Which type of tissue consists of closely packed, flattened cells that are mainly avascular?

Epithelial tissue is characterized by closely packed cells with very little extracellular matrix between them, which provides a barrier and protects underlying structures. This tissue type is avascular, meaning it lacks blood vessels; instead, it receives nutrients through diffusion from nearby tissues. The tightly packed nature of epithelial cells not only provides protection but also allows for selective permeability, enabling the regulation of the movement of substances in and out of the underlying tissues.

The specific arrangements of epithelial tissue can vary, leading to different functions. For example, simple squamous epithelium consists of a single layer of flat cells, making it ideal for functions like diffusion and filtration in areas such as the linings of blood vessels and the alveoli of the lungs.

Other tissue types have different structural and functional characteristics. Muscle tissue consists of cells that are elongated and specialized for contraction, nervous tissue is made up of neurons and glial cells responsible for transmitting impulses, and connective tissue is diverse, typically characterized by a significant amount of extracellular matrix providing support and connecting different tissues and organs.