Epithelium and Endothelium - High School Biology

An epithelia is a sheet of cells that are closely associated. Cells within an epithelium are polarized, indicating that the cells have a "direction". One side of the epithelium is called "apical" while the other side is "basal." The apical region is oriented toward a lumen, while the basal region is oriented toward a basement membrane associated with connective tissue. Cells within an epithelium are innervated, but not vascularized; therefore, they must receive signals through other methods of intercellular signaling, such as tight junctions.

The epidermis is composed of five layers of flattened epithelial cells. This stratified squamous epithelium allows the skin to be a barrier for the body and prevent pathogens and foreign materials from entering.

Stratified epithelium has multiple layers, and squamous epithelium is flattened.

Epithelium describes the inner and outer surfaces of the body including; the skin, colons, and the stomach. These surfaces have distinct upper and lower sides, are kept in place by connective tissue, and are made up almost entirely of cells. However, they are not supported by blood cells.

Nervous tissues such as the brain, spinal cord, and nerves function to transmit impulses for coordination, sensory reception, and motor actions. Epithelial tissue protects (skin), secretes (glands), absorbs (kidneys), and produces germ cells (embryo).

The primary function of epithelial tissue is to provide linings that separate the body from the external environment. Epithelial tissue lines blood vessels, the respiratory and digestive tracts, the excretory system, and composes the skins. Epithelial tissue also lines the ducts that connect the body to the exterior, such as those that allow the pancreas to secrete enzymes into the small intestine.

Epithelial tissue is avascular (does not have a blood supply); thus epithelial tissue is always found in close association with connective tissue that has a rich blood supply. Epithelial cells cover organs and are thus subject to lots of friction. As a result they are continually being renewed, and divide quickly. In particular, skin cells and the epithelium lining the digestive tract have a high rate of division.

Adipose tissue is the anatomical term assigned to fatty tissue. Keep in mind that adipose tissue is a type of connective tissue. Adipose tissue serves as an energy store, helps maintenain of core body temperature, and protects internal organs from damage caused by trauma. Epithelial tissue is responsible for covering body cavities, and comprising glandular tissue. Nervous tissue is responsible for receiving sensory input from the environment, integration of the sensory input, control of muscles and glands, maintenance of body homeostasis, and emotions/personality. Smooth and striated are subsets of muscle tissue: skeletal and cardiac muscle are striated, whereas smooth muscle is not.

Epithelial tissue forms membranes that cover the body, as well as lines body cavities and glands. Simple squamous epithelium consists of thin, flattened cells, that are one layer thick and allow for the easy movement of substances. This is especially important for gas exchange in the alveoli of the lungs and in the capillaries of the body. The minimal barrier provided by simple squamous epithelium allows molecules to easily diffuse across the membranes.

Stratified squamous epithelium is many layers thick and forms the skin and lining of mucous membranes. Simple cuboidal epithelium functions in absorption and secretion in the kidney tubules and salivary glands. Simple columnar epithelium forms a thick layer that lines the esophagus, stomach, intestines, and uterus. Pseudostratified columnar epithelium possess beating cilia and transport particles in the respiratory tract and reporductive system.

Keratinized stratified squamous epithelium makes up the outer layer of skin. Other types of epithelial tissues are found throughout the digestive and reproductive tracts, and other areas of the body.

Ciliated epithelium are located in the respiratory tract. There are hair-like processes, cilia, located on the surface that wave and function to sweep materials, such as mucous and dust, across the surface. Cuboidal epithelial calls are as high as they are wide. Cuboidal cells line the ducts of glands and organs and function in the absorption and transportation of substances. Squamous epithelial cells are flat and allow substances to freely move across their surface. These cells are located in the lungs and the kidneys. Stratified epithelial cells form layers, with mitosis taking place in the lowest layer. Skin is formed from stratified epithelial cells. Transitional epithelial cells adjust to mechanical changes such as stretching. This type of tissue is found only in the urinary system.

Since capillaries are the site of diffusion of substances (oxygen and carbon dioxide, for example), they need to have extremely thin walls. Their walls are made of simple (single-layered) epithelium. The other options would be too thick to facilitate diffusion of substances.

Since we are looking for a single layer of cells, we must rule out any area of the body that is subject to lots of friction. The skin is continually being rubbed and pushed by everything we touch, and the esophagus sustains abrasions from all the food we eat.

Microvilli are tiny projections on the apical surface of cells that increase the cell surface area. Since most of the absorption of food and nutrients occurs in the small intestine, it makes sense that we would want to increase the surface area of that region. Furthermore, since we are absorbing, we would want a single layer of cells to facilitate the diffusion of substances across the layer of epithelium. Thus, the correct answer is that simple columnar epithelium with microvilli will be found in the small intestine.

All of these areas of the body are subject to lots of friction. Thus, there would be multiple (stratified) layers of epithelium lining them.