Most specialized cells are associated with other cells of a similar kind to form tissues. Corresponding to the four general categories of differentiated cells, there are four general classes of tissues: (1) muscle tissue, (2) nerve tissue, (3) epithelial tissue, and (4) connective tissue. It should be noted that the term "tissue" is used in different ways. It is formally defined as an aggregate of a single type of specialized cell. However, it is also commonly used to denote the general cellular fabric of any organ or structure, for example, kidney tissue or lung tissue, each of which in fact usually contains all four classes of tissue.
We will emphasize later in this chapter that the immediate environment of each individual cell in the
Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition
A Framework for Human Physiology CHAPTER ONE
body is the extracellular fluid. Actually this fluid is interspersed within a complex extracellular matrix consisting of a mixture of protein molecules (and, in some cases, minerals) specific for any given tissue. The matrix serves two general functions: (1) It provides a scaffold for cellular attachments, and (2) it transmits to the cells information, in the form of chemical messengers, that helps regulate their migration, growth, and differentiation.
The proteins of the extracellular matrix consist of fibers—ropelike collagen fibers and rubberband-like elastin fibers—and a mixture of other proteins that contain chains of complex sugars (carbohydrates). In some ways, the extracellular matrix is analogous to reinforced concrete. The fibers of the matrix, particularly collagen, which constitutes one-third of all bodily proteins, are like the reinforcing iron mesh or rods in the concrete, and the carbohydrate-containing protein molecules are the surrounding cement. However, these latter molecules are not merely inert "packing material," as in concrete, but function as adhesion/recognition molecules between cells and as important links in the communication between extracellular messenger molecules and cells.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.