A tendon, a slab of belly fat, the gristle in your ear and the blood in your veins look nothing alike. They belong to one tissue family anyway. This page covers the connective tissue types: what they share, the fibers and cells they are built from, and how each type's makeup explains what it does, from the loose packing under your skin to cartilage, bone and blood.
What makes a tissue connective
Compare a slice of tendon with the lining of your trachea from the last topic. The epithelium is almost all cells, packed edge to edge. The tendon is almost all material between cells: tight bundles of protein fibers, with a few flattened cells tucked in rows between them.
That is the signature of connective tissue: relatively few cells scattered through a large amount of extracellular matrix (extra- = outside), the nonliving material the cells make and release around themselves. The matrix, not the cells, gives each connective tissue its properties. A tendon is strong because of its fibers. Cartilage is springy because of its water-holding gel. Bone is hard because its matrix is mineralized.
Two more features set connective tissue apart from epithelium. It has no free surface; it sits inside the body, under and around other tissues. And most of it has a blood supply, though the amount ranges from rich (bone) to poor (tendons) to none (cartilage).
Connective tissues fall into three groups:
- Connective tissue proper: the loose and dense tissues that bind and fill.
- Supportive connective tissue: cartilage and bone, the firm tissues that bear weight.
- Fluid connective tissue: blood, whose matrix is a liquid.
The matrix: ground substance plus fibers
The extracellular matrix has two parts.
Ground substance fills the space between cells and fibers. It is a clear gel of water, trapped by large, sugar-rich protein molecules that hold water the way a sponge does. Its consistency varies: watery in loose tissue, rubbery in cartilage, rock hard in bone once minerals are added. Nutrients and wastes diffuse through it between capillaries and cells.
Fibers run through the ground substance and set how the tissue handles force.
Three kinds of connective tissue fibers
- Collagen fibers (kolla = glue, -gen = producing). Collagen is the most abundant protein in your body. Its molecules twist into ropes, and the ropes bundle into thick, white fibers. They are flexible but barely stretch, and they are very strong when pulled. The collagen in the tendon at the back of your heel carries several times your body weight each time you push off in a run.
- Elastic fibers. These are made mainly of the protein elastin, whose molecules coil and uncoil. An elastic fiber can stretch to about one and a half times its length and snaps back when released. They branch and look thin and yellowish.
- Reticular fibers (reticulum = little net). These are thin strands of a finer form of collagen, coated with sugars. They branch and link into delicate nets that hold cells in place in soft organs.
A useful rule: collagen resists pulling, elastic fibers allow stretch and recoil, and reticular fibers make a scaffold.
The cells: builders, defenders and fat stores
Every connective tissue has resident cells, and most of them came from the same embryonic source (see mesenchyme, below).
- Fibroblasts (fibro- = fiber, -blast = builder) are the most common cells in connective tissue proper. They make the fibers and the ground substance. A fibroblast is flat and spindle-shaped, with long branches. When it slows down and mainly maintains the matrix, it is called a fibrocyte (-cyte = cell). A cut stirs nearby fibrocytes back into active fibroblasts that lay down new collagen.
- Macrophages (macro- = large, phag- = eat) are large cells that engulf bacteria, dead cells and debris by phagocytosis, then break them down in lysosomes. Any cell that eats this way is a phagocyte. Many macrophages develop from a type of white blood cell that leaves the blood and settles in a tissue; others settle in their tissue before birth and renew themselves there. This is a short version: macrophages return in full in the immune system chapter, where you'll see how they help start the body's defenses.
- Adipocytes (adip- = fat) are fat cells. Each stores triglyceride in a droplet.
- White blood cells also move out of capillaries and wander through connective tissue, especially where there is damage.
Loose connective tissue
In loose connective tissue, fibers are few and loosely woven, and there is plenty of ground substance. It comes in three types.
Areolar tissue: the body's packing material
Pinch the skin on the back of your hand and lift it. It slides over the deeper structures because a layer of areolar tissue (areola = small open space) lies underneath. Figure 1 shows its makeup: all three fiber types in a loose web, fibroblasts, macrophages, some fat cells, and a watery ground substance. It sits under almost every epithelium and wraps around vessels and organs. Its ground substance holds much of your interstitial fluid, so when fluid leaks out of capillaries, this is the tissue that swells and puffs up.
Adipose tissue: stored fat
Adipose tissue is loose connective tissue packed with adipocytes. In the common white form, each adipocyte holds one huge droplet of triglyceride that squeezes the nucleus and cytoplasm into a thin rim at the edge. It stores energy, insulates, and cushions organs such as the kidneys and the backs of your eyes. It also releases signals that affect appetite and energy use; you'll meet them in the endocrine chapter.
Brown adipose tissue (brown fat) is different. Its cells hold many small droplets and are packed with mitochondria, whose iron-containing proteins make the tissue look brown. In its mitochondria, a special protein lets hydrogen ions leak back across the inner membrane without making ATP. The energy from the electron transport chain is released as heat instead. Babies carry brown fat between the shoulder blades and around the neck. Adults keep smaller patches in the neck and upper chest, and cold switches them on.
Reticular tissue: a soft scaffold
Reticular tissue is a net of reticular fibers with the cells that make them. It forms the internal framework of soft organs whose cells need support but also need to move around: the spleen, the liver, the small bean-shaped filters of the immune system and the soft interior of many bones.
Dense connective tissue
Dense connective tissue is packed with thick collagen fibers, with little ground substance and few cells, mostly fibroblasts. The fiber direction decides what it resists.
- Dense regular tissue has collagen bundles lined up in parallel, all in one direction, with fibroblasts squeezed in rows between them. It is enormously strong when pulled along that direction and weak sideways. It forms tendons, ligaments and aponeuroses.
- Dense irregular tissue has thick collagen bundles running in many directions, like felt. It resists pulling from any direction. It forms the dermis of your skin and the tough capsules around organs such as the kidneys.
- Some dense tissues are rich in elastic fibers instead. They form the stretchy walls of your largest arteries and a few ligaments that must stretch and recoil.
Compare the two in Figure 2. The tissue in a tendon is pulled one way, by one muscle. The dermis is stretched in every direction as you move, so its bundles run every way.
| Loose connective tissue | Dense connective tissue | |
|---|---|---|
| Fibers | Few, thin, loosely woven | Many, thick, tightly packed collagen |
| Fiber arrangement | Random web | Parallel (regular) or in all directions (irregular) |
| Ground substance | Plenty | Little |
| Cells | Many kinds: fibroblasts, macrophages, adipocytes, white cells | Mostly fibroblasts |
| Blood supply | Good | Poor |
| Main job | Cushion, fill space, hold fluid, store fat, support soft organs | Resist strong pulling forces |
| Examples | Areolar tissue under epithelia, adipose tissue, reticular tissue of the spleen | Tendons, ligaments, dermis, organ capsules |
Tendons and ligaments
Both are dense regular connective tissue. They differ in what they connect.
- A tendon connects a muscle to a bone. It carries the muscle's pull to the bone, always in the same line.
- A ligament connects one bone to another. It holds bones together where they meet and limits how far they can move apart.
- An aponeurosis (apo- = off, neur- = sinew; plural aponeuroses) is a tendon spread into a broad, flat sheet. The flat, tendon-like sheets in the front of your abdominal wall are aponeuroses.
Because dense regular tissue has few cells and few blood vessels, a torn tendon or ligament heals slowly, over months, and the repair is often weaker than the original.
Cartilage
Press the tip of your nose, then bend the top of your ear. Both are cartilage: firm but flexible supportive connective tissue. Its cells, chondrocytes (chondr- = cartilage), sit alone or in small groups in pockets in the matrix. The matrix holds collagen fibers in a ground substance packed with large water-binding molecules. When you load cartilage, water is squeezed out; when the load comes off, water flows back and the cartilage springs back. That makes cartilage an excellent shock absorber.
Cartilage has no blood vessels. Oxygen and nutrients have to diffuse through the matrix from outside. The one partial exception is the outer rim of each knee meniscus, which gets a few vessels from the tissue around the joint. Tears there can heal, while tears in the inner part usually do not. Most cartilage is wrapped in a perichondrium (peri- = around), a layer of dense irregular tissue that carries blood vessels and contains cells that can become new chondrocytes.
There are three types, named for what dominates the matrix:
| Hyaline cartilage | Fibrocartilage | Elastic cartilage | |
|---|---|---|---|
| Fibers in matrix | Fine collagen fibers, too thin to see in a routine slide | Thick, visible bundles of collagen | Collagen plus a dense web of elastic fibers |
| Look | Smooth, glassy, bluish-white | Rows of chondrocytes between coarse fibers | Chondrocytes among dark, branching fibers |
| Property | Smooth, slightly flexible, resists compression | Strongest; resists compression and pulling | Most flexible; bends and springs back |
| Perichondrium | Yes, except on joint surfaces | No | Yes |
| Where found | Articular cartilage, tip of the nose, rings in the trachea wall, most of the embryo's skeleton | Intervertebral discs, menisci of the knee, the joint at the front of the pelvis | Ear flap, epiglottis |
Where cartilage is found
Hyaline cartilage (hyalos = glass) is the most common. As articular cartilage (articul- = joint), it caps the ends of bones where they meet and glide, giving a surface smoother than ice. It also stiffens the nose and forms the C-shaped rings that hold your trachea open. In an embryo, most of the future skeleton first forms as hyaline cartilage, which is later replaced by bone.
Fibrocartilage mixes cartilage with thick collagen bundles. It sits where the body needs both cushioning and resistance to pulling. Each intervertebral disc (inter- = between) is a pad of fibrocartilage between two vertebrae. Each knee has two menisci (menisc- = crescent; singular meniscus): C-shaped fibrocartilage pads that deepen and cushion the contact between the thigh bone and the shin bone.
Elastic cartilage holds many elastic fibers, so it bends and snaps back. It shapes your ear flap and forms the epiglottis, which flips down over your airway every time you swallow.
Bone (osseous) tissue
Osseous tissue (os-, oste- = bone) is the hardest connective tissue. Its matrix is collagen fibers hardened with crystals of calcium-containing minerals. The collagen resists pulling and bending, so bone doesn't shatter; the minerals resist squeezing, so bone doesn't bend under your weight. Without the minerals, a bone would bend like rubber. Without the collagen, it would crack like chalk.
Unlike cartilage, bone is richly supplied with blood vessels, and its living cells sit in tiny cavities throughout the matrix. That is why a broken bone heals in weeks while torn cartilage may never fully heal. The bone chapter covers the cells of bone, bone structure and how bone grows.
Blood as a connective tissue
It seems odd to group blood with bone. But blood fits the definition: cells scattered in a large amount of extracellular matrix, developing, like most other connective tissues, from the middle layer of the embryo. Its matrix is plasma, the liquid part of blood, with water, salts, nutrients and dissolved proteins. Its cells are the red cells (carry oxygen), the white cells (defense) and the platelets (cell fragments that help seal leaks). That makes blood a fluid connective tissue, and its job is transport.
Blood has no visible fibers while it flows. Its fiber-forming protein stays dissolved in plasma until it is needed. When a vessel is cut, platelets stick to the damaged wall and to each other. The dissolved protein then turns into long, sticky threads that tangle around the platelets and trap red cells. The result is a blood clot, a jelly-like plug that seals the leak. This is a short version; the full sequence of clotting returns in the blood chapter.
Mesenchyme: where connective tissues come from
In the early embryo, a loose tissue called mesenchyme (mes- = middle, -enchyme = poured-in tissue) fills the spaces between developing structures. It is embryonic connective tissue: star-shaped mesenchymal cells in a watery ground substance with few fibers. Mesenchymal cells are stem cells. As they divide and differentiate, they become fibroblasts, fat cells, chondrocytes and the cells that build bone. The stem cells that make blood cells arise from the same middle layer of the embryo. That shared origin is a big part of why bone, fat, cartilage and blood count as one tissue family. Small numbers of mesenchymal-type stem cells persist in adult tissues and help with upkeep.
A common mix-up: blood isn't "just a fluid"
Students often say blood can't be connective tissue because it is liquid and has no fibers. The definition of connective tissue is about cells in a large amount of extracellular matrix. Blood fits it. Plasma is its matrix, and its fiber protein is present in dissolved form, forming threads during clotting.