Difficulty: Intermediate | Prerequisites: Basic cell biology, connective tissue types (Chapter 4 material)
This chapter covers what bones are made of, how they are classified, and how they grow and remodel throughout life. It sits at the foundation of the musculoskeletal unit: you need to understand bone structure before joints (Ch. 9) or muscles (Ch. 10–11) make sense. The skeletal system is not just a scaffold. Bones are living, dynamic organs with their own blood supply, nerve supply, and continuous turnover. If you skipped connective tissue in earlier chapters, go back and review the extracellular matrix concept first.
Bones are organs made of bone tissue, cartilage, and other connective tissues. They are classified by shape (long, short, flat, irregular, sesamoid) and built from two tissue types: compact (dense outer shell) and spongy (internal lattice). Bones grow through either endochondral or intramembranous ossification, and are continuously remodelled by osteoblasts (build) and osteoclasts (break down).
Skeletal system organs
Bones, cartilages, joints, and ligaments. The skeleton is not just bone; it includes all the connective structures that hold it together.
In simple terms, the "skeletal system" is the full package: the hard bones plus the cartilage caps, the joints where bones meet, and the ligaments that bind them.
Cartilage
A firm but flexible connective tissue found in the nose, ears, tracheal rings, intervertebral discs, articular surfaces of joints, costal cartilages of the ribs, and the pubic symphysis. In adults, cartilage is much more limited in distribution than in a developing foetus.
Perichondrium
A dense irregular connective tissue membrane that surrounds cartilage (except at articular surfaces and fibrocartilage). It contains an outer fibrous layer for structural support and an inner cellular (chondrogenic) layer that produces new chondrocytes for appositional growth.
Think of it as the "skin" of cartilage, supplying it with nutrients via diffusion since cartilage itself is avascular.
Compact (cortical) bone
Dense bone tissue forming the hard outer shell of every bone. Under the microscope it is organised into osteons (Haversian systems): concentric rings (lamellae) of mineralised matrix around a central (Haversian) canal carrying blood vessels and nerves.
Spongy (cancellous/trabecular) bone
An open lattice of bony struts called trabeculae, found in the interior of bones. The spaces between trabeculae are filled with red or yellow bone marrow. Spongy bone has no osteons; osteocytes receive nutrients by diffusion from marrow blood vessels.
Osteon (Haversian system)
The structural unit of compact bone. Each osteon consists of a central canal surrounded by concentric lamellae, with osteocytes housed in lacunae connected by tiny channels called canaliculi.
Periosteum
A double-layered membrane covering the outer surface of bones (except at articular cartilage surfaces). The outer fibrous layer is dense irregular connective tissue. The inner osteogenic layer contains osteoblasts and osteoprogenitor cells. Serves as the attachment point for tendons and ligaments via Sharpey's fibres.
Endosteum
A thin connective tissue membrane lining the internal surfaces of bone: the inner surface of compact bone, the trabeculae of spongy bone, and the walls of the medullary cavity. Contains osteoblasts and osteoclasts. Thinner and simpler than periosteum, but equally important for bone remodelling.
Osteoblasts
Bone-forming cells. They secrete osteoid (the organic component of bone matrix, mainly type I collagen) which then becomes mineralised with hydroxyapatite crystals. When an osteoblast becomes trapped in the matrix it secreted, it matures into an osteocyte.
Osteoclasts
Large, multinucleated cells derived from monocyte/macrophage lineage that break down (resorb) bone. They sit on the bone surface in small depressions called Howship's lacunae and secrete acids and enzymes to dissolve the mineral and organic components.
Think of it as: osteoblasts build, osteoclasts clear.
Osteocytes
Mature bone cells trapped in lacunae within the mineralised matrix. They communicate with each other and with surface cells via gap junctions in their long cytoplasmic processes that run through canaliculi. Osteocytes are mechanosensors; they detect strain and signal for remodelling.
Epiphyseal plate (growth plate)
A disc of hyaline cartilage between the epiphysis and diaphysis of a growing long bone. This is where longitudinal bone growth occurs through a sequence of cartilage proliferation, hypertrophy, calcification, and replacement by bone.
Epiphyseal line
The remnant of the epiphyseal plate after growth has ceased (typically by late adolescence). It is a thin line of compact bone visible on X-ray or cross-section, indicating that longitudinal growth at that site is complete.
Endochondral ossification
Bone formation by replacing a hyaline cartilage model with bone tissue. This is how most bones of the body form, including all long bones, vertebrae, and the bones of the pelvis.
Intramembranous ossification
Bone formation directly from mesenchymal (fibrous) tissue, without a cartilage intermediate. This is how the flat bones of the skull and the clavicle form. Osteoblasts differentiate directly within the membrane and begin depositing osteoid.
Diaphysis
The shaft (midportion) of a long bone. Composed of a thick collar of compact bone surrounding a medullary (marrow) cavity.
Epiphysis
The expanded ends of a long bone. Made primarily of spongy bone covered by a thin shell of compact bone, with articular cartilage on the joint surface.
Medullary cavity
The hollow interior of the diaphysis, lined by endosteum, containing yellow marrow (fat) in adults.
Support: provides the rigid framework that holds the body upright and gives shape
Protection: surrounds vital organs (skull around brain, ribcage around heart and lungs, vertebral column around spinal cord)
Movement: serves as levers for muscles; muscles pull on bones to produce movement at joints
Mineral storage and release: bone matrix is a reservoir of calcium and phosphate, released into the blood under hormonal control (parathyroid hormone, calcitonin)
Blood cell formation (haematopoiesis): red bone marrow within spongy bone produces red blood cells, white blood cells, and platelets
Energy storage: yellow marrow in the medullary cavity stores triglycerides (fat)
Long bones: longer than they are wide; have a shaft (diaphysis) and two expanded ends (epiphyses). Examples: femur, humerus, radius, phalanges, metatarsals, metacarpals
Short bones: roughly cube-shaped. Examples: carpals (wrist), tarsals (ankle)
Sesamoid bones: a special type of short bone embedded within a tendon. Example: patella (kneecap)
Flat bones: thin, flattened, often curved. Examples: sternum, ribs, scapulae
Irregular bones: complex shapes that do not fit the other categories. Examples: vertebrae, hip bones (os coxae), facial bones
Compact bone forms the external layer of every bone and makes up the bulk of the diaphysis of long bones
Organised into osteons: central canal, concentric lamellae, lacunae with osteocytes, canaliculi
Between osteons: interstitial lamellae (remnants of old osteons that were partially resorbed during remodelling)
Circumferential lamellae run around the entire bone just deep to the periosteum (outer) and endosteum (inner)
Spongy bone is found in the interior of epiphyses, in the diploe of flat bones, and within irregular and short bones
Trabeculae align along lines of mechanical stress (Wolff's law), making spongy bone surprisingly strong for its weight
No osteons; nutrients reach osteocytes by diffusion from blood vessels in the surrounding marrow
Diaphysis: tubular shaft of compact bone surrounding the medullary cavity
Epiphyses: proximal and distal ends; spongy bone interior with compact bone shell; articular cartilage covers joint surfaces
Metaphysis: the transitional zone between diaphysis and epiphysis; location of the epiphyseal plate in growing bone
Periosteum: covers the diaphysis and non-articular surfaces
Endosteum: lines the medullary cavity and internal bone surfaces
Nutrient artery: enters through a nutrient foramen in the diaphysis; supplies the medullary cavity and inner compact bone
Flat bones: two layers of compact bone (tables) sandwiching spongy bone (diploe). No medullary cavity in the usual sense
Short, irregular, and sesamoid bones: thin shell of compact bone over a spongy bone interior; no diaphysis or epiphysis distinction
Feature | Periosteum | Endosteum |
|---|---|---|
Location | External bone surface | Internal bone surfaces (medullary cavity, trabeculae, canals) |
Layers | Two (outer fibrous, inner osteogenic) | Single thin layer |
Contains | Osteoblasts, osteoprogenitor cells, nerve fibres, blood vessels | Osteoblasts, osteoclasts |
Attachment | Tendons and ligaments attach via Sharpey's fibres | None |
Function | Bone growth (appositional), repair, anchoring | Bone remodelling, repair |
Bone markings arise from mechanical stress (muscle pull, ligament attachment) and from the passage of blood vessels and nerves. They fall into three categories:
Projections (processes) for muscle/ligament attachment: tuberosity (rough, rounded), crest (narrow ridge), trochanter (very large, on the femur only), spine (sharp, slender), epicondyle (raised area above a condyle)
Projections that help form joints: head (rounded articular projection on a narrow neck), condyle (rounded articular surface), facet (smooth, flat articular surface)
Depressions and openings: fossa (shallow basin-like depression), foramen (round opening for vessels/nerves), fissure (narrow slit-like opening), groove/sulcus (furrow), meatus (canal-like passageway)
Endochondral ossification
Begins with a hyaline cartilage model
A bone collar of compact bone forms around the diaphysis via the perichondrium
Cartilage in the centre calcifies and dies, forming cavities
A periosteal bud (blood vessels, osteoblasts, osteoclasts) invades the cavity, creating the primary ossification centre
Secondary ossification centres appear in the epiphyses after birth
Bones formed this way: most bones of the body, including limb bones, vertebrae, pelvis, and bones of the base of the skull
Intramembranous ossification
Mesenchymal cells cluster and differentiate directly into osteoblasts within a fibrous membrane
Osteoblasts secrete osteoid, which mineralises to form trabeculae of woven bone
Woven bone is later remodelled into mature lamellar bone
Bones formed this way: flat bones of the skull (frontal, parietal, occipital, temporal), mandible, and clavicle
The epiphyseal plate is present in growing bones. It has distinct zones (resting, proliferative, hypertrophic, calcification, ossification) that allow the bone to grow in length
The epiphyseal line is what remains after the plate closes. It is a thin marker of bony tissue and indicates that longitudinal growth at that location is finished
Closure timing varies by bone and by sex; generally completes between ages 18 and 25
Healthy bone depends on a balance between osteoblast (building) and osteoclast (resorbing) activity
If osteoclast activity exceeds osteoblast activity: bone density decreases. This is the mechanism behind osteoporosis, where bones become fragile and prone to fracture
If osteoblast activity exceeds osteoclast activity: bone becomes abnormally dense and thick. This occurs in osteopetrosis ("marble bone disease"), where excessively dense bone crowds out marrow space, impairing blood cell production
Paget's disease: involves excessive, disorganised remodelling, producing structurally weak bone despite increased mass
The balance between osteoblasts and osteoclasts is the target of osteoporosis drugs. Bisphosphonates (e.g. alendronate) work by inhibiting osteoclasts, slowing bone resorption and allowing osteoblasts to keep building. This is also why weight-bearing exercise is prescribed to prevent bone loss: mechanical stress on bone stimulates osteoblast activity and trabecular alignment (Wolff's law in action).
Students often think bones are dead, static structures. They are not. Bone is a living tissue with its own blood supply, and it remodels continuously throughout life.
Students confuse the medullary cavity with spongy bone. The medullary cavity is the hollow centre of the diaphysis (filled with yellow marrow in adults). Spongy bone is a lattice of trabeculae found in the epiphyses and interior of other bones.
Students mix up periosteum and perichondrium. Periosteum covers bone. Perichondrium covers cartilage. Both have similar layered structures but surround different tissues.
Students assume all bones form the same way. They do not. Flat skull bones and the clavicle form by intramembranous ossification (no cartilage model). Most other bones form by endochondral ossification (cartilage model replaced by bone).
⚠️ Be able to classify any named bone below the skull as long, short, flat, irregular, or sesamoid. Phalanges are long bones (longer than wide), not short bones.
⚠️ Know the difference between endochondral and intramembranous ossification and which bones use each pathway. The clavicle is a common trick: despite being a "long" bone, it forms by intramembranous ossification.
⚠️ Distinguish osteoblasts from osteoclasts and know the consequences of imbalance in either direction.
⚠️ Understand what happens when the epiphyseal plate closes (becomes the epiphyseal line), and know that this marks the end of longitudinal growth.
⚠️ Know the structural units: osteon for compact bone, trabeculae for spongy bone.
True or False: Spongy bone contains osteons.
False. Spongy bone is organised into trabeculae, not osteons. Osteons are the structural unit of compact bone.
Fill in the blank: Bone-building cells are called __________, and bone-resorbing cells are called __________.
Osteoblasts; osteoclasts.
True or False: The clavicle forms by endochondral ossification.
False. The clavicle forms by intramembranous ossification, despite being classified as a long bone.
Fill in the blank: The remnant of the epiphyseal plate in an adult bone is called the __________.
Epiphyseal line.
True or False: The periosteum lines the medullary cavity.
False. The endosteum lines the medullary cavity. The periosteum covers the outer surface of the bone.
Q: Name the organs of the skeletal system.
A: Bones, cartilages, joints (articulations), and ligaments.
Q: What are the five classifications of bone by shape? Give an example of each.
A: Long (femur), short (carpals), flat (sternum), irregular (vertebrae), sesamoid (patella).
Q: Describe the structural difference between compact and spongy bone at the microscopic level.
A: Compact bone is organised into osteons (Haversian systems), each with a central canal surrounded by concentric lamellae, lacunae housing osteocytes, and canaliculi connecting them. Spongy bone is a network of trabeculae with no osteons; osteocytes receive nutrients by diffusion from nearby marrow vessels.
Q: What is the difference between periosteum and endosteum in terms of location and function?
A: Periosteum is a double-layered membrane on the external bone surface. It functions in appositional growth, repair, and serves as the attachment site for tendons/ligaments. Endosteum is a thin membrane lining internal bone surfaces (medullary cavity, trabeculae, canals) and is active in bone remodelling.
Q: A patient has a condition in which osteoclast activity far exceeds osteoblast activity. What would you expect to happen to their bones?
A: Bone density would decrease, making bones porous and fragile. This is consistent with osteoporosis.
Q: What is the functional significance of the epiphyseal plate?
A: The epiphyseal plate is the site of longitudinal bone growth. Chondrocytes proliferate, hypertrophy, and are replaced by bone on the diaphyseal side, lengthening the bone. Once the plate ossifies into the epiphyseal line, longitudinal growth ceases.
Q: Explain why the clavicle is an unusual case in ossification.
A: The clavicle is classified as a long bone by shape, but it forms via intramembranous ossification rather than endochondral ossification, which is atypical for long bones.
This material connects directly to Chapter 9 (Joints), because bone markings and bone shape determine how joints are structured and what movements they permit. It also connects to Chapter 10 (Muscular System), since muscles attach to bone markings via tendons (anchored through the periosteum). Understanding Wolff's law here will help with exercise physiology concepts later in the course.
skeletal system, bone classification, long bone anatomy, compact bone, spongy bone, cancellous bone, trabecular bone, cortical bone, osteon, Haversian system, lamellae, lacunae, canaliculi, periosteum, endosteum, osteoblast, osteoclast, osteocyte, epiphyseal plate, growth plate, epiphyseal line, endochondral ossification, intramembranous ossification, bone markings, diaphysis, epiphysis, metaphysis, medullary cavity, Wolff's law, osteoporosis, osteopetrosis, Paget's disease, APK2100c, anatomy and physiology, University of Florida