Difference Between Primordial Follicle and Primary Follicle

Main Difference – Primordial Follicle vs Primary Follicle

Primordial follicle, primary follicle, secondary follicle, tertiary follicle, and Graafian follicle are the developmental stages of folliculogenesis in mammals. Graafian follicle is the follicle stage which is ready for ovulation. Primordial follicle is the first stage of folliculogenesis. The number of primordial follicles in an ovary is determined at birth. Under the influence of different hormones, the initial recruitment of primordial follicles occurs after puberty. Primary follicles are developed from primordial follicles. The main difference between primordial follicle and primary follicle is that primordial follicle is a smaller, dormant follicle with a single layer of flattened granulosa cells whereas primary follicle is much larger, mitotic follicle with cuboidal granulosa cells.

Key Areas Covered

1. What is a Primordial Follicle
      – Definition, Development, Anatomy
2. What is a Primary Follicle
      – Definition, Development, Anatomy
3. What are the Similarities Between Primordial and Primary Follicle
      – Outline of Common Features
4. What is the Difference Between Primordial and Primary Follicle
      – Comparison of Key Differences

Key Terms: Follicle-Stimulating Hormone (FSH), Folliculogenesis, Granulosa Cells, Ovarian Follicles, Ovary, Primary Follicle, Primordial Follicle, Zona Pellucida

What is a Primordial Follicle

A primordial follicle is an ovarian follicle that is surrounded by a single layer of flattened granulosa cells. Ideally, an ovarian follicle has to progress through several developmental stages until it becomes a matured follicle, which is suitable for ovulation. This developmental process is called folliculogenesis and primordial follicle is the first stage of the folliculogenesis. Ovulation is the release of the mature oocyte from an ovary.  The stages of folliculogenesis are shown in figure 1.

Figure 1: Folliculogenesis

Typically, an ovary bears a definite number of primordial follicles at birth. A primordial follicle contains a primary oocyte (25 μm in diameter), which is arrested in the diplotene stage of meiosis. It is surrounded by a single layer of flattened squamous epithelial cells called granulosa cells and a basal lamina. Ideally, the diameter of a human primordial follicle is 0.03 mm. Since primordial follicles do not have an independent blood supply, they have a less exposure to the endocrine system.

The process of the reinitiation of the development of a primordial follicle is called the initial recruitment, which starts at puberty. Primordial follicles are the majority of the ovarian follicles throughout the life. The initial recruitment of primordial follicles is continuous throughout the life. The first visible sign of initial recruitment is the change of the granulosa cells from squamous to cuboidal. The 8 granulosa cells of the primordial follicle reach 19.    

What is a Primary Follicle

A primary follicle is an immature ovarian follicle, which is surrounded by single or several layers of cuboidal granulosa cells. Based on the number of granulosa cell layers, the developing ovarian follicles can be categorized into three classes: primary follicles, secondary follicles, and early tertiary follicles. The primary follicle is composed of a single cuboidal granulosa cell layer. The granulosa cells begin to express receptors for the follicle-stimulating hormone (FSH). A primary follicle is shown in figure 2.

Figure 2: Primary Follicle

During the primary follicle stage, oocyte exhibits high levels of RNA synthesis. As a result, the zona pellucida (ZP) protein is expressed and the polymerization of the ZP protein near the surface of the oocyte forms an extracellular matrix coat called zona pellucida. The carbohydrate moiety of ZP-3 in the zona pellucida serves as a species-specific, sperm-binding molecule.

Similarities Between Primordial Follicle and Primary Follicle

• Both primordial follicle and primary follicle are two stages of folliculogenesis.
• Both primordial follicle and primary follicle occurs in the ovary.
• Both primordial follicle and primary follicle are surrounded by granulosa cells.
• Both primordial follicle and primary follicle are arrested at the diplotene stage of meiosis.
• Both primordial follicles and primary follicles are incapable of dividing by mitosis.

Difference Between Primordial Follicle and Primary Follicle

Definition

Primordial Follicle: Primordial follicle refers to an ovarian follicle surrounded by a single layer of flattened granulosa cells.

Primary Follicle: Primary follicle refers to an immature ovarian follicle surrounded by single or several layers of cuboidal granulosa cells.

In Folliculogenesis

Primordial Follicle: Primordial follicles are the first stage of folliculogenesis.

Primary Follicle: Primary follicles are the second stage of folliculogenesis.

Development

Primordial Follicle: Primordial follicles are developed in the fetal stage.

Primary Follicle: Primary follicles are developed after puberty.

Correlation

Primordial Follicle: The ovary contains a definite number of primordial follicles at birth.

Primary Follicle: Primary follicles are developed from primordial follicles under the influence of hormones.

Size

Primordial Follicle: Primordial follicles are small (0.03-0.05 mm in diameter).

Primary Follicle: Primary follicles are much larger than the primordial follicles (0.1 mm in diameter).

Significance

Primordial Follicle: Typically, primordial follicles are dormant.

Primary Follicle: Primary follicles are a type of mitotic follicle.

Granulosa Cells

Primordial Follicle: Primary follicle is surrounded by a single layer of flattened granulosa cells.

Primary Follicle: Primordial follicle is surrounded by a layer of cuboidal granulosa cells.

Exposure to Endocrine System

Primordial Follicle: Primordial follicles are less exposed to the endocrine system.

Primary Follicle: The granulosa cells of the primary follicle develop receptors for the FSH hormone.

Levels of RNA Synthesis

Primordial Follicle: Primordial follicles are a kind of dormant follicles. Therefore, they do not show a significant synthesis of RNA.

Primary Follicle: Primary follicles exhibit a significant level of RNA synthesis.

Zona Pellucida

Primordial Follicle: Primordial follicle lacks a zona pellucida.

Primary Follicle: Primary follicle contains a zona pellucida.

Conclusion

Primordial follicle and primary follicle are two stages of folliculogenesis in mammals. Primordial follicles are developed during the fetal stage of life. Primary follicle develops from the primordial follicle. The primordial follicle is a kind of a small, dormant follicle, which is surrounded by a squamous granulosa cell layer. But, the primary follicle is an active follicle with high levels of RNA synthesis. It is surrounded by a zona pellucida and cuboidal granulosa cell layer. The main difference between primordial follicle and primary follicle is their anatomy and the physiology.

Reference:

1. “Follicle Growth and Development.” GLOWM, Available here.

Image Courtesy:

1. “Figure 28 02 04” By OpenStax College – Anatomy & Physiology, Connexions Web site.  Jun 19, 2013. (CC BY 3.0) via Commons Wikimedia 
2. “Primary follicle (transition)” By Jpogi at English Wikipedia – Jpogi’s own picture. Transferred from en.wikipedia to Commons by Akira Kouchiyama. (Public Domain) via Commons Wikimedia