Male Reproductive System
Male Reproductive System
The male reproductive system, like the female, consists of organs whose function is to create a new individual, that is, to complete reproduction. This system consists of a pair of testicles and ejaculatory ducts (epididymis, ductus deferens (vas deferens), and ejaculatory ducts), seminal vesicles, prostate, bulbourethral glands, and penis.
Testes
The male gonads, testicles, or testicles, begin their development in the abdominal cavity, near the kidneys. During the last two months before birth, or shortly after birth, they descend into the scrotum through the inguinal canal, a pouch that extends below the abdomen, to the back of the penis. Although this location of the testicles, outside the abdominal cavity, seems vulnerable to injury, it provides a temperature about 3°C lower than normal body temperature. This low temperature is essential for the production of viable sperm.
The scrotum consists of the skin and subcutaneous tissue. A vertical septum, or partition, of subcutaneous tissue in the center, divides it into two parts, each containing a testis. Smooth muscle fibers, called dartos muscles, contract in the subcutaneous tissue to give the scrotum a wrinkled appearance. When these fibers are relaxed, the scrotum is smooth. Another muscle, the cremaster muscle, is composed of skeletal muscle fibers and controls the position of the scrotum and testicles. When it's cold or when a man is sexually aroused, these muscles contract to pull the testicles closer to the body to warm them.
Structure
Each testicle 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 testicle and expands inward to form septa that divide the organ into lobes. Each testis has about 250 lobules. Each lobule contains 1 to 4 highly coiled seminiferous tubules that form a straight duct, leading to the rete testis. A short afferent duct exits the testes.
Interstitial cells are the cells that produce male sex hormones, and they are located between the seminiferous tubules in a lobule.
Spermatogenesis
Sperm are produced by the reproductive processes and spermatogenesis within the seminiferous tubules. A transverse section of the seminiferous tubule shows that it is filled with cells in various stages of development. Along with these cells, there are large cells that extend from the periphery of the tubule to the lumen. These large cells are supporting, or permanent, cells (Sertoli's cells), which support and nourish other cells.
Early in embryonic development, Primordial germ cells in the testes enter the testes and differentiate into spermatogonia. They remain dormant until puberty.
Spermatogonia are diploid cells, each with 46 chromosomes (23 pairs) located around the seminiferous tubules. At puberty, hormones stimulate these cells to divide through mitosis. Some of the daughter cells produced by mitosis remain in the periphery as spermatogonia. Others are pushed to the lumen, and some undergo a transformation and become primary spermatocytes. Because they are produced by mitosis, primary spermatocytes, like spermatogonia, are diploid and contain 46 chromosomes.
Each primary spermatocyte undergoes the first meiotic division, meiosis I, to produce two secondary spermatocytes, each containing 23 chromosomes (haploid). Just before this division, the genetic material is transcribed so that each chromosome consists of two strands, called chromatids, that are joined by a centromere. During meiosis I, one chromosome, consisting of two chromatids, moves into each secondary spermatocyte.
A secondary spermatocyte divides in the second meiotic division to produce two spermatids. There is no duplication of genetic material in this division, but the centromere divides in such a way that each cell receives one stranded chromatid. As a result of two meiotic divisions, each primary spermatocyte produces four spermatids. During spermatogenesis, two cell divisions occur, but only one copy of the DNA occurs so that each spermatoid contains 23 chromosomes (haploid), one from each pair in the original primary spermatocyte. Each successive stage in sperm is pushed towards the center of the tube so that the more immature cells are at the periphery and the more differentiated cells are closer to the center.
Spermatogenesis (and oogenesis in females) differs from mitosis in that the resulting cells have only half the number of chromosomes as the original cell. When the sperm cell nucleus fuses with the egg cell nucleus, the full number of chromosomes is restored. If sperm and egg cells were produced by mitosis, each successive generation would have double the number of chromosomes of the previous one.
The final stage of sperm development is called spermiogenesis. In this process, spermatozoa form mature spermatozoa or sperm. A mature sperm cell has a head, midsection, and tail. The head, also called the nuclear region, consists of 23 chromosomes surrounded by a nuclear membrane. The tip of the head is covered with an acrosome, which contains enzymes that help the sperm penetrate the female gamete. The midpiece, the metabolic region, contains mitochondria that provide adenosine triphosphate (ATP). The tail, or locomotor region, uses a common flagellum for locomotion. Sperm are released into the lumen of the seminiferous tubules and leave the testicles. They then enter the epididymis where they undergo their final maturation and become capable of fertilizing the female gamete.
Sperm production begins at the level of puberty and continues throughout a man's life. The entire process, starting with a primary spermatocyte, takes about 74 days. After ejaculation, sperm can survive in the female reproductive tract for about 48 hours.
Duct System
Sperm cells travel through a series of ducts to reach the outside of the body. After they leave the testicles, sperm travel through the epididymis, ductus deferens, ejaculatory duct, and urethra.
Epididymis
Sperm leave the testicles through a series of ducts that enter the epididymis. Each epididymis is a long (about 6 m) tube firmly attached to form a comma-shaped organ located along the upper and posterior margins of the testes. When sperm leave the testicles, they are immature and unable to fertilize an egg. They complete their maturation process and become fertile as they pass through the epididymis. Mature sperm are stored in the base or tail of the epididymis.
Ductus Deferens
The ductus deferens, also known as the vas deferens, is a fibromuscular tube that is continuous (or contiguous) with the epididymis. It begins at the base (tail) of the epididymis and then turns sharply upward along the posterior margin of the testes. The ductus deferens enter the abdominal cavity through the abdominal cavity and pass along the lateral pelvic wall. It crosses the ureter and back of the urinary bladder and then descends along the back wall of the bladder to the prostate gland. Just before reaching the prostate gland, each ductus deferens enlarges to form an ampulla. Sperm are stored in the proximal part of the ductus deferens, near the epididymis, and peristaltic movements propel the sperm through the tube.
The proximal part of the ductus deferens is a component of the spermatic cord, which contains the vascular and neural structures that supply the testes. The spermatic cord consists of the ductus deferens, testicular artery and veins, lymph vessels, testicular nerves, the cremaster muscle that elevates the testicles during heat and sexual stimulation, and connective tissue.
Ejaculatory Duct
Every vas deferens, at the ampulla, and joins the vas from the ipsilateral seminal vesicle (one of the accessory glands). Ejaculatory ducts are a part of the male reproductive system, which is located in the back of the pelvis. They are made up of two tubes.
Urethra
The urethra extends from the urinary bladder to the external urethra at the tip of the penis. It is the passage of sperm and fluids from the reproductive system and urine from the urinary system. When reproductive fluids are passing through the urethra, the sphincters contract tightly to prevent urine from entering the urethra.
The male urethra is divided into three parts. The prostatic urethra is the proximal part that passes through the prostate gland. It receives the ejaculatory duct, which contains sperm and secretions from the seminal vesicles and the prostate gland through several ducts. The anterior segment, the membranous urethra, is a short tract that passes through the pelvic floor. The longest part is the penile urethra (also called the spongy urethra or cavernous urethra), which extends the length of the penis and opens outward onto the external urethra. The Ducts from the bulbourethral glands open into the penile urethra.
Accessory Glands
The accessory glands of the male reproductive system are the seminal vesicles, the prostate gland, and the bulbourethral glands. These glands secrete fluids that enter the urinary tract.
Seminal Vesicles
Paired seminal vesicles are saccular glands behind the urinary bladder. Each gland has a small duct that joins the ductus deferens at the ampulla to form the ejaculatory duct, which then empties into the urethra. The fluid secreted by the seminal vesicles is viscous and contains fructose, which provides an energy source for the sperm. prostaglandins, which contribute to sperm motility and viability; and proteins that cause a mild clotting reaction in semen after ejaculation.
Prostate
The prostate gland is a firm, dense structure located just inferior to the urinary bladder. It is about the size of a walnut and surrounds the urethra as it leaves the urinary bladder. The substance of the prostate gland empties into numerous short ducts into the prostatic urethra. Prostatic secretions are thin, milky, and alkaline. They work to increase sperm motility.
Bulbourethral Glands
The paired bulbourethral (couphers) glands are small, about the size of a pea, and are located near the base of the penis. A small duct from each gland enters the end of the penis closest to the urethra. In response, when sexual stimulation occurs, the bulbourethral glands secrete an alkaline mucus-like fluid that is important. This fluid neutralizes the acidity of urine residue in the urethra, helps neutralize the acidity of the vagina, and provides some lubrication for the tip of the penis during intercourse.
Seminal Fluid
Seminal fluid, or semen, is a slightly alkaline mixture of sperm cells and secretions from accessory glands. Secretions from the seminal vesicles make up about 60 percent of the volume of semen, with most of the rest coming from the prostate gland. Sperm and secretions from the bulbourethral glands contribute only a small amount.
The volume of semen in a single ejaculation can vary from 1.5 to 6.0 ml. There are usually 50 to 150 million sperm per milliliter of semen. A sperm count below 10 to 20 million per milliliter usually presents with fertility problems. Although only one sperm actually enters the ovum and fertilizes it, it takes several million sperm in one ejaculation to ensure that fertilization will occur.
Penis
The penis, the male penis, is a cylindrical pendant organ located behind the scrotum and functions to transport sperm into the vagina. The penis consists of three columns of the penis that are wrapped in connective tissue and covered by skin. The two dorsal columns are the corpora cavernosa. A single, midline ventral column surrounds the urethra and is called the corpus spongiosum.
The penis consists of a root, body (shaft), and glans penis. The root of the penis connects it to the pubic arch, and the body is the visible, pendulous part. The Corpus spongiosum expands at the distal end level to form the glans penis. The urethra, which extends the entire length of the corpus spongiosum, opens at the tip of the penis through the external urethra. A loose layer of skin, called the prepuce or foreskin, covers the penis.
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