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. 

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.

The endothelium is a form of squamous epithelium consisting of flat cells that line the blood and lymphatic vessels. Endothelial cells are metabolically active and produce compounds that relax vascular smooth muscle and inhibit the adhesion of platelets.

Adipose, or fat cells, are modified for long-term energy storage and are considered connective tissue. Cone cells are found in the eye and serve as light receptors for the nervous system. Stem cells can develop into more specifically differentiated daughter cells. Dendritic cells help T-cells respond to foreign antigens as part of the immune system. 

There are four main categories of tissue: connective tissue, epithelial tissue, nervous tissue, and muscle tissue. Connective tissue is classified by loosely associated cells in an abundant medium, generally used to fill spaces and connect structures to one another within the body. Collagen fibers are particularly abundant in connective tissue. Adipose cells (fat), blood, cartilage, and bone are all specific types of connective tissue. Extracellular matrix is generally also considered connective tissue, but can vary in composition to some extent by location and includes loose connective tissue and dense connective tissue.

In contrast, epithelial tissue serves to line and protect regions of the body form the external environment. Any region of the body that is directly or indirectly connected to the environment is lined with epithelial cells, including the skin, digestive tract, respiratory tract, and excretory tract. Since material from the external environment (food) can enter the stomach, we know that the stomach lining must be made of epithelium.

Connective tissue supports and connects other tissues and parts of the body. Connective tissue includes bone, cartilage, and fat. Epithelial cells form the epidermis of the skin and the linings of the digestive and respiratory tracts. Canaliculi are channels that allow osteocytes (bone cells) to remain in contact and transmit nutrients and gasses. Proteins are essential for the growth of new tissue or the repair of damaged tissues. Minerals are generally inorganic, but the mineral hydroxyapatite forms the matrix of bones.

Skin is part of the integumentary systemand is composed of stratified squamous epithelium.

Connective tissue is defined as a population fo cells suspended in an extensive extracellular matrix. For example, blood is classified as connective tissue because it consists of red blood cells (erythrocytes) suspended in a liquid matrix (plasma). Bone, cartilage, ligaments, and adipose tissue (fat) are also considered connective tissue.

Connective tissue supports and connects other tissues and parts of the body. Connective tissue includes adipose (fat), cartilage, bone, and blood. Muscles produce movement. Muscles are attached to bone, in the walls of hollow internal organs, and in the heart. Nervous tissue transmits impulses in from sensory receptors and out to produce motor responses. Types of nervous tissue include the brain, spinal cord, and nerves. Epithelial tissue protects, secretes, absorbs, and excretes. Examples of epithelial tissues include skin, glands, and mucous membranes. In dentistry, the term "hard tissue" is used to describe the calcified tissue components of the tooth: enamel, dentin, and cementum.

Connective tissue is used to support, protect, and transport within the body. Bone tissue forms skeletons. Cartilage connects and supports bones, and adipose tissue stores energy and provides cushioning. Epithelial tissue refers to another type of tissue.