Connective Tissue Cells


Of the connective tissue cells, fibroblasts are perhaps the most numerous cells of the body overall. Billions of them inhabit the connective tissue found in all of our organs. A fibroblast has a flattened, dark-staining nucleus and a sparse cytoplasm. They begin their lives in close proximity to each other but gradually are pushed apart by a protein that they produce. This protein, called collagen, forms thick, tough fibers found in the environment of the fibroblasts. Collagen fibers make up most of what we call leather and are very strong. They provide a structural support for softer portions of the body.

Fat Cells

A fat cell is a spherical cell that contains a large cytoplasmic droplet filled with oils, or fat. This fat represents a storage form of nutrients. When other cells in the body run out of nutrients, fat cells release fat molecules into the bloodstream. Muscle cells and nerve cells can then use these molecules as fuel to burn and to power their functions.

Fat cells also produce a hormone called leptin, which is released into the bloodstream as the fat cells get bigger and bigger. Leptin is carried to the brain, where it functions as a signal informing the brain that fat cells are filled with fat molecules. This signifies that there is no shortage of fuel in the body. High levels of leptin suppress feeding behavior, so that excess calories are not consumed.

In some cases, the appetite-suppressing effect of leptin becomes dysfunctional.In genetically abnormal strains of rodents, a defective form of leptin is made by fat cells, or else the brain receptors for leptin don’t work properly. As a result, the brain mistakenly interprets an absence of signaling by leptin to mean that there are no spare calories in fat cells. Consequently, a rat or mouse with these defects will overeat enormously and become very fat. In humans with obesity, there is some speculation that this leptin-signaling system is not working properly. The causes for this are not yet known. Most obese people have normal or even supranormal blood levels of leptin, and leptin receptors seem to be functioning. More research on this topic may shed light upon causes of obesity.

Cartilage Cells

Cartilage cells (chondrocytes) inhabit a stronger, more rigid type of connective tissue called cartilage. These cell begin their lives looking much like fibroblasts but then acquire a spherical, pale-staining cytoplasm and a round nucleus. They secrete a special type of collagen into their environment, along with long chains of sugar molecules. Cartilage cells are found in the cartilage of the ribs and at the ends of bones. During aging, cartilage can become abnormal and may be lost from the ends of bones, causing the pain of arthritis.

Cartilage cells produce a protein called chondromodulin that prevents blood vessels from tunneling into cartilage. This poses a potential problem for cartilage cells: if they are far from blood vessels, how are they to obtain the oxygen and nutrients they need for survival? Fortunately, the sugar molecules around the chondrocytes allow the slow passage of oxygen and nutrients from blood vessels outside of the cartilage.

Bone Cells

Bone cells (osteocytes) live in small cavities in bone, another form of rigid connective tissue. They are surrounded by collagen that has acquired massive deposits of calcium, turning the extracellular environment into kind of a rocky prison for these cells. Fortunately for osteocytes, this rocky material does permit the entry of capillaries into bone, so that osteocytes can receive their vital nutrients from the bloodstream. Even osteocytes far away from a blood vessel can still receive nutrients through a network of tiny channels, termed canaliculi. These canaliculi run through the calcified components of bone to connect osteocytes to each other and to blood vessels. Transfer of nutrients through these canaliculi keeps the osteocytes alive.


Blood represents a special type of connective tissue. Blood cells are found flowing through blood vessels that are lined with a simple squamous epithelium. Most blood cells are red blood cells. These cells destroy their cell nuclei during development! They become simple containers filled with hemoglobin, an oxygen-binding molecule that helps the red blood cells carry oxygen around the body.

A small proportion of blood cells are white blood cells. These cells often have oddly shaped cell nuclei, and like red blood cells, are born in bone marrow before being released into the bloodstream. White blood cells help the body fight off infections by bacteria and other organisms. These cells can eat and destroy bacteria or else produce chemicals called antibodies that bind to invading foreign molecules.

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Written by John Young

John K. Young is a retired professor of Cell Biology. He worked in the Department of Anatomy at Howard University College of Medicine in Washington, DC for 35 years, teaching his students about cells. During his career, Dr. Young published scientific articles about a part of the brain called the hypothalamus and also wrote a number of books about cells and about the brain.

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