MENSTRUAL CYCLE

Menstrual cycle is defined as cyclic events that take place in a rhythmic fashion during the reproductive period of a woman’s life. Menstrual cycle starts at the age of 12 to 15 years, which marks the onset of puberty. The commencement of menstrual cycle is called menarche. Menstrual cycle ceases at the age of 45 to 50 years. Permanent cessation of menstrual cycle in old age is called menopause.

MENSTRUAL CYCLE

 

DURATION OF MENSTRUAL CYCLE

Duration of menstrual cycle is usually 28 days. But, under physiological conditions, it may vary between 20 and 40 days.

CHANGES DURING MENSTRUAL CYCLE

During each menstrual cycle, series of changes occur in ovary and accessory sex organs.

These changes are divided into 4 groups:

1. Ovarian changes

2. Uterine changes

3. Vaginal changes

4. Changes in cervix.

All these changes take place simultaneously.

OVARIAN CHANGES DURING

MENSTRUAL CYCLE

Changes in the ovary during each menstrual cycle occur in two phases:

A. Follicular phase

B. Luteal phase.

Ovulation occurs in between these two phases.

FOLLICULAR PHASE

Follicular phase extends from the 5th day of the cycle until the time of ovulation, which takes place on 14th day. Maturation of ovum with development of ovarian follicles takes place during this phase.

Ovarian Follicles

Ovarian follicles are glandular structures present in the cortex of ovary. Each follicle consists of the ovum

surrounded by epithelial cells, namely granulosa cells. The follicles gradually grow into a matured follicle

through various stages.

Different follicles:

1. Primordial follicle

2. Primary follicle

3. Vesicular follicle

4. Matured follicle or graafian follicle.

1. Primordial Follicle

At the time of puberty, both the ovaries contain about 400,000 primordial follicles. Diameter of the primordial follicle is about 15 to 20 μ and that of ovum is about 10 μ. Each primordial follicle has an ovum, which is incompletely surrounded by the granulosa cells. These cells provide nutrition to the ovum during childhood. Granulosa cells also secrete the oocyte maturation inhibiting factor, which keeps ovum in the immature stage. All the ova present in the ovaries are formed before birth. No new ovum is developed after birth. At the onset of puberty, under the influence of FSH and LH the primordial follicles start growing through various stages.

2. Primary Follicle

Primordial follicle becomes the primary follicle, when ovum is completely surrounded by the granulosa cells. During this stage, the follicle and the ovum increase in size. Diameter of the follicle increases to 30 to 40 μ and that of ovum increases to about 20 μ. The follicle is not covered by a definite connective tissue capsule. Changes taking place during development of primary follicle

i. Proliferation of granulosa cells and increase in size of the follicle

ii. Increase in size of the ovum

iii. Onset of formation of connective tissue capsule around the follicle.

Primary follicles develop into vesicular follicles.

3. Vesicular Follicle

Under the influence of FSH, about 6 to 12 primary follicles start growing and develop into vesicular follicles. Changes taking place during the development of vesicular follicle

i. Changes in granulosa cells

ii. Changes in ovum

iii. Formation of capsule.

i. Changes in granulosa cells

a. First, the proliferation of granulosa cells occurs

b. A cavity called follicular cavity or antrum is formed in between the granulosa cells

c. Antrum is filled with a serous fluid called the liquor folliculi

d. With continuous proliferation of granulosa cells, the follicle increases in size

e. Antrum with its fluid also increases in size

f. Ovum is pushed to one side and it is surrounded

by granulosa cells, which forms the germ hill or cumulus oophorus

g. Granulosa cells, which line the antrum form membrana granulosa

h. Cells of germ hill become columnar and form corona radiata.

ii. Changes in ovum

a. First, the ovum increases in size and its diameter increases to 100 to 150 μ

b. Nucleus becomes larger and vesicular

c. Cytoplasm becomes granular

d. Thick membrane is formed around the ovum, which is called zona pellucida

e. A narrow cleft appears between ovum and zona pellucida. This cleft is called perivitelline space.

Formation of capsule

Spindle cells from the stroma of ovarian cortex are modified and form a covering sheath around the follicle. The covering sheath is known as follicular sheath or theca folliculi.

Theca folliculi divides into two layers:

a. Theca interna

b. Theca externa.

a. Theca interna

Theca interna is the inner vascular layer with loose connective tissue. This layer also contains special type of epithelial cells with lipid granules and some delicate collagen fibers. Epithelial cells become secretory in nature and start secreting the female sex hormones, especially estrogen. Hormones are released into the fluid of antrum.

b. Theca externa

Theca externa is the outer layer of follicular capsule and consists of thickly packed fibers and spindleshaped cells. After about 7th day of menstrual cycle, one of the vesicular follicles outgrows others and becomes the dominant follicle. It develops further to form graafian follicle. Other vesicular follicles degenerate and become atretic by means of apoptosis.

Graafian Follicle

Graafian follicle is the matured ovarian follicle with maturing ovum. It is named after the Dutch physician and anatomist, Regnier De Graaf.

Changes taking place during the development of graafian follicle

i. Size of the follicle increases to about 10 to 12 mm. It extends through the whole thickness of ovarian cortex

ii. At one point, the follicle encroaches upon tunica albuginea and protrudes upon surface of the ovary. This protrusion is called stigma. At the stigma, the tunica albuginea becomes thin

iii. Follicular cavity becomes larger and distended with fluid

iv. Ovum attains maximum size

v. Zona pellucida becomes thick

vi. Corona radiata becomes prominent

vii. Small spaces filled with fluid appear between the cells of germ hill, outside the corona radiata.

These spaces weaken the attachment of the ovum to the follicular wall

viii. Theca interna becomes prominent. Its thickness becomes double with the formation of rich capillary network

ix. On the 14th day of menstrual cycle, graafian follicle is ready for the process of ovulation.

OVULATION

Ovulation is the process by which the graafian follicle ruptures with consequent discharge of ovum into the abdominal cavity. It is influenced by LH. Ovulation occurs on 14th day of menstrual cycle in a normal cycle of 28 days. The ovum enters the fallopian tube.

Process of Ovulation

Mechanism of ovulation is not known clearly.

Stages of ovulation

1. Rupture of graafian follicles takes place at the stigma

2. Follicular fluid oozes out

3. Germ hillock is freed from wall

4. Ovum is expelled out into the abdominal cavity along with some amount of fluid and granulosa cells

5. From abdominal cavity, the ovum enters the fallopian tube through the fimbriated end.

Ovum becomes haploid before or during ovulation by the formation of polar bodies. After ovulation, the

ovum is viable only for 24 to 48 hours. So it must be fertilized within that time. Fertilized ovum is called zygote. Zygote moves from fallo pian tube and reaches the uterus on 3rd day after ovula tion. It is implanted in the uterine wall on 6th or 7th day. If fertilization does not occur, ovum degenerates. Generally, only one ovum is released from one of the ovaries.

LUTEAL PHASE

Luteal phase extends between 15th and 28th day of menstrual cycle. During this phase, corpus luteum is

developed and hence this phase is called luteal phase.

Corpus Luteum

Corpus luteum is a glandular yellow body, developed from the ruptured graafian follicle after the release of ovum. It is also called yellow body.

Development of Corpus Luteum

Soon after the rupture of graafian follicle and release of ovum, the follicle is filled with blood. Now the follicle is called corpus hemorrhagicum. The blood clots slowly. Corpus hemorrhagicum does not degenerate immediately. It is transformed into corpus luteum. Follicular cavity closes gradually by the healing of the wound. Blood clot is gradually replaced by a serous fluid containing fibrin. Corpus luteum obtains a diameter of 15 mm and remains in the ovary till the end of the cycle.

Structure of Corpus Luteum

In the corpus luteum, granulosa cells and theca interna cells are transformed into lutein cells called granulosa lutein cells and theca lutein cells. The process which transforms the granulosa and theca cells into lutein cells is called luteinization. Granulosa lutein cells contain fine lipid granules and the yellowish pigment granules. The yellowish pigment granules give the characteristic yellow color to corpus

luteum. Theca lutein cells contain only lipid granules and not the yellow pigment. Follicular cavity is greatly reduced with irregular outline. It is filled with the serous fluid and remnants of blood clots.

Functions of Corpus Luteum

1. Secretion of hormones

Corpus luteum acts as a temporary endocrine gland. It secretes large quantity of progesterone and small

amount of estrogen. Granulosa lutein cells secrete progesterone and theca lutein cells secrete estrogen.

LH influences the secretion of these two hormones.

2. Maintenance of pregnancy

If pregnancy occurs, corpus luteum remains active for about 3 months, i.e. until placenta develops. Hormones secreted by corpus luteum during this period maintain the pregnancy. Abortion occurs if corpus luteum becomes inactive or removed before third month of pregnancy, i.e. before placenta starts secreting the hormones.

Fate of Corpus Luteum

Fate of corpus luteum depends upon whether ovum is fertilized or not.

1. If the ovum is not fertilized

If fertilization does not take place, the corpus luteum reaches the maximum size about one week after

ovulation. During this period, it secretes large quantity of progesterone with small quantity of estrogen. Then, it degenerates into the corpus luteum menstrualis or spurium. The cells decrease in size and the corpus luteum becomes smaller and involuted. Afterwards, the corpus luteum menstrualis is transformed into a whitish scar called corpus albicans. The process by which corpus luteum undergoes regression is called luteolysis.

2. If ovum is fertilized

If ovum is fertilized and pregnancy occurs, the corpus luteum persists and increases in size. It attains a

diameter of 20 to 30 mm and it is transformed into corpus luteum graviditatis (verum) or corpus luteum

of pregnancy. It remains in the ovary for 3 to 4 months. During this period, it secretes large amount of

progesterone with small quantity of estrogen, which are essential for the maintenance of pregnancy. After 3 to 4 months, placenta starts secreting these hormones and corpus luteum degenerates.

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