Secondary Follicle

Definition - What does Secondary Follicle mean?

A secondary follicle is an intermediate stage in the progressive development of the early follicle (primordial follicle) to the tertiary follicle that ultimately releases the fully-grown egg to the fallopian tube. A secondary follicle is a round cellular structure seen in the ovary that contains a maturing egg surrounded by multiple layers of cells and connective tissue, namely the granulosa cells and the theca of the follicle. The arrest of follicular development at the secondary follicle stage can cause women to have menstrual cycles without a release of an egg leading to infertility. Growth and maturation of the eggs past the stage of a secondary follicle is a target in infertility treatments.

FertilitySmarts explains Secondary Follicle

Before puberty, a female ovary has a few hundred thousand follicles that contain immature oocytes (eggs). These follicles are known as primordia follicles. Only about 400 of these primordial follicles reach the final stage of development.

In response to follicle-stimulating hormone (FSH) and luteinizing hormone (LH) release patterns by the pituitary gland, a few primordial follicles start to mature at the beginning of every ovarian cycle through a process called follicular recruitment, but only a few of these primordial oocytes actually complete maturation. After activation of the follicle, the primordial follicle increases in size and the granulosa cells begin to enlarge. At this stage the follicle is known as a primary follicle.

As the primary follicle develops to become the secondary follicle, it acquires an outer layer made of cells and connective tissue known as the theca of the follicle, which then becomes two distinct layers, the theca interna and theca interna. Blood vessels grow inside the theca to nourish the oocyte. Granulosa cells enlarge and increase in number to form multiple layers surrounding the oocyte. The egg is fully grown at the last stages of the secondary follicle.

The formation of a fluid-filled cavity inside the follicle, also known as the antrum, marks the progression of the secondary follicle to the tertiary stage of follicular development. A tertiary follicle is also called the antral follicle or the Graafian follicle. During this stage, granulosa and theca cells continue to multiply and the antrum enlarges in size. At the end of the follicular phase of the ovarian cycle, the Graafian follicle ruptures and releases the egg into the fallopian tubes.

Fertility medication used in assisted reproductive technology (ART) acts by increasing the number of follicles that pass through the secondary follicular stage. Eggs that have matured past the primary follicle stage are used for in vitro fertilization (IVF).

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