Female reproductive system

Introduction

The female reproductive system has six major functions:

• Production of female gametes, the ova, by the process of oogenesis

• Reception of male gametes, the spermatozoa

• Provision of a suitable environment for the fertilisation of ova by spermatozoa

• Provision of an environment for the development of the fetus

• Expulsion of the developed fetus to the external environment

These functions are all integrated by an elegant system of hormonal and nervous mechanisms. The female reproductive system may be divided into three structural units on the basis of function:

• The ovaries, which are the site of oogenesis, are paired organs lying on either side of the uterus adjacent to the lateral wall of the pelvis. In sexually mature mammals, ova are released by the process of ovulation in a cyclical manner, either seasonally or at regular intervals throughout the year. This cycle is suspended during pregnancy. The ovaries are also endocrine organs, producing the hormones oestrogen and progesterone. Both ovulation and ovarian hormone production are controlled by the cyclical release from the anterior pituitary of the gonadotrophic hormones luteinising hormone (LH) and follicle stimulating hormone (FSH). Oestrogen and progesterone in turn regulate LH and FSH production by feedback mechanisms. Thus, ovulation is coordinated with preparation of the uterus to receive the fertilised ovum.

• The genital tract extends from near the ovaries to an opening at the external surface and provides an environment for reception of male gametes, fertilisation of ova, development of the fetus and expulsion of the fetus at birth. The genital tract begins with a pair of Fallopian tubes, also called oviducts or uterine tubes, which conduct ova from the ovaries to the uterus where fetal development occurs. Fertilisation of ova by spermatozoa occurs within the Fallopian tubes. The uterus is a muscular organ, the mucosal lining of which undergoes cyclical proliferation under the influence of ovarian hormones. This provides a suitable environment for implantation of the fertilised ovum and subsequent development of the placenta. This is the means by which the developing fetus is nourished throughout gestation. At birth (parturition), strong contractions of the muscular uterine wall expel the fetus through the lower part of the uterus, the uterine cervix, into the birth canal or vagina. The vagina is an expansile muscular tube specialised for the reception of the penis during coitus and for the passage of the fetus to the external environment. At the external opening of the vagina there are thick folds of skin, the labia which, along with the clitoris, constitute the vulva.

• The breasts are highly modified apocrine sweat glands which, in the female, develop at puberty and regress at menopause. During pregnancy, the secretory components expand greatly in size and number in preparation for milk production (lactation).

In the non-pregnant state, the female reproductive system undergoes continuous cyclical changes from puberty to menopause. When ovulation is not followed by the implantation of a fertilised ovum, the thickened mucosal lining, the endometrium, degenerates and a new ovulation cycle commences. In humans, the thickened endometrium is shed in a period of bleeding known as menstruation. The first day of bleeding marks the beginning of a new cycle of endometrial proliferation which is known as the menstrual cycle. In humans, the standard menstrual cycle is of 28 days duration, but there is considerable variation among normal individuals. Ovulation usually occurs at the midpoint of the cycle.

In other mammals, the proliferated uterine mucosa is absorbed rather than shed, and the female is receptive to the male only during the period of ovulation, which is known as oestrus (or heat). The remaining part of the cycle is called the dioestrus, and the whole cycle is known as the oestrus cycle.

The general anatomy of the female genital tract is illustrated in Figs 19.1 and 19.2.

Embryology of the female genital tract

Most of the structures of the female genital tract are derived from the paired Müllerian or paramesonephric ducts. These give rise to the right and left Fallopian tubes and fuse centrally to form the structures of the uterus, cervix and vagina.

Occasionally, congenital anomalies can occur because of failure of this process of embryological fusion. This can result in a persistent short septum dividing the fundus of the uterus or, in more extreme cases, reduplication of the uterus, cervix and vagina may occur.

The structures of the male genital tract are derived from the Wolffian or mesonephric duct system; in the female, these structures normally regress during fetal life. Small embryological remnants of the Wolffian ducts may persist into adulthood and can be noted in sites such as the paratubal tissue (or mesosalpinx) and in the lateral walls of the cervix. An awareness of these embryological remnants is important for diagnostic pathologists, as the unexpected finding of small epithelial-lined tubules in unusual sites may lead to misdiagnosis.

FIG. 19.1 Female reproductive system, sagittal view

FIG. 19.2 Female reproductive system, coronal view

The ovaries

Whilst most of the structures of the female reproductive system arise from the Müllerian duct system as described above (see textbox), the ovaries arise from the genital ridge, a thickening in the mesothelium high on the posterior wall of the peritoneal cavity. The ovaries later descend to their adult position in the pelvis. As a result, the blood supply and lymphatic drainage of the ovaries is derived from the upper abdomen, with the paired ovarian arteries arising from the abdominal aorta, just proximal to the origin of the renal arteries. This rather unexpected fact is important in understanding why ovarian (and, indeed, testicular) tumours typically spread to para-aortic lymph nodes, rather than to local lymph nodes in the pelvic or inguinal area.

FIG. 19.3 Ovary (illustrations (a) and (b) opposite)
(a) Monkey, Azan (LP) (b) Human, H&E (LP)
The ovaries of all mammals have a similar basic structure. There are, however, considerable variations in accordance with species differences in the ovarian cycle and the stage in the cycle at which the ovary is examined. These micrographs compare the ovarian appearance of the monkey with that of the human.
The ovaries, which are some 3 to 5 cm long in humans, have a flattened ovoid shape. The body of the ovary consists of spindle-shaped cells, fine collagen fibres and ground substance that together constitute the ovarian stroma. The stromal cells resemble fibroblasts, but some contain lipid droplets. Bundles of smooth muscle cells are also scattered throughout the stroma. In the peripheral zone of the stroma, known as the cortex, there are numerous follicles that contain female gametes in various stages of development. In addition, there may also be post-ovulatory follicles of various kinds, namely corpora lutea (responsible for oestrogen and progesterone production, see Fig. 19.8), degenerate and former corpora lutea (corpora albicantes, see Fig. 19.11) and degenerate (atretic) follicles (see Fig. 19.10).
The superficial cortex is more fibrous than the deep cortex and is often called the tunica albuginea. However, unlike the testis, this is not an anatomically distinct capsule. On the surface of the ovary is an epithelial covering, misleadingly called germinal epithelium, which is a continuation of the peritoneum.
In the monkey ovary, numerous follicles F are seen in various sizes and states of development. In contrast, developing follicles are difficult to see in the human ovary (b) at this magnification. An active corpus luteum CL and several degenerating corpora lutea D and corpora albicantes A dominate this human ovary.
The central zone of the ovarian stroma, the medulla M, is highly vascular and contains hilus cells, which are morphologically very similar to Leydig cells of the testis. The ovarian artery (a branch of the aorta) and ovarian branches of the uterine artery form anastomoses in the mesovarium and the broad ligament L. From this arterial plexus, approximately 10 coiled arteries, the helicine arteries H enter the hilum of the ovary, best seen in micrograph (a). Smaller branches form a plexus at the corticomedullary junction, giving rise to straight cortical arterioles that radiate into the cortex. Here, they branch and anastomose to form vascular arcades, giving rise to a rich network of capillaries around the follicles. Venous drainage follows the course of the arterial system, the medullary veins being large and tortuous. Lymphatics arise in the perifollicular stroma, draining to larger vessels which coil around the medullary veins. Innervation of the ovary is by sympathetic fibres that not only supply blood vessels but also terminate on smooth muscle cells in the stroma around the follicles, possibly playing some part in follicular maturation and ovulation. In micrograph (b), the nearby Fallopian tube FT is included in the plane of section.

A corpus albicans CL corpus luteum D degenerating corpus luteum F follicle FT Fallopian tube H helicine artery L broad ligament M medulla

During early fetal development, primordial germ cells called oogonia migrate into the ovarian cortex where they multiply by mitosis. By the fourth and fifth months of human fetal development, some oogonia enlarge and assume the potential for development into mature gametes. At this stage, they are called primary oocytes and commence the first stage of meiotic division (see Ch. 2). By the seventh month of fetal development, a single layer of flattened follicular cells surrounds the primary oocytes to form primordial follicles, of which there are approximately 500 000 in the human ovary at birth. This encapsulation arrests the first meiotic division and no further development of primordial follicles then occurs until after the female reaches sexual maturity (puberty). The process of meiotic division is only completed during follicular maturation, leading up to ovulation and fertilisation. Thus, all the female germ cells are present at birth, but the process of meiotic division is only completed some 15 to 50 years later! In contrast, in males, meiotic division of germ cells commences only after sexual maturity and formation and maturation of spermatozoa are accomplished within about 70 days (see Ch. 18). Female germ cells may undergo degeneration (atresia) at any stage of follicular maturation.