Connective tissues connect, support, and bind body structures together. Unlike other types of tissues, connective tissues are classified more by the stuff in which the cells lie — the extracellular matrix — than by the cells themselves. In most cases, the cells that produce that matrix are scattered within it.
The load-bearing strength of connective tissue comes from a fibrous protein called collagen. All connective tissues contain a varying mix of collagen, elastic, and reticular fibers.
Following are the primary types of connective tissue:
Areolar, or loose, tissue: This tissue exists between and around almost everything in the body to bind structures together and fill space. It’s made up of wavy ribbons called collagenous protein fibers, cylindrical threads called elastic fibers, and amorphous ground substance, a semisolid gel. Various cells including lymphocytes, fibroblasts, fat cells, and mast cells are scattered throughout the ground substance.
Dense regular connective tissue: Made up of parallel, densely packed bands or sheets of fibers, this type of tissue is found in tendons as bundles of collagenous fibers attaching muscles to bone and in ligaments extending from bone to bone, surrounding a joint, and anchoring organs. The two main types of ligament are the following:
White ligaments are rich in collagenous fibers that are sturdy and inelastic.
Yellow ligaments (ligamenta flava) are rich in elastic fibers and tough, although they allow elastic movement. They’re found mostly in the vertebral column.
Dense regular connective tissue has great tensile strength that resists lengthwise pulling forces. Ligaments are more elastic than tendons.
Dense irregular connective tissue: Also known as dense fibrous connective tissue, it consists of fibers that twist and weave around each other, forming a thick tissue that can withstand stresses applied from any direction. This tissue makes up the strong inner skin layer called the dermis as well as the outer capsule of organs like the kidney and the spleen.
Adipose tissue: Composed of fat cells, this tissue forms padding around internal organs, reduces heat loss through the skin, and stores energy in fat molecules called triglycerides. Fat molecules fill the cells, forcing the nuclei against the cell membranes and giving them a ringlike shape. Adipose has an intracellular matrix rather than an extracellular matrix.
Reticular tissue: Literally translated as “weblike” or “netlike,” reticular tissue is made up of slender, branching reticular fibers with reticular cells overlaying them. Its intricate structure makes it a particularly good filter, which explains why it’s found inside the spleen, lymph nodes, and bone marrow.
Cartilage: These firm but flexible tissues, made up of collagen and elastic fibers, have no nerve cells or blood vessels (a state called nonvascular or avascular). Cartilage contains openings called lacunae (from the Latin word lacus for “lake” or “pit”) that enclose mature cells called chondrocytes, which are preceded by cells called chondroblasts.
A membrane known as the perichondrium surrounds cartilage tissue, which also contains a gelatinous protein called chondrin. There are three types of cartilage:
Hyaline cartilage: The most abundant cartilage in the body, it’s collagenous and made up of a uniform matrix pocked with chondrocytes. It lays the foundation for the embryonic skeleton, forms the rib (or costal) cartilages, makes up nose cartilage, and covers the articulating surfaces of bones.
Fibrocartilage: As the name implies, fibrocartilage contains thick, compact collagen fibers. The spongelike structure, with the lacunae and chondrocytes lined up within the fibers, makes it a good shock absorber. It’s found in the intervertebral discs of the vertebral column and in the symphysis pubis at the front of the pelvis.
Elastic cartilage: Similar to hyaline cartilage but with a much greater abundance of elastic fibers, elastic cartilage has more tightly packed lacunae and chondrocytes between parallel elastic fibers. This cartilage — which makes up structures where a specific form is important such as the outer ear, Eustachian tube, and epiglottis — tends to bounce back to its original shape after being bent.
Bone, or osseous, tissue: Essentially, bone is mineralized connective tissue formed into repeating patterns called Haversian systems or osteons. In the center of each system is a large opening, the Haversian canal, that contains blood vessels, lymph vessels, and nerves.
The central canal is surrounded by thin layers of bone called lamellae that contain the lacunae, which in turn contain osteocytes (bone cells). Smaller canaliculi connect the lacunae and circulate tissue fluids from the blood vessels to nourish the osteocytes.
Blood: Yes, blood is considered a type of connective tissue. Like other connective tissues, it has an extracellular matrix — in this case, plasma — in which erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (platelets) are suspended. (Blood also is considered a vascular tissue because it circulates inside arteries and veins.)
Roughly half of blood’s volume is fluid or plasma while the other half is suspended cells. Erythrocytes are concave on both sides and contain a pigment, hemoglobin, which supplies oxygen to the body’s cells and takes carbon dioxide away. There are approximately 5 million erythrocytes per cubic millimeter of whole blood.
Thrombocytes, which number approximately 250,000 per cubic millimeter, are fragments of cells used in blood clotting. Some leukocytes are large phagocytic cells (literally “cells that eat”) that are part of the body’s immune system. There are, however, relatively few of them — less than 10,000 per cubic millimeter.