LESSON 26 Male reproductive system.pdf

_____________ LESSON 26 _____________ MALE REPRODUCTIVE SYSTEM The male reproductive system is made up of the testes, intratesticular and extratesticular genital ducts, accessory glands, and penis. TESTIS The testis is a combination of exocrine gland since it generates spermatozoa and endocrine sin...

_____________ LESSON 26 _____________ MALE REPRODUCTIVE SYSTEM The male reproductive system is made up of the testes, intratesticular and extratesticular genital ducts, accessory glands, and penis. TESTIS The testis is a combination of exocrine gland since it generates spermatozoa and endocrine since it produces hormones such as testosterone. Each testis is surrounded by a connective tissue capsule, the tunica albuginea. This tunica presents a thickening, the mediastinum testis, from which fibrous septa originate and reach the albuginea on the opposite side. In this way, each testis is divided into pyramidal compartments, the lobuli testis. Each lobule is occupied by seminiferous tubules, 2 to 4, immersed in loose connective tissue, in which the interstitial or Leydig cells are arranged. Figure 1: Detail of the testis showing the tunica albuginea (arrow) and seminiferous tubules (*). 1 The seminiferous tubules are made up of a special epithelium, a germinal epithelium in which the spermatozoa originate, a basement membrane and a layer of connective tissue with some smooth muscle fibers. The germinal epithelium is made up of 1. spermatogenic cells and 2. Sertoli cells. The spermatogenic cells are arranged in 4-8 layers. The sequence of events in the development of the spermatozoa from the spermatogonia or stem cells is called spermatogenesis. Spermatogonia are located near the basement membrane. They are rounded, small and undifferentiated cells. These cells undergo mitosis at sexual maturity and can give rise to 1. cells that continue to divide, remaining as stem cells of other spermatogonia (type A spermatogonia) and 2. cells that differentiate into primary spermatocytes (type B spermatogonia). Primary spermatocytes enter meiosis. Secondary spermatocytes arise from the first meiotic division and from the second division, the spermatids, with half the chromosomes of the species. Each spermatid will undergo a differentiation process called spermiogenesis, which leads to the formation of spermatozoa. The cells resulting from the divisions of type B spermatogonia do not divide completely, they remain united by fine cytoplasmic bridges (incomplete cytokinesis). These intercellular bridges allow communication between primary and secondary spermatocytes and spermatids derived from a single spermatogonia. When the maturation process of the spermatids ends, the disappearance of the cytoplasm and the bridges leads to the separation of the spermatozoa. Figure 2: Detail of a seminiferous tubule showing numerous mitoses of the germinal epithelium and Sertoli cells (arrows). 2 The spermiogenesis can be summarized with the following changes: The Golgi complex produces an electrodense vesicle, the acrosome, which is nothing more than a large primary lysosome. This vesicular structure moves towards the nucleus, covering its cranial surface, and thus forming the cephalic or acrosomal cap. Simultaneously, the two centrioles are located at the caudal pole of the nucleus and the distal centriole originates the flagellum. The cytoplasm envelops the flagellum and part of it is detached from the cell. The mitochondria are arranged in spiral around the initial part of the flagellum. Finally, the nucleus flattens and condenses and acquires the shape of an almond and the outer fibers system and the fibrous sheath develop. Spermatozoa vary in size, depending on the species, between 60 µm (boar, horse) and 75 µm (ruminants). Using the light microscope, the spermatozoon has two essential portions: the head and the tail. With the electron microscope, it is observed that the tail is subdivided into the neck, the middle piece, the principal piece and the end piece. HEAD. The shape of the nucleus determines the shape of the spermatozoon head, which is species-specific and subject to wide variations. The cranial pole of the nucleus is covered by the acrosome, which contains hydrolytic and proteolytic enzymes, necessary for the penetration of spermatozoon through the zona pellucida of the oocyte during fertilization. NECK. It consists of a centrally located centriole and nine thick, longitudinally oriented outer dense fibers. MIDDLE PIECE. It has the characteristic structure of a flagellum: two central microtubules and nine pairs of peripheral microtubules. Microtubules are surrounded by nine outer dense fibers surrounded, in turn, by numerous mitochondria arranged in a helical pattern. PRINCIPAL PIECE. It has an identical structure to that of the middle piece but, in addition, it has the characteristic "peripheral fibrous sheath" that is formed by the fusion of two of the outer dense fibers with semi-circular bands of structural proteins. pair. END PIECE. It only contains the nine pairs of microtubules and the central The flagellum of the spermatozoon has a motor function, and the location of the mitochondria is related to the supply of energy to the flagellar movement. 3 Figure 3: Scheme of the spermatozoon in which all its component parts are detailed. The Sertoli cells, the other part of the germinal epithelium, have a pyramidal shape and rest on the basement membrane of the seminiferous tubule. Its nucleus is spherical or triangular, with a prominent nucleolus, and its cytoplasm presents numerous invaginations, where the cells of the spermatogenic line are located. In their cytoplasm, they present numerous primary and secondary lysosomes. Sertoli cells perform three main functions: 1. Provide support, protection and control spermatozoa nutrition. 2. Phagocytize and digest, thanks to their lysosomes, the cytoplasmic remains that are released from spermatids. 3. Secreting a fluid that carries sperm and contains a protein that avidly binds to testosterone, increasing the concentration of this hormone in the seminiferous tubule, where it is necessary for spermatogenesis. Sertoli cells are linked together by occluding zonules. These junctions separate a basal compartment from an adluminal (apical) compartment and constitute a diffusion barrier also known as the blood-testis barrier. Spermatogonia multiplication takes place in the basal compartment. The bloodtestis barrier selectively prevents the entry of substances into the adluminal compartment, where the vital processes of meiosis and spermiogenesis take place. Primary spermatocytes pass through these intercellular junctions probably by a mechanism for opening these junctions similar to the operation of a zipper, which closes again underneath the spermatocytes before they reach the adluminal compartment. Among the seminiferous tubules are connective tissue, nerves and blood and lymphatics vessels. The capillaries of the testis are fenestrated, allowing the free passage of macromolecules to the interstitial space. Interstitial or Leydig cells are found in the stroma with rounded or polygonal morphology, a central nucleus, and a cytoplasm rich in lipid inclusions. The cytoplasm of these cells has the typical characteristics of steroid-secreting cells (smooth endoplasmic reticulum vesicles and mitochondria with tubular cristae). These cells produce testosterone, the hormone responsible for the sexual characteristics of the male. 4 The seminiferous tubules lead to the intratesticular ducts, which are the straight tubules, the rete testis and the ductuli efferentes. In its initial part, the wall of the straight tubules is made up solely of Sertoli cells. However, most of the wall of these tubules is lined by cuboidal epithelial cells, supported by dense connective tissue. It follows the rete testis, which lies within the testicular mediastinum. It is formed by simple squamous epithelium that rests on its corresponding basement membrane. Eight to 15 ductuli efferentes emerge from the rete testis, which present a simple epithelium formed by alternating groups of cuboidal cells with microvilli and ciliated columnar cells. The cilia are actively moving, displacing the spermatozoa in the direction of the ductus epididymis. GENITAL PASSAGES The tubes that carry spermatozoa produced in the testicle are 1. the epididymis and 3. the ductus or vas deferens. The epididymis is composed of ductuli efferentes and the ductus epididymis. The epididymal ductuli efferentes are continuation of the intratesticular ductuli efferentes, have the same histological characteristics and are continuous with the long and coiled ductus epididymis, which is surrounded by a thick albuginea tunic. The ductus epididymis is a single tube lined by a columnar pseudostratified epithelium that in its apical portion presents microvilli, improperly called stereocilia. These cells have phagocytic (cytoplasmic fragments released during spermatogenesis), absorption and secretion (spermatozoon nutrition) function. This epithelium rests on a lamina propria of connective tissue underneath which is a layer of smooth muscle fibers. The ductus deferens has a mucosa lined by a columnar pseudostratified epithelium with stereocilia. The lamina propria-submucosa is composed of connective tissue rich in elastic fibers. The muscularis layer is highly developed and is made up of a middle, circular layer, and two longitudinal layers, internal and external. The last layer is a connective tissue adventitia. The terminal portion of the ductus deferens contains simple branched tubuloalveolar glands in the propria-submucosa. ACCESSORY GLANDS The ejaculate is made up of spermatozoa and seminal plasma. Seminal plasma is produced by secretions from the epididymis and accessory glands of the male. These glands are the glandular portion of the ductus deferens, the seminal glands, the prostate, and the bulbourethral gland. 5 SEMINAL GLANDS (or VESICLES). Absent in carnivores. They are paired glands and are surrounded by a capsule of connective tissue. They are compound tubuloalveolar glands. The glandular epithelium is pseudostratified and the excretory ducts present cuboidal epithelium in most species. The secretion product of the seminal glands is rich in fructose which serves as an energy source for the ejaculated spermatozoa. PROSTATE. Two portions can be distinguished, according to topographic rather than histological criteria: the external or compact portion that surrounds the pelvic urethra and that presents a capsule of connective tissue covering it, and the internal or disseminated portion that is located in the submucosa of the pelvic urethra. Both portions are tubuloalveolar glands. The secretory portion is lined by a cuboidal epithelium while the excretory ducts are lined by a stratified columnar epithelium. The function of prostate secretion is to neutralize the seminal plasma and initiate the active movements of the ejaculated spermatozoa. BULBOURETHRAL GLANDS. Absent in the dog. They are paired glands located dorsolateral to the bulbar portion of the urethra. They are compound tubuloalveolar glands surrounded by a fibroelastic capsule. The secretory portion is lined by a simple columnar epithelium that ends in a bulbourethral duct lined by a transitional epithelium. The mucous and proteinaceous secretion product of the glands serves to lubricate the urethra and vagina. PENIS It is formed, essentially, by three cylindrical masses of erectile tissue and the urethra, and externally surrounded by skin. Two erectile masses are located dorsally and are called the corpora cavernosa of the penis. The other, ventral, is called corpus spongiosum of the urethra and surrounds the penile urethra all the way; in its terminal portion it dilates forming the glans. The corpora cavernosa and corpus spongiosum are surrounded by dense connective tissue, the tunica albuginea of the penis. This tunica forms a septum that penetrates between the two penile corpora cavernosa. This septum is not continuous, presenting interruptions that establish communications between the corpora cavernosa of the penis. The corpora cavernosa and corpus spongiosum are made up of venous sinuses. The prepuce is a retractable fold of the skin of the penis, which has connective tissue, with smooth muscles in its internal part, and in the skin that covers the glans, small sebaceous glands are observed. 6