An organ is a structure composed of two to four types of tissues working to perform functions that are beyond the scope of an individual tissue type. A set of related organs working cooperatively toward the performance of even more complex functions constitutes an organ system.
Organs come in many different forms. The stomach, with its composition of epithelium , connective tissue , nervous tissue, and smooth muscle tissue, is a familiar example. Bones are organs; although they consist primarily of osseous tissue, bones have a vast supply of nervous tissue in their nerves, fibrous tissue lining their cavities, and muscle and epithelial tissue in their blood vessels. The skin (integument) is an organ consisting of an epithelium (epidermis) overlying a thick layer of connective tissue (dermis) rich with blood vessels and accessory structures such as secretory glands.
Even the glands within the integument can be considered organs; any gland is primarily secretory epithelium surrounded by connective tissue for support and protection. Likewise, the blood vessels and nerves in these organs are organs unto themselves.
This "organ within an organ" motif is also exhibited in the sense organs. For example, within the eyeball is an organ called the retina, an association of neural and epithelial tissue that detects light entering the eyeball.
James A. Crowder
Saladin, Kenneth S. Anatomy and Physiology: The Unity of Form and Function. New York: WCB McGraw-Hill, 1998.