Biology - An Illustrated Guide to Science (794127), страница 11
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They hang outside thebody in the scrotum.● Sem in iferou s tu bu les in the testes arelined with cells that divide by meiosisto produce sperm atozoa. Sperm passfrom the testis into a convoluted tubecalled the epididym is, and from thereinto another tube, the vas deferen s(sperm duct).
A healthy male canproduce millions of spermatozoaevery day between puberty and oldage.● Sperms are dormant while they arestored in the epididymis and onlybecome active when mixed withsecretions from the seminal vesiclesalong the vas deferens. This seminalfluid combines with the sperm,prostatic fluid from the prostate gland,and mucus secreted by the Cowper’sglands to form semen, which isdischarged from the urethra duringejaculation.●Ejaculation●The penis is normally flaccid or soft,but when excited, blood is pumpedinto it at high pressure making itlarger and stiffer. This also allowssperm to pass from the testis to theoutside world in an ejaculation.seminal vesiclebladderrectumpubic boneprostate glandurethraCowper’s glandpenisanusvas deferens(sperm duct)scrotumepididymistestisFront viewbladderseminalvesicleprostateglandvas deferens(sperm duct)© Diagram Visual Information Ltd.epididymisforeskinscrotumtestis63Human reproductivesystem: femaleCONTINUITYKey wordscervixfallopian tubeovaryuterusFemale reproductive systemSide viewovaryfallopian tube(oviduct)AnatomyUnlike the male, the organs thatproduce the gametes in females arefound deep inside the body.
These arethe ovaries. There are two of themfound above the bladder in theabdominal cavity.● A tube called a fallopian tu be(oviduct) connects each ovary to thetop of the u teru s. Eggs produced bythe ovaries pass down this tube.● The uterus is a thick-walled structuresealed by the cervix at the lower end.Roughly the shape and size of a smallpear when the woman is not pregnant,it can swell to many times this in thefinal stages of pregnancy.● The cervix leads from the uterus intothe vagina, which connects with theoutside world.●funnel offallopian tubebladderuteruspubic bonecervixurethrarectumclitorisvaginavaginal openinganusExternal structuresFront viewThe external structures of thereproductive system in females aresimpler than the male.
The opening ofthe vagina is bounded by a number offlaps of tissue called the labia. Theclitoris is an area that is particularlysensitive.● The bladder connects to the outsidevia the urethra in this area as well.●fallopian tube(oviduct)ovaryfunnel offallopiantubevaginalopeninglabiacervixuterus(womb)© Diagram Visual Information Ltd.vagina64CONTINUITYKey wordsepididymisseminiferoustubulespermatozoontestisSpermatogenesis: testisTestis: vertical sectionMale humanvas deferensepididymisouter wallof testisTestis anatomySem in iferou s tu bu les are very longtubes found in the testes of malemammals.
They all drain into theepididym is, which is a wider,convoluted tube that rests on the backedge of the testis. Mature but inactivesperm atozoa are stored here untilthey are passed out of the bodythrough the vas deferens and urethraduring an ejaculation.● Seminiferous tubules have a space inthe middle where growingspermatozoa can develop and mature.Between these tubules are othertissues (Leydig cells) that nourishthem and produce the hormonetestosterone, which maintains thesecondary sexual characteristics ofmales (facial hair, etc.).●testis:verticalsectionSperm productionSperm production is affected bytemperature and is most effective atslightly below body temperature—which is why the testes hang outsidethe body.● Studies have suggested that manymales are producing less sperm thanprevious generations.
The wearing oftight pants and underwear that holdthe testes close to the body has beensuggested as a cause of this change.Another possible cause is the presencein the environment of pollutants thatare similar to female hormones.seminiferous tubules●Seminiferous tubule transverse sectionsecondary spermatocytesand spermatidsgerm cellsspermatogoniaspermatozoon© Diagram Visual Information Ltd.primary spermatocyteLeydig cellouter wall of testis65Spermatogenesis: spermCONTINUITYKey wordsSpermatogenesis: schematicepididymisenzymehaploidmeiosismitochondrionmitosisseminiferoustubulespermatidspermatozoonSperm formationSpermatogenesis is the name given tothe process of producing sperm.● Cells lining the sem in iferou s tu bu lesin the testes divide in the first instanceby m itosis to form spermatogonia.These then develop into cells calledprimary spermatocytes.● A single primary spermatocyte dividesby m eiosis to produce four haploidcells called sperm atids.● The spermatids mature intosperm atozoa— the male gametes— bygrowing a tail and reducing theamount of cytoplasm surrounding thenucleus to a minimum.
Thespermatozoa are stored in the centerof the seminiferous tubules andepididym is until released (seepage 64).●spermatogoniaphase ofmultiplicationby mitosisphase ofgrowthprimary spermatocytemeiosis Isecondaryspermatocytemeiosis IIspermatid(haploid)spermatozoonSpermatozoonnucleus(haploid)headSpermatozoa consist of a singlehaploid nucleus in the head with a tailbehind it that can move and so propelthe sperm toward the ovum producedby the female.● The acrosome at the front of the headcontains a package of en zym es that areable to digest the outer skin of theegg, thereby allowing the nucleus topass into the cell.● A collection of m itochon driasurround the filaments of the tail inthe middle part of the spermatozoa.These provide energy in the form ofATP, which allows the tail to thrashsideways to drive the sperm forward.mitochondrionneckcentrioleaxial filamenttail sheathmiddle piecetail© Diagram Visual Information Ltd.acrosomeSperm structure●phase ofmaturation66CONTINUITYKey wordsgametegerm cellmeiosismitosisoogoniumovaryovumprimary oocytesecondary oocyteOogenesis: meioticdivisionOogenesis: schematicgerm cellOogenesisOogenesis is the name given to theprocess by which ova are formed.● Human females produce eggsregularly, roughly every 30 days, frompuberty to menopause.
All of theseeggs develop from germ cells(gam etes) in the two ovaries.● Germ cells initially divide by m itosis toproduce oogon ia, cells that developinto prim ary oocytes (immature ova).At a woman’s birth, there arehundreds of thousands of primaryoocytes present in the ovarian tissues.●Meiotic divisionThe next stage in the process is theproduction of secon dary oocytes bym eiosis. These cells are haploid—theyhave half the chromosomes of theirparent cells.
One of these haploid cellswill develop into the ovum; the otherthree will be enclosed within thevitelline membrane surrounding thiscell.● The ovum is a large cell with a goodsupply of cytoplasm that helps to actas a food store for the first few criticaldays of development.● The other nuclei become the polarbodies, redundant cells that remainmuch smaller than the ovum.● The vitelline membrane can bedigested by enzymes in the acrosomeof the sperm to allow the spermnucleus to enter. Once one sperm hasentered, the membrane is modified toprevent entry of other nuclei.● Around the outside of the vitellinemembrane is a clear area of mucilagecalled the zona pellucida.© Diagram Visual Information Ltd.●phase ofmultiplicationby mitosisoogoniaphase ofgrowthprimary oocytemeiosis Isecondaryoocytefirst polarbodymeiosis IIphase ofmaturationovumsecond polar bodiesSecondary oocyte prior to fertilizationpolar bodiesnucleuscytoplasmplasma membranevitelline membranezona pellucida67Oogenesis: ovarian cycleCONTINUITYKey wordsHuman femalefallopian tubehormoneovaryovumpituitary glandThe ovarian cyclesection throughovary, fallopiantube, andpart of uterusantrumprimary oocytethecagerminaIepitheliumGraafianfollicleprimaryfolliclesecondaryfollicleprimordialfolliclestertiaryfolliclefolliclecellsactive corpusIuteumsecondaryoocyteantrum (fluid-filled sac)secondaryoocyteThe production of human eggs iscontrolled by horm on es.
These act ontissues in the ovaries to switch onprocesses that, in turn, producehormones that regulate the cycle.● Hormones produced by the pitu itaryglan d in the brain cause thedevelopment of primordial folliclesinto primary follicles. These gothrough various stages until a matureGraafian follicle is formed.● A Graafian follicle contains a matureovu m , and it moves to the edge of theovary where it ruptures to release theovum and form a yellow body calledthe corpus luteum.
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