Читать книгу Human Developmental Biology - Danton Inc. O'Day - Страница 17

The ovary doesn't appear to be a very organized structure. Each stage of oogenesis and follicle maturation is haphazardly strewn throughout both the left and right ovaries. However, since it is not the goal here to be reproductive anatomists, we will use a typical textbook example of the ovary which organizes components for us (Figure 3.3).

Figure 3.3. The structure of the ovary and development of ovarian follicles.

The above figure emphasizes the sequential events in follicle development. During embryogenesis the first follicles that form are called primordial follicles: the granulosa is one cell layer thick and the oocyte is arrested (has stopped developing) at prophase I of meiosis. Over time there is an increase in the number of granulosa cells as they become stratified and become referred to as the theca. As well, the fluid filled cavity or antrum around the oocyte continues to fill with follicular fluid. In keeping with these changes, the follicles are progressively named primary, secondary and tertiary (or mature) follicles. In the figure, the follicle cells (green), follicular fluid (cyan) and developing oocyte (magenta) have been colored to clearly demonstrate the changes in the follicle as the egg matures and is finally ovulated. Subsequently, the follicle undergoes changes as it becomes a hormone producing corpus luteum (gold, dark green).

The ovaries also play a role in regulating the status of the uterus. Estrogen, released from the maturing follicles, causes the uterine epithelial lining to proliferate in preparation for the arrival of a fertilized egg. Progesterone released from the corpus luteum will further mature the uterine lining causing it to enter the secretory phase which will be able to interact with the blastocyst should fertilization occur and embryonic development begin. These hormonal changes plus many others controlled by the pituitary lead to the monthly menstrual cycle of females, a topic covered in detail in physiology books.