Ovary Introduction .
[1]. Ovary is the gonad or primary sex organs in females. A woman has two ovaries. Ovaries have two functions, gametogenic and endocrine functions.
[2]. Gametogenic function is the production and release of ovum or egg, which is the female gamete (reproductive cell). Endocrine function of ovaries is the secretion of female sex hormones.
Functional Anatomy of Ovary .
[1]. 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 hilum to the broad ligament, by means of mesovarium and ovarian ligament.
[2]. Each ovary has two portions:
1. Medulla
2. Cortex.
Medulla .
[1]. Medulla or zona vasculosa is the central deeper portion of the ovary. It has the stroma of loose connective tissues.
[2]. It contains blood vessels, lymphatics, nerve fibers and bundles of smooth muscle fibers near the hilum.
Cortex .
[1]. Cortex is the outer broader portion and has compact cellular layers. It is interrupted at the hilum, where the medulla is continuous with mesovarium.
[2]. Cortex is lined by the germinal epithelium underneath a fibrous layer known as ‘tunica albuginea’.
Cortex consists of the following structures :
1. Glandular structures, which represent ovarian follicles at different stages .
2. Connective tissue cells .
3. Interstitial cells, which are clusters of epithelial cells with fine lipid granules formed mainly from theca interna .
Ovarian Follicle .
[1]. In the intrauterine life, outer part of cortex contains the germinal epithelium, which is derived from the germinal ridges. When fetus develops, the germinal epithelium gives rise to a number of primordial ova.
[2]. The primordial ova move towards the inner substance of cortex. A layer of spindle cells called granulose cells from the ovarian stroma surround the ova. Primordial ovum along with granulosa cells is called the primordial follicle .
[3]. At 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 300,000 to 400,000.
[4]. 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.
[5]. 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.
[6]. Atresia occurs at all levels of follicles. Usually, the degenerated follicles disappear without leaving any scar.
Functions of Ovaries .
Ovaries are the primary sex organs in females. Functions of ovaries are:
1. Secretion of female sex hormones
2. Oogenesis
3. Menstrual cycle.
Ovarian Hormones .
Ovary secretes the female sex hormones estrogen and progesterone. Ovary also secretes few more hormones, namely inhibin , relaxin
and small quantities of androgens.
Estrogen Hormone .
Source of Secretion of Estrogen Hormone .
[1]. In a normal non-pregnant woman, estrogen is secreted in large quantity by theca interna cells of ovarian follicles and in small quantity by corpus luteum of the ovaries.
[2]. Estrogen secretion is predominant at the later stage of follicular phase before ovulation . Estrogen is derived from androgens, particularly androstenedione, which is secreted in theca interna cells.
[3]. Androstenedione migrates from theca cells to granulosa cells, where it is converted into estrogen by the activity of the enzyme aromatase.
A small quantity of estrogen is also secreted by adrenal cortex.
[4]. In pregnant woman, a large amount of estrogen is secreted by the placenta.
Chemistry of Estrogen Hormone .
Estrogen is a C18 steroid.
Different Forms of Estrogen Hormone .
Estrogen is present in three forms in plasma:
1. β-estradiol
2. Estrone
3. Estriol.
All the three forms of estrogen are present in significant quantities in plasma. The quantity and potency of β-estradiol are more than those of estrone and estriol.
Plasma Level of Estrogen Hormone .
[1]. Plasma level of estrogen in females at normal reproductive age varies during different phases of menstrual cycle. In follicular phase, it is 30 to 200 pg/mL .
[2]. In normal adult male, estrogen level is 12 to 34 pg/mL .
Half-life of Estrogen Hormone .
Half-life of estrogen is 30 to 60 minutes.
Synthesis of Estrogen Hormone .
[1]. The different forms of estrogen are synthesized from the cholesterol or acetate.
[2]. If estrogen is formed from acetate, first acetate is converted into cholesterol.
Pathway for synthesis of estrogen Hormone .
Acetate → Cholesterol → Pregnenolone → Progesterone → Testosterone → Estrogen .
[3]. During synthesis of estrogen, progesterone and testosterone are synthesized first . Then, before leaving the ovaries, almost all the testosterone and much of the progesterone are converted into estrogen.
[4]. About 1/15 of testosterone is secreted into the plasma of the female by the ovaries.
Transport in Plasma of Estrogen Hormone .
[1]. Estrogen is transported mainly by the plasma protein, albumin. A small quantity of estrogen is also transported
by globulin.
[2]. The binding of estrogen with the plasma protein is loose, so that the hormones are released into the tissues easily.
Metabolism of Estrogen Hormone .
[1]. 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.
[2]. Most of the remaining part is excreted in the urine. Liver also converts the potent active beta estradiol into the almost inactive estrogen, the estriol.
Function of Estrogen .
[1]. Major function of estrogen is to promote cellular proliferation and tissue growth in the sexual organs and in other tissues, related to reproduction.
[2]. In childhood, the estrogen is secreted in small quantity. 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 uterus:
1. Enlargement of uterus to about double of its childhood size due to the proliferation of endometrial cells .
2. Increase in the blood supply to endometrium .
3. Deposition of glycogen and fats in 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 their sensitivity to oxytocin .
7. Increase in the contractility of the uterine muscles. All these changes prepare 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 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 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 proliferation .
3. Reduces the pH of 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 characters in females.
Secondary sexual characters in female
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. Scalp hair grows profusely .
2. Skin: Skin becomes soft and smooth. Vascularity of skin also increases .
3. Body shape: Shoulders become narrow, hip broadens, thighs converge and the arms diverge. Fat deposition increases in breasts and
buttocks .
Pelvis:
a. Broadening of pelvis with increased transverse diameter
b. Round or oval shape of pelvis
c. Round or oval=shaped pelvic outlet. Thus, pelvis in females is different from that of males, which is funnel shaped.
4. Voice: Larynx remains in prepubertal stage, which produces high-pitch voice.
6. Effect on Breast .
Estrogen causes:
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 development of lobules and alveoli of the breasts, to some extent. However, progesterone is necessary for the full growth of breast and prolactin is necessary for its function.
7. Effect on Bones .
[1]. 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.
[2]. As a result, the growth of the females usually ceases few years earlier than in the males. In old age, the estrogen is not secreted or it
becomes scanty.
[3]. It leads to osteoporosis, in which the bones become extremely weak and fragile. Because of this, the bones are highly susceptible for fractures .
8. Effect on Metabolism .
1. On protein metabolism :- Estrogen induces anabolism of proteins, by which it increases the total body protein.
2. On fat metabolism :- Estrogen causes 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 fat deposition.
9. Effect on Electrolyte Balance .
Estrogen causes sodium and water retention from the renal tubules. This effect is normally insignificant but
in pregnancy, it becomes more significant.
Mode of Action of Estrogen .
[1]. Estrogen receptors situated on nuclear membrane of target cells are of two types namely α and β estrogen receptors.
[2]. The α-estrogen receptors are present in uterus, liver, heart and kidneys. The β estrogen receptors are present in ovaries and other tissues. Estrogen acts through genes.
Regulation of Estrogen Secretion .
[1]. Estrogen secretion is regulated by follicle-stimulating hormone (FSH) released from anterior pituitary. Release of FSH is stimulated by the gonadotropin-releasing hormone (GnRH) secreted from hypothalamus.
[2]. 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 secretion of FSH and GnRH by negative feedback.
[3]. Inhibin secreted by granulosa cells also decreases estrogen secretion, by inhibiting the secretion of FSH and GnRH .
Progesterone Hormone .
Source of Secretion of Progesterone Hormone .
[1]. In non-pregnant woman, a small quantity of progesterone is secreted by theca interna cells of ovaries during the first half of menstrual cycle, i.e. during follicular stage.
[2]. But, a large quantity of progesterone is secreted during the latter half of each menstrual cycle, i.e. during secretory phase by the corpus luteum. Small amount of progesterone is secreted from adrenal cortex also.
[3]. In pregnant woman, large amount of progesterone is secreted by the corpus luteum in the first trimester.
[4]. In the second trimester, corpus luteum degenerates. Placenta secretes large quantity of progesterone in second and third trimesters.
Chemistry of Progesterone Hormone .
Progesterone is a C21 steroid.
Half-life of Progesterone Hormone .
Half-life of progesterone is 4 to 5 minutes.
Synthesis of Progesterone Hormone .
Progesterone is synthesized from acetate or cholesterol in the ovaries, along with estrogen .
Plasma Level of Progesterone Hormone .
[1]. Plasma level of progesterone in females at normal reproductive age varies during different phases of menstrual cycle.
[2]. In follicular phase, it is about 0.9 ng/mL . In normal adult male, progesterone level is 0.3 ng/mL.
Transport in Blood of Progesterone Hormone .
Like estrogen, progesterone is also transported in the blood by the plasma proteins – albumin and globulin.
Metabolism of Progesterone Hormone .
[1]. Within few minutes after secretion, almost all the progesterone is degraded into other steroids, which do not have progesterone effect. The degradation occurs in liver.
[2]. 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 .
[1]. Progesterone promotes the secretory activities of mucosal lining of the fallopian tubes.
[2]. Secretions of fallopian tubes are necessary for nutrition of the fertilized ovum, while it is in fallopian tube before implantation.
2. Effect on the Uterus .
Progesterone promotes the 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 :
[1]. Increases the thickness of the endometrium by increasing the number and size of the cells . Thickness of endometrium increases from 1 mm thickness at the beginning of secretory phase to about 5 to 6 mm at the end of secretory phase.
[2]. Increases the size of uterine glands and these glands become more tortuous .
[3]. Increases the secretory activities of epithelial cells of uterine glands .
[4]. Increases the deposition of lipid and glycogen in the stromal cells of endometrium .
[5]. Increases the blood supply to endometrium. It is due to increase in size of the vessels and vasodilatation
[6]. 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 cervical mucosa and thereby inhibits the transport of sperm into uterus. This effect is utilized in the contraceptive actions of minipills .
4. Effect on the Mammary Glands .
[1]. Progesterone promotes the development of lobules and alveoli of mammary glands by proliferating and enlarging the alveolar cells. It also makes the breasts secretory in nature.
[2]. It makes the breasts to swell by increasing the secretory activity and fluid accumulation in the subcutaneous tissue.
5. Effect on Hypothalamus .
Progesterone inhibits the release of LH from hypothalamus through feedback effect. This effect is utilized for its contraceptive action.
6. Thermogenic Effect .
[1]. Progesterone increases the body temperature after ovulation. The mechanism of thermogenic action is not known.
[2]. It is suggested that progesterone increases the body temperature by acting on hypothalamic centers for temperature regulation.
7. Effect on Respiration .
During luteal phase of menstrual cycle and during pregnancy, progesterone increases the ventilation via respiratory center. This decreases the partial pressure of carbon dioxide in the alveoli .
8. Effect on Electrolyte Balance .
[1]. Progesterone 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.
[2]. 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 .
[1]. The progesterone receptors situated on the nuclear membrane of target cells are of two types, namely A-progesterone receptors and B-progesterone receptors.
[2]. Exact location of each type of progesterone receptor is not clear. Like estrogen, progesterone also acts through genes.
Regulation of Progesterone Secretion .
[1]. LH from anterior pituitary activates the corpus luteum to secrete progesterone.
[2]. Secretion of LH is influenced by the gonadotropin-releasing hormone secreted in hypothalamus. Progesterone inhibits the release of LH from anterior pituitary by negative feedback .
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