Ovary Physiology Notes

Functional Anatomy Of Ovary

Ovaries are flattened ovoid bodies with dimensions of 4 cm in length, 2 cm in width, and 1 cm in thickness. Each ovary is attached at the hilum to the broad ligament by means of mesovarium and ovarian ligament.

At hilum. the connective tissue of the mesovarium and the ovarian ligament is continuous with ovarian stroma. The mesovarium gives rise to the germinal epithelium. On cross section, each ovary shows two zones:

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1. Medulla
2. Cortex.

Medulla

The medulla is otherwise known as zona vasculosa. It is the central deeper portion of the ovary. It has the stroma of loose connective tissues. It contains blood vessels, lymphatics, nerve fibers, and bundles of smooth muscle fibers near the hilum.

Cortex

It is the outer broader portion and has compact cellular layers. It is interrupted at the hilum, where the medulla is continuous with mesovarium.

The cortex is lined by the germinal epithelium underneath a fibrous layer known as the tunica albuginea. The cortex consists of the following structures:

1. Glandular structures which represent ovarian follicles at different stages of development
2. Connective tissue cells
3. Interstitial cells are clusters of epithelial cells with fine lipid granules formed mainly from theca Interna.

In intrauterine life, the outer part of the cortex contains the germinal epithelium, which is derived from the germinal ridges. When the fetus develops, the germinal epithelium gives rise to a number of primordial ova.

• The prior cordial ova move towards the inner substance of the cortex. A layer of spindle cells from the ovarian stroma gets arranged around the ova.
• The spindle cells are called granulosa cells. The primordial ovum along with granulosa cells is called the primordial follicle.
• At the 7th or 8th month of intrauterine life, about 6 million primordial follicles are found in the ovary. But, at the time of birth, only 1 million primordial follicles are seen in both the ovaries, and, the rest of the follicles degenerate.
• At the time of puberty, the number decreases further to about 3,00,000-4,00,000. After menarche, during every menstrual cycle, one of the follicles matures and releases its ovum. During every menstrual cycle, only one ovum is released from any one of the ovaries.
• During every cycle, many of the follicles degenerate. The degeneration of the follicles is called atresia and the degenerated follicles are known as atretic follicles.
• The atretic follicles become fibrous and, the fibrotic follicles are called the corpus fibrosa. Atresia occurs at all levels of follicles. Usually, the degenerated follicles disappear without leaving any scars.

The functions of ovaries are:

1. Secretion of female sex hormones
2. Oogenesis
3. Menstrual cycle.

Sex hormones are discussed in this chapter. Oogenesis and the menstrual cycle are explained in the next chapter.

Ovarian Hormones

The ovary secretes the female sex hormones estrogen and progesterone. Ovary also secretes a few more hormones namely, inhibin, relaxin, and small quantities of androgens.

Estrogen

Chemistry and Source of Secretion

• Estrogen is a C18 steroid. In a normal nonpregnant female, estrogen is secreted in large quantities by theca interna cells of ovarian follicles and in small quantities by the corpus luteum of the ovaries. The secretion of estrogen is predominant at the later stage of the follicular phase before ovulation (Chapter 80).
• Estrogen is derived from androgens, particularly androstenedione which is secreted in theca interna cells. Androstenedione migrates from theca cells to granulosa cells where it is converted into estrogen by the activity of the enzyme called aromatase.
• A small quantity of estrogen is also secreted by the adrenal cortex. In pregnancy, a large amount of estrogen is secreted by the placenta.

Different Forms

Estrogen is present in three forms in plasma:

1. βestradiol
2. Estrone
3. Estriol.

All three forms of estrogen are present in significant quantities in plasma. The quantity and potency of (3 estradiol are more than those of estrone and estriol.

Plasma Level

The plasma level of estrogen in females at normal reproductive age varies during different phases of the menstrual cycle. In the follicular phase, it is 30-200 pg/mL (Fig. 80-4). In normal adult males estrogen level is 12-34 pg/mL.

Synthesis

The different forms of estrogen are synthesized from cholesterol or acetate. If estrogen is formed from acetate, first acetate is converted into cholesterol. The pathway is:

Acetate Cholesterol -> Pregnenolone

Estrogen <— Testosterone <— Progesterone

During synthesis of estrogen, progesterone and testosterone are synthesized first (Fig. 70-2). Then, before leaving the ovaries, almost all the testosterone and much of the progesterone are converted into estrogen. About 1/15 of testosterone is secreted into the plasma of the female by the ovaries.

Transport in Plasma

Estrogen is transported mainly by the plasma protein, albumin. A small quantity of estrogen is also transported by the globulin. The binding of estrogen with the plasma protein is loose so that the hormones are released into the tissues easily.

Metabolism

• Estrogen is degraded mainly in the liver. Here, it is conjugated with glucuronides and sulfates. About one-fifth of the conjugated products are excreted in the bile.
• Most of the remaining part is excreted in the urine. The liver also converts the potent active beta-estradiol into the almost inactive estrogen, estriol.

Functions Of Estrogen

• The major function of estrogen is to promote cellular proliferation and tissue growth in the sexual organs and in other tissues related to reproduction. In childhood, the
• estrogen is secreted in small quantities. During puberty, the secretion increases sharply resulting in changes in the sexual organs. Effects of estrogen are:

1. Effect on Ovarian Follicles

Estrogen promotes the growth of ovarian follicles by increasing the proliferation of the follicular cells. It also increases the secretory activity of theca cells.

2. Effect on Uterus

Estrogen produces the following changes in the uterus:

1. Enlargement of the uterus to about double its childhood size: It is due to the proliferation of endometrial cells
2. Increase in the blood supply to the endometrium
3. Deposition of glycogen and fats in the endometrium
4. Proliferation and dilatation of blood vessels of endometrium
5. Proliferation and dilatation of the endometrial glands, which become more tortuous with increased blood flow
6. Increase in the spontaneous activity of the uterine muscles and sensitivity to oxytocin
7. Increase in the contractility of the uterine muscles due to an increase in actomyosin concentration,

All these changes prepare the uterus for pregnancy.

3. Effect on Fallopian Tubes

Estrogen:

1. Acts on the mucosal lining of the fallopian tubes and increases the number and size of the epithelial cells, especially the ciliated epithelial cells lining the fallopian tubes
2. Increases the activity of the cilia, so that the movement of the ovum in the fallopian tube is facilitated
3. Enhances the proliferation of glandular tissues in fallopian tubes.

All these changes are necessary for the fertilization of the ovum.

4. Effect on Vagina

Estrogen:

1. Changes the vaginal epithelium from cuboidal into stratified type. The stratified epithelium is more resistant to trauma and infection
2. Increases the layers of the vaginal epithelium by the proliferation
3. Reduces the pH of the vagina making it more acidic.

All these changes are necessary for the prevention of certain common vaginal infections such as gonorrheal vaginitis. Such infections can be cured by the administration of estrogen.

5. Effect on Secondary Sexual Characters

Estrogen is responsible for the development of secondary sexual characteristics in females.

The secondary sexual characters in females are:

1. Hair distribution: Hair develops in the pubic region and axilla. In females, pubic hair has the base of the triangle upwards. Body hair growth is less.
2. Scalp hair grows profusely. Even though estrogen has an effect on hair distribution, it mostly depends upon the androgens secreted from the ovary and adrenal cortex in small quantities
3. Skin: Estrogen renders softness and smoothness to the skin. It also increases the vascularity of the skin
4. Body shape: The shoulders become narrow, the hip broadens, the thighs converge, and the arms diverge. The fat deposition increases in the breasts and buttocks
5. Pelvis: Estrogen has a specific effect on the pelvis which results in:
   1. Broadening of the pelvis with an increased transverse
   2. diameter
   3. The round or oval shape of the pelvis n. Round or oval-shaped pelvic outlet.
6. Thus, the pelvis in females is different from that of males, which is funnel-like shaped.
7. Voice: The larynx remains in the prepubertal stage, which produces high pitch voice.

6. Effect on Breast

In childhood, the breasts in males and females are alike. In females at the time of puberty, under the influence of estrogen and other hormones like growth hormone and thyroxin, the breasts develop into an adult type.

The effects of estrogen on breasts are:

1. Development of stromal tissues of breasts
2. Growth of an extensive ductile system
3. Deposition of fat in the ductile system

All these effects prepare the breasts for lactation Estrogen causes the development of lobules and alveoli of the breasts to some extent. However, progesterone is necessary for the full growth of the breast and prolactin is necessary for its function.

7. Effect on Bones

• Estrogen increases osteoblastic activity. So, at the time of puberty, the growth rate increases enormously. But, at the same time, estrogen causes early fusion of the epiphysis with the shaft.
• This effect is much stronger in females than the similar effect of testosterone in males. As a result, the growth of females usually ceases a few years earlier than in males.
• In old age, the estrogen is not secreted or it becomes scanty. It leads to osteoporosis in which the bones become extremely weak and fragile. And, because of this, the bones are highly susceptible to fractures.

8. Effect on Metabolism

1. On protein metabolism: Estrogen induces anabolism of proteins. It increases the total body protein, with a mild positive nitrogen balance. It is because of the growth-promoting effect of estrogen on the sexual organs, the bones, and a few other tissues of the body.
2. On fat metabolism: Estrogen causes the deposition of fat in the subcutaneous tissues, breasts, buttocks and thighs. The overall specific gravity of the female body is considerably lesser than that of males because of the fat deposition.

9. Effect on Electrolyte Balance

Like mineralocorticoids, estrogen causes sodium water retention by the renal tubules. This effect is not insignificant but in pregnancy, it becomes more sig. vf cant.

Mode of Action of Estrogen

• The estrogen receptors situated on the nuclear membrane of target cells are of two types – a and p estrogen receptors. The estrogen receptors are present in the uterus, liver, heart, and kidneys. The p estrogen receptors are present in ovaries and other tissues.
• Estrogen binds with estrogen receptors and forms the hormone-receptor complex which in turn binds with DNA and induces transcription of mRNA.

Regulation of Estrogen Secretion

• The secretion of estrogen is regulated by FSH released from the anterior pituitary. The release of FSH is stimulated by the gonadotropic-releasing hormone (GnRH) secreted from the hypothalamus.
• The theca cells and granulosa cells have many FSH receptors. After binding with the receptors, FSH acts via cAMP and stimulates the secretory activities of theca and granulosa cells.
• Estrogen inhibits the secretion of FSH and GnRH by negative feedback. Inhibin secreted by granulosa cells also decreases estrogen secretion by inhibiting the secretion of FSH and GnRH.

Progesterone

Chemistry and Source of Secretion

• Posiorone is a C21 steroid. A small quantity of, j istcrone is secreted by theca interna cells of ovaries; d’,e first half of the menstrual cycle, i.e. during follicular a large quantity of progesterone is secreted the latter half of each menstrual cycle, i.e. during. Secretory phase by the corpus luteum. A small amount of progesterone is secreted from the adrenal cortex also.
• During pregnancy, a large amount of progesterone is secreted by the corpus luteum in the first trimester. In the second trimester corpus luteum degenerates. Placenta secretes a large quantity of progesterone in the second and third trimesters.

Synthesis

Progesterone is synthesized from acetate or cholesterol in the ovaries along with estrogen.

Plasma Level

The plasma level of progesterone in females at normal reproductive age varies during different phases of the menstrual cycle. In follicular phase, it is about 0.9 ng/mL (Fig. 80-4). In normal adult male progesterone level is 0.3 ng/mL.

Transport in Blood

Like estrogen, progesterone is also transported in the blood by the plasma proteins, albumin and globulin.

Metabolism

Within a few minutes after secretion, almost all the progesterone is degraded into other steroids, which do not have a progesterone effect. The degradation occurs in the liver. The main end product of progesterone degradation is pregnanediol, which is conjugated with glucuronic acid and excreted in the urine.

Functions Of Progesterone

Progesterone is concerned mainly with the final preparation of the uterus for pregnancy and the breasts for lactation. The effects of progesterone are:

1. Effect on Fallopian Tubes

Progesterone promotes secretory activities of the mucosal lining of the fallopian tubes. The secretions of fallopian tubes are necessary for the nutrition of the fertilized ovum while it is in the fallopian tube before implantation.

2. Effect on the Uterus

Progesterone promotes secretory activities of uterine endometrium during the secretory phase of the menstrual cycle. Thus, the uterus is prepared for implantation of the fertilized ovum.

Progesterone:

Increases the thickness of the endometrium by increasing the number and size of the cells. The thickness of endometrium increases from 1 mm thickness at the beginning of the secretory phase to about 5-6 mm at the end of the secretory phase

1. Increases the size of uterine glands and these glands become more tortuous
2. Increases secretory activities of epithelial cells of uterine glands
3. Increases the deposition of lipids and glycogen in the stromal cells of the endometrium
4. Increases the blood supply to the endometrium. It is due to the increase in the size of the vessels and vasodilatation
5. Decreases the frequency of uterine contractions during pregnancy. Because of this, the expulsion of the implanted ovum is prevented.

3. Effect on Cervix

Progesterone increases the thickness of the cervical mucosa and thereby inhibits the transport of sperm into the uterus. This effect is utilized in the contraceptive actions of minipills.

4. Effect on the Mammary Glands

Progesterone promotes the development of the lobules and alveoli of the mammary glands by proliferating and enlarging the alveolar cells.

It also makes the breasts secretory in nature. It makes the breasts swell by increasing secretory activity and fluid accumulation in the subcutaneous tissue.

5. Effect on Hypothalamus

Progesterone inhibits the release of LH from the hypothalamus through the feedback effect. This effect is utilized for its contraceptive action.

6. Thermogenic Effect

Progesterone increases the body temperature after ovulation. The mechanism of thermogenic action is not known. It is suggested that progesterone increases the body temperature by acting on hypothalamic centers for temperature regulation.

7. Effect on Respiration

During the luteal phase of the menstrual cycle and during pregnancy, progesterone increases ventilation via the moratory center. This results in decreased PC02 in the alveoli.

8. Effect on Electrolyte Balance

Prcalerone increases the reabsorption of sodium and water from the renal tubules. However, in large doses, it is believed to cause excretion of sodium and water. This may be due to an indirect effect, i.e. progesterone combines with the same receptors, which bind with aldosterone. So, the action of aldosterone is blocked leading to the excretion of sodium and water.

Mode of Action of Progesterone

The progesterone receptors situated on the nuclear membrane of target cells are of two types namely A and B progesterone receptors. The exact location of each type of progesterone receptor is not clear.
Like estrogen, progesterone also binds with receptors and forms the hormone-receptor complex which in turn binds with DNA and induces transcription of mRNA.

Regulation of Secretion of Progesterone

LH from the anterior pituitary activates the corpus luteum to secrete progesterone. Secretion of LH is influenced by the gonadotropic-releasing hormone secreted in the hypothalamus. Progesterone inhibits the release of LH from the anterior pituitary by negative feedback.