The tunica intima is a part of the venous system and refers to the innermost layer of the interior of veins and arteries. It is one of three layers that together form the tubular structure of vein and artery tissue. The material of the inside layer is made of endothelial cells that are characterized by their extremely elastic properties. Due to the fragile nature of this material, the tunica intima can not be fully separated from the tunica media, the middle layer of the vein tissue and can only be examined in labs in small pieces. It has been discovered that this thin, transparent layer of endothelium tissue actually has three distinct layers of its own.
The thickest part of this incredibly thin membrane is the fenestrated layer, and this is where the tunica intima gets its elastic nature. This elasticity is due to tiny fibers arranged in longitudinal lines to allow as much movement as possible in the vein's and artery's role of moving blood throughout the body. Even for such a tiny section of membrane, it is possible to see individual layers within this particular layer of tissue under a microscope. The larger blood vessels, such as the aorta near the heart, have considerable thickness thanks to the fenestrated layer, while those vessels that are smaller only contain a very thin layer of this connective tissue.
The middle layer of the tunica intima is the subendothelial layer and is noted for its branched cells that make up the matrix, or inter space, of the tissue. Thinner than the fenestrated layer, in the smallest blood vessels of the body, this layer only reaches a single layer of stellate cells in thickness. In the larger circulatory vessels, this layer becomes more developed and is then classified as a connective tissue. The function of this section is to support the less stable inner layer during the normal flow of blood.
The innermost section of the tunica intima mainly consists of endothelium cells, a type of skin cell found on the inside of the body. These cells have varying shapes and may appear oval, fusiform, or polygonal in a microscope when stained with silver nitrate. No matter what shape the membrane of each of the cells take, they all have a round or oval nucleus in common. It is this layer of cells and tissue that comes in direct contact with the blood stream. When studied after tissue death, it has been noted that this layer deteriorates in longitudinal fissures or wrinkles.