Tissues are groups of cells that can be grouped together and classified by their form and function. There are four major types of tissues: connective, epithelial, muscular, and nervous.

Connective tissue includes loose connective tissue, dense connective tissue, elastic connective tissue, reticular connective tissue, adipose (fat) tissue, cartilage, bone, lymph, and blood. Connective tissue can be identified as a loose arrangement of cells within an abundant extracellular matrix. The types of cells and composition of the matrix define the type of connective tissue. For example, the protein elastin is abundant in elastic connective tissue, while the protein collagen is abundant in cartilage and loose connective tissue. In blood, the matrix is composed of plasma. In bone, the matrix is composed of hydroxyapatite crystal.

Nervous tissue includes all types of neurons and neuroglia. These cells are specialized to transmit electrical signals and protect the transmission of these signals. Skin is composed of several layers of epithelium. Epithelial tissue is designed to form barriers between the body and the outside environment.

Blood, bone, tendons, ligaments, and cartilage are all types of connective tissue. Bone and cartilage provide the framework for the body, as well as protect the internal organs of the body. Blood is responsible for transportation of oxygen and other nutrients throughout the body. Tendons transfer the force created by muscle tissue to the bones, allowing for movement.

Connective tissue is not responsible for communicating pain to the brain, as the sensation of pain is created by signals from the nervous system. The four types of tissue are connective, epithelial, muscular, and neural.

Connective tissue binds, supports, and fills spaces in the body. It consists of cells surrounded by an extracellular matrix. The make-up of this matrix is essential to defining the properties of the given connective tissue type. The extracellular matrix of blood is aqueous and filled with platelets and protein factors, while the matrix of bone is composed of hydroxyapatite crystal.

The connective tissue is found throughout the body in between other tissues and cell types. Connective tissue functions to both connect and separate the different tissue types.

Connective tissue has three main components: fibers, ground substance, and cells (fibroblasts, adipocytes, macrophages, mast cells, and leukocytes). Typically, connective tissue contains a relatively low proportion of cells, and a very high proportion of ground substance and fibrous proteins, such as collagen. Blood and lymph is sometimes considered a component of connective tissue as well.

Cells are also a critical component of connective tissue and are typically suspended in the ground substance. Cells that are a part of connective tissue include fibroblasts (cells that produce fibers), mast cells (involved in histamine-mediated inflammatory response), and adipocytes (fat cells). Staining methods can show these cell types within the connective tissue. Other connective tissue cells include osteocytes (bone cells), and chondrocytes (cartilage cells).

In connective tissues, ground substance is an extracellular matrix that suspends and supports cells. Ground substance does not contain fibers and is composed mostly of water and glycoproteins.

The connective tissue includes fibers that act as a matrix and support structure for cells suspended in ground substance. These fibers are categorized as collagenous fibers, elastic fibers, and reticular fibers. Collagenous fibers are the most abundant in the extracellular matrix, comprise tendons and ligaments, where play structural roles. Elastic fibers are present in areas of the body that undergo cycles of stretching and recoiling, such as arteries. Reticular fibers form meshlike networks that provide the base for other support structures. 

Collagenous fibers are composed primarily of collagen. Collagenous fibers bind bones together and connect different tissues to one another. Examples of collagenous fibers include tendons, cartilage, and blood vessels.

Collagenous fibers connect tissue types and bind elements of the musculoskeletal system to one another. Types of collagenous fibers in connective tissue include blood vessels, ligaments, and tendons. Stratified epithelium is comprised of layers of epithelial cells, which function as a covering/lining to a body structure that is subject to lots of friction.