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MHT-CET : Biology Entrance Exam

MHT - CET : Biology - Reproduction Page 1

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Introduction

 

  • Reproduction is the process by which organisms produce new individuals of the same kind.
  • Reproduction is an essential life process, which results in maintaining the species.

 

 

Human Reproduction

 

  • Human beings reproduce by sexual method.
  • It involves two gametes, i.e. sperms from the male and ova from the female.
  • During fertilisation, the sperm and ova fuse to form a zygote, which develops into an embryo.
  • The mother gives birth to the young one.
  • Human beings are, thus, viviparous.

 

 

Male Reproductive System

 

The male reproductive system of man consists of the following parts:

  1. Scrotum:
    • It is a pouch which is thin walled and hangs downwards from the abdominal cavity.
    • It is divided by a vertical partition into two scrotal sacs, which enclose testes.
    • The scrotum gives protection to the testes.
    • It also keeps them at a lower temperature than that of the body for the development and maturation of sperms.
  2. Testes:
    • A pair of testes is present in the scrotum.
    • They are attached to the wall of the scrotum by a band of connective tissue called gubernaculum.
    • Testes produce sperms and the male sex hormone, testosterone.
  3. Vasa Efferentia:
    • Sperms are carried into the epididymis through numerous fine ducts that arise from the base of the testes. They are called vasa efferentia.
  4. Epididymis:
    • It is a long, coiled tube, which forms a compact cap-like mass at the top of each testes.
    • It stores the sperms. The sperms undergo maturation here.
  5. Vas Diferentia:
    • These are a pair of long, coiled tubes, which arise from cauda epididymis, run through inguinal canal in the abdominal cavity and join with the duct of seminal vesicle to form the ejaculatory ducts.
    • The ejaculatory ducts open into urethra.
  6. Urethra:
    • It is the downward extension of the urinary bladder.
    • It serves as a common passage for releasing semen as well as urine at different times.
  7. Penis:
    • It is the copulatory organ located in front of the scrotum.
    • The glance penis is covered by a loose retractable skin fold called prepuce at its anterior.
    • The urethral opening is present at the tip of the glance penis.

 

 

Accessory Sex Glands

 

  1. Seminal Vesicles:
    • These are a pair of sacculated glands present behind the urinary bladder.
    • Their secretion forms major part of the seminal fluid or semen.
    • As it contains fructose, it serves as a source of energy for the mobility of sperms.
    • It lubricates the passage of sperms and protects them from the effect of acidity present in the female genital tract.
  2. Prostate Gland:
    • It surrounds and opens at the base of urethra.
    • Its secretion is alkaline in nature, and therefore, neutralises the acidity of the vaginal passage.
  3. Cowper's Gland:
    • They are a pair of pear-shaped glands located at the base of the penis.
    • The secretions of these glands lubricate the passage of sperms.

 

 

Histology of Testes

 

  • Externally, the testis is covered by tunica albuginea (a fibrous capsule)
  • Internally, it is made up of seminiferous tubules.
  • Interstitial cells are present in between the seminiferous tubules.
  • These cells secrete the hormone testosterone.
  • Sertoli cells nourish the developing sperms.

 

 

Female Reproductive System

 

The reproductive system of a female consists of the following parts:

  1. Ovaries:
    • These are a pair of oval bodies located near the kidneys on either side of the uterus.
    • They produce ova.
    • They also secrete the female sex hormones - oestrogen and progesterone.
  2. Oviducts:
    • These are a pair of ducts which extend horizontally from the ovaries to the uterus.
    • They are commonly known as fallopian tubes, where maturation and fertilisation of ovum takes place.
  3. Uterus or Womb:
    • It is a large, highly distensible muscular bag, which lies between the rectum and the urinary bladder.
    • The fertilised egg gets implanted in the uterus and develops into an embryo.
  4. Vagina:
    • It is a muscular tube which lies below the cervix.
    • It receives semen from the male during copulation.
    • It also serves as a birth canal during delivery of a baby.
  5. Vulva:
    • It is the external female genitalia in which the vagina and urethra open separately.
    • A small conical projection called clitoris lies just above the urethral opening.
    • It is the site of stimulation during copulation and corresponds to the penis of the male.

 

 

Accessory Sex Glands

 

  1. Bartholin's Glands or Vestibular Glands:
    • These are a pair of glands situated on either side of the vaginal opening.
    • Their secretion keeps the vulva moist.
  2. Mammary Glands:
    • These are a pair of glands located in the pectoral region on the ventral side.
    • They grow during puberty but become active only after child birth and secrete milk for the infant.

 

 

Histology of Ovary

 

  • Human ovary is divided into two parts: the outer cortex and the inner central region called medulla.
  • A number of follicles are seen in the cortex region in different stages of development.
  • Out of these, only one attains maturity every month.
  • This mature follicle is called graafian follicle.
  • It encloses the ovum at the secondary oocyte stage.
  • The ovum is surrounded by three cellular layers: zona pellucida, corona radiata and discus proligerous.

 

 

Female Reproductive Cycle (Menstrual Cycle)

 

  • The cyclic changes that occur in the ovaries and wall of the uterus (endometrium) every month in the females from menarche to menopause is called the female reproductive cycle or menstrual cycle.
  • It consists of four phases:
  1. Menstrual Phase:
    • In the absence of fertilisation, the endometrium and the blood vessels of uterine wall break.
    • This results in bleeding through vagina (menstrual flow or menses), which lasts for
      3
      - 5 days.
    • There is a decrease in the level of progesterone and estrogen.
  2. Follicular or Proliferative Phase:
    • Estrogen stimulates the repair of endometrium, which assumes a thickness of about
      2
      -3 mm.
    • Level of FSH in the blood also increases and it stimulates the development of primary follicle into graafian follicle.
    • It occurs between the 6th - 14th day of the menstrual cycle.
  3. Ovulatory Phase:
    • LH stimulates the graafian follicle to rupture and the ovum is released from the ovary.
    • It occurs on 14th/15th day of the cycle.
    • LH also stimulates the empty graafian follicle to convert into corpus luteum.
  4. Secretory Phase:
    • The level of progesterone secreted by corpus luteum increases and some amount of estrogen is also secreted.
    • If fertilisation occurs, then the progesterone prepares the endometrium for maintaining pregnancy.
    • If fertilisation does not occur, the corpus luteum degenerates into corpus albicans. The endometrium breaks again, level of estrogen and progesterone fall and menses starts again around 28th day of the cycle.

 

 

Embryonic Development

 

  • Embryonic development refers to the series of changes by which a single-celled, diploid zygote develops into a multicellular young one.

 

 

Structure of Human Sperm (Male Gamete)

 

  • Human sperm is haploid, microscopic and motile.
  • It consists of a head, neck, middle piece and tail.
  • Head bears acrosome at the tip, which helps the sperm penetrate the egg during fertilisation.
  • Neck is short and contains a centriole.
  • Middle piece is characterised by the presence of mitochondria, which provide energy for the movement of sperms.
  • Tail is very long, about 10 times longer than the middle piece.
  • It consists of a flagellum, which is covered by a fibrous sheath.
  • The terminal part of the tail is called the end piece, where the fibrous sheath is absent.

 

 

 

Structure of an Ovum (Female Gamete)

 

  • The ovum is spherical in shape.
  • It contains a nucleus and cytoplasm that contains a little yolk.
  • Vitelline membrane is the limiting membrane of the ovum.
  • Zona pellucida is present outside the vitelline membrane.
  • Polar bodies are present within the perivitelline space present between vitelline membrane and zona pellucida.
  • Corona radiata is the outermost layer of the ovum, which is formed by follicular cells.

 

 

 

Fertilisation

 

  • Fertilisation is the process of fusion of male and female gamete, i.e. sperm and ova to form a zygote.
  • It occurs in the fallopian tube.
  • It restores the diploid (2n) condition of the zygote.
  • The mixing of male and female genital material brings about variations in the offspring.

 

 

 

Cleavage

 

  • Cleavage refers to the mitotic divisions of the fertilised egg.
  • It is holoblastic in human beings, which means at each division the egg divides into two complete cells.
  • At the end of first cleavage, two cells or blastomeres are formed.
  • The second cleavage results in formation of four blastomeres.
  • Between the two- and four-celled stages, there is also a three-celled stage, as one of the cells complete its division before the other.
  • At the end of the fourth cleavage, the embryo looks like a morula.
  • The morula stage is reached within three to five days after fertilisation.

 

 

 

Blastulation

 

  • Morula reaches the uterus and absorbs the uterine fluid.
  • It results in increase in size and rearrangement of cells.
  • The outer cells of morula form trophoblast.
  • The inner mass of cells give rise to embryo proper.
  • A cavity filled with uterine fluid appears in the morula. It is called blastocyst.
  • Blastulation is complete after five to six days of fertilisation and results in a hollow, fluid-filled embryo or blastula.

The attachment and sinking of the blastocyst in the endometrium of the uterus is called implantation, which takes place six to eight days following fertilisation.

 

 

 

Gastrulation

 

  • The process by which the blastula is converted into three germ layered embryo (gastrula) is called gastrulation.
  • Some cells of the inner mass of the blastocyst get flattened to form the first germ layer, the endoderm.
  • The rest of the cells of the inner mass become columnar and compact and form the second germ layer, the ectoderm.
  • A space appears between ectoderm and trophoblast at the top of the embryo, which is called amniotic cavity. It is filled with amniotic fluid.
  • The entire embryo develops from the embryonic disc which is formed together by ectoderm and endoderm.
  • Towards the posterior side, some cells of the ectoderm proliferate to form an elevated ridge which projects into the amniotic cavity. This is called the primitive streak which is formed on about 15th day after fertilisation.
  • As the development proceeds, the cells from the primitive streak enter the space between the ectoderm and endoderm and form the third germ layer, the intra-embryonic mesoderm.

 

 

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