OOGENESIS AND STRUCTURE OF OVUM

     Oogenesis:

   Formation of female gametes (ova) and maturation of ovum is called Oogenesis. It takes place in-the ovary.

   Period pf oogenesis:

1)   It is different in different animals. In human female it starts during third month of foetal development.

2)  At that time ovaries contain 6-7 million germ cells, however many undergo atresia (degeneration of germ cells) before birth.

3)  At the time of birth there are 2 million primary follicles, but 50% of these are atretic.

4)  Atresia continues and at the time of puberty ovaries contain less than 3 lakh primary follicles.

5)  Oogenesis completed only after the onset of puberty and only one out of 400 is mature.

    So Oogenesis is a discontinuous process. Oogenesis involves 3 phases.

    i. Multiplication phase: It is initial phase.

    Germinal epithelial cells divides to produce Oogonia (2n).

   ii. Growth phase: Oogonium cell grows and develops into primary oocyte (2n).

    Remaining oogonia form nutritive follicular epithelium

   iii. Maturation phase:

1)   Primary oocyte undergoes 1st meiotic division and produces two unequal haploid cells.

2)  Large cell is called as secondary oocyte and small cell is called as first polar body.

3)  Secondary oocyte divides further to form mature ovum and second polar body.

4)  1st and 2nd polar body degenerate.

5)  Mature ovum function as female gamete.

   Ovulation: Ovulation occurs when the ballooning ovary wall ruptures and expel the ovum into the peritoneal cavity.

  

 

   Structure of Ovum /unfertilized egg / secondary oocyte.

1)   Ovum was first discovered by Karl Ernst Von Baer.

2)  The ovum or egg is the largest cell of human body.

3)  It is spherical and non-motile cell.

4)  It is about 0.1 mm (100 m) to 0.2m[200 m] in diameter

5)  It is covered by its own membrane called plasmalemma.

6)  It contains cytoplasm. The cytoplasm is differentiated into the outer region containing a number of mucopolysaccharide, cortical granules called cortex or exoplasm and inner opaque endoplasm or ooplasm.

7)  Nucleus is large eccentric in position. It is present in anterior half. So egg or ovum has a polarity.

8)  The side of ovum with nucleus and polar body is called animal pole and opposite side is called vegetal pole.

9)  Nucleus with nucleolus is called germinal spot.

10) The ovum is covered by three membrane these are

   a. Vitelline membrane or plasma membrane or oolemma:

    Ovum is surrounded thin transparent membrane and secreted by ovum itself is called Vitelline membrane or plasma membrane or oolemma.

   b. Zona pellucida:

    It is middle, colorless, non-cellular, thick and transparent layer called secondary membrane. It is secreted by follicular cells of the ovary. It is protective in function.

   c. Corona radiata:

1)   It is outer thick cellular coat of egg called tertiary membrane.

2)  It is part of granulosa cells which remains around the ovum at the time of ovulation.

3)  Its cells are radially elongated and are held together by mucopolysaccharide called hyaluronic acid.

4)  A small space present between ooplasm and vitelline membrane is called perivitelline space in which polar bodies are lodged.

5)  Viability / Fertilizability of human egg is only 24 hours.

6)  It remains alive for 3 days (upto 72 hours).

7) Kenth (1969) stated that human egg is alecithal i.e. yolk is almost absent.

   Functions of ovum:

    1. It develops a new organism when fertilized with sperm cell

    2. It helps to restore diploid condition in zygote

    3. It transmits the maternal characters to the offspring.

4. It contributes mitochondria to zygote - important in aerobic respiration as well as cytoplasmic inheritance.

(REFER DIAGRAMS FROM TEXTBOOK)

 

1.            Which is the site of oogenesis?

2.           What are the different stages of oogenesis?

3.           At which stage oogonia formation begins in human female?

4.           Which hormone influences growth of oogonium?

5.           Which cell is called primary oocyte?

6.           What is the distinguishing feature of meiotic-I division in oogenesis?

7.           What is the purpose of unequal division?

8.           Which cell is produced at the end of meiosis I?

9.           359 At which stage secondary oocyte is released from ovary?

10.        At which stage division is arrested?

11.         Which is the largest cell of body?

12.         What is the size of ovum?

13.        What is the nature of human ovum?

14.        What is the position of nucleus of ovum?

15.        Which is the term applied to ovum nucleus?

16.        Why ovum shows polarity?

17.        What is animal half and animal pole?

18.        What is vegetal half and vegetal pole?

19.        What is egg envelope?

20.       What are the different layers of egg envelope from outside to inside?

21.        What is the nature of zona pellucida?

22.       What is the nature of corona radiate?

23.       What is the position of perivitelline or previtelline space?

24.       Which structure is located in perivitelline space?

25.       How zona pellucid is formed?

26.       How corona radiata is formed?

27.       Which material binds corona radiate cells?