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Bone and Cartilage

General Anatomy and Musculoskeletal System - THIEME Atlas of Anatomy

1.7 Primordial Skeleton, Prenatal Skeletal Development, and Ossification Centers

Primordial skeleton of an 11-week fetus showing ossification centers (alizarin red stain) and limb bud development

The supporting tissue of the human skeleton (mainly cartilage and bone tissue) arises from embryonic connective tissue (mesenchyme), derived from the middle germ layer (mesoderm). Mesenchymal cells initially differentiate into chondroblasts, which form a miniature model of the skeleton from hyaline cartilage - the "primordial skeleton." This cartilage template is later replaced by bone tissue - a process called endochondral osteogenesis, which forms the largest part of the human skeleton (torso, limbs, and skull base).

Timeline of ossification:

Ossification of the primordial skeleton begins at the end of the 8th embryonic week
Starts from primary ossification centers in the diaphyseal region of long bones (perichondral/desmal osteogenesis)
Then endochondral ossification forms primary ossification centers in the diaphysis
By the 12th week, ossification centers exist in all long bones
Secondary ossification centers (in the epiphyses) only appear after birth - exception: the distal femur and proximal tibial epiphysis exist at birth
At birth, most tarsal bones and all carpal bones are entirely cartilaginous
Only the bones of the cranial roof, parts of the facial bones, and the clavicle ossify directly (membranous osteogenesis - mesenchyme cells become osteoblasts directly)

Limb development: Limb buds appear at the end of the 4th embryonic week as paddle-shaped outpouches of the lateral trunk. The humerus develops first, followed by the ulna, then the radial and carpal elements. Fingers and toes form through interdigital apoptosis - programmed cell death separating the five rays. Failure of apoptosis causes syndactyly (fused digits). Absence of entire limbs = amelia; partial absence = meromelia.

Note: Achondroplasia - a genetic disorder disrupting endochondral ossification - is one of the most frequent causes of disproportionate short stature (short limbs, large skull). Cell division and maturation of chondrocytes in the growth plates is disrupted, meaning all bones produced by endochondral osteogenesis are affected.

1.8 Bone Development and Remodeling

Stages of long bone development - from cartilage model (a) to fully ossified adult bone with articular cartilage (i)

Bone development and remodeling are closely interrelated. During growth, immature woven bone (fiber bone) is constantly replaced by lamellar bone. In the mature skeleton, approximately 10% of the entire adult skeleton is remodeled each year - the skeleton is completely renewed over roughly a 10-year period. This process is a functional adaptation to dominant stress patterns. It also prevents material fatigue, repairs microinjuries, and provides a rapidly available source of calcium.

A - Development of a Long Bone (Endochondral Osteogenesis)

Long bones (humerus, tibia, etc.) form primarily by indirect bone formation - replacing a pre-existing cartilaginous model. However, portions still form by direct bone formation (the perichondrial bone collar allowing growth in thickness).

Stage	Event
a	Cartilaginous model of bone in embryonic skeleton
b	Formation of a perichondrial bone collar (directly from mesenchyme)
c	Differentiation to hypertrophic chondrocytes; mineralization of cartilaginous ECM
d	Ingrowth of a diaphyseal vessel; formation of a primary ossification center
e	Development of proximal and distal growth centers (epiphyseal plates)
f	Appearance of the proximal epiphyseal ossification center (secondary ossification center)
g	Formation of the distal epiphyseal ossification center
h	Closure of the distal epiphyseal plate
i	Closure of the proximal epiphyseal plate (end of skeletal growth, ~18-23 years for most tubular bones)

Key distinction: Osteogenesis = formation of an individual bone. Ossification = formation of bone tissue.

B - Structure of the Epiphyseal Plate

The epiphyseal plate (growth plate) has distinct zones from deep to superficial:

Resting zone - reserve cartilage, adjacent to secondary ossification center
Proliferative zone - active cell division, chondrocytes arranged in columns
Hypertrophic zone - enlarged chondrocytes, matrix mineralization
Zone of calcified cartilage - mineralized matrix, vascular invasion
Zone of ossification - osteoblasts deposit bone on calcified cartilage scaffolding

C - Two Pathways of Osteogenesis (Schematic)

Membranous (direct) osteogenesis: Mesenchymal fibrous tissue → directly to woven bone → membranous bone formation (positional growth in thickness)

Endochondral (indirect) osteogenesis: Hyaline cartilage → woven bone → functional remodeling (in response to greater stresses, e.g., increasing body weight) → Lamellar (mature) bone

Note: Most bones form by indirect osteogenesis. The few exceptions (clavicle, bones of the calvaria) still have portions that develop from direct mesenchyme transformation.

D - Development of an Osteon (after Hees)

The process of functional remodeling begins when blood vessels and osteoclasts invade woven bone:

a Longitudinal section through a resorption canal (osteoclasts drill a vascularized channel equal in diameter to the future osteon)
b Cross section at the resorption canal level
c Transformation zone: osteoprogenitor cells → osteoblasts
d Osteogenic zone: osteoblasts produce bony lamellae concentrically around the vascular canal
e Newly formed osteon

F - Growth and Remodeling in Cancellous (Lamellar) Bone

3D representation of cancellous bone tissue showing osteoblasts, osteoclasts, osteocytes, Haversian vessels, and endosteum in the context of remodeling

Cancellous trabeculae have no direct vascular supply - they are nourished by diffusion from the adjacent medullary cavity. This limits their thickness to only about 200-300 µm. Remodeling of a trabecula involves:

Osteoclasts sitting in Howship lacunae, resorbing bone
Osteoid (uncalcified bone matrix) laid down by osteoblasts
Endosteum lining the marrow spaces
Osteocytes embedded in lamellae, connected by cytoplasmic processes

4.1 The Bony Skeleton and Structure of Tubular Bones

C - Types of Bone

Type	Examples
Long bones (tubular)	Limb bones (femur, humerus, tibia, etc.)
Short bones	Carpal and tarsal bones
Flat bones	Scapula, ilium, bones of the calvaria
Irregular bones	Vertebrae
Pneumatic bones	Facial skeleton and paranasal sinuses (air-filled)
Sesamoid bones	Patella (incorporated in tendons)
Accessory bones	Anomalous supernumerary bones (calvaria and foot); result from failure of fusion of certain ossification centers

D - Structure of a Typical Tubular Bone (illustrated for the Femur)

Full structure of tubular bone (femur): periosteum, compact bone, cancellous bone, osteons, Haversian canals, Volkmann canals, osteocytes, with bone marrow compartments

Gross structure:

Proximal and distal epiphyses - covered by articular cartilage; contain cancellous bone with red (hematopoietic) bone marrow; the bony epiphyseal line marks where the growth plate closed
Diaphysis - the shaft; contains the medullary cavity (yellow/fat marrow in adults) surrounded by compact bone
Apophysis - a bony projection (non-articular eminence) at the epiphysis

Cancellous (spongy) bone (section b):

Lamellar architecture with trabeculae arranged like plywood contiguous plates
Nourished by diffusion from the medullary cavity
Trabeculae are ~200-300 µm thick
Contains active osteoblasts and osteoclasts on trabecular surfaces

Compact bone (section c - 3D diagram):

Structural units = osteons (~1 cm long, 250-350 µm diameter)
Each osteon has a central Haversian canal (longitudinally oriented, carries vessels)
Haversian canals are interconnected by short transverse/oblique Volkmann canals and also connect to vessels of the periosteum and medullary cavity
Each osteon: central Haversian canal surrounded by ~5-20 concentric lamellar systems of osteocytes + ECM
Osteocytes are interconnected by numerous fine cytoplasmic processes
Interstitial lamellae = remnants of old osteons between new ones
External/internal circumferential lamellae = parallel lamellae at the outer and inner surfaces

Periosteum (section e):

Stratum fibrosum (outer fibrous layer) - dense connective tissue
Cambium layer (inner layer) - contains osteoblasts
Sharpey fibers - collagen fibers that anchor the periosteum into the bone matrix

4.4 True Joints: Structure of the Joint Capsule and Hyaline Cartilage

The joint cavity of true joints is enclosed by a joint capsule with two distinct layers:

Joint Capsule Layers

Fibrous membrane (outer layer - Stratum fibrosum):

Dense connective tissue
Reinforced by ligaments
Mechanically stabilizes the joint

Synovial membrane (inner layer - Stratum synoviale):

Intima (innermost layer): contains 1-3 layers of synoviocytes:
- Type A synoviocytes - macrophage-like, phagocytic
- Type B synoviocytes - fibroblast-like, secrete synovial fluid components
Subintima: loose connective tissue containing fat cells, blood vessels, subsynovial fibroblasts
Produces synovial fluid (viscous, rich in hyaluronic acid) for lubrication and cartilage nutrition

Key clinical note: The synovial membrane can regenerate into old age, even after complete removal (synovectomy - necessary in cases of chronic rheumatoid arthritis). The hyaline cartilage, however, cannot regenerate.

Synovial reactions: The synovial membrane reacts to stimuli by increased secretion (articular effusion). Depending on the stimulus:

Mechanical or allergic: clear, thin effusion
Infectious: turbid, pus-filled
Injury (e.g., cruciate ligament rupture): hemarthrosis (bloody effusion)
Pain results from overstretching of the joint capsule and release of inflammatory mediators (prostaglandins, histamine, bradykinin, cytokines)

Hyaline Cartilage

With the exception of the jaw (TMJ) and sternoclavicular joint (which are covered by fibrous cartilage), all articular surfaces are covered by hyaline cartilage. Its thickness ranges from:

1-2 mm in phalangeal joints
5-7 mm in the femoropatellar joint

Hyaline cartilage consists of:

Extracellular matrix (ECM): ~95% of volume - water, collagen type II, proteoglycans
Chondrocytes: ~5% of volume

Zones of hyaline cartilage (determined by the arcade-like collagen fibril arrangement):

Zone	Description
Zone I (tangential)	Collagen fibrils run parallel to the surface; chondrocytes flattened
Zone II (transitional)	Fibrils transition, chondrocytes rounded
Zone III (radial)	Collagen fibrils run almost vertically (radially); columns of chondrocytes
Zone IV (mineralization)	Calcified cartilage, connected to subchondral bone; the tide mark (boundary line) separates zones III and IV
Subchondral bone	The underlying bone plate

Collagen fibril details: Visible only under transmission electron microscopy. In light microscopy they appear "masked" (refractive index matches surroundings). After enzymatic liberation with hyaluronidase, TEM shows empty "cartilage holes" surrounded by a thick collagen fibril mesh.

Proteoglycans - Aggrecan: The most important proteoglycan of hyaline cartilage. Aggrecan consists of a core protein with multiple glycosaminoglycan (GAG) chains (chondroitin sulfate and keratan sulfate) that bind to a hyaluronan backbone via link proteins, forming huge aggregates that attract water and give cartilage its compressive stiffness.

Biphasic viscoelastic model (after Mow): Hyaline cartilage behaves as a biphasic material:

Solid phase = collagen fibril network + proteoglycans
Fluid phase = interstitial water Under compression, fluid is squeezed out; upon release, it flows back in. This biphasic behavior accounts for the viscoelastic (time-dependent) properties of cartilage.

Important limitation: Hyaline cartilage (outside the primordial skeleton) has no perichondrium and no blood vessels. Chondrocytes must be nourished exclusively by synovial fluid (via diffusion and convection). This is why hyaline cartilage has virtually no regenerative capacity following injury - a major reason articular cartilage defects progress to osteoarthritis.

1.9 Ossification of the Limbs

Epi- and apophyseal ossification centers follow a predictable timetable:

Upper limb ossification centers (approximate timing):

Appear at different times (fetal months through postnatal years)
Females typically ossify earlier than males
Synostosis (closure) also occurs earlier in females

Lower limb:

Distal femoral and proximal tibial epiphyseal ossification centers are present at birth (unique among epiphyseal centers)
Other lower limb centers appear in the months/years following birth

1.10 Basic Skeletal Structure and Position of the Limbs

The human limb follows the basic pentadactyl tetrapod pattern:

Segment	Name	Upper Limb	Lower Limb
Proximal	Stylopodium	Arm - Humerus	Thigh - Femur
Middle	Zygopodium	Forearm - Radius + Ulna	Leg - Tibia + Fibula
Distal	Autopodium	Hand	Foot
-	Basipodium	Carpus	Tarsus
-	Metapodium	Metacarpals	Metatarsals
-	Acropodium	Phalanges	Phalanges

Common congenital malformations:

Polydactyly - extra digits
Syndactyly - fused digits (failure of interdigital apoptosis)

Source: General Anatomy and Musculoskeletal System, THIEME Atlas of Anatomy (ISBN 9781626237186)

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