Uterine fibroids are benign tumours or growths that develop in the wall of the uterus. Benign means that they are not cancerous or precancerous. Fibroids are often symptomless but they can cause unpleasant and sometimes painful symptoms.In order to understand fibroids it is helpful to know more about the anatomy of a normal uterus. The majority of the uterus is formed of smooth muscle and this is known as the myometrium. The outer layer of the myometrium is called the serosa. The lining of the uterus is a layer known as the endometrium. It is the endometrium that is shed during a period at the time of menstruation. The endometrium is not made of muscle.
The cavity inside the uterus is usually known as the uterine cavity or sometimes as the endometrial cavity. Diagrams of the uterus usually show the uterine cavity as a triangular space but really it is a potential space because most of the time the uterus is collapsed and there is only a small amount of fluid lying between the endometrial layers of the uterus.
Sometimes the endometrium is referred to as the mucosal surface of the uterus. The uterus is made up of the body or fundus and the cervix. The cervix is the lower part of the uterus that protrudes into the vagina.
During pregnancy the uterus enlarges considerably and nourishes the growing fetus. The muscle cells of the uterus both increase in number and increase in size. This process of hyperplasia and hypertrophy may be important in helping to explain the reasons for growth of uterine fibroids. One theory of reasons for fibroid development hypothesises that fibroids are confused myometrial cells that behave as if the woman is pregnant all the time.
Fibroids are considered to be what is known as clonal tumours. A clone is a collection or group of cells which are genetically identical. Each fibroid is thought to arise from one single smooth muscle cell within the uterus. This process differs from the development of cancerous tumours. When malignant tumours develop one cell becomes abnormal and grows out of control with the potential to spread throughout the body. It is possible for many separate fibroids to grow and develop at the same time. Each fibroid develops from a different muscle cell.
Medical studies have found that more than 80% of women will have some evidence of fibroids when their uteruses are examined very closely. Most fibroids are small and cause no symptoms. It is usually only when fibroids become large that they tend to cause problems, but sometimes small fibroids can cause problems.
Fibroids vary greatly in size and location. When doctors or medical journals describe fibroids the size is usually described in centimetres or millimetres. it is unusual for fibroids to be spherical and their size is usually reported in three dimensions ( length, width and depth). If only one dimension is given it is usually the largest dimension.
The symptoms caused by fibroids depend on both the size and location of the fibroid.
There are 4 different locations and types of fibroid.
See diagram above for illustration of the different locations of fibroids. fibroids that distort the endometrium or that protrude into the endometrial cavity are called sub mucosal fibroids because they are below the mucosal surface. Subserosal fibroids protrude from the outer layer of the uterus. They result in the uterus having an irregular shape and can often be felt during a pelvic examination. Intramural fibroids are those that are contained within the uterine wall. In general these fibroids cause enlargement of the uterus but do not usually make it feel irregular. The term pedunculated means that the fibroid has a stalk which connects it to the uterus.
The histology of fibroids (how they look under the microscope).
The main component of fibroids is the smooth muscle cell. The uterus, bladder and stomach are all organs within the body that are composed of smooth muscle cells. Muscles such as the biceps muscle in the upper arm are formed of skeletal muscle cells. The arrangement of smooth muscle cells within an organ such as the uterus is organised in a way that enables the uterus to stretch, and also contract during childbirth. Skeletal muscles are designed to pull and exert force to enable movement of limbs (for example).
Under the microscope smooth-muscle cells appear as long stretched out cells with a nucleus in the centre. They are grouped as interlacing bundles which are woven together. When examined microscopically fibroids appear differently from normal uterine muscle in that they have an increased amount of extracellular matrix (ECM). This extracellular matrix is composed of proteins and other substances which are found between cells that cause them to stick together. ECM typically has an increased quantity of collagen. It is this collagen that makes fibroids fibrous. It is also thought that ECM acts as a site of storage for a variety of substances that seemed to be important in causing the symptoms of fibroids. ECM is now known to be an extremely active and important component of any organ or tissue.
The way that fibroids appear under the microscope varies significantly. Some fibroids have many fibroid muscle cells with small bands of ECM between the muscle bundles. Other fibroids have lots of ECM with small areas of fibroid muscle cells. It does however appear that ECM is an important component of fibroids.
It is known that excess production of ECM results in in conditions other than fibroids. Surgical wounds and lacerations heal with scar tissue which includes ECM. If too much scar tissue forms after surgery then adhesions can develop or a keloid scar may form. Keloid scars are large raised hypertrophic scars. In keloid scars there is excessive production of scar tissue. It is possible that fibroids and keloid scars are related. Both fibroids and keloid scars are more frequent in black women and both seem to be related to an abnormality in control of ECM production.
The endometrium (lining of the uterus) is also an important factor in affecting the behaviour of fibroids. It is important to understand that it is the endometrial layer that is shared with each period. Therefore there could not be any abnormal menstrual bleeding in women with fibroids without the endometrium. When the endometrium is shed each month it is the upper two thirds of the endometrium which is lost. This is called the functional endometrium. The basal layer of the endometrium. which is about a third of the endometrium, remains and this then directs the buildup or regeneration of the endometrial lining for the next period of the menstrual cycle.
The endometrium under the microscope, typically appears normal in women with fibroids. In submucosal fibroids however sometimes there is an unusual type of endometrial lining which does not have the normal glandular structures. The medical term for this is aglandular functionalis (functional endometrium without glands). If a woman is having problems with heavy periods then this is sometimes due to a submucosal fibroid. Chronic endometritis is another pattern of endometrium that can suggest that a submucosal fibroid is present. But chronic endometritis is more often associated with infections of the uterus and other problems such as where pregnancy tissue not been cleared from the uterus (retained products of conception). Even if the endometrial lining looks normal under the microscope, an examination of the way that particular molecules are expressed, shows that the lining of the uterus in women with fibroids is different in important functional ways from that of normal uterine lining. These abnormalities may be the explanation for the infertility,pregnancy related problems and abnormal bleeding experienced by women with fibroids.
Fibroids vary considerably in position appearance and size. When examined under the microscope there are even more differences apparent. These differences probably result in the differences in the behaviour of different fibroids. More research needs to be done into understanding the biology of fibroids so that they can be categorised better. A greater understanding of the differences between different types of fibroids is important in understanding why some women are asymptomatic and some women have problems with fibroids such as pain bleeding and in fertility.