Source: APK2100C Study Guide SLOs, University of Florida
Difficulty: Introductory–Intermediate | Prerequisites: Basic understanding of connective tissue types (Ch. 4 material helpful)
Tags: skeletal system, bone tissue, cartilage, hyaline cartilage, elastic cartilage, fibrocartilage, compact bone, spongy bone, osteon, Haversian system, osteoblast, osteoclast, ossification, endochondral, intramembranous, epiphyseal plate, bone markings, periosteum, endosteum, bone classification
The skeletal system is the structural scaffold of the body, and this chapter is the foundation for everything that follows in joints (Ch. 9) and muscles (Ch. 10–11). You need to understand not just what bones are made of, but how they grow, how they remodel, and how their microscopic architecture relates to their mechanical function. If you are behind on connective tissue types, revisit that material first, because cartilage and bone are both specialised connective tissues and the vocabulary carries over directly.
The skeletal system consists of bones, cartilages, and joints. Bone tissue comes in two forms (compact and spongy), each with distinct microscopic structures and locations. Bones grow via two ossification pathways, are maintained by osteoblasts and osteoclasts working in balance, and their surface features (markings) reflect the mechanical forces they experience.
Chondrocyte
The predominant cell type in cartilage, responsible for maintaining the cartilage matrix. Think of it as the resident maintenance cell living inside the cartilage.
Hyaline cartilage
The most common type of cartilage, found at the ends of long bones, in growth plates, costal cartilages, and respiratory structures. In simple terms, this is the smooth, glassy cartilage that caps your joint surfaces.
Elastic cartilage
Cartilage with extra elastin fibres, found in the epiglottis and outer ear. Think of it as the bendy, springy cartilage in structures that need to flex and snap back.
Fibrocartilage
Tough, dense cartilage found in the pubic symphysis, knee menisci, and intervertebral discs. In simple terms, this is the heavy-duty shock absorber cartilage.
Perichondrium
A layer of irregular dense connective tissue proper surrounding most cartilage. It has an outer fibrous layer and an inner chondrogenic layer. Think of it as a protective wrapper that also feeds the cartilage by diffusion. Note: it is not found on articular cartilage or fibrocartilage.
Osteon (Haversian system)
The structural unit of compact bone: a cylindrical structure running parallel to the bone's long axis, containing a central canal, concentric lamellae, osteocytes in lacunae, and canaliculi. Think of it as a tiny tunnel system with rings of bone layered around a central blood supply.
Central (Haversian) canal
The channel at the centre of an osteon that carries blood vessels and nerves through compact bone.
Lamellae
Concentric layers of bone matrix surrounding the central canal in an osteon. In simple terms, the rings you would see if you sliced through an osteon like tree rings.
Osteocyte
A mature bone cell sitting in a small pocket (lacuna) between lamellae. It maintains the bone matrix day to day.
Canaliculi
Tiny canals connecting osteocytes to each other and to the central canal, allowing nutrient and waste exchange. Think of them as the communication tunnels between bone cells.
Trabeculae
Thin, branching plates of bone tissue forming the open lattice framework of spongy bone. They align along lines of mechanical stress.
Diaphysis
The shaft of a long bone, composed primarily of compact bone.
Epiphysis
The expanded end of a long bone, containing spongy bone and covered with articular cartilage.
Metaphysis
The transition zone between diaphysis and epiphysis. Contains the epiphyseal plate in children and the epiphyseal line in adults.
Medullary cavity
The hollow central space within the diaphysis, containing yellow bone marrow (fat storage) in adults and red bone marrow (blood cell production) in children.
Periosteum
A thick, two-layered membrane covering the external bone surface (absent at joint surfaces). The outer fibrous layer is dense irregular CT with blood vessels and nerves. The inner (osteogenic) layer contains osteoblasts and osteoclasts.
Endosteum
A thin, osteogenic membrane lining internal bone surfaces, including the central canals of osteons, all spongy bone trabeculae, and the medullary cavity. Contains osteoblasts, osteoclasts, and osteoprogenitor cells.
Osteoblast
A bone-forming cell that secretes collagen and minerals (calcium, phosphate) to build new bone matrix. In simple terms, the construction crew of bone tissue.
Osteoclast
A bone-resorbing cell that breaks down bone tissue by secreting HCl and lysosomal enzymes, releasing calcium into the bloodstream. Think of it as the demolition crew of bone tissue.
Osteogenesis (ossification)
The process by which bone forms, occurring during embryonic development, childhood growth, lifelong remodelling, and fracture repair.
Intramembranous ossification
Bone formation directly within mesenchyme (membrane-like tissue), without a cartilage template. Produces most skull bones and the clavicle.
Endochondral ossification
Bone formation within a hyaline cartilage template, which is gradually replaced by bone. Produces all bones from the base of the skull downward.
Epiphyseal plate (growth plate)
A layer of hyaline cartilage between the diaphysis and epiphysis of long bones in children and adolescents, responsible for longitudinal bone growth.
Epiphyseal line
The bony remnant of the epiphyseal plate once growth has ceased and all cartilage has been replaced by bone. Marks the end of growth in length.
Hemopoiesis
The production of blood cells (red blood cells, white blood cells, platelets) in red bone marrow.
The skeletal system comprises three organ types:
Bones – provide the rigid framework for the body
Cartilages – reduce friction at joints
Joints – structures that allow movement
Hyaline cartilage – ends of long bones, growth plates, costal cartilages (connecting ribs to sternum), respiratory structures
Elastic cartilage – epiglottis (the flap preventing food from entering the airway), outer ear
Fibrocartilage – pubic symphysis, knee menisci, intervertebral discs
Support – rigid framework for soft tissues and organs
Protection – shields organs from injury (e.g. skull protecting the brain, ribs protecting the heart and lungs)
Movement – attachment points for muscles, enabling movement through joint leverage
Mineral storage – stores calcium and phosphorus, released into the bloodstream as needed
Hemopoiesis – red bone marrow produces red blood cells, white blood cells, and platelets
Energy storage – yellow bone marrow stores lipids as an energy reserve
Metabolism – regulates calcium and phosphate homeostasis
Long bones – longer than they are wide, tubular shape
Examples: humerus (arm), femur (thigh), phalanges (fingers)
Short bones – roughly cube-shaped, similar width and length
Examples: carpals (wrist), tarsals (ankle)
Sesamoid bones are a subtype of short bone, shaped like a sesame seed
Examples: patella (kneecap), pisiform (wrist), hallux sesamoids (base of big toe)
Flat bones – thin, flattened, sometimes curved
Examples: sternum, ribs, scapula
Irregular bones – complex shapes that don't fit other categories
Examples: vertebrae, facial bones, certain pelvic bones
Gross anatomy: dense, solid, few spaces, appears smooth and white
Microscopic anatomy:
Osteons (Haversian systems) are the structural unit – cylindrical, running parallel to the bone's long axis
Each osteon contains a central (Haversian) canal with blood vessels and nerves, surrounded by concentric lamellae
Osteocytes sit in lacunae between lamellae, connected by canaliculi
Interstitial lamellae fill spaces between osteons; circumferential lamellae encircle the bone's outer and inner surfaces
Location: primarily in the diaphysis of long bones; forms the outer layer of all bones
Gross anatomy: porous, lightweight, honeycomb or lattice structure, filled with red or yellow bone marrow between trabeculae
Microscopic anatomy:
No osteons – instead, a network of trabeculae
Osteocytes in lacunae within trabeculae, connected by canaliculi
Trabeculae orient along lines of stress to resist forces in multiple directions
No central canal – nutrients diffuse through capillaries in the surrounding endosteum
Location: epiphyses of long bones; interior of flat, short, and irregular bones; inner layer of flat bones (e.g. skull)
Epiphysis – expanded ends, spongy bone inside, articular (hyaline) cartilage on the surface
Diaphysis – long tubular shaft, primarily compact bone
Medullary cavity – hollow centre within the diaphysis, lined by endosteum, contains yellow marrow in adults / red marrow in children
Metaphysis – transition zone; contains the epiphyseal plate (children) or epiphyseal line (adults)
Periosteum – covers the external surface except at joint surfaces
Endosteum – lines the medullary cavity and all internal bone surfaces
No medullary cavity
Compact bone on either side, spongy bone in the interior
Periosteum – thick, external, two layers (outer fibrous, inner osteogenic), absent at joint surfaces
Endosteum – thin, internal, osteogenic, lines central canals of osteons, spongy bone trabeculae, medullary cavity, and the epiphysis of long bones inside short, irregular, and flat bones
Structural features on bone surfaces, formed by interaction with muscles, tendons, ligaments, and mechanical stresses during movement and growth.
Projections – raised areas serving as attachment points for muscles, tendons, and ligaments
Examples: tuberosity (tibial tuberosity), tubercle (tubercle of rib), epicondyle (medial epicondyle of the humerus)
Joint surfaces – smooth, rounded areas for articulation with another bone
Examples: head (head of femur), facet (of the vertebrae), condyle (lateral condyle of femur)
Depressions and openings – indentations or holes for blood vessels, nerves, or other structures
Examples: foramen (foramen magnum), fossa (olecranon fossa), groove (costal groove of ribs)
Intramembranous ossification – bone forms directly within mesenchyme; osteoblasts secrete bone matrix without a cartilage precursor
Location: most skull bones and the clavicle
Endochondral ossification – bone forms within a hyaline cartilage template, which is gradually replaced
Location: all bones from the base of the skull downward
Epiphyseal plate – hyaline cartilage growth plate present only in children and adolescents, responsible for longitudinal bone growth
Epiphyseal line – the bony remnant once growth is complete and all cartilage has ossified; marks the end of growth in length
Osteoblasts build bone; osteoclasts break it down. The balance between them determines bone health.
Osteoblast overactivity – excessively dense bones (osteopetrosis), fractures, pain, deformities, nerve compression
Osteoblast underactivity – delayed fracture healing, poor bone growth in children, weak bones
Osteoclast overactivity – weak, brittle bones (osteoporosis), fractures (especially spine, hip, wrist), loss of height, stooped posture
Osteoclast underactivity – abnormally dense bones prone to fracture (osteopetrosis), nerve compression from bone thickening
Longitudinal growth (getting longer): a two-step process at the epiphyseal plate – cartilage grows on the epiphysis side, and is replaced by bone on the diaphysis side
Appositional growth (getting wider): osteoblasts in the periosteum add bone to the outer surface (circumferential lamellae), while osteoclasts remove bone from the inner diaphyseal wall at roughly the same rate
Students often confuse osteoblasts and osteoclasts. Remember: osteoblasts build, osteoclasts cleave (break down). Both names share "osteo-" (bone), but the second half of each word is the clue.
Students sometimes think spongy bone is weak or unimportant. It is lightweight and structurally efficient, with trabeculae aligned precisely along stress lines.
The perichondrium is often assumed to surround all cartilage. It does not cover articular cartilage (at joints) or fibrocartilage.
Osteopetrosis can result from either osteoblast overactivity or osteoclast underactivity – both produce abnormally dense bone but through opposite mechanisms.
⚠️ Know the difference between compact and spongy bone at both the gross and microscopic level, and where each is found.
⚠️ Be able to classify any bone below the skull as long, short (including sesamoid), flat, or irregular.
⚠️ Distinguish periosteum from endosteum by location, thickness, and layers.
⚠️ Know which ossification type produces which bones (intramembranous = skull + clavicle; endochondral = everything else from base of skull down).
⚠️ Understand the clinical consequences of osteoblast/osteoclast imbalance (osteoporosis vs. osteopetrosis).
⚠️ The epiphyseal plate is present only during childhood and adolescence – this is a commonly tested detail.
True or false: Spongy bone contains osteons arranged in cylindrical structures.
Fill in the blank: The __________ is a thin, osteogenic membrane lining the medullary cavity and internal bone surfaces.
True or false: The perichondrium is found surrounding articular cartilage at joints.
Fill in the blank: Intramembranous ossification produces most __________ bones and the __________.
True or false: When an osteoclast is overactive, the result is osteoporosis (weak, brittle bones).
Answers: 1. False (spongy bone has trabeculae, not osteons). 2. Endosteum. 3. False (perichondrium is absent from articular cartilage and fibrocartilage). 4. Skull; clavicle. 5. True.
Q: Name the three organs of the skeletal system and state a primary function of each.
A: Bones (provide the framework/support for the body), cartilages (reduce friction at joints), and joints (allow movement).
Q: A patient presents with abnormally dense bones that fracture easily and nerve compression symptoms. What two possible dysregulations could explain this?
A: Osteoblast overactivity or osteoclast underactivity. Both produce osteopetrosis (excessively dense but brittle bone).
Q: Describe the microscopic structure of an osteon.
A: An osteon is a cylindrical structure running parallel to the bone's long axis. It contains a central (Haversian) canal with blood vessels and nerves, surrounded by concentric lamellae of bone matrix. Osteocytes reside in lacunae between lamellae and communicate via small canals called canaliculi.
Q: How does a long bone increase in length during childhood?
A: Through activity at the epiphyseal plate: cartilage proliferates on the epiphysis side of the plate while being replaced by bone on the diaphysis side. This two-step process lengthens the bone until the plate ossifies completely.
Q: Where would you find fibrocartilage in the body? Give three examples.
A: Pubic symphysis, menisci of the knee joint, and intervertebral discs.
This material connects directly to Chapter 9 (Joints), because joint classification depends on whether bones are joined by fibrous tissue, cartilage, or a synovial cavity, and articular cartilage at synovial joints is hyaline cartilage covered by the concepts here. The osteoblast/osteoclast balance also ties into calcium homeostasis, which you will revisit in endocrine physiology (parathyroid hormone, calcitonin). Understanding bone markings (projections, depressions, joint surfaces) is essential for Chapter 11, where you need to know muscle origins and insertions.
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