A section through the ovary reveals two distinct regions:  The medulla or medullary region is located in the central portion of the ovary and contains loose connective tissue, a mass of relatively large contorted blood vessels, lymphatic vessels, and nerves (Fig. 23.2).
 The cortex or cortical region is found in the peripheral portion of the ovary surrounding the medulla. The cortex contains the ovarian follicles embedded in a richly cellular connective tissue (Plate 92, page 872). Scattered smooth muscle fibers are present in the stroma around the follicles. The boundary between the medulla and cortex is indistinct.
“Germinal epithelium” instead of mesothelium covers the ovary.
The surface of the ovary is covered by a single layer of cuboidal and, in some parts, almost squamous cells. This cellular layer, known as the germinal epithelium, is continuous with the mesothelium that covers the mesovarium. The term germinal epithelium is a carryover from the past when it was incorrectly thought to be the site of germ cell formation during embryonic development. It is now known that the primordial germ cells (both male and female) are of extragonadal origin and that they migrate from the embryonic yolk sac into the cortex of the embryonic gonad, where they differentiate and induce differentiation of the ovary. A dense connective tissue layer, the tunica albuginea, lies between the germinal epithelium and the underlying cortex (Plate 92, page 872).  Tumors that arise from the epithelial surface of the ovary  account for more than 70% of ovarian cancers. The origin of surface epithelial tumors may be related to repeated disruption and repair of the germinal epithelium that occurs during ovulations.
Ovarian follicles provide the microenvironment for the developing oocyte.
Ovarian follicles of various sizes, each containing a single oocyte, are distributed in the stroma of the cortex. The size of a follicle indicates the developmental state of the oocyte. Early stages of oogenesis occur during fetal life when mitotic divisions massively increase the number of oogonia (see the section on oogenesis). The oocytes present at birth remain arrested in development at the first meiotic division (see page 92). During puberty, small groups of follicles undergo cyclic growth and maturation. The first ovulation generally does not take place for a year or more after menarche. A cyclic pattern of follicular maturation and ovulation is then established that continues in parallel with the menstrual cycle. Normally, only one oocyte reaches full maturity and is released from the ovary during each menstrual cycle. Obviously, the maturation and release of more than one egg at ovulation may lead to multiple zygotes. During the reproductive life span, a woman produces only about 400 mature ova. Most of the estimated 600,000 to 800,000 primary oocytes present at birth do not complete maturation and are gradually lost through atresia, the spontaneous death and subsequent resorption of immature oocytes. This process begins as early as the fifth month of fetal life and is mediated by apoptosis of cells surrounding the oocyte. Atresia reduces the number of primary oocytes in a logarithmic fashion throughout life from as many as 5 million in the fetus to less than 20% of that number at birth. The oocytes that remain at menopause degenerate within a few years.
Histologically, three basic types of ovarian follicles can be identified on the basis of developmental state:  primordial follicles; growing follicles which are further subcategorized as pri mary and secondary (or antral) follicles; and mature follicle or Graafan follicles.
Some histologists and clinicians identify additional stages in the continuum of follicular development. In the cycling ovary, follicles are found at all stages of development, but primordial follicles predominate.
The primordial follicle is the earliest stage of follicular development.