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

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

Here are the steps to understand why:

1. The human body is composed of four basic types of tissues: epithelial, connective, muscle, and nervous tissue.

2. Epithelial tissue covers the body surface and forms the lining for most internal cavities.

3. One of its main functions is protection. For example, the skin is an organ made of epithelial tissue which protects the body from dirt, dust, bacteria and other microbes that may be harmful.

4. Epithelial tissues also help in absorption of water and nutrients, secretion of waste products, and are also involved in sensory perception.

5. Importantly, epithelial tissues act as gatekeepers of the body controlling permeability and allowing selective transfer of materials across their layers. They regulate the movement of materials in and out of certain body regions.

6. Therefore, epithelial tissue is the major tissue type that acts as "a gatekeeper".