Reproductive System

Last Updated on Tuesday, 7 February 2012 10:44 Written by Sandesh Monday, 5 April 2010 07:25

Human Reproductive system

Reproduction is the process by which an organism produces offsprings of its own kind.

Human reproductive system consists of a pair of primary reproductive organs or gonads, accessory glands and ducts. The gonads produce gametes and secrete sex hormones.

Male reproductive organs: male reproductive organs consists of- a pair of testes, epididymis, vas deferens, ejaculatory ducts, urethra and penis.

It also includes accessory glands like seminal vesicles, prostate glands and bulbourethral glands.

1. Testes: A pair of testes is suspended in an outpouching of the skin called as scrotum. Testes are covered by three layers- outer tunica vaginalis, middle tunica albuginea and inner tunica vasculosa. Each testis consists of 200 to 300 lubules. Each lobule contains 2 to 3 coiled seminiferous tubules. Seminiferous tubules are lined with germinal epithelium and sertoli cells.  Germinal epithelium gives rise to spermatozoa. Group of interstitial cells or cells of Leydig are present between seminiferous tubules which secrete testosterone.  Seminiferous tubules are joined together to form straight tubules which forms network of tubules called as rete-testes. Rete testes give rise to 12 to 20 efferent ductules which enter the epididymis.
2. Epididymis: they are a pair of comma shaped structure of coiled tubes. It serves as an organ for storage of sperms. It is divided into head, body and tail. Tail is continuous with vas deferens.
3. Vas deferens: It is a thick muscular walled duct which transmits sperms from epididymis to ejaculatory duct. Each vas deferens ascends along the testis and runs upwards.
4. Ejaculatory ducts: Vas deferens of each side unites with a duct of seminal vesicle to form an ejaculatory duct. Both ejaculatory ducts open into the upper part of urethra.
5. Urethra: It is a long tube of about 18 to 20 cm in length. It arises from neck of urinary bladder to the tip of penis. It is divided into three parts- upper prostatic part, middle membranous part and lower penile part.
6. Penis: It is an erectile muscular organ used for micturition and copulation. Internally the penis is composed of three cylindrical masses of tissues. These tissues are erectile, which contain blood sinuses bounded in fibrous tissue. Distal end of penis is enlarged called as glans. It covered by a loose-fitting skin called prepuce or foreskin.

Accessory reproductive glands of males:

1. Seminal vesicles: These are convoluted accessory glands located between bladder and rectum. They secrete an alkaline fluid containing fructose, ascorbic acid etc. that mixes with sperms. Duct of seminal vesicle join with vas deferens to form ejaculatory duct.
2. Prostate gland: It is a single gland present below the neck of the urinary bladder. It secretes a thin milky fluid which helps sperms in motility.
3. Bulbourethral glands (Cowper’s glands): These are a paired rounded glands situated beneath the prostate on either side of membranous urethra. Ducts of these glands open into the penile urethra. They secrete mucus like secretion during sexual arousal for lubrication.

Female reproductive organs: female reproductive system consists of a pair of ovaries, fallopian tubes, uterus, vagina and vulva. It also includes accessory glands like Skene’s glands and Bartholin’s glands.

1. Ovaries: Ovaries are a pair of gonads present on each side of the uterus in the upper pelvic cavity. They produce female gametes called ova. They are attached to the broad ligament of the uterus by a double layered fold of peritoneum called as mesovarium. They are connected to lateral pelvic wall by suspensory ligament of ovary and lateral wall of uterus by ligament of ovary.

Ovary is covered by a single layer of germinal epithelium. Ovary has outer cortex and inner medulla. Cortex contains numerous ovarian follicles of different stages of development such as primordial follicles, maturing follicles, Graafian follicles and corpus luteum.

1. Fallopian tubes (oviducts): These are paired tubes which extend laterally from the uterus. They transport ova from the ovary into the uterus. Each fallopian tube has three distinct parts- infundibulum, ampulla and isthmus. Infundibulum is a funnel shaped opening of fallopian tube. It contains a number of finger like projections called as fimbriae. Ampulla is a wide and long portion of fallopian tube. Isthmus is a narrow, thick walled portion that opens into the uterus.
2. Uterus: It is also known as womb. It is a muscular pear shaped structure situated in the pelvic cavity, between urinary bladder and rectum. It is the site of menstruation, implantation of fertilized ovum and development of fetus.  The uterus is divided into three parts- upper dome shaped fundus, middle body and lower narrow cervix. In between body and cervix is a narrow constricted part called isthmus. Cervix opens to vagina. The cavity of the uterus is called uterine cavity. Uterus wall consists of three layers – outermost perimetrium, middle thick muscular myometrium and innermost mucus lining is endometrium. Endometrium again consists of two layers- a stratum basalis and a stratum functionalis. Stratum functionalis forms innermost layer of uterine cavity which shed off during menstruation.
3. Vagina: It is a muscular canal lined with mucus membrane. It is about 10 cm long. It is a female copulatory organ. It extends from vulva to the uterus. it serves as birth canal.
4. Vulva: It is an external genitalia which contains mons pubis, labia majora, labia minora and clitoris.

Accessory reproductive glands of female:

1. Skene’s glands (lesser vestibular gland): They are homologous to the prostate glands of the males. They are situated in the wall of urethra which secretes mucus.
2. Bartholin’s glands (Greater vestibular glands): They are homologous to the bubourethral glands of the males.  They are situated on either side of vaginal orifice. They secrete mucus for the lubrication of vulva.
3. Breast (mammary glands): They are the largest accessory reproductive organs in female. They are a pair of round, spongy and swollen organs for milk production. Each breast consists of 15 to 20 lobes of lactiferous tubules which are surrounded by fatty tissues. All tubules dilate and open at the tip of breast called as nipple through milk ducts. Breasts are developed in girls after puberty by the action of hormone oestrogen. The lobules become active after giving birth of a young one.

Spermatogenesis: This is the process of formation of spermatozoa. Sperms are produced in seminiferous tubules. Seminiferous tubules are lined with germinal epithelium and sertoli cells. Cells of germinal epithelium give rise to spermatozoa.

There are interstitial cells in between seminiferous tubules. These cells produce testosterone when stimulated by Luteinizing hormone (LH) from pituitary. Similarly FSH from pituitary stimulates sertoli cells to produce testosterone binding proteins which bind testosterone. Testosterone is necessary for spermatogenesis. During spermatogenesis germinal epithelium develops into spermatogonia, primary spermatocytes, secondary spermatocytes, spermatids and finally spermatozoa.

Oogenesis: This is the process of formation of ova in ovary. There are about 1 million primary oocytes in two ovaries of a girl by birth. They are surrounded by primordial follicles. These primary oocytes develop into a mature ovum every month during her fertile period.

Process of Fertilization:

Fertilization in female body takes place in the upper part of the fallopian tube. The sperms are transferred to the genital tract of female through a muscular organ called penis during copulation. Ova are produced in the ovaries and discharged into the body cavity which enters into the fallopian tube.

If the ovulation occurs shortly after intercourse, ovum may be fertilized by one sperm. Fertilization occurs in upper part (ampulla) of fallopian tube within 24 hours after ovulation. During sexual intercourse, nearly 300 million sperms are introduced into vagina. But only few hundreds reach near the ovum.

[Ovum is surrounded by gelatinous zona pellucida, a corona radiata and several layers of granulosa cells (cumulus oophorous cells)]. A sperm has to penetrate through these layers to fertilize the ovum. Acrosomal enzymes dissolve the intercellular material covering the ovum. Sperms undergo certain physiological changes in female genital tract called sperm activation or capacitation. These changes enable the sperm to penetrate the cellular layers arround ovum. This sperm binds with zona pellucida, which is a species specific binding. After binding, sperm penetrate through the zona pellucida with the help of acrosomal proteolytic enzymes. The head of the sperm is drawn into the ooplasm. After penetration, the ovum which is actually a secondary oocyte completes its second meiotic division with next few hours.  Once the sperm penetrate, there are certain changes in the surface of the ovum and thezona pellucida hardens which block the entry of other sperms.

After penetration, the sperm nucleus develops into male pronucleus and nucleus of ovum into female pronucleus. They which migrate to the centre of the ooplasm and fuse to form a diploid zygote.zygote is ready for its further development and passes down to the uterine tube and the uterus.

If the fertilization takes place, graafian follicles do not die and keep producing progesterone. This is called as true graafian follicle. Progesterone level makes the wall of uterus (stratum functionalis) very vascular and prepares uterine it for implantation of zygote. Thus the zygote is implanted in uterine wall for further development.

Menstrual cycle:

In female, one ovum is produced once in 28 days. The female reproductive system passes through regular monthly sequential events. This cyclic reproductive stage is called as menstrual cycle.it begins at puberty and continues until menopause. Duration of menstrual cycle ranges from 28 to 36 days. During this period, changes take place in the ovary and endometrium of uterus.

Mentrual cycle can be studied in following phases:

1. Menstrual phase: This phase involve the shedding off of endometrium of uterus with the discharge of about 25 to 65ml of blood with tissue and mucus. It is caused due to sudden decrease in the level of progesterone and oestrogen. This phase last for about first 5 days of the cycle.
2. Proliferative (oestrogenic) phase:  this is also called as preovulatory phase. During this phase, FSH from pituitary stimulates primordial follicle to develop into mature graafian follicle. Now, graafian follicle secretes oestrgen which stimulates stratum basilis of the endometrium to proliferate new layer of stratum functionalis. This phase last from 6^th to 13^th day of menstrual cycle.
3. Ovulatory phase: There is sudden secretion of LH from pituitary called LH surge. This stimulates graafian follicle to release ovum. This occurs in the 14^th day of the cycle.
4. Secretory (Luteal) phase: After ovulation graafian follicle develops into corpus luteum. Corpus luteum secretes hormone progesterone. Progesterone prepares the endometrium of uterus for implantation of fertilized ovum (if fertilized). This increase the vascularity of the endometrium.
5. Premenstrual (post ovulatory) phase: If fertilization does not occur, corpus luteum degenerates and becomes corpus albicanus. Such degenerating corpus luteum is called as false corpus luteum. Corpus albicanus does not secrete progesterone. So when the progesterone level goes down then the highly developed stratum functionalis of endometrium of uterus starts disintegrating. Thus, the endometrium wall sheds off with the discharge of blood tissues and mucus.