Ch 27

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1

Sexual Reproduction

A.Sexual reproduction is a process in which organisms produce offspring by means of germ cells called gametes.
B.The organs of reproduction are grouped as gonads (produce gametes and secrete hormones), ducts (transport, receive, and store gametes), and accessory sex glands (produce materials that support gametes).

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II. MALE REPRODUCTIVE SYSTEM

A. The male structures of reproduction include the testes, a system of ducts (ductus epididymis, ductus deferens, ejaculatory duct, urethra), accessory sex glands (seminal vesicles, prostate gland, bulbourethral glands), and several supporting structures, including the penis.

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B. Scrotum

1.The scrotum is a cutaneous outpouching of the abdomen that supports the testes; internally, a vertical septum divides it into two sacs, each containing a single testis.
2.The reproduction and survival of spermatozoa require a temperature that is lower than normal core body temperature. The temperature of the testes is regulated by the cremaster muscle, which elevates them and brings them closer to the pelvic cavity or relaxes, causing the testes to move farther from the pelvic cavity.

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C. Testes

1.The testes, or testicles, are paired oval-shaped glands (gonads) in the scrotum.
2.The testes develop high on the embryo’s posterior abdominal wall and usually begin their descent into the scrotum through the inguinal canals during the latter half of the seventh month of fetal development.
3.The testes contain seminiferous tubules - in which sperm cells are made.
4.Embedded among the spermatogenic cells in the tubules are large Sertoli cells or sustentacular cells.

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5. Spermatogenesis

The process by which the seminiferous tubules of the testes produce haploid sperm.
a.It begins in the diploid spermatogia (stem cells).They undergo mitosis to reserve future stem cells and to develop cells (2n primary spermatocytes) for sperm production.
b.The diploid primary spermatocytes undergo meiosis I forming haploid secondary spermatocytes.
c.Meiosis II results in the formation of the haploid spermatids. The spermatids are connected by cytoplasmic bridges.
d.The final stage of spermatogenesis is spermiogenesis which is the maturation of the spermatids into sperm.
e.The release of a sperm from its connection to a Sertoli cell is known as spermiation.

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6. Sperm

a. Mature sperm consist of a head, midpiece, and tail. They are produced at the rate of about 300 million per day and, once ejaculated, have a life expectancy of 48 hours within the female reproductive tract. Their function is to fertilize a secondary oocyte.

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D. Hormonal Control of the testes

1. At puberty, gonadotropin releasing hormone stimulates anterior pituitary secretion of follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FHS initiates spermatogenesis, and LH assists spermatogenesis and stimulates production of testosterone.
2. Testosterone controls the growth, development, functioning, and maintenance of sex organs; stimulates bone growth, protein anabolism, and sperm maturation; and stimulates development of male secondary sex characteristics. Negative feedback systems regulate testosterone production.
3. Inhibin is produced by sustentacular (Sertoli) cells. Inhibition of FSH by inhibin helps to regulate the rate of spermatogenesis.

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III. Reproductive System Ducts in Males

The duct system of the testes includes the seminiferous tubules, straight tubules, and rete testis.

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B. Epididymis

1. The epididymis is a comma-shaped organ that lies along the posterior border of the testis.
2. Sperm are transported out of the testes through the efferent ducts in the epididymis which empty into a single tube called the ductus epididymis.
3. The ductus epididymis is lined by stereocilia and is the site of sperm maturation and storage; sperm may remain in storage here for at least a month, after which they are either expelled or degenerated and reabsorbed.

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C. Ductus Deferens

1. The ductus (vas) deferens, or seminal duct, stores sperm and propels them toward the urethra during ejaculation.

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D. Spermatic cord

1. The spermatic cord is a supporting structure of the male reproductive system, consisting of the ductus deferens, the testicular artery, autonomic nerves, veins that drain the testes, lymphatic vessels, and the cremaster muscle.
2. The principle method of sterilization of males is a vasectomy in which both ductus deferens are cut and tied. .This results in no sperm entering the ejaculate (Clinical Connection)

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E. Ejaculatory ducts

1. The ejaculatory ducts are formed by the union of the ducts from the seminal vesicles and ducti deferens; their function is to eject spermatozoa into the prostatic urethra

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F. Urethra

1. The male urethra is the shared terminal duct of the reproductive and urinary systems which serves as a passageway for semen and urine. The male urethra is subdivided into three portions: prostatic, membranous, and spongy (cavernous).

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Penis

1.The penis is the male organ of copulation that consists of a root, body, and glans penis. It is used to introduce spermatozoa into the vagina.
2.Covering the glans penis is the loosely fitting prepuce, or foreskin. Circumcision is a surgical procedure in which part of the entire prepuce is removed (for either religious or hygienic reasons). There is no consensus among physicians regarding the need for circumcision or the use of anesthesia during the procedure (Clinical Connection).
3.Expansion of its blood sinuses under the influence of sexual excitation is called erection; ejaculation, propulsion of semen from the urethra to the exterior, is a sympathetic reflex.

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FEMALE REPRODUCTION SYSTEM

A. The female organs of reproduction include the ovaries (gonads), uterine (Fallopian) tubes, uterus, vagina, vulva, and mammary glands.

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B. Ovaries

1.The ovaries are paired glands that are homologous to the testes.
2.The ovaries are located in the upper pelvic cavity, on either side of the uterus. They are maintained in position by a series of ligaments.
a.The germinal epithelium covers the surface of the ovary but does not give rise to ova. It is followed by the tunica albuginea, ovarian cortex (contains ovarian follicles), and ovarian medulla (contains blood vessels, lymphatics, and nerves).
b.Ovarian follicles, in various degrees of development, lie in the cortex and contain oocytes at different stages of gametogenesis.
c.A mature (Graafian) follicle expels a secondary oocyte by a process called ovulation.
d.A corpus luteum contains the remnants of an ovulated follicle and produces progesterone, estrogens, relaxin, and inhibin until it degenerates into a corpus albicans.

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3. Oogenesis and follicular development

a. Oogenesis occurs in the ovaries. It results in the formation of a single haploid secondary oocyte.
b. The oogenesis sequence includes reduction division (meiosis I), equatorial division (meiosis II), and maturation.
c. While oogenesis is occurring, the follicle cells surrounding the oocyte are also undergoing developmental changes. The sequence of follicular cell changes is: primordial, primary, secondary, and mature (Graffian) follicles, and corpus luteum and corpus albicans.

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C. Uterine Tube

1. The uterine (Fallopian) tubes transport ova from the ovaries to the uterus and are the normal sites of fertilization.
2. Histologically the uterine tubes are composed of three layers: the internal mucosa, the middle muscularis, and the outer serosa.
3. Ciliated cells and peristaltic contractions help move a secondary oocyte toward the uterus.

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D. Uterus

1. The uterus (womb) is an organ the size and shape of an inverted pear that functions in the transport of spermatozoa, menstruation, implantation of a fertilized ovum, development of a fetus during pregnancy, and labor.

Histologically, the uterus consists of an outer promethium, middle myometrium, and inner endometrium.
a. The myometrium consists of three muscle layers.
b. The endometrium is divided into the stratum functionalis (shed during menstruation) and the stratum basalis (gives rise to a new stratum functionalis after each menstruation).

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E. Vagina

1.The vagina functions as a passageway for spermatozoa and the menstrual flow, the receptacle of the penis during sexual intercourse, and the lower portion of the birth canal.
2.The mucosa of the vagina is continuous with that of the uterus and lies in a series of transverse folds called rugae.
a.Mucosa dendritic cells are APCs (antigen-presenting cells) that participate in the transmission of viruses, such as HIV, to a female during intercourse with an infected male.
b.The mucosa contains large stores of glycogen which decompose into organic acids which set up a hostile acid environment for sperm. Alkaline components of semen neutralize the acidity and increase sperm viability.
3.The vaginal orifice is often partially covered by a thin fold of vascularized mucous membrane called the hymen. If the orifice is completely covered, this imperforate hymen must be surgically opened to permit menstrual flow.

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F. Vulva

1. The term vulva refers to the external genitalia of the female.
2. It consists of the mons pubs, labia majora, labia minora, clitoris, vestibule, vaginal and urethral orifices, hymen, bulb of the vestibule, and the paraurethral (Skene’s), greater vestibular (Bartholin’s), and lesser vestibular glands

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G. Perineum

1.The perineum is the diamond-shaped area between the thighs and buttocks of both males and females that contains the external genitals and anus.
2.During childbirth the emerging fetus may cause excessive stretching and tearing of the perineum. A physician may make a surgical incision (episiotomy) in this region to prevent excessive, jagged tears (Clinical Connection).

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H. Mammary Glands

1.The mammary glands are modified sudoriferous (sweat) glands that lie over the pectoralis major and serratus anterior muscles.
2.Milk-secreting cells, referred to as alveoli, are clustered in small compartments (lobules) within the breasts.
3.Myoepithelial cells surround the glands and promote milk expression.
4.The essential functions of the mammary glands are synthesis of milk, secretion and ejection of milk, which constitute lactation.

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V. THE FEMALE REPRODUCTIVE CYCLE

A. The general term female reproductive cycle encompasses the ovarian and uterine cycles, the hormonal changes that regulate them, and cyclical changes in the breasts and the cervix.
1. The ovarian cycle is a series of events associated with the maturation of an ovum.
2. The uterine (menstrual) cycle involves changes in the endometrium to prepare for the reception of a fertilized ovum.

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B. Hormonal Regulation of the Female Reproductive Cycle

1.The menstrual and ovarian cycles are controlled by GnRH from the hypothalamus, which stimulates the release of FSH and LH by the anterior pituitary gland.
a.FSH stimulates the initial development of ovarian follicles and secretion of estrogens by the ovaries.
b.LH stimulates further development of ovarian follicles, ovulation, and the secretion of estrogens and progesterone by the ovaries.
c.At least six different estrogens have been isolated from the plasma of human females, with three in significant quantities: beta-estradiol, estrone, and estriol.
e.Progesterone works with estrogens to prepare the endometrium for implantation and the mammary glands for milk synthesis.
f.A small quantity of relaxin is produced monthly to relax the uterus by inhibiting contractions (making it easier for a fertilized ovum to implant in the uterus). During pregnancy, relaxin relaxes the pubic symphysis and helps dilate the uterine cervix to facilitate delivery.
g.Inhibin inhibits secretion of FSH and GnRH and, to a lesser extent, LH. It might be important in decreasing secretion of FSH and LH toward the end of the uterine cycle.

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C. Phases of the Female Reproductive Cycle

The female reproductive cycle may be divided into four phases.
a.The menstrual cycle (menstruation) lasts for approximately the first 5 days of the cycle.
b.The preovulatory phase, or proliferative phase, is the time between menstruation and ovulation. This phase is more variable in length that the other phases, lasting from days 6-13 in a 28-day cycle.
c.Ovulation is the rupture of the vesicular ovarian (Graafian) follicle with release of the secondary oocyte into the pelvic cavity, usually occurring on day 14 in a 28-day cycle.
d.Following ovulation, the vesicular ovarian follicle collapses (and blood within it forms a clot) to become the corpus hemorrhagicum. The clot is eventually absorbed by the remaining follicle cells. In time, the follicular cells enlarge, change character, and form the corpus luteum, or yellow body, under the influence of LH. Stimulated by LH, the corpus luteum secretes estrogens and progesterone.
e.The postovulatory phase is the most constant in duration and lasts from days 15-28 in a 28-day cycle, the time between ovulation and onset of the next menstrual period.

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VI. AGING AND THE REPRODUCTIVE SYSTEMS

A.Puberty refers to the period of time when secondary sexual characteristics begin to develop and the potential for sexual reproduction is reached.
B.In females, the reproductive cycle normally occurs once each month from menarche, the first menses, to menopause, the last menses.
C. In males, declining reproduction function is subtle. Males often retaining reproductive capacity into their 80s or 90s.