The major tissue type that acts as "a gatekeeper" because it regulates the movement of materials in and out of certain body regions is tissue.

The major tissue type that acts as "a gatekeeper" regulating the movement of materials in and out of certain body regions is epithelial tissue. Epithelial tissue layers are found in various parts of the body, including the skin, lining of the mouth, and the lining of internal organs.

This tissue serves several crucial functions, including protection, secretion, absorption, and filtration. For instance, the epithelial cells lining the intestines selectively absorb nutrients while blocking harmful substances. Similarly, in the respiratory tract, epithelial tissue helps filter inhaled air and secrete mucus, playing a vital role in maintaining respiratory health.

Epithelial tissue is characterized by closely packed cells with minimal extracellular matrix, forming a continuous sheet. Its selective permeability is essential for maintaining homeostasis in the body, allowing for the controlled exchange of substances, which is vital for the body's overall function and health.