FOLLICULOGENESIS AND MENSTRUAL CYCLE

Development of the female reproductive tract

The reproductive tract develops through the differentiation of wolffian ducts (male reproductive tract anlage) and müllerian ducts (female reproductive tract anlage). The female reproductive system consists of the ovaries , the ducts (oviduct, uterus and vagina) and the external genitalia (labia majora, labia minora and clitoris) .

Knowledge of the developmental sequence from the indifferent stage to the fully developed stage is helpful in understanding the structural anomalies that can be clinically observed. The molecular aspects of the development of the ovary, female genital ducts and external genitalia are summarized in the next sections.

Development of the ovary ( Primer 22-A )

The differentiation of a testis or an ovary from the indifferent gonad is a complex developmental process involving various genes and hormones.

Wnt4 is a major player in the ovarian-determination pathway and sexual differentiation. Wnt4 is a member of the Wingless (Wnt) family of proteins (see Chapter 3 , Cell Signaling | Cell Biology | Pathology).

The testis-determining factor (TDF) , encoded by the sex-determining region of the Y chromosome gene (SRY) , together with the sex-determining region Y-box 9 gene (Sox9) , are responsible for the development of the indifferent gonads into testes. You have already learned that Sox9 participates in the development of the skeleton (see Chondrogenesis in Chapter 4 , Connective Tissue).

As discussed in Chapter 21 , Sperm Transport and Maturation, the cortical region of the primitive gonad develops into an ovary. The cortical region of the indifferent gonad initially contains the primary sex cords (fifth week of development).

One week later, cells of the primary cell cords degenerate and are replaced by secondary sex cords that surround individual oogonia .

Oogonia result from the mitotic division of migrating primordial germinal cells derived from the yolk sac. Primordial germinal cells contain two X chromosomes. A genetic defect recognized in prepubertal and pubertal girls with Turner's syndrome is the absence of all or part of a second X chromosome (45, X) and a lack of Barr bodies (see Box 22-A ).

In the fetal ovary, oogonia enter meiotic prophase I to become primary oocytes . Primary oocytes are arrested after completion of crossing over (exchange of genetic information between non-sister chromatids of homologous chromosomes). Meiotic prophase arrest continues until puberty , when one or more ovarian follicles are recruited to continue folliculogenesis.

Development of the female genital ducts

During development, the cranial ends of the müllerian ducts (para-mesonephric ducts) remain separated to form the oviducts . The oviducts open into the coelomic cavity (the future peritoneal cavity). The caudal segments of the müllerian ducts (mesonephric ducts) fuse to develop into the uterovaginal primordium , which becomes the uterus and upper part of the vagina (see Box 22-B ). The broad ligaments of the uterus, derived from two peritoneal folds, approach each other when the müllerian ducts fuse.

The primitive cloaca is divided by the urorectal septum into two regions:

• 1.

  The ventral urogenital sinus .

• 2.

  The dorsal anorectal canal .

The urorectal septum fuses with the cloacal membrane (the future site of the perineal body), which is divided into the dorsal anal membrane and the larger ventral urogenital membrane . By week 7, the membranes rupture.

The contact of the uterovaginal primordium with the urogenital sinus results in the formation of the vaginal plate . The canalization of the vaginal plate results in the development of the middle and lower portions of the vagina:

• 1.

  The solid mass of cells of the vaginal plate extends from the urogenital sinus into the uterovaginal primordium.

• 2.

  The central cells of the vaginal plate disappear, forming the lumen of the vagina.

• 3.

  The peripheral cells persist and form the vaginal epithelium.

The urogenital sinus also gives rise to the urinary bladder, urethra, vestibular glands and hymen.

Development of the external genitalia

By week 4, the genital tubercle , or phallus , develops at the cranial end of the cloacal membrane . Then, labioscrotal swellings and urogenital folds develop at either side of the cloacal membrane.

The genital tubercle enlarges in both the female and male. In the absence of androgens, the external genitalia are feminized: the phallus develops into the clitoris . The urogenital folds form the labia minora and the labioscrotal swellings develop into the labia majora .

The ovaries ( 22-1 )

Each ovary is lined by a simple squamous-to-low cuboidal epithelium (called ovarian surface epithelium , see Box 22-C ) and a subjacent connective tissue layer, the tunica albuginea .

A cortex and a medulla without distinct demarcation can be visualized in a cross section. The broad cortex contains connective tissue and primordial follicles housing primary oocytes (at the end of meiotic prophase I). The medulla consists of connective tissue, interstitial cells, nerves, lymphatics and blood vessels reaching the ovary through the hilum .

The functions of the ovary are:

• 1.

  The production of the female gamete.

• 2.

  The secretion of estrogens and progesterone.

• 3.

  The regulation of postnatal growth of reproductive organs.

• 4.

  The development of secondary sexual characteristics.

The ovarian cycle ( 22-2 and 22-3 )

The three phases of the ovarian cycle are:

• 1.

  The follicular phase (folliculogenesis) .

• 2.

  The ovulatory phase (ovulation) .

• 3.

  The luteal phase (luteinization) .

The follicular phase consists of the sequential development of several primordial follicles into a:

• 1.

  Primary (unilayered) follicle .

• 2.

  Secondary (multilayered) follicle .

• 3.

  Pre-antral follicle .

• 4.

  Antral follicle .

• 5.

  Pre-ovulatory follicle (graafian follicle) .

The following structural changes occur during the development of the ovarian follicles:

Primordial follicles . Each primordial follicle, about 25 μm in diameter, is surrounded by a simple squamous layer of granulosa cells secreting anti-müllerian hormone (AMH) (see Box 22-B ). Several primordial follicles are recruited each cycle to initiate the process of folliculogenesis.

Primary (unilayered) follicles . Primordial follicles become primary follicles when the single layer of squamous granulosa cells changes into a simple cuboidal layer . A basal lamina is seen between the granulosa cell layer and the stroma of the ovary. At the same time, the zona pellucida initiates its assembly, separating gradually the primary oocyte from the granulosa cells.

Secondary (multilayered) follicles . Granulosa cells proliferate into a stratified cuboidal epithelium . A cellular shell or theca (theca folliculi; Greek theke , box) surrounds the follicle.

The theca begins to organize into two distinct layers:

• 1.

  The theca interna , a vascularized cell layer adjacent to the basal lamina, supporting the granulosa cells of the follicle.

• 2.

  The theca externa , a fibrous cellular layer continuous with the ovarian stroma.

Pre-antral follicles . Small intercellular spaces, named Call-Exner bodies , develop between the granulosa cells. These spaces contain follicular fluid (liquor folliculi). Protein-rich follicular fluid derives from the blood vessels of the theca interna, reaching the antrum by osmotic gradient (see 22-2 ).

AMH is secreted by granulosa cells during early folliculogenesis. AMH production is highest in pre-antral and early antral stages of folliculogenesis. AMH blood levels are regarded as a useful clinical marker of the ovarian reserve of primordial follicles.

Antral follicle . The Call-Exner bodies coalesce into a single space called antrum . At this stage, granulosa cells, stimulated by FSH, actively synthesize and secrete estrogen (see 22-3 ).

Pre-ovulatory follicle (graafian follicle) . The antrum reaches its maximum size. The fluid of the antrum segregates the granulosa cells into three specific regions:

• 1.

  The cumulus oophorous , a cluster of granulosa cells anchoring the primary oocyte to the wall of the follicle. The cumulus prevents the primary oocyte from floating freely in the antrum fluid. It is also the nutrient delivery channel to the primary oocyte.

• 2.

  The mural granulosa cells , lining the wall of the follicle.

• 3.

  The corona radiata , the layer of granulosa cells firmly anchored to the zona pellucida by zonapenetrating cellular processes).

A pre-ovulatory or graafian follicle reaches about 20 mm in diameter, as compared to the 25 μ m in diameter of a primordial follicle.

The theca externa forms a connective tissue capsule-like layer, continuous with the ovarian stroma. In contrast, the theca interna is a well-vascularized cell layer adjacent to the basal lamina of the follicle. It consists of elongated cells with small lipid droplets in the cytoplasm acquiring the characteristics of steroid-secreting cells (see 22-3 ).

In summary , folliculogenesis occurs around a centrally located primary oocyte arrested at the end of meiotic prophase I. It involves a progressive increase in the population of estrogen-producing granulosa cells, the assembly of a thick glycoprotein-containing zona pellucida coat and the development of the vascularized steroid-producing theca interna.

A basal lamina separates theca cells from granulosa cells. The zona pellucida separates the primary oocyte from the granulosa cells.

Upon the formation of the antrum, granulosa cells become segregated into two cell populations:

• 1.

  The clustered granulosa cells, surrounding the zona pellucida–encased primary oocyte. The clustered granulosa cells secrete a hyaluronic acid–rich product that enables the capture of the ovulated ovum in the fallopian tube.

• 2.

  The mural granulosa cells, lining the outer perimeter of the follicle. The mural cells are in close proximity to cells of the theca interna. As discussed later, this spatial relationship is largely responsible for the production of steroid hormones.