Oogenesis - Anatomy & Physiology

Introduction

Oogenesis is the process of producing the female gametes, the Ovum, from the primordial germ cells. The majority of the steps in oogenesis, up to the point of producing primary oocytes, occur pre-natally. Therefore, females are born with all of the Primary Oocytes that they will ever have as primary oocytes do not divide further. These Primary Oocytes are committed to a gamete fate and either become secondary oocytes or degenerate. Oocyte development occurs within follicles and so oogenesis and folliculogenesis occur in conjunction with each other, interacting via reciprocal induction. The follicular granulosa cells produce important growth factors and supporting substances to facilitate oocyte development and visa-versa.

Stages of Oogenesis

Stage 1

• Primordial germ cell undergoes mitosis to produce two Oogonia. These cells are all Diploid.
• Occurs Pre-natally.

Stage 2

• Each Oogonia also undergoes mitosis to produce two diploid Primary Oocytes.
• Occurs Pre-natally.

Stage 3

• Each Primary Oocyte starts to undergo meiosis I replicating their DNA, but they are arrested at the first Meiotic Prophase.
• Occurs Pre-natally.

Stage 4

• The meiotic block is removed by the onset of puberty and the first Luteinising Hormone surge. The primary oocyte completes its first meiotic division producing a Secondary oocyte and the first polar body. The Secondary Oocyte enters Meiosis II and is arrested at metaphase II. The secondary oocyte is diploid.
• The oocyte enlarges due to a inrease (x50) in cytoplasmic volume. The oocyte is now 100-150µm.
• Ovulation occurs at this stage once the first meiotic division has been completed and secondary oocyte formed within the dominant follicle. The exception to this is in the case of the Bitch where ovulation occurs after stage 3 with the Primary Oocyte arrested in prophase I.
• Occurs after puberty is reached.

Stage 5

• The secondary oocyte completes meiosis II producing the mature gamete, the Ova, which is haploid.
• This stage only ocurs once fertilisation of the secondary oocyte by the sperm has occurred.

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