When it is cold or a man is sexually aroused, this muscle contracts to pull the testes closer to the body for warmth. Each testis is an oval structure about 5 cm long and 3 cm in diameter. A tough, white fibrous connective tissue capsule , the tunica albuginea , surrounds each testis and extends inward to form septa that partition the organ into lobules.
There are about lobules in each testis. Each lobule contains 1 to 4 highly coiled seminiferous tubules that converge to form a single straight tubule, which leads into the rete testis. Short efferent ducts exit the testes. Interstitial cells cells of Leydig , which produce male sex hormones, are located between the seminiferous tubules within a lobule.
Sperm are produced by spermatogenesis within the seminiferous tubules. A transverse section of a seminiferous tubule shows that it is packed with cells in various stages of development. Interspersed with these cells, there are large cells that extend from the periphery of the tubule to the lumen.
These large cells are the supporting, or sustentacular cells Sertoli's cells , which support and nourish the other cells. Early in embryonic development, primordial germ cells enter the testes and differentiate into spermatogonia, immature cells that remain dormant until puberty. Slide 27 Testis, epididymis, vas deferens, infant. With low power objective identify the thick dense regular connective tissue capsule surrounding the testis; this is the tunica albuginea. With higher power note that it is covered by mesothelium , the visceral layer of the processus vaginalis.
Identify the area of the mediastinum thickened part of the tunica albuginea which contains the rete testis not well preserved in this slide. Note also the muscular vas deferens and the head of the epididymis on this slide. This testis is from an infant so the seminiferous tubules are not well developed.
Slide 29 Testis, adult. Note the highly coiled structure of the seminiferous tubules seen in cross section. The cell-type population varies from tubule to tubule depending in the stage of the spermatogenic cycle. The endocrine testis consists of the Leydig cells which are found in clusters within the interstitial tissue between seminiferous tubules. The exocrine testis consists of two cell types: the Sertoli cell and the germ cells. Identify the following germ cell types within the seminiferous tubules:.
Spermatogonia most numerous cell type, adjacent to the basement membrane. The sequence of amino acids determines the type of protein. Protein is synthesized according to the sequence of nucleoti.. Seed plants are vascular plants.
They differ from the other vascular plants in producing seeds that germinate into a new.. Plants, like animals, produce hormones to regulate plant activities, including growth. They need these hormones to respo.. Multicellular organisms evolved. The first ones were likely in the form of sponges.
The seminal vesicles and prostate gland add fluids to the sperm to create semen. The testes are located in a skin-covered, highly pigmented, muscular sack called the scrotum that extends from the body behind the penis. The dartos muscle makes up the subcutaneous muscle layer of the scrotum.
It continues internally to make up the scrotal septum, a wall that divides the scrotum into two compartments, each housing one testis. Descending from the internal oblique muscle of the abdominal wall are the two cremaster muscles, which cover each testis like a muscular net. By contracting simultaneously, the dartos and cremaster muscles can elevate the testes in cold weather or water , moving the testes closer to the body and decreasing the surface area of the scrotum to retain heat.
Alternatively, as the environmental temperature increases, the scrotum relaxes, moving the testes farther from the body core and increasing scrotal surface area, which promotes heat loss. Externally, the scrotum has a raised medial thickening on the surface called the raphae.
Figure 2. This anterior view shows the structures of the scrotum and testes. They produce both sperm and androgens, such as testosterone, and are active throughout the reproductive lifespan of the male. Figure 3. This sagittal view shows the seminiferous tubules, the site of sperm production.
Formed sperm are transferred to the epididymis, where they mature. They leave the epididymis during an ejaculation via the ductus deferens. Paired ovals, the testes are each approximately 4 to 5 cm in length and are housed within the scrotum. They are surrounded by two distinct layers of protective connective tissue. The outer tunica vaginalis is a serous membrane that has both a parietal and a thin visceral layer. Beneath the tunica vaginalis is the tunica albuginea, a tough, white, dense connective tissue layer covering the testis itself.
Not only does the tunica albuginea cover the outside of the testis, it also invaginates to form septa that divide the testis into to structures called lobules. Within the lobules, sperm develop in structures called seminiferous tubules. During the seventh month of the developmental period of a male fetus, each testis moves through the abdominal musculature to descend into the scrotal cavity. The tightly coiled seminiferous tubules form the bulk of each testis. They are composed of developing sperm cells surrounding a lumen, the hollow center of the tubule, where formed sperm are released into the duct system of the testis.
Specifically, from the lumens of the seminiferous tubules, sperm move into the straight tubules or tubuli recti , and from there into a fine meshwork of tubules called the rete testes.
Sperm leave the rete testes, and the testis itself, through the 15 to 20 efferent ductules that cross the tunica albuginea. Inside the seminiferous tubules are six different cell types. These include supporting cells called sustentacular cells, as well as five types of developing sperm cells called germ cells. Germ cell development progresses from the basement membrane—at the perimeter of the tubule—toward the lumen.
Surrounding all stages of the developing sperm cells are elongate, branching Sertoli cells. Sertoli cells are a type of supporting cell called a sustentacular cell, or sustenocyte, that are typically found in epithelial tissue. Sertoli cells secrete signaling molecules that promote sperm production and can control whether germ cells live or die. They extend physically around the germ cells from the peripheral basement membrane of the seminiferous tubules to the lumen.
Tight junctions between these sustentacular cells create the blood—testis barrier , which keeps bloodborne substances from reaching the germ cells and, at the same time, keeps surface antigens on developing germ cells from escaping into the bloodstream and prompting an autoimmune response.
Spermatogonia are the stem cells of the testis, which means that they are still able to differentiate into a variety of different cell types throughout adulthood.
Spermatogonia divide to produce primary and secondary spermatocytes, then spermatids, which finally produce formed sperm. The process that begins with spermatogonia and concludes with the production of sperm is called spermatogenesis. As just noted, spermatogenesis occurs in the seminiferous tubules that form the bulk of each testis. One production cycle, from spermatogonia through formed sperm, takes approximately 64 days. A new cycle starts approximately every 16 days, although this timing is not synchronous across the seminiferous tubules.
Sperm counts—the total number of sperm a man produces—slowly decline after age 35, and some studies suggest that smoking can lower sperm counts irrespective of age. The process of spermatogenesis begins with mitosis of the diploid spermatogonia. However, mature gametes are haploid 1 n , containing 23 chromosomes—meaning that daughter cells of spermatogonia must undergo a second cellular division through the process of meiosis. Figure 4.
Meiosis has two rounds of cell division: primary spermatocyte to secondary spermatocyte, and then secondary spermatocyte to spermatid. This produces four haploid daughter cells spermatids. The location of the primary spermatocytes is near the basement membrane, and the early spermatids are approaching the lumen tissue source: rat. Two identical diploid cells result from spermatogonia mitosis.
One of these cells remains a spermatogonium, and the other becomes a primary spermatocyte , the next stage in the process of spermatogenesis. As in mitosis, DNA is replicated in a primary spermatocyte, and the cell undergoes cell division to produce two cells with identical chromosomes.
Each of these is a secondary spermatocyte. Now a second round of cell division occurs in both of the secondary spermatocytes, separating the chromosome pairs. This second meiotic division results in a total of four cells with only half of the number of chromosomes.
Each of these new cells is a spermatid. Although haploid, early spermatids look very similar to cells in the earlier stages of spermatogenesis, with a round shape, central nucleus, and large amount of cytoplasm.
A process called spermiogenesis transforms these early spermatids, reducing the cytoplasm, and beginning the formation of the parts of a true sperm. The fifth stage of germ cell formation—spermatozoa, or formed sperm—is the end result of this process, which occurs in the portion of the tubule nearest the lumen. Eventually, the sperm are released into the lumen and are moved along a series of ducts in the testis toward a structure called the epididymis for the next step of sperm maturation.
Sperm are smaller than most cells in the body; in fact, the volume of a sperm cell is 85, times less than that of the female gamete. Approximately to million sperm are produced each day, whereas women typically ovulate only one oocyte per month as is true for most cells in the body, the structure of sperm cells speaks to their function.
Sperm have a distinctive head, mid-piece, and tail region.
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