Humans are sexually reproducing and viviparous organisms. Their reproductive events include the formation of gametes (gametogenesis), i.e. sperms in males and ovum in females. Transfer of sperms into the female genital tract (insemination) and fusion of male and female gametes (fertilization) leads to the formation of a zygote. This is followed by the formation and development of the blastocyst and its attachment to the uterus wall (implantation).embryonic development (gestation) and delivery of the baby (parturition).
THE MALE REPRODUCTIVE SYSTEM
The male reproductive system is located in the pelvic region. It includes a pair of testes, along with accessory ducts, glands, and the external genitalia. The primary sex organs are testes in males and ovaries in females. Besides producing gametes, they also secrete sex hormones. The growth of gonads, their maintenance, and their functions are regulated by gonadotropins (FSH, LH) of the anterior lobe of the pituitary.
The other reproductive organs which perform important functions in reproduction but neither produce gametes nor secrete sex hormones are called secondary sex organs. These include the prostate, seminal vesicles, vas deferens, and penis in males, and the fallopian tubes, uterus, vagina, and mammary glands in females.
The testes are situated outside the abdominal cavity within a pouch called the scrotum. The scrotum helps in maintaining the low temperature of the testes (2 – 2.5°C lower) than the normal internal body temperature, which is necessary for spermatogenesis. The slightly cooler temperature of the scrotum is necessary for the development of normal sperm. The testes start their development in the abdominal cavity. But during the 7th, the month of the foetal life, they descend into the scrotal sacs in presence of testosterone hormone.
Hence, the testes of human males are extra-abdominal. If they fail to descend, this condition is called cryptorchidism which leads to sterility. Scrotum remains connected with the abdomen or pelvic cavity by the inguinal canal. Blood vessels, nerves, and conducting tubes pass through it. Cremaster muscles and connective tissues form the spermatic cord and surround all structures passing through the inguinal canal. Cremaster muscles and dartos muscles of the scrotal sac help in the positioning of testes.
Whenever the outside temperature is low, these contract to move the testes close to the abdominal or pelvic cavity. When the outside temperature is high, these relax moving the testes away.
1. Gubernaculum: A fibrous cord that extends from the caudal end of the epididymis to the scrotal wall.
2. Inguinal canal: Oblique passage through the lower abdominal wall. In males, it is the passage through which the testes descend into the scrotum and contain the spermatic cord.
3. There are certain mammals in which the testes remain permanently in the abdomen and do not cause any defect. Examples:- are elephants, aquatic mammals like whales, dolphins, seals, and prototherians, or egg-laying mammals like Ornithorhynchus.
4. In mammals that breed seasonally, the testes descend into the scrotum only during the breeding season, for example, bat and otter.
In adults, each testis is oval in shape, with a length of about 4 to 5 cm and width of about 2 to 3 cm. The testis is covered by a dense covering. They are enclosed in an outer tough capsule of collagenous connective tissue, the tunica albuglnea. Each testis has about 250 compartments called testicular lobules, these compartments contain highly coiled tubules called seminiferous tubules.
Each lobule contains one to three seminiferous tubules in which the sperms are produced. Each seminiferous tubule is lined on its inside by two types of cells called male germ cells (spermatogonia) and Sertoli cells. Spermatogonia lining these tubules give rise to spermatozoa which are released into the lumen of the tubule.
In between spermatogenic cells, Sertoli or sustentacular or nurse cells are present which provide nourishment to developing spermatozoa and regulate spermatogenesis by releasing inhibin to check FSH over-activity. The other functions of sertoli cells are
- To absorb the parts being shed by developing spermatozoa.
- To release anti mullerian factor (AMF) to prevent development of mullerian duct/oviduct in male.
- To release Androgen Binding Protein (ABP).
- To form blood-testis barrier.
Castration is removal of testes. It causes failure of development of secondary sex organs and characters and remove the ability to reproduce due to deficiency of testosterone.
Chotr boys were often castrated in medieval Europe to retain their high-pitch voice for singing. Castration often changes the aggressive bull into a docile ox. The latter lacks the male character of aggressiveness due to deficiency of testosterone. The docile ox can be conveniently used for ploughing and drawing of bullock carts.
Mullerian ducts (or paramesonephrlc ducts) are paired ducts of the embryo. In the female, they develop to form the Fallopian tubes, uterus, cervix and the upper portion of the vagina; in the male, they degenerate. These ducts are made of tissue of mesodermal origin.
The Wolffian duct (also known as archinephric duct, Leydig’s duct, mesonephric duct, or nephric duct) is a paired organ found in mammals including humans during embryogenesis. It connects the primitive kidney Wolffian body (or mesonephros) to the cloaca. In male, the Wolffian duct develops into the trigone of urinary bladder, a part of the bladder wall and vas deferens and in female the Wolffian duct develops into the trigone of urinary bladder, a part of the bladder. However, further development differentiates between the sexes in the development of the urinary and reproductive organs.
Groups of polyhedral cells called Interstitial cells of Leydlg, are located in the connective tissue around the seminiferous tubules.They constitute the endocrine tissue of the testis. Leydig cells synthesise and secrete testicular hormones called Androgens into the blood.
Seminiferous tubules unite to form several straight tubules called tubuli recti which open into irregular cavities in the posterior part of the testis which is a highly anastomosing labyrinth of cuboidal epithelium lined channels called rete testis. Several tubes called vasa efferentia arise from it and conduct spermatozoa out of the testis. Tubuli recti, rete testis and ductuli efferentes form the Intra-testicular genital duct system.
The extratesticular duct system consists of tubes which conduct sperms from the testes to the outside. It starts with ducts known as vasa efferentia. From each testis, 10-12 vasa efferentia confluent to form a folded and coiled tube called epididymis behind each testis. The epididymis consists of three parts: (i) Caput (ii) Corpus (iii) Cauda.
The epididymis stores the sperms temporarily. From cauda epididymis, a partially coiled tube called vas deferens ascends into the abdomen through inguinal canal, passes over the urinary bladder, the ductus deferens/vas deferens dilates to form ampulla, which receives the duct from the seminal vesicle behind the urinary bladder and forms an ejaculatory duct. The final portion of ampulla passes through the prostate to open into the urethra shortly after its origin from the urinary bladder.
Urethera : –
Male urethera provides a common pathway for the flow of urine and semen. It is much longer in male than in female, measuring about 20 cm.
- First part is surrounded by prostate gland and is called prostatic/glandular part of urethera.
- Membranous urethera is the second part which is situated behind the lower part of pubic symphysis and is smallest
- Penile urethra is situated in the penis and is the longest part.
The urethra receives the ducts of the prostate and Cowper’s glands, passes through the penis and opens to the outside.
This is the copulatory organ of man. It is a cylindrical, erectile, pendulous organ suspended from pubic region in front of scrotum. It remains small and limp (flaccid) but on sexual arousal, it becomes long, hard and erect, ready for copulation (coitus or intercourse). Erect human penis is, on an average, about 15 cm long.
The penile mass is itself encased in a fibrous sheath, called tunlca albuglnea. The interior of the penis is formed of three cylindrical cords of spongy, erectile (cavernous) tissues. Two of these cords are thicker and situated parallely on right and left sides, forming the thick part of penis that remains in front when penis is limp, but become superio-posterior when penis is erect. These two cords are called corpora cavernosa. The fibres of tunica albuginea surround both the cords jointly and also form a separate sheath around each cord. Some fibres form a partition called septum penis between these cords. The third, smaller cord forms that part of penis which remains inferio-anterior in erect penis. Urethra runs through this cord. Hence, this cord is called corpus urethrae or sponglosum.
The extended part of corpus spongiosum is enlarged, forming a bulging, conical structure called glans penis. The surface of glans is formed of thin, smooth and shiny, hairless skin. The base line of glans is referred to as the neck of the penis. The loose skin of penis becomes folded here to form a loose, retractile skin covering upon the glans, called foreskin or prepuce. At the tip of glans penis is the slit like external urethral orifice or meatus by which urethra opens out and discharges urine or semen.
Tyson’s gland or Preputial glands, present in the skin of penis neck, secrete a white sebaceous substance called smegma. Microbial infection in smegma can cause irritation due to inflammation.
Accessory Glands of male
1. Seminal vesicles : These are paired, tubular, coiled glands situated behind the bladder. They secrete viscous fluid which constitutes the main part of the ejaculate. Seminal fluid contains fructose, citric acid, inositol and prostaglandins.
2. Prostate gland : The prostate gland is a chestnut shaped gland and is a collection of 3040 tubuloalveolar glands which lie at the base of the bladder and surrounds the first part of the urethra. It contributes an alkaline component to the semen. (Although, the alkalinization of semen is primarily accompalished through secretion from the seminal vesicles.) The alkaline secretions of prostate gland help the sperms to become active and counteract any adverse effects that the urine may have on the sperms. The prostatic fluid provides a characteristic odour to the seminal fluid. Prostate gland secretes citrate ions, calcium, phosphate Ions and profibrinolysin.
Prostatltls : Inflammation of prostate gland.
3. Bulbourethral glands or Cowper’s glands : The two bulbourethral glands are pea sized structures lying adjacent to the urethra at the base of penis. They secrete a viscous mucus which acts as a lubricant.
The duct system, accessory glands and penis are secondary male sex organs. Their growth, maintenance and functions are promoted by testosterone, secreted by Leydlg cells. On the other hand, the growth, maintenance and functions of semlnlferous tubules and Leydlg cells are regulated respectively by FSH and ICSH of anterior pituitary.
Semen is a mixture of sperms and seminal fluid, which is the liquid portion of semen that consists of secretions of the seminiferous tubules. seminal vesicles. prostate gland and bulbourethral glands. The average volume of semen in an ejaculation is 2.5 – 5 ml, with a sperm count (concentration) of 200 to 300 million sperms. Out of these sperms, for normal fertility, atleast 60 percent sperms must have normal shape and size and atleast 40 percent of them must show vigorous motility. When the number of sperms falls below 20 million/ml, the male is likely to be infertile.
Semen has a slightly alkaline pH of 7.2-7.7, due to the higher pH and larger volume of fluid from the seminal vesicles. The prostatic secretion gives semen a milky appearance whereas the fluids from the seminal vesicles and bulbourethral glands give it a sticky consistency. Semen provides sperms with transportation medium and nutrients. It neutralizes the hostile acidic environment of the male urethra (due to presence of urine) and the female vagina.
Delivery of Sperm : The urethra passes through the penis, an erectile copulatory organ that deposits the semen in the female reproductive tract. The penis is the male external genitalia, made up of three cylinders of special spongy tissue. Filling of blood in these tissue helps in erection of the penis that facilitate insemination. The enlarged end of the penis is called the glans penis,covered with a loose fold of skin called foreskin or prepuce. Semen is forcefully expelled from the penis by the contractions of smooth muscles that line the urethra. This process is ejaculation.
THE FEMALE REPRODUCTIVE SYSTEM
The female reproductive system consists of a pair of ovaries, a duct system consisting of a pair of fallopian tubes (oviducts), a uterus, cervix and vagina. A pair of mammary glands are accessory genital glands.
The ovary is the primary female sex organ. It produces ova and secretes the female sex hormones, estrogens and progesterone which are responsible for the development of secondary female sex characters and cause marked cyclic changes in the uterine endometrium. The human ovaries are small, almond-like flattened bodies, about 2 to 4 cm in length and is connected to the pelvic wall and uterus by ligaments.
1. Location : Ovaries are located near kidneys and remain attached to the lower abdominal cavity through mesovarium.
2. Structure : The free surface of the ovaries is covered by a germinal epithelium composed of a single layer of cubical cells.
This epithelium is continuous with the mesothelium lining called peritoneum. The epithelium encloses the ovarian stroma. The stroma is divided into two zones a peripheral cortex and an inner medulla. Immediately below the germinal epithelium, the cortex is covered by a connective tissue called tunica albuginea.The cortex contains numerous spherical or oval, sac-like masses of cells known as ovarian follicles. The medulla consists of loose connective tissue, elastic fibres, numerous blood vessels and some smooth muscle fibres.
Internal Structure :-
(a) Ovarian follicle : The ovarian follicle contains a large, centrally placed ovum, surrounded by several layers of granular cells (follicular granulosa or discus proligerus or cumulus oophorus). It is suspended in a small cavity called the antrum. Antrum is filled with a fluid known as liquor folliculi. The secondary oocyte in the tertiary follicle also forms a new membrane called zona pellucida. The follicle bulges onto the surface of the ovary. Such a follicle is called the mature Graafian follicle (after de Graaf, who reported them in 1672 and considered them to be eggs).
(b) Corpus luteum : The ovum is shed from the ovary by rupture of the follicle. The shedding of the ovum is called ovulation and occurs nearly 14 days before the onset of the next menstrual cycle. After the extrusion of the ovum, what remains in the Graafian follicle is called corpus luteum (yellow body). The cytoplasm of the corpus luteum is filled with a yellow pigment called luteln. The corpus luteum grows for a few days and if the ovum is fertilized and pregnancy results, it continues to grow. But if the ovum is not fertilized, the corpus luteum persists only for about 14 days and during this period, it secretes progesterone and small amount of estrogen. At the end of its functional life, the corpus luteum degenerates and is converted into a mass of fibrous tissue called corpus albicans (white body).
Fallopian Tubes (Oviducts)
These are one pair of long (10 to 12 cm), ciliated, muscular and tubular structures which extend from the periphery of each ovary to the uterus. Each oviduct is suspended by mesosalpinx and is differentiated into three parts :
(i) lnfundlbulum : The part of oviduct closer to the ovary is the funnel shaped infundibulum. The edges of infundibulum possess finger-like projections called fimbriae. Fimbriae help in the collection of the ovum after ovulation. lnfundibulum opens into the abdominal cavity by an aperture called ostium.
(ii)Ampulla : The infundibulum leads to a wider part of the oviduct called ampulla.
(iii) Isthmus : It is the last and narrow part having narrow lumen that links to the uterus. The tube is involved in conduction of the ovum or zygote towards the uterus by peristalsis and ciliary action. It is also the site of fertilization. (Fertilization occurs at the junction of ampulla and isthmus).
It is a large hollow, muscular, highly vascular and inverted pear shaped structure present in the pelvis between the bladder and rectum. It is suspended by a mesentery, the myometrium. It has the following three parts.
(i) Fundus : It is upper, dome-shaped part above the opening of fallopian tubes.
(ii) Corpus/Body : It is the middle and main part of uterus.
(Iii) Cervix : It is lower, narrow part which opens in body of uterus by Internal os and in vagina below by external os. It Is mainly formed of the most powerful sphincter muscles in the body. The cavity of the cervix is called Cervical canal which along with vagina forms the birth canal.
Wall of uterus : The wall of uterus is formed of outer peritoneal layer, perimetrium; middle muscular myometrium of smooth muscle fibres, and inner highly vascular and glandular endometrium. The endometrium undergoes cyclical changes during menstrual cycle while myometrium exhibits strong contractions during delivery of the baby. Implantation of embryo occurs in uterine fundus. It is the site of foetal growth during pregnancy. It also takes part in placenta formation and expulsion of the baby during parturition.
It is a long (8.5 cm), fibro-muscular lube. It extends backward in front of rectum and anal canal from cervix to the vestibule. It is a highly vascular tube lined internally by mucus membrane which is raised into transverse folds called vaginal rugae. It is lined with stratified squamous epithelium (Non Keratinised). Vagina is devoid of glands.
Vaginal orifice is covered partially by a membranous diaphragm called hymen. The hymen is often ruptured during the first coitus (intercourse). However, it can also be broken by a sudden fall or jolt, insertion of a vaginal tampon, active participation in some sports like horseback riding, cycling etc. In some women the hymen persists even after coitus, In fact, the presence or absence of hymen is not reliable indicator of virginity or sexual experience.
Vagina acts both as copulation canal (as it receives the sperms from penis during copulation) and as birth canal along with cervix (during parturition).
The external genital structures of the female reproductive system are collectively called the vulva. The female external genitalia or vulva includes mons pubis, labia majora, labia minora, hymen and clitoris. Mons pubis is a cushion of fatty tissue covered by skin and pubic hair. The labla majora are fleshy folds of skin, which extend down from mons pubis and surround the vaginal opening.
The labia minora are paired folds of tissue in the form of lips under the labia majora. The opening of vagina is often covered partially by a membrane called hymen. The clitoris is a tiny finger-like structure which lies at the upper junction of the two labia minora above the urethral opening. It is formed of two erectile bodies and is covered by skin fold called prepuce. It has a depression, the vestibule, in front of anus. Vestibule has two apertures-upper external urethral orifice and lower vaginal orifice.
Vestibule is bounded by two pairs of moist skin folds called labia minora and labia majora. Labia majora is homologous to scrotum.
Labia minora fuse anteriorly to form a skin fold called prepuce in front of a small erectile organ, the clitoris which is homologous to penis as both are supported by corpora cavernosa. Labia minora also fuse posteriorly to form a membranous fold called fourchette.
The area between the fourchette and the anus is called perineum. There is fleshy elevation above the labia majora and is known as mons veneris (mons pubis) which has pubic hair.
Vestibular Glands : These are of two types-greater and lesser. Greater vestibular or Bartholin’s glands are a pair of small reddish yellow glands on each side of vaginal orifice and secrete alkaline secretion for lubrication and neutralising urinary acidity. Lesser vestibular glands or paraurethral or skene’s glands are small mucus glands present between urethral and vaginal orifices.
There are a pair of rounded prominences present over the pectoralis major muscles on the front wall of the thorax. These remain in rudimentary form in male. In females, these remain undeveloped till puberty. At puberty, these start developing under the influence of oestrogen and progesterone hormones. On the extemal side, each breast has a projection, the ‘nipple‘ surrounded by rounded hyperpigmented area called areola and appear deep pink or light brown. On the surface of the areola, numerous sebaceous glands, called areolar glands are present.
Internally, the breast consists of the glandular tissue forming mammary glands, the fibrous tissue (connective tissue) and the fatty or adipose tissue. Mammary glands are modified sweat glands.
(a) The glandular tissue comprises about 15-20 lobes in each breast. Each lobe is made up of a number of lobules. Each lobule is composed of grape like clusters of milk secreting glands termed alveoli. When milk is produced it passes from the alveoli into the mammary tubules and then into the mammary ducts. Near the nipple, mammary ducts expand to form mammary ampullae ( = lactiferous sinuses) where some milk may be stored before going to lactiferous ducts. Each lactiferous duct typically carries milk from one of the lobes to exterior.
(b) The fibrous tissue (connective tissue) supports the alveoli and the ducts.
(c) The fatty or adipose tissue is found between the lobes and covers the surface of the gland. The amount of the adipose tissue detennines the size of the breasts.
Main functions of the mammary glands are secretion and ejection (release) of milk. These functions are called lactation. Lactation is associated with pregnancy and child birth. Milk production is stimulated largely by the hormone prolactin secreted by anterior lobe of the pituitary gland. The ejection of milk is stimulated by the hormone oxytocin, released from the posterior lobe of the pituitary gland.
Human milk consists of water and organic and inorganic substances. Its main constituents are fat (fat droplets), casein (milk protein), lactose (milk sugar), mineral salts (sodium, calcium, potassium, phosphorus, etc) and vitamins. Milk is poor in iron content. Vitamin C is present in very small quantity in milk. The process of milk secretion is regulated by the nervous system. It is also influenced by the psychic state of the mother. The process of milk production is also influenced by hormones of the pituitary gland (already mentioned), the ovaries and other endocrine glands. A nursing woman secretes 1 to 2 litres of milk per day.
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