The female internal genital organs

The internal genital organs are the uterus, ovaries and Fallopian tubes. The uterus is the size and shape of a medium sized pear although it is flattened from front to back. The narrow end of the uterus is the cervix and the upper part is called the body. The cervix, the neck of the womb, protrudes into the top of the vagina (Figure 2.1). In a child, the cervix forms about two-thirds of the length of the uterus but as the uterus enlarges following puberty, the proportions are reversed. After the menopause the uterus becomes small again. During childbirth the cervix is stretched open. When fully open it becomes continuous with the vagina to form the birth-canal through which the baby descends into the world.

Figure 2.2 Female Anatomy - Side (Lateral) View

You may be able to feel the cervix by introducing a finger inside the vagina. It is a firm structure that feels like the tip of the nose. The small central depression , the external os, is the entrance to the cavity of the uterus. The cervical canal is the inner tube of the cervix leading from the vagina to the cavity of the uterus. Cervical mucus is secreted by tiny glands lining the canal. During reproductive years this mucus is watery around the time of ovulation (egg release) - at other times it is thick and acts as a barrier to sperm and infection.

The body of the uterus has a central cavity, the endometrial cavity, which is triangular with the front and back walls touching. The endometrial cavity has a very special covering - the endometrium. During menstruation most of the endometrium is shed leaving just a residual basal layer which regenerates the endometrium during each menstrual cycle (Figure 2.3). The endometrium has the ability to allow an early embryo to become implanted and to receive nourishment before the placenta (afterbirth) develops.

The Fallopian tubes, or oviducts, extend for about 10 cm from each cornu (Latin, horn) of the body of the uterus, outwards to their openings near the ovaries(Figure 2.1) . At their openings there are very fine finger-like fronds called fimbria. The inner surface of the tubes have fine hair-like structures called cilia which help to move eggs, released from the ovaries, along the tubes and into the cavity of the uterus.

The ovaries release ova (eggs) and female sex hormones. Primordial follicles consist of a primary oocyte (egg) and pre-granulosa cells which surround the egg and when it matures it is these cells that will produce the sex hormones (oestrogens and progesterone - Q 2.9 and Q 2.13) . Primordial follicles first appear in the female fetus 16 weeks after conception and it is believed that all the follicles are formed within six months of birth. A baby girl will have between two and three million primordial follicles but by puberty the number has fallen to about 250,000. These follicles lie dormant until they go into a phase of maturation. During reproductive years, one or two follicles will become 'dominant' each month and are released at 'ovulation'. There may be about 300 episodes of ovulation during a woman's reproductive years. If we were to assume ova were scheduled to start their active phase of development on a regular basis, this would e Quate to 20 each day or 600 each month. During a process called atresia, eggs are lost at some stage without maturing to ovulation.

There remain many unanswered questions about reproductive physiology. Perhaps the most intriguing of these relates to the timing of individual egg maturation. Although the exact mechanism remains to be established, from a functional point of view, each primordial follicle behaves as if it has its own egg-timer so that it will start its maturation process at a pre-determined time(Figure 2.4); ;>. Some ova have relatively small egg-timers resulting in the egg maturing whilst the woman is in her teens and others may have larger egg-timers resulting in egg maturation perhaps in her thirties or forties.

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