Endometrial cancer is a cancer that develops from the endometrium, the inner lining of the uterus (womb). 

About the Uterus and Endometrium

The uterus is a hollow organ, about the size and shape of a medium-sized pear. The uterus
has 2 main parts. The lower end of the uterus, which extends into the vagina, is called the cervix.The upper part is the body of the uterus, also known as the corpus. (Corpus is the Latin word for body.) The body of the uterus has 2 layers. The inner layer is called the endometrium. (Endo is Greek for inside and metrium is Greek for uterus.) The outer is called the myometrium. (Myo is Greek for muscle.) The myometrium is the thick layer of muscle that contracts to push the baby out during delivery.

Changes in hormones during a woman's menstrual cycle cause the endometrium to change.During the early part of the menstrual cycle the ovaries secrete estrogens. This causes the endometrium to thicken. At the mid-point of the cycle, estrogen production decreases and the ovaries produce larger amounts of progesterone. This hormone causes the innermost layer of thelining to change (such as grow blood vessels) to nourish an embryo in case a pregnancy occurs .

If pregnancy does not occur, by the end of the cycle, production of estrogen and progesterone 
from the ovaries decreases. This causes cells of the inner layer of endometrium to die, and the dead tissue is shed from the uterus (womb) as the menstrual flow. This cycle repeats throughout a woman's life until menopause.

Cancers of the Uterus and Endometrium

Nearly all endometrial cancers are adenocarcinomas (cancers of glandular cells). About 90% are endometrial adenocarcinomas. Sometimes, in addition to glandular cells, there are squamous cells (the type of cells found on the surface of the cervix and the skin). If the squamous cells look benign (noncancerous) under a microscope, and the glandular cells look cancerous these tumors are called adenoacanthomas. If the squamous areas and glandular areas both look malignant (cancerous), these tumors are called adenosquamous carcinomas . Although these look different they are considered the same endometrial adenocarcinomas. There are other kinds of glandular cancers called papillary serous adenocarcinomas and clear cell. These are less common types of endometrial cancer and are different from the usual kind because they tend to grow and spread more rapidly. The above cancers of the endometrium develop from its glandular cells. Doctors call this layer of glandular cells the endometrial epithelium. 

Three less common uterine cancers that do not come from glandular tissue of the endometriumare called uterine sarcomas. Uterine sarcomas include1. Stromal sarcomas which develop in the stroma (supporting connective tissue) of theendometrium.2. Malignant mixed mesodermal tumors (MMMT, or carcinosarcomas) which may combine features of endometrial carcinoma and those of sarcomas.3. Leiomyosarcomas which start in the myometrium or muscular wall of the uterus.