Human Reproduction – NCERT/Advance Level Notes

Introduction

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

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.

Facts:-

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.

Penis :- 

This  is the copulatory organ of man. It is a cylindricalerectilependulous 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.

male reproductive organ – penis

Internal Structure:-

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:-

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.

Ovaries:-

The ovary  is the primary  female sex organ. It produces ova and  secretes the female sex  hormonesestrogens 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).

Uterus  (Hystera/Womb)

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 layerperimetriummiddle 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.

Vagina:-

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).

External  Genitalia:-

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  majoraLabia 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.

Accessory Glands

Vestibular Glands : These are of two  types-greater and lesserGreater 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.

Mammary Glands/Breasts

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.

Gametogenesis:-

will upload very soon…..

2 thoughts on “Human Reproduction – NCERT/Advance Level Notes”

    1. Thank you, Johnnyerofe! I’m glad you found the phrase “Humans are sexually reproducing and viviparous organisms. Their reproductive events include the formation of gametes (gametogenesis)” to be excellent. It’s indeed a concise and accurate way to describe the reproductive process in humans.

      Thank you for sharing your appreciation, and if you have any further questions or thoughts on this topic, feel free to share them!

Leave a Comment

Your email address will not be published. Required fields are marked *