Types of Epithelial Tissue

Epithelial tissue can be divided into two groups depending on the number of layers of which it is composes. Epithelial tissue which is only one cell thick is known as simple epithelium. If it is two or more cells thick such as the skin, it is known as stratified epithelium.

Germ Cell Origin: Epithelial tissues are derived from all three primary germ cell layers.

Ectoderm: The epithelial cells of the skin and oral cavity (epidermis) are derived from ectoderm. Epithelial cells covering the cornea and lens, as well as sensory receptors of the eyes, ears, and nose, are also ectodermal in origin.
Mesoderm: The epithelial lining of blood vessels (endothelium) is derived from mesoderm. The epithelial lining of the pleural and peritoneal cavities (mesothelium) also originate from mesodermal cells.
Endoderm: The epithelial lining of the respiratory system and digestive tracts - as well as the functional cells (parenchyma) of the liver, pancreas, gallbladder, thyroid, and parathyroid, are derived from endoderm.