1 Ch 27 The Reproductive System Related Issues:

2 STUDENTS DO THE FOLLOWINGErectile Dysfunction p. 1226 Male Menopause p. 1228 Cervical Cancer p Female Menopause p. 1243 Etopic Pregnancy p. 1264 Apgar Score p. 1284 Genetic Counselors p. 1295

4 C. Production of GametesHomologous Chromosomes one maternal, one paternal (23 pairs of homologous chromosomes = 46 total) Have same traits, but possibly different genes for those traits Diploid chromosomal number, 2n: 46 or 2 X 23 pairs Most body cells = diploid (2n)

6 Each DNA molecule duplicates and the two duplicates stay together 5. REVIEW OF MITOSIS D2 M1 Each DNA molecule duplicates and the two duplicates stay together Duplicated DNA condense into Chromosomes Chromosomes line up in middle the two sides of the chromosome are pulled apart the the duplicated DNA are now separate from each other The separated duplicate DNA molecules are are still considered chromosomes and they move to opposite sides of the cell cell divides producing 2 identical cells D1 M2 M1 D1 M2 D2

7 6. Meiosis: two divisions required1st Division = Meiosis I DNA molecules duplicate and condense into chromosomes with duplicates staying together The Chromosomes of each homologous pair find each other and temporarily attach The Homologous pairs line up in middle of the cell together and are then pulled apart they move to opposite ends of the cell and then the cell splits into 2 cells Now, Each of the 2 cells has only one chromosome of each homologous pair They They are now genetically different The chromosomes still consist of duplicate DNA molecules. The duplicates must now be D1 M2 M2 D2 M1 D1 M2 D2 M1 D1 separated.

8 2nd Division = Meiosis II Chromosomes line up individually The chromosomes are still made up of the two duplicated DNA, so one more division is required. (This division is just like Mitosis) 2nd Division = Meiosis II Chromosomes line up individually duplicated DNA are pulled apart from each other and they move to opposite ends of the cell. The cell divides forming 4 cells with two of the cells duplicates of the other two. The duplicated DNA has been separated and used to make more cells

9 Mitosis vs. Meiosis Meiosis I Meiosis II Figure 27.5 (1 of 2)Mother cell (before chromosome replication) DNA replication DNA replication 2n = 4 MITOSIS MEIOSIS Chromosome with duplicated DNA homologous Chromosomes attach to each other Meiosis I Chromosomes align in middle The pairs line up in the middle Sides of a chromosomes separate and the DNA duplicates are no longer together Homologous chromosomes separate but DNA duplicates remain together within each chromosome Daughter cells of mitosis Daughter cells of meiosis I each chromosome has its sides split apart – the DNA duplicates are separated from each other 2n 2n Meiosis II n n n n Daughter cells of meiosis II (usually gametes) Figure 27.5 (1 of 2)

10 II. Anatomy of the Male Reproductive System A. The Scrotum1. = Sac of skin + superficial fascia Contains: testes Function: 2. Two Muscles Dartos Muscle: Smooth muscle wrinkles scrotal skin Cremaster Muscle: Skeletal muscle elevates testes Cremaster muscle Superficial fascia containing dartos muscle Scrotum Skin

11 Tunics: Surround testes STUDENTS DOB. Testes Tunics: Surround testes STUDENTS DO Tunica albuginea Tunica vaginalis Seminal vesicle Urethra Ejaculatory duct Prostate Bulbourethral gland Ductus (vas) deferens Epididymis Testis Scrotum

12 fluid filled lumen where sperm are Cells: B. Testes … 2. Lobules: 250–300 per testes 3. Seminiferous Tubules 1-4 per Lobule fluid filled lumen where sperm are Cells: spermatogenic cells within stratified epithelium w/ Myoid cells: muscle outer layer Serotoli cells: are suporting cells called sustenacular cells that surround the developing sperm Spermatic cord Tunica albuginea Tunica vaginalis

13 Seminiferous Tubules …Serotoli Cells … Function: promote sperm development and create a sperm-testes barrier to protect them from the immune system Cells Interstitial Endocrine Cells = Cells of Leydig produce androgens 4. Rete testis: sperm travel there from seminiferous tubules 5. Descent of Testes into Scrotum: seven months in utero 6. Sperm path: Seminiferous tubules → Rete Testis  Epididymis Epididymis Rete testis

15 Spermatic cord = to-and-from testes and through iguinal canalSpermatic cord = to-and-from testes and through iguinal canal. Contains: Venous plexus, testicular artery, Autonomic nerve lymphatics Cremaster muscle Ductus Deferans

16 D. Accessory Glands 1. Seminal Vesicles - viscous alkaline fluid:Fructose Coagulating enzyme: coagulate sperm → stick to vagina Prostaglandins: decrease viscosity of cervix mucus 70% of semen volume 2. Prostate – milky fluid: Citrate-- nutrient prostate-specific antigen (PSA) Helps activate sperm 30% semen volume Prostate Cancer Seminal vesicle Prostate Bulbourethral gland and duct

19 F. Semen has sperm (20 – 150 million per event)Fructose Alkalinity (pH= ) Vagina = pH Antibiotic chemicals to kill some bacteria Suppress female immune response Enhance sperm motility Clotting factors: coagulate it Amount per ejaculation = 2-5 ml Each ml has M sperm

21 B. Spermatogenesis in seminiferous tubuleAll this begins at puberty Spermatogonium (stem cell) Cytoplasm of adjacent sustentacular cells Sustentacular cell nucleus Basal lamina Type B daughter cell Type A daughter cell remains at basal lamina as a stem cell 1. Spermatogonium = stem cells at basal lamina of tubule Mitosis yields Type A and B daughter cells Type A = stem cell; 2. Type B = primary spermatocyte These do Meiosis I to form 2 secondary spermatocyte that vary genetically Tight junction between sustentacular cells Primary spermatocyte Secondary spermatocytes Early spermatids Late spermatids Spermatozoa Lumen of seminifer- ous tubule (c) A portion of the seminiferous tublule wall, showing the spermato- genic cells surrounded by sustentacular cells (colored gold) Figure 27.7c

22 4. Spermatids then physically change to become spermatozoon, spermB. Spermatogenesis … Spermatogonium (stem cell) Cytoplasm of adjacent sustentacular cells Sustentacular cell nucleus Basal lamina 3. Secondary Spermatocytes undergo Meiosis II to each form 2 identical spermatids 4. Spermatids then physically change to become spermatozoon, sperm 5. Sperm mature in epididymis 100 to 300 Million sperm produced per day Type B daughter cell Type A daughter cell remains at basal lamina as a stem cell Tight junction between sustentacular cells Primary spermatocyte Secondary spermatocytes Early spermatids Late spermatids Spermatozoa Lumen of seminifer- ous tubule (c) A portion of the seminiferous tublule wall, showing the spermato- genic cells surrounded by sustentacular cells (colored gold) Figure 27.7c

23 5. Sperm … Approximately 24 days Golgi apparatus Acrosomal vesicle Mitochondria Acrosome Nucleus 1 2 Centrioles Spermatid nucleus Microtubules Midpiece Head (a) 3 Flagellum Major regions Head: genetic region w/ helmetlike acrosome = hydrolytic enzymes for egg penetration Midpiece: metabolic region; mitochondria Tail: locomotor region; flagellum Excess cytoplasm Tail 4 5 6 7 (b) Figure 27.8a, b

24 6. Role of Sustentacular (a.k.a. Sertoli) CellsSustentacular cell Functions, absorb excess cytoplasm from sperm, secrete testicular fluid with Androgen Binding Protein protect sperm from immune system, Blood-Testis Barrier-from tight junctions between Sustentacular cells and dividing spermatogonia: Prevents sperm antigens from activating the immune system 2. Basal compartment— spermatogonia and 1◦ spermatocytes Adluminal compartment—meioticaly active cells and lumen Sustentacular cell nucleus Tight junction between sustentacular cells

25 6. Interstitial (a.k.a. Leydig) CellsInterstitial Cells produce androgens in response to : Pituitary Hormones = FSH and LH Pituitary Hormones stimulated by hormones of the Gonadotrophin Releasing Hormone (GnRH) Seminiferous tubules Interstitial cells

26 C. Hormonal Regulation of Male Reproductive Function1. HPG Axis– hypothalmus- pituitary-gonad REVIEW Hypothalamus releases gonadotropin-releasing hormone (GnRH) GnRH stimulates anterior pituitary to secrete FSH and LH FSH sustentacular cells release androgen-binding protein (ABP), concentrates testosterone near spermatogenic cells LH interstitial cells release testosterone GnRH 1 Anterior pituitary Via portal blood LH FSH 2 Inhibin 8 6 7 Testosterone Interstitial cells 4 Sustentacular cell 3 Spermatogenic cells 5 Seminiferous tubule Stimulates Inhibits

27 C. Hormonal Regulation of Male reproductive Function … 1. HPG Axis …Testosterone spermatogenesis (+ other somatic/psychological effects) Feedback inhibition on hypothalamus and pituitary due to: Rising levels of testosterone Inhibin (released when sperm count is high) GnRH 1 Anterior pituitary Via portal blood LH FSH 2 Inhibin 8 6 7 Testosterone Interstitial cells 4 Sustentacular cell 3 Spermatogenic cells 5 Seminiferous tubule Stimulates Inhibits

29 III. Female Reproductive Anatomy ∙ IntroductionOvaries: female gonads Produce: Secrete: Accessory ducts include Uterine tubes Uterus Vagina Uterine tube Ovary store Uterus Vagina

32 B. Uterine Tubes (fallopian tubes or oviducts)1. Isthmus: constricted region where tube joins uterus 2. Ampulla major part that expands towards the ovary and ends at the Infundibulum Usual site of fertilization 3. Infundibulum funnel-shaped portion adjacent to ovary and lying over distal end of ovary fibriae create currents to move oocyte into uterine tube Isthmus Ampulla Uterine tube Infundibulum Fimbriae

35 3. Supports of the Uterus STUDENTS DOMesometrium—lateral support: portion of broad ligament Lateral cervical (cardinal) ligaments: from cervix and superior part of vagina to walls of pelvis (continuous w/ mesometrium) Uterosacral ligaments secure uterus to sacrum Round ligaments bind to anterior wall Mesometrium Round ligament of uterus Uterosacral ligament Lateral cervical (cardinal) ligament

37 Stratum functionalis (functional layer) Responds to ovary hormonec) Endometrium … Lumen of uterus Stratum functionalis (functional layer) Responds to ovary hormone Shed during menstruation kinks & spasms of spiral arteries lack of progesterone Stratum basalis (basal layer) Forms new functionalis Unresponsive to ovarian hormones Capillaries Spiral (coiled) artery Straight artery Endometrial vein Smooth muscle fibers Radial artery Arcuate artery Uterine artery (b)

38 D. Vagina = birth canal + organ of copulationb/n bladder and rectum from cervix to exterior Layers of wall Fibroelastic adventitia Smooth muscle muscularis Stratified squamous mucosa with rugae Hymen: incomplete partition near vaginal orifice Vaginal fornix: upper end of vagina surrounding cervix Acidity: Epithelia secrete glycogen, bacteria breakdown to lactic acid, pH = Posterior fornix Anterior fornix Vaginal Orifice Vagina

41 IV. Physiology of the Female Reproductive SystemOVARIAN CYCLE: Events associated with the maturation of an egg Occurs Monthly UTERINE CYCLE: Series of cyclic changes that the uterus undergoes each month DO SLIDE # ST: for slides

42 A. Oogenesis = production of female gametesBegins in fetus Oogonia (2n ovarian stem cells) multiply and store nutrients Primary oocytes develop in primordial follicles (single layer of squamous support cells) Primary oocytes begin meiosis but stall in prophase I Puberty If adequate fat: Leptin Hypothalamus releases GnRH Meiotic events Follicle development in ovary Before birth Oogonium (stem cell) Mitosis Follicle cells Oocyte Primary oocyte Infancy and childhood (ovary inactive) Primary oocyte (arrested in prophase I; present at birth) Primordial follicle Each month from puberty to menopause Start w/ 7,000,000 oocytes, reduced to 250,000 by puberty, 500 actually released during ovulation…

43 Oogenesis If unfertilized, 2° oocyte deterioratesMeiotic events Each month:GnRH  LH & FSH a few primary oocytes activated One selected per month to: Finish Meiosis I Begin Meiosis II Result = two haploid cells Secondary oocyte First polar body ‘secondary oocyte arrests in metaphase II and ovulated If penetrated by sperm, oocyte completes meiosis II, yielding Ovum (functional gamete) Second polar body Before birth Oogonium (stem cell) Mitosis Primary oocyte Why the unequal division of cytoplasm? Growth Infancy and childhood (ovary inactive) Primary oocyte (arrested in prophase I; present at birth) Each month from puberty to menopause Primary oocyte (still arrested in prophase I) Meiosis I (completed by 1 primary oocyte in response to LH surge) Secondary oocyte (arrested in metaphase II) First polar body Ovulation Meiosis II completed (only if sperm penetration occurs) Polar bodies (all polar bodies degenerate) Ovum Second polar body

44 Figure 27.17 Meiotic events Follicle development in ovary Before birthOogonium (stem cell) Follicle cells Mitosis Oocyte Primary oocyte Primordial follicle Growth Infancy and childhood (ovary inactive) Primary oocyte (arrested in prophase I; present at birth) Primordial follicle Each month from puberty to menopause Primary follicle Primary oocyte (still arrested in prophase I) Secondary follicle Spindle Vesicular (Graafian) follicle Meiosis I (completed by one primary oocyte each month in response to LH surge) Secondary oocyte (arrested in metaphase II) First polar body Ovulation Meiosis II of polar body (may or may not occur) Sperm Ovulated secondary oocyte Meiosis II completed (only if sperm penetration occurs) In absence of fertilization, ruptured follicle becomes a corpus luteum and ultimately degenerates. Polar bodies (all polar bodies degenerate) Second polar body Ovum Degenating corpus luteum Figure 27.17

45 B. Ovarian Cycle Monthly series of events = maturation of an eggTwo consecutive phases (in a 28-day cycle) Follicular phase = follicle growth (days 1–14) Ovulation occurs midcycle Luteal phase = corpus luteum activity (days 14–28) Theca folliculi 3 4 2 Primary oocyte 1 Zona pellucida Antrum Secondary oocyte 5 8 6 Secondary oocyte 7 Corona radiata

46 1. Follicular Phase (day 1-14 give or take)Primordial follicle w/ Primary Oocyte becomes primary follicle Primordial follicle activated (FSH) Squamouslike cells become thicker Follicle enlarges → primary (1) follicle cuboidal (single layer) Primary oocyte begins Meiosis I 1 Primordial follicles Primary follicle 2

47 Follicular Phase (days 1-14 or so) …2° follicle 3 Follicular Phase (days 1-14 or so) … 1° follicle → 2° (secondary) follicle Stratified epithelium (granulosa cells) forms around oocyte Granulosa cells and oocyte guide one another’s development Primary Oocyte stalled in Meiosis I 2° follicle → late 2° follicle Connective tissue, theca folliculi, and granulosa cells cooperate to produce estrogens Zona pellucida (transparent glycoprotein layer) forms around oocyte Fluid accumulates b/n granulosa cells late 2° follicle 4

48 Late 2° follicle becomes vesicular (Graafian) follicle Follicular Phase … Late 2° follicle becomes vesicular (Graafian) follicle Antrum (fluid-filled space) forms, isolates oocyte with corona radiata (capsule of granulosa cells) which are on a stalk Vesicular follicle bulges from external surface of ovary Produces estrogen 1° oocyte begins to complete meiosis I 5 Mature vesicular follicle carries out meiosis I; ready to be ovulated

49 2. Ovulation Primary Oocyte finishes Meiosis I and proceeds to Meiosis II becoming a Secondary Oocyte Does not complete Meiosis II Occurs just before Ovulation Ovary wall ruptures, expels 2° oocyte with Corona Radiata Triggered by LH surge (1–2% of ovulations = more than one 2° oocyte = fraternal twins if fertilized) Theca folliculi 3 4 2 Primary oocyte 1 Zona pellucida Antrum Secondary oocyte 5 6 Follicle ruptures; secondary oocyte ovulated 8 6 Secondary oocyte 7 Corona radiata

50 3. Luteal Phase Ruptured follicle collapsesGranulosa cells + internal thecal cells = corpus luteum Corpus luteum secretes progesterone and estrogen If no pregnancy, corpus luteum degenerates into corpus albicans (scar tissue) in 10 days – The lack of estrogen and progesterone causes the endometrium also begins eroding Theca folliculi 3 4 2 Primary oocyte 1 Zona pellucida Antrum Secondary oocyte 5 7 Corpus luteum (forms from ruptured follicle) 8 6 Secondary oocyte 7 Corona radiata

51 Lives about 24 hours after ovulation 3. Luteal Phase … If fertilization occurs, the secondary oocytes completes meiosis II to become a mature Ovum Lives about 24 hours after ovulation Embryo will implant causing Pregnancy The corpus luteum produces hormones until placenta takes over at ~ 3 months Theca folliculi 3 4 2 Primary oocyte 1 Zona pellucida Antrum Secondary oocyte 5 7 Corpus luteum (forms from ruptured follicle) 8 6 Secondary oocyte 7 Corona radiata

52 C. Establishing the Ovarian CycleDuring childhood, until puberty Ovaries secrete small amounts of estrogens Estrogen inhibits release of GnRH At puberty Leptin from adipose tissue decreases estrogen inhibition GnRH, FSH, and LH are released If inadequate adipose cycle delayed ~4 years later = menarche & adult cycle Theca folliculi 3 4 2 Primary oocyte 1 Zona pellucida Antrum Secondary oocyte 5 8 6 Secondary oocyte 7 Corona radiata

53 Hormonal Interactions: 28-Day Ovarian CycleDay 1: GnRH↑  release FSH + LH FSH + LH  growth of several follicles Late secondary follicle begins to produce estrogen Estrogen production increases Hypothalamus GnRH Travels via portal blood 1 Anterior pituitary 1 2 FSH LH Thecal cells 3 Slightly elevated estrogen and rising inhibin levels. Granulosa cells Convert androgens to estrogens Androgens Inhibin

54 Hormonal Interactions: 28-Day Ovarian Cycle …4. Initially,  estrogen levels Inhibits FSH + LH release from pituitary inhibin released by granulosa does the same Stimulates synthesis/storage of FSH and LH in pituitary Enhances FSH affect on follicles = further estrogen output 5. Estrogen levels reach a high threshold level suddenly becomes positive feedback Hypothalamus GnRH Travels via portal blood 1 Anterior pituitary 1 2 FSH LH Thecal cells 3 Slightly elevated estrogen and rising inhibin levels. Granulosa cells Convert androgens to estrogens Androgens Inhibin

55 Hormonal Interactions: 28-Day Ovarian Cycle …Triggers a Sudden burst of GnRH → LH and FSH surge forth from stored supplies LH surge causes Ovulation =Dominant follicle ruptures Ruptured follicle → corpus luteum Corpus luteum releases progesterone, estrogen, and inhibin; hypthalamus, FSH & LH inhibited If no fertilization/implant, corpus luteum degenerates (day 26-28), blockade of GnRH/FSH/LH ends, new cycle begins Hypothalamus 4 Positive feedback by ↑estrogen output. 5 LH surge 8 ↑Progesterone ↑Estrogen ↑Inhibit 6 Ruptured follicle 7 Corpus luteum Mature follicle Ovulated secondary oocyte

56 LH Ovarian Cycle consistent w/ Uterine Cycle– menstrual cycle Both start on Day 1 FSH Estrogens Progesterone Endometrium Menstrual flow Menstrual phase Proliferative phase Secretory phase Figure 27.20a

57 D. Uterine (Menstrual) Cycle = cyclic changes to endometriumThree phases Days 1–5: menstrual phase – low hormone levels, stratum functionalis shed Days 6–14: proliferative (preovulatory) phase – estrogen ↑, new functional layer + progesterone receptors Estrogens Progesterone If no pregnancy, spiral arteries kink = dead epithelia, final dilation of arteries ruptures capillaries Endometrial glands Spiral Arteries Menstrual Flow = shed stratum functionalis Functional layer Basal layer Days Menstrual phase Proliferative phase Secretory phase

58 D. Uterine (Menstrual) Cycle = cyclic changes to endometrium ,,,Three phases … Days 15–28: secretory (postovulatory) phase – progesterone ↑ = glands secrete glycogen, mucus plug reforms (endometrium prep-ed for implantation) Estrogens Progesterone If no pregnancy, spiral arteries kink = dead epithelia, final dilation of arteries ruptures capillaries Endometrial glands Spiral Arteries Menstrual Flow = shed stratum functionalis Functional layer Basal layer Days Menstrual phase Proliferative phase Secretory phase

59 D. Uterine (Menstrual) Cycle = cyclic changes to endometrium ,,,Three phases … If no pregnancy, spiral arteries kink = dead epithelia, final dilation of arteries ruptures capillaries Cycle starts again with Menstrual Flow = shed stratum functionalis Estrogens Progesterone Endometrial glands Spiral Arteries Functional layer Basal layer Days Menstrual phase Proliferative phase Secretory phase

60 E. Effects of Sex Hormones 1. Effects of EstrogensPromote oogenesis/follicle growth Maturation of female reproductive tract/breasts Induce 2° sex characters – widened, lightened pelvis; subcutaneous fat deposits of hips/breasts Support growth spurt at puberty Enhances Ca2+ uptake

61 2. Effects of ProgesteroneWorks w/ estrogen to regulate uterine cycle Effects of placental progesterone during pregnancy Inhibits uterine motility Helps prepare breasts for lactation

62 F. Female Sexual ResponseClitoris, vaginal mucosa, and breasts engorge with blood Vestibular gland lubricates vestibule Orgasm not required for conception; but unlike male, no refractory period = multiple orgasms possible

63 V. Health Issues 1. Breast CancerUsually arises from epithelial cells of small ducts Risk factors include: Early onset of menstruation/late menopause No pregnancies or first pregnancy late in life Family history of breast cancer 10% due to hereditary defects, mutations to genes BRCA1 and BRCA2 esp. Still 70% have no known risk factors (a) Mammogram procedure Malignancy 13% of U.S. women get it, most common malignancy, 2nd among cancer deaths (b) Film of normal breast (c) Film of breast with tumor

64 2. Menopause When menses cease for entire year No equivalent in malesDeclining estrogen levels  Atrophy: of reproductive organs/breasts Mood: Irritability and depression in some Blood Vessels: Hot flashes as skin blood vessels undergo intense vasodilation Loss of cells: Gradual thinning of skin and bone loss Increased cholesterol levels/falling HDL

67 end

68 Review Questions The ________ are the primary male sex organs and contain __________ ________ that actually manufacture sperm. testes seminiferous tubules

69 Review Questions Gametes are generated through a cell division process called _________ which creates up to ___ haploid cells, each with _____ chromosomes in humans. meiosis 4 23

70 Review Questions _____ stimulates the anterior pituitary to release ____ that stimulates sustenacular cells to release _____, and ___ that stimulates ____________________ cells to release testosterone. GnRH FSH ABP LH interstitial (Leydig)

71 Review Questions All follicles are found in the ________ of the ovary.The stratum __________ of the __________ is shed during menstruation. The ________ of the infundibulum is responsible for ‘catching’ the oocyte during ___________. cortex functionalis endometrium fimbriae ovulation

72 Participate The development of a true ____ does not occur unless fertilization occurs. A primary follicle is surrounded by a layer of simple ________ cells. A __________ follicle is distinguished by an ______ and the corona _______. Ruptured follicles become the _______ _________ during the luteal phase. ova cuboidal vesicular antrum radiata corpus luteum

73 Review Questions _____ from the hypothalmus controls the release of ____ that stimulates follicle growth and ___ that induces the thecal cells to produce ___________. After initially inhibiting the hypothalamus and pituitary, ________ exerts a positive feedback effect that triggers a surge in ___ that leads to __________. GnRH FSH LH androgens estrogen LH ovulation

75 Mitosis vs. Meiosis Figure 27.5 (1 of 2) Mother cell(before chromosome replication) Chromosome replication Chromosome replication 2n = 4 MITOSIS MEIOSIS Replicated chromosome Tetrad formed by synapsis of replicated homologous chromosomes Prophase Prophase I Chromosomes align at the metaphase plate Metaphase I Tetrads align at the metaphase plate Metaphase Sister chromatids separate during anaphase Homologous chromosomes separate but sister chromatids remain together during anaphase I Daughter cells of mitosis Daughter cells of meiosis I 2n 2n No further chromosomal replication; sister chromatids separate during anaphase II Meiosis II n n n n Daughter cells of meiosis II (usually gametes) Figure 27.5 (1 of 2)

76 Meiosis: Spermatocytes to SpermatidsBasal lamina Spermatogonium (stem cell) Type A daughter cell remains at basal lamina as a stem cell Mitosis Type B daughter cell Growth Enters meiosis I Primary spermatocyte Meiosis I completed Secondary spermatocytes Meiosis II Early spermatids Late spermatids Spermatozoa (b) Events of spermatogenesis, showing the relative position of various spermatogenic cells