Chapter 46. Animal Reproduction

Transcription

1 Chapter 46 Animal Reprductin Lecture Outline Overview: Pairing Up fr Sexual Reprductin As humans, we tend t think f reprductin in terms f the mating f males and females and the fusin f sperm and eggs. Hwever, animal reprductin takes many frms. In sme species, individuals change their sex during their lifetime, whereas in thers, such as sea slugs, an individual is bth male and female. There are animals that can fertilize their wn eggs, as well as thers that can reprduce withut any frm f sex. Fr certain species, such as hneybees, nly a few individuals within a large ppulatin reprduce. A ppulatin utlives its members nly by reprductin, the generatin f new individuals frm existing nes. Cncept 46.1 Bth asexual and sexual reprductin ccur in the animal kingdm Sexual reprductin is the frmatin f ffspring by the fusin f haplid gametes t frm a diplid zygte. The female gamete, the unfertilized egg, is a large and nnmtile cell. The male gamete is the sperm, which is usually small and mtile. Asexual reprductin is the frmatin f individuals whse genes cme frm a single parent. There is n fusin f sperm and egg. Reprductin relies entirely n mittic cell divisin. Diverse mechanisms f asexual reprductin enable animals t prduce identical ffspring rapidly. Many invertebrates reprduce asexually by fissin, in which a parent separates int tw r mre apprximately equal-sized individuals. Budding is a frm f asexual reprductin in which new individuals split ff frm existing nes. Stny crals, which can grw t be mre than 1 m acrss, are cnidarian clnies f thusands f cnnected individuals. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc. 46-1

2 In anther frm f asexual reprductin, invertebrates such as spnges release specialized grups f cells that grw int new individuals. In fragmentatin, the bdy breaks int several pieces, fllwed by regeneratin f lst bdy parts. If mre than ne piece grws and develps int a new animal, the result is reprductin. Numerus spnges, cnidarians, bristle wrms, and sea squirts reprduce by fragmentatin and regeneratin. In parthengenesis, an egg develps withut being fertilized. Amng invertebrates, parthengenesis ccurs in species f bees, wasps, and ants, prducing either haplid r diplid prgeny. If haplid, ffspring develp int adults that prduce eggs r sperm withut meisis. Male (drne) hneybees are fertile haplid adults that arise by parthengenesis, while female hneybees, including bth the sterile wrkers and the fertile queens, are diplid adults that develp frm fertilized eggs. Parthengenesis has been bserved in abut ne in a thusand vertebrate species. Recently, a female Kmd dragn and hammerhead shark prduced parthengenetic ffspring in captivity, despite being kept apart frm males. Sexual reprductin is an evlutinary enigma. Sex must enhance reprductive success r survival r it wuld rapidly disappear. Cnsider an animal ppulatin in which half the females reprduce sexually and half reprduce asexually, prducing tw ffspring each. The tw ffspring f the asexual female are bth daughters, each able t give birth t mre reprductive daughters. In cntrast, half f the sexual female s ffspring are male. The number f sexual ffspring remains the same at each generatin because bth a male and a female are required t reprduce. Thus the asexual cnditin increases in frequency at each generatin. Despite this twfld cst, sex is maintained in the vast majrity f eukarytic rganisms. Mst hyptheses abut the advantages f sex fcus n the unique cmbinatins f parental genes frmed during meitic recmbinatin and fertilizatin. By prducing diverse ffspring, sexual reprductin may enhance the reprductive success f parents when envirnmental factrs, such as pathgens, change rapidly. Asexual reprductin wuld be mst advantageus in stable, favrable envirnments. Beneficial gene cmbinatins arising thrugh recmbinatin may speed up adaptatin. The theretical advantage f this is significant nly when the rate f beneficial mutatins is high and the ppulatin size is small. Shuffling f genes during sexual reprductin may allw ppulatins t rid themselves f harmful genes mre readily. Experiments t test these and a range f ther hyptheses are under way in many labratries. Mst animals exhibit variatin in reprductive activity. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc. 46-2

3 Mst animals exhibit cycles in reprductive activity, usually related t changing seasns. Animals can thus cnserve resurces and reprduce when mre energy is available and when envirnmental cnditins favr the survival f ffspring. Fr example, ewes (female sheep) have a reprductive cycle lasting days. Ovulatin, the release f mature eggs, ccurs at the midpint f each cycle. The ewes give birth t lambs in the early spring, the time when their chances f survival are ptimal. Reprductive cycles are cntrlled by hrmnes, which are regulated by envirnmental cues such as changes in day length, seasnal temperature, rainfall, and lunar cycles. Because seasnal temperature is ften an imprtant cue fr reprductin, climate change can decrease reprductive success. Researchers in Denmark have demnstrated just such an effect n caribu (wild reindeer). In spring, caribu migrate t calving grunds t eat spruting green plants, give birth, and care fr their new calves. Changes in the length f daylight trigger the migratin, while the seasnal rise in temperature that thaws the tundra causes plants t sprut. Prir t 1993, the arrival f the caribu at the calving grunds cincided with the brief perid during which the plants were nutritius and digestible. Between 1993 and 2006, average spring temperatures in the calving grunds increased by mre than 4 C, and the plants nw sprut tw weeks earlier. Since the length f daylight is unaffected by climate change, the timing f the caribu migratin has nt changed. The result is a timing mismatch between new plant grwth and caribu birthing. Withut adequate nutritin fr the nursing females, prductin f caribu ffspring has declined by 75%. Reprductive cycles are als fund amng animals that can reprduce bth sexually and asexually such as the water flea Daphnia. Daphnia reprduce by parthengenesis under favrable cnditins and sexually during times f envirnmental stress. A different type f reprductive cycle is fund amng animals that nly reprduce asexually. Several genera f fishes, amphibians, and lizards engage in a frm f parthengenesis that prduces diplid zygtes. Fifteen species f whiptail lizards reprduce exclusively by parthengenesis. There are n males in these species, but the lizards carry ut curtship and mating behavirs typical f sexual species f the same genus. During the breeding seasn, ne female f each mating pair mimics a male. An individual adpts female behavir prir t vulatin, when the level f the female sex hrmne estradil is high, then switches t male-like behavir after vulatin, when the level f prgesterne is high. Ovulatin is mre likely t ccur if the individual is munted during the critical time f the hrmne cycle; islated lizards lay fewer eggs than thse that g thrugh the mtins f sex. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc. 46-3

4 These parthengenetic lizards evlved frm species having tw sexes, and they still require certain sexual stimuli fr maximum reprductive success. Sexual reprductin presents a prblem fr sessile r burrwing animals r parasites that may have difficulty encuntering a member f the ppsite sex. An evlutinary slutin t this prblem is hermaphrditism, in which ne individual functins as bth a male and a female. Sme hermaphrdites can self-fertilize, but mst mate with anther member f the same species. In such a mating, each individual receives and dnates sperm, resulting in twice as many ffspring as wuld be prduced if nly ne set f eggs were fertilized. Anther reprductive pattern invlves sex reversal, in which an individual changes its sex during its lifetime. The bluehead wrasse (Thalassma bifasciatum) exhibits sex reversal frm female t male. This cral reef fish lives in harems cnsisting f a single male and several females. When the male dies, a female wrasse underges sex reversal and becmes the new male. It is the largest (and usually ldest) female in the harem that underges sex reversal. What advantage did this ffer in evlutin f this wrasse? Because it is the male that defends a harem against intruders, a larger size may be mre imprtant fr males than females in ensuring successful reprductin. Certain yster species als underg sex reversal frm male t female. Oysters reprduce as males and then later becme female. Because the number f gametes prduced generally increases with size much mre fr females than fr males, sex reversal in this directin maximizes gamete prductin. The result is enhanced reprductive success: Because ysters are sedentary animals and release their gametes int the surrunding water rather than mating directly, releasing mre gametes tends t result in mre ffspring. Cncept 46.2 Fertilizatin depends n mechanisms that bring tgether sperm and eggs f the same species Fertilizatin, the unin f sperm and egg, can be internal r external. In external fertilizatin, eggs and sperm are bth released int the envirnment. In species with internal fertilizatin, sperm are depsited in r near the female reprductive tract, and fertilizatin ccurs within the tract. Successful fertilizatin requires careful timing. A mist habitat is required fr external fertilizatin, t prevent gametes frm drying ut and t allw the sperm t swim t the eggs. In species with external fertilizatin, timing is crucial t ensure that mature sperm and eggs encunter ne anther. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc. 46-4

5 Individuals clustered in the same area may release their gametes int the water at the same time in respnse t chemical signals r envirnmental cues, a prcess knwn as spawning. When external fertilizatin is nt synchrnus acrss a ppulatin, individuals may engage in mating behavirs that lead t fertilizatin f the eggs f ne female by ne male. This allws mate chice and, by triggering the release f bth sperm and eggs, increases the prbability f successful fertilizatin. Internal fertilizatin is an adaptatin t terrestrial life that enables sperm t reach an egg in a dry envirnment. Internal fertilizatin requires cperative and sphisticated reprductive systems, including cpulatry rgans that deliver sperm and receptacles fr sperm strage and transprt. Mating animals may use phermnes, chemical signals released by ne rganism that influence the behavir r physilgy f ther individuals f the same species. Phermnes are small, vlatile r water-sluble mlecules that disperse int the envirnment and, like hrmnes, are active in minute amunts. Many phermnes functin as male attractants. Develping embrys are prtected in varius ways. All species prduce mre ffspring than survive t reprduce. Species with external fertilizatin tend t prduce very large numbers f gametes, but few survive. Internal fertilizatin tends t invlve the prductin f fewer zygtes with higher survival. Eggs fertilized internally are sheltered frm ptential predatrs. Internal fertilizatin is ften assciated with mechanisms that prvide greater prtectin f the embrys and parental care f the yung. The internally fertilized eggs f many species f terrestrial animals exhibit adaptatins that prtect against water lss and physical damage during their external develpment. In birds and ther reptiles, as well as mntremes, the zygte is prtected by a shell and a set f internal membranes. The fertilized eggs f fishes and amphibians have a gelatinus cat and lack internal membranes. Sme animals retain the embry within the female reprductive tract. Marsupial mammals retain their embrys in the uterus fr nly a shrt time. The embrys crawl ut and cmplete fetal develpment attached t a mammary gland in the mther s puch. The embrys f eutherian mammals develp entirely within the uterus, nurished by the mther s bld supply thrugh the placenta. The embrys f sme fishes and sharks als cmplete develpment internally, but withut nutrient exchange between mther and yung. Many animals prvide parental care t their ffspring. Birds feed their yung; mammals nurse their ffspring. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc. 46-5

6 A female gastric brding frg carried her tadples in her stmach until they underg metamrphsis and hp ut f her muth as yung frgs. The frgs are nw extinct. Many invertebrates als prvide parental care. Animals shw variatin in reprductive systems. A grup f cells dedicated t serve as precursrs fr va and sperm is ften established very early in embry frmatin. Cycles f grwth and mitsis amplify the number f cells available fr making eggs r sperm. The simplest reprductive systems d nt even cntain discrete gnads, the rgans that prduce gametes in mst animals. Mst plychaete wrms have separate sexes but lack distinct gnads; eggs and sperm develp frm undifferentiated cells lining the celm. Mature gametes may be shed thrugh the excretry penings, r the swelling mass f eggs may split a prtin f the bdy pen, spilling the eggs int the envirnment. Mst animals pssess sets f accessry tubes and glands that carry, nurish, and prtect the gametes and smetimes the develping embrys. Mst insect species have separate sexes with cmplex reprductive systems. In males, sperm develp in a pair f testes and are passed alng a ciled duct t tw seminal vesicles fr strage. During mating, sperm are ejaculated int the female reprductive system. Eggs develp in a pair f varies and are cnveyed thrugh ducts t the uterus, where fertilizatin ccurs. In many insect species, the female reprductive system includes a spermatheca, a sac in which sperm may be stred fr extended perids and released under apprpriate cnditins. In many nnmammalian vertebrates, the digestive, excretry, and reprductive systems have a cmmn pening t the utside, the claca, which was present in ancestral vertebrates. Mst mammals have a separate pening fr the digestive tract; mst female mammals have separate penings fr the excretry and reprductive systems. In mst vertebrates, the uterus is partly r cmpletely divided int tw chambers. In mammals (including humans) that prduce nly ne r a few yung at a time, birds, and many snakes, the uterus is a single structure. Male reprductive systems differ mainly in the cpulatry rgans. Many nnmammalian vertebrates lack a penis and turn the claca inside ut t ejaculate. Animals ften mate with mre than ne member f the ther sex; mngamy is relatively rare. Hwever, mechanisms have evlved t diminish the chance f a female mating successfully with anther male. Sme male insects transfer secretins that make a female less receptive t curtship, thus reducing the likelihd f her mating again. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc. 46-6

7 Researchers have fund that females may influence the relative reprductive success f their mates. Rhnda Snk and David Hsken, cllabratrs wrking in the United Kingdm and Switzerland, studied female fruit flies that cpulated with ne male and then anther. The researchers fund that female fruit flies play a majr rle in determining the reprductive utcme f multiple matings. The prcesses by which gametes and individuals cmpete during reprductin are nly partly understd and remain a vibrant research area. Cncept 46.3 Reprductive rgans prduce and transprt gametes Human reprductin invlves intricate anatmy and cmplex behavir. The reprductive anatmy f the human female includes external and internal reprductive structures. The external reprductive structures cnsist f tw sets f labia surrunding the clitris and vaginal pening. The internal reprductive rgans cnsist f a pair f gnads, which prduce eggs and reprductive hrmnes, and a system f ducts and chambers, which receive and carry gametes and huse the embry and fetus. The varies, the female gnads, flank the uterus and are held in place by ligaments. Each vary cntains many fllicles, cnsisting f an cyte surrunded by supprt cells. The fllicles nurish and prtect the cyte during genesis, the frmatin and develpment f an vum. At birth, a wman s varies cntain 1 2 millin fllicles. Abut 500 fully mature between puberty and menpause. Usually ne fllicle matures and releases its egg during each menstrual cycle in the prcess f vulatin. Prir t vulatin, cells f the fllicle prduce the primary female sex hrmne, estradil. After vulatin, the remaining fllicular tissue develps int the crpus luteum, which secretes additinal estrgens and prgesterne t help maintain the uterine lining during pregnancy. If pregnancy des nt ccur, the crpus luteum disintegrates and a new fllicle matures during the next cycle. At vulatin, the egg is released int the abdminal cavity near the pening f the viduct. The cilia-lined funnel-like pening f the viduct draws in the egg. Cilia and the wavelike cntractins f the viduct cnvey the egg thrugh the viduct t the uterus. The highly vascularized inner lining f the uterus is called the endmetrium. The neck f the uterus, the cervix, pens int the vagina. The vagina is a muscular, elastic chamber that frms the birth canal and is the repsitry fr sperm during cpulatin. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc. 46-7

8 The vagina pens t the utside at the vulva, the cllective term fr the external female genitalia. A pair f thick, fatty labia majra enclse and prtect the rest f the vulva. The vaginal and urethral pening are lcated within a cavity brdered by by a pair f slender flds called the labia minra. Until ruptured by sexual intercurse r vigrus physical activity, the vaginal pening is partially cvered by a thin sheet f tissue called the hymen. Lcated at the upper intersectin f the labia minra, the clitris cnsists f erectile tissue supprting a runded glans cvered by a small hd f skin, the prepuce. During sexual arusal, the clitris, vagina, and labia minra all engrge with bld and enlarge. Sexual arusal als induces the vestibular glands near the vaginal pening t secrete lubricating mucus, thereby facilitating intercurse. Mammary glands are present in bth sexes but nrmally prduce milk nly in females. Within the glands, small sacs f epithelial tissue secrete milk, which drains int a series f ducts that pen at the nipple. The breasts cntain cnnective and fatty tissue in additin t the mammary glands. Because the lw level f estradil in males prevents the develpment f the fat depsits, male breasts remain small. The male s external reprductive rgans cnsist f the scrtum and penis. The internal reprductive rgans cnsist f gnads that prduce sperm and reprductive hrmnes, accessry glands that secrete prducts essential t sperm mvement, and ducts t carry the sperm and glandular secretins. The male gnads, r testes, prduce sperm in highly ciled tubes called seminiferus tubules. Leydig cells scattered in cnnective tissue between the tubules prduce teststerne and ther andrgens. The scrtum, a fld in the bdy wall, maintains testis temperature abut 2 0 C belw that f the rest f the bdy. The testes develp in the bdy cavity and descend int the scrtum just befre birth. A testis within a scrtum is a testicle. In many rdents, the testes are drawn back int the cavity between breeding seasns, interrupting sperm maturatin. Sme mammals whse bdy temperature is lw enugh t allw sperm maturatin such as mntremes, whales, and elephants retain the testes in the abdminal cavity. Frm the seminiferus tubules f the testes, the sperm pass thrugh the ciled tubules f the epididymis. It takes 3 weeks fr sperm t pass thrugh the 6-m-lng tubules f each epididymis. During this passage, sperm cmplete their maturatin and becme mtile. Sperm acquire the ability t fertilize an egg nly when expsed t the chemical envirnment f the female reprductive system. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc. 46-8

9 Ejaculatin prpels sperm frm each epididymis thrugh a muscular duct, the vas deferens. Each vas deferens extends behind the urinary bladder and jins with a duct frm the seminal vesicle t frm an ejaculatry duct, which pens int the urethra. The urethra drains bth the excretry and reprductive systems at the tip f the penis. Accessry sex glands add secretins t semen. A pair f seminal vesicles cntributes abut 60% f ttal semen vlume. Seminal fluid is thick, yellwish, and alkaline. It cntains mucus, fructse (an energy surce fr sperm), a cagulating enzyme, ascrbic acid, and prstaglandins. The prstate gland secretes directly int the urethra. Prstatic fluid is thin and milky, cntaining anticagulant enzymes and citrate, a sperm nutrient. Prstate prblems are cmmn in men lder than 40, with benign prstate enlargement in virtually all men lder than 70. Prstate cancer is ne f the mst cmmn cancers in men. The bulburethral glands are a pair f small glands alng the urethra belw the prstate. Prir t ejaculatin, the bulburethral glands secrete clear mucus that neutralizes any acidic urine remaining in the urethra. Bulburethral fluid als carries sme sperm released befre ejaculatin, which is ne reasn the withdrawal methd f birth cntrl has a high failure rate. The human penis cntains the urethra as well as three layers f spngy erectile tissue. During sexual arusal, the erectile tissue fills with bld frm arteries. The increasing pressure seals ff the veins that drain the penis, causing it t engrge with bld. The engrgement f the penis with bld causes an erectin, essential fr the insertin f the penis int the vagina. Temprary imptence can result frm the cnsumptin f alchl r ther drugs, emtinal prblems, and aging. Lng-term erectile dysfunctin can be treated with drugs such as Viagra, which prmte the vasdilating actin f the lcal regulatr NO (nitric xide), enhancing relaxatin f smth muscles in the bld vessels f the penis and allwing bld t enter the erectile tissue and sustain an erectin. The penis f sme mammals pssesses a baculum, a bne that helps stiffen the penis. A male usually ejaculates 2 5 ml f semen, with each milliliter cntaining millin sperm. The main shaft f the penis is cvered by relatively thick skin; the sensitive head, r glans, is cvered by thinner skin. The glans is cvered by the freskin, r prepuce, which may be remved by circumcisin. Reprductin in mammals invlves tw distinct types f gametes. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc. 46-9

10 Many f the differences in reprductive anatmy between males and females reflect the distinct structure and functin f the tw types f gametes. Sperm are small and mtile and must pass frm the male t the female. Eggs prvide the initial fd stres fr the embry. They are large and carry ut their functin within the female reprductive system. Eggs must mature in synchrny with the tissues f the female reprductive system that supprt the fertilized embry. Egg and sperm develpment invlves distinct patterns f meitic divisin during gametgenesis, the prductin f gametes. Spermatgenesis, the frmatin and develpment f sperm cells, is a cntinuus and prlific prcess in the adult male. Hundreds f millins f sperm are prduced each day within tubules ciled in tw testes. Fr a single sperm, the prcess takes abut seven weeks frm start t finish. Ogenesis is the develpment f mature cytes (eggs). Immature eggs frm in the vary f the female embry, but they d nt cmplete their develpment until years, and ften decades, later. Spermatgenesis differs frm genesis in three majr ways. 1. In spermatgenesis, all fur prducts f meisis develp int mature gametes. In genesis, cytkinesis during meisis is unequal, with almst all the cytplasm segregated t a single daughter cell. This large cell becmes the egg; the ther prducts f meisis, smaller cells called plar bdies, degenerate. 2. Spermatgenesis ccurs thrughut adlescence and adulthd. During genesis in human females, mittic divisins are cmplete befre birth and the prductin f mature gametes ceases at abut age Spermatgenesis prduces mature sperm frm precursr cells in a cntinuus sequence, whereas genesis has lng interruptins. Cncept 46.4 The interplay f trpic and sex hrmnes regulates mammalian reprductin The crdinated actins f hrmnes frm the hypthalamus, anterir pituitary, and gnads gvern human reprductin. The hypthalamus secretes gnadtrpin-releasing hrmne (GnRH), which directs the anterir pituitary t secrete the gnadtrpins, fllicle-stimulating hrmne (FSH) and luteinizing hrmne (LH). These tw hrmnes regulate gametgenesis directly, thrugh target tissues in the gnads, and indirectly, thrugh the regulatin f sex hrmne prductin. The principal sex hrmnes are sterid hrmnes: in males, andrgens, especially teststerne, and in females, estrgens, especially estradil, and prgesterne. The sex hrmnes regulate gametgenesis directly and indirectly. In many vertebrates, andrgens are respnsible fr male vcalizatins, such as the territrial sngs f birds and the mating calls f frgs. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc

11 During develpment f the human embry, andrgens direct the appearance f the primary sex characteristics f males, the structures directly invlved in reprductin. These include the seminal vesicles and ther ducts as well as the external reprductive anatmy. At puberty, sex hrmnes in bth males and females induce the frmatin f secndary sex characteristics, the physical and behaviral features that are nt directly related t the reprductive system. In males, andrgens cause deepening f the vice, develpment f facial and pubic hair, and grwth f muscle (thrugh stimulatin f prtein synthesis). Andrgens als prmte specific sexual behavirs and sex drive as well as increase general aggressiveness. In females, estradil stimulates breast and pubic hair develpment. Estradil als influences female sexual behavir, induces depsitin f fat in the breasts and hips, increases water retentin, and alters calcium metablism. Tw clsely linked cycles characterize reprductin in human females. On reaching sexual maturity, human males carry ut gametgenesis cntinuusly, while human females prduce gametes in cycles. Ovulatin ccurs after the endmetrium has started t thicken and develp a rich bld supply, preparing the uterus fr the pssible implantatin f an embry. If pregnancy des nt ccur, the uterine lining is slughed ff, and anther cycle begins. The cyclic shedding f the endmetrium frm the uterus, which ccurs in a flw thrugh the cervix and vagina, is called menstruatin. The menstrual r uterine cycle refers t the changes that ccur in the uterus. Menstrual cycles average 28 days (althugh cycles vary, ranging frm days). Cyclic events in the varies define the varian cycle. Hrmne activity links the tw cycles, resulting in the synchrnizatin f varian fllicle grwth and vulatin with the establishment f a uterine lining that can supprt embrynic develpment. The reprductive cycle begins with the release frm the hypthalamus f GnRH, which stimulates the pituitary t secrete small amunts f FSH and LH. FSH stimulates fllicle grwth, aided by LH, and the cells f the grwing fllicles start t make estradil. There is a slw increase in the secretin f estradil during the fllicular phase, the part f the varian cycle in which fllicles grw and cytes mature. Lw levels f estradil inhibit secretin f the pituitary hrmnes, keeping FSH and LH levels lw. The secretin f estradil by the grwing fllicle rises sharply, and levels f FSH and LH sht up. The high cncentratin f estradil stimulates the secretin f gnadtrpins by acting n the hypthalamus t increase its utput f GnRH, stimulating the secretin f FSH and LH. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc

12 LH secretin is especially high because the high cncentratin f estradil increases the sensitivity f LH-releasing cells in the pituitary t GnRH. Fllicles als respnd mre strngly t LH at this stage because mre f their cells have receptrs fr this hrmne. The increase in LH cncentratin caused by increased estradil secretin frm the grwing fllicle is a rare example f psitive feedback. LH induces the final maturatin f the fllicle and vulatin, which takes place abut a day after the LH surge. The fllicle and adjacent wall f the vary rupture, releasing the secndary cyte. Fllwing vulatin, during the luteal phase f the varian cycle, LH stimulates the transfrmatin f the fllicle int the crpus luteum, a glandular structure. Under cntinued stimulatin by LH, the crpus luteum secretes prgesterne and estradil. As the levels f these hrmnes rise, they exert negative feedback n the hypthalamus and pituitary, inhibiting the secretin f LH and FSH. This negative feedback prevents anther egg frm maturing when a pregnancy may already be underway. Near the end f the luteal phase, the crpus luteum disintegrates, causing estradil and prgesterne levels t decline. The pituitary and hypthalamus are liberated frm the inhibitry effects f these hrmnes. The pituitary begins t secrete enugh FSH t stimulate the grwth f new fllicles in the vary, initiating the next varian cycle. Prir t vulatin, varian sterid hrmnes stimulate the uterus t prepare fr the prspect f an embry. This starts the uterine cycle. Estradil secreted in increasing amunts by grwing fllicles signals the endmetrium t thicken. The fllicular phase f the varian cycle is thus crdinated with the prliferative phase f the uterine cycle. After vulatin, estradil and prgesterne secreted by the crpus luteum stimulate develpment and maintenance f the uterine lining, including enlargement f arteries and grwth f endmetrial glands. The glands secrete a nutrient fluid that can sustain an early embry befre it implants in the uterine lining. The luteal phase f the varian cycle is crdinated with the secretry phase f the uterine cycle. The rapid drp in varian hrmnes as the crpus luteum disintegrates causes arteries in the endmetrium t cnstrict, depriving it f bld. Much f the uterine lining disintegrates, and the uterus, in respnse t prstaglandin secretin, cntracts. Small bld vessels in the endmetrium cnstrict, releasing bld that is shed alng with endmetrial tissue and fluid. The result is menstruatin, r the menstrual flw phase f the uterine cycle, and the beginning f a new cycle. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc

13 During menstruatin, new varian fllicles begin t grw. The first day f menstruatin is designated day 1 f the new uterine and varian cycle. Abut 7% f reprductive-age wmen suffer frm endmetrisis, a disrder in which sme cells f the uterine lining migrate t an abdminal lcatin that is abnrmal r ectpic. In its new lcatin, the ectpic tissue still respnds t stimulatin by hrmnes in the bldstream, swelling and breaking dwn each varian cycle, and leading t pelvic pain and bleeding int the abdmen. Treatments fr endmetrisis, invlving hrmnal therapy r surgery, lessen discmfrt. Research is directed at learning why endmetrisis ccurs. Menpause is the cessatin f vulatin and menstruatin. After abut 500 cycles, menpause usually ccurs in wmen between ages 46 and 54. During these years, the varies lse their respnsiveness t FSH and LH, and menpause results frm a decline in estradil prductin by the vary. In mst species, females and males retain their reprductive capacity thrughut life. One evlutinary hypthesis fr human menpause is that cessatin f reprductin allwed a wman t prvide better care fr her children and grandchildren, thus increasing the chance f survival f individuals bearing her genes and increasing her fitness. In all female mammals, the endmetrium prir t vulatin, but nly humans and sme ther primates have menstrual cycles. Other mammals have estrus cycles. If pregnancy des nt ccur, the uterus reabsrbs the endmetrium with little fluid flw. Estrus cycles are assciated with mre prnunced behaviral cycles than are menstrual cycles. The perid f sexual activity, estrus, is the nly time the female is receptive t mating. In cntrast, human females may be sexually receptive thrughut the menstrual cycle. The length and frequency f reprductive cycles vary widely amng mammals. Bears and wlves have ne estrus cycle per year; elephants have several. Rats have estrus cycles thrughut the year, each lasting nly five days. In males, the FSH and LH secreted in respnse t GnRH are bth required fr nrmal spermatgenesis. FSH prmtes the activity f Sertli cells, cells within the seminiferus tubules that nurish develping sperm. LH regulates Leydig cells, interstitial cells lcated between the seminiferus tubules. In respnse t LH, Leydig cells secrete teststerne and ther andrgens, which prmte spermatgenesis within the tubules. Tw negative-feedback mechanisms maintain andrgen prductin at an ptimal level. Teststerne regulates bld levels f GnRH, FSH, and LH thrugh inhibitry effects n the hypthalamus and anterir pituitary. Inhibin, a hrmne that in males is prduced by Sertli cells, acts n the anterir pituitary gland t reduce FSH secretin. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc

14 Tgether, these negative-feedback circuits maintain andrgen prductin at ptimal levels. Human sexual respnse is very cmplex. Teststerne, prlactin, and xytcin each appear t influence sexual functin in males and females, but their precise rles have yet t be defined. The study f human sexual respnse has largely fcused n the physilgical changes assciated with sexual activity. The arusal f sexual interest in humans is cmplex, invlving a variety f psychlgical as well as physical factrs. Reprductive structures in the male and female that are quite different in appearance ften serve similar functins, reflecting their shared develpmental rigin. Fr example, the same embrynic tissues give rise t the glans f the penis and the clitris, the scrtum and the labia majra, and the skin n the penis and the labia minra. The general pattern f human sexual respnse is similar in bth genders. Tw types f physilgical reactin predminate in bth sexes: vascngestin, the filling f tissue with bld, and mytnia, increased muscle tensin. Bth smth and skeletal muscles may shw sustained r rhythmic cntractins, including thse assciated with rgasm. The sexual respnse can be divided int fur phases: excitement, plateau, rgasm, and reslutin. Excitement prepares the vagina and penis fr citus. Vascngestin leads t erectin f the penis and clitris; enlargement f the testicles, labia, and breasts; and vaginal lubricatin. Mytnia may result in nipple erectin r tensin in the arms and legs. In the plateau phase, these respnses cntinue as a result f direct stimulatin f the genitalia. In females, plateau includes vascngestin f the uter third f the vagina, expansin f the inner tw-thirds f the vagina, and elevatin f the uterus t frm a depressin that receives sperm at the back f the vagina. Stimulatin by the autnmic nervus system increases breathing and heart rate. Orgasm is characterized by rhythmic, invluntary cntractins f the reprductive structures in bth sexes. In male rgasm, emissin is the cntractin f the glands and ducts f the reprductive tract, which frces semen int the urethra, while ejaculatin ccurs with the cntractin f the urethra and expulsin f semen. In female rgasm, the uterus and uter vagina cntract, but the inner tw-thirds f the vagina d nt. Orgasm is the shrtest phase f the sexual respnse cycle, usually lasting nly a few secnds. In bth sexes, cntractins ccur at abut 0.8-secnd intervals and may als invlve the anal sphincter and several abdminal muscles. Reslutin cmpletes the cycle and reverses the respnses f earlier stages. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc

15 Vascngested rgans return t their nrmal sizes and clrs; muscles relax. Mst f the changes f reslutin are cmpleted in 5 minutes, but sme may take as lng as an hur. Fllwing rgasm, the male enters a refractry perid, lasting anywhere frm a few minutes t hurs, during which erectin and rgasm cannt be achieved. Female d nt experience a refractry perid, s multiple rgasms within a shrt perid f time are pssible. Cncept 46.5 In placental mammals, an embry develps fully within the mther s uterus In human cpulatin, 2 5 ml f semen is transferred, with millin sperm per ml. The alkalinity f the semen helps neutralize the acidic envirnment f the vagina, prtecting the sperm and increasing their mtility. Ejaculated semen cagulates, helping t keep ejaculate in place until sperm reach the cervix. In the cervix, anticagulants liquefy the semen, and the sperm begin swimming thrugh the female tract. Fertilizatin r cnceptin ccurs in the viduct. Twenty-fur hurs later, cleavage begins. Three t fur days after fertilizatin, the embry reaches the uterus as a ball f 16 cells. By 5 days after fertilizatin, the blastcyst frms as a sphere f cells cntaining a cavity. After a few mre days, the blastcyst implants in the endmetrium. The implanted embry secretes hrmnes t cntrl the mther s reprductive system. The embrynic hrmne human chrinic gnadtrpin (hcg) acts like pituitary LH t maintain secretin f prgesterne and estrgens by the crpus luteum fr the first few mnths f pregnancy. Sme HCG is excreted in the urine, where it can be detected by pregnancy tests. In placental mammals, pregnancy r gestatin is the cnditin f carrying ne r mre embrys in the uterus. A human pregnancy averages 266 days r 38 weeks. Many rdents have gestatin perids f 21 days, while cws have a gestatin perid f 270 days, and elephant gestatin lasts 600 days. Nt all fertilized eggs are capable f cmpleting develpment; many pregnancies terminate spntaneusly due t chrmsmal r develpmental abnrmalities. Occasinally, a fertilized egg ldges in the fallpian tube, resulting in a tubal, r ectpic, pregnancy. Such pregnancies cannt be sustained and may rupture the viduct, resulting in serius internal bleeding. A number f cnditins, including endmetrisis, increase the likelihd f tubal pregnancy. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc

16 Bacterial infectins arising during childbirth, frm medical prcedures, r as a sexually transmitted disease can scar the viduct, making ectpic pregnancy mre likely. STDs are the mst significant preventable causes f infertility. In wmen between 15 and 24 years f age, 700,000 cases f chlamydia and gnrrhea are reprted annually in the United States. The number f wmen infected with these STDs is actually significantly higher because mst wmen with these bacterial infectins have n symptms. Amng wmen wh remain untreated, up t 40% develp an inflammatry disrder that can lead t infertility r ectpic pregnancies. Human gestatin is divided int three trimesters f three mnths each. Fr the first 2 4 weeks f develpment, the embry btains nutrients directly frm the endmetrium. The uter layer f the blastcyst, the trphblast, invades the endmetrium and later frms the placenta, a disk-shaped rgan weighing clse t 1 kg. The placenta allws diffusin f material between maternal and embrynic circulatins, supplying nutrients, prviding immune prtectin, exchanging respiratry gases, and dispsing f metablic wastes fr the embry. Bld frm the embry travels t the placenta thrugh arteries f the umbilical crd and returns via the umbilical vein. Splitting f the embry during the first mnth f develpment can result in identical r mnzygtic twins. Fraternal r dizygtic twins arise when tw fllicles mature in a single cycle, leading t separate fertilizatin and implantatin f tw genetically distinct embrys. Organgenesis, the develpment f the bdy rgans, ccurs during the first trimester. By the end f week eight, all the majr structures f the adult are present in rudimentary frm and the embry is called a fetus. The heart begins beating by the furth week; a heartbeat can be detected at 8 t 10 weeks. By the end f the first trimester, the fetus is well differentiated but nly 5 cm lng. The mther is als underging majr changes. High levels f prgesterne initiate changes in the maternal reprductive system, including increased mucus in the cervix t frm a prtective plug against infectin, grwth f the maternal part f the placenta, enlargement f the uterus, and cessatin f varian and menstrual cycling. The breasts enlarge rapidly and may be tender. Abut three-furths f all pregnant wmen experience nausea during the first trimester. The cnnectin between mther and develping fetus via the placenta als allws harmful substances t pass between them. Cnsuming alchl during pregnancy pses a majr risk: Alchl that reaches the develping central nervus system f the fetus can cause fetal alchl syndrme, a disrder that can result in mental retardatin and ther serius birth defects. Smking during pregnancy is assciated with lw birth weight and ther health prblems. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc

17 During the secnd trimester, the fetus grws rapidly t 30 cm and is very active. The mther may feel mvements during the early part f the secnd trimester, althugh fetal activity is nt visible thrugh the abdminal wall until ne t tw mnths later. Hrmnal levels stabilize as hcg levels decline; the crpus luteum deterirates; and the placenta takes ver the secretin f prgesterne, which maintains the pregnancy. During the third trimester, the fetus grws rapidly t abut 3 4 kg in weight and 50 cm in length. Fetal activity may decrease as the fetus fills the space available t it. Maternal abdminal rgans becme cmpressed and displaced, leading t frequent urinatin and digestive blckages. Childbirth begins with labr, a series f strng, rhythmic uterine cntractins that push the fetus and placenta ut f the bdy. Research suggests that labr begins when the fully develped fetus prduces hrmnes and certain lung prteins that initiate an inflammatry respnse in the mther. A cmplex interplay f lcal regulatrs (prstaglandins) and hrmnes (estradil and xytcin) induces and regulates further cntractins f the uterus. The actin f xytcin frms a psitive feedback lp with uterine cntractins stimulating secretin f xytcin that in turn stimulates further cntractins. Labr has three stages. The first stage is the thinning and pening up f the cervix. The secnd stage is the expulsin r delivery f the baby by strng uterine cntractins. The third stage is delivery f the placenta. Lactatin is unique t mammals. In respnse t suckling by the newbrn, as well as changes in estradil levels after birth, the hypthalamus signals the anterir pituitary t secrete prlactin, which stimulates the mammary glands t prduce milk. Suckling als stimulates the secretin f a psterir pituitary hrmne, xytcin, which triggers release f milk frm the mammary glands. Reprductive immunlgists are wrking t understand why mammalian mthers d nt reject the embry as a freign bdy, despite its paternal antigens. The symptms f rheumatid arthritis, an autimmune disease f the jints, becme less severe during pregnancy, suggesting that regulatin f the immune system may be altered by pregnancy. Cntraceptin, the deliberate preventin f pregnancy, can be achieved in several ways. Sme methds f cntraceptin prevent the develpment r release f female r male gametes; thers prevent fertilizatin by keeping sperm and egg apart; still thers prevent implantatin f an embry. Fertilizatin can be prevented by abstinence frm sexual intercurse r by any f several barriers that keep sperm and egg apart. Temprary abstinence is called the rhythm methd f birth cntrl. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc

18 With natural family planning, cuples refrain frm intercurse during the time cnceptin is mst likely t ccur. The egg can survive in the viduct fr hurs and sperm fr as lng as five days. Ovulatin can be detected by nting changes in cervical mucus and bdy temperature during the menstrual cycle. Natural family planning has a pregnancy rate f 10 20%. Sme cuples use vulatin timing methds t increase the prbability f cnceptin. Citus interruptus, r withdrawal f the penis frm the vagina befre ejaculatin, is an unreliable methd f preventing fertilizatin. Sperm may be present in secretins that precede ejaculatin. Barrier methds f cntraceptin that blck sperm frm meeting the egg have pregnancy rates f less than 10%. The cndm is a thin latex r natural membrane sheath that fits ver the penis t cllect semen. Latex cndms are the nly cntraceptives that are highly effective in preventing the spread f sexually transmitted diseases, including AIDS. The diaphragm is a dme-shaped rubber cap that fits int the upper prtin f the vagina befre intercurse. Bth cndms and diaphragms are mre effective when used in cnjunctin with a spermicide. Other barrier devices include the vaginal puch, r female cndm. After cmplete abstinence frm sexual intercurse, the mst effective means f birth cntrl are sterilizatin, intrauterine devices (IUDs), and hrmnal cntraceptives. Sterilizatin is almst 100% effective. The IUD, with a pregnancy rate f 1% r less, is the mst cmmnly used reversible methd f birth cntrl utside the United States. Placed in the uterus by a dctr, the IUD interferes with fertilizatin and implantatin. Pregnancy rates f 1% r lwer are als achieved with birth cntrl pills. The mst cmmnly used birth cntrl pills are a cmbinatin f a synthetic estrgen and prgestin (prgesterne-like hrmne). This cmbinatin acts by negative feedback t stp the release f GnRH by the hypthalamus and thus f FSH and LH by the pituitary. The preventin f LH release prevents vulatin. The inhibitin f FSH secretin by the lw dse f estrgen in the pills prevents fllicles frm develping. A similar cmbinatin f hrmnes is als available as an injectin, in a ring inserted int the vagina, and as a skin patch. Cmbinatin birth cntrl pills can als be used in high dses as mrning-after pills, which prevent fertilizatin r implantatin with an effectiveness f abut 75%. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc

19 A different hrmne-based cntraceptive cntains nly prgestin, which causes thickening f a wman s cervical mucus t blck sperm frm entering the uterus. Prgestin decreases the frequency f vulatin and causes changes in the endmetrium that interfere with implantatin if fertilizatin ccurs. Hrmne-based cntraceptives have bth beneficial and harmful side effects. Female smkers have an increased risk f dying frm cardivascular disease if they use ral cntraceptives. Birth cntrl pills slightly increase a wman s risk f abnrmal bld cltting, high bld pressure, heart attack, and strke. Despite these increased risks, ral cntraceptives eliminate the dangers f pregnancy, and wmen wh take birth cntrl pills have mrtality rates abut half thse f pregnant wmen. The pill als decreases the risks f varian and endmetrial cancers. Research aimed at finding a male cntraceptive has fcused n hrmne cmbinatins that suppress gnadtrpin release and thereby blck spermatgenesis. Teststerne included in such cmbinatins inhibits reprductive functins f the hypthalamus and pituitary, while maintaining secndary sexual characteristics. Sterilizatin is the permanent preventin f gamete prductin r release. Tubal ligatin in wmen usually invlves sealing shut r tying ff a sectin f the viducts t prevent eggs frm traveling int the uterus. Vasectmy in men is the cutting and tying ff f each vas deferens t prevent sperm frm entering the urethra. Bth male and female sterilizatin prcedures are relatively safe and free frm harmful effects. Secretin f sex hrmnes and sexual functin are unaffected by either prcedure, with n change in menstrual cycles in females r ejaculate vlume in males. Althugh tubal ligatin r vasectmy are cnsidered permanent, bth prcedures can be reversed by micrsurgery. Abrtin is the terminatin f a pregnancy in prgress. Spntaneus abrtin, r miscarriage, ccurs in as many as ne-third f all pregnancies, ften befre the wman is even aware she is pregnant. Each year, abut 850,000 U.S. wmen have abrtins perfrmed by physicians. RU486, r mifepristne, is a drug that terminates pregnancy within the first 7 weeks. RU486 blcks prgesterne receptrs in the uterus, preventing prgesterne frm maintaining the pregnancy. RU486 is taken with a small amunt f prstaglandin t induce uterine cntractins. Genetic diseases and develpmental prblems can be diagnsed while the fetus is still in the uterus. Many genetic diseases and develpmental prblems can nw be diagnsed while the fetus is still in the uterus. Ultrasund imaging generates images using sund frequencies abve the nrmal hearing range that are cmmnly used t analyze the fetus s size and cnditin. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc

20 In amnicentesis and chrinic villus sampling, a needle is used t remve fetal cells frm fluid r tissue surrunding the embry. A bld sample frm the mther can cntain fetal cells, which can be identified with specific antibdies and then tested fr genetic disrders. A few fetal bld cells leak acrss the placenta int the mther s bldstream. Diagnsing genetic diseases in a fetus pses ethical questins. Mst detectable disrders cannt be treated in the uterus; many cannt be crrected even after birth. Parents must decide whether t terminate a pregnancy r t raise a child wh may have prfund defects and a shrt life expectancy. Reprductive technlgy can help with a number f fertility prblems. An inability t cnceive ffspring affects ne in ten cuples. The causes f infertility vary, and the likelihd f a reprductive defects is equal fr men and wmen. Fr wmen, the risk f reprductive difficulties as well as genetic abnrmalities f the fetus increases steadily past age 35; the prlnged perid f time cytes spend in meisis is largely respnsible. Hrmne therapy may increase sperm r egg prductin; surgery can crrect ducts that have failed t frm prperly r becme blcked. Many infertile cuples turn t assisted reprductive technlgies, which invlve the surgical remval f eggs (secndary cytes) frm a wman s varies after hrmnal stimulatin. The eggs are fertilized and returned t the wman s bdy. Unused eggs, sperm, and embrys frm such prcedures can be frzen fr later attempts. In in vitr fertilizatin (IVF), cytes are mixed with sperm in culture dishes, the fertilized eggs are incubated until the eight-cell stage, and then the eggs are transferred int the wman s uterus. If mature sperm are defective, scarce (fewer than 20 millin per milliliter f ejaculate), r even absent, fertility may be restred by intracytplasmic sperm injectin (ICSI). In this prcedure, the head f a sperm is drawn up int a needle and injected directly int an cyte t achieve fertilizatin. Thugh cstly, IVF prcedures have enabled hundreds f thusands f cuples t cnceive children. Abnrmalities arising as a cnsequence f the IVF prcedure are rare. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc