Female Reproductive System
This chapter will expand upon what we learned about the ovary in the Endocrine System chapter.
The female reproductive system has several functions: hormone production (ovary), gamete production (ovary), fetal development and menstruation (uterus), and milk production (breast). The major hormones produced by the female reproductive system are progesterone and estrogen. In the following sections, we will discuss the cell types responsible for hormone production. However, this chapter will not discuss the stimuli for and patterns of hormone release, which fluctuate throughout the uterine/menstrual cycle.
Oogenesis
The ovary is the site of oogenesis, which is the process of female gamete (oocyte) production. Production and release of oocytes is driven by hormones produced by the pituitary gland (FSH and LH) and the ovary (progesterone and estrogen). The ovary contains millions of diploid (2n) oocytes arrested in prophase I of meiosis (meiotic arrest). At birth all oocytes are found as part of primordial follicles, in which the oocyte is surrounded by a thin layer of flattened cells called pre-granulosa follicular cells (Figure 1 top panel).
At puberty, hormone production (FSH) causes a small number (~ 10-20) of primordial follicles each month to mature into primary (1°) follicles. Primary follicles are surrounded by one or two layers of cuboidal granulosa cells (green cells in Figures 1, 2, 3). The oocyte is surrounded by a transparent (clear) zone/ring called the zona pellucida (Latin for “transparent zone”, light pink ring in Figures 1, 2, 3) that plays an important role in fertilization and development (Figure 1 bottom panel).
Figure 1: Primordial and primary follicles of the ovary with and without illustration overlay
During follicle development, granulosa cells begin producing estrogen. As the follicle continues to develop, the granulosa cell layer increases in thickness as additional cells are added (Figure 2, green cells). The follicle is now considered a secondary (2°) follicle. The secondary follicle is surrounded by an additional layer of hormone-producing cells called theca cells (Figure 2, yellow cells). In response to LH, the theca cells produce progesterone and androgen precursors that the granulosa cells turn into estrogen. Notice the oocyte in the secondary (2°) follicle has grown in size and the zona pellucida is thicker and more noticeable (Figure 2). The secondary follicle begins to develop filled pockets of a fluid called antrum.
Figure 2: Secondary follicle of the ovary with and without illustration overlay
As the ovarian cycle continues, one dominant follicle is selected to mature. The follicle continues to mature, the layer of theca cells thickens (Figure 3, yellow cells). The follicle is now considered mature follicle. Different textbooks will use different names for the mature follicle, and it is known as a tertiary (3°), antral or Graafian follicle. The tertiary follicle is easy to recognize because the granulosa cells (green cells) now surround a continuous fluid filled space called the antrum.
The oocyte of the tertiary follicle is surrounded by an inner ring of granulosa cells called the corona radiata (Figure 3, green cells). The cells of the corona radiata remain attached to the oocyte after ovulation and are important for fertilization and early embryo development.
The tertiary follicle is the site of lots of activity. The granulosa cells of the tertiary follicle produce lots of estrogen, which stimulates the oocyte to continue developing. In the tertiary follicle, the oocyte continues the process of meiosis. Remember until this point the diploid (2n) oocyte has been arrested in prophase I. In the tertiary follicle, the oocyte finishes meiosis I and begins meiosis II before it is suspended/arrested in metaphase II. Now the haploid (n) oocyte is ready to be ovulated. Ovulation is triggered by an intense surge in LH production by the anterior pituitary. Meiosis II will only complete in the ovulated oocyte (n) if it is fertilized by a sperm (n). When the nuclei of the sperm (n) and ovum (n) fuse, the resulting diploid cell is called a zygote (2n).
Figure 3: Tertiary (Graafian or antral) follicle of the ovary with and without illustration overlay
The Uterus
The uterus is the site of fetal development if the oocyte is fertilized or menstruation if the oocyte is not fertilized. The uterus has two major layers surrounded by a serous membrane. The inner layer, the endometrium contains glands and blood vessels (Figure 4 upper panel). While the deepest parts of the endometrium remain, most of the tissue of the endometrium is shed during each menstrual cycle (menses or menstruation) if no embryo implants into the endometrium. Depending on the slide you are viewing, the thickness of the endometrium may vary. The outer layer, the myometrium, contains a very thick layer of smooth muscle cells, which contract during childbirth (Figure 4 lower panel).
Figure 4: The uterus with and without illustration overlay
Chapter illustrations By:
Juan Manuel Ramiro Diaz, Ph.D.
