There are four tissue types
Specialized cells rarely function alone, but rather as groups of similar cells called tissues. There are many different kinds of cells, but they are all classified as one of four tissue types: epithelial, muscle, connective, and nervous (Focus: Key Figure 39.2):
Epithelial tissues are sheets of cells that create barriers between different compartments and frequently have secretory functions.
Muscle tissues contract to generate forces and movement.
Connective tissues provide structure and support.
Nervous tissues convey and process information.
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focus: key figure
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Question
Q: If we consider the functions of skeletal muscle in running, what are roles played by the three other tissue types in this process?
The muscles are connected to the bones and the bones are connected to each other by connective tissue. Bone is connective tissue, and without bones, the muscles would not be able to exert forces on the environment. Blood is also a connective tissue and supplies the muscles with oxygen and removes wastes and heat. Epithelial tissues line the blood vessels. Neural tissues carry the motor commands to control the contractions of the muscles.
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These are very general groupings that reflect developmental origins, but within each group there are various specializations that accommodate different functions. For example, connective tissue includes bones, cartilage, fat, and blood. Epithelial tissue includes the outer layers of skin (epidermis) and many types of secretory cells that produce and release substances internally, such as hormones, and externally, such as sweat and digestive juices. You might think of digestive juices as being secreted internally, but the lumen of the digestive tract is continuous with the external environment. You will learn a lot about the diversity of functions accomplished through specializations of these four tissue types in the chapters that follow.