Chapter 21: The Male Reproductive System PDF

VetBooks.ir C H A P T E R 21 The Male Reproductive System TESTES Interstitial Tissue Seminiferous Tubules Spermatogenesis The Clonal Nature of Male Germ Cells Spermiogenesis Sertoli Cells INTRATESTICULAR DUCTS 439 440 441 442 446 447 448 449 EXCRETORY GENITAL DUCTS Epididymis Ductus or Vas Deferens...

VetBooks.ir C H A P T E R 21 The Male Reproductive System TESTES Interstitial Tissue Seminiferous Tubules Spermatogenesis The Clonal Nature of Male Germ Cells Spermiogenesis Sertoli Cells INTRATESTICULAR DUCTS 439 440 441 442 446 447 448 449 EXCRETORY GENITAL DUCTS Epididymis Ductus or Vas Deferens 450 450 451 T he male reproductive system consists of the testes, genital ducts, accessory glands, and penis (Figure 21–1). Testes produce sperm but also contain endocrine cells secreting hormones such as testosterone, which drives male reproductive physiology. Testosterone is important for spermatogenesis, sexual differentiation during embryonic and fetal development, and control of gonadotropin secretion in the pituitary. A metabolite of testosterone, dihydrotestosterone, also begins to act on many tissues during puberty (eg, male accessory glands and hair follicles). The genital ducts and accessory glands produce secretions required for sperm activity and contract to propel spermatozoa and the secretions from the penile urethra. These secretions provide nutrients for spermatozoa while they are confined to the male reproductive tract. Spermatozoa and the secretions of the accessory glands make up the semen (L. seed), which is introduced into the female reproductive tract by the penis. ››TESTES Each testis (or testicle) is surrounded by a dense connective tissue capsule, the tunica albuginea, which thickens on the posterior side to form the mediastinum testis. From this fibrous region, septa penetrate the organ and divide it into about 250 pyramidal compartments or testicular lobules (Figures 21–2 and 21–3). Each lobule contains sparse ACCESSORY GLANDS Seminal Vesicles Prostate Gland Bulbourethral Glands PENIS 451 452 453 455 456 SUMMARY OF KEY POINTS 457 ASSESS YOUR KNOWLEDGE 459 connective tissue with endocrine interstitial cells (or Leydig cells) secreting testosterone, and one to four highly convoluted seminiferous tubules in which sperm production occurs. The testes develop retroperitoneally in the dorsal wall of the embryonic abdominal cavity and are moved during fetal development to become suspended in the two halves of the scrotal sac, or scrotum, at the ends of the spermatic cords (Figure 21–2). During migration from the abdominal cavity, each testis carries with it a serous sac, the tunica vaginalis, derived from the peritoneum. This tunic consists of an outer parietal layer lining the scrotum and an inner visceral layer, covering the tunica albuginea on the anterior and lateral sides of the testis (Figure 21–2). Having evolved in cold-blooded animals, certain molecular events in the process of sperm formation cannot occur at the core body temperature of 37°C. A permissive temperature of about 34°C is maintained in the scrotal sac by various mechanisms. Each testicular artery is surrounded by a rich pampiniform venous plexus containing cooler blood from the testis, which draws heat from the arterial blood by a countercurrent heat-exchange system. Evaporation of sweat from the scrotum also contributes to heat loss. Relaxation or contraction of the thin dartos muscle of the scrotum and the cremaster muscles of the spermatic cords move the testes away from or closer to the body, respectively, allowing further control on testicular temperature. 439 21_Mescher_ch21_p439-459.indd 439 26/04/18 11:56 am VetBooks.ir 440 CHAPTER 21 FIGURE 21–1 The Male Reproductive System The male reproductive system. Ureters Urinary bladder Pubic symphysis Ampulla of ductus deferens Seminal vesicle Ejaculatory duct Prostate gland Ductus deferens Urogenital diaphragm Bulbourethral gland Urethra Anus Penis Epididymis Glans Testis Scrotum The diagram shows the locations and relationships of the testes, epididymis, glands, and the ductus deferens running from the scrotum to the urethra. The ductus deferens is located along the › ›› MEDICAL APPLICATION An excessive accumulation of serous fluid in one or both sides of the scrotal sac, termed a hydrocele, is the most common cause of scrotal swelling and a condition easily corrected surgically. Cryptorchidism (Gr. kryptos, hidden + orchis, testis), the failure of one or both testes to descend from the abdomen, occurs in about 4% of male neonates, but in most of these individuals the testes move to the scrotum during the first year. Bilateral cryptorchidism causes infertility if not surgically corrected by 2-3 years of age. Interstitial Tissue The interstitial tissue of the testis between the seminiferous tubules consists of sparse connective tissue containing fibroblasts, lymphatics, and blood vessels including fenestrated 21_Mescher_ch21_p439-459.indd 440 anterior and superior sides of the bladder as a result of the testes descending into the scrotum from the abdominal cavity during fetal development. capillaries. During puberty interstitial cells, or Leydig cells, develop as large round or polygonal cells with central nuclei and eosinophilic cytoplasm rich in small lipid droplets (Figures 21–2b and 21–4). These cells produce the steroid hormone testosterone, which promotes development of the secondary male sex characteristics. Testosterone is synthesized by enzymes present in the smooth ER and mitochondria similar to the system in adrenal cortical cells. Testosterone secretion by interstitial cells is triggered by the pituitary gonadotropin, luteinizing hormone (LH), which is also called interstitial cell-stimulating hormone (ICSH). Testosterone synthesis thus begins at puberty, when the hypothalamus begins producing gonadotropin-releasing hormone. In the late embryonic testes gonadotropin from the placenta stimulates interstitial cells to synthesize the testosterone needed for development of the ducts and glands of the male reproductive system. These fetal interstitial cells are very active during the third and fourth months of pregnancy, then 26/04/18 11:56 am FIGURE 21–2 441 Testes and seminiferous tubules. C H A P T E R VetBooks.ir Testes 2 1 Spermatic cord The Male Reproductive System Testes Blood vessels and nerves Ductus deferens SG Head of epididymis Duct of epididymis Seminiferous tubule Straight tubule SC M Efferent ductule Mediastinum testis (housing rete testis) Body of epididymis LS Septum PS Lobule Visceral layer of tunica vaginalis IC Parietal layer of tunica vaginalis Tunica albuginea b Tail of epididymis a The anatomy of a testis is shown. (a) The diagram shows a partially cutaway sagittal section of the testis. (b) A seminiferous tubule cross section shows spermatogonia (SG) near the periphery, near regress and become quiescent cells resembling fibroblasts until puberty when they resume testosterone synthesis in response to the pituitary gonadotropin. › ›› MEDICAL APPLICATION Both interstitial cell tumors and Sertoli cell tumors are rare. Most (95%) testicular cancer involves germ cell tumors, which only appear after puberty and are much more likely to develop in men with untreated cryptorchidism. Seminiferous Tubules Sperm are produced in the seminiferous tubules at a rate of about 2 × 108 per day in the young adult. Each testis has from 250 to 1000 such tubules in its lobules, and each tubule measures 150-250 μm in diameter and 30-70 cm in length. The combined length of the tubules of one testis totals about 250 m. 21_Mescher_ch21_p439-459.indd 441 nuclei of Sertoli cells (SC), primary spermatocytes (PS), and late spermatids (LS) near the lumen, with interstitial cells (IC) in the surrounding connective tissue. (X400; H&E) Each tubule is actually a loop linked by a very short, narrower segment, the straight tubule, to the rete testis, a labyrinth of epithelium-lined channels embedded in the mediastinum testis (Figures 21–2a and 21–3). About 10-20 efferent ductules connect the rete testis to the head of the epididymis (Figure 21–2a). Each seminiferous tubule is lined with a complex, specialized stratified epithelium called germinal or spermatogenic epithelium (Figure 21–2b). The basement membrane of this epithelium is covered by fibrous connective tissue, with an innermost layer containing flattened, smooth muscle-like myoid cells (Figure 21–2b), which allow weak contractions of the tubule. The germinal epithelium consists of two types of cells: Large nondividing Sertoli cells (Figure 21–4), which physically and metabolically support developing sperm cell precursors Dividing cells of the spermatogenic lineage (Figure 21–5a) 26/04/18 11:56 am VetBooks.ir 442 CHAPTER 21 FIGURE 21–3 The Male Reproductive System Lobules converging at rete testis. M ST ST RT S S ST S ST The dense capsule of the testis, the tunica albuginea, thickens on the posterior side as the mediastinum (M) testis, from which many thin septa (S) subdivide the organ into about 250 lobules. Each lobule contains one to four convoluted seminiferous tubules (ST) The cells of the spermatogenic lineage, comprising four or more concentric layers of cells in the germinal epithelium, develop from progenitor cells to fully formed sperm cells over a period of approximately 10 weeks. As shown in Figure 21–5 spermatogenesis, the first part of sperm production involves mainly mitosis and meiosis and is followed by spermiogenesis, the final differentiation process occurring in the haploid male germ cells. Spermatogenesis Spermatogenesis begins at puberty with proliferation of stem and progenitor cells called spermatogonia (Gr. sperma + gone, generation), small round cells about 12 μm in diameter. These cells occupy a basal niche in the epithelial wall of the tubules, next to the basement membrane and closely associated with Sertoli cell surfaces (Figures 21–5, 21–6, and 21–7). Different stages of spermatogonia development can be recognized by subtle changes in shape and staining properties of their nuclei. Spermatogonia with dark, ovoid nuclei act as stem cells, dividing infrequently and giving rise both to new 21_Mescher_ch21_p439-459.indd 442 ST in a sparse connective tissue interstitium. Each tubule is a loop attached by means of a short straight tubule to the rete testis (RT), a maze of channels embedded in the mediastinum testis. From the rete testis the sperm move into the epididymis. (X60; H&E) stem cells and to cells with more pale-staining, ovoid nuclei that divide more rapidly as transit amplifying (progenitor) cells (Figure 21–7). These type A spermatogonia each undergo several unique clonal divisions that leave most of the cells interconnected as a syncytium. These become type B spermatogonia, which have more spherical and pale nuclei. Each type B spermatogonium then undergoes a final mitotic division to produce two cells that grow in size and become primary spermatocytes, which are spherical cells with euchromatic nuclei (Figures 21–6 and 21–7). Primary spermatocytes replicate their DNA, so each chromosome consists of duplicate chromatids, and enter meiosis, during which homologous chromosomes come together in synapsis, DNA recombination occurs, and two rapid cell divisions produce haploid cells (see Chapter 3). The primary spermatocyte has 46 (44 + XY) chromosomes, the diploid number, and a DNA content of 4N. (The letter N denotes either the haploid number of chromosomes, 23 in humans, or the amount of DNA in this set.) Soon after their formation, these cells enter the first meiotic prophase 26/04/18 11:56 am FIGURE 21–4 443 Interstitial cells and Sertoli cells. C H A P T E R VetBooks.ir Testes CT M The Male Reproductive System Testes ST ST 2 1 M IC IC IC M a b S S c d (a) Seminiferous tubules (ST) are surrounded by stroma containing many interstitial cells (IC), typically located near capillaries, which secrete androgens. The seminiferous tubule wall consists of a unique germinal epithelium composed of columnar Sertoli cells and dividing spermatogenic stem cells. Seen around the seminiferous tubules are myoid cells (M) with elongated nuclei, the contractions of which help move fluid and mature sperm in the tubules. (X400; H&E) (b) A plastic section shows lipid droplets filling the cytoplasm of the clumped interstitial cells (IC), or Leydig cells, in the connective tissue (CT) between tubules. Such cytoplasm is typical of steroidsecreting endocrine cells and here indicates cells actively secreting testosterone. The epithelium of a nearby seminiferous tubule is immediately surrounded by myoid cells (M). (X400; PT) (c) Immunohistochemistry of a seminiferous tubule wall with antibodies against prosaposin, a glycoprotein abundant in Sertoli cells. The yellow fluorescent stain indicates the tall columnar shape of Sertoli cells (S) and the dendritic nature of their apical ends. Sertoli 21_Mescher_ch21_p439-459.indd 443 e cells support all spermatogenic cells physically and metabolically, phagocytize debris, and have endocrine roles affecting spermatogenesis and fetal development of the male reproductive tract. (400X; Immunofluorescence) (d) Immunohistochemistry of the seminiferous tubule with the same antibodies labeled with peroxidase shows the tall Sertoli cells in brown. This bright-field preparation also shows the close association of the Sertoli cells with the numerous spermatogenic cells, whose nuclei are stained with hematoxylin. (400X; Immunoperoxidase & hematoxylin) (e) Lower magnification of the same preparation as part c shows the distribution and density of Sertoli cells (S) in the seminiferous tubules. (200X; Immunofluorescence) (FIGURE 21–4c-e, used with permission from Dr. Richard Sharpe and Chris McKinnell, University of Edinburgh, MRC Centre for Reproductive Health, UK.) 26/04/18 11:57 am VetBooks.ir 444 CHAPTER 21 FIGURE 21–5 The Male Reproductive System Spermatogenesis and spermiogenesis. Interstitial cells Developing acrosomal cap Interstitial space Spermatid nucleus Mitochondria 46 Spermatogonium Basal compartment 1 Tight junctions Sertoli cell 46 2 Developing flagellum 23 Acrosomal cap Meiosis II 3 Adluminal compartment Spermatid 23 23 23 Spermatid nucleus Meiosis I 23 Secondary spermatocyte Developing acrosomal cap Primary spermatocyte 46 23 23 4 23 Spermatid nucleus 23 23 Developing flagellum Spermatids differentiating 23 23 23 Nucleus Excess cytoplasm 23 Acrosome Mitochondria Sperm Acrosome (a) Spermatogenesis 1 The spermatogenic progenitor cells, called spermatogonia, are diploid cells containing 46 chromosomes (23 pairs). Mitotic divisions of these cells produce new spermatogonia and cells called primary spermatocytes which are committed to meiosis. Nucleus Midpiece 2 Each new primary spermatocyte transiently disassembles the tight junctions of the blood-testis barrier and moves from the tubule’s basal compartment to the adluminal compartment. At the same time these cells replicate their DNA, enter meiosis I, and undergo synapsis. The first meiotic division produces two haploid secondary spermatocytes each with 23 chromosomes. Tail (flagellum) 3 Meiosis II is rapid as chromatids in the secondary spermatocyte chromosomes separate into two smaller haploid cells, the spermatids. 4 Near the lumen but still embedded in Sertoli cells, the spermatids differentiate and undergo the morphological changes required to become motile and capable of fertilization. (a) The diagram shows two large, columnar Sertoli cells with their surfaces binding many germ cells in various stages of spermatogenesis. Near the basement membrane are spermatogonia, which divide by mitosis to produce both more spermatogonia and also primary spermatocytes undergoing meiosis to produce secondary spermatocytes and then haploid spermatids differentiating as sperm. Newly formed spermatocytes temporarily disassemble the tight junctions between Sertoli cells comprising the “blood-testis barrier” in order to move 21_Mescher_ch21_p439-459.indd 444 Head Mitochondria Sperm (b) Spermiogenesis from the basal compartment to the adluminal compartment of the tubule. (b) Spermiogenesis is the process of cell differentiation by which spermatids become sperm. The major changes that occur during spermiogenesis are shown here. These involve flattening of the nucleus, formation of an acrosome resembling a large lysosome, growth of a flagellum (tail) from the basal body, reorganization of the mitochondria in the midpiece region, and shedding of unneeded cytoplasm. 26/04/18 11:57 am FIGURE 21–6 445 Seminiferous tubules: Sertoli cells and spermatogenesis. C H A P T E R VetBooks.ir Testes 2 1 SC LS SG M PS SC The Male Reproductive System Testes ES SG LS PS SG SC M F PS PS M a (a, b) In these cross sections of seminiferous tubules, most of their cell types can be seen. Outside the tubules are myoid cells (M) and fibroblasts (F). Inside near the basement membrane are many prominent spermatogonia (SG), small cells that divide mitotically but give rise to a population that enters meiosis. The primary spermatocytes (PS) remain for 3 weeks in prophase of the first meiotic division during which recombination occurs. Primary spermatocytes are the largest spermatogenic cells and are usually abundant at all levels between the basement membrane and the lumen. Each that lasts about 3 weeks. Most spermatocytes seen in sections of testis are in this phase of meiosis. The primary spermatocytes are the largest cells of the spermatogenic lineage and are characterized by the presence of partially condensed chromosomes in various stages of synapsis and recombination (Figure 21–6). Homologous chromosomes separate in the first meiotic division, which produces smaller cells called secondary spermatocytes (Figures 21–5a and 21–7) with only 23 chromosomes (22 + X or 22 + Y), but each still consists of two chromatids so the amount of DNA is 2N (see Chapter 3). Secondary spermatocytes are rare in testis sections because 21_Mescher_ch21_p439-459.indd 445 b divides to form two secondary spermatocytes, which are rarely seen because they undergo the second meiotic division almost immediately to form two haploid spermatids. Newly formed round, early spermatids (ES) differentiate and lose volume in becoming late spermatids (LS) and finally highly specialized sperm cells. All stages of spermatogenesis and spermiogenesis occur with the cells intimately associated with the surfaces of adjacent Sertoli cells (SC) that perform several supportive functions. (Both X750; H&E) they are very short-lived cells that remain in interphase only briefly and quickly undergo the second meiotic division. Division of each secondary spermatocyte separates the chromatids of each chromosome and produces two haploid cells called spermatids each of which contains 23 chromosomes (Figures 21–5a, 21–6, and 21–7). Because no S phase (DNA replication) occurs between the first and second meiotic divisions, the amount of DNA per cell is reduced by half when the chromatids separate and the cells formed are haploid (1N). With fertilization, a haploid ovum and sperm produced by meiosis unite and the normal diploid chromosome number is restored. 26/04/18 11:57 am VetBooks.ir 446 CHAPTER 21 FIGURE 21–7 The Male Reproductive System Clonal nature of spermatogenesis. Type A spermatogonia (stem cells) Type A spermatogonia (progenitor cells) Mitosis Type B spermatogonia Primary spermatocytes First meiotic division Secondary spermatocytes Second meiotic division Spermatids Mature spermatozoa Spermiogenesis (differentiation) + Residual bodies A subpopulation called type A spermatogonia act as stem cells, dividing to produce new stem cells and other type A spermatogonia that undergo transit amplification as progenitor cells. Mitosis in these cells occurs with incomplete cytokinesis, leaving the cytoplasm of most or all of these cells connected by cytoplasmic bridges. Type A spermatogonia divide mitotically two or three more times, then differentiate as type B spermatogonia that undergo a final round of mitosis to form the cells that then The Clonal Nature of Male Germ Cells The stem cells produced by mitotic divisions of spermatogonia remain as separate cells. However, all subsequent divisions of the daughter cells, which become transit amplifying progenitor cells, have incomplete cytokinesis after telophase and the 21_Mescher_ch21_p439-459.indd 446 enter meiosis and become primary spermatocytes (only two are shown), still interconnected. The intercellular bridges persist during the first and second meiotic divisions, but they are lost as the haploid spermatids differentiate into sperm (spermiogenesis) and shed excess cytoplasm as residual bodies. The interconnections of the cells allow free cytoplasmic communication among the cells and help coordinate their progress through meiosis and spermiogenesis. cells remain attached to one another by intercellular bridges of cytoplasm (Figure 21–7). These allow free cytoplasmic communication among the cells during their remaining mitotic and meiotic divisions. Although some cells degenerate without completing spermatogenesis and some cells may 26/04/18 11:57 am Spermiogenesis, the final phase of sperm production, is the temperature-sensitive process by which spermatids differentiate into spermatozoa, which are highly specialized to deliver male DNA to the ovum. No cell division occurs during this FIGURE 21–8 In the Golgi phase the cytoplasm contains a prominent Golgi apparatus near the nucleus, mitochondria, paired centrioles, and free ribosomes. Small proacrosomal vesicles from the Golgi apparatus coalesce as a single membrane-limited acrosomal cap close to one end of the nucleus (Figures 21–5b and 21–8). The centrioles migrate to a position farthest from the acrosomal cap and one acts as a basal body, organizing the axoneme of the flagellum which is structurally and functionally similar to that of a cilium (see Chapter 2). The Male Reproductive System Testes Spermiogenesis process, and as with spermatogenesis the cells involved remain associated with Sertoli cells. The haploid spermatids are small (7-8 μm in diameter) cells near the lumen of the seminiferous tubules (Figures 21–5a and 21–6b). Spermiogenesis includes formation of the acrosome (Gr. akron, extremity + soma, body), condensation and elongation of the nucleus, development of the flagellum (L, whip), and the loss of much of the cytoplasm. The end result is the mature spermatozoon, which is released from the Sertoli cell surface into the tubule’s lumen. Spermiogenesis is commonly divided into four phases: 2 1 separate, clones of approximately a hundred cells may remain linked through meiosis. The complete significance of this spermatogenic syncytium is not clear, but the cytoplasmic bridges allow the haploid cells to be supplied with products of the complete diploid genome, including proteins and RNA encoded by genes on the X or Y chromosome missing in their haploid nuclei. The germ cells finally become separated from one another during differentiation (Figure 21–7). The cellular events and changes between the final mitoses of spermatogonia and the formation of spermatids take about 2 months. The spermatogenic cells are not randomly distributed in the spermatogenic epithelium. Cells at different stages of development are typically grouped together along the tubule, with the intercellular bridges helping to coordinate their divisions and differentiation. 447 C H A P T E R VetBooks.ir Testes Differentiating spermatid. A M N F The head of a late spermatid during spermiogenesis is completely enclosed by Sertoli cell cytoplasm. As shown by TEM, the sperm nucleus (N) is half covered by the thin Golgi-derived acrosomal cap (A). The flagellum (F) can be seen emerging from a basal body 21_Mescher_ch21_p439-459.indd 447 near the nucleus on the side opposite the acrosome. A perinuclear bundle of microtubules and actin filaments called the manchette (M) transports vesicles, mitochondria, and keratins into position as the spermatid elongates. (X7500) 26/04/18 11:57 am 448 CHAPTER 21 The Male Reproductive System VetBooks.ir In the cap phase the acrosomal cap spreads over about half of the condensing nucleus (Figures 21–5b and 21–8). The acrosome is a specialized type of lysosome containing hydrolytic enzymes, mainly hyaluronidase and a trypsin-like protease called acrosin. These enzymes are released when a spermatozoon encounters an oocyte and the acrosomal membrane fuses with the sperm’s plasma membrane. They dissociate cells of the corona radiata and digest the zona pellucida, both structures that surround the egg (see Chapter 22). This process, the acrosomal reaction, is one of the first steps in fertilization. In the acrosome phase the head of the developing sperm, containing the acrosome and the condensing nucleus, remains embedded in the Sertoli cell while the growing axoneme extends into the lumen of the tubule (Figure 21–6b). Nuclei become more elongated and very highly condensed, with the histones of nucleosomes replaced by small basic peptides called protamines. Flagellum growth continues distally in the tail and mitochondria aggregate around it proximally to form a thickened middle piece where the ATP for flagellar movements is generated (Figure 21–5). In the maturation phase of spermiogenesis, unneeded cytoplasm is shed as a residual body from each spermatozoon and remaining intercellular bridges are lost. Fully formed, but not yet functional or mobile, sperm (Figure 21–5) are released into the lumen of the seminiferous tubule. › ›› MEDICAL APPLICATION Decreased semen quality, which is frequently idiopathic (arising from unknown causes), is a major cause of male infertility. Common features of poor semen quality include oligospermia (ejaculate volume > 2 mL), sperm cell density less than 10-20 million/mL, abnormal sperm morphology, and flagellar defects that impair sperm motility. cells are seen to contain abundant SER, some rough ER, welldeveloped Golgi complexes, numerous mitochondria, and lysosomes (Figure 21–8). Their nuclei are typically ovoid or triangular, euchromatic, and have a prominent nucleolus, features that allow Sertoli cells to be distinguished from the neighboring germ cells (Figure 21–6). Important in Sertoli cell function are elaborate tight occluding junctions between their basolateral membranes, which form a blood-testis barrier within the seminiferous epithelium (Figure 21–5a). The tightest blood-tissue barrier in mammals, this physical barrier is one part of a system preventing autoimmune attacks against the unique spermatogenic cells, which first appear after the immune system is mature and central self-tolerance is well established. Spermatogonia lie in a basal compartment of the tubule, below the tight junctions and not sealed off from the vascularized interstitial tissue containing lymphocytes and other immune cells. Newly formed primary spermatocytes temporarily disassemble the adhesion molecules of the local occluding junctions and move into the tubule’s adluminal compartment while still adhering to Sertoli cells (Figure 21–5a). Like the spermatogonia, all spermatocytes and spermatids lie within invaginations of the Sertoli cells surfaces. Adluminal migration occurs without compromising the blood-testis barrier, which is all the more impressive when one remembers that the germ cells remain linked by intercellular bridges. Sertoli cells are also connected and coupled ionically by gap junctions, which may help regulate the transient changes in the occluding junctions and synchronize activities in the spermatogenic cells. As the flagellar tails of the spermatids develop, they appear as tufts extending from the apical ends of the Sertoli cells. Related to their role in establishing the blood-testis barrier, Sertoli cells have three general functions: Support, protection, and nutrition of the develop- Sertoli Cells The Sertoli cells, named after Enrico Sertoli (1842-1910) who first demonstrated their physiologic significance, are tall “columnar” epithelial cells, which nourish the spermatogenic cells and divide the seminiferous tubules into two (basal and adluminal) compartments (Figure 21–4c to e). All cells of the spermatogenic lineage are closely associated with the extended surfaces of Sertoli cells and depend on them for metabolic and physical support. Sertoli cells adhere to the basal lamina and their apical ends extend to the lumen, as shown immunohistochemically in Figure 21–4c to e. In routine preparations the outlines of Sertoli cells surrounding the spermatogenic cells are very poorly defined (Figures 21–6 and 21–8). Each Sertoli cell supports 30-50 developing germ cells. Ultrastructurally Sertoli 21_Mescher_ch21_p439-459.indd 448 ing spermatogenic cells: Because spermatocytes, spermatids, and developing sperm are isolated from plasma proteins and nutrients by the blood-testis barrier, they depend on Sertoli cells for production or transport into the lumen of metabolites and nutritive factors such as the iron-transport protein transferrin. Thus, while protecting spermatogenic cells from circulating immune components, Sertoli cells supply many plasma factors needed for cell growth and differentiation. Exocrine and endocrine secretion: Sertoli cells continuously release into the seminiferous tubules water that carries new sperm out of the testis. Production of nutrients and androgen-binding protein (ABP), which concentrates testosterone to a level required for spermiogenesis, is promoted by follicle-stimulating hormone (FSH). As endocrine cells, they secrete the 39-kDa glycoprotein inhibin, which feeds back on the anterior pituitary gland to suppress FSH synthesis and release. In the fetus Sertoli cells also secrete a 140-kDa glycoprotein called müllerian-inhibiting substance (MIS) that 26/04/18 11:57 am 449 causes regression of the embryonic müllerian (paramesonephric) ducts; in the absence of MIS these ducts persist and become parts of the female reproductive tract. Phagocytosis: During spermiogenesis, excess cytoplasm shed as residual bodies is phagocytosed and digested by Sertoli cell lysosomes. No proteins from sperm normally pass back across the blood-testis barrier.      TABLE 21–1 Summary of histology and functions of male genital ducts. Location Epithelium Support Tissues Function(s) Seminiferous tubules Testicular lobules Spermatogenic, with Sertoli cells and germ cells Myoid cells and loose connective tissue Produce sperm Straight tubules (tubuli recti) Periphery of the mediastinum testis Sertoli cells in proximal portions, simple cuboidal in distal portions Connective tissue Convey sperm into the rete testis Rete testis In mediastinum testis Simple cuboidal cells with microvilli and single cilia Dense irregular connective Channels with sperm tissue from all seminiferous tubules Efferent ductules From rete testis to head of epididymis Alternating patches of simple cuboidal nonciliated and simple columnar ciliated Thin circular layer of smooth muscle and vascular loose connective tissue Absorb most fluid from seminiferous tubules; convey sperm into the epididymis Epididymal duct Head, body, and tail of the epididymis Pseudostratified columnar, with small basal cells and tall principal cells bearing long stereocilia Circular smooth muscle initially, with inner and outer longitudinal layers in the tail Site for sperm maturation and shortterm storage; expels sperm at ejaculation Ductus (vas) deferens Extends from epididymis to ejaculatory ducts in prostate gland Pseudostratified columnar, with fewer stereocilia Fibroelastic lamina propria and three very thick layers of smooth muscle Carries sperm by rapid peristalsis from the epididymis to the ejaculatory ducts Ejaculatory ducts In prostate, formed by union of ductus deferens and ducts of the seminal vesicles Pseudostratified and simple columnar Fibroelastic tissue and smooth muscle of the prostate stroma Mix sperm and seminal fluid; deliver semen to urethra, where prostatic secretion is added 21_Mescher_ch21_p439-459.indd 449 The Male Reproductive System Intratesticular Ducts Acute or chronic inflammation of the testis, orchitis, frequently involves the ducts connecting this organ to the epididymis. Common forms of orchitis are produced by infective agents and occur secondarily to a urinary tract infection or a sexually transmitted pathogen such as Chlamydia or Neisseria gonorrhoeae entering the testis from the epididymis or via the lymphatics. Acute epididymitis is a result of sexually transmitted infections such as gonorrhea or Chlamydia infection and causes intrascrotal pain and tenderness. Persistent inflammation of the epididymis, such as that associated with gonorrhea infections, includes massive invasion by leukocytes into the infected duct, stimulating fibrosis that obstructs the epididymis and is a common cause of male infertility. The intratesticular ducts are the straight tubules (or tubuli recti), the rete testis, and the efferent ductules (Figure 21–2), all of which carry spermatozoa and liquid from the seminiferous tubules to the duct of the epididymis (Table 21–1). The loops of seminiferous tubules join the rete testis by the short straight tubules, which are lined initially only by Sertoli cells (Figure 21–9). These empty into the rete testis, an interconnected network of channels lined with cuboidal epithelium and supported by connective tissue of the mediastinum (Figure 21–9). The rete testis drains into about 20 efferent ductules lined by an unusual epithelium in which groups of nonciliated cuboidal cells alternate with groups of taller ciliated cells and give the tissue a characteristic scalloped appearance (Figure 21–10). The nonciliated cells absorb some of the fluid secreted by the Sertoli cells of seminiferous tubules. This absorption and the ciliary activity create a fluid flow, which carries sperm passively out of the testis toward the epididymis. A thin layer of circularly oriented smooth muscle cells in the walls of efferent ductules aids the movement of sperm into the duct of the epididymis. 2 1 › ›› MEDICAL APPLICATION ››INTRATESTICULAR DUCTS C H A P T E R VetBooks.ir Intratesticular Ducts 26/04/18 11:57 am VetBooks.ir 450 CHAPTER 21 FIGURE 21–9 The Male Reproductive System Seminiferous tubules, straight tubules, and rete testis. R R R R R CT T CT R R R T S S a (a) The seminiferous tubules (S) drain into short, much narrower straight tubules (T), which connect to the rete testis (R), a network of channels embedded along with blood vessels (V) in the connective tissue (CT) of the mediastinum testis. (X120; H&E) b tubule (T). Initially, the straight tubule wall has only tall Sertoli cells devoid of germ cells. The wall becomes a simple cuboidal epithelium near its connection to the rete testis (R), which is also lined with simple cuboidal epithelium. (X300; H&E) (b) At higher magnification the enclosed portion of part a shows the transition from wide seminiferous tubule (S) to the straight ››EXCRETORY GENITAL DUCTS The excretory genital ducts are those of the epididymis, the ductus (or vas) deferens, and the urethra. They transport sperm from the scrotum to the penis during ejaculation. Epididymis The long, highly coiled duct of the epididymis, surrounded by connective tissue, lies in the scrotum along the superior and posterior sides of each testis (Figure 21–2). Nearly 6 m in length, the epididymis includes a head region where the efferent ductules enter, a body where sperm cells undergo further subtle modifications, and a tail where sperm are stored until ejaculation. The epididymal duct is lined with pseudostratified columnar epithelium consisting of columnar principal cells, with 21_Mescher_ch21_p439-459.indd 450 characteristic long stereocilia and small round stem cells (Figure 21–11). The principal cells remove most of the water and residual bodies entering the epididymis with the sperm and secrete various products, greatly changing the fluid in which sperm are suspended. In addition to the secretion of many proteins and glycoproteins by exocytosis, these epithelial cells release abundant 25-300-nm-diameter exosomes and other vesicles from multivesicular bodies and by apocrine secretion. These contain many different proteins, glycoproteins, glycolipids, and lipids, their composition varying along the epididymis. Passage of sperm through the duct of the epididymis normally takes 2-4 weeks, during which time the spermatozoa cell membranes bind and integrate many components secreted by the principal cells and released from their exosomes. Without these modifications sperm cannot 26/04/18 11:57 am FIGURE 21–10 Rete testis and efferent ductules. E b (a) Rete testis (R) channels have simple cuboidal epithelium and are usually embedded in dense connective tissue. (X350; Mallory trichrome) (b) The efferent ductules (E) draining the rete testis have a lining with a characteristic scalloped appearance in section, consisting of patches of simple cuboidal cells with water-absorbing microvilli alternating with patches of taller cells with cilia (arrows). (X350; H&E) participate in fertilization, but the significance of most factors involved is not well-understood. In general, changes within spermatozoa while passing through the epididymis include development of competence for forward motility, final modifications within the acrosome, and importantly, reorganization of the cell membrane sur- rounding the sperm head, including the addition of cholesterol and other “decapacitation factors”, which block the acrosomal reaction, a key event in fertilization. The fertilizing ability of spermatozoa is inhibited until their cell membranes are modified again as part of the capacitation process, which occurs in the female reproductive tract. 21_Mescher_ch21_p439-459.indd 451 Ductus or Vas Deferens Each ductus (or vas) deferens, a long straight tube with a thick, muscular wall and a relatively small lumen, leaves the scrotum and continues toward the prostatic urethra where it empties (Figure 21–1). As shown in Figure 21–12, its mucosa is slightly folded longitudinally, the lamina propria contains many elastic fibers, and the epithelial lining is pseudostratified with some cells having sparse stereocilia. The very thick muscularis consists of longitudinal inner and outer layers and a middle circular layer. The muscles produce strong peristaltic contractions during ejaculation, which rapidly move sperm along this duct from the epididymis. The ductus (vas) deferens forms part of the spermatic cord, which also includes the testicular artery, the pampiniform plexus, and nerves (Figure 21–2). Following the general path along which the embryonic testes descend, each ductus passes over the urinary bladder where it enlarges as an ampulla (L. a small bottle) where the epithelium is thicker and more extensively folded (Figure 21–13). Within the prostate gland, the ends of the two ampullae merge with the ducts of the two seminal vesicles, joining these ducts to form the ejaculatory ducts which open into the prostatic urethra. The histology of the intratesticular and excretory ducts is summarized in Table 21–1. The Male Reproductive System Accessory Glands a The epididymal duct is surrounded by a thin, circular layer of smooth muscle cells, supplemented in the tail region with the addition of inner and outer longitudinal layers. At ejaculation peristaltic contractions of these muscle layers rapidly empty stored sperm from this tail region, which is continuous with the ductus deferens. 2 1 R 451 C H A P T E R VetBooks.ir Accessory Glands › ›› MEDICAL APPLICATION The accessibility of the ductus (vas) deferens in the spermatic cords allows for the most common surgical method of male contraception: vasectomy. In this procedure a very small incision is made through the scrotal skin near the two ducts and each vas is exposed, cut, and the two ends (or only the end leading to the abdomen) are cauterized and tied. After vasectomy sperm are still produced, but they degenerate and are removed by macrophages in the epididymis (and in the scrotal sac if the short portion of the vas is left open-ended.) Inflammatory and other changes occur in the mucosa of the epididymis, but serious adverse effects of vasectomy are usually minimal. A vasectomy may be reversed by surgically reconnecting the two ends of each ductus deferens. However, even successful surgery very often fails to restore fertility, due to incomplete sperm maturation in the epididymis changed by postvasectomy inflammation. ››ACCESSORY GLANDS The accessory glands of the male reproductive tract produce secretions, which become mixed with sperm during ejaculation to produce semen and are essential for reproduction. 26/04/18 11:57 am VetBooks.ir 452 CHAPTER 21 FIGURE 21–11 The Male Reproductive System Epididymis. B TV P V c SM V DE S a (a) The long, coiled duct of the epididymis (DE), where sperm undergo maturation and short-term storage, is enclosed by connective tissue with many blood vessels (V) and covered by a capsule and the tunica vaginalis (TV). The duct is lined by a pseudostratified columnar epithelium with long stereocilia (arrows). (X140; H&E) (b) The columnar epithelium of the epididymal duct is surrounded by a thin circular layer of smooth muscle (SM) cells and its lumen The accessory genital glands are the seminal vesicles (or glands), the prostate gland, and the bulbourethral glands (Figure 21–13). Seminal Vesicles The two seminal vesicles consist of highly tortuous tubes, each about 15-cm long, enclosed by a connective tissue capsule. The unusual mucosa of the tube displays a great number of thin, complex folds, which fill much of the lumen (Figure 21–14). The folds are lined with simple or pseudostratified columnar epithelial cells rich in secretory granules. The lamina propria contains elastic fibers and is surrounded by smooth muscle 21_Mescher_ch21_p439-459.indd 452 b contains sperm (S). The smooth muscle becomes thicker and a longitudinal layer develops in the body and tail of the epididymis. (X400; H&E) (c) The inset photo shows the epithelium with tall principal cells (P) with stereocilia and fewer small basal stem cells (B). Intraepithelial lymphocytes are also commonly seen in the epididymal duct. (X500; H&E) with inner circular and outer longitudinal layers, which empty the gland during ejaculation. The seminal vesicles are exocrine glands in which production of their viscid, yellowish secretion depends on testosterone. Fluid from seminal vesicles typically makes up about 70% of the ejaculate and its components include the following: Fructose, a major energy source for sperm, as well as inositol, citrate, and other metabolites Prostaglandins, which stimulate activity in the female reproductive tract Fibrinogen, which allows semen to coagulate after ejaculation 26/04/18 11:57 am FIGURE 21–12 453 Ductus deferens. C H A P T E R VetBooks.ir Accessory Glands LP 2 1 E The Male Reproductive System Accessory Glands b M L-SM C-SM L-SM LP A a (a) A cross section of the vas deferens shows the mucosa (M), a thick muscularis with inner and outer layers of longitudinal smooth muscle (L-SM) and an intervening layer of circular smooth muscle (C-SM), and an external adventitia (A). The muscularis is specialized for powerful peristaltic movement of sperm at ejaculation. (X60; H&E) c (b) The lamina propria (LP) is rich in elastic fibers and the thick epithelial lining (E) shows longitudinal folds (X150; Mallory trichrome). (c) Higher magnification of the mucosa shows that the epithelium is pseudostratified with basal cells and many columnar cells, some with stereocilia. (X400; H&E) Prostate Gland The prostate gland is a dense organ surrounding the urethra below the bladder. It is approximately 2 cm × 3 cm × 4 cm in size and weighs about 20 g. The prostate consists of a collection of 30-50 tubuloacinar glands embedded in a dense fibromuscular stroma in which smooth muscle contracts at ejaculation (Figure 21–13b). Ducts from individual glands may converge but all empty directly into the prostatic urethra, which runs through the center of the prostate. As shown in Figure 21–15, the glands are arranged in three major zones around the urethra: The transition zone occupies only about 5% of the prostate volume, surrounds the superior portion of 21_Mescher_ch21_p439-459.indd 453 the urethra, and contains the periurethral mucosal glands. The central zone comprises 25% of the gland’s tissue and contains the periurethral submucosal glands with longer ducts. The peripheral zone, with about 70% of the organ’s tissue, contains the prostate’s main glands with still longer ducts (Figure 21–16). The tubuloacinar glands of the prostate are lined by simple or pseudostratified columnar epithelium and produce a fluid that contains a complex mixture of exosomes, various glycoproteins, enzymes, and small molecules such as prostaglandins and is stored until ejaculation. One clinically important 26/04/18 11:57 am VetBooks.ir 454 CHAPTER 21 FIGURE 21–13 The Male Reproductive System Accessory glands of the male reproductive tract. Urinary bladder Ureter F Ampulla Seminal vesicle Ejaculatory duct Prostate gland Prostatic urethra Bulbourethral gland (b) Seminal vesicle CA Membranous urethra Urogenital diaphragm G Ductus deferens G Corpus cavernosum Epididymis (c) Prostate gland Testis M M Penis Corpus spongiosum Spongy urethra Glans (d) Bulbourethral gland (a) Posterior view (a) Three sets of glands connect to the ductus deferens or urethra: the paired seminal vesicles, the prostate, and the paired bulbourethral glands. The first two types of glands contribute the major volume to semen and the latter produces a secretion that lubricates the urethra before ejaculation. (c) The prostate gland has many individual tubuloacinar glands (G), some containing concretions called corpora amylacea (CA). (d) The small, paired bulbourethral glands consist mainly of mucous acini (M). (b,c,d: X80; H&E) (b) Seminal vesicles have a large number of characteristically thin mucosal folds (F) with a large surface area. product of the prostate is prostate-specific antigen (PSA), a 34-kDa serine protease, which helps liquefy coagulated semen for the slow release of sperm after ejaculation. Small amounts of PSA also leak normally into the prostatic vasculature; elevated levels of circulating PSA indicate abnormal glandular mucosa typically due to prostatic carcinoma or inflammation. 21_Mescher_ch21_p439-459.indd 454 Small spherical concretions, up to 2 mm in diameter and often partially calcified, are normally present in the lumens of many prostatic tubuloacinar glands (Figure 21–16). These concretions, called corpora amylacea, containing primarily deposited glycoproteins and keratan sulfate, may become more numerous with age but seem to have no physiologic or 26/04/18 11:58 am FIGURE 21–14 455 Seminal vesicles. LP C H A P T E R VetBooks.ir Accessory Glands LP SM 2 1 a SM SM b The seminal vesicles are paired exocrine glands, which secrete most seminal fluid, including sperm nutrients. (a) A low-power micrograph shows that each consists of a coiled duct with a mucosa organized with many thin folds (arrows) in the lumen (L) and surrounded by two layers of smooth muscle (SM). (X20; H&E) FIGURE 21–15 gland. Organization of the prostate Seminal vesicle Vas deferens Peripheral zone Ejaculatory ducts Central zone Transitional zone Prostatic urethra Membranous urethra The prostate consists of 30-50 branched tubuloacinar glands organized into three layers, shown here schematically. Around the prostatic urethra is the transition zone containing mucosal glands. Surrounding most of that zone is the intermediate central zone, which contains the submucosal glands. The outermost and largest layer is the peripheral zone, which contains the most numerous main glands. 21_Mescher_ch21_p439-459.indd 455 c (b, c) The mucosal folds include smooth muscle (SM), a thin lamina propria (LP), and a simple columnar epithelium of principal secretory cells. (Both: X300. b: H&E; c: PSH) The Male Reproductive System Accessory Glands L clinical significance. The prostate is surrounded by a fibroelastic capsule, from which septa extend and divide the gland into indistinct lobes. Like the seminal vesicles, the prostate’s structure and function depend on the level of testosterone. › ›› MEDICAL APPLICATION The prostate gland is prone to three common problems: (1) chronic prostatitis, usually involving bacteria or other infectious agents; (2) nodular hyperplasia or benign prostatic hypertrophy, occurring mainly in the periurethral mucosal glands where it often leads to compression of the urethra and problems with urination; and (3) prostate cancer (adenocarcinoma), the most common cancer in nonsmoking men, occurring mainly in glands of the peripheral zone. Bulbourethral Glands The paired round bulbourethral glands (or Cowper glands), 3-5 mm in diameter, are located in the urogenital diaphragm (Figure 21–13) and empty into the proximal part of the penile urethra. Each gland has several lobules with tubuloacinar secretory units surrounded by smooth muscle cells and lined by a mucus-secreting simple columnar epithelium that is also 26/04/18 11:58 am VetBooks.ir 456 CHAPTER 21 FIGURE 21–16 The Male Reproductive System Prostate gland. LP S E M LP CA CA G LP S E LP a b M (a) The prostate has a dense fibromuscular stroma (S) in which are embedded a large number of small tubuloacinar glands (G). (X20; H&E) (b) A micrograph of one prostatic gland, showing a corpus amylaceum (CA) concretion and the secretory epithelium (E) surrounded testosterone-dependent (Figure 21–13d). During erection the bulbourethral glands, as well as numerous, very small, and histologically similar urethral glands along the penile urethra, release a clear mucus-like secretion, which coats and lubricates the urethra in preparation for the imminent passage of sperm. ››PENIS The penis consists of three cylindrical masses of erectile tissue, plus the penile urethra, surrounded by skin (see Figure 21–1). Two of the erectile masses—the corpora cavernosa—are dorsal; the ventral corpus spongiosum surrounds the urethra (Figure 21–17). At its end the corpus spongiosum expands, forming the glans (Figure 21–13a). Most of the penile urethra is lined with pseudostratified columnar epithelium. In the glans, it becomes stratified squamous epithelium continuous with that of the thin epidermis covering the glans surface. Small mucus-secreting urethral glands are found along the length of the penile urethra. In uncircumcised men the glans 21_Mescher_ch21_p439-459.indd 456 M c by a thin lamina propria (LP) and thick smooth muscle (M). (X122; H&E) (c) Higher magnification shows the lamellar nature of a corpus amylaceum (CA) and the pseudostratified columnar epithelium underlain by lamina propria (LP). (X300; Mallory trichrome) FIGURE 21–17 Structure of the penis. Dorsal vein (blue), artery (red), and nerve (yellow) Tunica albuginea Central artery Venous spaces Deep dorsal vein Corpus cavernosum Deep fascia Superficial fascia Corpus spongiosum Skin Spongy urethra A diagram of the penis in transverse section shows the relationships of the three erectile bodies, the tunica albuginea, and major blood vessels. Compare this section to the longitudinal diagram in Figure 21–13. 26/04/18 11:58 am FIGURE 21–18 Penis. V S TA CC TA U S CS › ›› MEDICAL APPLICATION The corpus spongiosum (CS) is on the ventral side of the penis and surrounds the urethra (U). Two corpora cavernosa (CC) occupy most of the dorsal side and are surrounded by dense, fibrous tunica albuginea (TA). Along the dorsal side run the major blood vessels (V) and deep in each corporal mass of erectile tissue are smaller blood vessels (V), including the central arteries. The penis is covered loosely by skin (S), which distally forms the large foreskin fold and becomes thin over the glans. (X15; H&E) is covered by the prepuce or foreskin, a retractable fold of thin skin with sebaceous glands on the internal surface. The corpora cavernosa are each surrounded by a dense fibroelastic layer, the tunica albuginea (Figures 21–17 and 21–18). All three erectile tissues consist of many venous cavernous spaces lined with endothelium and separated by The Male Reproductive System SUMMARY OF KEY POINTS Testes In each testis approximately each of 250 lobules contains one or more very long, convoluted seminiferous tubules in a sparse, vascular stroma containing testosterone-producing interstitial cells (of Leydig). Seminiferous tubules consist of spermatogenic epithelium containing columnar Sertoli cells, each of which supports and nourishes many germ cells embedded at its surface. Tight junctions between Sertoli cells establish two compartments within seminiferous tubules: a basal compartment with spermatogonia and an adluminal compartment with spermatocytes and spermatids. 21_Mescher_ch21_p439-459.indd 457 At the beginning of an erection acetylcholine from parasympathetic nerves in the penis causes the vascular endothelial cells of the helicine arteries and cavernous tissue to release nitric oxide (NO). Diffusing into the adjacent smooth muscle cells, NO activates guanylate cyclase to produce cyclic GMP, which causes these cells to relax, resulting in increased blood flow, filling of the cavernous spaces, and penile erection. Erectile dysfunction, or impotence, can result from diabetes, anxiety, vascular disease, or nerve damage during prostatectomy. The drug sildenafil may alleviate the problem by inhibiting the phosphodiesterase degrading cyclic GMP in the smooth muscle cells of helicine arteries and erectile tissue. The subsequent higher level of cGMP promotes relaxation of these cells and enhances the neural effect to produce or maintain an erection. The Male Reproductive System Penis V TA trabeculae with smooth muscle and connective tissue continuous with the surrounding tunic (Figure 21–19). Central arteries in the corpora cavernosa branch to form nutritive arterioles and small coiling helicine arteries, which lead to the cavernous vascular spaces of erectile tissue. Arteriovenous shunts are present between the central arteries and the dorsal veins. Penile erection involves blood filling the cavernous spaces in the three masses of erectile tissue. Triggered by external stimuli to the CNS, erection is controlled by autonomic nerves in these vascular walls. Parasympathetic stimulation relaxes the trabecular smooth muscle and dilates the helicine arteries, allowing increased blood flow and filling the cavernous spaces. This enlarges the corpora cavernosa and causes them to compress the dorsal veins against the dense tunica albuginea, which blocks the venous outflow and produces tumescence and rigidity in the erectile tissue. Beginning at ejaculation, sympathetic stimulation constricts the helicine arteries and trabecular muscle, decreasing blood flow into the spaces, lowering the pressure there, and allowing the veins to drain most blood from the erectile tissue. 2 1 CC 457 C H A P T E R VetBooks.ir Penis Sertoli cells also produce androgen-binding protein, which con centrates testosterone, phagocytose shed debris from differentiating spermatids, and secrete fluid, which carries sperm along the tubules. Stem cells called spermatogonia undergo mitosis and give rise to primary spermatocytes, which undergo a first meiotic division to form haploid secondary spermatocytes. After a very short interval, secondary spermatocytes undergo the second meiotic division to produce small, round spermatids, which differentiate while still associated with Sertoli cells. A spermatid undergoes spermiogenesis by greatly condensing its nucleus, forming a long flagellum with a surrounding mitochondrial middle piece, and forming a perinuclear acrosomal cap. 26/04/18 11:58 am