A tissue is a group of similar cells and cell products that arise from the same region of the embryo and work together to perform a specific structural or physiological role in an organ. The four primary tissues are epithelial, connective, nervous, and muscular tissue (table 5.1). These tissues differ from each other in the types and functions of their cells, the characteristics of the matrix (extracellular material) that surrounds the cells, and the relative amount of space occupied by cells versus matrix. In muscle and epithelium,
1 histo = tissue + logy = study of the cells are so close together that the matrix is scarcely visible, while in connective tissues, the matrix usually occupies much more space than the cells do.
The matrix is composed of fibrous proteins and, usually, a clear gel variously known as ground substance, tissue fluid, extracellular fluid (ECF), interstitial2 fluid, or tissue gel. In cartilage and bone, it can be rubbery or stony in consistency. The ground substance contains water, gases, minerals, nutrients, wastes, and other chemicals. In summary, a tissue is composed of cells and matrix, and the matrix is composed of fibers and ground substance.
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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.