Question

The second maturation division of the mammalian ovum completes

• A. shortly after ovulation before the ovum makes an entry into the
  fallopian tube
• B. after the ovum is penetrated by a sperm
• C. after the nucleus of the sperm has fused with that of the ovum
• D. in the Graafian follicle following the first maturation division

Answer 1

The correct option is B after the ovum is penetrated by a sperm

i) Oogenesis is initiated during the embryonic stage when a couple of million gamete mother cells (oogonia) are formed within each fetal ovary.

ii) These cells are diploid, they start division and enter into prophase-I of the meiotic division and get temporarily arrested at that stage, called primary oocytes.

iii) Each primary oocyte then gets surrounded by a layer of granulosa cells and then called the primary follicle.

iv) The primary follicles get surrounded by more layers of granulosa cells and a new theca and called secondary follicles.

v)The secondary follicle soon transforms into a tertiary follicle which has a fluid filled cavity called antrum.

At this stage the primary oocyte within the tertiary follicle grows in size and completes its first meiotic division resulting in the formation of a large haploid secondary oocyte and a tiny first polar body.

vi) The secondary oocyte proceeds with meiosis II (second maturation division) but the division gets arrested at metaphase stage.

It is in this stage of oocyte that the ovum is shed during ovulation. It passes into oviduct, where the cell cycle resumes the second maturation only after the entry of sperm.