Glands are multicellular organs or individual cells that secrete substances for use in the body. Most glands are made up of epithelial tissue and their seat creations include mucin, hormones, electrolytes, enzymes, and waste products. Glands can be divided into two groups, endocrine glands, and exocrine glands. Endocrine glands secrete their products, mainly hormones, directly into the bloodstream or interstitial fluid that surrounds cells. Some endocrine glands include the adrenal, thyroid, and pituitary glands. Exocrine glands typically secrete their products into a duct or onto a surface, such as the skin, but not into the bloodstream. Exocrine glands can be unicellular or multicellular. Unicellular exocrine glands do not contain ducks and the multicellular glands do contain a duct system. An example of a unicellular exocrine gland is the goblet cell, which can be found in epithelial tissue and secrets mucin, which forms a mucous layer, coating certain tissue areas, such as the air passageway in the trachea, in the respiratory system. Multicellular exocrine glands can be classified in two different ways, by their form, or by their secretion method. Let's look at classification by form. There are two different forms, simple glands, which have a single duct, and compound glands, which have branching ducts. There are also two different forms of the secretory portion of the gland. Tubular, where the diameter of the duct and secretary portion are similar, and acinar, or alveolar where the secretory portion of the gland forms a sac like shape. So, they're simple straight tubular, coiled tubular, and simple branch tubular, which still has only one duct, making it a simple gland. And simple a acinar and simple branched acinar, again, only one duct. And for compound glands with more than one duct there are compound tubular, compound acinar, and compound tubuloacinar, with both tubular and a sinner regions. Now for classification by secretion method, in which there are three basic types. Merocrine, apocrine, and holocrine. Merocrine glands release their secretions through the process of exocytosis, where secretory vesicles are released through the plasma membrane of the cell without damaging the cell itself. The salivary glands are an example of merocrine glands. Apocrine glands actually pinch off a portion of the cell to release their secretory products. The cell repairs itself and continues to release its products in the same manner, mammary and some sweat glands are examples of apocrine glands. The third type is holocrine glands. In holocrine glands cells accumulate a secretory product and the entire cell ruptures to release the product. The ruptured cells are replaced by other epithelial cells through cell division. Examples of holocrine glands include certain glands in the skin and in the eyelids. We will cover the structure and functions of individual glands in future videos. And that be the basics on the structure and classification of glands.