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THE REPRODUCTIVE SYSTEM & DEVELOPMENT

INTRODUCTION

1. The human reproductive system produces, stores, nourishes, and transports functional gametes.Components of the system include the gonads, ducts, accessory glands, and external genitalia.

THE REPRODUCTIVE SYSTEM OF THE MALE

1. The male reproductive system includes the testes, the epididymis, the ductus deferens, the ejaculatory duct, the seminal vesicles, the prostate, and the bulbourethral glands. The scrotum and penis are components of the male external genitalia.

Male Reproductive Anatomy

The Testes

1. The testes are situated within the scrotum. Internally the scrotum contains two separate chambers, one for each testis. A superficial ridge, the raphe, marks the line of demarcation between the chambers.

2. The dartos muscle gives the scrotum a wrinkled appearance. The cremaster muscle pulls the testes closer to the body. Movement towards or away from the body keep temperatures at the testes several degrees below core body temperature.

3. The testes form inside the body cavity and pass through the inguinal canal as they descend into the scrotum. The testes remain connected with internal structures via the spermatic cord.

Optional Clinical Discussion: cryptorchidism

4. The tunica albuginea surrounds each testis and attaches it to the epididymis. Septa extend from the tunica albuginea to the mediastinum, creating a series of lobules. Seminiferous tubules within each lobule are the sites of sperm production.

5. From the seminiferous tubules sperm pass through the straight tubules to the rete testis. Efferent ducts connect the rete testis to the epididymis.

6. Between the seminiferous tubules there are interstitial cells that secrete androgens, principally testosterone.

7. Seminiferous tubules contain spermatogonia, stem cells involved in spermatogenesis.

The Anatomy of a Spermatozoon

1.Each spermatozoon has a head, middle piece, and tail.

The Male Duct System

1. From the testis the spermatozoa enter the epididymis. The distal portion of the epididymis merges with the ductus deferens.

2. It takes 2 weeks for a spermatozoon to complete its development and leave the epididymis.

3. The ductus (vas) deferens begins at the epididymis and passes through the inguinal canal as one component of the spermatic cord.

4. After emerging from the canal the ductus deferens passes posteriorly and curves along the lateral surface of the urinary bladder towards the prostate. Near the prostate it becomes enlarged, forming the ampulla.

5. The junction of the base of the seminal vesicle and the ampulla creates the ejaculatory duct. This duct penetrates the wall of the prostate to empty into the urethra.

6. The urethra extends from the urinary bladder to the tip of the penis.There are prostatic, membranous, and penile subdivisions of the urethra.

7. Along its path the urethra receives the secretions of the prostatic and bulbourethral glands.

8. The skin of the penis resembles that of the scrotum in its smooth muscle content and loose attachment to underlying structures.

9. Beneath the superficial fascia there are two erectile corpora cavernosa and a single corpus spongiosum that surrounds the urethra. Dilation of the erectile tissue with blood produces an erection.

The Accessory Glands:

1. The fluids contributed by the seminiferous tubules and epididymis account for only about 5 percent of the final volume of semen.

2. The seminal vesicles provide up to 60 percent of the seminal volume at ejaculation.The fluid contains nutrients important for the activation and nutrition of spermatozoa.

3. The prostate gland contributes around 30% of the volume of semen.Prostatic secretions contain buffers, enzymes, prostaglandins, and a compound with antibiotic properties.

4. The bulbourethral (Cowper's) glands provide a small quantity of mucus lubricant.

Semen:

1. A typical ejaculation releases 2-5 ml of semen containing 20- 100 million sperm per ml.

Male Reproductive Physiology

Male Sexual Function:

1. During arousal erotic thoughts or sensory stimulation lead to parasympathetic activity that produces erection.

2. Stimuli accompanying copulation lead to emission and ejaculation, primarily under the control of the sympathetic division of the ANS.

3. Ejaculation and orgasm occurs as the bulbocavernosus and ischiocavernosus muscles contract.

The Role of Hormones:

1. Important regulatory hormones include GnRF, FSH, and ICSH. GnRF stimulates the pituitary production of FSH and ICSH.

2. ICSH triggers the release of testosterone from the interstitial cells of the testes. Testosterone promotes the maturation of spermatozoa, maintains the accessory glands, and produces male secondary sexual characteristics and behaviors.

3. FSH causes an increase in the rate of spermatogenesis. Active sustentacular cells release nhibin, a hormone that suppresses the production of FSH.

THE REPRODUCTIVE SYSTEM OF THE FEMALE

1. The female reproductive system includes the ovaries, uterine tubes, uterus, vagina, external genitalia, andvarious accessory glands.

Female Reproductive Anatomy

1. The ovaries, uterine tubes, and uterus are suspended within a sheet of mesentery, the broad ligament. Folds and aggregations of fibrous tissue within that mesentery contribute to the mesovarium and other supporting ligaments.

The Ovaries:

1. The ovaries are held in position by the ovarian and suspensory ligaments. Blood vessels and nerves arrive via the suspensory ligament, entering the ovary at the hilus.

2. Each ovary is covered by a tunica albuginea. Internally it can be divided into a peripheral cortex and an inner medulla. Production of gametes occurs in the cortex.

The Uterine Tubes:

1. Each uterine tube has an infundibulum with fimbriae, an ampulla, an isthmus, and a short intramural section that penetrates the uterine wall before communicating with the cavity of the uterus.

2. It takes roughly 4 days for an oocyte to travel from the infundibulum to the uterine chamber. Unless fertilization occurs shortly after its arrival in the uterine tube, the oocyte will degenerate.

The Uterus:

1. The uterus provides mechanical protection and nutritional support for the developing embryo. It is located between the rectum and the urinary bladder. In anteflexion the uterus curves across the superior surface of the bladder.

2. The position of the uterus is stabilized by the broad ligament, the uterosacral ligaments, the round ligaments, and the lateral ligaments.

3. Major anatomical landmarks of the uterus include the body, isthmus, cervix, external orifice (os), uterine cavity, cervical canal, and internal orifice.

4. The uterine wall can be divided into an inner endometrium and a muscular myometrium.The endometrium contains a superficial functional zone and a deeper basilar zone. The proportions of these zones change during the menstrual cycle.

The Vagina:

1. The vagina extends from the cervix to the external genitalia. Prior to the onset of sexual activity a fold of epithelium, the hymen, partially blocks the entrance to the vagina.

The External Genitalia:

1. The components of the vulva include the vestibule, the labia minora, the clitoris, the lesser and greater vestibular glands, and the labia majora.

The Mammary Glands:

1. The mammary glands are situated within the hypodermis of the skin of the chest.

2. The glandular tissue consists of separate lobes, each containing multiple lobules. Each lobule is drained by a lactiferous duct that drains into a larger lactiferous sinus.

Female Reproductive Physiology

Female Sexual Function:

1. The phases of female sexual function resemble those of the male, with parasympathetic arousal and muscular contractions associated with orgasm.

The Ovarian Cycle:

1. Oogenesis occurs within ovarian follicles. Stages of follicular development include primordial, primary, secondary and tertiary (Graafian) follicles.

2. At ovulation the oocyte and the surrounding follicular cells of the corona radiata are released through the ruptured ovarian wall.

3. The oocyte gets transferred to the oviduct for transport to the uterus. The follicular cells remaining within the ovary form the orpus luteum that later degenerates into a corpus albicans of scar tissue.

4. The hypothalamic secretion of GnRF triggers the pituitary secretion of FSH and the synthesis of LH. FSH initiates follicular development, and activated follicles and ovarian interstitial cells produce estrogens.

5. Estrogens stimulate protein synthesis, oogenesis, the appearance of female secondary sexual characteristics, and the secretion of LH.

6. Each month as a group of primordial follicles begin developing into primary follicles the estrogen concentrations begin to rise. As the time of ovulation approaches estrogen levels peak and cause a sudden upsurge in LH secretion. The follicular wall then ruptures, and ovulation occurs.

7. After ovulation LH induces the formation of the corpus luteum. This glandular structure secretes progestins and smaller amounts of estrogens. LH levels fall after 2 days, but the corpus luteum survives for two weeks before deteriorating into a corpus albicans.

8. As the levels of estrogens and progestins decline, the pituitary production of GnRF accelerates, and another cycle begins.

The Menstrual Cycle:

1. A typical 28-day menstrual cycle begins with the onset of menses and the destruction of the functional zone of the. This process of menstruation continues for several days.

2. During the following proliferative, preovulatory, or follicular, phase the functional zone undergoes repair and thickens.

3. In the secretory, postovulatory, or luteal, phase the endometrial glands reach their full development and secrete actively. At the close of the secretory phase the functional zone begins to atrophy, heralding the arrival of another menstrual cycle.

4. Menstruation begins at menarche and continues until menopause.

5. The hormones secreted by the follicular cells are responsible for coordinating the ovarian and menstrual cycles. The decline in estrogen and progestin levels causes the endometrial disintegration at menses.

6. Side effects of the hormonal cycles include alterations in core body temperature that can be monitored to determine the approximate date of ovulation.

Pregnancy, Hormones, and Maternal Systems:

1. If pregnancy occurs, implantation of the blastocyst occurs in the endometrial wall. The placenta that develops produces several hormones that modify the ovarian and menstrual cycles.

2. Human chorionic gonadotropin (HCG) maintains the corpus luteum for several months. The continued production of progesterone keeps the endometrial lining intact and prevents menstruation.

3. After a few months HCG production declines, but placental estrogen and progesterone secretion increases.The placenta also produces relaxin and human placental lactogen (HPL).

4. HPL and prolactin from the anterior pituitary are primarily responsible for preparing the mammary glands for milk production.

5. During pregnancy hormonal changes bring the mammary gland to functional readiness. For the first few days after delivery the mammary glands produce a special secretion, colostrum.Colostru contains more protein and less fat than regular milk, and many of the proteins are immunoglobulins.

6. Milk contains water, proteins, amino acids, lipids, sugars, and salts. It also contains lysozymes with antibacterial properties.

7. After delivery, suckling of the infant stimulates receptors that cause the pituitary release of oxytocin. Oxytocin triggers the contraction of cells in the walls of the lactiferous ducts and sinuses in the milk ejection reflex.

8. Lactation continues until weaning, usually one to two years after delivery.

Aging and Menopause:

1. At menopause the ovaries no longer contain functional follicles, and estrogen and progesterone production declines. GnRF, FSH, and LH concentrations rise, secondary sexual characteristics fade, and a variety of metabolic, CNS and cardiovascular symptoms appear. In many cases these changes can be partially alleviated by hormonal replacement therapies.

DEVELOPMENT

1. The process of development regulates the appearance and gradual modification of an individual's anatomical and physiological characteristics from conception through maturity.

2. Prenatal development occurs before birth and postnatal development occurs thereafter.

3. Development involves the multiplication and differentiation of cells.Inheritance involves the transfer of genetic information from generation to generation.Genetics is the study of the mechanisms of inheritance.

MEIOSIS AND THE FORMATION OF GAMETES

1. Meiosis is a special form of cell division leading to the production of gametes containing one-half of the normal chromosome number of ordinary somatic cells.

2. The first meiotic division is reductional, and the second equational.

3. In the male spermatogonia differentiate into primary spermatocytes before undergoing meiosis I. The division of a single primary spermatocyte produces a pair of secondary spermatocytes. In meiosis II each secondary spermatocyte divides to yield a pair of spermatids that will mature into spermatozoa.

4. In the female oogonia differentiate into primary oocytes. Each primary oocyte that enters meiosis-I generates a secondary oocyte and a polar body. Subsequent division of the secondary oocyte produces a mature ovum and another polar body.

Fertilization

1. Fertilization normally occurs in the uterine tubes within a day of ovulation.

2. Sperm arriving in the vagina must undergo capacitation before they are fully functional. They must also ascend the female reproductive tract before arriving at the fertilization site.

3. Out of 200 million spermatozoa introduced into the vagina only around a hundred reach the vicinity of the egg. If the sperm count falls below 20 million/ml the male will be functionally sterile because fertilization requires a group effort.

4. The acrosomal caps of the spermatozoa release hyaluronidase that separates the cells of the corona radiata and exposes the oocyte membrane. When a single spermatozoon contacts that membrane fertilization occurs.

GENETICS, DEVELOPMENT, AND INHERITANCE

1. The genes within the nucleus of the somatic cells determine the genotype. The visible expression of the genetic programming represents the phenotype.

Genes and chromosomes

Autosomal chromosomes:

1. Each somatic cell contains 23 pairs of chromosomes. A pair consists of homologous chromosomes. Twenty-two of the pairs are autosomal chromosomes.

2. The corresponding genes located on separate homologous chromosomes are known as alleles. If both alleles carry identical instructions the individual is homozygous; if they carry conflicting instructions they are heterozygous.

3. At fertilization each gamete carries one of the two parental alleles for each autosomal characteristic. Using a Punnet square enables us to predict the characteristics of the offspring based on the genetic composition of the parents.

Sex chromosomes:

1. The twenty-third pair of chromosomes are known as the sex chromosomes. Sex chromosomes are either X or Y. In a male this pair consists of XY, while a female has XX.

2. The X chromosome carries genes that affect somatic structures. These characteristics are called X-linked because they have no corresponding alleles on the Y chromosome. In a male, X-linked characteristics will always affect the phenotype.

Induction and the Regulation of Development

1. Development and differentiation occur because there are changes in the genetic activity of some cells and not others. The activity within the nucleus of each cell varies depending on the chemical messages arriving from the cytoplasm.

2. The pathways of cleavage and early development are largely determined by variations in the composition of the oocyte cytoplasm.

3. Induction involves turning specific genes on or off in response to changes in the local environment. Compounds produced by other cells or introduced into the intercellular fluids may act as inducers.

Competence and Developmental Timing:

1. Induction is a selective process because the cells responding must have the competence to be influenced by a particular inducer.

2. Competence depends upon when the inducing stimulus is produced as well as where. Over time a series of mutual inductions can direct the development of complex structures.

PRENATAL DEVELOPMENT

1. The nine month gestation period that follows can be divided into three trimesters.The first trimester establishes the basic rudiments of major organ systems. The functional development of those systems continues throughout the second trimester. The third trimester is dominated by rapid fetal growth.

The First Trimester

Cleavage and blastocyst formation:

1. Cleavage subdivides the cytoplasm of the zygote in a series of mitotic divisions.

2. The blastocyst consists of an outer trophoblast and an inner cell mass.

Implantation:

1. During implantation the blastocyst burrows into the uterine endometrium. The trophoblast erodes the maternal tissues and absorbs nutrients. As the trophoblast enlarges and spreads, maternal blood flows through open lacunae.

Placentation:

1. The trophoblast establishes the basic structure of the placenta. Completion of its formation requires the participation of embryonic tissues originating at the inner cell mass.

2. After gastrulation the blastodisc contains an embryo composed of ectoderm, mesoderm, and endoderm.

3. These primary germ layers participate in the formation of four extraembryonic membranes: the yolk sac, the amnion, the allantois, and the chorion.

4. Blood vessels migrating within the mesoderm of the allantois spread across the inner surface of the cellular trophoblast. The appearance of these vessels represents the first step in the

ormation of a functional placenta.

5. Chorionic villi extend outward into the maternal tissues,forming an intricate, branching network through which maternal blood flows.

6. As development proceeds the umbilical cord contains the circulatory connections between the embryo and the placenta.

7. The trophoblast synthesizes HCG, estrogens, progesterone, HPL, and relaxin.

Embryogenesis:

1. During the first trimester the basic body plan becomes established, and organogenesis begins.

The Second and Third Trimesters

1. In the second trimester the organ systems near functional completion. The fetus grows rapidly, increasing its weight 50-fold to around 0.64 kg (1.4 lb).

2. During the third trimester the organ systems become functional, and the fetus grows to roughly 3.2 kg (7 lb).

Pregnancy and Maternal Systems:

1. The developing fetus is totally dependent upon maternal organs for nourishment, respiration, and waste removal. Maternal adaptations include increased blood volume, respiratory rate, tidal volume, nutrient intake, and glomerular filtration.

2. The uterus quadruples in length and grows to almost 20 times its original weight.

3. The myometrial stretching is associated with a gradual increase in smooth muscle sensitivity, and spontaneous contractions begin to appear. Progesterone produced by the placenta has an inhibitory effect on the uterine muscles.

4. Estrogens produced by the placenta late in gestation overcome the inhibitory effects of progesterone, aided by the release of uterine prostaglandins and oxytocin by the posterior pituitary.

5. At some point the conditions favor the onset of labor contractions. Once true labor begins a positive feedback mechanism continues to provide stimulation until parturition occurs.

LABOR AND DELIVERY

1. During true labor the uterine contractions are strong and they occur at regular intervals.

2. Labor can be divided into the dilation stage, the expulsion stage, and the placental stage.

Common Problems with Labor and Delivery

1. Complications of labor and delivery include forceps deliveries, breech births, and premature delivery.

Multiple Births:

1. Multiple births may be monozygotic (identical) or dizygotic (fraternal). Monozygotic twins may result from separation of blastomeres during cleavage or the division of the embryonic disc early in development.

POSTNATAL DEVELOPMENT

1. Postnatal development involves a series of life stages, including infancy and childhood, adolescence, and maturity. Senescence begins at maturity and ends in the death of the individual.

Infancy and Childhood

1. The neonatal period extends from birth to 1 month of age. Infancy continues to age 2 years, and childhood extends until puberty.

2. In the transition from fetus to neonate the respiratory, circulatory, digestive, and urinary systems must begin functioning independently.The newborn infant must also begin thermoregulating. Suggestion: Review the information from earlier chapters pertaining to specific changes occurring at delivery.

3. Growth during infancy and childhood occurs under the direction of growth hormone, adrenal steroids, and thyroid hormones. Because different tissues vary in their responses to these hormones, body proportions gradually change.

Monitoring Postnatal Development:

1. Pediatrics focuses on postnatal development from infancy through adolescence. Minor variations from "normal" development are not serious.

Adolescence and Maturity

1. Adolescence begins at puberty with the increased production of GnRF by the hypothalamus, coupled with an increased pituitary sensitivity to this hormone.

2. The rise in FSH and LH/ICSH levels leads to the secretion of sex hormones by the gonads. The combination of sex hormones, growth hormone, thyroxine, and adrenal hormones causes a sudden acceleration in the growth rate.

3. Maturity is usually associated with the cessation of growth, although a precise definition is difficult to establish. Aging (senescence) begins almost at once following maturity.

4. Aging reduces the efficiency and capabilities of the individual. All physiological systems are affected, but not at identical times nor at similar rates.

5. Aging appears to involve changes in cellular activity. Important processes include a reduction in the size of some cell populations, decreased mitoses among stem cells, accumulated mutations, and alterations in genetic activity.

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