Reproduction PDF

The Reproductive System Chapters 27-28  Reproduction Reproduction is a characteristic of living things – variety of methods Human sexual reproduction – results in offspring with genetic material from 2 parents – provides genetic diversity – im...

The Reproductive System Chapters 27-28  Reproduction Reproduction is a characteristic of living things – variety of methods Human sexual reproduction – results in offspring with genetic material from 2 parents – provides genetic diversity – important for the survival and evolution of species  The two sexes Male and female gametes combine genes to form a fertilized egg (zygote) – male gamete is motile - sperm – female gamete (egg or ovum) contains nutrients for the developing zygote female mammals also provides shelter for developing fetus (uterus and placenta)  Overview of the reproductive system Primary sex organs – produce gametes (testes or ovaries) Secondary sex organs – genitalia- (essential for reproduction) – male – ducts/glands that make/deliver sperm eg penis – female - uterus and vagina receive sperm and nourish the fetus Secondary sex characteristics – develop at puberty to attract a mate Eg facial hair, scent glands, body morphology  Role of the sex chromosomes Our cells contain 23 pairs of chromosomes – 22 pairs of autosomes – 1 pair of sex chromosomes (XY males: XX females) males produce 50% Y carrying sperm and 50% X carrying all eggs carry the X chromosome Sex of the child is determined when the egg is fertilized  Hormones and sex differentiation Gonads develop at 6 weeks (near kidneys) Male & female are indistinguishable for the first 8 to 10 weeks of development SRY gene (Sex-determining Region of Y gene) – in males, codes for a protein that causes development of testes (testes-determining factor (TDF)) secrete testosterone Female development occurs in the absence of male hormones Androgen-insensitivity syndrome Genetically male (XY), feminine appearance (complete AIS) Testosterone is secreted Target cells lack receptors for the hormone No masculizing effects occur  Descent of the gonads Gonads initially develop high in the abdominal cavity and then migrate into the pelvic cavity (ovaries) or scrotum (testes) Descent of the gonads begins as early as 6 weeks – Testes pass into the scrotum guided by the gubernaculum (embryonic connective tissue cord). At the same time, testicular arteries and veins, lymphatic vessels, nerves, spermatic ducts, and internal abdominal muscles elongate Ovaries descend to lesser extent – Lodge on inferior brim of the lesser pelvis. Gubernaculum becomes a pair of ligaments that supports the ovary and the uterus Cryptorchidism—undescended testes – Occurs in about 3% of male births – In most cases the testes descend during the first year of infancy – If not, testosterone injection or simple surgery can draw testes into the scrotum – Uncorrected cases lead to sterility or testicular cancer Male reproductive system  Penis Functions – Reproduction - Introduce sperm into female body – Organ of excretion - urethra  Testis Oval organ, 4 cm long x 2.5 cm in diameter Covered by tunica albuginea (white fibrous capsule) contains – seminiferous tubules where sperm are produced germinal epithelium composed of germ cells in the process of becoming sperm – sustentacular cells promote sperm cell development  Testis and associated structures Seminiferous tubules drain into network called rete testis Low BP of testicular artery results in poor O2 supply – sperm develop large mitochondria – help survive the hypoxic environment of the female reproductive tract Histology of the testis  Histology of the testes Blood–testis barrier (BTB) is formed by tight junctions between nurse cells (sustenacular cells) Functions: – Separates sperm from immune system – Prevents antibodies and other large molecules in the blood from getting to germ cells – Germ cells are immunologically different from body cells and would be attacked by the immune system Temperature regulation in the scrotum Cremaster: strips of the internal abdominal oblique muscle – In cold temperatures - contracts, draws testes upward toward body. In warm temperatures, relaxes suspending testes further from body. Dartos fascia: subcutaneous layer of smooth muscle – Contracts when cold, wrinkling the scrotum, holding testes against warm body. Reduces surface area of the scrotum and heat loss Pampiniform plexus : a network of veins from the testes that surrounds the testicular artery – counter current heat exchange which cools the blood reaching the testes by 1.5° to 2.5°C  Heat exchange of pampiniform plexus Testicular thermoregulation is necessary since sperm are not produced at core body temperature  Male duct system Efferent ductules collect sperm from the rete testes for transport to the epididymis Epididymis (coiled duct posterior to testis) –site of sperm maturation & storage (fertile for 60 days) Accessory glands Seminal vesicles – posterior to bladder Prostate gland – below bladder Bulbourethral glands Functions – Secretion of nutrients to nourish sperm  Puberty Reproductive system remains dormant for years after birth – surge of pituitary gonadotropins (triggered by GnRH) begins development 10-12 in most boys; 8-10 in most girls Puberty = period from onset of gonadotropin secretion until menarche (first menstrual period) or first production of viable sperm  Hormones Testosterone in men – Development - enlargement secondary sexual organs Eg sex organs, muscle mass, scent and sebaceous glands – sustains libido and spermatogenesis GnRH produced by hypothalamus in both sexes – Stimulation of anterior pituitary causes secretion of FSH & LH (follicle-stimulating & luteinizing hormone) – In males: LH stimulates interstitial cells to produce testosterone FSH stimulates sustentacular cells - spermatogenesis – In females: FSH stimulates follicles to secrete estrogen & progesterone Ripens egg and prepares uterus  Aging and sexual function Decline in testosterone secretion – peak secretion at 7 mg/day at age 20 – declines to 1/5 of that by age 80 Rise in FSH and LH secretion after age 50 produces male climacteric (menopause) – mood changes, hot flashes & “illusions of suffocation” Impotence (erectile dysfunction) – 20% of those in 60s and 50% of those in 80  Menopause Midlife change in hormone secretion accompanied by cessation of menstruation) – average age of 52 Age related depletion of follicles means less secretion of estrogen & progesterone – atrophy of uterus, vagina & breasts – bone mass declines, risks of CV disease increase – hot flashes (sudden dilation of cutaneous arteries) occur several times a day HRT = low dose estrogen & progesterone therapy  Mitosis and meiosis Mitosis produces 2 genetically identical daughter cells – occurs in tissue repair & embryonic growth Meiosis produces gametes – haploid cells required for sexual reproduction – 2 cell divisions (after only one replication of DNA) meiosis I separates homologous chromosome pairs2 haploid cells meiosis II separates duplicated sister chromatids4 haploid cells – meiosis keeps chromosome number constant from generation to generation after fertilization – occurs in gametes of males and females Meiosis I Meiosis I Meiosis II Meiosis II Spermatogenesis & sustentacular Cells  Spermiogenesis Transformation of spermatids into spermatozoa  The spermatozoon Head – 4 to 5 microns long structure – pear-shaped front end – containing the nucleus and acrosome nucleus contains haploid set of chromosomes acrosome contains enzymes that penetrate the egg Tail contains – Axoneme containing mitochondria (to produce ATP) – flagella (sperm motility) Spermatozoon  Semen or seminal fluid 2-5 mL of fluid expelled during copulation – 60% seminal vesicle fluid, 30% prostatic & 10% sperm normal sperm count is 50-120 million/mL (< 25 million/mL is associated with infertility) Other components of semen – fructose provide energy for sperm motility – fibrinolysin liquefies semen within 30 minutes – prostaglandins stimulate female peristaltic contractions – spermine is a base stabilizing sperm pH at 7.2 to 7.6 Female reproductive system Produce & deliver gametes Provide nutrition & room for fetal development Give birth Nourish the infant  Ovary Ovaries produce eggs & female hormones – almond-shaped organ, 3 cm x 1.5 cm x 1 cm Each egg develops in a fluid-filled follicle which bursts to release egg (ovulation) Anatomy of ovary  Uterine or fallopian tubes (oviducts) 10 cm long, muscular tube with ciliated cells Major portions of tube – near uterus forms a narrow isthmus – middle portion is body (ampulla) – flared distally into infundibulum  The uterus Uterus—thick muscular chamber that opens into roof of the vagina – Usually tilts forward over urinary bladder – Harbors fetus, provides a source of nutrition, and expels the fetus at the end of its development – Pear-shaped organ Fundus—broad superior curvature Body (corpus)—middle portion Cervix—cylindrical inferior end  Histology of the uterine wall Perimetrium is external layer Myometrium is middle muscular layer – 1 cm thick in nonpregnant uterus – smooth muscle (produces labor contractions) Endometrium – simple columnar epithelium – stratum functionalis is shed with each period – stratum basalis is deeper layer from which cells regenerate with each menstrual cycle Histology of the endometrium Normal & abnormal PAP smears  Vagina or birth canal Structure – 8-10 cm long, distensible muscular tube Functions – Reproduction allows for discharge of menstrual fluid, receipt of semen and birth of baby – Excretion urethra embedded in its anterior wall  Oogenesis Monthly event producing 1 haploid egg by meiosis Embryonic development of ovary – female germ cells arise from yolk sac of embryo – differentiate into oogonia & multiply in number – transform into primary oocytes(eggs) -- early meiosis I – most degenerate (atresia) by time reach childhood – by puberty 400,000 oocytes remain FSH stimulates completion of meiosis – proceeds to meiosis II & ceases until fertilization – after fertilization , releases 2nd polar body  Oogenesis and follicle development  Reproductive cycle Averages 28 days (20 - 45) Follicular phase (2 weeks) – menstruation occurs during first 3 to 5 days of cycle – follicles grow Postovulatory phase (2 weeks) – corpus luteum stimulates endometrial thickening – endometrium lost if no pregnancy  Follicular and menstrual phase Follicular -from beginning of menstruation (day 1) to ovulation(14) - most variable part of cycle Menstrual phase day 1 to 5 – discharge of menstrual fluid  Preovulatory phase From days 6 to 14, one follicle advances to graafian stage & protrudes from surface of ovary Egg stopped at metaphase II stage of meiosis  Ovarian cycle -- ovulation Results from a spike of LH (caused by  estrogen from follicle) –  blood flow causes follicle to swell rapidly; – Follicle ruptures and oocyte is swept into uterine tube  Signs of ovulation Couples attempting to conceive a child (or avoid pregnancy) may wish to know when ovulation occurs – Cervical mucus becomes thinner and more stretchy – Resting body temperature rises 0.4° to 0.6°F Best measured first thing in the morning before arising from bed Record for several days to see the difference – Mittelschmerz—twinges of pain at time of ovulation Best time for conception – Within 24 hours after the cervical mucus changes and the basal temperature rises Histology of ovarian follicles Ovulation of a human follicle  Postovulatory phase Luteal phase - corpus luteum forms from ruptured follicle - produces progesterone Premenstrual phase – if no pregnancy, corpus luteum corpus albicans  progesterone menstruation  Menstrual cycle Endometrium thickens (2-3mm) - result of estrogen from developing follicles In secretory phase (days 15-25) reaches 5-6 mm in thickness due to progesterone  Premenstrual phase Progesterone level falls due to atrophy of corpus luteum Spiral arteries constrict causing endometrial ischemia Pools of blood accumulate in endometrium  Menstrual phase Blood, serous fluid and endometrial tissue are discharged Average woman loses 40 mL of blood (contains fibrinolysin so it does not clot)  Endometrial changes (a) Proliferative phase (b) Secretory phase (c) Menstrual phase  Pregnancy and childbirth Gestation (pregnancy) lasts an average of 266 days from conception to childbirth Measured from first day of the woman’s last menstrual period (LMP) Birth is predicted to occur 280 days from LMP – 3 three month intervals called trimesters  Hormones of pregnancy HCG (human chorionic gonadotropin) – secreted by trophoblast within 9 days of conception – prevents involution of corpus luteum Estrogens – increases to 30 times normal before birth – corpus luteum is source for first 12 weeks until placenta takes over – causes uterine, mammary duct & breast enlargement  Hormones of pregnancy Progesterone secreted by placenta & corpus luteum suppresses secretion of FSH & LH preventing follicular development prevents menstruation & thickens endometrium HCS (human chorionic somatomammotropin) called human placental lactogen Stimulates placenta and secreted in direct proportion to its size Other endocrine organs thyroid gland increases 50% in size  BMR of mother parathyroid glands enlarge & stimulate osteoclasts to release additional calcium from mother’s bones Aldosterone secretion  fluid retention & leads to  in mother’s blood volume Hormone levels and pregnancy  Childbirth & uterine contractility Parturition is process of giving birth by means of contraction of mother’s uterine & abdominal muscles Progesterone inhibits contractions while estrogen stimulates contractions Nearing full term -- posterior pituitary release more oxytocin & uterus produces more receptors – directly stimulates myometrial contractions – fetus secretes cortisol enhancing estrogen & oxytocin secretion  Lactation Synthesis and ejection of milk from mammary glands – high estrogen levels in pregnancy cause mammary glands to grow Colostrum forms in late pregnancy – similar to breast milk but contains 1/3 less fat, thinner – nutrition for first 1 to 3 days after birth – contains immunoglobulins Synthesis of breast milk is promoted by prolactin (from pituitary) Milk ejection is controlled by a neuroendocrine reflex