Gametogenesis- Oogenesis

GAMETOGENESIS- OOGENESIS

Q. 1

Primary oocyte :

 A

ls formed after single meiotic division

 B

Maximum in number in 5 month fetus

 C

ls in prophase arrest

 D

Option B and C both

Q. 1

Primary oocyte :

 A

ls formed after single meiotic division

 B

Maximum in number in 5 month fetus

 C

ls in prophase arrest

 D

Option B and C both

Ans. D

Explanation:

Ans. is b and c i.e. Maximum in number at 5th month of the fetus; and ls in the prophase arrest

The process involved in the development of mature ovum is called Oogenesis.
The primitive germ cells take their origin from the yolk sac at about the end of 3rd week and migrate to the developing gonadal ridge, at aboul the end ol 4th week.

Oogenesis :

Primordial

       

Enters the gonad of a genetic female & differentiate into

       

Oogonlum tl4 XX

       Mitosis (not meiosis, as given in option)

Primary Oocyte 44 xx

                                                    (At birth no more mitosis occur & all oogonium are replaced by primary oocyte)

Enters Ist meiotic division

   ↓

Anested in prophase

   ↓

Ist meiotic division is completed after puberty, just prior to ovulationo

                                            releasing

             ___________________________________________

    ↓                                   

Secondary Oocyte 22 X
First Polar Body 22 X

                 2nd meiotic oivision

Arrested in metaphase

      ↓ At the time of  Fertilisation

       _________________________________

  ↓                          

Ovum 22 X
2nd Polar Body 22 X

lmportant facts

  • Oogenesis begins in the ovary at 6-8 A weeks of gestationo
  • Maximum number of oocytes/oogonia are in the ovary at 5th month of ddvelopmento (20 weeks of gestation)o.
  • Al birth total content ol both ovaries is 2 million primary oocytes.o
  • At puberty is further decreased and is  ~ 300000 – 500000, of which only 500 are distined to mature during an individual’s life time.o

Q. 2

At birth, oocytes are in which stage of develop­ment:

 A

Prophase of 1st meiotic division

 B

Oogonia

 C

Telophase of 2nd meiotic division

 D

Resting phase between prophase and metaphase of 1st meiotic division

Q. 2

At birth, oocytes are in which stage of develop­ment:

 A

Prophase of 1st meiotic division

 B

Oogonia

 C

Telophase of 2nd meiotic division

 D

Resting phase between prophase and metaphase of 1st meiotic division

Ans. D

Explanation:

Ans:D.)Resting phase between prophase and metaphase of 1st meiotic division.

OOGENESIS

  • The process involved in the development of a mature ovum is called oogenesis.
  • The primitive germ cells take their origin from the yolk sac at about the end of 3rd week and their migration to the developing gonadal ridge is completed round about the end of 4th week.
  • In the female gonads, the germ cells undergo a number of rapid mitotic divisions and differentiate into oogonia.
  • The number of oogonia reaches its maximum at 20th week, numbering about 7 million.
  • While the majority of the oogonia continue to divide, some enter into the prophase of the first meiotic division and are called primary oocytes.
  • These are surrounded by flat cells and are called primordial follicles and are present in the cortex of the ovary.
  • At birth, there is no more mitotic division and all the oogonia are replaced by primary oocytes which have finished the prophase of the first meiotic division and remain in resting phase (dictyotene stage) between prophase and metaphase.
  • Total number of primary oocytes at birth is estimated to be about 2 million.
  • The primary oocytes do not finish the first meiotic division until puberty is reached.
  • At puberty, some 400,000 primary oocytes are left behind, the rest being atretic. Out of these, some 400 are likely to ovulate during the entire reproductive period.

Q. 3

Primary Oocyte remains in ovary in which stage

 A

Metaphase

 B

Anaphase

 C

Interphase

 D

Prophase

Q. 3

Primary Oocyte remains in ovary in which stage

 A

Metaphase

 B

Anaphase

 C

Interphase

 D

Prophase

Ans. D

Explanation:

Prophase


Q. 4

Following statements describe the various stages through which oocytes pass. Choose the incorrect statement

 A

Primary oocyte is arrested at prophase till puberty

 B

Primary oocyte is hormonally induced to resume the first meiotic division during onset of puberty

 C

Secondary oocyte enters the second meiotic division   just   before   ovulation   and   arrests   at metaphase

 D

Fusion of sperm & ovum initiates the resumption of second meiotic division at fertilization

Q. 4

Following statements describe the various stages through which oocytes pass. Choose the incorrect statement

 A

Primary oocyte is arrested at prophase till puberty

 B

Primary oocyte is hormonally induced to resume the first meiotic division during onset of puberty

 C

Secondary oocyte enters the second meiotic division   just   before   ovulation   and   arrests   at metaphase

 D

Fusion of sperm & ovum initiates the resumption of second meiotic division at fertilization

Ans. A

Explanation:

Ans:A.)Primary oocyte is arrested at prophase till puberty.

At birth, the primary oocytes  have finished the prophase of the first meiotic division and remain in resting phase (dictyotene stage) between prophase and metaphase.

OOGENESIS

  • The process involved in the development of a mature ovum is called oogenesis.
  • The primitive germ cells take their origin from the yolk sac at about the end of 3rd week and their migration to the developing gonadal ridge is completed round about the end of 4th week.
  • In the female gonads, the germ cells undergo a number of rapid mitotic divisions and differentiate into oogonia. The number of oogonia reaches its maximum at 20th week, numbering about 7
  • million.
  • While the majority of the oogonia continue to divide, some enter into the prophase of the first meiotic division and are called primary oocytes.
  •  At birth, there is no more mitotic division and all the oogonia are replaced by primary oocytes which have finished the prophase of the first meiotic division and remain in resting phase (dictyotene stage) between prophase and metaphase.
  • Total number of primary oocytes at birth is estimated to be about 2 million.
  • The primary oocytes do not finish the first meiotic division until puberty is reached. At puberty, some 400,000 primary oocytes are left behind, the rest being atretic. Out of these, some 400 are likely
  • to ovulate during the entire reproductive period.
  • Maturation of the oocytes: The essence of maturation is reduction of the number of chromosomes to half. Before the onset of first meiotic division, the primary oocytes double its DNA by replication, so they
  • contain double the amount of normal protein content.
  • The first stage of maturation occurs with full maturation of the ovarian follicle just prior to ovulation but the final maturation occurs only after fertilization.
  • The primary oocyte undergoes first meiotic division giving rise to secondary oocyte and one polar body. Ovulation occurs soon after the formation of the secondary oocyte.
  • Secondary Oocyte enters second meiotic division and arrests at metaphase 3 hours before ovulation
  • The secondary oocyte completes the second meiotic division (homotypical) only after fertilization by the sperm in the Fallopian tube.
  • In the absence of fertilization, the secondary oocyte does not complete the second meiotic division and degenerates as such.


Q. 5

Polar bodies are formed during ?

 A

Spermatogenesis

 B

Organogenesis

 C

Oogenesis

 D

Morphogenesis

Q. 5

Polar bodies are formed during ?

 A

Spermatogenesis

 B

Organogenesis

 C

Oogenesis

 D

Morphogenesis

Ans. C

Explanation:

The first polar body is formed at the end of meiosis I and the second polar body is formed at the end of meiosis II.

Ref: An Introduction to Human Embryology for Medical Students, 7th Edition, Pages 12, 14; Anatomy and Physiology Laboratory Manual By Robert Amitrano, 7th Edition, Page 614; An Introduction to Human Disease: Pathology and Pathophysiology Correlations By Leonard V. Crowley, 8th Edition, Pages 49, 57.


Q. 6

In a female child at birth oocyte is in a stage of

 A

Anaphase 2nd meiotic

 B

Prophase 1st meiotic

 C

Oogony

 D

Maturation

Q. 6

In a female child at birth oocyte is in a stage of

 A

Anaphase 2nd meiotic

 B

Prophase 1st meiotic

 C

Oogony

 D

Maturation

Ans. B

Explanation:

B. i.e. Prophase 1st meiotic


Q. 7

Cells which surround the oocyst in graafian follicle are called ‑

 A

Discus proligerus

 B

Cumulus oophoricus

 C

Luteal cells

 D

Villus cells

Q. 7

Cells which surround the oocyst in graafian follicle are called ‑

 A

Discus proligerus

 B

Cumulus oophoricus

 C

Luteal cells

 D

Villus cells

Ans. B

Explanation:

Ans. is ‘b’ i.e., Cumulus oophoricus

Oocyte lies eccentrically in the graafian (ovarian) follicle.

It is surrounded by some granulsa cells that are given the name cumulus oophoriacus (or cumulus ovaricus).

The cells that attach it to the wall of the follicle are given the name discus proligerus.


Q. 8

After first meiotic division, the primary oocyte remains arrested in ‑

 A

Diplotene stage

 B

Pachytene stage

 C

Metaphase

 D

Telophase

Q. 8

After first meiotic division, the primary oocyte remains arrested in ‑

 A

Diplotene stage

 B

Pachytene stage

 C

Metaphase

 D

Telophase

Ans. A

Explanation:

Ans. is ‘a’ i.e., Diplotene stage

  • Oogenesis refers to the process of formation of ova from the primitive germ cells. Unlike fetal testis (in which spermatogenesis beings at puberty), the fetal ovary begins oogenesis by 10 weeks of gestation.
  • The sequence of events in oogenesis are :
  1. The primitive germ cells undergo mitotic divisions to form oogonia (diploid cells). Oogonium is unique in that it is the only female cell in which both X’ chromosomes are active.
  2. The oogonia proliferate by mitosis to form primary oocytes (diploid cells).
  3. Primary oocytes formed from the oogonia enter a prolonged prophase (diplotene stage) of the first meiotic division and remain in this stage until ovulation occurs after puberty.
  4. Primary oocytes completes the first meiotic division at puberty just before ovulation to form secondary oocyte (haploid cell) and 1st polor body.
  5. Secondary oocyte immediately begins second meiotic division but this division stops at metaphase and is completed only if the mature ovum (ootid) is fertilized with sperm. At that time second polor body (polocyte) is extruded and the fertilized ovum proceeds to form a new individual. Fertilization normally occurs in the ampulla of fallopian tube.

Q. 9

In oogenesis, first meiotic division completes ‑

 A

After ovulation

 B

Before ovulation

 C

During ovulation

 D

At fertilization

Q. 9

In oogenesis, first meiotic division completes ‑

 A

After ovulation

 B

Before ovulation

 C

During ovulation

 D

At fertilization

Ans. B

Explanation:

Ans. is ‘b’ i.e., Before ovulation


Q. 10

Number of oocytes at birth ‑

 A

2 million

 B

6 million

 C

5 million

 D

10 million

Q. 10

Number of oocytes at birth ‑

 A

2 million

 B

6 million

 C

5 million

 D

10 million

Ans. A

Explanation:

Ans. is ‘a’ i.e., 2 million

In humans, no new ova are formed after birth. During fetal development, the ovaries contain over 7 million primordial follicles.

However,-many undergo atresia (involution) before birth and others are lost after birth.

At the time of birth, there are 2 million ova, but 50% of these are atretic.

The million that are normal undergo the first part of the first meiotic division at about this time and enter a stage of arrest in prophase in which those that survive persist until adulthood.

Atresia continues during development, and the number of ova in both of the ovaries at the time of puberty is less than 300,000.


Q. 11

One primary oocyte forms how many ovum/ova ‑

 A

1

 B

2

 C

3

 D

4

Q. 11

One primary oocyte forms how many ovum/ova ‑

 A

1

 B

2

 C

3

 D

4

Ans. A

Explanation:

Ans. is ‘a’ i.e., 1



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