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Topic 6.6, 11.4 Sexual reproduction A population transcends the limit of finite life spans only by reproduction, the creation of new individuals from existing ones. Sexual reproduction is the creation of offspring by the fusion of two haploid (n) sex cells, or gametes, to form a diploid (2n) zygote (fertilized egg). The male gamete, the sperm, is a relatively small cell that moves by means of a flagellum. The female gamete, the unfertilized egg, or ovum (plural, ova) is a much larger cell that is not selfpropelled. The zygote—and the new individual it develops into— contains a unique combination of genes carried from the parents via the egg and sperm. Sexual Reproduction Sexual reproduction increases genetic variability among offspring. Meiosis and random fertilization can generate enormous genetic variation. The variability produced by the reshuffling of genes in sexual reproduction may provide greater adaptability to changing environments. In humans, internal fertilization occurs by which sperm are deposited in to the female reproductive tract, and gametes unite within the tract. Requires copulation, or sexual intercourse. Also requires complex reproductive systems, including organs for gamete storage and transport and organs that facilitate intercourse. Both females and males have gonads (ovaries or testes) where the gametes are produced, a system of ducts that house and conduct the gametes, and structures that facilitate copulation. Reproductive anatomy of the human female A woman’s ovaries are each about an inch long, with a bumpy surface. The bumps are follicles, each consisting of a single developing egg cell surrounded by one or more layers of follicle cells that nourish and protect the developing egg cell. In addition to producing egg cells, the ovaries produce hormones. Specifically, the follicle cells produce the female sex hormones estrogen. Most or all of the 400,000 follicles a woman will ever have are though to be formed before her birth, but only several hundred will release egg cells during her reproductive years. Starting at puberty and continuing until menopause, one follicle (or rarely tow are more ) matures and releases its egg cells about every 28 days. An egg cell is ejected from the follicle in a process called ovulation. Reproductive anatomy of the human female After ovulation, the remaining follicular tissue grows within the ovary to form a solid mass called the corpus luteum (Latin for “yellow body”): Secretes the hormone progesterone, which helps maintain the uterine lining during pregnancy, and additional estrogen. If the egg is not fertilized, the corpus luteum degenerates, and a new follicle matures during the next cycle. **we will discuss ovulation and female hormonal cycles in more detail later on…** Reproductive anatomy of the human female Each ovary lies next to the opening of an oviduct, also called a fallopian tube. The oviduct opening resembles a funnel fringed with fingerlike projections. The projections touch the surface of the ovary, but the ovary is actually separated from the opening of the oviduct by a tiny space. When ovulation occurs, the egg cell passes across the space and the oviduct, where cilia sweep it toward the uterus. Fertilization usually occurs in the upper third of the oviduct. The resulting zygote starts to divide, thus becoming an embyro, as it moves along within the oviduct. Reproductive anatomy of the human female The uterus, also known as the womb, is the actual site of pregnancy. The uterus is only about 3 inches long in a woman who has never been pregnant, but during pregnancy it expands considerably to accommodate a baby. The uterus has a thick muscular wall, and its inner lining, the endometrium, is richly supplied with blood vessels. The embryo implants (digests a place for itself) in the endometrium, and development is completed there. The term embryo is used for the stage of development from the first division of the zygote until body structures begin to appear, about the ninth week in humans. From the ninth week until birth, a developing human is called a fetus. Reproductive anatomy of the human female The uterus is the normal site of pregnancy. However, in about 1% of pregnancies, the embryo implants somwhere else, resulting in an ectopic pregnancy. Most ectopic pregnancies occur in the oviduct and are called tubal pregnancies. Ectopic pregnancies require surgical removal; otherwise, they can rupture surrounding tissues, causing severe bleeding and even death of the mother. Reproductive anatomy of the human female The narrow neck of the uterus is the cervix, which opens into the vagina. The vagina is a thin-walled, but strong, muscular chamber that serves as the birth canal through which the baby is born. It is also the repository for sperm during copulation. The vagina opens to the outside just behind the opening of the urethra, the tube through which urine is excreted. A pair of slender skin folds, the labia minora, border the openings, and a pair of thick, fatty ridges, the labia majora, protect the vaginal opening. Until sexual intercourse or vigorous physical activity ruptures it, a thin piece of tissue called the hymen partly covers the vaginal opening. Bartholin’s glands, near the vaginal opening, secrete mucus during sexual arousal, lubricating the vagina and facilitation intercourse. The vagina, labia minora, and a structure called the clitoris all engorge with blood and enlarge during sexual activity. The sole function of the clitoris is sexual arousal Reproductive anatomy of the human female Refer to p. 538 Figure 27.2A Reproductive anatomy of the human female Refer to p. 539 Figure 27.2C Reproductive anatomy of the human male The male gonads, or testes, are each housed outside the abdominal cavity in a sac called the scrotum. Sperm cannot develop at human core body temperature, but the scrotum keeps the sperm-forming cells cool enough to function normally. Reproductive anatomy of the human male Path of sperm from one of the testes out of the male’s body: From each testis, sperm pass into a coiled tube called the epididymis, which stores the sperm while they continue to develop. Sperm leaves the epididymis during ejaculation, the expulsion of sperm-containing fluid from the penis. At that time, muscular contractions propel the sperm from the epididymis through another duct called the vas deferens. The vas deferens passes upward into the abdomen and loops around the urinary bladder. Next to the bladder, the vas deferens joins a short duct from a gland, the seminal vesicle. The two ducts unit to form a short ejaculatory duct, which joins its counterpart conveying sperm from the other testis. The union of the two ejaculatory ducts forms the urethra, which conveys both urine and sperm out through the penis, although not at the same time. Thus, unlike the female, the male has a connection between the reproductive and excretory systems. Reproductive anatomy of the human male In addition to the testes and ducts, the male reproductive systems contains three sets of glands: the seminal vesicles, the prostate gland, and the bulbourethral glands.: The two seminal vesicles secrete a thick fluid that contains fructose, which provides most of the energy used by the sperm. The prostate gland secretes a thin fluid that further nourishes the sperm. The two bulbourethral glands secrete a clear, alkaline mucus that balances the acidity of any traces of urine in the urethra. Reproductive anatomy of the human male The sperm and the glandular secretions make up semen, the fluid discharged (ejaculated) from the penis during orgasm. About 2-5 mL (1 teaspoonful) of semen are discharged during a typical ejaculation. About 95% of the fluid consists of glandular secretions. The other 5% is made up of 50-130 million sperm, only one of which may eventually fertilize a single egg. The alkalinity of the semen helps neutralize the acidic environment of the vagina, protecting the sperm and increasing their motility. Reproductive anatomy of the human male The human penis consists mainly of tissue that can fill with blood to cause an erection during sexual arousal. Erection is essential for insertion of the penis into the vagina. Like the clitoris, the penis consists of a shaft that supports the glans, or head. The glans is richly supplied with nerve endings and is highly sensitive to stimulation. As in the female, a fold of skin called the prepuce, or foreskin, covers the glans. Circumcision, the surgical removal of the prepuce, arose from religious traditions. Scientific studies have not proved that circumcision has an overally positive or negative impact on a man’s health or hygiene. Reproductive anatomy of the human male Ejaculation occurs in two stages: 1.At the peak of sexual arousal, muscles in the epididymis, seminal vesicles, prostate gland, and vas deferens contract. These contractions force secretions from the glands into the vas deferens and propel sperm from the epididymis. At the same time, a sphincter muscle at the base of the bladder contracts, preventing urine from leaking into the urethra from the bladder. Another sphincter also contracts, closing off the entrance of the urethra into the penis. The section of the urethra between the two sphincters fills with semen and expands. 2.In the second stage, the expulsion stage, the sphincter at the base of the penis relaxes, admitting semen into the penis. Simultaneously, a series of strong muscle contractions around the base of the penis and along the urethra expels the semen from the body. Reproductive anatomy of the human male Hormones control sperm production by the testes: Influence by signals from other parts of the brain, the hypothalamus secretes a releasing hormone that regulates release of folliclestimulating hormone (FSH) and luteinizing hormone (LH) by the anterior pituitary. FSH increase sperm production by the testes, while LH promotes the secretion of androgens, mainly testosterone. Androgens stimulate sperm production. In addition, androgens carried in the blood help maintain homeostasis by a negative-feedback mechanism, inhibiting secretion of both the releasing hormone and LH. Under the control of this chemical regulatory system, the testes produce hundreds of millions of sperm every day, from puberty well into old age. Gametogenesis Spermatogenesis Formation of sperm cells, takes about 65-75 days in the human male. Gametogenesis Spermatogenesis Sperm develop in the testes in coiled tubes called the seminiferous tubules. Diploid cells that begin the process are located near the outer wall of the tubules. These cells multiply constantly by mitosis, and each day about 3 million of them differentiate into primary spermatocytes, the cells that undergo meiosis. Meiosis I or a primary spermatocyte produces two secondary spermatocytes, each with the haploid number of chromosomes (n=23) The chromosomes are still in their duplicated state, each consisting of two identical chromatids. Meiosis II then forms four cells, each with the haploid number of single- chromatid chromosomes. A sperm cell develops by differentiation of each of these haploid cells and is gradually pushed toward the center of the seminiferous tubule. From there it passes in to the epididymis, where it matures, becomes motile, and is stored until ejaculation. Gametogenesis Oogenesis The development of mature ova (egg cells) Gametogenesis Oogenesis Most of the process occurs in the ovary. Oogenesis actually begins prior to birth, when a diploid cell in each developing follicle begins meiosis. At birth, each follicle contains a dormant primary oocyte, a diploid cell that is resting in prophase of meiosis I. A primary oocyte can be hormonally triggered to develop further. After puberty, about overy 28 days, FSH (follicle-stimulating hormone) from the pituitary stimulates one of the dormant follicles to develop. The follicle enlarges, and the primary oocyte completes meiosis I and begins meiosis II. Meiosis then halts again at metaphase II. In the female, the division of the cytoplasm in meiosis I is unequal, with a single secondary oocyte receiving almost all of it. The smaller of the two duaghter cells, called the first polar body, receives almost no cytoplasm. Gametogenesis Oogenesis (continued)… The secondary oocyte is the stage released by the ovary during ovulation. It enters the oviduct, and if a sperm cell penetrates it, the secondary oocyte completes meiosis II. Meiosis II yields a second polar body and the actual ovum. The haploid nucleus of the ovum can then fuse with the haploid nucleus of the sperm cell, producing a zygote. Polar body formation leaves the ovum with nearly all the cytoplasm and thus the bulk of the nutrients contained in the original diploid cell. Gametogenesis Oogenesis (continued…) Development of an ovarian follicle An actual ovary would have thousands of dormant follicles, each containing a primary oocyte. Usually, only one follicle has a dividing oocyte at any one time, and as it develops, that follicle stays in one place in the ovary. Meiosis I occurs as the follicle matures About the time the secondary oocyte forms, the pituitary hormone LH (luteinizing hormone) triggers ovulation, the rupture of the follicle and expulsion of the secondary oocyte. The ruptured follicle then develops into a corpus luteum. Unless fertilization occurs, the corpus luteum degenerates before another follicle starts to develop. Gametogenesis Development of an ovarian follicle Gametogenesis Key differences between spermatogenesis and oogenesis: 1. only one ovum results from each diploid cell that undergoes meiosis. The other products of oogenesis, the polar bodies, degenerate. In spermatogenesis, all four products of meiosis develop into mature gametes 2. although the cells from which sperm develop continue to divide by mitosis throughout the male’s life, this is not the case for the comparable cells in the human female 3. oogenesis has long “resting” periods, whereas spermatogenesis produces mature sperm in an uninterrupted sequence. Ovarian and Menstrual cycle The reproductive cycle is actually one integrated cycle involving cycles in tow different reproductive organs: the ovaries and the uterus. in discussing oogenesis, we were locating at the ovarian cycle, cyclic events that occur about every 28 days in the human ovary . Hormonal messages synchronize the ovarian cycle with related events in the uterus called the menstrual cycle. The hormones are complex and involve intricate feedback mechanisms. Refer to Table 27.5 on p. 544 Ovarian and Menstrual cycle The ovarian cycle is divided into two phases separated by ovulation: the pre-ovulatory phase, when a follicle is growing and a secondary oocyte is developing, and the post-ovulatory phase, after the follicle has become a corpus luteum. Events in the menstrual cycle are synchronized with the ovarian cycle. By convention, the first day of a woman’s “period” is designated day 1 of the menstrual cycle. Uterine bleeding, called menstruation, usually persists for 3-5 days. This corresponds to the pre-ovulatory phase of the ovarian cycle. During menstruation, the endometrium (inner lining of the uterus) breaks down and leaves the body through the vagina. The menstrual discharge consists of blood, small clusters of endometrial cells, and mucus. After menstruation, the endometrium regrows. It continues to thicken through the time of ovulation, reaching a maximum at about 20-25 days. If an embryo has not implanted in the uterine lining by this time, menstruation begins again, marking the start of the next ovarian and menstrual cycles. Ovarian and Menstrual Cycle Hormones that regulate ovarian and menstrual cycles: The five hormones listed in Table 27.5 synchronize events in the ovarian cycle (the growth of the follicle and ovulation) with events in the menstrual cycle (preparation of the uterine lining for possible implantation of an embryo). A releasing hormone from the hypothalamus in the brain regulates secretion of the two pituitary hormones FSH and LH. The blood levels of FSH, LH, and two other hormones— estrogen and progesterone—coincide with specific events in the ovarian and menstrual cycles. Ovarian and Menstrual Cycle Hormonal events before ovulation: 1.The releasing hormone from the hypothalamus stimulates the anterior pituitary to increase its output of FSH and LH. 2. True to its name, FSH stimulates the growth of an ovarian follicle, in effect starting the ovarian cycle. In turn, the follicle secretes estrogen. Early in the pre-ovulatory phase, the follicle is small and secretes relatively little estrogen. 3. As the follicle grows, it secretes more and more estrogen, and the rising but still relatively low level of estrogen exerts negative feedback on the pituitary. This keeps the blood levels of FSH and LH low for most of the pre-ovulatory phase. As the time of ovulation approaches, hormone levels change drastically, with estrogen reaching a critical peak just before ovulation. This high level of estrogen exerts positive feedback on the hypothalamus which then 4. makes the pituitary secrete bursts of FSH and LH. 5. Then, ovulation occurs. Ovarian and Menstrual Cycle Hormonal events at ovulation and after: LH: stimulates the completion of meiosis, transforming the primary oocyte in the follicle into a secondary oocyte. it also signals enzymes to rupture the follicle, allowing ovulation to occur, and triggers the development of the corpus luteum from the ruptured follicle. it also promotes the secretion of progesterone and estrogen by the corpus luteum. Estrogen and progesterone High levels of these hormones in the blood following ovulation have a strong influence on both ovary and uterus. The combination of both hormones exerts negative feedback on the hypothalamus and pituitary, producing 6. falling FSH and LH levels. This drop prevents follicles from developing and ovulation from occurring during the post-ovulatory phase. Also, the LH drop is followed by gradual degeneration of the corpus luteum. Near the end of the post-ovulatory phase, unless an embryo has implanted in the uterus, the corpus luteum stops secreting estrogen and progesterone. Ovarian and Menstrual Cycle Hormonal events at ovulation and after 7.When the levels of estrogen and progesterone drop, the endometrium begins to slough off. Menstrual bleeding begins soon thereafter, on day 1 of a new cycle. 8. As blood levels of FSH and LH drop, they hypothalamus once gain can stimulate the pituitary to secrete more FSH and LH, and a new cycle begins. Ovarian and Menstrual Cycle Control of the menstrual cycle: Hormonal control of the menstrual cycle is simpler than that of the ovarian cycle. The menstrual cycle is directly controlled by estrogen and progesterone alone. Starting around day 5 of the cycle, the endometrium thickens in response to the rising levels of estrogen and later progesterone. When the levels of these hormones drop, the endometrium begins to slough off. Menstrual bleeding begins soon thereafter, on day 1 of a new cycle. Estrogen and Progesterone Feedback Relationships Menstrual Cycle Ovarian and Menstrual Cycle Fertilization Embryonic development begins with fertilization, the union of a sperm and an egg to form a diploid zygote. Fertilization combines haploid sets of chromosomes from two individuals and also activates the egg by triggering metabolic changes that start embryonic development. Fertilization The Properties of Sperm Cells Of all of the millions of sperm that surround a human egg cell, only one will enter and fertilize the egg. All the other sperm will die; the one sperm that penetrates the egg adds its unique set of genes to those of the egg and contributes to the next generation. In a mature human sperm, we can see how form fits function: The sperm’s streamlined shape is adaptation for swimming through fluids in the vagina, uterus, and oviduct of the female. The sperm cell’s thick head contains a haploid nucleus and is tipped with a vesicle, the acrosome, which lies just inside the plasma membrane. It contains enzymes that help the sperm penetrate the egg. The neck and middle piece of the sperm contain a long, spiral mitochondrion. The sperm absorbs high-energy nutrients, especially the sugar fructose, from the semen. Thus fueled, its mitochondrion provides ATP from movement of the tail, which is actually a flagellum. By the time a sperm has reached the egg, it has consumed much of the energy available to it. But a successful sperm will have enough energy left to penetrate the egg and deposit its nucleus in the egg’s cytoplasm. Fertilization Fertilization The Process of Fertilization: To reach the egg nucleus, the sperm nucleus must pass through three barriers: the egg’s jelly coat (yellow), a middle region of glycoproteins called the vitelline layer (pink), and the egg cell’s plasma membrane. Fertilization The Process of Fertilization: 1. As a sperm approaches and then 2. contracts the jelly coat of the egg, the acrosome in the sperm head releases a cloud of enzyme molecules that digest a cavity into the jelly. 3. When the sperm head reaches the vitelline layer, species-specific protein molecules on its surface bind with specific receptor proteins on the vitelline layer. The binding between these proteins ensures that sperm of other species cannot fertlize the egg. 4. After the specific binding occurs, the sperm proceeds through the vitelline layer and the sperm’s plasma membrane fuses with that of the egg. 5. Fusion of the two membranes makes it possible for the sperm nucleus to enter the egg. Fusion of the sperm and egg plasma membranes triggers a number of important changes in the egg. Two such changes prevent other sperm from entering the egg. About 1 second after the membranes fuse, the entire egg plasma membrane becomes impenetrable to other sperm cells Fertilization The Process of Fertilization (continued): 6. Shortly thereafter, the vitelline layer hardens and separates from the plasma membrane. The space quickly fills with water, an the vitelline layer becomes the so-called fertilization envelope, another barrier impenetrable to sperm. If these events did not occur an egg were fertilized with more than one sperm, the resulting zygote nucleus would contain too many chromosomes, and the zygote could not develop normally. Membrane fusion also triggers a burst of metabolic activity in the egg. In preparation for the enormous growth and development that will follow fertilization, the egg gears up from near dormancy, increasing cellular respiration and protein synthesis. 7. Next, the egg and sperm nuclei fuse, producing the diploid nucleus of the zygote. Fertilization- Animation http://www.uchsc.edu/ltc/Fertilization.html http://www.dnatube.com/video/1127/Human-Reproduction- Fertilization-and-Fetal-Development Pregnancy Pregancy, or gestation, is the carrying of developing young within the female reproductive tract. It begins at conception, the fertilization of the egg by a sperm and continues until birth. Duration of pregnancy varies considerably among animal species, in humans, it averages 266 days (38 weeks) from conception, or 40 weeks from the start of the last menstrual cycle. Conception occurs in the oviduct. Cleavage starts about 24 h ours after fertilization and continues as the embryo moves down the oviduct toward the uterus. By the sixth or seventh day after fertilization the embryo has reached the uterus, and cleavage has produced about 100 cells. Embryonic DevelopmentCleavage The transformation from a zygote to the multicellular state is truly phenomenal. Embryonic DevelopmentCleavage Cleavage is a rapid succession of cell divisions that produces a ball of cells—a multicellular embryo—from the zygote. DNA replication, mitosis, and cytokinesis occur rapidly, but gene transcriptin virtually shuts down, and few new proteins are synthesized. As a result, the embryo of most animals does not grow larger during cleavage. Nutrients stored in the egg nourish the dividing cells, and the cell divisions partition the zygote into many smaller cells. The first cleavage is completed after 36 hours, and each succeeding division takes less time. After 3 days, successive cleavages have produced a solid mass of cells called the morula, which is still about the same size as the original zygote. Embryonic DevelopmentBlastocyst Embryonic DevelopmentBlastocyst Refer to figure 27.16A Implantation of the Blastocyst (after 6-8 days): After several days in the uterus, the morula develops into the blastocyst. It has a fluid-filled cavity, an inner cell mass that will actually for the baby, and an outer layer of cells called the trophoblast which secretes enzymes that enables the blastocyst to implant the endometrium, the uterine lining. About a week after conception, the blastocysts makes contact with the uterine lining and pushes deeply into it, ensuring a close maternal-fetal contact. The trophoblast cells eventually form part of the placenta, the organ that provides nourishiment and oxygen to the embryo and helps dispose of its metabolic wastes. Extraembryonic membranes develop as attachments to the embryo and help support it. The amnion, yolk sac, and chorion take shape. Embryonic DevelopmentBlastocyst Gastrulation: Under way by 9 days of conception. Three embryonic layers- ectoderm, endoderm, and mesoderm. The embryo itself develops from the three inner cell layers. Ectoderm layer will form the outer part of the embryo’s skin. Ecotderm layer is continuous with the amnion. Similarly, the embryo’s digestive tract will develop from the endoderm layer, which is continuous with the yolk sac. The bulk of most other organs will develop form the central layer of mesoderm. Refer to figure 27.16E on p. 557 Embryonic Development-Embryo About a month after conception The extraembryonic membranes have formed; The amnion encloses and protects the embryo in a fluid-filled space Usually breaks just before child birth, and the amniotic fluid “water” leaves the mother’s body through her vagina. Yolk sac contains no yolk, but is given the same name as the homologous structure in other vertebrates. Produces the embryo’s first blood cells and its first germ cells, the cells that will give rise to the gamete-forming cells in the gonads. Allantois Forms part of the umbilical cord—the lifeline between the embryo and the placenta. It also forms part of the embryo’s urinary bladder. Chorion The outermost extraembryonic membrane develops from the trophoblast and from mesoderm cells derived from the yolk sac. Becomes the embryo’s part of the placenta Cells in the chorion secrete a hormone called human chorionic gonadotropin (HCG) which maintains production of estrogen and progesterone by the corpus luteum of the ovary during the first few months of pregnancy. Without these hormones, menstruation would occur and the embryo would abort spontaneously. Levels of HCG in maternal blood are so high that some is excreted in the urine, where it can be detected by pregnancy tests. Embryonic Development-About a month after conception The chorion contain knobby growths on the outside called chorionic villi which are larger and contain mesoderm. Contain embryonic blood vessels formed from the mesoderm. By this stage, the placenta is fully developed. Starting with the chorion and extending outward, the placenta is a composite organ consisting of chorionic villi closely associated with the blood vessels of the mother’s endometrium. The villi are actually bathed in tiny pools of maternal blood. The mother’s blood and the embryo’s blood are not in direct contact. However, the chorionic villi absorb nutrients and oxygen from the mother’s blood and pass these substances to the embryo via the chorionic blood vessels. The chorionic blood vessels carry wastes away from the embryo, and the wastes diffuse into the mother’s bloodstream and are excreted by her kidneys. The placenta allows protective antibodies to pass from mother to fetus. However, the placenta cannot always protect the embryo from substances circulating in the mother’s blood. HIV, German measles virus, alcohol, and drugs Embryo at 4 weeks Human Development The previous slides show human development through the first four weeks. Now, we will look at the rest of human development in the uterus. Human development from conception to birth is divided into three trimesters of about 3 months each. Human Development- First Trimester First month Second month Human Development-First Trimester The first trimester is the time of the most radical change for both mother and embryo. At 5 weeks, the embryo is a highly organized multicellular embryo that has developed from a single cell. Extraembryonic membranes surround the embryo and most of the umbilical cord is attached to the placenta. The embryo is about 7 mm (.28 in) long and has a number of features in common with the somite stage of a frog embryo. The embryo has a notochord and a coelom, both formed from the mesoderm. Its brain and spinal cord have begun to take shape from a tube of ectoderm. Has four stumpy limp buds, a short tail, and elements of gill pouches. The gill pouches will eventually develop into parts of the throat and middle ear. Human Development-First Trimester At 9 weeks, the developing human is now called a fetus. The large pinkish structure on to the left of the fetus is the placenta, attached to the fetus by the umbilical cord. The clear sac around the fetus is the amnion. By this time, the fetus is decidedly human It is about 5.5cm (2.2in) long and has all of its organs and major body parts, including a disproportionately large head. It now has developed segmental muscles and the bones of the back and ribs. The limb buds have become tiny arms and legs with fingers and toes. Beginning at about 9 weeks, the fetus can move its arms and legs, turn its head, frown, and make sucking motions with its lips. By the end of the first trimester, the fetus looks like a miniature human being, although its head is still oversized for the rest of the body. The sex of the fetus is usually evident by this time, and its heartbeat can be detected with a stethoscope. Human DevelopmentTrimester 3rd month 4th month nd 2 Human DevelopmentTrimester Fifth month Sixth month nd 2 Human Development- 2nd Trimester The main developmental changes during the second and third trimesters involve an increase in size and general refinement of the human features—nothing as dramatic as the changes of the first trimester. At 14 weeks, 2 weeks into the 2nd trimester, the fetus is now about 6 cm (2.4 in) long. The placenta takes over the task of maintaining itself by secreting progesterone, rather than receiving it from the corpus luteum. At the same time, the placenta stops secreting HCG and the corpus luteum, no longer needed to maintain pregnancy, degenerates. Human Development- 2nd Trimester At 20 weeks, well into the second trimester, the fetus is about 19 cm (7.6 in) long, weighs about half a kilogram (1 lb), and has the face of an infant, complete with eyebrows and eyelashes. Its arms, legs, fingers, and toes have lengthened. It also has fingernails and toenails and is covered with fine hair. Fetuses of this age are usually quite active. The mother’s abdomen has become markedly enlarged, and she may often feel her baby move. Because of the limited space in the uterus, the fetus flexes forward into the so-called fetal position. By the end of the second trimester, the fetus’s eyes are open and its teeth are forming. Human DevelopmentTrimester Seventh month Eight month rd 3 Ninth month Human Development- 3rd Trimester The third trimester (28 weeks to birth) is a time of rapid growth as the fetus gains the strength it will need to survive outside the protective environment of the uterus. Babies born prematurely—as early as 24 weeks– may survive, but they require special medical care after birth. During the third trimester, the fetus’s circulatory system and respiratory system undergo changes that will allow the switch to air breathing. The fetus gains the ability to maintain its own temperature, and its bones begin to harden and it muscles thicken. It also loses much of its fine body hair, except on its head. The head itself changes it proportions. The fetus becomes less active as it fills the space in the uterus. At birth, babies average about 50 cm (20 in) in length and weigh 3-4 kg (6-8 lb) Childbirth The birth of a child is brought about by a series of strong, rhythmic contractions of the uterus, called labor. Several hormones are thought to play key roles. One hormone, estrogen, reaches its highest level in the mother’s blood during the last weeks of pregnancy. An important effect of this estrogen is to trigger the formation of numerous oxytocin receptors on the uterus. Cells of the fetus produce the hormone oxytocin, and late in pregnancy, the mother’s pituitary gland secretes it in increasing amounts. Oxytocin stimulates powerful contractions of smooth muscles in the wall of the uterus. It also stimulates the placenta to make prostaglandins, local tissue regulators that also stimulate uterine muscle cells, making the muscles contract even more. Childbirth The hormonal induction of labor involves positive- feedback control. In this case, oxytocin and prostaglandins cause uterine contractions that in turn stimulate the release of more and more oxytocin and prostaglandins. The result is climactic—the intense muscle contractions that propel a baby from the uterus (womb) Childbirth Three stages of labor: As the process begins, the cervix (neck of the uterus) gradually opens, or dilates. 1. The first stage, dilation, is the time from the onset of labor until the cervix reaches its full dilation of about 10 cm. Dilation is the longest stage of labor, lasting 6-12 hours or even considerably longer. 2. The period from full dilation of the cervix to delivery of the infant is called the expulsion stage. Strong uterine contractions, lasting about 1 minute each, occur every 2-3 minutes, and the mother feels an increasing urge to push or bear down with her abdominal muscles. Within a period of 20 minutes to an hour or so, the infant is forced down and out of the uterus and vagina. An attending physician or midwife clamps and cuts the umbilical cord after the baby is expelled. 3. The final stage is the delivery of the placenta, usually within 15 minutes after the birth of the baby. Childbirth Hormones continue to be important after the baby and placenta are delivered. Decreasing levels of progesterone and estrogen allow the uterus to start returning to its prepregnancy state. Less progesterone in the maternal blood also allows the pituitary hormone prolactin to promote milk production by the mammary glands. About 2-3 days after birth, the mother begins to secrete milk under the direct influence of both oxytocin and prolactin. Reproductively challenge About 15% of couples who want children are unable to conceive, even after a year of unprotected sex. In most cases, such infertility can be traced to problems with the man. His testes may not produce enough sperm (a “low sperm count”), or those that are produced may be defective. Underproduction of sperm is frequently caused by the man’s scrotum being too warm, so a switch of underwear from briefs (which hold the scrotum close to the body) to boxers may help. In other cases, infertility is caused by impotence, also called erectile dysfunction, the inability to maintain an erection. Reproductively challenge Female infertility can result from a lack of ova, a failure to ovulate, or blocked oviducts (often caused by scarring due to sexually transmitted diseases). Other women are able to conceive, but cannot support a growing embryo in the uterus. Reproductively challenge Reproductive technologies can help many cases of infertility. Drug therapies (including Viagra) and penile implants can be used to treat impotence. If a man produces no functioning sperm, the couple may elect to use another man’s sperm that had been anonymously donated to a sperm bank. If a woman produces ova but is unable to support a growing fetus, she and her partner may hire a surrogate mother. In such cases, the couple enters into a legal contract with a woman who agrees to be implanted with the couple’s embryo and carry it to birth. However, a number of states have laws surrogate motherhood owing to the serious ethical and legal problems that can arise. If a woman has normal ova that are not being released properly, hormone injections can induce ovulation. Such treatments frequently result in multiple pregnancies. If a woman has no ova of her own, they, too can be obtained from a donor for fertilization and injection into the uterus. While sperm can be collected without any danger to the donor, collection of ova involves surgery and therefore pain and risk for the donating woman. Reproductively challenge Assisted reproductive technology (ART) Procedures that involve surgically removing eggs (secondary oocytes) from a woman’s ovaries, fertilizing the eggs, and returning them to the woman’s body. Eggs, sperm, and embryos from such procedures can be frozen for later pregnancy attempts. The most common ART procedure, in vitro fertilization, a woman’s eggs are mixed with sperm in culture dishes (in vitro) and incubated for several days to allow fertilized eggs to start developing. When they have developed into embryos of at least 8 cells each, the embryos are carefully inserted into the woman’s uterus. In ZIFT (zygote intrafallopian transfer), eggs are also fertilized in vitro, but zygotes are then transferred immediately to the woman’s oviducts. In GIFT (gamete intrafallopian transfer), the eggs are not fertilized in vitro. Instead, the eggs and sperm are placed in the woman’s oviducts, in the hope that fertilization will occur there. Reproductively challenge Recent research has showed increased risks (lower birth weights and higher rates of birth defects) for babies born from IVF (compared with “natural” conception). Despite such risks and the high cost (typically $10,000 per attempt, whether it succeeds or not), IVF techniques are now performed at medical centers throughout the world and results in the birth of thousands of babies each year.
