Understanding the Menstrual Cycle: Phases, Hormones, and Fertility

The menstrual cycle is the process by which people develop female gametes (oocytes, ova) and produce a series of changes designed to establish a possible pregnancy. The beginning of the cycle is defined as the first day of menstruation, and the end of the cycle is the day before the start of the next menstruation. The mean cycle length is 28 days, although this may be longer or shorter.

The first menstruation is termed menarche, and the last is known as menopause. Depending on the changes in the ovary and uterus, the menstrual cycle is divided into two phases:

• Follicular phase: From the first day of menstruation until ovulation, which consists of menstruation and the proliferative phase.
• Luteal phase: From ovulation until the first day of menstruation, which occurs while the secretory phase.

Ovum

The corona radiata is the structure surrounding an unfertilized animal egg. It consists of two or three layers of follicle cells attached to the outer protective layer of the egg, the zona pellucida, which is made up of proteins and polysaccharides. Its main purpose is to provide proteins to the egg cell.

• The yolk is the protoplasm of the ovum of animals.
• The polar body is the product of meiosis I and contains (n) chromosomes. It is a residual product that will disappear later.
• The nucleus contains the genetic material and has a number n of chromosomes. In its interior is the germinal spot or nucleolus, which looks dark and is directed toward one pole of the cell to, once fertilized, give rise to the embryo.

Hormones

• FSH (Follicle-Stimulating Hormone): Starts the maturation of a group of follicles and primary oocytes, starting the menstrual cycle.
• LH (Luteinizing Hormone): Stimulates the maturation of the developed follicle and resumes its primary oocyte meiosis. Causes ovulation, the remains of the follicle become the corpus luteum, and keeps it active.
• Progesterone: Appears in the luteal phase. It stimulates and promotes the development of endometrial glands to secrete their mucus to encourage the implementation of the fertilized egg or blastocyst.
• Estrogens: A group of several hormones, most notably, estradiol and estriol. Their function is to promote the growth of follicles and primary oocytes, stimulate endometrial development, foster the implementation of the egg, and cause increased FSH and LH to produce ovulation.

Follicular Phase

This phase runs from the first day of menstruation until ovulation occurs. At the beginning of the follicular phase, there is an increase in the levels of FSH. This hormone initiates maturation in the ovaries of a group of follicles and the oocytes they contain.

In follicular maturation, follicular cells secrete more estrogen, which promotes the growth of follicles and the oocytes they contain, stimulates the development of the endometrium, and stimulates the secretion of FSH and LH (Luteinizing Hormone).

The hormone LH stimulates the maturation of the developed follicle and produces meiosis II in the primary oocyte, thus forming a polar body and a secondary oocyte; causes ovulation and intervenes in the formation of the corpus luteum. At the end of the follicular phase, only one follicle ripens.