A tissue is a collection of similar cells grouped to perform a common function. Different tissues are made of their own specialized cells that are adapted for a given function.
All animal cells are basically similar. Each cell has a cell wall or plasma membrane that surrounds the cell and contains various receptors that interact with the outside area. A nucleus, Golgi apparatus, mitochondria, and other structures are contained in each cell. Beyond that, cells are specialized in structure for a given function.
The study of tissues is called histology. Studying the structure of tissue is done by staining a thin specimen of the tissue and placing it under a microscope. An experienced histologist can look at a specimen and immediately determine from which organ it was taken. A histologist can also see and diagnose a disease if it is present in the tissue. The study of disease processes is called pathology.
Tissues are divided by function into a number of categories. Muscle tissue, for example, makes up the muscles of the body. A muscle belongs to one of three categories-voluntary muscle which can be controlled for movement or lifting, involuntary muscle which is not under conscious control (such as the muscle tissue in the digestive organs), and cardiac muscle which forms the heart.
Connective tissues compose the bones, tendons, and ligaments that make up the support of the body. The body also consists of adipose tissue or fat.
Nervous tissue forms the brain, spinal cord, and the nerves that extend through the body. Digestive tissue is found in the digestive system including the stomach, intestines, liver, pancreas, and other organs involved in digestion. Vascular tissue comprises the blood-forming portion of the bone marrow and the blood cells themselves. Epithelial tissue forms sheets that cover or line other tissues. The skin and the lining of the stomach are both examples of epithelial tissue. Various reproductive tissues form the ovaries, testes and the resulting gametes (ova and sperm).
Combined, these tissues form the human body and carry out its functions.