What is connective tissue, and why does an anatomy textbook put bone, fat, tendons and blood in the same family? The short answer is that "connective" describes how a tissue is built, not what it looks like. A connective tissue is a group of cells spread out in a large amount of material called matrix, and that one design covers solids, soft padding and liquids alike. Below are the four tissue families, the parts every connective tissue shares, the named types from loose to dense to bone, and why blood counts as one.

Plenty of people are asking right now. According to Google Trends, for US searches over the past seven days related to the seed term "connective tissue," the rising query "fibrous connective tissue" was up 40%. The top related queries included "what is connective tissue" at 100, "bone connective tissue" at 46, "dense connective tissue" at 46, "blood connective tissue" at 46, "is blood connective tissue" at 33, "connective tissue function" at 31, "connective tissue types" at 30, "types of connective tissue" at 28 and "loose connective tissue" at 21.

Those numbers are relative, not counts. A rising percentage shows growth compared with the previous period, so +40% means that query was searched noticeably more than the period before. A top score is scaled from 0 to 100 against the most searched query in the set, which is set at 100. A score of 21 means a query was searched about a fifth as often as the top one, not that 21 people searched it.

This piece is anatomy vocabulary only. The main sources are the OpenStax textbook Anatomy & Physiology 2e (sections 4.1, "Types of Tissues," and 4.3, "Connective Tissue Supports and Protects"), the National Cancer Institute's SEER Training Modules pages on tissues and connective tissue, and the Cleveland Clinic's "Connective Tissue" explainer.

What is connective tissue? Where it sits among the four tissue families

Start one level up. The NCI SEER Training Modules define a tissue as "a group of cells that have similar structure and that function together as a unit," and state that "there are four main tissue types in the body: epithelial, connective, muscle, and nervous." OpenStax uses the same four.

• Epithelial tissue forms sheets of cells that cover body surfaces, line internal cavities and passageways, and form certain glands, per OpenStax.
• Connective tissue, in the OpenStax wording, "binds the cells and organs of the body together."
• Muscle tissue is excitable tissue that contracts to produce movement.
• Nervous tissue is excitable tissue that carries electrochemical signals between body regions.

The Cleveland Clinic calls connective tissue "a catchall term for tissues that hold your body together and link its different parts." OpenStax notes that all connective tissues come from the same embryonic layer, the mesoderm, the "middle" germ layer in its section 4.1, which a figure there describes as giving rise to "connective tissues, bone, cartilage, blood."

The three parts every connective tissue shares

What makes a tissue connective is its recipe. OpenStax says connective tissues typically share "three characteristic components: cells, large amounts of amorphous ground substance, and protein fibers." Together, the ground substance and fibers make up the matrix that surrounds the cells. SEER puts it another way: connective tissues are "characterized by an abundance of intercellular matrix with relatively few cells."

OpenStax names three main fiber types, all secreted by cells called fibroblasts:

• Collagen fibers, which OpenStax says have "great tensile strength" and resist stretching.
• Elastic fibers, which return to their original shape after being stretched or compressed.
• Reticular fibers, which "remain narrow and are arrayed in a branching network."

OpenStax adds that "the most abundant cell in connective tissue proper is the fibroblast."

Connective tissue function, in textbook terms

Ask about connective tissue function and the textbooks give a short list. OpenStax says connective tissues "support and connect other tissues," that protection "is another major function," and that specialized fluid connective tissues handle the transport of fluid, nutrients, waste and chemical messengers. It also notes that fat cells "store surplus energy in the form of fat." SEER says connective tissues "bind structures together, form a framework and support for organs and the body as a whole."

Types of connective tissue: loose, dense, cartilage, bone and fluid

Textbooks group the connective tissue types differently. OpenStax uses three broad categories, sorted by ground substance and fiber type: connective tissue proper, supportive connective tissue, and fluid connective tissue. The Cleveland Clinic uses two, connective tissue proper and specialized connective tissue. SEER simply lists them: "loose connective tissue, adipose tissue, dense fibrous connective tissue, elastic connective tissue, cartilage, osseous tissue (bone), and blood."

Loose connective tissue

In loose connective tissue, OpenStax says, "the fibers are loosely organized, leaving large spaces in between." It lists three kinds: areolar, adipose and reticular. Areolar tissue "shows little specialization," with fibers laid out in a random, web-like pattern. Adipose tissue "consists mostly of fat storage cells," and OpenStax separates white fat, which stores lipid as a single large drop, from brown fat, which stores it as many small droplets. Reticular tissue forms "a mesh-like, supportive framework for soft organs such as lymphatic tissue, the spleen, and the liver."

Dense connective tissue and fibrous connective tissue

OpenStax says dense connective tissue "contains more collagen fibers than does loose connective tissue." It adds that "there are two major categories of dense connective tissue: regular and irregular." In dense regular tissue the fibers run parallel to each other, and "ligaments and tendons are made of dense regular connective tissue." In dense irregular tissue the fiber direction is random, and "the dermis of the skin is an example." SEER calls this group dense fibrous connective tissue, which is why you will see fibrous connective tissue used as another name for the dense types.

Cartilage

Cartilage is a supportive connective tissue. Its cells are chondrocytes, and OpenStax explains that the spaces they sit in are called lacunae. OpenStax also notes that cartilage is avascular, meaning it has no blood vessels of its own, so nutrients diffuse through the matrix. It names "three main types of cartilage tissue": hyaline cartilage, found in the rib cage and nose and where bones meet at movable joints; fibrocartilage, found in the knee menisci and intervertebral discs; and elastic cartilage, found in the external ear.

Bone connective tissue

Bone is connective tissue with a hardened matrix. OpenStax describes its ground substance as mineralized and containing hydroxyapatite, "a form of calcium phosphate." Bone cells, called osteocytes, sit in lacunae much like chondrocytes do. OpenStax names two forms: compact bone, which "is solid and has greater structural strength," and spongy or cancellous bone, which "looks like a sponge under the microscope."

Is blood connective tissue? Why the liquid counts

This is the question that surprises most people. The answer from every source here is yes. The Cleveland Clinic puts it plainly: "Yes, blood is technically a fluid connective tissue." OpenStax states that "blood and lymph are fluid connective tissues," and SEER includes blood in its list of connective tissue types.

The reason goes back to the recipe. Blood connective tissue has cells spread through a matrix, just like bone or tendon. The difference is that the matrix is liquid. OpenStax describes blood cells that "circulate in a liquid extracellular matrix," and names the formed elements: erythrocytes (red blood cells), leukocytes (white blood cells) and platelets, which it calls "cell fragments involved in blood clotting." Lymph, per OpenStax, "contains a liquid matrix and white blood cells."

So the family is not defined by texture. A tendon, a rib's cartilage, a thigh bone and the blood moving past them all follow the same plan of cells in a matrix. The matrix just changes from fibrous to rubbery to mineral to liquid.

Our take

We like this topic because the answer to what is connective tissue is a design, not a look. Cells sit in a matrix of ground substance and protein fibers, and changing that matrix gives you everything from fat to bone to blood.

For the tissue that bone builds into, read our piece on what the skeletal system is. For the tissue family we only named here, see our guide to skeletal, smooth and cardiac muscle. And since every fiber in this article is a protein fiber, our explainer on what amino acids are covers the building-block vocabulary behind proteins.

Why we carry it at PRSPR

This piece connects to our Inflammation Reset Stack - Anti-Inflammatory Bundle as part of our anatomy vocabulary series. The stack is a four-product bundle of Sarsaparilla (Nature's Iron), Magnesium Glycinate, Black Seed Oil (Nigella Sativa) and Seamoss Capsules. It comes as three amber capsule jars (Magnesium Glycinate, Black Cumin Seed Oil liquid veggie caps and Sea Moss Capsules) and one amber 1 fl oz dropper bottle of Sarsaparilla. This article is about tissue types in a textbook, not about what is in the jars.

Like everything we carry, it is made in a GMP-certified facility and third-party batch tested, so what's on the label is what's in the bottle. It sits in our Bundles collection and our Vitamins & Supplements collection, listed at $122.36.

To read each label in full, see the Inflammation Reset Stack at PRSPR.

FAQ

What is connective tissue in simple terms?

It is one of the four main tissue types. Per OpenStax, connective tissues share three components: cells, ground substance and protein fibers, with the ground substance and fibers forming a matrix around the cells.

Is blood connective tissue?

Yes. OpenStax calls blood and lymph fluid connective tissues, and the Cleveland Clinic says blood is "technically a fluid connective tissue." Its cells move through a liquid matrix.

What are the main types of connective tissue?

SEER lists loose connective tissue, adipose tissue, dense fibrous connective tissue, elastic connective tissue, cartilage, bone and blood. OpenStax groups them as connective tissue proper, supportive and fluid.