104. Development of The Female Genital System: Ovarian Differentiation

  1. Ovarian differentiation: see Section 98 for description of indifferent gonads
    1. IN EMBRYOS THAT LACK Y-CHROMOSOMES, development occurs slowly in the areas of the gonadal ridges, and the female gonad, which is initially undifferentiated, develops into an identifiable ovary at about week 10 of embryonic life, when its characteristic cortex becomes evident. The testes, on the other hand, are recognizable by week 7. The development and differentiation of the ovary are determined by the genetic constitution of its XX sex chromosomes and are characterized by the following
      1. The primary sex cords are not prominent in the embryonic female gonads, but extend into the medulla to form a rudimentary rete ovarii. Both the rete ovarii and the primary sex cords normally degenerate and disappear
        1. The first proliferation of sex cords of the indifferent period are reoriented toward the center of the gland and form the medullary cords. The latter, as well as the rete ovarii and its mesonephric connections, regress to eventually constitute Rosenmuller's body or the so-called epo?phoron
          1. During fetal life, there are proliferation of the coelomic germinal epithelium into the underlying mesenchyme and the appearance of a second outgrowth of cords, the cortical or secondary sex cords (Pfl?ger's tubes). These eventually occupy the cortex of the gland
          2. As the cortical cords increase in size, primordial germ cells are incorporated into them
          3. At about week 16 (early month 5), the cortical cords, which were originally trabecular, begin to break up into isolated cell clusters called primordial follicles. These follicles consist of large cells with clear cytoplasm, the oogonia, which are derived from primordial germ cells that migrated to this location from the yolk sac wall, and supporting nutrient cells that form a single layer of flattened follicular cells derived from the secondary cortical or sex cords
          4. The primordial follicles are distributed in a connective tissue stroma and demonstrate the cellular duality of the undifferentiated gonadal primordium

            Their number is limited, and about 300,000 to 2,000,000 are present at birth

            They contain a specific reproductive cell, the oocyte, at the primary stage, with 46 chromosomes

            Of the initial stock of primordial follicles, approximately 300 develop between puberty and menopause to produce fertilizable ova

            Active mitosis of oogonia occurs during fetal life, producing the thousands of primitive germ cells. No oogonia form postnatally in full-term humans. Although many oogonia degenerate before birth, those that do remain enlarge to become primary oocytes.

            When the primary oocyte is surrounded by 1 or 2 layers of cuboidal or low columnar follicular cells, it is called a primary follicle. Most follicles remain quiescent until puberty

            The mesenchyme around the follicles becomes the ovarian stroma

            After birth, the germinal epithelium flattens out to a single cuboidal layer of cells that is continuous with the mesothelium of the peritoneum at the ovarian hilum. The germinal epithelium is separated from the follicles in the cortex by a thin fibrous capsule, the tunica albuginea

            As the ovary separates from the regressing mesonephros, it is suspended by its own mesentery, the mesovarium

development of the female genital system: ovarian differentiation: image #1