Key Difference – Primordial Follicle vs Primary Follicle
It is important to know about the process of folliculogenesis in order to understand the difference between Primordial Follicle and Primary Follicle; hence, we will first look at this process. The human ovaries are located behind and below the fallopian tube, and have two principle functions; oogenesis and endocrine function. Oogenesis is the maturation of ova from the primitive germ cells. This process is also called the ovarian cycle. It can be divided into three stages, namely; preovulatory phase, ovulation, and postovulatory phase. During the preovulatory phase, primordial follicles start maturing, but only one follicle completes the maturation while the rest undergo atrophy. This process of maturation of the primordial follicle into graafian follicle is called folliculogenesis. There are several different follicles based on their stage of maturation, namely; primordial follicles, primary follicle, secondary follicle, tertiary follicle, and Graafian follicle. This article describes the difference between primordial and primary follicles. The key difference between Primordial Follicle and Primary Follicle is that the primordial follicle is the first follicle of folliculogenesis while primary follicle is the second follicle of folliculogenesis.
What is Primordial Follicle?
Primordial follicles are the initial reproductive units of the ovary that occur during the fetal development. These follicles remain infertile, until puberty. At the time of sexual maturity, both ovaries contain about 300,000 primordial follicles, but only 400-450 of these follicles reaches maturity during the folliculogenesis. After the onset of puberty, primordial follicles start to develop rapidly into primary molecules. Each primordial follicle contains a primary oocyte enclosed by a single-cell layer known as follicular cells. The primary oocyte is in the prophase of the first meiotic division. Both primary oocyte and the follicular cell layer are surrounded by a thin membrane called basal lamina. It is believed that the follicular cells nourish the oocyte and keep it immature by secreting oocyte maturation inhibiting factor (OMIF).
What is Primary Follicle?
Primary follicles are formed from primordial follicle during the folliculogenesis. During this transformation, the follicular cells convert into columnar cells and go through mitotic division to form multilayered granulosa cells. Primary follicle has an enlarged oocyte when compared to oocyte size in primordial follicles. With further maturation of primary follicles, a homogenous membrane appears between the granulosa cells and oocyte. This membrane is called zona pellucida. The primary follicle with zona pellucida is now referred to as a multilaminate primary follicle.
What is the difference between Primordial Follicle and Primary Follicle?
Definition of Primordial Follicle and Primary Follicle
Primordial Follicle: The primordial follicle is the first follicle of folliculogenesis.
Primary Follicle: The primary follicle is the second follicle of folliculogenesis.
Characteristics of Primordial Follicle and Primary Follicle
Origin:
Primordial Follicle: Primordial follicles are formed during the fetal development and remain immature until puberty
Primary Follicle: Primary follicles are formed from primordial follicles after the onset of puberty. However, not every primordial follicle develops into the primary follicle.
Size:
Primordial Follicle: Primordial follicle is smaller than the primary follicle.
Primary Follicle: Primary follicle is larger than the primary follicle.
Oocyte:
Primordial Follicle: Primordial follicle has small oocyte surrounded by follicular cells,
Primary Follicle: Primary follicle has a larger oocyte than the primordial follicle and is surrounded by granulosa cells. Zona pellucida is only present in the primary follicles.