The female reproductive system produces egg cells, which are the female gametes. In mammals, when fertilization happens, the embryo is implanted in the female reproductive ducts, where further development occurs until birth. Several components of the female reproductive system can be distinguished: ovaries (the female gonads), reproductive ducts, and external genitalia (Figures 1 and 2).
1. Ovary
Ovaries perform two main functions: production of gametes (egg cells) and secretion of hormones like estrogens and progestagens. Ovaries are two organs located in the pelvic cavity, on both sides of the uterus.
Each ovary is divided into the ovarian surface epithelium, tunica albuginea, cortical region, and medullary region. The surface epithelium is the outermost layer of the ovary, made up of cuboidal or squamous cells. The tunica albuginea is a layer of dense connective tissue found immediately below the surface epithelium. The ovarian cortex comes next. It is a more or less dense stroma containing ovarian follicles in various developmental stages. Each ovarian follicle is composed of somatic cells surrounding an oocyte. The oocytes produced during embryonic development and arrested in meiotic prophase I are stored in the cortex. During each menstrual cycle, one or more of these arrested follicles resumes meiosis. Surrounded by the cortex, the ovarian medulla is found in the inner part of the ovary, although it is difficult to find a distinct limit between the medulla and cortex. The ovarian medulla contains abundant blood vessels and nerves, which enter and exit the ovary through the hilum. There are no ovarian follicles in the medulla.
Oocytes develop from oogonia, the female germ cells, during the embryonic period. In humans, at the sixth month of fetal development, proliferation halts and oogonia become primary oocytes by meiosis. The meiotic process is arrested in prophase I. Arrested oocytes are distributed through the cortex of the ovary. They remain arrested in this location until maturation starts in the first menstrual cycle. Meiosis is resumed only for those oocytes to be released in each ovulation.
The somatic cells of the ovarian follicle generate the environment for proper oocyte development. The size of an ovarian follicle is indicative of the stage of the development of the oocyte. The maturation process of the follicles is divided morphologically into three stages: primordial, growing (primary, secondary, and tertiary follicles), and mature (Graafian follicles).
Primordial follicles are found right below the tunica albuginea. They consist of one cell-thick layer of somatic cells wrapping one oocyte, which is arrested in prophase I.
Primary follicles show an increase in the size of the oocyte, and the somatic cells wrapping the oocyte change from flattened to cuboidal shapes. Furthermore, the zona pellucida starts to be visible between the oocyte and the somatic cells. As maturation progresses, somatic cells of the follicle proliferate and organize into several layers, known as the granulosa layer surrounding the zona pellucida. Stroma cells surrounding the follicle reorganize to form a multi-layered sheath made up of flattened somatic cells known as theca. A basal membrane separates the theca from the granulosa. The oocyte keeps maturing throughout the process. Secondary follicles, also known as antral follicles, develop a cell-free space among the granulosa cells. This space is the antrum. Now, the oocyte stops growing.
The mature follicle, or Graafian follicle, is so big that it pushes the ovarian tunica albuginea outward. The antrum takes almost all the inner follicular space, and the oocyte, with its nearest somatic cells, known as the corona radiata, is disconnected from the granulosa. Theca cells secrete androgens, while granulosa cells produce estrogens. The oocyte resumes meiosis and completes the first meiotic division to become a secondary oocyte. It is in this stage that the oocyte is released from the follicle, a process known as ovulation.
Following the release of the egg, the somatic cells of the follicle become the corpus luteum. They produce progesterone and estrogens that stimulate the cells that form the uterus walls so that embryo implantation can take place. If there is no fertilization, the corpus luteum degenerates several days after ovulation.
2. Oviducts
Egg cells released during ovulation are collected in the fallopian tube, also known as the uterine tube, which leads them to the uterus.
The fallopian tube is divided into several parts. The infundibulum, or pavilion, fetches the egg cell immediately after ovulation. The longest part of the Fallopian tube is the ampullary region. Fertilization occurs in this segment. The isthmus is near the uterus and the narrower part of the fallopian tube. The interstitial, or intramural, part is the uterine connecting region.
The wall of the fallopian tubes consists of a serous layer, a smooth muscle layer, and a mucosal layer with longitudinal folds made up of connective tissue and a simple columnar epithelium.
The uterus is where most of the embryonic development takes place. It is found between the fallopian tubes and the vagina. The uterus is divided into a superior part, or body, and an inferior part, or cervix. The wall of the uterus is thick and divided into three layers: the endometrium, or uterine mucosa, is a layer composed of simple columnar epithelium and connective tissue; the myometrium, or muscle layer, is continuous with the muscle of the fallopian tubes; and the perimetrium, which is the peritoneal layer. If fertilization takes place, the embryo invades the uterine wall to form the placenta. The placenta is composed of two components: the chorion, derived from the embryo, and the decidua, derived from the endometrium layer.
The vagina is the receptacle for the male sexual organ and communicates the uterus with the vestibule of the vulva. The vaginal wall is formed of a thin mucosa with a stratified, usually keratinized, squamous epithelium. Wrapping the muscle layer, there is an adventitia layer of connective tissue.
3. Outer genitalia
The vulva is the outer female reproductive organ. It includes the mons pubis, labia majora, labia minora, clitoris, and the vestibule 0f the vulva. The mons pubis is a rounded elevation produced by subcutaneous adipose tissue. Labia are skin folds with sweat and sebaceous glands. The clitoris is an erectile organ homologous to the penis, with cavernous bodies. The vulvar vestibule is covered by stratified squamous epithelium. The external genitalia contain many sensory receptors.
Reproductive 