Mammalian Reproductive System

Mammals are unisexual. It means male and female sexes are separate. Each sex has repro­ductive organs (gonads), reproductive ducts and accessory structures. The gonads are called the primary sex organs. There are male gonads (testes) in males and female gonads (ovaries) in females. They produce gametes and sex hormones. The growth maintenance and functions of the gonads are regulated by gonadotropins of the anterior lobe of pituitary gland. The organs which neither produce gametes nor secrete sex hormones but perform important functions in reproduction are called secondary sex organs. The latter include the prostate, seminal vesicles, vas deferentia and penis in males, and the Fallopian tubes, uterus, vagina and mammary gands in females. The characters which distinguish the male from the female externally are called ac­cessory or external sex characters. They are also called secondary sex characters. The male gamete is the spermatozoon & female gamete is the Ovum.

Human Male Reproductive System it consists of the following parts:

1. Scrotum. It is a pouch of deeply pigmented skin divided into two separate sacs. Each sac contains one testis. The normal temperature of the testes in the scrotum is about 2° to 2.5 0C lower than the internal body temperature— the ideal temperature for developing sperms. When the body is chilled, the smooth muscle contracts and brings the testes closer to the pelvic cavity. Movement towards the pelvic cavity when the temperature drops allows the testes to absorb heat from the rest of the body so that the sperm cells do not become chilled. The scrotum remains connected with the abdomen or pelvic cavity by the inguinal canals. The spermaticord, formed from the spermatic artery, vein and nerve bound together with connective tissue passes into the testis through inguinal canal.

Important notes by AKB sir:-
  1. Internally scrotum is lined by dartos muscle & spermatic fascia.
  2. Dartos muscle helps in regulation of scrotum internal temperature during cold season.
  3. It becomes contracted in cold & during warm season, it becomes relaxed.
  4. Cremaster muscles line inside the wall of scrotal & inguinal canal region & help in elevation of testes.
  1. Testes. There are two testes, one being larger than the other; in adult each weighs 10-40 gms (average 25 gm), weight decreases in old age its size is 4 – 5 cm X 2 – 3 cm. It has rich sympathetic innervation.Testes are primary sex organs in man. During early foetal life the testes develop in the abdominal cavity just below the kidney then they descend into the scrotum. There is a pair of testes that are suspended in the scrotum by the spermatic cords. A fibrous cord that extends from the caudal end of the testis to the scrotal wall is called gubernaculum or Mesorchium made of elastic fibre.

Important Note by AKB sir:-

1.The spermatic cord is made up of elastin fibre & spermatic fascia.

2. The contents of cord are vas deferens, gonadial vein, gonadial arteries, nerves & lymphatics.

 

Important Note by AKB sir:-
  1. When the testes does not descend to reach the scrotum but remain in abdominal cavity at the time of this conditions is called undescended testes or cryptorchidism. Such testis cannot develop and function properly and may develop malignancy.
  2. Orchiopexy: When the undescended testes are brought into scrotal sac by surgical process during childhood this process called as orchiopexy.
  3. Castration : Crushing of testes in bulls to convert them to bullocks. (This makes than more obidient due to fall in the level of testosterone)

 

(L.S of testis )

 

(i) Tunicae or Coverings of Testes. The testis is surrounded by three layers,

(a) The tunica vaginalis is the outer covering of the testis.

(b) The tunica albuginea is a fibrous cover¬ing surrounding the testis situated under the tunica vaginalis.

(c) The tunica vasculosa consists of a network of capillaries supported by delicate connective tissue which lines the tunica albug-inea.

(ii) Testicular Lobules. Ingrowth of the tunica albuginea forms septa dividing the glandular portion of the testis into 200 to 300 testicular lobules.

Important Note by AKB sir:-

1.The testis that descend into the scrotum of some animals only in the breeding-season, e.g. Order Chiroptera Order Rodentia

2.Testes of some animals are permanently placed in the abdominal-cavity. Their scrotum is vestigeal-organ, e.g. Prototherians, animals of the order Edentata (Armadillo, Pangolin, Sloth), Order Proboscida (Elephant) , Order Sirenia and Order Cetacea.

3.Castration : Crushing of testes in bulls to convert them to bullocks. (This makes than more obidient due to fall in the level of testosterone)

 

Important Note by AKB sir:-
  1. Each scrotum is connected to the abdominal cavity through a passage termed as inguinal-canal. Through this canal the testis descend down into the scrotal-sacs at the time of birth.
  2. Spermatic cord in males passes through the inguinal canal.
  3. Sometimes the inguinal canal may tearjeading to abnormal protrusion of abdominal contents like loops of intestine into the inguinal canal. This condition is called as inguinal hernia.

 2. Blood supply. It is highly vascular and capillaries of the testes are not fenestrated. The spermatic arteries to the testes are tortuous and blood in them runs parallel but in the opposite direction to blood in the pampiniform plexus of spermatic veins. This anatomic arrangement may permit countercurrent exchange of heat and testosterone.

(iii) Seminiferous tubules. Each testicular lobule of testis contains two to three seminiferous tubules(70-80 cm long), blood vessels, nerves and connective tissue. Wall of each seminiferous tubule is formed of a single layered germinal epithelium. Majority of cells in this epithelium are cuboidal, however, at certain places, there are present large pyramidal Sertoli or nurse cells. Sertoli cells secrete androgen binding protein (ABP) that concentrates testosterone in the seminiferous tubules. Sertoli cells also secrete another protein called inhibin which suppresses FSH synthesis. The cuboidal cells undergo mitosis to produce spermatogonia. Spermatogonia grow into primary spermatocytes which undergo meiosis, producing haploid cells, first secondary spermatocytes and then spermatids. The latter convert into spermatozoa (sperms). Sertoli cells nourish the developing sperms. Total number of seminiferous tubules in each testis is about 750 to 1000 in man if is 6m in length.

 

 

(T.S. of mammalian seminiferous tubules.)

AKB’s Stroke :-

The seminiferous epithelium contains Sertoli Cells whose features are :

(i) These are large columnar cells which extend from the basement membrane to the centre of the lumen of the seminiferous tubules.

(ii) The cells are rich in glycogen which nourish the germ cells, the sperms.

(iii) They secrete androgen binding protein-ABP, inhibin B and MRF

 (iv) They also secrete an enzyme aromatase (CYP 19) that converts androgens to oestrogen.

 (v) They provide blood testis barrier (see below) to chemicals and phagocytose defective sperms.

(vi) They cause controlled release of mature sperms into the lumen of seminiferous tubules (spermiation).

 

Important Note by AKB sir:-

1. Because of relatively tight connections between the Sertoli cells and other cells lining the seminiferous tubular wall (Blood Testis Barrier), proteins and other substances penetrate poorly into the area near the wall of the tubule. However, testosterone and other steroids penetrate this barrier with ease due to the fact that the Leydig cells are close to the tubules.

2. Blood testis barrier helps maintaining the composition of the fluid in the lumen of the seminiferous tubules and also protects the germ cells from blood-borne noxious agents. It also prevents antigenic products of germ cell division from entering the circulation and generating an autoimmune response.

(iv)  Interstitial Cells or Leydig's Cells (Endocrine portion of the testis). These are endocrinal cells which develop from the mesoderm of the embryo and are usually arranged round the blood vessels. They are abundant in the 4th month of foetal life, fewer in the newborn and continue to diminish by the end of childhood; their number again increases at puberty. In between the seminiferous tubules in the connective tissue, there are present small groups of rounded interstitial or Leydig's cells which secrete androgens (e.g., testosterone) — male sex hormones.

Function:-

(a) They secrete androgenic hormone – testosterone at about the time of puberty.

(b) It helps in growth of the accessory organs of reproduction (epididymis, seminal vesicles, prostate, penis) and is responsible for the appearance of the secondary male sex characters.

(c) Testosterone secretion is depressed by undernutrition, specially by vitamin B deficiency.

(v)  Rete testis and vasa efferentia.The seminiferous tubules are closed at one end but on the other side they join to a network the rete testis from where fine ciliated ductules, the vasa efferentia arise.

3.Epididymes. The epididymis is a mass of long narrow closely coiled tubule which lies along the inner side of each testis approximately 7 metres long. At the anterior end of the testis it is called caput epididymis or Globus-major, in which the vasa efferentia open. The middle part of the eipdidymis is known as corpus epididymis. The posterior end of the epididymis is called as cauda epididymis or Globus minor. The epididymis stores the sperms and also secretes a fluid which is considered to nourish the sperms. This epididymis is also responsible for functional maturation of sperm.

Important Note by AKB sir:- Oestrogen content of the fluid in the rete testis is high. Here the fluid is reabsorbed and the sperms are concentrated. If the sperm directly enter the epididymis, they get diluted in the large volume of the fluid and infertility may occur.

Function :-

  • Spermatozoa (sperms) in the seminiferous tubular lumen are moved along the tubules to the Ductuli Efferentes which lead to the epididymis.
  • The spermatozoa are then stored in the tail of epididymis, where they can remain viable for a month.
  • Its wall contains:
  • (i) Smooth muscle, and
  • (ii) Secretory columnar epithelium, its secretion nourishes the spermatozoa and helps them to mature.
  • Here non-motile spermatozoa become motile when they are exposed to 02.
Important Note by AKB sir:-

Epididymal secretions are:

(a) rich in potassium and have a high K+/Na+ ratio;

(b)high concentration of glyceryl-phosphoryl-chlorine, a potential source of energy

(c) contain testosterone which helps in the maturation of spermatozoa.

 (An enzyme which splits off choline and releases glycerophosphate is found in the endometrium, this could explain the enhanced sperm motility in utero.)

 4. Vasa deferentia. A vas deferens emerges from the cauda epididymis on each side and leaves the scrotal sac and enters the abdominal cavity through the inguinal canal. The vas deferens passes upwards and ascends medially towards the posterior wall of the urinary baldder where it is joined by duct from the seminal vesicle to form the ejaculatory duct. Vasa deferentia carry sperms. It serves as a secondary storehouse for spermatozoa which will be released at the time of ejaculation. Terminal dilated part of vas deferens are called ampulla. Vas deferens runs upward & enters into abdominal cavity. Both vas deferens coil around the ureter of their respective sides and then dilate to form ampulla. Ampulla of each side receives the seminal vesicle of that side. The vas deferens now forms ejaculatory duct and opens into prostatic urethra.

 

Important Note by AKB sir:-

(i) Vas deferens and Epididymis both develop from the wolfian-duct of mesonephros.

(ii) Epididymis can temporarily stores the sperms for as long as one month and here the functional maturation of sperms takes place. 18 to 24 hr. after functional maturation of sperm, sperms proceed further to store in vas deferens, specially in its ampulla part.

(iii) The wall of epididymis is made up of 2 layers-outer circular muscle layer and inner epithelium.

(iv) Wall of vas-deferens is also made up of 2 layers-outer circular muscle layer and epithelium.

(v) The sperms reach the abdominal cavity due to the pulsation of the vas-deferens.

Important Note by AKB sir:-

Vasectomy:-

(i)  simple procedure for sterilization in males, in which vas deferens is ligated and sectioned.

(ii) It may cause some degeneration of Seminiferous tubules without affecting the testosterone secretion from the Leydig cells and secretions of other related glands i.e. seminal vesicles; prostate and Cowper's glands.

 

Important Note by AKB sir:-

For first two months after vasectomy viable spermatozoa may be released from the ampulla, therefore, during this period conventional methods should be used for contraception purposes

 

Differences between Vasa efferentia and Vasa deferentia

Vasa Efferentia

Vasa Deferentia

1. They arise from the rete testis.

2. They vary from 15 to 20 in number.

3. Vasa efferentia are fine.

4. Their lining bears many ciliated cells.

5. It carries spermatozoa from the rete testis to

the epididymis.

1. They arise from the cauda epididymes.

2. They are only 2 in number.

3. Vasa deferentia are thick.

4. Their lining has many stereocilia.

5. It carries spermatozoa from cauda epididymis to the ejaculatory duct.

 5.Ejaculatory ducts. The ejaculatory ducts are two short tubes each formed by the union of the duct from a seminal vesicle and a vas deferens. They pass through the prostate gland and join the prostatic part of the urethra. The ejaculatory ducts are composed of the fibrous, muscular and columnar epithelial tissue. Ejaculatory ducts carry sperms and secretion of seminal vesicles.

6. Seminal Vesicles:-

1. These are two lobulated glands (about 7 cm in length), situated between the urinary bladder and rectum which secrete fluid into the ejaculatory duct.

2. Its secretion is thick and sticky (viscid) and rich in:

(i) potassium,

(ii) fructose, it is oxidised by the mitochondria of the middle piece of spermatozoa,

(iii) phosphoryl choline,

(iv) citric acid,

(ii), (iii) (iv) provide energy for movement of the spermatozoa.

(v) ascorbic acid, and

(vi) hyaluronidase enzyme, splits mucopolysaccharides, therefore, sperms can penetrate through the

cervical mucus plug.

3. It also synthesizes and secretes prostaglandins which increase formation of cAMP. This leads to the contraction of uterus during coitus and therefore, sperms can be sucked into the uterus.

4. Its secretion contributes to 60% of semen's total volume.

SEMINAL FLUID (SEMEN)

It is the fluid that is ejaculated at the time of orgasm.

  1. It consists of the products of the seminiferous tubules (i.e. the sperms) and the secretions of the seminal tract and related glands
  2. Characteristic features:-

Composition

Colour :- White, opalescent

Specific gravity :- 1028

pH :- 7.35 to 7.50

Average volume :- 2.5 to 3.5 mL per ejaculate.

Sperm Count:- 80-120 million/mL (average 100 million/mL); motile with more than 80% sperms having normal morphology.

Important Note by AKB sir:- Volume of semen and sperm count decreases rapidly with repeated ejaculation. 50% of men with counts of 20-40 million/mL and essentially all of those with count less than 20 million/mL are sterile.

Normal sperm count 20 to 120 million/ml.

Oligospermia < 20 million/ml.( are sterile.)

Azospermia - either absence or near absence of sperms

Other components:

From seminal vesicles (form 60% of semen's total volume:

(i) Fructose (1.5-6.5 mg/mL)

(ii) Phosphoryl-choline

(iii) Ergothioneine

(iv) Ascorbic acid

(v) Flavins

(vi) Prostaglandins.

From prostate (form 20% of semen's total volume)

(vii) Spermine

(viii) Citric Acid

(ix) Cholesterol, phospholipids

(x) Fibrinolysin, fibrinogenase

(xi) Zinc

(xii) Acid phosphatase

(xiii) Phosphate(act as buffers)

(xiv) Bicarbonate (act as buffers)

(xv) Hyaluronidase.

Important Note by AKB sir:- The human male ejaculates about 200 to 300 million sperms during a coitus of which, for normal fertility,at least 60 percent sperms must have normal shape and size and for at least 40 percent of them must show vigorous motility.
  1. Reaction: Alkaline fluid - Importance: Sperms get rapidly immobilized in an acid medium; acid prostatic fluid being neutralized by secretions of other related glands .
  2. The fluid part of the semen is contributed chiefly by the prostate and seminal vesicles.
  3. It is rich in fructose (fructose concentration exceeds that of glucose in blood): significance - spermatozoa have very little cytoplasm and use fructose as their metabolic fuel.
  4. It is liquid when ejaculated but soon coagulates in vitro or in the vagina; however, after 15 minutes, it undergoes secondary liquefaction.
  1. Urethra. The male urethra provides a common pathway for the flow of urine and the secretions of the male reproductive organs called semen. It is much longer in male than in the female, measuring about 20 cm. The urethra includes three parts (i) The first part is surrounded by the prostate gland and is called the prostatic urethra which carries urine only, (ii) The second part is the membranous urethra which is situated behind the lower part of the pubic symphysis. The membranous urethra is the smallest urethra, (iii) The third part is the penile urethra which is situated in the penis. There are two urethral sphincters. The internal sphincter consists of smooth muscle fibres situated at the neck of the bladder above the prostate gland. The external sphincter consists of striated muscle fibres surrounding the membranous part of the urethra. Membranous urethra and penile urethra carry both urine and semen.
  1. Penis. The penis, in addition to conducting urine from the body, is the male copulatory organ. It transfers semen into the reproductive tract of the female during sexual intercourse. The penis contains three cylindrical masses of erectile tissue— two dorsal corpora cavernosa and one ventral corpus spongiosum. These bodies are surrounded by fibrous tissue. The corpus spongiosum, which contains the penile urethra, is enlarged at the distal end of the penis to form the glans penis. The penis is covered with smooth skin, the prepuce or foreskin. During sexual arousal the three bundles of tissue in the penis become engorged with blood.

(T.S. of mammalian penis)

Accessory Sex glands

(i) The seminal vesicles are one pair of sac like structures near the base of the bladder;their ducts join the vasa deferentia to form the ejaculatory ducts. They produce a thick secretion which forms the bulk of semen. The secretion of the seminal vesicles contains fructose, prostaglandins, amino acids, proteins, ascorbic acid and citric acid. The fructose is a source of energy for the sperm. The prostaglandins stimulate uterine contractions and thus may help the sperm to be moved towards the female's oviducts, where fertilization takes place.

(ii) The prostate gland is a single large gland that surrounds the urethra.This gland is located below the urinary bladder. It is unpaired & made up of 5 lobes in man. It makes about 30% part of semen. It produces a milky secretion which forms a considerable part of semen. This secretion contains citric acid, lipids and enzymes (acid phosphatase, amylase, protease) and prostaglandins as well as significant numbers of bicarbonate ions, which give the semen its aklakine pH. In the secretion of prostate-gland Calcium and phosphate, clotting enzyme and profibrinolysina are also present. The secretion of the prostate gland combines with the secretion of seminal vesicle and so the semen gets coagulated. In the coagulated semen, the mobility of sperms is reduced and so their energy is conserved. After some time due to fibrinolysins, semen again liquefies and in this semen now the sperms can move. A number of small ducts carry fluid from the prostate to the urethra. Secretion of the prostate gland nourish and activates the spermatazoa to swim.

(iii) A pair of bulbourethral glands or Cowper's glands are present in the bulbous part of urethra. Their secretions, which contain mucus for lubrication, enter the semen through ducts attached to the membranous portion of the urethra. It is slightly alkaline (pH is 7.2). This destroys the acidity of the urethra and cleans it for the movement of sperms

(iv) Perineal or Rectal glands:-These are found both in male and female rabbits. Perineal gland is found around external genital organ & perineal region, while rectal glands are found around external genital organs & anus. During the breeding season, these glands secrete odoriferous liquid which has pheromones or Eciohormones in it. The pheromone develops sexual attraction between opposite sexes & also develops desire for copulation.

Note :- In man, Perineal or Rectal glands are absent.

 

Differences between Primary Sex Organs and Secondary Sex Organs

Primary Sex Organs

Secondary Sex Organs

1. They produce gametes.

2. They also secrete sex hormones.

3. Testes in male and ovaries in female are examples of primary sex organs.

1. They do not produce gametes. They are concerned with the conduction of gametes.

2. They do not secrete sex hormones.

3. Epididymes, vasa deferentia, penis, etc. are secondary sex organs in male and oviducts, uterus, etc. are examples of secondary sex organs in female.

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