🦴 Anatomy introductory Lesson 1 of 8 2 min read

The Skeletal System

The 206 bones of the adult skeleton support and protect the body, store minerals, and make blood, while joints and bone cells keep the framework alive and adaptable.

Reading level

What you'll learn

Distinguish the axial from the appendicular skeleton and give examples of each.
List the major functions of bone, including support, protection, mineral storage, and blood cell formation.
Identify the parts of a long bone and the three main bone cell types.
Describe the stages of fracture healing and the role of calcium and vitamin D.
Explain how osteoporosis develops and why it raises fracture risk.

Overview

The adult human skeleton consists of 206 bones organized into two divisions:

Axial skeleton (80 bones) — skull, vertebral column, rib cage; protects CNS and thoracic organs
Appendicular skeleton (126 bones) — pectoral and pelvic girdles, upper and lower limbs; enables locomotion and manipulation

Functions of Bone

Support — structural framework for the body
Protection — cranium shields the brain; vertebrae protect the spinal cord; sternum and ribs protect heart and lungs
Movement — levers for muscle action
Mineral homeostasis — 99% of body calcium and 85% of phosphorus stored in bone; regulated by PTH and calcitonin
Hematopoiesis — red bone marrow in cancellous bone produces blood cells
Energy storage — yellow bone marrow stores fat

Bone Tissue Types

Type	Structure	Location
Compact (cortical) bone	Dense osteon (Haversian) systems	Diaphysis of long bones; outer shell
Spongy (cancellous) bone	Trabecular lattice with red marrow	Epiphyses; flat bones

Long Bone Anatomy

Diaphysis — shaft; compact bone surrounding medullary cavity (yellow marrow in adults)
Epiphysis — ends; covered by articular cartilage
Epiphyseal plate (growth plate) — hyaline cartilage; site of longitudinal growth; closes at ~18–25 years
Periosteum — fibrous outer covering; attachment point for tendons and ligaments
Endosteum — internal lining; contains osteoprogenitor cells

Bone Cells

Cell	Function
Osteoblast	Synthesizes bone matrix (osteoid); becomes osteocyte when surrounded by matrix
Osteocyte	Maintains bone matrix; mechanosensory
Osteoclast	Resorbs bone (acid + protease secretion); multinucleated; derived from monocytes

Joints

Where bones meet, joints determine how much movement is possible. Fibrous joints (skull sutures) are essentially immovable, cartilaginous joints (pubic symphysis, intervertebral discs) allow slight movement, and synovial joints (knee, shoulder, hip) are freely movable and lined with articular cartilage and lubricating synovial fluid. Synovial joints are where muscles produce most visible motion, linking the skeletal and muscular systems.

Fractures and Healing

Fracture healing proceeds through: hematoma → fibrocartilaginous callus → bony callus → bone remodeling (months). Vitamin D and calcium are essential for mineralization.

Clinical Relevance

Osteoporosis — reduced bone mass (T-score ≤ −2.5 on DEXA) increases fracture risk, especially vertebral, hip, and wrist. Bisphosphonates (e.g., alendronate) inhibit osteoclast activity and slow bone loss. Because bone is a living tissue that constantly remodels, weight-bearing exercise and adequate calcium and vitamin D throughout life help maximize peak bone mass and slow later decline.

Going deeper advanced

Extra depth for when you're ready — expanded automatically in Advanced mode.

The RANKL–OPG axis governs remodeling

Osteoblasts and their precursors control osteoclasts through the RANKL–RANK–osteoprotegerin (OPG) system. RANKL binds RANK on osteoclast precursors to drive their differentiation and resorptive activity, while OPG acts as a decoy receptor that blocks RANKL and restrains bone loss. Many pro-resorptive signals, including PTH and inflammatory cytokines, shift the RANKL:OPG ratio toward resorption, which is why the monoclonal antibody denosumab (an anti-RANKL agent) is effective in osteoporosis.

Calcium homeostasis is a three-hormone loop

Serum calcium is held near 8.5–10.5 mg/dL by parathyroid hormone (PTH), calcitriol (active vitamin D), and calcitonin. A fall in ionized calcium triggers PTH, which raises calcium by stimulating osteoclastic resorption, increasing distal tubular reabsorption, and promoting renal 1-alpha-hydroxylase conversion of 25-OH vitamin D to calcitriol, which in turn boosts intestinal calcium absorption. Calcitonin from thyroid parafollicular C cells opposes this by inhibiting osteoclasts, though its physiologic role in adults is minor.

Key terms

Axial skeleton: The 80 bones of the skull, vertebral column, and rib cage that form the body's central axis and protect vital organs.
Appendicular skeleton: The 126 bones of the limbs and the pectoral and pelvic girdles that enable movement and manipulation.
Diaphysis: The shaft of a long bone, made of compact bone surrounding the marrow cavity.
Epiphyseal plate: The growth plate of hyaline cartilage where long bones lengthen until it closes in early adulthood.
Osteoblast: A bone-forming cell that lays down new bone matrix and becomes an osteocyte once trapped within it.
Osteoclast: A large multinucleated cell that breaks down (resorbs) bone by secreting acid and enzymes.
Hematopoiesis: The production of blood cells, which occurs in the red marrow of spongy bone.
Osteoporosis: A condition of reduced bone mass and density that makes bones fragile and prone to fracture.

Check your understanding

5 questions · answers reveal instantly.

1.Which of the following is part of the axial skeleton?
2.Which bone cell is responsible for resorbing (breaking down) bone tissue?
3.Where does the lengthwise growth of a long bone occur during childhood?
4.Roughly what percentage of the body's calcium is stored in bone?
5.Which is the correct order of fracture healing?

Citations & References

Links open publicly available educational and peer-reviewed sources.