Animal Reproduction

ANIMAL REPRODUCTION Overview of Animal Reproduction 1. Both asexual and sexual reproduction occur in the animal kingdom Asexual reproduction involves the formation of individuals whose genes all come from one parent. There is no fusion of sperm and egg. Sexual reproduction is the formation of offspring by the fusion of haploid gametes to form a zygote. Ovum: female gamete - usually large and nonmotile. Spermatozooan: male gamete - usually small and motile. Sexual reproduction increases genetic variation among offspring. 2. Diverse mechanisms of asexual reproduction enable animals to produce identical offspring rapidly Invertebrates: Fission: asexual reproduction in which a parent separates into two or more approximately equal-sized individuals. Budding: asexual reproduction in which new individuals split off from existing ones. Gemmules of sponges are an example of a type of asexual reproduction that involves the release of specialized cells that can grow into new individuals. Fragmentation: the breaking of the body into several pieces, some or all of which develop into complete adults. Requires regeneration of lost body parts. Advantages of asexual reproduction: Can reproduce without needing to find a mate. Can have numerous offspring in a short period of time. In stable environments, allows for the perpetuation of successful genotypes. 3. Reproductive cycles and patterns vary extensively among mammals Reproductive cycles are controlled by both environmental and hormonal cues. Animals may be solely asexual or sexual. Or they may alternate between the two modes depending on environmental conditions. Daphnia reproduce by parthenogenesis under favorable conditions and sexually during times of environmental stress. Parthenogenesis is the process by which an unfertilized egg develops into an (often) haploid adult. Parthenogenesis plays a role in the social organization of species of bees, wasps, and ants. Male honeybees are haploid and female honeybees are diploid. Several genera of fishes, amphibians, and lizards reproduce by a form of parthenogenesis that produces diploid zygotes. Sexual reproduction presents a problem for sessile or burrowing animals or parasites who may have difficulty encountering a member of the opposite sex. Hermaphroditism: one individual is functional as both a male and a female. Some self-fertilize. Most mate with another member of the same species. Each individual receives and donates sperm. Results in twice as many offspring as would occur if only one set of eggs were fertilized. Sequential hermaphroditism: an individual reverses its sex during its lifetime. Protogynous: female first sequential hermaphroditism. Protandrous: male first sequential hermaphroditism B. Mechanisms of Sexual Reproduction 1. Internal and external fertilization both depend on mechanisms ensuring that mature sperm encounter fertile eggs of the same species Internal fertilization requires cooperative behavior that leads to copulation. External fertilization requires a moist habitat that will protect a developing egg from desiccation and heat stress. Specific mating behaviors assure that sperm and egg will be in the same place at the same time. Pheromones: chemical signals released by one organism that influence the behavior of other individuals of the same species. Many act as male attractants. 2. Species with internal fertilization usually produce fewer zygotes but provide more parental protection than species with external fertilization Internal fertilization usually results in the production of fewer zygotes than does internal fertilization. However, the survival rate is lower for external fertilization than it is for internal fertilization. The externally fertilized eggs of fishes and amphibians are surrounded by a gelatinous coat. The internally fertilized amniote eggs of birds, reptiles, and monotremes are protected by calcium and protein shells. In mammals the embryo is retained within the females reproductive tract. Parental care of offspring may occur regardless of whether fertilization is external or internal. 3. Complex reproductive systems have evolved in many animal phyla The least complex reproductive systems lack gonads. Polychaete worms lack gonads. Eggs and sperm develop from undifferentiated cells lining the coelom. Some reproductive systems, such as that seen in parasitic flatworms, can be very complex. Most insects have separate sexes with complex reproductive systems. In many species the female reproductive system includes a spermatheca, a sac in which sperm may be stored for a year or more. The basic plan of all vertebrate reproductive systems is very similar. However, there are variations. In many non-mammalian vertebrates the digestive, excretory, and reproductive systems share a common opening to the outside, the cloaca. Mammals have separate openings for the digestive and reproductive systems. Female mammals also have separate openings for the excretory and reproductive systems. C. Mammalian Reproduction 1. Human reproduction involves intricate anatomy and complex behavior • Reproductive Anatomy of the Human Male. The scrotum and the penis are the external components of the reproductive system. The internal reproductive organs consist of gonads, accessory sex glands, and ducts. Testes are the male gonads. Consists of many highly coiled seminiferous tubules surrounded by layers of connective tissue. Sperm form in seminiferous tubules. Leydig cells scattered between seminiferous tubules produce androgens. Testes are located in the scrotum, outside the body cavity. This keeps testicular temperature cooler than the body cavity. The testes develop in the body cavity and descend into the scrotum just before birth. From the seminiferous tubules, sperm pass to the coiled tubules of the epididymis. It takes about 20 days for sperm to pass through the tubules of the epididymis. In the epididymis, sperm become motile and gain the ability to fertilize. Ejaculation propels sperm from the epididymis to the vas deferens. The vas deferens run from the scrotum and behind the urinary bladder. Here each vas deferens joins with a duct from the seminal vesicle to form an ejaculatory duct. The ejaculatory ducts open into the urethra. The urethra drains both the excretory and reproductive systems. Accessory sex glands add secretions to semen. A pair of seminal vesicle contribute about 60% of total semen volume. Seminal fluid is thick, yellowish, and alkaline. It contains mucus, fructose, a coagulating enzyme, ascorbic acid, and prostaglandins. The prostate gland secretes directly into the urethra. Prostatic fluid is thin and milky. This fluid contains anticoagulant enzymes and citrate. Prostate problems are common in males over 40. Benign prostate enlargement occurs in virtually all males over 70. Prostate cancer is one of the most common cancers in men. The bulbourethral glands are a pair of small glands along the urethra below the prostate. Prior to ejaculation they secrete a clear mucus that neutralizes any acidic urine remaining in the urethra. Bulbourethral fluid also carries some sperm released before ejaculation. This is one of the reasons why the withdrawal method of birth control has a high failure rate. A male usually ejaculates about 2-5 mL of semen, each milliliter containing about 50-130 million sperm. Once in the female reproductive tract: Prostaglandins in semen thin the mucus at the opening of the uterus and stimulate uterine contractions When ejaculated semen coagulates: This makes it easier for uterine contractions to move it along. Anticoagulants liquefy the semen and the sperm begin swimming. The alkalinity of semen helps neutralize the acidic environment of the vagina. The penis is composed of three layers of spongy erectile tissue. During sexual arousal the erectile tissue fills with blood from arteries. The resultant increased pressure seals off the veins that drain the penis. The engorgement of the penis with blood causes an erection. An erection is essential for the insertion of the penis into the vagina. The penis of some mammals possesses a baculum, a bone that helps stiffen the penis. Impotence can result from the consumption of alcohol and other drugs, and from emotional, nervous system, or circulatory problems. Treatment includes drugs and penile implant devices. Viagra® acts by promoting the action of nitric oxide. External structure of the penis: The shaft of the penis is covered by relatively thick skin. The sensitive glans penis is covered by thinner skin. The glans is covered by the prepuce which may be removed by circumcision. There is no verifiable health benefit to circumcision. • Reproductive Anatomy of the Human Female. External reproductive structures consist of two sets of labia surrounding the clitoris and vaginal opening. Internal reproductive organs consist of a pair of gonads and a system of ducts and chambers. The role of the ducts and chambers is to conduct the gametes and house the embryo and fetus. Ovaries are the female gonads. Located in the abdominal cavity. Flanking, and attached by mesentery to, the uterus. Each ovary is enclosed in a tough protective capsule. Each ovary contains follicles. Each follicle consists of one egg cell surrounded by one or more layers of follicle cells. Follicles produce the primary female sex hormones, estrogens. Follicle cells nourish and protect the developing egg cell. A woman is born with about 400,000 follicles. Only several hundred of these will release eggs during a female’s reproductive years. Usually one follicle matures and releases its egg during each menstrual cycle. After ovulation the remaining follicular tissue develops into the corpus luteum. Secretes estrogens and progesterone. Maintain the uterine lining during pregnancy. If pregnancy does not occur the corpus luteum disintegrates. At ovulation the egg is released into the abdominal cavity near the opening of the oviduct. The cilia-lined funnel-like opening of the oviduct draws in the egg. Cilia convey the egg through the oviduct to the uterus. Endometrium: highly vascularized inner lining of the uterus. The neck of the uterus, the cervix, opens into the vagina. The vagina is a thin-walled chamber that forms the birth canal and is the repository for sperm during copulation. The vaginal opening: Until ruptured is partially covered by the hymen. The vaginal and urethral openings are located within the vestibule. The vestibule is surrounded by the labia minora. The labia majora enclose and protect the labia minor and vestibule. The clitoris is found at the front edge of the vestibule. Its structure is similar to that of the penis. During sexual arousal: The clitoris, vagina, and labia engorge with blood and enlarge. Bartholin’s glands secrete mucus into the vestibule. Provides lubrication and facilitates intercourse. Mammary glands. Are present in both males and females. Are not a component of the reproductive system. Contain epithelial tissue that secrete milk. Milk drains into a series of ducts opening at the nipple. • Human Sexual Response. Vasocongestion: filling of tissue with blood. Myotonia: increased muscle tension. Both smooth and skeletal muscle may show sustained or rhythmic contractions. Phases of the sexual response: excitement, plateau, orgasm, and resolution. Excitement: preparation of the vagina and penis for coitus. Vasocongestion is evident in the erection of the penis and clitoris, and the enlargement of the testes, labia, and breasts. Myotonia may result in nipple erection or tension in the arms and legs. Plateau. Stimulation by the autonomic nervous system increases breathing and heart rate. In females: vasocongestion of the outer third of the vagina, expansion of the inner two-thirds of the vagina, and elevation of the uterus. Forms a depression that receives sperm at the back of the vagina. Orgasm: the shortest phase of the sexual response cycle. Characterized by rhythmic, involuntary contractions of the reproductive structures. The anal sphincter and abdominal muscles may also contract. Male orgasm: Emission: contraction of the glands and ducts of the reproductive tract. Forces semen into the urethra. Ejaculation: contraction of the urethra and expulsion of semen. Female orgasm: Uterus and outer vagina contract. Resolution: reverses the responses of earlier stages. Vasocongested organs return to their normal sizes and colors. Muscles relax. 2. Spermatogenesis and oogenesis both involve meiosis but differ in three significant ways Spermatogenesis is the production of mature sperm cells from spermatogonia. A continuous and prolific process in the adult male. Each ejaculation contains 100 – 650 million sperm. Occurs in seminiferous tubules. As spermatogenesis progresses, the developing sperm cells move from the wall to the lumen of a seminiferous tubule. Sperm structure: Haploid nucleus. Tipped with an acrosome. Contains enzymes that help the sperm penetrate to the egg. A large number of mitochondria provide ATP to power the flagellum. Oogenesis is the production of ova from oogonia. Differs from spermatogenesis in three major ways: At birth an ovary contains all of the primary oocytes it will ever have. Unequal cytokinesis during meiosis results in the formation of a single large secondary oocyte and three small polar bodies. The polar bodies degenerate. Oogenesis has long “resting” periods. 3. A complex interplay of hormones regulates reproduction The Male Pattern. Androgens secreted by Leydig cells are responsible for primary and secondary sex characteristics. Primary sex characteristics: Development of the vasa deferentia and other ducts. Development of the external reproductive structures. Sperm production. Secondary sex characteristics: Deepening of the voice. Distribution pattern of facial and pubic hair. Muscle growth. Androgens are also responsible for sexual behavior and general aggressiveness. The Female Pattern. A cyclic pattern of hormone secretion and reproductive events. Humans and many other primates have menstrual cycles. If pregnancy does not occur the endometrium is shed through the cervix and vagina: menstruation. Other mammals have estrous cycles. If pregnancy does not occur the endometrium is reabsorbed by the uterus. Associated with more pronounced behavioral cycles than are menstrual cycles. More pronounced seasonal and climatic effects than those associated with menstrual cycles. Humans females may be sexually receptive throughout their cycles. Most mammals will copulate only during the period surrounding ovulation. This period of sexual activity is called estrus. The Reproductive Cycle of the Human Female. Menstrual cycle: changes that occur in the uterus. Day 1: the first day of menstruation. Menstrual flow phase. Menstrual bleeding. Usually lasts for a few days. Proliferative phase. Regeneration and thickening of the endometrium. About 1 – 2 weeks in duration. Secretory phase. Continued endometrial thickening, increased vascularization of the endometrium, endometrium develops glands that secrete a glycogen-rich fluid, and a duration of about 2 weeks. If, by the end of the secretory phase, an embryo has not implanted in the uterus, a new menstrual flow commences. Ovarian cycle. Follicular phase. Several ovarian follicles begin to grow. The developing egg enlarges. Coat of follicle cells thickens. Usually only one follicle continues to develop, the others disintegrate. The follicular phase ends with ovulation. Follicle and adjacent wall of the ovary rupture Secondary oocyte is released. Luteal phase. Follicular tissue remaining in the ovary develops into the corpus luteum. Secretes estrogens and progesterone. Hormonal coordination of the menstrual and ovarian cycles involves five hormones. Gonadotropin-releasing hormone (GnRH) secreted by the hypothalamus. Follicle-stimulating hormone (FSH) secreted by the anterior pituitary. Luteinizing hormone (LH) secreted by the anterior pituitary. Estrogens secreted by the ovaries. Progesterone secreted by the ovaries. Follicular phase of the ovarian cycle. GnRH stimulates secretion of small amounts of FSH and LH. FSH stimulates the growth of immature ovarian follicles. The growing follicles secrete small amounts of estrogens. Inhibits secretion of FSH and LH. FSH and LH levels remain relatively low. The rate of secretion of estrogens by the growing follicle rises steeply. Stimulates the secretion the GnRH. Stimulates the secretion of FSH and LH. LH secretion is especially high. LH induces the final maturation of the follicle and ovulation. The follicular phase of the ovarian cycle is coordinated with the proliferative phase of the menstrual cycle. Secretion of estrogens during the follicular phase stimulates endometrial thickening. Following ovulation, LH stimulates the formation of the corpus luteum, Luteal phase of the ovarian cycle. LH stimulates the corpus luteum to secrete estrogens and progesterone. High levels of estrogens and progesterone inhibit FSH and LH secretion. Near the end of the luteal phase the corpus luteum disintegrates. Concentrations of estrogens and progesterone decline abruptly. FSH secretion increases and initiates a new follicular phase. The luteal phase of the ovarian cycle is coordinated with the secretory phase of the menstrual cycle. The estrogens and progesterone of the luteal phase stimulate development and maintenance of the endometrium. With the disintegration of the corpus luteum estrogens and progesterone levels decline. Menstruation occurs and a new menstrual cycle begins. Estrogens are also responsible for female secondary sex characteristics. Deposition of fat in the breasts and hips. Increased water retention. Affects calcium metabolism. Stimulates breast development. Mediates female sexual behavior. Menopause: cessation of ovarian and menstrual cycles. Usually occurs between ages 46 and 54. Due to ovaries decreased responsiveness to gonadotropins. 4. Embryonic and fetal development occur during pregnancy in humans and other eutherian (placental) mammals From Conception to Birth. In placental mammals, pregnancy is the condition of carrying one or more embryos. Pregnancy is preceded by conception and continues until birth. A human pregnancy averages 266 days. Human gestation is divided into three trimesters. First trimester. Fertilization occurs in the oviduct. 24 hours latter the zygote begins cleavage. 3- 4 days after fertilization the zygote that reaches the uterus as the embryo is a ball of cells. It takes about 1 week past fertilization for the blastocyst to form. After 5 more days it implants in the endometrium. For the first 2-4 weeks of development, the embryo obtains nutrients from the endometrium. Then the placenta provides for the diffusion of material between maternal and embryonic circulations. Organogenesis occurs during the first trimester. By week 4: the heart is beating. By the end of week 8: all of the major structures of the adult are present in rudimentary form. The rapidity of development makes this a time when the embryo is especially sensitive to environmental insult. Maternal changes during the first trimester. The embryo secretes human chorionic gonadotropin (HCG). Maintains the corpus luteum and thus maintains the endometrium. High levels of progesterone cause. Increased mucus in the cervix. Growth of the maternal part of the placenta. Enlargement of the uterus. Cessation of ovarian and menstrual cycling. Rapid enlargement of breasts, which are often very tender. Second trimester. Fetus grows rapidly and is very active. Hormonal levels stabilize as HCG declines. Corpus luteum deteriorates. Placenta secretes progesterone, which maintains the pregnancy. Third trimester. Fetus grows rapidly. Fetal activity may decrease as the fetus fills the space available to it. Maternal abdominal organs become compressed and displaced. Terminates with parturition. Parturition occurs as a result of labor. First stage: opening up and thinning of the cervix. Ending in complete dilation. Second stage: Expulsion of the baby as a result of strong uterine contractions. Third stage: Expulsion of the placenta. Reproductive Immunology. The trophoblast may inhibit a maternal immune response against the embryo. May function by interfering with the mother’s T lymphocytes. By stimulating the development of suppressor T cells in the uterus? By breaking down supplies of tryptophan, an amino acid necessary for T cell survival and function? Contraception can work in several ways. Chemical contraceptives, birth control pills, and tubal ligation. Prevent the release of eggs. A vasectomy prevents the release of sperm. Barrier methods prevent egg and sperm from meeting. Prevent embryo from implanting. Abort the embryo. Fertilization can also be prevented by: Abstinence Temporary abstinence: rhythm method = natural family planning Withdrawal, coitus interruptus. 5. Modern technology offers solutions for some reproductive problems Prenatal diagnosis of genetic and congenital abnormalities. Invasive techniques. Amniocentesis. Chorionic villus sampling. Noninvasive techniques. Ultrasound imaging. Maternal blood contains fetal blood cells that can be tested. Infertility treatment. Sperm donors. In vitro fertilization.