1. Topic 4: Cartilage and Bone
Animal Histology BIOL 241
Topic 4:
Cartilage and Bone
Dr. Issa Al-Amri
Department of Biological Sciences & Chemistry
College of Arts & Sciences

2. Introduction Cartilage and bone form supporting tissues of the body.
In these specialized connective tissues, extracellular elements dominate their microscopic appearance
Cartilage:
Cartilage is avascular (lacks blood vessels and nerve).
Has firm extracellular matrix less flexible than connective tissue proper.
Contains chondrocytes embedded in the matrix.
Functions primarily to support soft tissues and assist in development and growth of long bones.
Three types of cartilage: hyaline cartilage, elastic cartilage and fibrocartilage. They vary in matrix components.

3. Cartilage Types of Cartilage: A. Hyaline cartilage
Most abundant cartilage in the body and serves as temporary skeleton in fetus until replaced by bone.
Hyaline cartilage looks translucent (glass-like).
Structure:
Matrix: ground substance (proteoglycan aggregates and chondronectin), and fibers (collagen type II).
Perichondrium: dense, irregular connective tissue surrounds hyaline cartilage (except at articular surfaces) with collagen type I, fibroblasts and blood vessels.
Cells: chondroblasts and chondrocytes.

4. Cartilage Chondroblasts: secrete collagen and the matrix
Chondrocytes: secrete collagen and the matrix
2. Sites and functions :
Ends of ribs at their attachments with sternum
Nose
Larynx
Trachea and bronchi, C-shape cartilage, provides support for soft tissues of trachea.
Ends of long bones in joints
Fetal skeleton

6. Hyaline cartilage :
Figure 1 : Embryonic hyaline cartilage
- Emb. connective tissue (ECT)
- perichondrium (P)
- chondrocytes (arrow)
- matrix (arrowhead)
Figure 2: Hyaline cartilage - Trachea
- pseudostratified ciliated
columnar epithelium (Ep)
- blood-filled vein (V)
- chondrocytes (C)
- isogenous group (IG)
- Perichondrium (P)
- matrix (arrow)
Figure 3: Hyaline cartilage - Rabbit
- fibrous (F), chondrogenic (CG)
- chondroblasts, chondrogenic
cells (arrow)
Figure 4: Hyaline cartilage - Trachea
- connective tissue (CT)
- goblet cells (arrow)
- chondrogenic layer (Cg)
- chondrocytes(C), matrix (M)

8. Cartilage Histogenesis of hyaline cartilage:
Cartilage develop from mesenchymal tissue as follows:
Mesenchymal cells proliferate rapidly by mitotic division and become condensed.
The condensed cells differentiated into chondroblasts.
Chondroblasts synthesis matrix and cells, and cells in the centre become chondrocytes.
In developing cartilage, the central cells are chondrocytes while the peripheral cells are chondroblasts; the superficial layer of mesenchymal tissue will differentiate into perichondrium.

9. Cartilage B. Elastic cartilage 4. Degeneration of hyaline cartilage:
Occurs when chondrocytes undergo hypertrophy and die and matrix become calcified, process become more frequent with age.
Degeneration of hyaline cartilage is a normal process of endochondral bone formation.
B. Elastic cartilage
Similar to hyaline cartilage except for the elastic fibres in its matrix (appears yellowish in color).
Although contain type II collagen, but less degenerating than hyaline cartilage and located in areas where flexible support required.

10. Cartilage C. Fibrocartilage Has no perichondrium.
Characterized by alternating rows of fibroblast-derived chondrocytes.
Matrix: small amount of matrix filled with parallel dense collagen fibers type I.
Fibrocartilage located in areas where support and tensile strength required (example: Intervertebral discs).

12. Elastic cartilage & Fibrocartilage:
Figure 1 : Elastic cartilage - Epiglottis
- chondrocytes(C)
- perichondrium (P)
- elastic fibre (E)
Figure 2: Elastic cartilage - Epiglottis
- fibrous (F)
- chndrogenic region (CG)
- chondrocytes (arrow)
Figure 3: Elastic cartilage – Epiglottis
- High mag of fig-1
- chondrocytes (C)
- nuclei (N)
- elastic fibres (E)
Figure 4: Fibrocartilage - Intervertibral disc
- collagen fibres (CF)

13. Bone
Bone
Bone is a hard connective tissue where fibres impregnated with mineral substances mainly calcium phosphate and calcium carbonate (calcified matrix). It is covered by periosteum and lined by endosteum.
It makes up the majority of adult skeleton.
Functions to protect vital organs, support fleshy structure, and provide calcium reserve (bone contain 99% of body calcium).
Dynamic tissue constantly undergo changes in shape.
Applied pressure results in bone resorption.
Applied tension results in bone formation.

14. Bone Bone structure Bone matrix
Inorganic (calcified) portion of bone matrix (65%) composed mainly of calcium.
Organic portion of bone matrix (35%) consists mainly of type I collagen. Has ground substance contains chondroitin sulfate and keratin sulfate.

15. Bone
Periosteum:
Layer of noncalcified connective tissue covering the outside of the bone; bone interior is lined by endosteum.
Sharpey`s fibres are bundles of periosteal collagen fibres penetrating the bone matrix and binding periosteum to bone.
composed of two layers: outer dense fibrous collagenous layer and inner cellular layer contain osteoprogenitor cells.
Functions to distribute blood vessels to bone.

17. Bone Bone cells Endosteum:
Thin specialized connective tissue line internal surfaces of bone cavities.
Supplies osteoprogenitor cells and osteoblasts for bone growth and repair.
Bone cells
Osteoprogenitor cells:
Spindle-shaped cells derived from embryonic mesenchyme and located in periosteum and endosteum.
Capable of differentiating into osteoblasts. At low oxygen tension, change into chondrogenic cells.

19. Bone Osteoblasts Differentiated cells arising from osteogenic cells.
Located at the bone surface and appear as a layer of cuboidal cells as they secrete organic matrix.
When active, during bone formation, they have a well-developed RER and Golgi complex which involves in protein synthesis for export.

22. Bone
Ostyocytes
Main cells of adult bone; long-living cells that reside within bony lacunae between the matrix lamellae, one osteocyte in each lacuna.
Have narrow cytoplasmic processes extend through canaliculi in calcified matrix.
Maintain communication with each other via gap junctions between their processes.
Nourished and maintained by nutrients, metabolites, and single molecules carried by extracellular fluid that flows through lacunae and canaliculi. Calcium released from bone enters extracellular fluid located within these spaces.
Contain abundant heterochromatin, RER, and small Golgi complex.

23. Bone
Ostyocytes

25. Bone Osteoclasts Multinucleated (5 - 50 nuclei) large, motile cells.
The cytoplasm is acidophilic.
Function:
Bone resorption (osteolysis)
Remodeling and shaping of bone
Phagocytosis of damaged bone

26. Bone
Morphology. Osteoclasts display four regions in electron micrographs:
Basal zone: where most of organelles of osteoclast.
Ruffled border: site of active bone resorption. Made up of cytoplasmic projections (known as Howships lacunae).
Clear zone (surrounds ruffled border): Contains actin filaments help osteoclasts maintain contact with bony surface and isolates region of osetolytic activity.
Vesicular zone: contains exocytotoxic vesicles transfer lysosomal enzymes to Howship’s lacunae and endocytotic vesicles that transfer degraded bone products from Howship’s lacunae to interior of the cell.

28. Osteoclast
Figure 1a : Osteoclast - long bone
- cell surrounding bony surface
(asterisk)
Figure 1b: Osteoclast
- higher mag of fig 1a.
- nucleus (N)
- nucleolus (n)
- ruffled border (RB)
- clear zone (CZ)
- vacuoles (v)
Figure 2: Osteoclast
- nuclei (N)
- basal region (BR)
- Howship’s lacunae (HL)
- ruffled boarder (arrowheads)

29. Bone Classification of bone
Gross observation: cross-sections reveals two types:
Spongy (cancellous) bone composed of interconnected trabeculae (filled with bone marrow).
Compact (dense) bone no trabeculae or bone marrow cavities.
Microscopic observation reveals two types:
Primary bone known as immature or woven bone:
Contains many osteocytes and large, irregularly arranged type I collagen bundles.
Has low mineral content.

34. Bone
Primary bone:
First compact bone produced during fetal development and bone repair.
Remodeled and replaced by secondary bone
Secondary known as mature or lamellar bone:
Compact bone of adults.
Has calcified matrix arranged in regular layers, or lamellae. Each lamellae 3 to 7 µm thick.
Contains osteocytes in lacunae between, and occasionally within, lamellae.

35. Bone
Organization of lamellae in compact long bone (diaphysis of long bones). Consists of the following elements:
Haversian systems (osteons):
Long cylindrical structures run parallel to the long axis of the diaphysis.
Surrounded by amorphous cementing substance.
Interconnected by Volkman canals.
Interstitial lamellae:
Irregularly shaped lamellae between Haversian systems.
Outer and inner circumferential lamellae:
located at the external and internal surfaces of the diaphysis, respectively.

38. Compact bone:
Figure 1 : Decalcified compact bone
- skeletal muscle (SM)
- fibrous periosteum (FP)
- osteogenic periosteum (OP)
- inner circumferential (IC)
- osteone (Os)
- marrow (M)
- interstitial lamellae (asterisk)
Figure 2: Decalcified compact bone
- haversian canal systems (Os)
- lamellae (L)
- interstitial lamellae (IL)
- haversian canal (HC)
- cementing line (arrowheads)
- Volkman’s canal (VC)
Figure 3: Decalcified compact bone
- octyocytes (Oc)
- blood vessel (BV)
- osteoblast (Ob)
- osteogenic cells (Op)
Figure 4: Undecalcified ground compact bone
- haversian canals (HC)
- lacunae (arrows)

39. Bone Histogenesis (bone growth) Bone growth occurs in two ways:
Intramemebranous bone formation.
Interstitial growth by increasing the length through ossification of epiphyseal plates.
2. Endochondrial bone formation.
Growth in width by increasing the diameter of long bones due to continuous bone deposition under the periosteum.
Histogenesis is accompanied by bone resorption. The combination of bone formation and resorption, termed remodeling, occurs throughout life, although it is slower in secondary than in primary bone.

40. Compact bone & Intramembranous
Ossification:
Figure 1 : Undecalcified ground bone
- lamellae (L)
- haversian canal (HC)
- canaliculi (C)
Figure 2: Intramembranous ossification
- trabeculae (T)
- embryonic connective
tissue (ECT)
- osteons (Os)
- haversian canals (HC)
- bolld vessels (BV)
- osteocytes (Oc)
- osteoblasts (Ob)
Figure 3: Intramembranous ossification
- periosteum (P)
- osteoid (Ot)
Figure 4: Intramembranous ossification
- osteoclasts (OcI)
- Howship’s lacunae (arrowheads)

41. Endochondral ossification:
Figure 1 : Epiphyseal ossification
- diaphysis (D)
- marrow (M)
- epiphysis (E)
- secondary centre of
ossification (2nd )
- epiphysal plate (ED)
- trabeculae (T)
Figure 2: Endochondral ossification
- bone marrow (M)
- periosteum (P)
Figure 3: Endochondral ossification-
- region where periosteum and
perichondrium meet (arrowheads)
- subperiosteal bone collar (BC)
- chondrocytes (arrows)
- zone of cell proliferation (ZP)
- zone calcifying cartilage (ZC)

42. Endochondral ossification:
Figure 1 : Endochondral ossification
- multinucleated osteoclast
(arrowheads)
- subperiosteal bone collar (BC)
- periosteum (P)
- blood vessels (BV)
Figure 2: Endochondral ossification
- darker staining bone (arrow)
contains osteocytes
- calcified cartilage (CC)
- osteoblasts (Ob)
- osteoid (Ot)
Figure 3: Endochondral ossification
- osteogenic differentiated into
osteoblasts (arrowheads)
- medullary cavity (MC)
- hematopoietetic tissue (HT)
- periosteal bone collar (BC)
- fibrous (FP)
- osteogenic (Og)

43. Bone
Bone remodeling
Bone constantly remodeled as necessary for growth and to alter its structural makeup to adapt to changing stresses in the environment throughout life.
Remodeling of bone is carried out by phagocytic cells derived from blood monocytes called osteoclasts. Osteoclasts are rich in lysosomes; they secrete collaginase enzymes.
As osteoclasts resorb bone, new bone is laid down by osteoblasts.

44. Bone Repair of bone fracture
At site of fracture, destruction of bone cells and matrix occurs.
Damaged blood vessels produce blood clot.
During repair; blood clot, damaged bone cells and matrix are removed by macrophages.
The osteogenic cells of periosteum and endosteum proliferate intensely, forming cellular tissue around the fracture and extending between the two ends of fracture.
A small fragment of cartilage is formed in the connective tissue of fracture forming temporary callus.
The osteogenic cells are reactivated to form bony callus (intramembranous) to replace the temporary callus after resorption.

45. Joints Joints Joints are regions where bones are joined together.
There are two major groups of joints: synarthroses and diarthroses.
Synarthrosis:
Immovable joints composed of connective tissue, cartilage, or bone. These joints unite the first rib to the sternum and connect the skull bones to each other.

46. Joints
Synarthrosis: sutures of skull

47. Joints Diarthroses (synovial joints)
Permit maximum movement and generally unite long bones. These joints are surrounded by a two-layered capsule, enclosing and sealing the articular cavity.
External (fibrous) layer: tough, fibrous layer of dense connective tissue.
Internal (synovial) layer: lined by a layer of squamous to cuboidal epithelial cells on its surface. Two cell types of this epithelium:
Type A cells are intensely phagocytic and have a well-developed Golgi complex, many lysosomes, and sparse RER.
Type B cells resemble fibroblasts and have a well-developed RER; thus cells probably secrete synovial fluid.

48. Diarthroses (synovial joint)