2.3 Connective Tissues

Connective tissue is tissue that supports other tissues, holds tissues together, or fills spaces. Among and outside the cells of the connective tissues, there is a material called matrix. The matrix is manufactured by the connective tissue cells. Each type of connective tissue has its own particular type of matrix.

TYPES OF CONNECTIVE TISSUE

There are several major types of connective tissue (CT). These include:

Fibrous CT (FCT)

Cartilage CT

Bone CT,

Fat CT

. Blood is sometimes considered an additional type of CT.

FIBROUS CONNECTIVE TISSUE (FCT)

Fibroblasts. The characteristic cells of FCT are fibroblasts. Fibroblasts are able to form elongated fibers.

Matrix. These fibers make up the matrix of FCT.

Fibers. The fibers are either white or yellow. White fibers are made from a protein called collagen. White fibers tend to have a fixed length. White fibers are not very easily stretched. Yellow fibers are made from a protein called elastin. Yellow fibers are elastic. They can be stretched and then they can snap back (like a rubber band).

The types of FCT are recognized by the arrangement of their fibers. These types include loose and dense areolar FCT.

Loose areolar FCT has an open irregular arrangement of its fibers. These fibers are found widely throughout the body. An example is the superficial fascia (subcutaneous layer). The superficial fascia is the connective tissue which lies beneath the skin. Loose areolar FCT is the filling substance around most organs and tissues of the body.

The fibers of dense FCT are closely packed and parallel. There are no significant spaces between the fibers. Examples of dense FCT are ligaments and tendons. A ligament is a band of dense FCT that holds the bones together at a joint. A tendon attaches a muscle to a bone.

CARTILAGE CONNECTIVE TISSUE

Cartilage cells (also called chondroblasts) are clustered in microscopic pockets within the cartilage matrix. The cartilage cells produce the material of the matrix.

The matrix produced by the cartilage cells appears homogeneous (the same throughout). The matrix also appears amorphous (shapeless).

The three types of cartilage CT are hyaline, fibrous, and calcified.

Hyaline cartilage CT appears homogeneous and clear.This type of cartilage helps to cover bone surfaces at joints. Hyaline cartilage is found as incomplete rings which keep the trachea (windpipe) open.

Fibrous cartilage CT includes dense masses of fibers (of FCT). It is more rigid than hyaline cartilage. The auricle of the external ear is stiffened with fibrous cartilage.

Calcified cartilage CT is cartilage that has been stiffened by the addition of calcium salts. This is not the same as bone tissue. An example is the cartilages of the larynx (the voice box) which become calcified with age.

BONE CONNECTIVE TISSUE

Osteoblasts are cells that make and repair bone. Osteoclasts are cells which tear down and remove bone. Bone is continually being remodeled as a person lives. Remodeling is in direct response to the stresses placed on the bone.

There are two major types of bone tissue. One is compact bone CT, which is dense. The other is cancellous bone CT, which is spongy. Compact bone CT forms the hard outer layers of bones as organs. Cancellous bone CT forms the inner, lighter portion of bones.

FAT CONNECTIVE TISSUE

A large fraction of the volume of a fat cell is occupied by a droplet of fat. This droplet has its own membrane, in addition to the outer membrane of the cell. The remaining components of the fat cell, including the nucleus, are found in an outer layer of cytoplasm surrounding the droplet of fat.

Fat connective tissue has a matrix of lipid (oil or fat). There may be yellow fat CT or brown fat CT.

Fat CT acts as a packing material among the organs, nerves, and vessels. Fat CT also helps to insulate the body from both heat and cold. Some fat CT serves as a high-energy storage area.

BLOOD "CONNECTIVE TISSUE"

Some experts consider blood to be a type of connective tissue. Blood will be discussed in lesson 9.