THE REPRODUCTIVE SYSTEM
The ability to reproduce is one of the unifying characteristics of all living things. Sexual reproduction produces offspring that are genetically different from their parents. Asexual reproduction produces offspring genetically identical to their parent.
Fission, budding, fragmentation, and the formation of rhizomes and stolons are some of the mechanisms that allow organisms to reproduce asexually. The hydra produces buds; starfish can regenerate an entire body from a fragment of the original body. Asexual reproduction allows an organism to rapidly produce many offspring without the time and resources committed to courtship, finding a mate, and mating. The lack of genetic variability in asexually reproducing populations can be detrimental when environmental conditions change quickly.
In sexual reproduction new individuals are produced by the fusion of haploid gametes to form a diploid zygote. Sperm are male gametes, ova are female gametes. Meiosis produces cells that are genetically distinct from each other; fertilization is the fusion of two such distinctive cells that produces a unique new combination of alleles, thus increasing variation on which natural selection can operate.
Rotifers will reproduce asexually when conditions are favorable by having females produce eggs by mitosis. When conditions deteriorate, rotifers will reproduce sexually and encase their zygotes inside a resistant shell. Once conditions improve, these eggs hatch into diploid individuals. Rotifers thus use sexual reproduction as way to survive a deteriorating environment.
Sexual reproduction offers the benefit of generating genetic variation among offspring, which enhances the chances of the population’s survival. Costs of this process include the need for two individuals to mate, courtship rituals, as well as a number of basic mechanisms described later.
Human Reproduction and Development
Human reproduction 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 and female sex hormones.
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.
Sperm production takes place in the testes. The testes are in the scrotum. The scrotum is a sac located outside of the male’s body. Since sperm can develop only in an environment with a temperature that is about 3C lower than normal body temperature, the exterior position of the scrotum provides an ideal location.
Located within each testis, a fine network of highly coiled tubes are the production facilities for sperm. Sperm are produced through the meiosis of cells lining these tubes. Meiosis produces haploid cells. One cell divides by meiosis and produce four cells that mature into sperm. In human males, the production of mature sperm takes about 74 days. A sexually mature male can produce about 300 million sperm per day, each of his life. A sperm is highly adapted for reaching and penetrating the female egg. It can live for about 48 hours inside the female reproductive tract. A sperm is composed of a head, a midpiece, and a tail. The head contains the nuclear material and is covered by a cap containing enzymes that facilitate penetration of the egg. A number of mitochondria are found in the midpiece; they provide energy for locomotion. The tail is a typical flagellum that propels the sperm along its way.
How Sperm Leave The Testes
Before the sperm mature, they move out of the testes through a series of coiled ducts that empty into a single tube called the epididymis. The epididymis is a coiled tube within the scrotum in which the sperm complete their maturation. Mature sperm remain in the epididymis until they are released from the body.
When sperm are released from the epididymis, they enter the vas deferens. The vas deferens is a duct that transports sperm from the epididymis toward the ejaculatory ducts and the urethra. Peristaltic contractions of the vas deferens force the sperm along. The urethra is the tube that transports sperm out of the male’s body. The urethra also transports urine from the urinary bladder. A muscle located at the base of the bladder prevents urine and sperm from mixing.
Fluids Transporting Sperm
As sperm travel from the testes, they mix with the fluids secreted by several different glands. The seminal vesicles are a pair of glands located at the base of the urinary bladder. They secrete a mucouslike fluid into the vas deferens. The fluid is rich in the sugar fructose, which provides energy for the sperm cells.
Another structure, the prostate gland, is a single, doughnut-shaped gland that surrounds the top portion of the urethra. The prostate secretes a thinner, alkaline fluid that helps sperm move and survive. Two tiny bulbourethral glands located beneath the prostate. These glands secrete a clear, sticky, alkaline fluid that protects sperm by neutralizing the acidic environment of the vagina. The combination of sperm and all of these fluids is called semen.
Male Sex Hormones
Hormones also are the control mechanism for the male’s reproductive system. In early male teen years, changes begin to occur to their bodies. Puberty is the period characterized by development of secondary sex characteristics as a result of production of follicle-stimulating hormone, luteinizing hormone, and other sex hormones. The changes associated with puberty are controlled by the sex hormones that are secreted by the endocrine system. In male, the onset of puberty causes the hypothalamus to produce several kinds of hormones that interact with the pituitary gland. The pituitary is a gland located at the base of the hypothalamus that secretes hormones used to influence many different physiological processes of the body. The hypothalamus secretes a hormone that causes the anterior lobe of the pituitary to release two other hormones: follicle-stimulating (FSH) and luteinizing (LH). When released into the bloodstream, FSH and LH are transported to the testes. In the testes, FSH causes the production of sperm cells. LH causes endocrine cells in the testes to produce the testosterone. The testosterone is the steroid hormone responsible for the growth and development of secondary sex characteristics in a male. These characteristics include the growth and maintenance of male sex organs; the production of sperm; the deepening of the voice; an increase in body hair; and an increase in muscle mass and in the growth rates of the long bones of the arms and legs. Evidence suggests that testoste1rone may also be responsible for an increase in increase in aggressive behavior.
The Female Reproductive System
The main function of the female reproductive system are to produce eggs, the female sex cells, and to provide an environment for a fertilized egg to develop. The female reproductive system includes two ovaries, two oviducts or fallopian tubes, the uterus, and the vagina. Egg production takes place in the two ovaries. Each ovary is about the size and shape of an almond. One ovary is located on each side of the lower part of the abdomen.
The open end of the oviduct is located close t each ovary. The oviduct is a tube that transports eggs from the ovary to the uterus. Peristaltic contractions of the muscles in the wall of the oviduct combine with beating cilia to move the egg in the tube.
Female mammals have a uterus in which the fetus develops during pregnancy. The uterus is situated between the urinary bladder and the rectum and is shaped like and inverted pear. The uterus is composed of a thick, muscular layer and a thin lining called the endometrium. It’s lower end, called the cervix, tapers to a narrow opening into the vagina. The vagina is a passageway between the uterus and the outside of the female’s body. 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.
Puberty In Females
Puberty in females begins when the hypothalamus signals the anterior lobe of the pituitary to produce and release the hormones FSH and LH. In females FSH stimulates the development of a follicle in the ovary. A follicle is a group of epithelial cells that surround an undeveloped egg cell. FSH also causes the release of the hormone estrogen from the ovary. Estrogen is the steroid hormone responsible for the secondary sex characteristics of females. These characteristics include the growth and maintenance of female sex organs; an increase in fat deposits in the buttocks, thighs, and breast; a broadening of the hips; and an increase in body hair. Before a female is born, her body begins to develop eggs. During this prenatal period, cells in her ovaries divide until the first stage of meiosis, prophase I, is reaches. At this point, the cells go into a resting stage. At this point the cells go into a resting stage. At birth, a female has all the potential eggs she will ever have. About once a month in a sexually mature female, the process of meiosis starts up again in one of the prophase I cells. The result is the production an egg that ruptures from the ovary and passes into the oviduct. This process of the egg rupturing through the ovary wall and moving into the oviduct is called ovulation. Fertilization occurs in the oviduct if the egg and sperm unite. All the activities of the human female reproductive system are part of a cycle. Once the egg has been released, the remaining part of the follicle develops into a structure called the corpus luteum. The corpus luteum secretes the steroid hormone, progesterone. Progesterone causes changes to occur in the lining of the uterus, which prepare it for receiving a fertilized egg. The series of changes in the female reproductive system that include producing an egg and preparing the uterus for receiving it is known as the menstrual cycle.
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 menstruation. During menstruation 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.
Fertilization, the fusion of the sperm and egg, usually occurs in the upper third of the oviduct. Thirty minutes after ejaculation, sperm are present in the oviduct, having traveled from the vagina through the uterus and into the oviduct. Sperm traverse this distance by the beating of their flagellum. Of the several hundred million sperm released in the ejaculation, only a few thousand reach the egg. One reason is that the fluids secreted by the vagina are acidic and destroy most of the delicate sperm. Some sperm survive because of the buffering effect of semen. The surviving sperm swims up the vagina and into the uterus. Only a few hundred sperm pass into the two the fallopian tubes. The egg is in one of the oviducts.
The head of the sperm contains enzymes that help it penetrate the egg. The sperm will lose its midpart and its tail as it penetrates the egg. After one sperm penetrates the egg, the egg’s membrane changes it electrical charge. That prevents the other sperm from getting in the egg. Also its nucleus combines with the egg’s nucleus to form a zygote. The fertilized egg moves to the uterus. As the zygote passes down the oviduct, it begins to divide by mitosis. They zygote obtains nutrients from fluids secreted by the mother during its journey. The zygote passes into the uterus by the sixth day. Continuous cell divisions cause a hollow ball of cells called a blastocyst to form. After seven to eight days after fertilization the blastocyst attaches to the uterine lining. Implantation is the attachment of the blastocyst to the lining of the uterus. A small inner mass of cells within the blastocyst will soon become a human embryo. The uterine lining becomes enlarged and prepared for implantation of the embryo in the trophoblast layer. Twelve days after fertilization, the trophoblast has formed a two-layered chorion. Human chrionic gonadotropin is secreted by the chorion, and prolongs the life of the corpus luteum until the placenta begins to secrete estrogen and progesterone. Home pregnancy tests work by detecting elevated hCG levels in the woman’s urine. Maternal and embryonic structures interlock to form the placenta, the nourishing boundary between the mother’s and embryo’s systems. The umbilical cord extends from the placenta to the embryo, and transports food to and wastes from the embryo.
In humans, pregnancy usually lasts about 280 days. The baby develops for about 266 days, calculated from the time of fertilization to birth. The period of time from fertilization to birth is divided into trimesters, each about three months long. Each trimester brings significant advancement in the development of the embryo. During the first trimester the three embryonic tissue layers form. Cellular differentiation begins to form organs during the third week. After one month the embryo is 5 mm long and composed mostly of paired somite segments. During the second month most of the major organ systems form, limb buds develop. The embryo becomes a fetus by the seventh week. Beginning the eighth week, the sexually neutral fetus activates gene pathways for sex determination, forming testes in XY fetuses and ovaries in XX fetuses. External genitalia develop. During the second trimester the fetus increases in size during this trimester, and bony parts of the skeleton begin to form. Fetal movements can be felt by the mother. The fetus’s body metabolism cannot yet maintain a constant body temperature, and its lungs have not matured enough to provide a regular respiratory rate. During the last trimester the fetus increases in size. Circulatory and respiratory systems mature in preparation for air breathing. Fetal growth during this time uses large parts of its mother’s protein and calcium intake. Maternal antibodies pass to the fetus during the last month, conferring temporary immunity. Sometimes in the ninth month, the fetus rotates its position so that its head is down, partly as a result of the shape of the uterus, but also because the head is the heaviest part of the body. By the end of the third trimester, the fetus weighs about 330 g and is about 51 cm long. All of its body systems have developed, and it can now survive independently outside the uterus.
Birth is the process by which a fetus is pushed out of the uterus and the other’s body into the outside world. Birth occurs in three stages. The dilation, expulsion, and the placental stage. The first stage of birth lasts from beginning of contractions to the full dilation of the cervix. Membranes of the amniotic fluid rupture, lubricating the vagina. The second stage occurs when the involuntary uterine contractions become forceful that they push the baby through the cervix into the birth canal. During the last stage the placenta and umbilical cord are expelled from the mother’s body.
Nursing mothers have their hormone levels and uterine size return to normal much faster than non-nursing mothers. Breasts develop the capability for milk secretion about the mid point of pregnancy. Secretion of milk does not occur until delivery, and the action of prolactin. Suckling by the infant causes production of oxytocin to promote release of milk into the ducts emptying into the nipple.
Growth and Aging
Once a baby is born, growth continues and learning begins. Human growth does not continue in a steady, linear process. It varies with age and with the individual and is somewhat sex dependant.
Infancy lasts from birth until age two. Infant reflexes concern finding the nipple: suckling can occur minutes after birth, and the rooting reflex is a tactile clue for the infant to turn its head toward a touch. During the first year the infant passes through a series of developmental stages. An infant will double its birth weight by five months and triple its weight in a year. By two years of age, the infant learns to control its limbs, roll over, sit, crawl, and walk.
Childhood is the period of growth and development that extends from infancy to adolescence. Childhood begins at two and lasts until puberty. The childhood years are a period of relatively steady growth. A child develops the ability to reason and to solve problems. Adolescence follows childhood. Growth accelerates during puberty. Boys increase in height by 3-5 inches per year, girls 2-4 inches. The heart doubles in size, and hormones stimulate sexual maturity. During the teen years, adolescents reach their maximum physical stature, which is determined by heredity, nutrition, and their environment. By the time a young person reaches adulthood, his or her organs have reached their maximum mass, and physical growth is complete.
As an adult ages, his or her body undergoes many distinct changes, The skin loses some of is elasticity, while less pigment is produced in the hair follicles. The bones become thinner and more brittle. Stature may be shorten because the discs between the vertebrae become compressed. Vision and hearing might diminish.