Connective tissue proper is a class of connective tissue that encompasses all mature connective tissues except bone, cartilage, blood, and lymph. This extensive class of tissues has two subclasses — loose and dense connective tissues — classified based on the protein fiber arrangement and the amount of ground substance.

The loose connective tissues have a meshwork of thin collagen and elastin fibers, which provide tensile strength for support and enough elasticity to move cells. They have three categories based on the cell types, density, and the type of protein fibers present— areolar connective tissue, adipose connective tissue, and reticular tissue. Areolar connective tissues act as packaging material in the body. In contrast, the primary role of adipose tissue is to reserve energy and insulate the body's temperature. The reticular tissue, with its reticular fibers, forms the stroma of organs.

Dense connective tissues have collagen fibers densely arranged in their relatively acellular matrices. These fibers are organized into thick bundles, imparting considerable strength. The regular dense connective tissue has densely packed fibers that are arranged in parallel, forming structures such as ligaments or tendons. In contrast, an irregular, intertwined array of these fibers is seen in irregular dense connective tissue, found in muscle fascia or the dermis. The third category of dense connective tissue is elastic tissue which predominantly contains elastic fibers and is found in the lungs and walls of arteries. This tissue is resilient and prevents the deformation of organs, even when stretched during physiological activities like breathing or blood transport.