extend anteriorly to the clitoris which they surround.
d. Vestibule – This is the name for the area surrounded by the labia minora. The vagina and urethra open into this area.
e. Clitoris - This is a small structure which lies at the anterior end of the labia minora. It is homologous to the penis and contains erectile tissue which fills with blood during sexual arousal.
f. Glands - There are many glands associated with the external genitalia. They are found on the inside of the labia majora and many open into the vestibule where they moisten and lubricate.
6. Mammary glands - These are not involved in the actual reproductive process but play a role in nourishing the offspring which is the product of reproduction. Each mammary (breast) is a skin covered rounded elevation located on the pectoral muscles. Each one consists of the following structures.
a. Nipple - This is the protruding tip (anterior) of each breast. It is surrounded by a circular areola. Both nipple and areola are pigmented and contain smooth muscle which can cause the nipple to become erect. The nipple functions as the exit for milk.
b. Adipose tissue - This is found internally distributed throughout a network or stroma made up of connective tissue. It is the amount of adipose tissue present that usually determines breast size.
c. Glandular tissue - These are the glands that produce milk. There are about twenty of them arranged radially around the nipple. Each is drained by a lactiferous duct which enters the nipple.
E. Sexual responses in females - Sexual arousal in females, like in males, is brought about by both tactile and psychological factors. The following events occur during arousal.
1. The clitoris becomes erect due to blood engorgement.
2. Erectile tissue at the opening of the vagina also becomes erect thereby narrowing the vaginal opening.
3. The lining of the vagina becomes lubricated by secretions of glands around the cervix of the uterus and around the vestibule.
4. After sufficient stimulation, especially of the clitoris, females undergo an orgasm which is similar to that of males except that no ejaculation takes place.
F. Hormonal control in females - Hormonal regulation is a function of FSH, LH, estrogen, and progesterone. Estrogen and progesterone are produced by the ovaries.
1. FSH stimulates maturation of the ovarian follicle and the ovum. It also stimulates production of estrogen by the follicle cells.
2. LH stimulates ovulation. It then stimulates the conversion of the ruptured follicle into a body termed the corpus luteum. The corpus luteum produces both estrogen and progesterone.
3. Estrogens - These constitute a family of related hormones, the most common one being, 17-beta estradiol. The estrogens function in the following areas.
a. The initiate puberty. This normally occurs a year or two earlier in females than in males. At puberty a surge of the gonadotropins cause increases in estrogen which has the following results.
(1) It stimulates the maturation and enlargement of the primary sex organs.
(2) Enlargement of the breast occurs as a result of its action.
(3) Estrogen initiates development of the female torso primarily by regulating skeletal and adipose tissue changes.
(4) It seems to stimulate body hair growth in the female pattern.
(5) There is a stimulation of skeletal growth in certain patterns, but the growth of long bones is terminated. This is one reason that females are typically shorter than males.
b. Estrogen also plays an important role in the menstrual cycle
4. Progesterone - This hormone is produced by the corpus luteum. It functions in the following ways.
a. It plays a key role in the menstrual cycle and during pregnancy.
b. It stimulates development of the secretory apparatus in the breast.
G. Menstrual cycle - Unlike males in which sex cell and sex hormone production is constant, the female is cyclic in production of both. This cycle of fertility occurs once every 28 days and involves careful coordination of all associated organs. The two salient features of the cycle are ovulation which occurs midway and menstruation or bleeding which occurs at the end of the cycle. There are three different but coordinated components.
1. Ovarian cycle - These are the events that occur in the ovary during the monthly cycle. They consist of ovum maturation, ovulation, and corpus luteum formation.
a. Maturation of the ovum - At the beginning of the cycle the primitive ovum (oocyte) is surrounded by a single layer of follicle cells. This is the primary follicle. FSH stimulates the follicle cells to proliferate and the ovum to mature. As the follicle matures (secondary follicle) it develops a fluid filled cavity in which the ovum is enclosed. At maturity (tertiary or Graafian follicle) the ovum sits on a pedestal inside of the fluid filled cavity.
b. Ovulation - Under hormonal stimulation (principally LH) the follicle ruptures and ovum washes into the uterine tube.
c. Corpus luteum - LH induces the ruptured follicle to fill with new cells and become the corpus luteum which is an endocrine gland that produces both estrogen and progesterone.
2. Sex hormone production - FSH causes the follicle cells to produce estrogen. LH causes the corpus luteum to produce both estrogen and progesterone.
3. Endometrial changes - The endometrium is where the fertilized egg (ovum) will implant. Estrogen stimulates proliferation of the endometrium during the first two weeks of the cycle. Progesterone maintains the integrity of the endometrium during the second two weeks of the cycle. If pregnancy does not occur then the endometrium sloughs off during the menstrual period.
4. Cycle details - The specifics of the cycle are presented below.
a. The discussion begins at the end of a cycle during the menses with all hormone levels low . The endometrium is being sloughed off. FSH levels begin to rise in response to increasing levels of Gn-RH.
b. The gradual increase in FSH stimulates several follicles to undergo the maturation process. Normally only one follicle will become mature, the others abort. The follicle cells begin to produce estrogen and inhibin.
c. The increasing levels of estrogen and inhibin depress FSH release but as estrogen levels rapidly increase cells which secrete LH become more sensitive to GnRH. Estrogen enhances their sensitivity. This causes a surge of LH and a lesser release of FSH. It is the LH surge that causes ovulation.
d. LH converts the ruptured follicle into the corpus luteum which begins to produce high levels of progesterone and estrogen. The progesterone acts on the endometrium to cause the secretory or glandular phase of the cycle. During this phase the endometrium becomes highly vascularized and epithelial glands develop. High levels of estrogen and progesterone inhibit the release of Gn-RH which results in a decline in both LH and FSH. This insures that no new follicles will begin maturation.
e. If fertilization does not occur then the corpus luteum degenerates. Progesterone and estrogen levels fall off. The decrease in progesterone results in spontaneous uterine contractions. These contractions constrict the small arteries supplying the endometrium which results in the degeneration and sloughing off of the endometrium and then menses begin again.
5. Menopause - This is the cessation of the menstrual cycle which occurs around age fifty. It is thought to be due to loss of responsiveness of the ovaries to the gonadotropins. Estrogen and progesterone levels become low and the estrogen dependent tissues slowly begin to atrophy. It may be accompanied by hot flashes, irritability, fatigue, and anxiety.
H. Gametogenesis - This is the formation of the sex cells. Gamete is a generic term that means sex cell. Formation of the egg and the sperm show similarities and differences. Both utilize a special type of cell division known as meiosis.
1. Meiosis - This is a special kind of cell division that occurs only in the gonads. The overall result is the reduction of the chromosome number by half.
a. The normal human chromosome number (diploid, 2N) is 46. Meiosis decreases this number to 23 (haploid, N) in sex cells.
b. In diploid cells there is a double set of genetic instructions. This is because the chromosomes which contain the genes exist in homologous pairs. Each member of a homologous pair carries genes that govern the same traits (not necessarily the same genes however). One member of each pair is obtained from each parent. Therefore we have 23 maternal chromosomes and 23 paternal chromosomes, a double set of genetic blueprints.
c. During meiosis the homologous pairs are separated and one sex cell receives one member of each homologous pair. This will yield 23 chromosomes, or one complete set of genetic instructions.
2. Sex determination - Two of the chromosomes are non-homologous, and are designated the sex chromosomes because their presence determines sex. These are the X and the Y chromosomes. A fertilized egg that receives two X chromosomes will become a female while an egg that receives one X and one Y will become a male.
a. Sex differences in cells - The extra X chromosome of the female diploid cell manifests itself in a visible fashion.
(1) Barr bodies - These are dark spots found in the nucleus of epithelial cells of females.
(2) Drumsticks - These are small appendages found on neutrophils in females.
b. The X chromosome is a complete chromosome with a full compliment of genes. The Y on the other hand is almost genetically inert. Therefore males are haploid for genes carried on the X chromosome, while females with two X chromosomes are diploid. This is why men are more likely to express rare sex linked traits which are carried on the X chromosome. Such traits are usually recessive and are masked in females by a normal gene on the second X chromosome. In males they will be expressed.
3. Abnormalities in chromosome number - Failures in the meiotic process can lead to sex cells with more or less than the 23 number. Generally a cell that is deficient in an entire chromosome will not be viable (unless the missing chromosome is a Y) but cells that have an extra chromosome may be viable, unite with a normal sex cell, and produce an individual with 47 chromosomes.
a. Trisomy 21 - This is an extra 21st chromosome and produces the condition known as Downe's syndrome. Certain characteristic physical features as well as mental retardation are usually present.
b. An extra Y chromosome does not seem to have much of an effect. Individuals are usually tall, over six feet, but otherwise appear normal.
c. A male with an extra X (XXY) suffers from Klinefelter's syndrome. These individuals are tall, somewhat feminine in certain respects, usually retarded, and sterile.
4. Spermatogenesis - This is the male aspect of gametogenesis. Sperm are generated by the millions from the germinal epithelium which makes up the walls of the seminiferous tubules. Salient features include the following.
a. The process begins with a diploid cell known as a spermatogonium. It begins meiosis and gives rise to a primary spermatocyte which is still a diploid cell.
b. The primary spermatocyte continues meiosis. It undergo the reductional division to give rise to two haploid secondary spermatocytes.
c. Each secondary spermatocyte now undergoes the second meiotic division (equational division). The result is the formation of four haploid spermatids.
d. Each spermatid undergoes a metamorphosis (spermiogenesis) which gives rise to the mature sperm.
e. In summary, one diploid spermatogonium gives rise to four haploid sperm.
5. Oogenesis - This is the female gametogenesis. The major difference between this process and spermatogenesis is that in oogenesis the cell divisions are unequal.The salient events of this process are as follows.
a. A diploid oogonium begins the meiotic process yielding a primary oocyte. This begins in the ovaries before the birth of the female, becomes arrested, and then continues onward at puberty.
b. The diploid primary oocyte undergoes reductional division to give rise to a single haploid primary oocyte and a polar body. The polar body is simply a set of chromosomes and a small amount of cell membrane. The secondary oocyte receives the bulk of the cytoplasmic material.
c. The secondary oocyte undergoes the equation division. This results in the ovum and another polar body. The first polar body may also divide. If this occurs then the sum result is the production of one ovum and three polar bodies. In reallity an ovum results only if fertilization occurs. Ovulation occurs while the female cell is still a secondary oocyte. If it is fertilized then it will complete mieosis and development.
d. In summary, oogenesis results in the production of a single, very large ovum, and three degenerate polar bodies.
I. Fertilization - This is the union of the sperm and the egg which restores the diploid chromosome number.
1. The process occurs high in the uterine tube.
2. Only one sperm actually fertilizes, but it requires the combined efforts of a large number of sperm to accomplish this task. This is because the ovum is surrounded by a layer of cells from the ruptured follicle known as the corona radiata.
a. The cells are held together by a substance known as hyaluronic acid.
b. Each sperm produces an enzyme, hyaluronidase, which breaks hyaluronic acid down and thereby separates the cells surrounding the ovum.
c. Because of the small size of each sperm, it takes the combined efforts of many individuals in order to breach the corona radiata.
d. Only a single sperm is permitted into the egg. Monospermy is assured by two different mechanisms.
(1) Fast block to polyspermy - As soon as the first sperm's plasma membrane contacts the egg membrane, sodium channels open allowing sodium to flood into the egg, depolarizing the membrane. The depolarization prevents other sperm from making contact.
(2) Cortical reaction - This occurs after the sperm has penetrated the membrane. Depolarization of the egg membrane also results in an increase in intracellular calcium. This calcium activates the egg for cell division. It also causes the rupture of cortical granules into the extracellular space beneath the zona pellucida. The released cortical material swells and pushes all sperm away from the egg membrane forming a permanent sperm barrier.
3. After fertilization, development begins. While still in the uterine tube the fertilized ovum (zygote) begins to divide mitotically, and by the time it reaches the uterus it has become a hollow ball of cells.
J. Implantation - This is the process by which the embryo burrows into the endometrium. Upon arrival at the uterus the embryo (trophoblast) produces an enzyme which erodes a hole into the endometrium. The embryo then implants itself into this hole.
K. Placentation - This is the formation of the placenta, the life line between the mother and fetus. The placenta is a series of membranes derived from both the mother and embryo. The principal membrane is the chorion which separates the maternal circulation from the fetal circulation. It is across the chorion that all exchange between mother and fetus occurs.
1. Endocrine functions - In addition to nourishment, the placenta functions as an endocrine organ. It produces the hormones necessary to sustain the pregnancy. These hormones are as follows.
a. Human chorionic gonadotropin (HCG) - This is similar to LH and maintains the corpus luteum. It is initially produced by cells of the embryo and then by the placenta. It is produced for about three months. At the end of this time the placenta is producing the corpus luteal hormones and therefore the corpus luteum is no longer necessary. HCG is usually detectable in the blood and urine a week after fertilization. Most pregnancy tests utilize antibodies against HCG.
b. Estrogen and progesterone - These are produced by the placenta beginning at about the second month.
c. Human placental lactogen (HPL) - In concert with other hormones this stimulates maturation of the breast.
d. Human chorionic thyrotropin (HCT) - Similar to TSH, this hormone increases metabolic rate throughout pregnancy.
e. Relaxin - This is produced at the later stages of pregnancy. It softens the ligaments than bind the pubic symphysis, increasing the amount of movement possible at this joint.
L. Other embryonic membranes - In addition to the chorion, three more fetal membranes are produced. They are as follows.
1. Amnion - This is a membrane that forms inside of the chorion and around the fetus. It is filled with fluid and the fetus literally floats inside of it. A fold in this membrane gives rise to the umbilical cord which connects the fetus to the chorion.
2. Yolk sac - This is a vestigial membrane in humans because there is virtually no yolk in the ovum. It does serve as a source of cells which migrate to the developing gonads. It is also the earliest site of blood cell formation.
3. Allantois - This is another membrane which is derived from the fetus. It serves as a storage site for embryonic wastes in most animals. In humans it serves as the structural base for the umbilical cord. It, along with the yolk sac, is found in the umbilical cord.
M. Parturition - This is the process of birth. The movement of the fetus from the uterus into the birth canal and its expulsion, is brought about strong contractions of the smooth muscle in the uterus (labor). The details are as follows.
1. Although the exact trigger for the process is not known, hormone levels begin to change prior to birth. Estrogen levels go up and progesterone levels decrease. This can lead to irritable uterine contractions, or false labor.
2. The increased estrogen levels stimulate uterine muscle cells to develop large numbers of oxytocin receptors.
3. Certain fetal cells begin to secrete oxytocin which in turn causes the placenta to release prostaglandins. Both are powerful stimulators of contraction. This marks the beginning of true labor.
4. Dilation of the cervix initiates a neural reflex that causes the release of oxytocin by the neurohypophysis. This greatly increases the force of the uterine contractions. As the force increases, more oxytocin is released, which in turn increases the force of contraction further. This is one of the rare cases of positive feedback found in the body.
5. The contraction of the uterus ruptures the fetal membranes. When the amnion ruptures, amniotic fluid is released and passes out the vagina.
6. As the infants head passes into the cervix, more oxytocin is released and contractions become stronger. This forces the expulsion of the baby.
7. Following birth, rapid and vigorous contractions of the uterus continue for a few minutes. This results in the expulsion of the placenta (afterbirth).
N. Twinning - Twins may be identical or fraternal.
1. Fraternal - Here two eggs have formed and each is fertilized, independently of the other, by two separate sperm. Each embryo forms its own placenta. Fraternal twins are genetically siblings who happen to be born at the same time.
2. Identical - The result from a single fertilization. Shortly after development begins, the small ball of cells that will be the future baby gets broken into two parts. Each part then goes on to form a complete individual. Identical twins share a common placenta. As they resulted from a single fertilization, they are genetically identical.
O. In vitro fertilization (IVF) - Infertility is a problem experienced by many couples. Modern technology has permitted the actual harvesting of eggs and sperm and subsequent fertilization in test tubes (in vitro). There are three major steps.
1. Controlled hyperstimulation of the ovaries using GnRH agonists. This leads to a high production of oocytes which can then be harvested from the ovaries. Harvesting is by means of an ultrasound probe which is inserted into the vagina. The probe permits visualization of the ovaries. The probe contains a needle which goes through the vagina and aspirates the oocytes. The success rate for this phase is about 90%.
2. Next comes in vitro fertilization using sperm from the father. Following fertilization the zygotes are allowed to go through initial divisions to form small embryos. This stage is about 75% successful.
3. Fresh or frozen embryos are transferred to the uterus. The success rate here is very low, 20 to 25%. The low success rate of implantation may be due to undesirable changes which occur in the endometrium as a result of hyperstimulation. Because of the low success rate, it is necessary to transplant several embryos. Recently, a technique called embryo cloning has been used. This techniques takes on early embryo, separates the cells, and allows each cell (or group of cells) to divide and form new embryos. This technique provides for a large supply of transplantable embryos.
4. A major problem in infertility is low sperm counts in men. During the last few decades, sperm counts have decline about 50% all over the developed world (unknown causes). This is now being overcome by sperm micromanipulation. Using a light microscope and high tech "joy sticks", 5 to 10 motile sperm are injected directly under the zona pellucida. While this zone normally blocks polyspermy, statistically it is necessary to inject about 5 sperm to insure fertilization.
5. Current research is aimed at increasing the success rate. This means studying the problems of implantation as this is where most failures occur. Two current directions show promise.
a. Embryo biopsy - About 50% of all oocytes are genetically defective. It is estimated that as many as 60% of normal pregnancies spontaneously abort due to implantation and defective embryo problems. Genetic screening of embryos prior to implantation should greatly increase the success rate.
b. Freeze eggs instead of embryos - A major part of the implantation problem seems to be due to the effects on the uterus of hyperstimulation of the gonads. Freezing oocytes would permit the endometrium to develop through a normal cycle. The major road block to freezing of oocytes is that when harvested they are still undergoing meiosis. Freezing seems to damage the spindle apparatus. If ways can be found to overcome this problem, then it is anticipated that the success rate will increase dramatically.
P. Contraception - This is the prevention of pregnancy. Listed below are a number of different means and their effectiveness.
1. Coitus interruptus - This oldest of methods involves the removal of the penis prior to ejaculation. Effectiveness ranges from 50-70%.
2. Condom - This is a rubber or natural sheath which covers the penis. It also prevents many sexually transmitted diseases. Effectiveness is 64-97%.
3. Spermaticides - These are agents that are introduced into the vagina either before or after intercourse. Effectiveness is 70-80%.
4. Sponge - This is a chemically impregnated sponge that is inserted into the vagina prior to intercourse. It forms both a chemical and physical barrier to sperm. Effectiveness is about 80-87%.
5. Diaphragm - This is a fitted rubber disk that is inserted prior to intercourse into the vagina. Effectiveness is 80-98%. It is usually used in conjunction with a spermatocide.
6. IUD - This stands for intrauterine device. It consists of loops, coils, or other pieces of hardware that are inserted by a physician into the uterus. Effectiveness is 95-96%. These are rarely used now because of adverse side effects.
7. Rhythm method - This is abstinence from sex except during the "safe" (non-ovulatory) period. Effectiveness is highly variable, probably from 50-98%. It requires a detailed knowledge of the physiological state of the body. This is the only method approved by the Catholic church.
8. Birth control pills - These are synthetic estrogen and progesterone combinations that work within the normal menstrual cycle to prevent maturation of ova and ovulation. Effectiveness ranges from 97-99%. There are other forms of hormonal prevention. Depro-provera may be injected every three months. Norplant may be implanted under the skin and lasts for up to five years. Both of these are progesterone only products.
9. Sterilization (vasectomy and tubal ligation) - These are usually over 99% effective.
10. RU486 - This is technically not a contraceptive method, but rather an abortion inducing agent. It is a pill that can be taken at home if a period does not come around, and it will induce a spontaneous abortion. Developed in France, it is currently not available in this country.