Chapter 28

The Reproductive Systems

Lecture Outline

INTRODUCTION

• Sexual reproduction is a process in which organisms produce offspring by means of germ cells called gametes.

• 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).

• Gynecology is the specialized branch of medicine concerned with the diagnosis and treatment of diseases of the female reproductive system. Urology is the study of the urinary system but also includes diagnosis and treatment of diseases and disorders of the male reproductive system.

MALE REPRODUCTIVE SYSTEM

• 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 (Figure 28.1).

Scrotum

• 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 (Figures 28.2).

• Skin contains dartos muscle causes wrinkling• Temperature regulation of testes

– sperm survival requires 3 degrees lower temperature than core body temperature

– cremaster muscle in spermatic cord• elevates testes on exposure to cold & during arousal

• warmth reverses the process

Testes

• The testes, or testicles, are paired oval-shaped glands (gonads) in the scrotum (Figure 28.3).

• The testes contain seminiferous tubules (in which sperm cells are made) (figure 28.5).

• Embedded among the spermatogenic cells in the tubules are large Sertoli cells or sustentacular cells (Figure 28.4).– The tight junctions of these cells form the blood-testis

barrier that prevents an immune response against the surface antigens on the spermatogenic cells.

Testes - cells

• The sustentacular cells – nourish spermatocytes, spermatids, and spermatozoa– mediate the effects of testosterone and follicle stimulating

hormone on spermatogenesis– phagocytose excess spermatids cytoplasm as

development proceeds– control movements of spermatogenic cells and the release

of spermatozoa into the lumen of the seminiferous tubule– secrete fluid for sperm transport and the hormone inhibin.

• The Leydig cells or interstitial endocrinocytes found in the spaces between adjacent seminiferous tubules secrete testosterone (Figure 28.4).

Spermatogenesis - Introduction

• Spermatogenesis is the process by which the seminiferous tubules of the testes produce haploid sperm. (Review the discussion of reproductive cell division in Chapter 3. Take special note of Figures 3.33 and 3.34)

• 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.

Sperm Morphology (Figure 28.8)• Adapted for reaching &

penetrating a secondary oocyte• Head contains DNA & acrosome

(hyaluronidase and proteinase enzymes)

• Midpiece contains mitochondria to form ATP

• Tail is flagellum used for locomotion

• 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.

Hormonal Control of Spermatogenesis

• Puberty– hypothalamus increases its stimulation of anterior

pituitary with releasing hormones (GnRH)– anterior pituitary increases secretion LH & FSH

• LH stimulates Leydig cells to secrete testosterone– an enzyme in prostate & seminal vesicles converts

testosterone into dihydrotestosterone (DHT is more potent.)

• FSH stimulates spermatogenesis– with testosterone, stimulates sertoli cells to secrete

androgen-binding protein (keeps hormones levels high)– testosterone stimulates final steps spermatogenesis

Hormonal Control of Spermatogenesis

• Testosterone – controls the growth, development, functioning, and

maintenance of sex organs– stimulates bone growth, protein anabolism, and sperm

maturation– stimulates development of male secondary sex

characteristics. – Negative feedback systems regulate testosterone production

(Figure 28.8).

• Inhibin is produced by sustentacular (Sertoli) cells. Inhibition of FSH by inhibin helps to regulate the rate of spermatogenesis.

Pathway of Sperm Flow through the Ducts of the Testis

• Seminiferous tubules • Straight tubules• Rete testis• Efferent ducts• Ductus epididymis• Ductus (vas) deferens

Anatomy• The ductus (vas) deferens, or seminal duct,

stores sperm and propels them toward the urethra during ejaculation (Figures 28.3a).

• The spermatic cord is a supporting structure of the male reproductive system, consisting – ductus deferens– testicular artery– autonomic nerves– veins and lymphatic vessels– cremaster muscle (Figure 28.2).

Ductus (Vas) Deferens

• Pathway of 18 inch muscular tube– ascends along posterior border of epididymis– passes up through spermatic cord and inguinal ligament– reaches posterior surface of urinary bladder– empties into prostatic urethra with seminal vesicle

• Lined with pseudostratified columnar epithelium & covered with heavy coating of muscle – convey sperm along through peristaltic contractions– stored sperm remain viable for several months

The prostate gland (Figure 28.9)• Is a donut shaped gland about the size of a golf ball

which is inferior to the urinary bladder and surrounds the prostatic urethra.

• It secretes a milky, slightly acidic fluid that contains: – citric acid, which can be used by sperm for ATP

production– acid phosphatase– several proteolytic enzymes, including:

• prostate-specific antigen (PSA), pepsinogen, lysozyme, amylase, and hyaluronidase which liquefy coagulated semen.

• Prostatitis is a common group of disorders which may be characterized by symptoms such as difficult urination, urinary frequency, and pain; or which may be asymptomatic.

Secretions - SummarySemen (seminal fluid) is a mixture • spermatozoa and accessory sex gland secretions that

provides the fluid in which spermatozoa are transported, provides nutrients, and neutralizes the acidity of the male urethra and female vagina

• antibiotic, seminal plasmin, and prostatic enzymes that coagulate and then liquefy semen to aid in its movement through the uterine cervix.

Once ejaculated, liquid semen coagulates within 5 minutes due to the presence of clotting proteins from the seminal vesicles. After about 10-20 minutes, semen re-liquifies because PSA and other proteolytic enzymes produced by the prostate gland break down the clot.

Semen Statistics

• Mixture of sperm & seminal fluid – slightly alkaline, milky appearance, sticky

• Typical ejaculate is 2.5 to 5 ml in volume• Normal sperm count is 50 to 150 million/ml• Coagulates within 5 minutes• Reliquifies in 15 minutes • Semen fertility analysis----bad news if

sperm show lack of forward motility, low count or abnormal shapes.

Penis

• The penis contains the urethra and is a passageway for the ejaculation of semen

• Body composed of three erectile tissue masses – filled with blood sinuses– lined by endothelial cells – surrounded by smooth muscle and elastic

connective tissue.

Female Reproduct

ive System

• Ovaries produce 2nd oocytes & hormones• Uterine tubes transport fertilized ova• Uterus where fetal development occurs• Vagina & external genitalia constitute the vulva• Mammary glands produce milk

Ovaries: Overview

• 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 (containing blood vessels, lymphatics, and nerves).

• Ovarian follicles lie in the cortex and consist of oocytes in various stages of development.– A mature (Graafian) follicle expels a secondary oocyte by a

process called ovulation.

• A corpus luteum contains the remnants of an ovulated follicle and produces progesterone, estrogens, relaxin, and inhibin until it degenerates into a corpus albicans.

Oogenesis and Follicular Development

• Oogenesis occurs in the ovaries. It results in the formation of a single haploid secondary oocyte.

• The oogenesis sequence includes reduction division (meiosis I), equatorial division (meiosis II), and maturation (Figure 28.15).

• While oogenesis is occurring, the follicle cells surrounding the oocyte are also undergoing developmental changes (Figures 28.13 and 28.14). The sequence of follicular cell changes is: primordial, primary, secondary, and mature (Graffian) follicles, and corpus luteum and corpus albicans.

• Table 28.1 summarizes the events of oogenesis and follicular development.

Follicular Stages

• primordial • primary• secondary• graafian • ovulation

• Corpus luteum is an

ovulation wound – fills in with hormone-secreting cells

• Corpus albicans is a white scar left after corpus luteum degenerates (when it is not needed.)

• An ovarian cyst is a fluid filled sac in or on an ovary. Most require no treatment, but the larger ones may require surgery.

Early Life History of Oogonia• Germ cells from yolk sac migrate to ovary &

become oogonia– In the female fetus, oogonia divide to produce

millions by mitosis but most degenerate (atresia)– Some develop into primary oocytes & stop in

prophase stage of meiosis I• 200,000 to 2 million are present at birth• 40,000 remain at puberty, but only 400 mature during

a woman’s life

• Each month, hormones cause meiosis I to resume in several follicles so that meiosis II is reached by ovulation

• Penetration by the sperm causes the final stages of meiosis to occur.

Histology & Function of Uterine Tube• The uterine (Fallopian) tubes transport ova

from the ovaries to the uterus and are the normal sites of fertilization (Figure 28.16).

Function -- events occurring in the uterine tube– fimbriae sweep oocyte into tube

– cilia & peristalsis move it along

– sperm reaches oocyte in ampulla, fertilization occurs within 24 hours after ovulation

– zygote reaches uterus about 7 days after ovulation

Uterus

• 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 (Figure 28.18).

• Anatomical subdivisions – fundus – body– isthmus– cervix

Anatomy of the Uterus

3 inches long by 2 in. wide and 1 in. thick

• subdivided– fundus– body– isthmus– cervix

• interior contains uterine cavity accessed by cervical canal (internal & external os)

Secretions and Functions• Secretory cells of the mucosa of the cervix produce a

cervical mucus (a mixture of water, glycoprotein, serum-type proteins, lipids, enzymes, and inorganic salts) – when thin, is more receptive to sperm– when thick, forms a cervical plug that physically impedes

sperm penetration– mucus supplements the energy needs of the sperm.

• Both the cervix and the mucus serve as a sperm reservoir, protect sperm from the hostile environment of the vagina, and protect sperm from phagocytes.

• The cervix and the mucus also play a role in capacitation.

Vagina (Figures 28.11, 28.16)• 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

• 4 inch long fibromuscular organ ending at cervix– mucosal layer

• stratified squamous epithelium & areolar connective tissue

• large stores of glycogen breakdown to produce acidic pH

– muscularis layer is smooth muscle allows considerable stretch

– adventitia is loose connective tissue that binds it to other organs

• lies between urinary bladder and rectum

Mammary Glands

• The mammary glands are modified sudoriferous (sweat) glands that lie over the pectoralis major and serratus anterior muscles (Figure 28.22).– Milk-secreting cells, referred to as alveoli, are

clustered in small compartments (lobules) within the breasts.

• The essential functions of the mammary glands are synthesis of milk, secretion and ejection of milk, which constitute lactation.

Female Reproductive Cycle - Introduction• 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 – Controlled by monthly hormone cycle of anterior

pituitary, hypothalamus & ovary

• Ovarian cycle– changes in ovary during & after maturation of oocyte

• The uterine (menstrual) cycle – involves changes in the endometrium– preparation of uterus to receive fertilized ovum– if implantation does not occur, the stratum

functionalis is shed during menstruation

Hormonal Regulation of Reproductive Cycle• GnRH secreted by the hypothalamus controls the

female reproductive cyclestimulates the release of FSH and LH by the anterior

pituitary gland (Figure 28.23).– FSH initiates growth of follicles that secrete estrogen

• estrogen maintains reproductive organs

– LH stimulates ovulation & promotes formation of the corpus luteum which secretes estrogens, progesterone, relaxin & inhibin

• progesterone prepares uterus for implantation and the mammary glands for milk secretion

• relaxin facilitates implantation in the relaxed uterus• inhibin inhibits the secretion of FSH

• At least six different estrogens have been isolated from the plasma of human females, with three in significant quantities: beta-estradiol, estrone, and estriol.

Estrogens have several important functions:

• Promotion of the development and maintenance of female reproductive structures, secondary sex characteristics, and the breasts.

• Increase protein anabolism and build strong bones.

• Lower blood cholesterol.

• Moderate levels of estrogens in the blood inhibit the release of GnRH by the hypothalamus and secretion of LH and FSH by the anterior pituitary gland.

• Progesterone works with estrogens to prepare the endometrium for implantation and the mammary glands for milk synthesis.

Phases of the Female Reproductive Cycle

• The female reproductive cycle may be divided into four phases (Figure 28.24).– The menstrual cycle (menstruation) lasts for

approximately the first 5 days of the cycle.• During this phase, small secondary follicles in each

ovary begin to develop.• the stratum functionalis layer of the endometrium is

shed, discharging blood, tissue fluid, mucus, and epithelial cells.

Menstrual Phase - 5 days• First day is considered beginning of the 28 day cycle

• In ovary– 20 follicles that began to develop 6 days before are now

beginning to secrete estrogen – fluid is filling the antrum from granulosa cells

• In uterus– declining levels of progesterone caused spiral arteries to

constrict -- glandular tissue dies– stratum functionalis layer is sloughed off along with 50 to

150 ml of blood (average.)

Preovulatory Phase - days 6-13

• In the uterus (proliferative phase)– the time between menstruation and ovulation.

This phase is more variable in length that the other phases.

– endometrial repair occurs

Preovulatory Phase - days 6-13

• In the ovary (follicular phase)– primary follicle develop into secondary follicle (occasionally

more than one) – The dominant follicle continues to increase its estrogen

production under the influence of an increasing level of LH (Figure 28.24).

– develops into a vesicular ovarian (Graafian) follicle, or mature follicle.

– by day 14, graafian follicle has enlarged & bulges at surface of the ovary (Figure 28.13).

– follicular secretion of estrogen & inhibin has slowed the secretion of FSH

– increasing estrogen levels trigger the secretion of LH

• In the uterus (proliferative phase)– increasing estrogen levels have repaired & thickened the

stratum functionalis to 4-10 mm in thickness

Ovulation

• 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.– high levels of estrogen during the last part of the

preovulatory phase exert positive feedback on both LH and GnRH to cause ovulation (Figure 28.25).

– GnRH promotes release of FSH and more LH by the anterior pituitary gland.

– The LH surge brings about the ovulation.

Ovulation• Following ovulation, the vesicular ovarian

follicle collapses (and blood within it forms a clot) to become the corpus hemorrhagicum (Figure 28.13).

• 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.

Phases - Postovulatory - days 15-28

• Time between ovulation and onset of the next menstrual period. (Figure 28.26): most constant timeline = lasts 14 days

• In the ovary (luteal phase)– both estrogen and progesterone are secreted in

large quantities by the corpus luteum.– if fertilization does not occur, then the corpus

albicans is formed• as hormone levels drop, secretion of GnRH, FSH & LH

rise

– if fertilization does occur, then the developing embryo secretes human chorionic gonadotropin (hCG) which maintains health of corpus luteum & its hormone secretions

Phases - Postovulatory - days 15-28

• Time between ovulation and onset of the next menstrual period. (Figure 28.26): most constant timeline = lasts 14 days

• In the uterus (secretory phase)– hormones from corpus luteum promote

thickening of endometrium to 12-18 mm• formation of more endometrial glands &

vascularization

– if no fertilization occurs, menstrual phase will begin

Fertilization• If fertilization and implantation do occur, the corpus luteum is maintained – The placenta will take over its hormone-producing function. – During this time, the corpus luteum, maintained by human

chorionic gonadotropin (hCG) from the developing placenta, secretes estrogens and progesterone to support pregnancy and breast development for lactation.

– Once the placenta begins its secretion, the role of the corpus luteum becomes minor.

• With reference to the uterus, this phase is also called the secretory phase because of the secretory activity of the endometrial glands as the endometrium thickens in anticipation of implantation.

Terminology of DevelopmentSummary• Gestation period

– fertilization to birth (38 weeks)

• Prenatal period (before birth)– embryological development

• first 2 months after fertilization (embryo)• all principal adult organs are present

– fetal development• from 9 weeks until birth (fetus)• placenta is functioning by end of 3rd month

• Neonatal period– first 42 days after birth

First Week of Development• Fertilization

– During fertilization, the genetic material from a haploid sperm cell (spermatozoon) and a haploid secondary oocyte merges into a single diploid nucleus.

– Fertilization normally occurs in the uterine (Fallopian) tube when the oocyte is about one-third of the way down the tube to the uterus, usually within 12 to 24 hours after ovulation. (Oocyte usually dies in 24 hours)

• The process leading to fertilization begins as peristaltic contractions and the actions of cilia transport the oocyte through the uterine tube.– Sperm swim up the uterus and into the uterine tube by the whip

like movements of their tails (flagella) and muscular contractions of the uterus.

Fertilization

• The functional changes that sperm undergo in the female reproductive tract that allow them to fertilize a secondary oocyte are referred to as capacitation.

• To fertilize an oocyte, a sperm must penetrate the corona radiata and zona pellucida around the oocyte (Figure 29.1a).

• A glycoprotein in the zona pellucida (ZP3) acts as a sperm receptor, binds to specific membrane proteins in the sperm head and triggers the acrosomal reaction, the release of the contents of the acrosome.

• The acrosomal enzymes digest a path through the zona pellucida allowing only one sperm to make its way through the barrier and reach the oocyte’s plasma membrane.

Twins• Fraternal twins (dizygotic)

– independent release of 2 oocytes fertilized by 2 separate sperm

– genetically as different as any 2 siblings

• Identical twins (monozygotic)– 2 individuals that develop from a single

fertilized ovum– genetically identical & always the same sex– if ovum does not completely separate,

conjoined twins (share some body structures)

Events Within the Egg

• Sperm entry, triggers oocyte to complete meiosis II and dump second polar body

• Once inside the oocyte, the sperm loses its tail & becomes a male pronucleus

• Fusion of male & female haploid pronuclei is the true moment of fertilization

• Fertilized ovum (2n) is called a zygote– zona pellucida still surrounds it

Formation of the Morula

• Rapid mitotic cell division of embryo is called cleavage

• 1st cleavage in 30 hours produces 2 blastomeres

• 2nd cleavage on 2nd day• By 3rd day has 16 cells • By day 4 has formed a solid

ball of cells called a morula

Blastocyst Formation

• As the number of cells in the morula increases, it moves from the site of fertilization down through the ciliated uterine tube toward the uterus and enters the uterine cavity.

• The morula develops into a blastocyst, a hollow ball of cells that is differentiated into – a trophoblast (which will form the future embryonic

membranes)– an inner cell mass or embryoblast (the future embryo)– an internal fluid-filled cavity called the blastocele (Figure

29.2e).

Development of the Blastocyst• A blastocyst is a hollow ball of cells

– enters the uterine cavityby day 5

– outer covering is the trophoblast

– inner cell mass– fluid-filled cavity is

the blastocele

• Trophoblast & part of innercell mass will develop into the fetal portion of placenta

• Most of the inner cell mass will become embryo.

Formation of Embryonic Membranes• Yolk sac

– site of early blood formation– gives rise to gonadal stem cells (spermatogonia &

oogonia)

• Amnion– develops from the epiblast– thin, protective membrane called the amnion – Initially the amnion overlies only the bilaminar embryonic

disc; as the embryo grows it eventually surrounds the entire embryo creating the amniotic cavity (Figure 29.11a inset).

– surrounds embryo with fluid: shock absorber, regulates body temperature & prevents adhesions

– fluid is filtrate of mother’s blood + fetal urine– May be examined for embryonic cells (amniocentesis)

Placenta & Umbilical Cord• Placenta forms during 3rd month

– chorion of embryo & stratum functionalis layer of uterus

• Chorionic villi extend into maternal blood filled intervillous spaces --- maternal & fetal blood vessels do not join & blood does not mix– diffusion of O2, nutrients, wastes– stores nutrients & produces hormones– barrier to microorganisms, except some viruses

• AIDS, measles, chickenpox, poliomyelitis, encephalitis

– not a barrier to drugs such as alcohol

• Placenta detaches from the uterus (afterbirth)

Labor and Parturition• Parturition means giving birth; labor is the process of

expelling the fetus

• Labor begins when progesterone’s inhibition is overcome by an increase in the levels of estrogen– progesterone inhibits uterine contraction– placenta stimulates fetal anterior pituitary which causes

fetal adrenal gland to secrete DHEA– placenta converts DHEA to estrogen– estrogen overcomes progesterone and labor begins

• A decrease in progesterone levels and elevated levels of estrogens, prostaglandins, oxytocin, and relaxin are all probably involved in the initiation and progression of labor.

Positive Feedback during Labor

• Uterine contraction forces fetal head into cervix (stretch)

• Nerve impulses reach hypothalamus causing release of oxytocin

• Oxytocin causes more contractions producing more stretch of cervix & more nerve impulses

Stages of Labor• Dilation

– 6 to 12 hours– regular contractions of the uterus– rupture of amniotic sac &

dilation of cervix (10cm)

• Expulsion– 10 minutes to several hours– baby moves through birth canal

• Placental– 30 minutes– afterbirth is expelled by

uterine contractions– constrict blood vessels that were torn– reduce the possibility of hemorrhage