FEMALE REPRODUCTIVE CYCLE

By Oba Mike

INTRODUCTION

During their reproductive years (which begins from puberty), nonpregnant females normally exibit cyclical changes in the ovaries and uterus. These changes begin from menarche and terminates at menopause.  Each cycle takes about a month and involves both oogenesis and preparation of the uterus to receive the fertilized ovum.

The changes that occur in the ovaries, form the ovarian cycle while the changes that occur in the uterus form the uterine or menstrual cycle. Both cycles are regulated by hormones secreted by the hypothalamus, anterior pituitary and ovaries.

DURATION

Both cycles take an average of 28 days. Some females have longer duration while others experience shorter duration. However, for descriptive purpose, we'll focus on a regular 28 day cycle.

OVARIAN CYCLE

Ovarian cycle above

The ovarian cycle comprises the series of monthly changes that occur in the ovaries during the reproductive years of a nonpregnant woman. These changes can be grouped into three phases:

1. Follicular phase

2. Ovulation (ovulatory) phase

3. Luteal phase

Follicular phase

This occurs from Day  1-13 in a 28 day cycle.

As the name suggests, this phase involves the development of primordial follicles into primary and secondary follicles. This development is under the influence of Follicle Stimulating Hormone (FSH) secreted by the anterior pituitary. The developing follicles begins to secrete estrogen.

Around day 6, a single secondary follicle in one of the two ovaries outgrow others to become the dorminant follicle. Estrogen and inhibin secreted by this dorminant follicle decreases the secretion of FSH which causes other less developed follicles to stop growing and undergo atresia.

The dominant follicle grows to become the mature (Graafian) follicle that enlarges to more than 20mm in diameter.

 At this point, there's increase in estrogen level. This increase stimulates the anterior pituitary to release more Luteinizing hormone (LH) leading to LH surge.

Ovulation

Occurs on day 14 in a 28 day cycle.

The mature (Graafian) follicle forms a blister like bulge at the surface of the ovary. Under the influence of LH, it ruptures and releases the secondary oocyte. The secondary oocyte remains surrounded by it's zona pellucida and corona radiata. Together with these structures, it is swept into the uterine tube and begins its descent into the uterus. Along the way, fertilization can occur.

Luteal Phase

This occurs from day 15-28 in a 28 day cycle.

The remnant of the mature follicle goes through a series of changes to become the corpus luteum under the influence of luteinizing hormone. 

First, a blood clot is formed to stop bleeding of the ruptured follicle. At this point of clotting, the entire structure is called corpus hemorrhagicum. 

Thereafter, the theca interna cells and granulosa cells unite and transform the entire structure into the corpus luteum. 

The newly formed corpus luteum, still under the influence of LH, secretes progesterone, estrogen, inhibin and relaxin. The presence of progesterone and estrogen together halts the production of FSH and LH.

Later events in the luteal phase depends on whether the oocyte is fertilized or not. 

In the absence of fertilization, the corpus luteum degenerates after two weeks into corpus albicans which is later phagocytized.

Following the breakdown of the corpus luteum, the level of progesterone, estrogen and inhibin decreases and FSH and LH secretion begins. A new cycle commences.

Assignment

Write on what happens if fertilization occurs.

UTERINE CYCLE

Uterine cycle below

The uterine cycle commonly known as menstrual cycle is a series of monthly changes in the endometrium of the uterus in preparation for the arrival of a fertilized ovum that will develop there until birth. 

Like the ovarian cycle, a typical uterine cycle ranges from 24 to 36 days. Note that both cycles occur concurrently. For easy description, we'll assume a 28 day cycle.

Phases

There are three phases in the uterine cycle:

1. Menstrual phase

2. Proliferative phase

3. Secretory phase

Menstrual phase:

The menstrual phase is the most obvious stage and occurs from day 1-5 in a 28 day cycle. 

It coincides with the early follicular phase of the ovarian cycle.

It is due to the low level of estrogen and progesterone in the blood which causes the stratum functionalis of the endometriun to disintegrate and rupture of its blood vessels. 

Menstrual flow consisting of blood, tissue fluid, mucus and ruptured stratum functionalis passes from the uterine cavity through the cervix and vagina to the exterior. If the ovum is not reabsorbed, it also forms part of menses.

This flow is called menstruation while the fluid is called menses.

Structurally, the endometrium is very thin at this phase (about 2-5mm) because only the stratum basalis remains. This is shown in the picture.

Proliferative phase

It occurs from day 6-13 and coincides with the late follicular phase of the ovarian cycle.

At this stage, the ovarian follicles have already started producing estrogen which stimulates the repair of the endometrium.

Cells of the stratum basalis undergo mitosis to produce a new stratum functionalis.

The thickness of the endometrium doubles to about 4-10mm (hence the name proliferating).

Secretory phase

This occurs from day 15-28 in a 28 day cycle and coincides with the luteal phase of the ovarian cycle.

Progesterone and estrogen produced by the corpus luteum promotes further growth and thickening of the endometrium. 

The endometrial glands begins to secrete watery mucus rich in glycogen which assists the passage of spermatozoa towards the ovulated and incoming egg(God is wonderful😮). This is the reason for the name of this phase.

The endometrium thickens to about 12-18mm.

If fertilization does not occur, degeneration of corpus luteum leads to the decline of progesterone and estrogen which causes menstruation and beginning of a new cycle.