Question

. Describe the sequence of events in the ovarian cycle, with reference the follicular phase, ovulation, and the luteal phase OVARIAN CYCLE: REMEMBER: DAY 1 is the FIRST day of woman’s period.

Stage Day (s) What are the hypothalamus and pituitary doing? What is happening in the ovary and what is it producing?

Follicular phase 1 

Ovulation

Luteal Phase

Answer 1

•OVARIAN CHANGES DURING MENSTRUAL CYCLE -Changes in the ovary during each menstrual cycle occur in three phases:

A. Follicular phase

B. Ovulation

C. Luteal phase.

•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. 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 from the pituitary, 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 cell.

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. 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.

iii. 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.

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.

4.GraafianFollicle- Graafian follicle is the matured ovarian follicle with maturing ovum . 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.

*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 fallopian tube and reaches the uterus on 3rd day after ovulation. 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.