What is the follicle growth cycle?

Everyone knows that eggs are an important part of women's pregnancy. The appearance of the embryo is due to the growth of follicles. Many women do not understand this situation, so some people will have abnormal follicles and lead to infertility. So what is the follicle growth cycle? The editor will explain it to you.

The development of follicles begins with the primordial follicle. During each menstrual cycle, usually only one primordial follicle develops and matures under the regulation of hormones. The primordial follicle goes through the primary follicle and secondary follicle stages, and finally develops into a pre-ovulatory follicle (mature follicle). The early stages of the development of the primordial follicle to the primary follicle are not controlled by pituitary gonadotropin, and their development depends on the internal memory factors of the follicle itself. By the late stage of primary follicle development, FSH receptors appear on granulosa cells and LH receptors appear on endometrial cells. By the secondary follicle stage, the number of FSH receptors on granulosa cells further increases. Under the synergistic effect of estrogen, FSH induces the appearance of LH receptors in granulosa cells. As the follicles mature, the number of LH receptors on granulosa cells and endometrial cells continues to increase. From the primary follicle development stage, the follicle is controlled by pituitary gonadotropin, which promotes its development and maturation.

At the beginning of the follicular phase, the concentrations of estrogen and progesterone in the blood are at low levels, and the feedback inhibition on the secretion of pituitary FSH and LH is weak. The FSH in the blood shows a trend of gradually increasing, and LH also increases after one or two days. In recent years, it has been discovered that there is a protein in follicular fluid that promotes the secretion of FSH, called FSH-releasing protein, which may play a certain role in the increase of FSH. In addition to the increase in FSH receptors, receptors for factors related to cell proliferation, such as insulin-like growth factor (IGF) and epithelial growth factor (EGF), also appear on the growing and developing granulosa cells of the follicle. Under the action of FSH and various growth factors, the granulosa cells develop and differentiate significantly, and produce aromatase, which can convert androgens (mainly androstenedione) produced by the endometrium and diffusely transported to the granulosa cells into estrogens. LH binds to the LH receptors on the endometrial cells and converts cholesterol into androgens through the cAMP-protein kinase system. Endometrial cells produce androgens, which are converted into estrogens in granulosa cells, a phenomenon known as the dual cell theory of estrogen secretion.

About a week before ovulation, the estrogen secretion of the follicles increases significantly and the concentration in the blood rises rapidly. At the same time, the level of FSH in the blood decreases. This is because: ① Estrogen selectively inhibits the secretion of FSH; ② Follicle-inhibiting hormone produced by granulosa cells also inhibits the secretion of FSH. It is worth pointing out that the FSH concentration is temporarily at a low level, but the estrogen concentration does not decrease, but continues to increase. The reason is that estrogen can enhance the differentiation and growth of endometrial cells and increase the number of LH receptors, thereby enhancing the process of synthesizing androgen and converting it into estrogen.

About one day before ovulation, the estrogen concentration in the blood reaches its peak. Under its action, the hypothalamus enhances the secretion of GnRH, which is transported to the adenohypophysis through the pituitary portal vein, stimulating the secretion of LH and FSH. The increase in LH secretion is the most obvious, forming an LH surge. If the animals are treated with anti-hormone serum beforehand, the LH peak no longer occurs. After the preoptic area of ​​the rhesus monkey hypothalamus was damaged, the LH peak also disappeared. Therefore, the LH peak is induced by the estrogen peak. The effect of estrogen in promoting the secretion of large amounts of LH is called the positive feedback effect of estrogen.

Before the LH peak appears, the oocyte has basically matured, but because the granulosa cells surrounding the oocyte secrete an oocyte maturation inhibitor (OMI), the oocyte maturation and division are interrupted in the early stage. At the moment the LH peak appears, the high concentration of LH immediately offsets the inhibitory effect of OMI, and the oocyte resumes the interrupted maturation and division. The mature follicles protrude to the surface of the ovary, forming transparent follicular spots. LH, in conjunction with progesterone, increases the activity of follicle wall-lytic enzymes (such as plasmin and proenzyme), leading to the dissolution and loosening of the follicle wall. In addition, LH can cause the follicles to secrete prostaglandins, which promote the contraction of the muscle-like cells in the follicle wall. As a result, the egg cell and the attached zona pellucida and corona radiata are expelled from the ruptured follicle wall. This process is called ovulation. The released egg is immediately captured by the fallopian tube fimbria and inhibited, while injection of exogenous LH or HCG (see below) can induce ovulation. It can be seen that the LH peak is a key factor in controlling ovulation.

In humans, 15-20 primordial follicles begin to grow and develop at the same time at the beginning of each menstrual cycle, but usually only one follicle develops into the dominant follicle, and finally matures and ovulates. The other follicles degenerate one after another and form atretic follicles. The reason for this is not yet fully understood.

During the follicular phase, the endometrium also undergoes corresponding changes, mainly manifested as endometrial thickening, an increase in the number of glands and their elongation. This period is called the proliferative phase.

What is the follicle growth cycle? The above introduces the changes in the follicle growth cycle. From the above content, we can clearly understand that it takes a process, and if some problems are encountered during this process, it may lead to the occurrence of diseases. I hope everyone can correctly understand this disease.