Anatomy of Ovaries: Description, Function & Physiology of Ovulation

Description of Ovaries

The Ovaries are the female glands or organs that lie within the peritoneal cavity in a small depression of the posterior wall of the broad ligament.

Location: They are situated at the fimbrated end of the Fallopian tube at about the level of the pelvic brim.

Shape: They are small almond like organs dull white in colour and with a corrugated surface

Size: They measure 3 cm in length 2 cm  in breath and 1 cm in thickness. They weigh about 6 gram

Gross structure

This varies with the age of the woman. From birth to puberty, the organs are smooth dull white and rather solid in consistency. During the menstrual phase ie from puberty to menopause, the organs are longer and rather on the surface, and in more like a walnut than an almond. 

During the post menopausal phase, the ovaries beco smaller and shruken and are covered with scar tissues, where month after month the graffian follicle have ruptured.

Microscopic structure

The ovaries are composed of the cortex and a medulla surrounded by a germinal epithelium.

  • The medulla: Is the part of the ovary that is directly attached to the Gonad ligament at the mesovarian. It consists of fibrous tissue and transmits the ovarian vessels, lymphatics and nerves which enter and leave the ovary from the broad ligament.
  • The cortex: Is the functional part of the ovary and consists of a sense stroma in which about 200,000 primordial follicles are embedded. Each follicle contains an ovum which is capable in later life of becoming mature and fertilized and then developing into a baby. The primordial follicles are separated from each other by stroma and enclosed with the tunica albuginea and germinal epithelium.

Changes begin in some of the primordial follicles right from the Uterine life and persist throughout infancy and early girlhood. These changes consist of increase in the number of calls around the ovum and is followed by the appearance of fluid between the cells known liquour folliculi and the primordial follicle is now known as the “graafian follicle”

The tunica albuginea is the outer part of the corvex formed by a sense fimbrous coat.

The germinal epithelium consists of a layer of low cubical cells which covers the tunica albuginea of the cortex and is continuous with the mesovarian. Therefore the germinal epithelium is the modified form of the peritoneum.  

Ovarian Attachments

  1. Mesovarian attaches the ovaries to the posterior layer of the broad ligament
  2. Mesosalpinx is the part of the broad ligament attaching the Ovaries to the Fallopian tube
  3. Ovarian ligaments attaches or suspends the ovaries from the cornua of the uterus
  4. Infundibulo pelvic ligament is a fold of peritoneum attaching the lateral poles the ovaries to the side wall of the pelvis

Relations of the ovaries

  • Medially: by the body of uterus and ovarian ligament
  • Laterally: by the Infundibulo ligament and the sidewall of the pelvis
  • Anteriorly: by the broad ligament and the mesovarian
  • Postery: by the peritoneal cavity and the intestines
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Blood supply: by the ovarian and Uterine arteries

Venous Drainage: by corresponding veins

Nerve Supply: by ovarian plexus

Lymphatic Drainage: into lumbar glands.

Functions of ovaries

  1. The production of ova for the purpose of fertilization
  2. The production of hormones oestrogen and progesterone 

Hormonal secretion of the Ovaries

The graffian follicles and corpora lutea are not only under the hormonal influence of pituitary gonadotropins, but also secrete hormones on their own account.

During the development of the graffian follicle, oestradiol is secreted by the cells of zona granulosa and the theca internal and stored in liquour folliculi. After ovulation it is secreted in lesser amount by the corpus luteum and is also formed in the cortex of the supra – renal glands. It is metabolized in the liver and excreted mainly in the urine in form of oestriol and oestreone and some 8 other related compounds. 

These substances are collectively called oestrogen and are responsible for the secondary sex characteristic of puberty in young girls and some changes that occur during the menstrual cycle and pregnancy.

Effects of oestrogen

Major effects of oestrogens are:

  • The production of typical feminine shape of the waist and hips with soft skin and smooth gentle curves resulting from the deposition of fat in the subcutaneous tissoat puberty.
  • The growth of the breasts mainly due to the development of duct system, also the growth of the nipples
  • The growth of the axillary and pubic hairs
  • The development of adult vulva and vagina, the squamous epithelium of the vagina which is thin in childhood becomes thicker.
  • It is responsible for the growth of the uterine muscles and breasts changes during pregnancy
  • It is responsible for the growth of the uterus at puberty and the proliferative changes at the endometrium during the menstrual cycle
  • It suppresses the action of prolactin until after delivery
  • It suppresses ovulation during pregnancy by inhibition of FSH from the anterior pituitary gland
  • It is responsible for retention of water and electrolytes in the body tissue

Progesterone —Key hormone of ovaries

The corpus luteum under the influence of LH produce another hormone known as progesterone and a small quantity of oestrogen.

The changes produced in the body by progesterone are mainly manifested in large quantities. Apart from pregnancy, it’s main function is endometrial changes as part of the menstrual cycle.

Effects of progesterone

  • It is responsible for the secretory changes in the endometrium prior to menstruation.
  • It causes slight tingling in the breasts before the onset of menstruation.
  • It causes a slight rise in body temperature of about 0.5.
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During pregnancy it is responsible for: 

  • The development of decidua in the Uterus for the embeddment of fertilized ovum
  • The relaxation of the tone of the smooth muscles all over the body.
  • The enlargement of the breasts by growth of the milk forming alveoli.
  • The retention of water and electrolytes in the body tissue in association with oestrogen.

The Ovarian Cycle

This is described in three parts as follows: 

  • Development of graffian follicle
  • Mechanism of ovulation
  • Development of corpus luteum

Development of graffian follicle

During fetal life the cortex of the ovary forms thousands of primitive eggs cells or primordial follicles which are present when the child is born. Every month from age of puberty onwards through child bearing life several follicles begins to develop as a rule, only one becomes fully matured and rises to the surface of the ovary. The one that matured is known as the graffian follicle (named after Dr.Von Graff). 

Structure of a mature graafian follicle

A mature graafian follicle is about 8-12mm. 

  • A large central cell, the ovum which consists of a large nucleus with prominent nucleoli
  • Immediately, outside the ovum is a very narrow space the periviteline space.
  • Surrounding the ovum and the periviteline space is a large clump of cells attached to one Pole to the lining of the follicle. These cells are the granulosa cells and the clump they form around the ovum is the DISCUS PROLIGERUS. The cells of the Discus Proligerus appears to radiate from the ovum by their mode of arrangement, a formation called the corona radiata. These corona cells contain amorphous material which forms a translucent membrane- the zona pellucida lying adjacent to the periviteline space.
  • The follicle itself is lined with the granulosa cells similar to the Discus Proligerus known as membrana granulosa or zona granulosa and scattered between these cells especially where the Discus are globules of clear fluid known as callexner bodies.
  • The follicle is filled with fluid- the liquor folliculi
  • Externally the zona granulosa cells rest upon a basement membrane which encloses the whole follicle- Externally limiting membrane.
  • Outside the follicle, the stroma of the ovum is compressed to form a capsule- the theca which is in 2 parts: the inner vascular layer- theca internal or vasculosa and theca external.

Mechanism of ovulation

Ovulation means ripening and discharging of an ovum from a graffian follicle into the peritoneal cavity. The reproductive age of a woman begins with menarche (appearance of first menses) to menopause (cessation of menstruation) at about 45-50 years.

During this period, one graffian follicle out grows it’s fellow and enlarges in size . It then protrudes to the surface of the ovary with the discus and the ovum.

As the follicle enlao it stretches the ovarian capsule (covering) until it becomes so thin and breaks finally. The follicle thus ruptures and the liquor folliculi, Discus and ovum are cast out into the peritoneal cavity. This process is known as ovulation.

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Ovulation takes place each month in alternate Ovaries so that each individual ovary ovulates at 2 monthly intervals.

The corpus luteum

After ovulation the walls of the graffian follicle collapses and become wrinkled at the same time some of the cells grow and develop yellowish coloured bodies. This yellowish wrinkled structure is known as corpus luteum, which under the influence of LH produce by the anterior pituitary produce progesterone which thickens the endometrial wall. It remins active for 14 days but if after that period fertilization does not take place, it does ,shrinks and become corpus albicans. 

This leaves a whitish scar on the surface of the ovary. On the other hand, if fertilization takes place corpus luteum persist for 12 weeks until placenta is formed to take over the hormonal function of the corpus luteum.

Hormonal control of ovulation

The growth and ripening of the ovarian follicle are under the hormonal control- Follicular stimulating hormone (FSH) from the anterior pituitary gland.

FSH is carried in the bloodstream from the pituitary to the ovary where it causes the follicle to grow and enlarge and at the same time stimulate the cells of th zona granulosa and the theca internal to produce oestradiol.

When the amount oestradiol reaches a certain level, the production of FSH is stopped and LH from anterior pituitary gland is produced. When the FSH is falling, LH is rising and at certain ratio , this cause the follicle to rupture and ovulation occurs.

After ovulation, the development and growth of corpus luteum occurs due to action of LH. This lasts for 14 days after which the secretion cease. FSH reappear and the cycle is repeated. 

Therefore, 2 weeks proceeding ovulation the ovary is under the influence of FSH and for the 2weeks after ovulation, it is under the influence of LH. The degeneration of the corpus luteum is due to withdrawal of LH, and it’s substitution by FSH causes fresh growth in another graffian follicle in the opposite ovary. In this way, the cycle continues throughout the sexual phase of a woman’s life.

The anterior pituitary is linked to the hypothalamus by a special system of nerves , the pituitary portal system. The production of the gonadotrophic hormones from the anterior pituitary is regulated by the hypothalamus through this portal veins.

As the hypothalamus is connected by nerves tract with the higher cortical centres it can be appreciated that emotional factors influences ovulation and interferes menstruation.

Summary

Healthy ovaries are key to getting pregnant. This is because only healthy ovaries can ovulate or release ovum needed during fertilization. Kindly share to your family and friends.