The endometrium is the membrane which covers the uterine cavity. The female endometrium starts to develop and grow during childhood. Then in the menstrual cycle, endometrium also undergoes periodic changes. Menstruation is the shedding of the uterine endometrium. A woman produces an egg during each phase of the menstrual cycle. Only when the woman is not pregnant can the next period comes on time. The menstrual cycle is stimulated by ovarian hormones.

Once the ovarian hormone is in disorder, menstruation will be in disorder, too. Then the endometrium will change, which is manifested in the thickening of the endometrium. While the manifestation of the thickening of the endometrium includes menostasia or “functional uterine bleeding”. In fact, “functional uterine bleeding” is a much common one. Endometrial hyperplasia can attack a woman at any phase of her life, such as adolescence, reproductive period, peri-menopausal period, or postmenopause, etc.  

Clinical observation finds patients with endometrial hyperplasia always accompanied by pelvic inflammatory disease or pelvic effusion, which indicates that the stimulation of inflammation is the major cause of endometrial hyperplasia. Clinical manifestation is the irregular and large amounts of abnormal uterine bleeding; the patient may have sustained bleeding long after menopause, or have extended menstruation and the bleeding time may up to one month. Sometimes the condition is serious enough to cause hemorrhagic shock.

Generally speaking, there are four types of endometrial hyperplasia: simple hyperplasia, complex thickening, glandular thickening, and atypical hyperplasia. The latter two types have higher risk of developing cancers, so they must be actively treated.

The pathology of endometrial thickening:

The endometrium as a whole is thickening, the thickness can up to 0.5 ~ 1cm and the surface is smooth and soft.    
Simple endometrial hyperplasia: both of endometrial glands and stroma proliferate; glands are increased significantly, varying in sizes and unevenly distributed. Occasionally, the glands will extend into cysts. Glandular epithelial cells are in the shape of a roller. The cells are often arranged in a pseudostratified layer because of lack of secretion. The interstitial cells are tightly arranged.  
 
Cystic hyperplasia of endometrium: it is characterized by the obvious expansion of crypt hyperplasia. In typical cases, there are scattered small holes which are visible in the thickened endometrium. For this reason, Cystic hyperplasia of endometrium is called Swiss-cheese hyperplasia.
Through the microscope, endometrial glands vary in shapes and sizes. The small endometrial glands have the same size with the early proliferative glands, and the diameter of the larger ones is several times or tenfold the length of that of the small ones. Both of the two types of endometrial glands are lined with pseudostratified columnar or cubic-storey epithelium. And they also lack secretion. There are large numbers of interstitial cells. And the cytoplasm is very little. Nuclear condensation.

Adenomatous hyperplasia of endometrium: it is characterized by the dense arranged glandular hyperplasia and interstitial substance is very little in amount. The number of this type hyperplasia is much larger than the first two types; the structure is also more complex. There is papillary hyperplasia in the cavity, while there is bud-shape hyperplasia in interstitial substance. The hyperplasia comes from Glandular epithelium. The interstitial substance is very scarce. Glandular cells are in the shape of roller and pseudo stratified. The cell shows characteristic vacuolation, and the mitotic figures of the cells are very common, without any significant atypia.

Atypical hyperplasia of endometrium: its structure of tissues is similar to that of the adenomatous hyperplasia of endometrium. Its glands are crowded and in irregular shapes. There are hyperplasias in branching or budding shape. Interstitial substance is significantly reduced. Glandular cells have obvious atypia. The nucleus is very large. The chromatin is very thick. Nucleolus is prominent. Epithelium is stratified. The cells lose polarity. The mitotic figures of the cells are very common. Sometimes, it is difficult to distinguish atypical hyperplasia of endometrium from well differentiated adenocarcinoma. The main difference is that the former has no interstitial infiltration. Some people do believe that atypical hyperplasia of endometrium is the preneoplastic changes of adenocarcinoma endometrium.