Slide 1: Ovary, scanning power
FEMALE REPRODUCTIVE SYSTEM HISTOLOGY
The male and female systems are anatomically and developmentally homologous: they have similar functions and develop from similar embryological tissues. Depending on the genetic makeup of the embryo (XY in a male, XX in a female), development proceeds along two different pathways. In the male, the embryonic gonads develop into testes. Cells associated with the testes secrete testosterone and a substance called anti-Mullerian factor, which direct the development of internal and external genitalia along the male model. In the female, the embryonic gonads develop into ovaries, testosterone and anti-Mullerian factor are not secreted, and the internal and external genitalia develop along the female model.
In the male, beginning at puberty, mitotic division of stem cells (spermatogonia) in testicular structures called seminiferous tubules produces cells called spermatocytes, which undergo meiotic cell division and differentiation to produce small haploid motile cells called spermatozoa. Spermatozoa are stored and undergo development in the epididymis, an organ that lies adjacent to the testes. Male gametes or sperm are produced continuously in large numbers. Production of the male androgen, testosterone, is accomplished by interstitial cells, which are located in the spaces between seminiferous tubules.
All of the gametes that a female will possess are present within the ovary at birth, contained in structures called follicles. The follicle, in addition to supporting gamete development, also produces hormones that direct follicle and uterine development. Gametogenesis begins before birth and is suspended until puberty, when cyclical gametogenesis resumes. During each ovarian cycle, several follicles are stimulated and undergo a process of growth, development and maturation. Usually only a single follicle completes development, resulting in the release of a single oocyte per follicle. The remnants of the follicle are transformed into the corpus luteum, which secretes hormones necessary for the maintenance of the uterus. As a result, female gametes are larger and non-motile and are produced discontinuously in much smaller numbers.
The following slides are sections of the cat reproductive system. While human females normally release only a single oocyte per menstrual cycle, female cats release multiple oocytes during each estrous cycle. Despite these differences, the processes governing oogenesis are fundamentally homologous.
Slide 1: Ovary, scanning power
The first slide is a low magnification view of a section through a cat ovary.
This low-power cross section of an ovary shows that the ovary is a solid structure. The blue arrows indicate regions within the ovary where follicles are at different stages of maturation. Follicular structure is unique to female gamete development. The follicle contains an oogonium that will complete meiosis in order to develop into a single viable ovum and several non-functional polar bodies. In the follicle, the oogonium is surrounded by a layer of cells call the granulosa which produces estrogens in response to stimulation from FSH. Females are born with between 1 and 2 million follicles.
Slide 2: Ovary, low power
This slide shows a higher magnification view of a portion of the same ovary. The labeled structures (A, B, and C) are ovarian follicles.
In this higher-power view, you can see the difference between a few different stages of follicular development. Follicles at A represent primordial follicles. The follicle at C is a primary follicle. Notice that a big difference is that the granulosa is much more developed and organized than in the primordial follicle while the egg really has not changed size too much. The follicle labeled B is somewhere between being a secondary follicle and a mature follicle. In this stage, note that the granulosa has become multilayered and there are pools of fluid forming called the antrum. Additionally, a second layer of cells is becoming apparent around the outside of the follicle. This layer, called the theca, forms from connective tissue that surrounds the granulosa.
Slide 3: Mature ovarian follicle
This slide shows a section through a mature follicle.
This slide demonstrates a mature follicle. C shows the now ready to be ovulated oocyte surrounded by a clear layer called the corona radiata. B indicates the total amount of granulosa development and A points to the now well-organized thecal layer. The large granulosa is responsible for high levels of estrogens observed during follicular development. In normal human follicular phase, many primordial follicles may become activated, but generally only one matures and is ovulated.
Slide 4: Corpus luteum
Following ovulation, those portions of the follicle that are not released from the ovary are transformed into the corpus luteum. This slide shows a low power view of one of these structures.
This slide shows the corpus luteum. The corpus luteum includes the remnants of the granulosa and thecal layers after ovulation. By this point in the luteal phase, high levels of LH in the female trigger progestin secretion from the corpus luteum.