THE REPRODUCTIVE SYSTEM

Reproduction is a fundamental process that allows all organisms to perpetuate continuity through their progeny or offspring. There are many different ways in which living organisms reproduce, however, there are two basic means of reproduction. What is parthenogenesis?

They are asexual and sexual reproduction.

Small organism including bacterial reproduce by asexual method (only one sex partner). In bacteria this method is called binary fission. Some plants produce asexually (by budding) and some small insects including worms also reproduce asexually.

Let us examine sexual reproduction (involves two gametes and exchange of genetic information). This presentation features two parts. Part I: a) general information on reproduction b) male and female reproductive organs c) Disorders of male and female reproductive systems Part II: Pregnancy and birth; Growth and development.

A. General Information

In higher animals sexual reproduction involves copulation of both male and female partners leading to the process of fertilization of the egg. In many small organisms (fish, frogs and other organism) the male sprays the sperm after the female has laid the eggs in a comfortable location. The essence of sexual reproduction is that each offspring requires two parents who contribute the genetic material to the offspring. Consequently, unlike asexual reproduction as shown in binary fission or budding where all offsprings are identical, sexual reproduction offers genetic combination and unidentical offsprings. Diversity in offsprings promotes survival and evolution of species as an added advantage.

The Gametes

The sex partners produce sex cells called gametes. The male sex cell is the spermatozoon (or sperm) and the female sex cell is the egg (ovum). These two cells are naturally endowed with capacities for their roles. The role of the sperm is to fertilize the egg, which would become a zygote (fertilized egg). The sperm is designed to swim, using its flagella, and also to deposit its head which carries 23 chromosomes. The egg on the other hand also carries 23 chromosomes. It is protected in a tissue that requires penetration by the sperm. It is thus provided with nourishment within its walls of confinement.

Reproductive systems

The reproductive systems consist of primary sex organs (gonads) which produce the sex organs and secondary sex organs.

• The primary sex organs in males involves the testes and ovaries in the female.
• The secondary sex organs are all other organs except the sex organs that are involved in reproduction. In males they include ducts, glands and the penis. In female they include fallopian tubes, uterus and vagina
• Secondary sex characteristics are features unique to male and females and appear at specific periods of development. They include facial hairs, deep voice and mustache in male and breast development among others in females.

Sex Determination

Each sex cell carries 23 chromosomes. 22 of these chromososomes are autosomes (ie chromosomes for body cells) and 1 (one) chromosome is for sex cell (sperm or egg).

• A sex chromosome is either an X or a Y. The egg contains X chromosomes and the sperm contain Y or X chromosome.
• If an egg is fertilized by a sperm that carries an X chromosome, then the zygote will be XX, indicating it is a female.
• However, if the egg is fertilized by a sperm carrying a Y chromosome, the zygote would have an XY chromosome indicating a male.
• The presence of an X or Y determines whether the gonad will be a testes or ovary.
• Therefore the gender of a child after conception is determined not by the egg but by the sperm.

Hormonal Influence

The masculinity of a zygote is determined not only by the Y chromosomes but also by SRY gene (sex determining region of Y) on the chromosome.

• SRY gene causes the development of testes by 8 to 10wks of pregnancy that begins the secretion of testosterone (the male hormone).
• Testosterone causes the development of tubes, ducts and other delivery systems
• In females, the absence of testes determining factor (TDF) lead to the development of ovaries in the presence of estrogen (estradiol).

Development of Male Genitalia

In the fetus the external genitalia are identical up to 8 weeks after pregnancy. Three folds are formed: phallus, urogenital and labioscrotal folds.

• Phallus (small shaft of tissue with swollen glans or head);
• Urogenital folds (a pair of medial tissue folds posterior to the phallus) and
• Labioscrotal fold (larger pair of folds lateral to the urogenital fold)
• Sexual differentiation occurs by the end of 9wks and by 12wks, distinct male or female genitalia are formed.
• In female, the structures above form the clitoris, labia minora and labia majora respectively. In male, the phallus elongates forming the penis; the urogenital folds forms the urethra and the labioscrotal folds forms the scrotal sac and testes.

B. THE REPRODUCTIV ORGANS

Anatomy of the Male Sex Organs

Scrotum

The scrotum is a hanging sac that houses the testes. The testes hang lower in the left sac. The sac has a rich supply of sensory innervation, hairs, sebaceous glands and dark pigmentation than the skin. It is divided into two halves by median septum that protects the testes.

Testes

· Testes are oval-shapes structures that consist of two layers: the tunica vaginalis, a thin serous sac derive from peritoneum and tunica albuginea, a fibrous capsule that covers each testes.

· The fibrous strands divided the testis into 250 to 300 wedge-shaped lobules.

· Sperm production (spermatogenesis) occurs in seminiferous tubules, series of convoluted within the lobules. The walls of the tubules are made of sustentacular (Sertolli) cells that provide nutrients to the germ cells (stem cells that would become spermatogonia). The cells also produce inhibin, an hormone that regulates spermatogenesis.

· The sperms undergo further maturational development in a tubular compartment called rete testes and are finally moved into the epididymis thru efferent ducts.

· Testes are supplied with testicular arteries and veins.

· Between seminiferous tubules are clusters of endocrine cells called interstitial cells (Leydig cells) that secrete androgen hormone of which testosterone is the most potent.

· Nerves from the autonomic nervous system (sympathetic and parasympathetic) also enervate on testes.

Spermatogenesis

Spermatogenesis involves the process of formation of sperms.

• The process begins within spermatogonia (embryonic cells) which line the sides of the seminiferous tubules.
• The sperm cells carry 46 chromosomes (diploid). Each spermatogonium undergo mitosis producing two primary spermatocytes (2n).
• The primary spermatocytes produce secondary spermatocytes in meiosis I in which the chromosome number is decreased by half (or 23).
• Each secondary spermatocyte produce two spermatids each carrying 23 chromosomes in meiosis II.
• The spermatids mature into sperms.

Accessory Organs

1. Accessory ducts: The accessory ducts transport the sperm from the testes to the point where they are ejaculated from the penis. The following ducts transport the sperm.

• Epididymus: stores sperm as they mature, serves as a duct for their passage and through peristalsis moves sperm towards the penis.
• Duct deferentia: these paired ducts are continuations of the epididymus and the main carriers of sperm
• Ejaculatory ducts: receive secretions from seminal vesicles and joins the urethra.
• Urethra: a tube through the penis that consist of prostatic, membranous and penile (spongy) tissues that delivers the sperm out of the body.

2. Accessory glands

Accessory glands add their secretions to the sperm as they pass through ducts. These glands are seminal vesicles, prostate gland and bulbourethral glands. The combination of fluids that is produced is called semen.

• Seminal vesicles: secretory sacs that produce alkaline secretion provide nourishment (fructose and vitamin C) for the sperm and help neutralize the vaginal acidity. Prostate gland make the sperm motile and neutralize vaginal acidity. Bulbourethral gland neutral any urine in the urethra and lubricate the urethra.
• Semen: secretions from the seminal vesicles, prostate gland and bulbourethral gland and sperm collectively make up semen.
• Semen is over 90% water, energy-rich fructose, vitamin C and inositol; calcium, zinc, magnesium, copper and sulfur and a high conc. of prostaglandins. The odor of semen is due to ammonia (derived from amines in testes).
• The average ejaculate is 3-4ml of semen (about a teaspoonful) and contains about 300-500 million sperms.
• A person who produce less than 3-4ml of semen may be infertile (although the amt of semen produced may still be able to fertilize an egg).

Penis

The penis has two functions: 1. transport urine from the bladder to the exterior during urination 2. transport semen from the ducts through the urethra during ejaculation. The penis contains three erectile tissues: two corpora carvernosa which runs parallel to the dorsum of the penis and corpus spongiosum which contains the urethra. The tip of the penis is called glans penis becomes erected when stimulated and engorged with blood.

Hormones

Testosterone secretion from the testes is controlled by follicle stimulating hormone (FSH) and leutinizing hormone (LH) from the pituitary gland controlled by gonadotropin releasing hormone (GnRH) from the hypothalamus.

Anatomy of Female Sex Organs

The female reproductive system is complex. It goes through monthly menstrual cycles, produce ova, maintain a fertilized embryo for nine months and care for the infant after delivery. The following are structures of the female reproductive organs.

• Ovaries: The paired ovaries are the female gonads. They produce the eggs (ova) and female hormones (estradiol and progestrone) and located in the pelvic part of the abdomen.
• Germinal layer is a special epithelium that covers the ovaries. Beneath this layer is a mass of tissue called stroma which contains the ova in various stages of development.
• Mesentery tissue attach each ovary to a mesovarium which is also attached to the uterus by an ovarian ligament The eggs are in the follicles undergoing various stages of development.
• Mesovarium contains veins, arteries, lymphatics and nerves to and from the helium
• Suspensory ligaments connect both the ovary and fallopian tube to the pelvic wall.
• The ovaries are supplied with ovarian arties, veins and nerves.

Secondary Sex Organs

The secondary sex organs include uterine tubes (fallopian tubes), uterus and vagina. The external genitalia include the clitoris, labia manora and majora.

• Fallopina tubes (Uterine tubes): It is also called oviduct and measures about 10 cm. The end of the fallopian tube is called infundibulum which contains brush-like projections called fimbriae. The middle part of the tube is called the ampulla and near the uterus it forms the isthmus.
• One for each egg carries egg from the ovaries to the uterus

Uterus:

• houses and nourishes the developing embryo. It is a thick muscular wall that consist of the fundus of the uterus, and an enlarged portion called the body of the uterus.
• The portion that opens to the vagina is called the cervix of the uterus. The uterine cavity is the space within the fundus and the body of the uterus.
• The junction of the uterine with the cervical canal is called the isthmus of uterus.
• The opening of the cervical canal into the vagina is called the uterine ostium. The wall of the uterus is composed of three layers:

1. Peritoneum (outermost serous layer) and consist of thin visceral peritoneum

2. Myometrium: composed of serous layers of smooth muscle of circular, longitudinal and spiral patterns.

3. Endometrium: innermost, mucosal tissue composed of two layers: a) the statum functionale consist of columnar epithelium that contains secretory glands. The tissue is under the influence of steroid hormones, sheds during menstruation and build up again; b) Stratum basale is the inner vascular layer that regenerates the stratum functionale after each menstruation. These reproductive organs are supported by pair broad and round ligaments.

Vagina

• Vagina: receives semen from the penis during intercourse; exit for menstrual flow and childbirth.
• The lower end of the vaginal mucosa forms a hymen that stretches across the orifice. The lower end also has transverse friction ridges called vaginal ruggae which stimulates the penis and help induce ejaculation.
• Vaginal epithelium is cuboidal in childhood but undergoes estrogen stimulation and transformation into stratified squamous epithelium.
• The mucosa of the vagina also has antigen presenting cells called dentritic cells, a route for possible transmisson of HIV

Vulva

• The external genitalia of female are collectively called vulva (pudendum) that includes the mons pubis, labia minora, majora, clitoris, vaginal and urethral orifices, accessory glands and erectile tissues.
• Mons pubis is largely made of adipose tissues; labia majora and minora are skin folds.
• Vestibule is the area enclosed by these tissues that contain vaginal and urinary orifices
• The clitoris is a center for sensory stimulation

Secondary Sex Characteristics

Mammary glands

• Located in the subcutaneous tissue with the breast
• The breast extend between the second and sixth ribs
• Glands are composed of lobes that contain tubular glands
• Mammary glands are connected to the nipple by ducts
• Estrogen stimulates breast development in female. The breast changes during pregnancy. Progesterone causes development of alveolar glands (alveolar glands and ducts enlarge upon estrogen stimulation).
• Prolactin is released but progesterone inhibits milk production by prolactin
• After birth of the baby, prolactin stimulates milk production. Oxytocin causes milk-ejection when the baby sucks the breast…this reflex action causes the posterior pituitary to release oxytocin.
• Breast remain non-functional in males

Development of Follicles

The ovary consists of two layers called cortex and medulla. development. Each follicle contains immature oocyte surrounded by one or more layers.

· Follicles are vesicles that have one layer. They are called granulose when surrounded by more than one layers.

· The follicles are at various stages of development.

· Primordial follicles: Vesicles containing oocyte with one layer of squamous tissue surrounding them.

· Primary follicle consist of two or more layers of cuboidal or columnar cells around the oocyte, and becomes a secondary follicle when it develops a fluid-filled space called antrium.

· Vesiclular (Graffian follicle): a mature follicle containing an oocyte. This follicle releases its oocyte once a month in a process called ovulation.

· Corpus luteum (yellow body) : the follicular structure that remains after the oocyte has been extruded. The corpus luteum secretes estrogens and progesterone that stimulate thickening of the uterus and prevents further ovulation and development of the mammary glands if pregnancy occurs.

· If pregnancy does not occur in 14 days after ovulation the corpus luteum regresses into a scar tissue called corpus albicans and menstruation occurs immediately.

· If pregnancy occurs, the corpus luteum persists for 2-3 days and degenerates as it is replace by the placenta.

Oogenesis:

Ovaries and Follicles

• Ova or egg cells are contained within hollow structures called follicles
• Primary oocytes have begun meitosis but are arrested at prophase I
• Unstimulated primary oocytes are contained in the primary follicles.
• In response to FSH stimulation, some primary follicles mature each month to become secondary follicles. Secondary follicles contain granulose cells and vesicles.
• Continued stimulation results in a graffian follicle that contains an antrium or cavity.
• A primary oocyte may be stimulated to become a secondary oocyte but meiosis is arrested at metaphase II; the other product of the first meiotic division is a non-functional structure called polar body.

Ovulation

· High levels of gonadotrophic hormones (FSH) and leutinizing (LH) from the anterior pituitary gland stimulate the Graffian follicles to rupture and release mature egg, the process called ovulation.

· After ovulation the oocyte is drawn into the opening of the uterine tube

· The empty ovulated follicle become a new ovarian structure called corpus luteum.

· Ovarian steroid hormones exert negative feedback control of the FSH and LH

· A hypothalamic-releasing hormone known as gonadotropin releasing hormone (GnRH) controls LH and FSH

Menstrual cycle: Pituitary gland and Ovary

· During the follicular phase, follicles grow under FSH stimulation to become secondary follicles. By day 13, one graffian follicle is usually produced.

· The growing follicles secrete an increasing amount of estradiol; this stimulates the release of LH from the pituitary prior to ovulation

· The LH surge exerts a feedback effect of estradiol that stimulate ovulation

· The corpus luteum also secretes progesterone and estradiol which inhibit FSH and LH secretion during the cycle.

Menstrual cycle and Endometrium

· During the follicular phase, increasing levels of estradiol stimulate a thickening of the endometrium

· The effects of progesterone and estradiol during the luteal phase cause endometrium to enter its secretory phase

· At the end of the cycle the corpus luteum dies; the decline in the level of estrodiol and progesterone stimulates menstruation.

C. General disorders

Male

· Impotence: inability of a mature male to achieve penile erection or inability to achieve ejaculation. Causes may be physical (abnormalities of penis, vascular irregularities, neurological disorders or diseases). Impotence is mostly psychological and requires counseling by sex therapist.

· Infertility: inability of sperm to fertilize an egg and may involve male or female. Most common cause is inadequate production of viable sperm. The problem may due to alcoholism, dietary deficiencies, local injury, varicocele, excessive heat, hormonal imbalance or excessive exposure to x-ray. Most infertility problems can be treated thru proper nutrition, hormone therapy or microsurgery.

· Sterility is similar to infertility except that it is a permanent condition. May be genetic problem or degenerative alterations in the seminiferous tubes (e.g mumps may cause permanent damage in testes)

Female

· Endometriosis: presence of endometrial tissues at sites other than the inner lining of the uterus (ectopic sites). Ectopic endometriosis is often found on the ovaries, on the outer layer of the uterus, on the abdominal wall, and on the urinary bladder. Bleeding occurs because the endometrial cells respond to hormone stimulation. Most common symptoms are dysmenorrhea (menstrual pain) and a feeling Hyterectomy of fullness during each menstrual period. Endometriosis can cause infertility. and possibly oophorectomy (removal of the ovaries) are possible solutions.

· Ovarian neoplasms: usually non-malignant fluid-filled cysts. Tumors can be palpated during gynecological exams. If the mass exceed 4cm in diameter, they may have to be removed surgically.

· Uterine neoplasms: extremely common problem of female reproductive tract and mostly benign include cysts, polyps and smooth muscle tumors. They very often provoke irregular menstruation and may pose problems related to infertility if they grow big.

· Cancer of the uterus: most common malignancy of female reprod. Tract and found most often as uterine cancer in the cervix of the uterus. Treatment depends on the stage of malignancy and health of the woman. Hysterectomy is one possible solution.