Basic tissues in organs

Bone is an organ because it is composed of several types of connective tissue (the tissue bone, bone marrow, and a connective tissue proper covering), blood vessels (composed of epithelium and muscle), and nerves.

Skin is composed of epithelial tissues, forming a surface epidermis and deeper glands along with a connective tissue layer that forms the dermis. Blood vessels and nerves are also present.

Blood vessels, like this muscular artery, are lined by an epithelium and have a wall composed of muscle and connective tissue.

Two of these organs have a free surface and, therefore, are lined by an epithelium. The free surface of the skin is the exterior surface of the body and the free surface of the artery is the lumen through which blood flows.

Several types of connective tissue proper are found in these three organs, forming a covering for bone, the dermis of skin, the hypodermis beneath the skin, and part of the wall of the artery.

Bone, another type of connective tissue, has a firm, calcified matrix that gives bone its rigidity. Bone cells are present within the matrix and on the bone surface.

The spaces within bone are filled with a special type of connective tissue called bone marrow.

Muscle is a frequent component of organs and forms a majority of the arterial wall. Muscle also forms an important component in the walls of other hollow organs, as seen in the previous image of the stomach. Smooth muscle is also present in the skin, where it is attached to hair follicles. Muscle is absent in bone, except in the walls of vessels supplying the bones.

Blood vessels supply all three of these organs, although they are only visible here in the sections of bone and skin.