Maybe it is a biology worksheet or a labeled chart on a clinic wall. Either way, you typed "what is the skeletal system" into a search bar and want a straight answer. Here it is, part by part, with the names that anatomy texts actually use.

Plenty of people are looking it up. According to Google Trends, for US searches over the past seven days related to the seed term "skeletal system", the top related queries included "the skeletal system" (100), "what is the skeletal system" (40), "skeletal system bones" (22), "skeletal system function" (22), "function of skeletal system" (20), "skeletal system cells" (19), "bones of the skeletal system" (17) and "skeletal system organs" (13). Among rising queries, "skeletal system key structures" was up 70% and "skeletal system with labels" was up 60%.

A quick note on those numbers. Google Trends scores are relative within each seed term: 100 marks the most searched query in that set, and the others are scaled against it. They are not search counts. A rising percentage measures growth in interest, not volume.

This piece sticks to plain anatomy and vocabulary: what belongs to the system, how bones are grouped and shaped, which cells live in bone tissue, how joints are classified, and what anatomy texts list as the system's jobs. Every definition comes with its source.

What is the skeletal system? The parts list

The clearest starting point is the National Cancer Institute's SEER Training Modules, an online anatomy course from the NCI. Its introduction to the skeletal system names four components: bones, cartilage, ligaments and tendons. The same page says the system accounts for about 20 percent of body weight.

So "the skeletal system" on a chart means more than bones alone:

• Bones: the rigid pieces of the framework, built from bone tissue.
• Cartilage: a firm connective tissue. SEER's joint page describes hyaline cartilage covering the ends of bones in freely movable joints, where it is called articular cartilage.
• Ligaments: SEER describes the outer layer of a joint capsule as made of "the ligaments that hold the bones together."
• Tendons: the cords that link muscle to bone. SEER's muscular system module says tendons form indirect attachments from muscles to the periosteum, the covering of a bone.

Marrow sits inside bones rather than beside them, and it gets its own section below. If you searched "skeletal system organs," the SEER material answers with this parts list rather than a single organ: bones, cartilage, ligaments and tendons working as one system.

How many bones of the skeletal system are there?

According to SEER's page on the divisions of the skeleton, "The adult human skeleton usually consists of 206 named bones." The module groups those bones into two divisions.

Skeletal system bones: axial, appendicular and five shapes

The two divisions

In SEER's scheme, the axial skeleton has 80 bones that form the vertical axis of the body: the bones of the head, the vertebral column, the ribs and the breastbone, or sternum. The appendicular skeleton has 126 bones: the free appendages (the upper and lower limbs) plus their attachments to the axial skeleton, which SEER calls girdles. Add the two and you get the 206 from the section above.

Classifying bones by shape

Anatomy texts also sort bones by shape. SEER's classification page lists four categories, and a fifth term, sesamoid, comes from the dictionary.

The name sesamoid is a description. Dictionary.com defines it as "shaped like a sesame seed" and traces the word through Latin to a Greek term meaning like a sesame seed.

Skeletal system cells and the tissue they build

SEER describes bone as living tissue with its own cell types and two textures.

The three bone cells by name

• Osteoblasts: SEER calls them "bone-forming" cells.
• Osteocytes: SEER calls them "mature bone cells."
• Osteoclasts: SEER says they "resorb or break down bone."

One memory trick: blasts build, clasts cut, and cytes are the settled residents. SEER's tissue page adds that an equilibrium between osteoblasts and osteoclasts maintains bone tissue.

Compact and spongy bone

SEER describes two textures. Compact bone is made of tightly packed units called osteons, or haversian systems, which together look like a solid mass. Spongy bone, also called cancellous bone, is lighter and less dense. It is built from plates called trabeculae and bars of bone, and its cavities hold red bone marrow.

Red and yellow marrow

The National Cancer Institute's Dictionary of Cancer Terms defines bone marrow as soft, spongy tissue with many blood vessels, found in the center of most bones, and names two types. Red bone marrow contains blood stem cells that can become red blood cells, white blood cells or platelets. Yellow bone marrow is made mostly of fat and contains stem cells that can become cartilage, fat or bone cells.

Where is the red marrow? SEER's introduction says that in adults, red marrow is limited to the spongy bone of the skull, ribs, sternum, clavicles, vertebrae and pelvis.

Skeletal system function and how joints are named

The function of skeletal system parts, as SEER lists them

Searches for "skeletal system function" and "function of skeletal system" usually want the textbook list. SEER's introduction gives five, and here they are as the module describes them:

1. Support: bones form a rigid framework that holds the body up against gravity.
2. Protection: the cranial bones surround the brain, the vertebrae surround the spinal cord, and the rib cage surrounds the heart and lungs.
3. Movement: bones work with muscles as simple mechanical lever systems to produce body movement. Skeletal muscles reach the bones through tendons.
4. Mineral storage: bone tissue stores minerals and releases them into the blood.
5. Blood cell formation: SEER uses the term hematopoiesis and places it mostly in red marrow.

Joints by how much they move

A joint, or articulation, is where bones meet. SEER's articulations page classifies joints by movement:

• Synarthroses are immovable. The bones sit very close together with a thin layer of fibrous connective tissue between them. The sutures of the skull are the example.
• Amphiarthroses are slightly movable. The bones are joined by hyaline cartilage or fibrocartilage. Examples include the costal cartilages linking ribs to the sternum, the symphysis pubis and the joints between vertebrae and intervertebral disks.
• Diarthroses are freely movable and, per SEER, make up most joints in adults. They are sometimes called synovial joints.

In a synovial joint, SEER describes articular cartilage on the bone ends, a joint cavity between them, and a dense fibrous capsule around the whole thing. The membrane lining that capsule secretes synovial fluid into the cavity for lubrication.

Reading a skeletal system with labels: the key structures

A chart with forty arrows is easier once you know which family each label belongs to. When you look for skeletal system key structures on a diagram, sort every label into one of these buckets:

• Division: is it axial (head, vertebral column, ribs, sternum) or appendicular (limbs and girdles)?
• Shape: long, short, flat, irregular or sesamoid?
• Tissue: compact bone, spongy bone, cartilage, or marrow inside the bone?
• Connector: a ligament (bone to bone, as part of a joint capsule) or a tendon (muscle to bone)?
• Joint type: immovable, slightly movable or freely movable?

Take the kneecap as a test case. On a labeled chart it is appendicular, and by Dictionary.com's definition it is sesamoid, a small bone formed in a tendon.

Our take

We like anatomy explained the way a good textbook does it: a short list of parts, each with a name and a plain definition. The skeletal system is a strong example. With four components, two divisions, five shapes and three cell names, most labeled charts read like a sentence instead of a puzzle. Every term above traces to a source you can check.

If you want more anatomy tours in the same style, read our plain anatomy tour of what the immune system is made of, which also visits marrow, our guide to what minerals are in nutrition, from major to trace, and our explainer on lean body mass vs skeletal muscle mass. Questions about your own body belong with a clinician.

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This article is about the body vocabulary people search for, not about any product. The bundle's listing is the place to read each of the four labels in full. 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. You can browse the rest of the range in our Vitamins & Supplements collection.

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FAQ

What is the skeletal system, in one sentence?

According to the NCI's SEER Training Modules, it is the body system made of bones, cartilage, ligaments and tendons, accounting for about 20 percent of body weight.

What are the bones of the skeletal system grouped into?

SEER groups the usual 206 named adult bones into the axial skeleton (80 bones: head, vertebral column, ribs, sternum) and the appendicular skeleton (126 bones: limbs and girdles).

What are the skeletal system cells called?

SEER names osteoblasts (bone-forming cells), osteocytes (mature bone cells) and osteoclasts (cells that break down bone).

What is the function of skeletal system structures?

SEER lists support, protection, movement with muscles, mineral storage and blood cell formation, which takes place mostly in red marrow.

What is the difference between red and yellow marrow?

Per the NCI Dictionary of Cancer Terms, red marrow contains blood stem cells, while yellow marrow is made mostly of fat and contains stem cells that can become cartilage, fat or bone cells.