The female reproductive system consists of two ovaries, two oviducts, the uterus, the vagina, and external genitalia (figure 1). The Oviducts come in close proximity to the ovaries with fimbriae extensions. The oviducts connect to the uterus, which narrows into the cervix, which then opens into the vagina.
In the Ovaries, gametes, called ova or eggs, are produced by meiosis in the process called oogenesis. Oogenesis involves the division of germ cells, called oogonia, by mitosis. Oogonia then replicate through the first stage of meiosis and are then termed primary oocytes. At puberty, one or a few of these oocytes are released in a process called ovulation, in which the oocytes finish meiosis one, and then complete meiosis two if fertilization occurs. In most mammals, Ovulation occurs throughout their entire life, but in humans, and a few other mammals, ovulation stops and reproduction is no longer possible, a phase called menopause 1 .
Origin of the Ovaries
In the embryo, at the end if the first month, primordial germ cells move from the yolk sac to the gonadal primordial, where they divide into oogonia. By the third month, they are primary oocytes and become surrounded by follicular cells. By the fifth month, the embryonic ovaries may have more than seven million oocytes. Many of these oocytes are later degraded in a process called atresia, and at puberty the ovaries contain approximately 300 000 oocytes. Usually one oocyte is released from the ovaries about once a month, for about 30-40 years. Thus, only about 450 oocytes are released in a lifecycle. Atresia remains an active process throughout the female life cycle, and degrades all other oocytes 2.
Histology and Function
The surface of the ovary is covered by simple squamous, or cubiodal epithelium, called the germinal epithelium. Under the epithelium is a layer of dense connective tissue which gives the ovaries it white color, called the tunica albuginea (Figure 2).
Figure 2. Ovary with main components 2.
Figure 3. Low Magnification of Ovary surrounded by fallopian tube tissue 6.
Ovarian follicles are basically an oocyte surrounded by one or more layers of follicular cells, also called granulosa cells (Figure 4). A basal lamina separates the follicle from the stroma. Primordial follicles are formed in the fetus, and consist of a primary oocyte surrounded by one layer of flat granulose cells in the cortical region. They are in first prophase of meiosis, and the chromosomes are not coiled and do not stain dark.
Figure 4. Cortical region of the ovary 2.
At puberty, follicle stimulating hormone produced by the pituitary starts the process of follicular growth, in which the nucleus of primordial follicles enlarges, mitochondria increase in number, and the granulosa cells divide by mitosis to form a single layer of cuboidal cells. The follicle is then a unilaminar primary follicle (Figure 5).
Figure 5. Types of Ovarian Follicles5.
Granulosa cells become stratified into a granulosa layer, and the follicle is called a multilaminar primary follicle. As the follicle increases in size it moves deeper into the ovary’s cortical region and liquid accumulates between granulosa cells, forming a large cavity in the follicle, called the antrum, which contains proteins and steroids secreted by the follicle. Simultaneously, in the stroma, fibroblasts surrounding the follicle differentiate to form the theca interna, and theca externa(Figure 6).
Figure 6. Higher Magnification of the Ovary, showing the different stages of development of the follicles 6.
One follicle develops more than the others, and is called the mature, preovulatory, or graafian follicle and it protrudes from the ovary. Before ovulation, the first meiotic division finishes. During ovulation, the oocyte of the mature follicle is released into the oviduct when the wall of the follicle bursts under hormonal control.
After Ovulation the Corpus Luteum is formed from the theca interna cells and is a temporary endocrine gland that is embedded in the cortical region. When under the stimulus of Lutenizing hormone it secrets progesterone and estrogens. If pregnancy does not occur, the cells of the corpus luteum degenerates, and menstruation occurs. If pregnancy does occur, human chronic gonadotropin (HCG) is secreted by cells of the embryo and the corpus luteum continues to grow and produce progesterone such that the endometrium is not shed and menstruation does not occur in preparation for pregnancy 2.
Hormones Involved
- Luteinizing hormone (LH) and follicle stimulating hormone (FSH) are not produced by the ovaries, but do greatly influence its development. They are produced by the anterior pituitary, and are secreted in response to gonadotropin releasing hormone (GnRH), which is secreted by the hypothalamus. LH targets the theca cells, and FSH targets the grandulosa cells surrounding the oocyte. Both hormones act to produce estrogen. LH also plays a large role in controlling ovulation. LH peaks at ovulation causing the decrease in estrogen, increase in progesterone, and release of prostaglandins and enzymes that breakdown the follicular and ovarian membranes so the oocyte can be released for ovulation. It also causes the grandulosa and theca cells to form into the corpus luteum.
- Estrogens secreted by the grandulosa cells of the follicle, stimulates the growth of the endometrium, which is the layer of glandular cells inside the uterus. This thickening prepares the female for possible pregnancy such that an egg can become embedded in the endometrium. It also causes the development of progesterone receptors. Progesterone secreted by the corpus luteum also aids in the thickening of the endometrium. Both Progesterone and Estrogen are steroid hormones.
- Inhibin is secreted by the granulose cells just prior to ovulation, which inhibits FSH secretion 1.
Ovarian Cancer- The Silent Killer
Ovarian Cancer has the unfortunate nickname the “silent killer” because in many people, there are no noticeably distinct symptoms until it has become fairly advanced. It claims 125 000 lives per year globally. Ovarian Cancer actually refers to a group of malignancies that affect the ovaries 3.
Classification
Generally, three types of cancer can be described based on the cells in which they develop. Cancer can arise in epithelial cells, sex cord –stromal cells (grandulosa, theca, and hilus cells), and germ cells (oocytes). Most ovarian cancers are epithelial cancers (about 90%).
Tumours are classified into four stages. Stage one malignant tumour is defined as being totally confined to the ovary. Stage two tumours extend into other tissues, such as the fallopian tube and the uterus. Stage three cancers occur when they spreads to the peritoneum or lymph nodes. Stage four tumours have spread to even further tissue.
Tumours can also be classified into subtypes: serous, endometrioid, mucinous, clear cell, squamous cell, mixed epithelial, and undifferentiated (Figure 7). About 50% are serous tumours, which resemble fallopian tube epithelium, and tend to be very aggressive in spreading throughout the pelvis. However, the endometriod and mucinous tumours spread more slowly. Serous tumours are highly responsive to treatment, while clear cell tumours are highly resistant 3.
Figure 7. The major subtypes of ovarian carcinomas 3.
Origin of Malignant Ovarian Tumours
There are many existing theories on the cause of development of malignant ovarian tumours. Traditionally, it was thought that all subtypes originated in the ovarian surface epithelium (OSE). During ovulation the rupture of the follicle causes physical damage in need of repair. Repeated several times throughout a lifetime, the OSE becomes highly plastic.
The OSE is also subjected to ovulation- related inflammatory cytokines and reactive oxygen, capable of damaging DNA. Also, the ovary over time develops invaginations into the cortical region, which can pinch off, become trapped, and form “cortical inclusion cysts” or CIC’s. Hormones may act on the CICs to cause proliferation of epithelial cells inside these CICs. Finally, some theories suggest that the fallopian tube is the site of tumour origin in which the fimbria are thought to be a major site for the development of serous tumours3.
Genetics is now being examined as a cause for the formation of malignant ovarian tumours. Quaye et al. states that there is is a 2-3 fold increase risk off having ovarian cancer when women have a single first-degree relative with the cancer. In twin studies, it was shown that genetics, rather than environment, plays a large role in the development of ovarian cancer 4.
References
4 Quaye L, Tyrer J, Ramus SJ, Song H, Wozniak E, DiCioccio RA, McGuire V, Høgdall E, Høgdall C, Blaakaer J,Goode EL, Schildkraut JM, Easton DF, Krüger-Kjaer S, Whittemore AS, Gayther SA, Pharoah PD(2009).
5
6 Dee,Leaven, Consoer, Heidger (2009). Virtual Slidebox of Histology. University of Iowa. http://www.path.uiowa.edu/virtualslidebox/nlm_histology/content_index_db.html.
