Arrange the stages of oogenesis in order of their occurance.

(A) Formation of ovum & II^nd polar body
(B) Meiosis II begins but held at Metaphase II
(C) Meiosis I started but held at Prophase I
(D) Formation of primary oocyte from oogonia
(E) Completion of Meiosis-I and formation of secondary oocyte and first polar body

Choose the correct answer from the options given below:

(D), (C), (E), (B), (A)

The correct answer is Option (3) → (D), (C), (E), (B), (A)

(D) Formation of primary oocyte from oogonia

(C) Meiosis I started but held at Prophase I

(E) Completion of Meiosis-I and formation of secondary oocyte and first polar body

(B) Meiosis II begins but held at Metaphase II

(A) Formation of ovum & II^nd polar body

The process of formation of a mature female gamete is called oogenesis. Oogenesis is initiated during the embryonic development stage when a couple of million gamete mother cells (oogonia) are formed within each fetal ovary; no more oogonia are formed and added after birth. These cells start division and enter into prophase-I of the meiotic division and get temporarily arrested at that stage, called primary oocytes. Each primary oocyte then gets surrounded by a layer of granulosa cells and is called the primary follicle . A large number of these follicles degenerate during the phase from birth to puberty. Therefore, at puberty only 60,000-80,000 primary follicles are left in each ovary. The primary follicles get surrounded by more layers of granulosa cells and a new theca and are called secondary follicles.

The secondary follicle soon transforms into a tertiary follicle which is characterised by a fluid filled cavity called antrum. The theca layer is organised into an inner theca interna and an outer theca externa.  At this stage that the primary oocyte within the tertiary follicle grows in size and completes its first meiotic division. It is an unequal division resulting in the formation of a large haploid secondary oocyte and a tiny first polar body. The secondary oocyte retains bulk of the nutrient rich cytoplasm of the primary oocyte. The secondary oocyte immediately begins Meiosis II but halts at Metaphase II. It will only complete Meiosis II if fertilization by a sperm occurs. Upon fertilization, Meiosis II completes, forming the mature ovum and the second polar body.

                                                  Oogenesis