Female reproductive system produces oocytes, female gametes. In mammals, if fertilization happens, the embryo is implanted in the female reproductive system ducts, where posterior development occurs until birth. Physiology of the female reproductive system is different depending on if there is fertilization or not. Several components can be distinguished: ovaries or female gonads, reproductive ducts and external genitalia.
Ovaries perform two main functions: production of gametes or oocytes and hormones like estrogens and progestogens (family of hormones where progesterone is included), which are steroid hormones. Ovaries are a pair of organs located in the pelvic cavity, at both sides of the uterus. Although their shape changes during the menstrual cycle, they are usually ovoids and, in humans, they are 3-4 cm in length, 1.5 cm wide, and 1 cm thick.
Each ovary is idivided into ovarian surface epithelium, tunica albuginea, cortical region, and medullar region. The surface epithelium is the outermost layer of the ovary, made up of cuboidal or squamous cells derived from visceral peritoneum (mesothelium). Tunica albuginea is a layer of dense connective tissue found inmediatly under the surface epithelium. The ovarian cortex comes next. It is a more or dense compact stroma containing ovarian follicles in different developmental stages. Each ovarian follicle is composed of somatic cells wrapping an oocyte. The oocytes produced during the embryonary development and arrested in meiotic prophase I are stored here. In every menstrual cycle, one or several of these arrested follicles restart meiosis and the follicle develops. Surrounding by de cortex, ovarian medulla is found in the inner part of the ovary, although is difficult to find a clear limit between medulla and cortex. Ovarian medulla is fibro-elastic tissue with abundant blood vessels and nerves, both entering and leaving the ovary through the hilium. There are no ovarian follicles in the medulla.
In humans, each ovary is attached to other structures by the so-called ligaments. The upper part of the ovary is attached to the peritoneum, the wall of the pelvis, by the suspensory ligament. The posterior portion of the broad ligament of the uterus forms the mesovarium, which also supports the ovary. Through these last two ligaments, nerves and blood vessels enter and leave the ovary. The lower part of the ovary is attached to the uterus by the utero-ovarian or ovarian ligament.
Oocytes are produced from oogonia, the female germinal cells, during the embryonary period. Oogonia are derived from the neural crests before the gonadal ridges are developed in the embryo. There is an initial period of proliferation by mitosis that yields about 5 to 7 millions oogonia. In humans, during the sixth month of fetal development, proliferation stops and oogonia become primary oocytes by meiosis. The meiotic process is arrested in prophase I, before the first meiotic division. Arrested oocytes are distributed in the outer part, or cortex, of the developing ovary. They remain arrested in this location until the maturation starts during the first menstrual cycle. Meiosis is restarted for those oocytes to be released (ovulated). In humans, it is of notice that the second meiotic division starts only if fertilization takes place.
Ovarian follicles are reponsible for the environment for the oocytes development. The size of an ovarian follicle is indicative of the stage of development of the oocyte. The maturation process of the follicle, included de oocyte, is divided morphologicaly in three stages: primordial, growing (primary, secondary and tertiary follicles), and mature or Graaf follicles.
Primordial follicles are found just under the tunica albuginea. They are one cell thick layer of somatic cells wrapping one oocyte, which is arrested in prophase I. Somatic cells area rather flattened to cuboidal, in close apposition to the oocyte. The follicle is separated from the surrounding stroma by a basal lamina.
Those follicles that start growing also begin to mature, and will eventually release the fully developed ovum during ovulation, at the end of this maturing process. Depending on the author and the developing stage, follicles receive different names. Primary follicles show an increase in the oocyte size and the follicle somatic cells covering the oocyte change from flattened to cuboidal shapes. Furthermore, the pellucida membrane starts to be visible between the oocyte and the somatic cells of the follicle. As maturation progresses, follicle somatic cells proliferate and reorganize into several layers, altogether known as granulosa cells. Granulosa cells are enclosed by the follicle basement membrane or basal membrane. Stroma cells surrounding the follicle reorganize and wrap the basal membrane to form a multi-layered structure made up somatic flattened cells known as theca. Theca may become thick, and an internal and and external part can be distinguished. During the process, besides growing in size, the oocyte has matured by reorganizing the cytoplasm, storing cortical granules below the cell membrane, forming microvilli, and other cell changes. Secondary follicles or antral follicles start to develop a space free of cells among the granulosa cells. This space is the antrum, and is formed when granulosa cells proliferation reach around 5 layers of cells. Now, oocyte stops growing. However, follicle somatic cells keep proliferating and follicle grows larger, including the antrum. The oocyte becomes surrounded by a group of granulosa cells that will later, during ovulation, form the corona radiata. These granulosa cells and the oocyte are now isolated from the rest of the granulosa cells by the antrum, excepting by a bridge of granulosa cells known as discus proligerus or cumulus oophorus.
The mature follicle or Graaf follicle is so big that it pushes the ovarian tunica albuginea out. Antrum takes almost all the inner follicular space and the oocyte, with its surrounding somatic cells of corona radiata, is disconnected from granulosa cells just before the ovulation. Theca cells are now well-developed. Both theca and granulosa cells release androgen and estrogen hormones respectively. After stimulation by luteinizing hormone released by the hypophysis, the oocyte restarts meiosis and undergoes the first meiotic division to become a secondary oocyte. It is in this stage when the oocyte is released from the follicle, process known as ovulation.
Then the oocyte has been released, the follicle somatic cells become the corpus luteum. They produce progesterone and estrogens that stimulate the cells that form the uterus walls, so that the implantation may take place. If there is no fertilization, the corpus luteum degenerates several days after the ovulation. If fertilization happens, the corpus luteum grows and continues releasing progesterone and estrogens, mainly during the first weeks of the pregnant period.
Oocytes released during ovulation are collected in the Fallopian tube, also known as uterine tubes, which lead them to the uterus. There are two Fallopian tubes, one for each ovary. In humans, they are 10 to 12 cm in length. Fertilization and part of the early embryonary development take place inside the Fallopian tubes. The embryo arrives at the uterus during the blastocyst stage, when the implantation occurs. Spermatozoids need to swim a long way, including the uterus and a long portion of the Fallopian tubes, before they contact the oocyte.
The Fallopian tube is divided in several parts. The infundibulum or pavilion is close to the ovary, has a funnel-like shape, and fetches the oocyte immediately after the ovulation. The longest part of the Fallopian tube is the ampullary region, about two thirds of the total length. Fertilization occurs in this segment. The isthmus is near the uterus and is the narrower part of the Fallopian tube. The interstitial or intramural part of the Fallopian tube enters the walls of the uterus and connects with the internal cavity of the uterus.
The wall of the Fallopian tubes are surrounded by a serous layer or visceral peritoneum made up of mesothelium and a thin layer of connective tissue. Toward the lumen, there is a layer of smooth muscle divided into a longitudinal external layer and a thicker circular internal layer. In contact with the lumen, there is a mucosal layer with longitudinal folds protruding into the lumen. Mucosa is made up of connective tissue and simple columnar epithelium with two types of cells, ciliated and non-ciliated. The movement of cilia propels the oocyte toward the uterus. Non-ciliated cells are secretory cells releasing nutritious substances for the oocyte. The proportion of these type of cells changes during the menstrual cycle. Estrogens promote more ciliated cells and an increase in cellular height. The oocyte can not move by itself so that it is pushed away by ciliated cells and peristaltic movements of the Fallopian tubes. Spermatozoa travel in the opposite direction, toward the ovary. Besides the propelling power of the flagellum, it is not known what mechanism pushes spermatozoa through Fallopian tubes to meet the oocyte.
The uterus is the compartment of the reproductive ducts where most of the embryonary development takes place. It is found between Fallopian tubes and vagina. In humans, it is around 7.5 cm in length. 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 in three layers: endometrium or uterus mucosa is a layer of simple columnar epithelium plus connective tissue, myometrium or muscle layer, which is continuous with the muscle of the Fallopian tubes, and perimetrium, which is the peritoneal layer. Endometrium and myometrium undergo major changes during the menstrual cycles. If fertilization takes place, the embryo attaches to the endometrium (implantation) and eventually invades the uterus wall to form the placenta. Placenta is constituted of two components: chorion, derived from the embryo, and decidua, derived from the uterus endometrium layer. The mucosa of the cervix is morphologically different because it has exocrine glands releasing mucous substances, more or less dense according to the point of the menstrual cycle, facilitating or inhibiting the entrance of sperm in the uterus lumen.
The vagina is the receptacle for the male sexual organ and communicates the uterus with the vulva vestibule, the part of the vulva located between the two labia minora. The vaginal wall is formed of a thin mucosa with a stratified squamous epithelium, usually keratinized. Below, there is a layer of smooth muscle divided into two sublayers, one circular and one longitudinal. The longitudinal one is thicker and it is continuous with the muscle layer of the uterus. There is striated muscle in the part of the vagina near the exterior. Wrapping the muscle layer, there is an adventitia layer of dense connective tissue and then smooth connective tissue. Vagina does not contain glands and its lubrication is caused by secretion of glands located in the cervix of the uterus. The external lubrication of the vagina entrance is produced by glands located between the labia of the vulva.
Vulva is the external female reproductive organ. It includes the mons pubis, labia major, labia minor, clitoris and vulva vestibule. Mons pubis is a rounded elevation produced by subcutaneous adipose tissue. Labia are cutaneous folds with sweat and sebaceous glands. The external labia (labia major) have hair follicles and smooth muscle. Clitoris is an erectile structure homologous to penis, with cavernous bodies and a clitoral glans (hood). Vulvar vestibulus is covered by stratified squamous epithelium with many small mucous glands and other more complex tubule-alveolar glands that release their content into the vaginal opening. These glands lubricate the external part of the female external organ. The external genitalia contain many touch and pressure sensory receptors like Meissner corpuscles, Pacini corpuscles and free endings axons.