4 Jacques Gilloteaux and James Coey A pair of ovaries are attached to the lateral, right and left pelvic cavities, by the suspensory ligaments covered by the peritoneum. The ovaries are ellipsoid‐shaped, 4 cm long, 2 cm wide, and about 1 cm thick, with some variations according to age, time of the reproductive cycle, and whether pregnancy is present. An old ovary is often covered with scars left by previous ovulations and pregnancies. The ovary is subdivided into a central medulla and a peripheral cortex delimited by a capsule of a dense irregular connective tissue layer or tunica albuginea (Figure 4.1). Their functions are: (1) as endocrine organs making and secreting the female steroids controlling the reproductive cycle, all reproductive tissues, especially those adapted to an eventual pregnancy, the mammary glands, and hence, maintenance of the female phenotype; and (2) producing female gametes. Figure 4.1 Human ovary. Reproduced with permission of Vecton/shutterstock.com. Gonadogenesis is covered in more detail in Chapter 1; this section will give a brief overview. The ovaries, like the testes (gonads), derive from three embryonic tissues and cells: At first, each gonad develops during the fifth week when a mesothelial thickening grows in the medial side of the mesonephros. Covered by an actively proliferating epithelium both tissues make an elongated bulge: the gonadal ridge. This structure is also named an indifferent gonad made of an external cortex (epithelium) and an internal medulla (mesenchyme). The embryos with a pair of XX sex chromosomes grow the cortex to differentiate into an ovary while the medulla regresses. Out of the wall of the umbilical vesicle, primordial germ cells migrate along the dorsal mesentery of the gut to the gonadal ridges. During the sixth week, these primordial germ cells penetrate the mesenchyme and parts of the gonadal cords. These cells become the oogonia. Follicular/granulosa cells derive from the surface epithelium of the primitive ovaries; this original population of surface cells also produce the germinative epithelium or mesothelium. Fertilization with a combination of two gametes with X sex chromosomes establish the female sex. However, before the seventh week, the gonads of the two sexes are identical in appearance as indifferent gonads. Both XX chromosomes are required for the development of the female phenotype. There is at least one autosomal gene (Rspo1) that plays a role in ovarian organogenesis. It is only by the tenth week that ovaries are identifiable histologically. When the gonadal cord extensions in the medulla form primitive rete ovarii, this structure undergoes degeneration about 5 weeks later. The cortical cords extending out of the surface epithelium increase in size and the primordial germ cells merge with them. Around the sixteenth week the cortical cords fragment into isolated cell clusters, now called primordial follicles. These structures, covered by flat cells, that derive from the surface or germinal epithelium and the primordial germ cells, constitute the primordial follicles each containing one oogonium. The ovary surface epithelium makes its basal lamina and the basement membrane with the capsule, a dense irregular connective tissue or tunica albuginea, are formed by the mesenchymal tissue and maintained by the cortical stroma later. The ovaries are the organs that secrete the female, cholesterol‐derived steroid hormones (oestrogens and progesterone) under the control of the gonadotropin‐releasing hormones (GnRH) (from the hypothalamus) and the gonadotropins (see Chapter 3) during reproductive life. Thus, the ovaries not only provide ova but also support sexual differentiation, fetal development, growth with sexual maturation, and maintenance of the female phenotype. The female germ cells issued from ootids maturing into ova are highly specialized cells that contain half the chromosomes of any other cells of the body, i.e. haploid cells. Each cell has 23 chromosomes (22 autosomes and 1 sex chromosome X). This number is required in order to unite with the male gamete to form a new living being (diploid) which carries either an X or Y sex chromosome, which determines the gender of the fertilized ovum. This then develops into either a male XY or female XX embryo. Distributed in the main cortical ovarian zone, oogonia (plural of oogonium) undergo gametogenesis, which in females is called oogenesis. The ovaries are completely enveloped by the mesothelium. This envelope appears histologically as a simple epithelium of flat cuboidal to low columnar cells called germinal epithelium. These cells are more keratinized after menopause. Like most mesothelial cells their surfaces are decorated by numerous microvilli and rare cilia. Numerous pinocytotic vesicles and mitochondria can be seen. Even though the term germinal epithelium is often taught to be a misnomer, it has a developmental relationship with the covering of the oogonia and several ovarian pathologies. Its basal membrane extends on a stroma of a dense irregular connective tissue layer: the ovary capsule or tunica albuginea. Near its attachment with the aforementioned ligament, each ovary has a hilum or structure containing the mesovarium, the ovarian blood vessels, lymphatics, and nerves. This hilum continues into the medulla, at first showing twisted tracts that decrease in diameter, while branching into fine capillaries and nerves into a stroma of dense to loose connective tissue. This stroma extends into the cortex and reaches the capsule. Amongst the initial parts of these vessels, especially in the hilum, vestiges of Wolffian ducts can appear as flat to cuboidal tubular remnants, along with aggregates of hilar cells. These hilar cells can be numerous and display a morphology similar to the interstitial Leydig cells of the testes, which probably also originated from modified, smooth muscle cells from the walls of arterioles. They have a round to oval shape, lots of endoplasmic reticulum, mitochondria with tubular cristae, and lipid and lipofuscin deposits. Some contain Reinke crystals and hyaline bodies. These hilar cells (also called hilus cells) distribute in the aforementioned medullar stroma constituting, after some local replication, the theca interna cells of the growing follicles, producing androgens (testosterone and metabolites). The cortical region encompasses: Female gametes and cyclically released hormones are produced from the ovarian cortex. Each month, these hormones control and stimulate oogenesis. The term oogenesis encompasses a series of events that makes each stem cell or oogonium an ootid, then an ovum or oocyte (haploid female sex cells). In order to become female gametes, oogonia, have to undergo meiosis. During this dynamic activity, these cells undergo morphological changes, reduce the nucleus DNA content by half (becoming haploid), and mature to permit fertilization. This differentiation process, initiated during the fetal period of development, is not completed until after puberty or for some, oogonia only, 40 years later. During follicular maturation, the following histological types can be recognized:
The Ovaries, Oocytes, and Folliculogenesis
Introduction
Development of Ovaries
Sex Determination
Endocrine Organs
Gametogenesis
The Ovarian Epithelium
The Hilum and Medulla
The Cortex
Oogenesis or Gamete Production
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