Reproduction

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CHAPTER 46 ANIMAL REPRODUCTION Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Section C1: Mammalian Reproduction 1. Human reproduction involves intricate anatomy and complex behavior 2. Spermatogenesis and oogenesis both involve meiosis but differ in three significant ways

Human reproduction involves intricate anatomy and complex behavior :

Reproductive Anatomy of the Human Male . The mammalian male reproductive system includes the external genitalia and the internal reproductive organs The scrotum and the penis are the external components of the reproductive system. The internal reproductive organs consist of the gonads (testes) that produce gametes (sperm cells) and hormones accessory sex glands that secrete products essential to sperm movement A set of ducts that carry the sperm and glandular secretion. Human reproduction involves intricate anatomy and complex behavior Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

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Fig. 46.8 Reproductive Anatomy of the Human Male

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Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 46.8

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The scrotum is a fold of the body wall which aids the reproductive process in different ways: Testes develop in the abdomen and descend into the scrotum just before birth. This is important since sperms can not develop at normal body temperature. By having the testes hanging outside the abdominal cavity in the scrotum, the temperature is 2 ° C lower and sperm production can occur In many rodents, the testes are drawn back into the abdominal cavity, and sperm maturation is interrupted between breeding seasons. Some mammals whose body temperature is low enough to allow sperm maturation, such as monotremes, whales and elephants retain the testes within the abdominal cavity permanently.

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Internal male reproductive organs are : The gonads or Testes (singular testis) are the male gonads. highly coiled tubules surrounded by layers of connective tissue. These tubules are the seminiferous tubules . Sperm form in seminiferous tubules. Leydig cells , scattered between seminiferous tubules produce testesterone and other androgens (male sex hormones). Sperm pass from the seminiferous tubules into the tubules of the epididymis . It takes 20 days for sperms to pass through the 6-m long tubules of each epididymis of a human male During this passage the sperm become motile and gain the ability to fertilize.

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Fig. 46.11 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

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At ejaculation, sperm are forced from the epididymis, through the vas deference which is a muscular duct running from epididymis and out of the scrotum around and behind the urinary bladder to the ejaculatory duct . Each ejaculatory duct forms by the joining of the vas deferens duct with the duct from the seminal vesicle. The two ejaculatory ducts opens into the urethra. The urethra is the tube that: Drains both the excretory and reproductive systems Runs through the penis and opens to the outside at the tip of the penis.

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There are three sets of accessory glands associated with the male system. These glands add their secretions to the semen (the fluid that is ejaculated): Their secretion is thick, yellow and alkaline They secrete a fluid containing mucous, a coagulating enzyme (causes semen to coagulate after deposited in female), fructose (provides energy for sperm) and prostaglandins (stimulates female uterine contractions to help move semen to the uterus Seminal vesicle secretion make up about 60% of the total semen volume. (1) A pair of seminal vesicle is located below and behind the bladder and empty into the ejaculatory duct

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(2) The prostate gland is the largest of the semen-secreting glands: Surrounds the upper portion of the urethra and empties directly into it. It secretes a thin, milky fluid that contains anticoagulating enzymes, citrate (a sperm nutrient) and is slightly acidic. Benign (noncancerous) enlargement of the prostate occurs in more than half of all men over the age of 40 and in virtually all men over 70. Prostate cancer is one of the most common cancer in men, it is treated surgically or with drugs that inhibit gonado-tropins, resulting in reduced prostate activity and size.

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(3) The bulbourethral glands are a pair of small glands below the prostate that empty into the urethra at the base of the penis: It secretes a clear mucous before ejaculation The fluid neutralizes any acidic urine remaining in the urethra. A man usually ejaculates about 2 – 5 ml of semen containing about 50 – 130 million sperm/ml. Once in the female reproductive tract , prostaglandins in the semen stimulate contractions of the uterine muscles, which help move the sperm up the uterus. The semen is slightly alkaline and this neutralizes the acidic environment of the vagina, protecting the sperm and increasing their motility. When first ejaculated the semen coagulates , making it easier for uterine contractions to move it along; then anticoagulants liquefy the semen and the sperm begins swimming through the female tract.

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The human penis is composed of three layers of spongy erectile tissue. During sexual arousal the erectile tissue fills with blood from arteries. The penis of some mammals (rodents, raccoons walruses ) possesses a baculum , a bone that helps stiffen the penis. Temporary impotence , a reversible inability to achieve an erection can result from the consumption of alcohol and certain drugs, and emotional problems: Several drugs (e.g.Viagra ® ) and penile implant devices are available for men with nonreversible impotence due to nervous system, or circulatory problems. External structure of the penis is covered by a thick skin: The head of the penis,the glans penis is covered by thinner skin. The glans is covered by the prepuce which may be removed by circumcision .

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Reproductive Anatomy of the Human Female . External reproductive structures consists of two sets of labia surrounding the clitoris and vaginal opening. Internal reproductive organs consist of a pair of gonads ( ovaries ) and a system of ducts and chambers. The role of the ducts and chambers are involved with gamete movement and embryo development . Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

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Fig. 46.9 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

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Fig. 46.9 Ovaries are the female gonads. Located in the abdominal cavity and enclosed in a tough protective capsule. . Attached by mesentery to, the uterus . Each ovary contains follicles.

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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 which will release eggs during a female ’ s reproductive years. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Starting at puberty and continuing to menopause , one follicle matures and releases its egg cell during each menstrual cycle .

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During ovulation , the egg is expelled from the follicle. After ovulation the remaining follicular tissue develops into the corpus luteum . Secretes estrogens and progesterone . Maintain the uterine lining during pregnancy. If the egg is not fertilized (pregnancy does not occur) the corpus luteum degenerates and a new follicle matures during the next cycle. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 46.10

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At ovulation the egg is expelled into the abdominal cavity near the funnel-shaped opening of the oviduct or fallopian tube. The cilia-lining the oviduct draws in the egg. Cilia convey the egg through the oviduct to the uterus . The uterus (or Womb) is a thick muscular organ that can expend during pregnancy to accommodate a 4 kg fetus. The inner uterine lining , the Endometrium is richly supplied with blood vessels

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The remaining female reproductive structures are: Cervix: neck of the uterus, the opens into the vagina . Vagina: thin-walled chamber that forms the birth canal and is the repository for sperm during copulation.

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Hymen : A vascularized membrane, usually covers the vaginal opening from birth until ruptured by vigorous physical activity or sexual intercourse. Vestibule : Chamber-like area bordered by the 2 pairs of skin folds covering vaginal opening and the separate urethral opening. Labia minora : The slender skin folds bordering the vestibule. Labia majora : A pair of thick , fatty ridges enclosing and protecting the labia minora and vestibule. Clitoris: bulb of erectile tissue found at the front edge of the vestibule. It is covered by a prepuce . It is richly supplied with nerve endings and is one of the most sensitive points of sexual stimulation. Bartholin ’ s glands are small glands located near the vaginal opening that secrete mucous into the vestibule during sexual arousal. The mucous facilitates intercourse by lubricating the vagina

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Mammary glands . Are present in both males and females but normally function only in women. They are important to mammalian reproduction, although not actually a part of the reproductive system. Consist of small sacs of epithelial tissue that secrete milk . Milk drains into a series of ducts opening at the nipple . In nonlactating female mammal, the mammary glands are composed primarily of fatty (adipose) tissue. In males, the low level of estrogen prevents the development of both the secretory apparatus and the fat deposits, so male breasts remain small, and nipples are not connected to ducts. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Spermatogenesis and oogenesis both involve meiosis but differ in three significant ways :

Spermatogenesis is the production of mature sperm cells in adult males. A continuous and process in the adult male. Each ejaculation contains 100 – 650 million sperm. Occurs in seminiferous tubules. Begins with the differentiation of primordial germ cells into spermatogonia in embryonic testes. Both types of cells are diploid. The spermatogonia are located near the outer wall of the seminiferous tubules. They increase in number through repeated mitosis throughout development and early life. When the male matures , spermatogonia begin to differentiate into primary spermatocyte which are diploid. Primary spermatocyte will pass through several stages before giving rise to mature spermatozoa: Spermatogenesis and oogenesis both involve meiosis but differ in three significant ways Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

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Fig. 46.11

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Each primary spermatocyte undergoes meiosis I to produce two haploid secondary spermatocytes. Each secondary spermatocyte undergoes meiosis II to form two spermatids . Thus, each primary spermatocyte forms four haploid spermatids through meiotic division. Each spermatid becomes associated with a large Sertoli cell from which it receives nutrients. All spermatids differentiate into mature spermatozoa . During spermatogenesis , the developing sperm are gradually pushed toward the center of the seminiferous tubule and make their way to epididymis, where they acquire motility . The process, from spermatogonia to motile sperm, takes 65 to 75 days in the human male.

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Spermatozoon (Sperm) structure: The thick head contains the haploid nucleus is tipped with an acrosome which contains enzymes to aid in egg penetration. Behind the head the sperm contains many mitochondria that provide ATP for movement of the tail (flagellum). Mammalian sperm shape varies from species to species. Fig. 46.12

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Oogenesis is the development of ova (mature, unfertilized egg cell). Begins in the embryo when primordial germ cells undergo mitotic divisions to produce diploid oogonia . Each oogonium will develop into primary oocyte by the time of birth of the female, resulting in all potential ova being present in the ovaries at birth. Between birth and puberty , primary oocytes enlarge and their surrounding follicles grow: They replicate their DNA and enter Prophase and remain there until activated by hormones (Puberty) After puberty during each ovarian cycle, FSH stimulates a follicle to enlarge and the primary oocyte completes meiosis I.

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Meiosis in oogenesis involves unequal cytokinesis Meiosis I produces a haploid secondary oocyte (large) and the first polar body (much smaller) Meiosis then stops again LH triggers ovulation and the secondary oocyte is released from the follicle. If the sperm cell penetrates the secondary oocytes membrane, meiosis II will occur and the small secondary polar body will separate from the ovum, this completes oogenesis.

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The three important differences between spermatogeneis and oogenesis. In oogenesis the unequal cytokinesis during meiosis results in most of the cytoplasm being in one daughter cell which will form the single ovum the other cells (polar bodies) will degenerate. In spermatogeneis all four products of meiosis I and II become mature spermatozoa. Spermatogenesis is a continuous process throughout the reproductive life of the male as spermatogonia continue to divide by mitosis. At the time of the female ’ s birth an ovary contains all of the primary oocytes it will ever have. Spermatogenesis occurs as an uninterrupted sequence ; in oogenesis long “ resting ” periods occur between the formation of the initial steps and final production of the ovum. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

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Fig. 46.13 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

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