Reproductive System Of Mammals
Human Reproduction of Class 12
Being viviparous the structural/functional organisation of reproductive organs is of highest grade.
Male Reproductive System
Consists of a pair of testis with epididymis, a pair of vas deferens a pair of seminal vesicle and ejaculatory duct, a multilobulated prostate gland,
a pair of Cowper’s gland and one urethra passing through the penis.
The male reproductive organs include the external and internal genitalia. External Genitalia are the penis and secrotum. Internal Genitalia on each side are epididymis, the ductus deferens (vas deferens or deferent duct), the seminal vesicle, the ejaculatory ducts, the prostate and male urethra.
Fig. Male Sex Organs: A. L.S. of rabbit’s reproductive organs; B. L.S. of human reproductive organs
Male gonads, homologous with the ovary of female. It is suspended in the scrotum by the spermatic cord. An adult testis weighs about 10 to 15 gm. Most commonly . it is extra abdominal. At the time of birth it descends in scrotal sac (scrotum); FSH helps in the process, because, spermatogenesis takes place at comparatively lower temperature than the body temperature. But, in elephant, sloths, dolphin, whale (cetacean), sirenian, hyracoidea testes are intra abdominal. In seasonal breeders (bat, rat, wolf, foxes) testis descends into scrotum only during breeding season. Testis remains connected to scrotal wall through gubernaculum representing mesorchium.Scrotum is separated from abdominal cavity by inguinal plate or ring with inguinal canal through which passes spermatic cord with sperm duct, blood vessels, nerves, cremaster muscle and connective tissue cord.
Coverings of the Testis
Testis is covered by three coats :
- Tunica vaginalis - outer covering of testis
- Tunica albuginea - fibrous covering surrounding the testis situated under tunica vaginalis.The posterior border of the tunica albuginea is thickened to form an incomplete vertical septum called the mediastinum testis. Numerous septa extend from the mediastinum to the inner surface of the tunica albuginea. They incompletely divide the testis into 200 to 250 lobules. Tunica vasculosa is the inner most, vascular coat of the testis lining its lobule.
Structure of Testis
Glandular part of the testis consists of 200-250 lobules. Each lobule contains two to three seminiferous tubules. The tubules are lined by cells which represent stages in the formation of sperm.
Fig. L.S. of Testis Fig. Enlarged section of a seminiferous tubule of rabbit
The seminiferous tubules join together at the apices of the tubules to form 20 to 30 straight tubules (tubulirecti) which enter the mediastinum. Here they anastomose with each other to form a network of tubules, called the rete testis. The rete testis gives rise to 15 to 20 ciliated efferent ductules called vasa efferentia which emerge near the upper pole of the testis and enter the epididymis. Sertoli cells (nurse cell or sustentacular cells) are the columnar nutritive cells emerging from germinal epithelium that keeps sperms with their head burried into it to draw nutrition. Inhibin secreted by this cells inhibits FSH synthesis i.e. it has negative feedback effect with FSH. The release of spermatozoa from sertoli cell into the lumen of seminiferous tubule is called spermiation. Structure and spermatogenic function of seminiferous tubule is maintained by FSH.
Epididymis (pl. = epididymes) Consists of a single narrow, long (~ 6 metre in man) tube tightly coiled as a mass along the antero-dorsal side of each testis In rat & rabbit it is
distinct into three parts :
- Caputa epididymis (head or globus major)
- Corpus epididymis (body)
- Cauda epididymis (tail or globus minor)
Storage and activation of sperms take place here. Its secretion also contributes to seminal fluid. If there is no ejaculation for longer time the sperms get absorbed by its wall. Prior to ejaculation sperms remain stored in last portion of epididymis and the vas deferens. Sperms acquire motility in epididymis Vas deferens (30 cm in man) Arising from the lower end of cauda epididymis as straight tube, it ascends to enter abdominal cavity via inguinal canal alongwith spermatic cord.
It is also glandular and contributes to seminal fluid.
After entwining with ureter it comes down and receives duct from seminal vesicle.
It acts as secondary store house for sperm.
It is cut or tied off as the birth-control measure called vasectomy.
Seminal vesicles (~ 5 cm in man)
Paired sac-like glandular part between urinary bladder and rectum; secrete a viscous fluid as the main (60 - 70%) part of the ejaculate (semen) containing protein, fructose, citrate, inositol, prostaglandins and several proteins. Sperms have fructokinase enzyme to utilize fructose sugar.
In rabbit, instead of seminal vesicle there is a dorsomedian pouch-like, uterus masculina (behind urinary bladder) that opens into urethra. Ejaculatory Duct (2 cm in man)
Union of vas deferens with the duct of seminal vesicles forms a short contractile ejaculatory duct. It helps in the emission of the seminal fluid. Urethra (20 cm in man)
In male it is the common pathway for release of urine and seminal fluid.
Divided as upper prostatic part, middle membranous part and the lower penile part. The basal part of prostatic urethra is dilated chamber like called prostatic utricle or vagina masculina.
In rat this is referred as ampullary gland. Both uterus masculina and vagina masculina are derived from embryonic Mullerian duct.
A chestnut shaped multilobular gland (with 30-40 tubulo-alveolar units) at the base of the urinary bladder surrounds the first part of the urethra. It secretes an odorous alkaline fluid as part (30 - 40%) of the seminal fluid; contains prostaglandins, citrate, calcium and phosphate ion, a clotting enzyme. In old age benign prostatic hypertrophy (BPH) causes obstruction in urination. Bulbourethral glands (or Cowper’s glands) A pair of pea-sized glands along the sides of urethra at the base of penis; secretes viscous alkaline fluid as lubricant which also neutralizes the acidic effect of urine in urethra and contributes to the seminal fluid to help in upkeeping of sperms.
Perineal glands :
Glands situated near anus in some mammals (rat, rabbit) and secrete odorous substances for sexual attraction Maturation of sperms Sperm is required to stay for a period of time in epididymis to achieve activity and motility. Human sperms can survive for 4 weeks in epididymis but, once ejaculated it survives only for 48 to 72 hrs. in female reproductive tract Fructose present in semen is not secreted elsewhere in human body hence its presence in vagina confirms sexual intercourse.
About 3-5 ml of semen is ejaculated at each coitus in man. 1 ml of semen has nearly 100 million sperm. The count below 20 million/ml (oligospermia) leads to infertility. The absence of sperm is called azoospermia.
Penis A vascular, spongy, erectile structure is a copulatory organ. Its long shaft has enlarged bulbous tip, glans penis, covered by the foreskin, prepuce. Consists of 3 columns of erectile tissue: one pair dorsal corpus cavernosa and a median ventral corpus spongiosum containing urethra. Outer margin of glans has Glands of Tyson which secrete an odoriferous cheese like substance called smegma. In rat these are called preputial glands In rodents, dog, walrus, there is also a cartilaginous or bony core called os penis or baculum or penile bone.