Human Reproductive and Development
Human reproductive employs internal fertilization, and depends on the integrated action of hormones, the nervous system, and the reproductive system. Gonads are sex organs that produce gametes. Male gonads are the testes, which produce sperm and male sex hormones. Female gonads are the ovaries, which produce eggs (ova) and female sex hormones.
1. The Male Reproductive System
Testes are suspended outside the abdominal cavity by the scrotum, a pouch of skin that keeps the testes close or far from the body at an optimal temperature for sperm development. Seminiferous tubules are inside each testis, and are where sperm are produced by meiosis. About 250 meters (850 feet) of tubules are packed into each testis. Spermatocytes inside the tubules divide by meiosis to produce spermatids that in turn develop into mature sperm.
Spermatogenesis
Sperm production begins at puberty at continues throughout life, with several hundred million sperm being produced each day. Once sperm form they move into the epididymis, where they mature and are stored.
Male Sex Hormones
The anterior pituitary produces follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Action of LH is controlled by the gonadotropin-releasing hormone (GnRH). LH stimulates cells in the seminiferous tubules to secrete testosterone, which has a role in sperm production and developing male secondary sex characteristics. FSH acts on cells to help in sperm maturation. Negative feedback by testosterone controls the actions of GnRH.
Sexual Structures
Sperm pass through the vas deferens and connect to a short ejaculatory duct that connects to the urethra. The urethra passes through the penis and opens to the outside. Secretions from the seminal vesicles add fructose and prostaglandins to sperm as they pass. The prostate gland secretes a milky alkaline fluid. The bulbourethral glandsemen. secretes a mucus-like fluid that provides lubrication for intercourse. Sperm and secretions make up
2. The Female Reproductive System
The female gonads, ovaries, are located within the lower abdominal cavity.
The ovary contains many follicles composed of a developing egg surrounded by an outer layer of follicle cells. Each egg begins oogenesis as a primary oocyte. At birth each female carries a lifetime supply of developing oocytes, each of which is in Prophase I. A developing egg (secondary oocyte) is released each month from puberty until menopause, a total of 400-500 eggs.
Ovarian Cycles
After puberty the ovary cycles between a follicular phase (maturing follicles) and a luteal phase (presence of the corpus luteum). These cyclic phases are interrupted only by pregnancy and continue until menopause, when reproductive capability ends. The ovarian cycle lasts usually 28 days. During the first phase, the oocyte matures within a follicle. At midpoint of the cycle, the oocyte is released from the ovary in a process known as ovulation. Following ovulation the follicle forms a corpus luteum which synthesizes and prepares hormones to prepare the uterus for pregnancy.
The secondary oocyte passes into the oviduct (fallopian tube or uterine tube). The oviduct is connected to the uterus.
The uterus has an inner layer, the endometrium, in which a fertilized egg implants. At the lower end of the uterus the cervix connects the uterus to the vagina. The vagina receives the penis during intercourse and serves as the birth canal.
External Genitals
The female external genitals are collectively known as the vulva. The labia minora is a thin membrane of folded skin just outside the vaginal opening. The labia majora cover and protect the genital area. A clitoris, important in arousal, is a short shaft with a sensitive tip covered by a fold of skin.
Hormones and Female Cycles
The ovarian cycle is hormonally regulated in two phases. The follicle secretes estrogen before ovulation; the corpus luteum secretes both estrogen and progesterone after ovulation. Hormones from the hypothalamus and anterior pituitary control the ovarian cycle. The ovarian cycle covers events in the ovary; the menstrual cycle occurs in the uterus.
Menstrual cycles vary from between 15 and 31 days. The first day of the cycle is the first day of blood flow (day 0) known as menarche. During menarche the uterine lining is broken down and shed as menstrual flow. FSH and LH are secreted on day 0, beginning both the menstrual cycle and the ovarian cycle. Both FSH and LH stimulate the maturation of a single follicle in one of the ovaries and the secretion of estrogen. Rising levels of estrogen in the blood trigger secretion of LH, which stimulates follicle maturation and ovulation (day 14, or midcycle). LH stimulates the remaining follicle cells to form the corpus luteum, which produces both estrogen and progesterone.
Estrogen and progesterone stimulate the development of the endometrium and preparation of the uterine inner lining for implantation of a zygote. If pregnancy does not occur, the drop in FSH and LH cause the corpus luteum to disintegrate. The drop in hormones also causes the sloughing off of the inner lining of the uterus by a series of muscle contractions of the uterus.
3. Fertilization and Pregnancy
If mature spermatozoa recovered from uterus or oviduct a few hours after coitus are capable of immediate fertilzation. The stages of fertilization are (a) spermatozoa approach an ovulated oocyte with its polar body, chromosomes on the second metaphase spindle, cortical granules and perivitelline space between the oolema of the oocyte and the zona pellucida, (b) penetrating the zona to lie in the pperivitelline space, (c) fuses with plasma membrane via its equatorial and postacrosomal membrane, thus activiting a series of calcium waves, (d) meiotic devision is completed, yielding the second polar body and female pronucleus, (e) chromosomes duplicate their DNA, pronuclei migrate together, their membranes break down as the first mitotic spindle forms. After its formation, chromatids separate and move apart, and a clavage furrow develops.
The growing conceptus faces a hazardous few days of early life. Not only must it pass from the oviduct to uterus at exactly the right time, but it must also establish adequate nutritional support for its growth and development. The very earliest days are marked by little growth at all, but the rising progesterone stimulates the secretory phase p\of uterus to provide a nutritious fluid support. The superimposition of an oestrogenic stimulus allows attachment and implantation to occur. The coceptus can then secure a physical hold on the mother, establish its own circulation and stimulate, at adjacent endometrial sites, the establishment of special maternal circulatory changes. The transition of metabolic exchange has been ochieved via placenta.
4. Lactation
Lactation provides the primary source of nutrition for the new-born and is also the focus around which mother-infant bonding occurs. The breast develops during pregnancy under influence of several hormones, especially sex steroid, prolactin and insulin in women. Breast are consistof alveoli are groups together in lobules and these are responsible for secreting milk which is conveyed to the nipple via lactiferous ducts. Milk contains of water, lactose, fats, amino acids, proteins, minerals and vitamins with energy value of 650 kcal/100 ml.
5. Sexually Transmitted Diseases
Sexually transmitted diseases (STDs) cause over $7 billion to be expended for treatment. STDs can affect the sex partners, fetus, and newborn infants. STDs are grouped into three categories.
Category One
STDs that produce inflammation of the urethra, epididymis, cervix, or oviducts. Gonorrhea and chlamydia are the most common STDs in this category. Both diseases can be treated and cured with antibiotics, once diagnosed.
Category Two
STDs that produce sores on the external genitals. Genital herpes is the most common disease in this class, affecting more than 25 million individuals in the US. Symptoms of herpes can be treated by antiviral drugs, but the infection cannot be cured. Syphilis is a bacterially caused infection, and can, if left untreated, cause serious symptoms and death. However, the disease is curable with antibiotics.
Category Three
This class of STDs includes viral diseases that affect organ systems other than those of the reproductive system. AIDS hepatitis B are in this category. Both can be spread by sexual contact or blood. Infectious individuals may appear symptom-free for years after infection.
