The endometrium is the inner
epithelial layer, along with its
mucous membrane, of the
mammal
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
ian
uterus. It has a basal layer and a functional layer: the basal layer contains
stem cell
In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type o ...
s which regenerate the functional layer.
The functional layer thickens and then is shed during
menstruation
Menstruation (also known as a period, among other colloquial terms) is the regular discharge of blood and mucosal tissue from the inner lining of the uterus through the vagina. The menstrual cycle is characterized by the rise and fall of hor ...
in humans and some other mammals, including
apes,
Old World monkeys, some species of
bat, the
elephant shrew and the
Cairo spiny mouse
The Cairo spiny mouse (''Acomys cahirinus''), also known as the common spiny mouse, Egyptian spiny mouse, or Arabian spiny mouse, is a nocturnal species of rodent in the family Muridae. It is found in Africa north of the Sahara, where its natur ...
. In most other mammals, the endometrium is reabsorbed in the
estrous cycle. During
pregnancy, the glands and
blood vessels in the endometrium further increase in size and number. Vascular spaces fuse and become interconnected, forming the
placenta, which supplies
oxygen and nutrition to the
embryo
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
and
fetus.