Follicles - Anatomy & Physiology

BACK TO THE OVARY

Introduction

The follicle is the functional unit of the ovary containing both gametes (Oocyte) and endocrine cells (Granulosa and Theca cells). A female is born with all the Primordial follicles it is ever going to have i.e Primordial Follicles do not devide anymore, they either develope further into Primary, secondary, and tertiary follicles or they degenerate. Therefore the number of follicles declines with age as follicles develope and degenerate during the oestrus cycle. When the supply of follicles is depleted then reproductive senescence occurs. In humans this phenomenon is known as "The Menopause".

General Structure

• Made up from two cell types; Granulosa cells and Theca cells aranged in two differnt layers.
  • The Granulosa cell layer is the inner cell layer found closest to the oocyte
  • The Theca cell layer is the outer cell layer

Follicular Development

1. Primordial Follicle

• Smallest and most immature follicle
• Oocyte surrounded by a Single layer of squamous cells

2. Primary Follicle

• Oocyte surrounded by a single layer of follicular cuboidal cells.

3. Secondary Follicles

• The oocyte had developed a thick translucent glycoprotein layer called the Zona Pellucida.
• Oocyte surrounded by two or more layers of follicular cuboidal cells. These cells differenciate into the inner Granulosa cells and outer Theca cells.

4. Tertiary/Antral Follicles

• The innermost cells of the Granulosa layer become firmly attached to the Zona Pellucida and are known as the Corona Radiata. The outermost Granulosa cells become attached to the basement membrane surrounding them and become known as the Theca Folliculi.
• Granulosa cells secrete fluid which accumulates within the follicle forming an Antrum.
• Theca cells become organised into Theca Interna and Theca Externa layers. The Theca Interna layer cells differenciate into steroidogenic cells produce the Androgen Androsteinedione in response to the Gonadotrophin Luteinising hormone (LH).
• Theca Cell layer becomes vascularised.
• Granulosa cells become responsive to Follicle Stimulating Hormone (FSH) and start producing 17β-Oestrodiol and small amounts of Inhibin and Progesterone.

Dominant Follicle

• Only a single Dominant follicle forms with each wave of folliculogenesis.