Difference Between Oocyte and Follicle

Main Difference – Oocyte and Follicle

Ovary is an organ that belongs to the female reproductive structure of animals. The four hormones, estrogen, progesterone, follicle-stimulating hormone (FSH), and luteinizing hormone (LH), regulate the functions of the ovary. Ovary repeatedly undergoes histological changes throughout the menstrual cycle of females. Medulla and cortex are the two components of an ovary. The follicles are developed in the cortex. Oocyte develops inside the follicle. Thereby, oocyte and follicle are two structures developed inside the cortex of an ovary. The main difference between oocyte and follicle is that oocyte is a cell that develops the ovum through meiosis whereas follicle is a small secretory cavity which surrounds the developing oocyte.

Key Areas Covered

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

Key Terms: Follicle, Folliculogenesis. Ovary, Oocyte, Ovum, Primary Oocytes, Primordial germ cells, Secondary Oocytes

What is Oocyte

Oocyte is a cell that develops the ovum through meiosis. The development of the oocyte occurs inside the follicle. Oogenesis is the process by which the oocyte matures into an ovum. Oocytes develop from the primordial germ cells. During embryonic development, primordial germ cells move into the gonads to become the oogonium. Oogonia are diploid cells. They increase their number by mitotic cell divisions to produce primary oocytes. A five months-aged fetus contains 7 million primary oocytes. But at birth, only 2 million primary oocytes are left in the ovary.

Video 1: Oogenesis

Primary oocytes undergo meiosis to produce haploid oocytes. After the first mitotic division, the primary oocyte is called the secondary oocyte. Meiosis I also produces a smaller cell called the polar body. Secondary oocytes are arrested at the meiotic division stage until puberty. After puberty, the second stage of meiotic division starts, developing the secondary oocytes into the ootids. The splitting of the secondary oocyte by the mitotic division also produces a polar body. The ovulation occurs at the ootid stage. The final stage of maturation of the ootid into ovum occurs after the fertilization.

What is a Follicle

Follicle is a small secretory cavity which surrounds the developing oocyte. It occurs in the cortex of the ovary. After puberty, hormones cause the maturation of follicles along with the primary oocytes. The maturation of both follicle along with the ovum causes the ovulation where the mature ovum is released from the ovary. Human Graafian follicle is shown in figure 2. The oocyte is the large, pink color cell at the top of the follicle.

Figure 2: Graafian Follicle

The developmental process of the follicle is referred to as folliculogenesis. Primordial (resting) follicle, primary follicle, secondary (pre-antral) follicle, tertiary (antral) follicle and pre-ovulatory (Graafian) follicle are the stages of folliculogenesis. The primordial follicle is arrested at the diplotene stage of meiosis 1. It contains a single layer of squamous, granulosa cell layer. The conversion of the squamous, granulosa cells into cuboidal, granulosa cells is called the initial recruitment of the folliculogenesis. At the stage of single layer of cuboidal, granulosa cell layer, the follicle is called the primary follicle. In the primordial follicle, zona pellucida is developed, surrounding the oocyte. The secondary follicle contains 2 to 10 layers of granulosa cells. In the tertiary follicle, a cavity appears inside the granulosa cell layer. The Graafian follicle is relatively a large follicle. The cavity of the granulosa cell layer contains follicular fluid. The rupture of the Graafian follicle causes ovulation.

Similarities Between Oocyte and Follicle

Both oocyte and follicle are two developing structures found inside the ovary.
The development of both oocyte and follicle occur in the ovary cortex.
Both oocyte and follicle undergo simultaneous development.
The development of both oocyte and follicle is regulated by hormones.

Definition

Oocyte: Oocyte is a cell which develops the ovum through meiosis.

Follicle: Follicle is a small secretory cavity which surrounds the developing oocyte.

Developing Process

Oocyte: The development process of the oocyte is called oogenesis.

Follicle: The developmental process of the follicle is called folliculogenesis.

Stages of the Development Process

Oocyte: Oogonia, primary oocyte, secondary oocyte, ootid, and ovum are the stages of the oocyte.

Follicle: Primordial follicle, primary follicle, secondary follicle, tertiary follicle, and Graafian follicle are the stages of the folliculogenesis.

Structures

Oocyte: Oocyte consists of a haploid nucleus, cytoplasm, vitelline membrane, a protective space, zona pellucida, and a corona radiata.

Follicle: Follicle consists of an oocyte, granulosa cells, theca tissue, antrum or the cavity, and the follicular fluid.

Nucleus

Oocyte: Oocyte contains the female nucleus, which fuses with the male nucleus.

Follicle: The nuclei of the follicles do not undergo fertilization.

Ovulation

Oocyte: Oocyte is released from the ovary during ovulation.

Follicle: Follicle remains in the ovary, becoming the corpus luteum.

Conclusion

Oocyte and follicle are two simultaneously developing structures of the ovary. Oocyte occurs inside the follicle and follicle occurs in the cortex of the ovary. Oocyte contains the haploid nucleus. Follicle is a secretory sac which surrounds the oocyte. The main difference between oocyte and follicle is their structure and function inside the ovary.

Reference:

1. Rachel Gurevich | Reviewed by Anita Sadaty, MD. “Do You Know How Human Egg Cells Develop?” Verywell, Available here.
2. “Follicle Growth and Development.” GLOWM, Available here.

Image Courtesy:

1. “Graafian Follicle, Human Ovary (3595817584)” By Ed Uthman from Houston, TX, USA – Graafian Follicle, Human OvaryUploaded by CFCF (CC BY 2.0) via Commons Wikimedia