Epithelium Characteristics Cells fit closely together and often form sheets The apical surface is the free surface of the tissue – The unattached surface Can be exposed to exterior or cavity of an organ The lower surface of the epithelium (the basal lamina) rests on a basement membrane – Consists of reticular connective tissue
Epithelium Characteristics Avascular (no blood supply) – Relies on circulatory system to obtain food and oxygen Surrounding connective tissues supply via diffusion from nearby capillaries Regenerate easily if well nourished (through normal cell division) – Those on the exterior of the skin are exposed to friction and other hostile substances and are therefore lost
Figure 3.17b Classification and functions of epithelia. Squamous Cuboidal Columnar (b) Classification based on cell shape
Figure 3.17c Classification and functions of epithelia. Remember these general characteristics and you will be able to classify each type a lot easier !! Diffusion and filtration Secretion in serous membranes Protection Secretion and absorption; ciliated types propel mucus or reproductive cells Protection; these tissue types are rare in humans Protection; stretching to accommodate distension of urinary structures (c) Function of epithelial tissue related to tissue type Number of layers Cell shape One layer: simple epithelial tissues More than one layer: stratified epithelial tissues Squamous Cuboidal Columnar Transitional
Figure 3.18a Types of epithelia and their common locations in the body. Nucleus of squamous epithelial cell Basement membrane Air sacs of lungs Nuclei of squamous epithelial cells (a) Diagram: Simple squamous Photomicrograph: Simple squamous epithelium forming part of the alveolar (air sac) walls (275×).
Figure 3.18c Types of epithelia and their common locations in the body. Basement membrane Mucus of a goblet cell Nucleus of simple columnar epithelial cell Simple columnar epithelial cells (c) Diagram: Simple columnar Photomicrograph: Simple columnar epithelium of the small intestine (575×).
Figure 3.18d Types of epithelia and their common locations in the body. (d) Diagram: Pseudostratified (ciliated) columnar Photomicrograph: Pseudostratified ciliated columnar epithelium lining the human trachea (560×). Basement membrane Pseudo- stratified epithelial layer Cilia Connective tissue
Figure 3.18e Types of epithelia and their common locations in the body. Basement membrane Connective tissue Stratified squamous epithelium (e) Diagram: Stratified squamous Photomicrograph: Stratified squamous epithelium lining of the esophagus (140×). Nuclei
Figure 3.18f Types of epithelia and their common locations in the body. Basement membrane Connective tissue Transi- tional epithelium Transitional epithelium (f) Diagram: Transitional Photomicrograph: Transitional epithelium lining of the bladder, relaxed state (270×); surface rounded cells flatten and elongate when the bladder fills with urine.