Human Reproduction

Vagina

The vagina serves as the excretory passage for menstrual fluid, receives the penis during sexual intercourse, and forms the inferior part of the birth canal —the cavity of the uterus and vagina through which the fetus passes (see Fig. 2.1 A and B ).

Uterus

The uterus (womb) is a thick-walled, pear-shaped organ ( Fig. 2.2 A and B ) that consists of two main parts:

• The body , the expanded superior two thirds

• The cervix , the cylindrical inferior third

The fundus of the uterus is the rounded part of the uterine body that lies superior to the orifices of the uterine tubes. The body of the uterus narrows from the fundus to the isthmus , the constricted region between the body and the cervix (see Fig. 2.2 A ). The lumen of the cervix, the cervical canal , has a constricted opening, the os (ostium), at each end. The internal os communicates with the cavity of the body of the uterus, whereas the external os communicates with the vagina. Walls of the uterine body consist of three layers:

• Perimetrium , a thin external peritoneal layer

• Myometrium , a thick smooth muscle layer

• Endometrium , a thin internal layer

At the peak of its development, the endometrium is 4 to 5 mm thick. During the luteal (secretory) phase of the menstrual cycle (see Fig. 2.8 ), three layers of the endometrium can be distinguished microscopically (see Fig. 2.2 C ) into the following:

• A compact layer, consisting of densely packed connective tissue around the neck of the uterine glands

• A spongy layer, composed of edematous connective tissue containing the dilated, tortuous bodies of the uterine glands

• A basal layer, containing the blind ends of the uterine glands

The compact and spongy layers—the functional layer —disintegrate and are shed at menstruation and after parturition (childbirth). The basal layer has its own blood supply and is not cast off during menstruation.

Uterine Tubes

The uterine tubes , measuring 10 cm long and 1 cm in diameter, extend laterally from the horns of the uterus (see Fig. 2.2 A ). Each tube opens into a horn at its proximal end and into the peritoneal cavity at its distal end. The uterine tube is divided into the following parts: infundibulum, ampulla, isthmus, and uterine part. The tubes carry oocytes from the ovaries and sperms to the fertilization site in the ampulla (see Fig. 2.2 B ). The uterine tube is lined with cilia and, together with the muscular contractions of the tube, conveys the dividing zygote to the uterine cavity.

Ovaries

The ovaries are almond-shaped reproductive glands that are located close to the lateral wall of the pelvis on each side of the uterus. The ovaries produce oocytes (see Fig. 2.5 ). When released from the ovary at ovulation, the secondary oocyte passes into one of the uterine tubes. These tubes open into the uterus, which protects and nourishes the embryo and fetus until birth. The ovaries also produce estrogen and progesterone, the hormones responsible for the development of secondary sex characteristics and regulation of pregnancy.

Female External Sex Organs

The female external sex organs are known collectively as the vulva ( Fig. 2.3 ). The labia majora , fatty external folds of skin, conceal the vaginal orifice , the opening of the vagina. Inside these labia are two smaller folds of mucous membrane, the labia minora . The clitoris , a small erectile organ, is situated at the superior junction of these folds. The vagina and urethra open into a cavity, the vestibule (cleft between the labia minora). The vaginal orifice varies with the condition of the hymen , a fold of mucous membrane that surrounds the orifice (see Fig. 2.3 ).