Unit 1 – Introduction to Epithelial Tissues
Introduction to Epithelial Tissues
Epithelial tissues are tissues that cover all exposed surfaces of the body including the external surface of the skin (epidermis) and the inner lining of all passageways, tubes, and chambers within the body. Most of the epithelial tissue images in the chapters that follow are taken from the inner lining of tubes and chambers and can thus be considered endothelial tissues (plural = endothelia; an epithelial tissue lining the inside of a tube or chamber). Essentially, all tubes and passageways comprising the digestive, respiratory, urinary, and reproductive tracts are lined with endothelia, along with all blood vessels and heart chambers of the cardiovascular system and lymphatic vessels of the lymphatic system. Epithelial tissues also line the fluid-filled ventricles of the brain and the large internal surfaces of the thoracic and abdominal cavities, known collectively as mesothelia (or mesothelial linings). Mesothelial linings include the visceral pleura, parietal pleura, epicardium, and pericardium of the thoracic cavity and the visceral peritoneum and parietal peritoneum of the abdominal cavity.
All epithelial tissues are comprised entirely of cells and characterized by cellular polarity within the tissue. In other words, there is a “top” and “bottom” to all epithelial tissues. At the “bottom” or basal surface of epithelial tissues is a thin strip of collagen and proteoglycans known as the basement membrane which anchors the basal surface of epithelial cells to the tissue below. The “top” surface of epithelial tissues, known as the apical surface, is exposed to the inner lumen of the chamber or tube it lines, or in the case of the epidermis of skin, to the outside world. Most epithelial tissues are named according to the shape of cells within the tissue (columnar, cuboidal, or squamous) and how many layers of cells there are within the tissue. Epithelial tissues with a single layer of cells that all contact the basement membrane are known as simple tissues. Epithelial tissues comprised of more than one layer of cells where only the bottom layer of cells contacts the basement membrane are known as stratified tissues.
In the chapters that follow, you can study images of six specialized categories of epithelial tissues found in the human body, each unique in the size, shape, and arrangement of cells within the tissue. The six categories of epithelial tissues covered in this text include: simple squamous epithelium, stratified squamous epithelium, simple cuboidal epithelium, simple columnar epithelium, pseudostratified ciliated columnar epithelium, and transitional epithelium. There is a chapter devoted to each of the above tissues where you can explore how to identify each tissue by site and learn the primary characteristics and functions of each tissue along with the location(s) where the tissue is typically found inside the human body. At the end of the unit is a chapter devoted to epithelial tissue practice quizzes and interactive learning activities.
