What Is the Follicular Phase?

The follicular phase lasts from 10 to 12 days, starting from the last day of menstrual cessation to the day of ovarian ovulation. In this phase, the granulosa cells of the follicles in this phase produce estrogen under the action of FSH and LH. Under the action of estrogen, the endometrium rapidly proliferates, blood vessels proliferate, and glands widen and lengthen, but not secreted.

Follicular phase

Follicular phase

a. Primitive follicles: near the white membrane, there is an egg cell in the center, and a layer of flat follicle cells on its periphery.

b. Growing follicles:

Primary growth follicles: one or more cubic follicular cells with red-stained zona pellucida between the egg cells and the follicle cells, and the periphery of the follicles

The development of follicles begins with primitive follicles. In humans, each menstrual cycle usually has only one primitive follicle that matures under the regulation of hormones. The primitive follicle passes through the primary and secondary follicular phases, and finally develops into a preovulatory follicle (

Primordialfollicle is composed of a large primary oocyte and a single layer of flat follicular cells surrounding it. There are about 1 million primitive follicles in the neonatal ovarian cortex, but most of them cannot reach the mature stage. They have shrunk and degenerate in succession, forming closed follicles. Hormone regulation is not required to develop follicles from primordial follicles.

What are the criteria for dominant follicles? Growing follicles increase in the size of primary oocytes, and significant RNA and protein synthesis occurs within the cells. After follicular cells become square or columnar, they rapidly proliferate into multiple layers. A thick film is formed between the oocyte and the follicular cell, called zonapellucida, which is a gel-like glycoprotein complex. Under the electron microscope, the microvilli on the surface of the oocyte and the protrusions of the follicular cells protruded into the zona pellucida, which facilitated the follicular cells to deliver nutrients to the oocytes. Zona pellucida plays an important role in identifying sperm species and preventing polyspermia. The follicular cells continue to proliferate, and the small space between the follicular cells gradually merges into a larger follicular cavity. The cavity is filled with follicular fluid, which contains hyaluronic acid, estrogen and nutrients. With the increase of the follicular fluid and the expansion of the follicular cavity, the standard of dominant follicles, the oocytes and some follicular cells around them are squeezed to one side of the follicle, and protrude toward the follicular cavity, called the cumulus. The rest of the follicular cells that make up the egg wall are densely arranged in several layers, called the granular layer. A layer of columnar follicular cells next to the zona pellucida, which is arranged radially, is called a radiation crown. The connective tissue around the follicles forms two layers of follicular membranes. The inner layer contains many cells and is rich in capillaries. The inner layer cells and the follicle cells work together to secrete estrogen; the outer layer has many fibers. From puberty, there are several to dozens of follicles growing and developing in each cycle, but only one follicle is mature and occasionally two, and the rest are closed.