Etymologically, the word gland comes from the Latin "glandula" , which is diminutive of "glans" or "glandis" which means "acorn". "Glandula" in Latin which means "acorn" was previously applied to the tonsils, then began to be applied to any organ whose function is to produce a secretion that spills to the skin, mucosa or blood. A gland is any plant or animal organ, consisting essentially of differentiated cells of the epithelial tissue, which is responsible for secreting and producing substances essential for the functioning of the body and expelling unnecessary ones.
The glands can be classified as: Endocrine, or closed glands, these do not have a conduit and discharge their secretion into the capillaries that surround the glands. There are also the Mixed glands, they are those that in their structure can produce both products that are secreted to the outside and to the blood. On the other hand are the Exocrine glands or open glands, which secrete their products to an excretory tube that secretes their product both on the surface and into the lumen of a hollow organ. These exocrine glands are subdivided depending on their different mechanisms to discharge their secreted products, for example we have the Apocrines frequently refers to the sweat glands, these are part of the body cells that are lost in the secretion process; then we have the Holocrines in these, the whole cell disintegrates to excrete its content, as in the sebaceous glands found in the chorion of the skin; and finally the Merocrines here the cells secrete their substances by exocytosis, just as in the mucous and serous glands.
The glands can also be divided into unicellular and multicellular, depending on their number of cells, unicellular are individual cells that are distributed between non-secreting cells, such as goblet cells. And the Pluricellulars, which have more than one cell, can differentiate between the disposition of the secretory cells and whether or not there is branching of the secretory ducts.
