The endometrium is a complex organ. It consists of a surface epithelium ( columnar epithelium), glands and blood vessels.

Here is a short explanation of a gland. The surface epithelium bends inward to form indentations. The cells at the base of these indentations, usually change and becomes cells with special secretory functions. In the endometrium these cells secrete large amounts of proteins.

The next drawing shows the endometrium immediately after the menstruation stopped. The endometrium is thin and the glands are wide and shallow with very little glandular activity.

Remember this layer covers the whole cavity inside the uterus. Only one of million glands are shown here.  

The endometrium growths thicker due to oestrogen stimulation. It continues to grow until ovulation occurs approximately two weeks after the menstruation began.

The next drawing shows the endometrium just before ovulation is going to take place. The scientific name for this stage is " the follicular phase".The glands are deeper and the cells at the base are changing into glandular cells (cells with secretory functions)

Following ovulation, a structure called the corpus luteum, develops in the ovary. The corpus luteum secretes oestrogen and progesterone. Progesterone increases the glandular activity of the glands, blood supply increases and arteries become tortuous (spiral arteries) The cavities of the glands are filled with proteins. The scientific name for this stage is " Secretory endometrium".

The next drawing shows the endometrium prior to menstruation during the secretory phase.

The spriral areteries are shown. There is secretions present in the lumen of the glands.