Lecture 9 – Animal Reproduction

Lecture 9 – Animal Reproduction Both asexual and sexual reproduction occur in the animal kingdom Asexual reproduction: 1. Fission: The separation of a parent into two or more individuals of approximately equal size. 2. Budding: occurs when a new individual splits off from an existing one. Ex. budding Hydra (Cnidarian) 3. Fragmentation: is the breaking of the body into several pieces, each developing into a complete adult. - Must be accompanied by regeneration , the regrowth of lost body parts. (ex. Sponges, cnidarians, annelids) Allows many animals to replace lost body parts. Advantages of asexual reproduction include: 1. Animals able to reproduce without mates 2. Quickly 3. Without expending energy for gamete production or fertilization 4. Produce large numbers of successful offspring. Sexual reproduction results in genetic recombination, which provides potential advantages: - An increase in variation in offspring, providing an increase in the reproductive success of parents in changing environments - An increase in the rate of adaptation - A shuffling of genes and the elimination of harmful genes from a population Reproductive Cycles and Patterns • Ovulation is the release of mature eggs at the midpoint of a female cycle • Most animals exhibit reproductive cycles related to changing seasons • Reproductive cycles are controlled by hormones and environmental cues • Animals may reproduce asexually or sexually, or they may alternate these methods • Sexual reproduction is a special problem for organisms that seldom encounter a mate • One solution ishermaphroditism, in which each individual has male and female reproductive systems • Some hermaphrodites can self-fertilize • Individuals of some species undergo sex reversals • Some species exhibit male to female reversal (for example, certain oysters), while others exhibit female to male reversal (for example, a coral reef fish) Fertilization, union of sperm and egg, can be: • In external fertilization, eggs shed by the female are fertilized by sperm in the external environment • In internal fertilization, sperm are deposited in or near the female reproductive tract, and fertilization occurs within the tract • Internal fertilization requires behavioral interactions and compatible copulatory organs Ensuring the Survival of Offspring • All species produce more offspring than the environment can handle, and the proportion that survives is quite small • Species with external fertilization produce more gametes than species with internal fertilization • Species with internal fertilization provide greater protection of the embryos and more parental care • The embryos of some terrestrial animals develop in amniote eggs with protective layers • Some other animals retain the embryo, which develops inside the female • In many animals, parental care helps ensure survival of offspring Male Reproductive System: Testes, system of ducts, accessory sex glands, and supporting structures, including the penis. scrotum: sac that hangs from the root of the penis. It is the supporting structure for the testes. A vertical septum divides it into two sacs (internally). The scrotum suspends the testicles on the outside of the body where a lower temperature can be maintained. This is vital for normal sperm production (approx.°F) 94 Testes: (testis is singular) exocrine function (production of sperm) and endocrine function (testosterone) - paired glands that develop high on the embryo's posterior abdominal wall and begin their descent into the scrotum in the 7th month of fetal development. - Cryptorchidism: when testes do not descend. If left untreated, it results in sterility, and 30-50 fold increase in the incidence of testicular cancer. Some will descend during the first year of life, or they may be surgically descended. - Seminiferous tubules: Within the testes . This is where sperm production takes place. - Spermatogenic cells (spermatogonia): sperm-forming cells. From most immature to mature: primary spermatocytes, secondary spermatocytes, spermatids, and sperm. By the time a sperm cells has nearly reached maturity, it is released into the lumen of the seminiferous tubule. - Spermatogenesis: the process by which haploid (n) sperm are produced within the seminiferous tubules. Sperm: mature at the rate of about 300 million per day. Once they leave the body, they survive for about 48 hours in the female's reproductive tract. - - the head contains the DNA and a lysosome-like structure acrosome) ( containing enzymes that aid in penetrating the oocyte. Numerous mitochondria in the midpiece provide the ATP for locomotion. The tail propels the sperm. Ducts • From the seminiferous tubules of a testis, sperm pass into the coiled tubules of the epididymis • During ejaculation, sperm are propelled through the muscular vas deferens and the ejaculatory duct, and then exit the penis through urethra the Accessory Glands • The twoseminal vesicles contribute about 60% of the total volume of semen • The prostate gland secretes its products directly into the urethra through several small ducts • The bulbourethral glands secrete a clear mucus before ejaculation that neutralizes acidic urine remaining in the urethra Semen is composed of sperm plus secretions from three sets of accessory glands. Average volume per ejaculation is 2.5 to 5 ml, with sperm counts ranging from 50-150 million sperm per ml (so a maximum of 750,000,000 sperm per ejaculate). - If counts fall below 20 million, the male is likely to be infertile. A large number is required because most will die on the way to the oocyte. Even though it only takes one sperm to fertilize an egg, it takes the combined action of a large number of sperm to break down the material surrounding the oocyte. This allows the entrance of one sperm. Puberty: Anterior pituitary secretion of FSH and LH. LH stimulates production of testosterone. FSH and testosterone stimulate spermatogenesis. Development: before birth, testosterone stimulates the male pattern of development of reproductive system ducts and the descent of the testes. Sexual Characteristics: testosterone at puberty brings about the development and enlargement of the male sex organs and secondary sexual characteristics (muscular, skeletal growth, pubic, axillary, facial, and chest hair, thickening of the skin, increased oil gland secretion, enlargement of the larynx (deepens the voice). Female Reproductive System: ovaries, uterine tubes, uterus, and vagina. Mammary glands are also considered part of the female reproductive system. ovaries: paired glands, resemble almonds in size and shape. They are homologous to the testes because the have the same embryonic origin. There is one on either side of the uterus. There are several ligaments that hold them in place. - ovarian follicles: consist of oocytes in various stages of development and their surrounding cells. Surrounding cells nourish the developing oocyte and begin to secrete estrogens as the follicle grows. - mature (Graafian) follicle: large, fluid-filled follicle that soon ruptures to expel a secondary oocyte (ovulation). - corpus luteum: remnants of an ovulated mature follicle.produces It progesterone, estrogens, andrelaxin until it degenerates and turns into fibrous tissue called corpus albicans. Ovarian cancer: sixth most common form of cancer in females and is the leading cause of death from all gynecological malignancies (excluding breast cancer). It is difficult to detect before it metastasizes. Oogenesis: formation of haploid secondary oocytes in the ovaries involves meiosis (like spermatogenesis). - Primary oocytes enter prophase of meiosis I during fetal development but do not complete it until after puberty. - 200,000-2,000,000 oogonia and primary oocytes remain in each ovary at birth. About 400 will mature and ovulate during a woman's reproductive lifetime. The remainder degenerate. - After puberty, under the influence of gonadotropin hormones, each month, meiosis resumes in one secondary follicle. - The primary oocyte completes meiosis I and two haploid cells of unequal size are produced. The smaller cell is called the first polar body and the larger cells is called the secondary oocyte, which receives most of the cytoplasm. - secondary oocyte proceeds to the metaphase of meiosis II and then stops at this stage. The follicle in which these events are taking place mature Graafian follicle will rupture to release its secondary oocyte. Meiosis II: usually one secondary oocyte is released into the pelvic cavity. Normally the cells are swept into the uterine (fallopian) tube. If fertilization does not occur, the secondary oocyte degenerates. - If sperm are present in the uterine tube and one penetrates the secondary oocyte (fertilization), meiosis II resumes. The secondary oocyte splits into 2 haploid cells of unequal size. The larger cell is called ovum the (mature egg) and the smaller is the second polar body. The nuclei of the sperm cells and the ovum then unite forming a diploid (2n) zygote. Uterine (Fallopian) Tubes: also calledoviducts, females have 2 that extend laterally from the uterus. They serve to transport secondary oocytes and fertilized ova from the ovaries to the uterus. - - they measure about 4 inches long. Their distal ends contain fimbrae, which are fingerlike projections that produce wave-like movement to draw oocytes into the tube's lumen. a sperm cell usually encounters and fertilizes an egg in the first third of the uterine tube, although fertilization in the abdominal cavity is not uncommon. - fertilization may take place for up to 24 hours following ovulation while the egg is still viable. - if fertilization occurs, thezygote will take approximately 7 days to reach the uterus. By that time, it will have undergone several divisions. Uterus: (womb) - Serves as part of the pathway for sperm to reach the uterine tubes and is also a site for menstruation, implantation of a fertilized ovum, development of the fetus during pregnancy, and labor. Pap smear: (Papanicolaou test) a few cells are removed from the part of the vagina surrounding the cervix and the cervix itself, and are examined microscopically. Malignant cells have a characteristic appearance and indicate an early stage of cancer before symptoms appear (neoplasm). Hysterectomy: surgical removal of the uterus; most common gynecological operation. Vagina: passageway for the menstrual flow and childbirth. Also receives semen from the penis during intercourse. - low pH that retards microbial growth and sperm. Breast cancer: one in seven women develop breast cancer. Of the newly reported cases, 75% have no family history of breast cancer. Mammography is recommended between the ages of 35-39 to be used as a baseline to which later mammograms may be compared. All females over the age of 20 should develop the habit of monthly breast selfexamination. Birth Control Methods