Essay on the Mammalian Reproductive System!
The reproduction system of sexually reproducing animal consists of:
(i) Primary sex organs (called gonads) which produce gametes and hormones.
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(ii) Secondary sex organs/ glands which participate in the reproduction but do not form gametes.
(iii) Accessory sex organs/ characters which distinguish the two sexes in appearance.
Male Reproductive system:
Primary Sex Organ – Testes:
Testes are paired structures which lie outside the abdominal cavity in a thin pouch of skin called scrotum. The testis is attached to the scrotum by a band of connective tissue known as gubernaculum testis. Scrotum communicates with abdominal cavity through inguinal canal.
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Testes are surrounded by a tough connective tissue capsule known as tunica albuginea.
Testis contains 1 to 3 seminiferous tubules which are embedded in loose interstitial connective tissue containing blood capillaries, nerve fibres and numerous small groups of large glandular cells called interstitial cells or cells of Leydig which secrete male sex hormone, testosterone and other androgens.
Testosterone shows the onset of puberty and also maintains spermatogenesis and libido (sexual urge or desire).
Puberty is the transition period from a non-reproductive to a reproductive state i.e. period when reproductive organ becomes functional.
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The lining of seminiferous tubule called germinal epithelium contains two types of cells – primary germ cells which undergo spermatogenesis to form spermatozoa and columnar indifferent cells (derived from coelomic epithelium) which enlarge to form Sertoli cells. Sertoli cells, also called subtentacular cells, are the functional component of seminiferous tubules.
It functions as nurse cells for differentiating spermatozoa, phagocytize defective sperm and secrete protein hormone inhibin (which inhibits FSH secretion). The phagocytic property of sertoti cells was due to presence of abundance of lysosomes (containing acid phosphatase and alkaline phosphatase) in the Sertoli cells cytoplasm.
The seminiferous tubules are situated in testicular lobules. Both ends of the tubule are connected to the central region of the testis and form a network of small ductules called the rete testis. From rete testis 10-20 vasa efferentia in the form of fine ciliated ductules arise to collect sperms and transfer them to the head of the epididymis.
The rete testis is an anastomosing network of channels that empties into the efferent ductules. The ductules connect the ducts of the epididymis which in turn connect with the ductus deferens.
Secondary Sex Organs/Glands
i. Epididymis (ductus epididymidis):
Each epididymis consists of a single tightly coiled narrow tube, enclosed in a fibrous covering. It lies along the top and side of testis and is divided into three parts – anterior caput epididymis, middle corpus epididymis and posterior cauda epididymis.
Epididymis serves the following functions:
a. Stores sperms prior to ejaculation.
b. Contributes to seminal fluid, for nourishing sperms and providing them motility.
Testis and epididymis are together called testicle.
Vas deferens (ductus deferens):
It is connected with epididymis at the tail end. It comes out of scrotum via inguinal canal into abdomen along-with spermatic cord (which traverses through the canal & connects testes with abdominal cavity). They conduct sperms.
Ejaculatory duct:
The union of the canals from seminal vesicles and vas deferens form the ejaculatory duct. The ejaculatory duct has a contractile mechanism that aids in the emission of the seminal fluid.
Urethra:
Urethra leads from urinary bladder through the prostate gland and into the penis. It consists of four parts – urinary, prostatic, membranous and penile. The urethra (membranous and penile part) in the male forms the outflow pathway for the urine and for the seminal fluid.
However it is physically impossible for a man to urinate and ejaculate at the same time because just prior to ejaculation the internal sphincter closes off the opening of the urinary bladder.
The sphincter does not relax until the ejaculation is completed. The closing of this internal sphincter prevents urine from entering the urethra and also prevents the backflow of ejaculatory fluid into the urinary bladder.
Seminal vesicles:
These are paired tubular coiled sac like structures, situated behind the bladder. They secrete viscous fluid which constitutes the main part of the ejaculate.
Seminal fluid contains fructose (as a source of energy which provide nourishment for the activity of sperm), citric acid and prostaglandins. The latter two stimulates the movements of sperms in female tract for transporting spermatozoa distally toward the uterine tube.
Prostate gland:
The prostate gland is a chestnut shaped gland which lies at the base of the bladder and surrounds the first part of the urethra. It contributes an alkaline component to the seminal fluid for sperm motility.
The substance made by the prostate helps the sperms to become active and counteracts any adverse effect urine may have on the sperms. The prostatic fluid provides a characteristic odour to the seminal fluid. Prostate gland secretes – citrate ion, calcium, phosphate ion, a clotting enzyme-profibrinolysin.
Bulbourethral glands (or Cowper’s glands):
The two bulbourethral glands are pea-sized structures lying adjacent to the urethra at the base of penis. It secretes a clear, viscous mucous which is lubricating in function.
Penis:
The penis is a cylindrical and highly vascularised copulatory organ. It consists of three columns of spongy tissues surrounding the urethra and a layer of skin on the outside. The penis contains three cylindrical strands of erectile tissue: two corpora cavernosa which run parallel on the dorsal part and the corpus spongiosum which contains the urethra.
The corpora cavernosa are surrounded by a dense relatively inelastic connective tissue called the tunica albuginea.
The corpus spongiosum does not have such a surrounding layer. The corpora cavernosa contains numerous vascular cavities called venous sinusoids.
The corpus spongiosum extends distally beyond the corpora cavernosa and is expanded into the tip of the penis, which is called glans penis.
The skin in this region is folded to form a retractable casing called the foreskin or prepuce.
During sexual excitement the elastic cavernosa spaces become filled with blood which then compresses the penile vein running through the centre of the body of penis. Since no blood flows out of the penis, it becomes enlarged and erect.
Female Reproductive System:
1. Primary Sex Organ – Ovary:
The two ovaries are small bodies which remain attached to the abdominal wall by an ovarian ligament called mesovarium (or fold of peritoneum).
It is covered by a layer of cubical epithelium called the germinal epithelium which is further covered by visceral peritoneum. Beneath this epithelium, a layer of connective tissue called tunica albuginea is present and under it is stroma.
The stroma consists of dense outer layer called the cortex and a less dense inner portion called the medulla. Interspersed throughout the cortex are many ovarian follicles in different stages of development.
These follicles are initially formed during embryonic development by proliferation of primordial germ cells of germinal epithelium but start maturing (once a month) after puberty only.
The peripheral part of ovary produces ova or eggs. Normally, only one egg matures in each ovary every alternate month. A maturing egg contained in a cellular sac is called the follicle.
As the egg grows larger the follicle also enlarges and gets filled with a fluid (called liquor follicle rich in mucoproteins, and secreted by granulosa cells) and is now called the Graafian follicle. Graafian follicle is known to be as mature follicle of ovary which occupies a single cavity called the antrum. The growth of graafian follicle is under the control of FSH hormone from anterior pituitary.
Graafian follicle consists of oocyte in the centre which is surrounded by layer of a vascular granulosa cells called corona radiata. Corona radiata is immediately surrounding the prominent zona pellucida which surrounds the oocyte.
Nucleus of ovum is known as germinal vesicle. The granulosa cell which joins the corona radiata with membrana granulosa is called cumulus oophorus. Membrana granulosa, the main source of estrogen, is separated from the thecal layer via basement membranes.
Thecal layer consists of theca externa (fibrous) and theca interna (granular or highly vascularised). LH acts on theca interna which synthesizes androgen and this androgen diffuses into granulosa cells which aromatize them into estrogen prior to ovulation.
Naturally occurring estrogens are 17b-estradiol, estrone and estriol. Estradiol is the principal and biologically most active estrogen whereas estriol is the weakest of all naturally occurring estrogens. Estriol is synthesised by placenta and the liver but not the ovary.
It is formed in the liver as a conversion product of estradiol and estrone in case of nongravid women. At the time of ovulation the graafian follicle ruptures to release ovum, corona radiata and cumulus oophorus and the remaining structure forms corpus luteum (composed of thecal lutein cells, derived from theca interna cells).
2. Secondary Sex Organs/Glands:
Fallopian tubes or uterine tubes or oviducts are about 12 cm long. Each oviduct is differentiated into four parts. These are infundibulum, ampulla, isthmus and uterine part. Cilia lining the funnel help to pick up and push the released ovum into the oviduct. Subsequently, the muscular contractions of the oviduct push the egg down into the uterus.
Uterine tubes also transport spermatozoa from the uterus toward the ovary and produce trophic substances that ensure the development of fertilized conceptus.
Uterus is a hollow pear-shaped muscular organ situated in the pelvic cavity between the urinary bladder and the rectum.
The wall of the uterus are composed of three layers of tissues – the perimetrium (outer); myometrium (middle one of smooth muscle fibre), and endometrium (inner).
It has two regions, an upper wider portion which receives the two oviducts and a small lower constricted part, the cervix or neck. Cervix encircles the cervical canal which connects uterine cavity to the vagina.
The uterus is the site for implantation of the pre-embryo and for the subsequent embryonic and fetal development.
Vagina is a muscular tube starting from the lower end of the uterus up to the outside. It is richly vascularised but lacks glands. The vagina receives the male penis during copulation.
The great elasticity of its wall also allows the passage of the baby during childbirth. The opening of the vagina in young females is partially closed by a thin membrane called hymen.
The hymen is frequently ruptured in childhood due to strenuous physical exercise or disease. During reproductive life the vagina contains Lactobacillus acidophilus which keeps the vaginal pH between 4.9 – 3.5 by producing lactic acid from glycogen. This acidity helps to prevent vaginal infections.
Vulva:
The external female genitalia are called the vulva. The sides of the vulva have two small fleshy folds, the labia minora (lesser lips) which are hidden by larger hairy folds the labia majora (greater lips). Labia majora are homologus to scrotal sacs of a male.
In the upper-most angle of the vulva, in front of the urethral opening is located a small erectile clitoris which is highly sensitive as if contains numerous sensory nerve endings for touch and pressure (eg. Meissner’s corpuscles). Clitoris is equivalent of male penis.
Bartholin’s glands are pair of small glands occurs one on each side of the vaginal opening. These glands correspond to Cowper’s gland of male. The secretion of this gland is thick, viscid and alkaline for lubrication during copulation and counteracting urinary acidity.
Mammary glands are modified sweat glands that lie over the pectoral muscles. They are also present in male but in a rudimentary form. Internally each gland has 15-20 lobulated milk glands each having a number of lobules containing number of alveoli.
Each milk gland or lobe sends a lactiferous duct towards nipple. Its secretion is under the control of prolactin hormone (of anterior pituitary) while milk ejection is under the control of oxytocin hormone of posterior pituitary. Essential function of mammary gland is milk production which has nutritive and immunologic functions.