bone microanatomy

Osteon (Haversian system)

Circumferential lamellae

Lamellae

Perforating (Sharpey’s) fibers

Compact bone

Periosteal blood vessel

Periosteum

Central (Haversian) canal

Perforating (Volkmann’s) canal

Blood vessel

Blood vessel continues into medullary cavity containing marrow

Spongy bone

Lamella

Osteocyte

Lacuna

Canaliculus

Central (Haversian) canal

Canaliculi opening on surface

Trabeculae of spongy bone

Endosteum

Lamellae

Osteocytes in lacunae

One lamella

Canaliculi (lines)

Harversion canal

osteoblasts

osteoclasts

osteocytes in lacunae

intramembranous ossification

bone develops from a mesenchymal or fibrous connective tissue. In the right microenvironment, mesenchyme cells turn into osteoblasts and begin forming bone. Many skull bones and the clavicle from this way.

Endochondral ossification

bone forms by replacing hyaline cartilage. Many long bones form this way in the fetus. Long bones grow in length using this method after birth.

intramembranous ossification

Mesenchymal cells aggregate, differentiate, and begin the ossification process. The bone expands as a series of spicules that spread into surrounding tissues.

(STEP 1/3)

intramembranous ossification

As the spicules interconnect, they trap blood vessels within the bone

(STEP 2/3)

intramembranous ossification

Overtime, the bone assumes the structure of spongy bone. Areas of spongy bone may later be removed, creating marrow cavities. Through remodeling, spongy bone formed in this way can be converted to compact bone.

(STEP 3/3)

Bone spicules

Mesenchyme

blood vessel

spicule of bone

osteoblasts

endochondral ossification

• begins in early embryonic development

• uses hyaline cartilage “skeleton” as models for bone construction

• requires breakdown of hyaline cartilage prior to ossification

• histologically, cartilage breaks down in the order: hypertrophy, calcification, and ossification. This order is repeated later at the epiphyseal cartilages during bone growth in length

hypertrophy, calcification, ossification

histologically, cartilage breaks down in this order:

endochondral ossification

As the cartilage enlarges, chondrocytes near the center of the shaft increase greatly in size. The matrix is reduced to a series of small struts that soon begin to calcify. The enlarged chondrocytes then die and disintegrate, leaving cavities within the cartilage

(STEP 1/6)

endochondral ossification

Blood vessels grow around the edges of the cartilage, and the cells of the perichondrium convert to osteoblasts. The shaft of the cartilage then becomes ensheathed in a superficial layer of bone

(STEP 2/6)

endochondral ossification

Blood vessels penetrate the cartilage and invade the central region; Fibroblasts migrating with the blood vessels differentiate into osteoblasts and begin producing spongy bone at a primary center of ossification. Bone formation then spreads along the shaft toward both ends.

(STEP 3/6)

endochondral ossification

Remodeling occurs as growth continues, creating a marrow cavity. The bone of the shaft becomes thicker, and the cartilage near each epiphysis is replaced by shafts of bone. Further growth involves increases in length and diameter.

(STEP 4/6)

endochondral ossification

Capillaries and osteoblasts migrate into the epiphyses, creating secondary ossification centers

(STEP 5/6)

endochondral ossification

Soon the epiphyses are filled with spongy bone. An articular cartilage remains exposed to the joint cavity; over time it will be reduced to a thin superficial layer. At each metaphysis, an epiphyseal cartilage separates the epiphysis from the diaphysis.

(STEP 6/6)

bone elongation at epiphyseal cartilages

• growth in length cartilage growth and subsequent replacement with bone at the epiphyseal cartilages

• the sequence of zones at the epiphysis and epiphyseal cartilages is:

  • resting (hyaline) cartilage

  • proliferation of chondrocytes

  • hypertrophy (cell size increases) of chondrocytes

  • calcification of the ECM of cartilage (ECM color changes)

  • ossification occurring on top of decaying calcified cartilage

epiphysis

epiphyseal cartilage

diaphysis

1. resting cartilage zone

hyaline cartilage connective tissue with scattered interstitial and appositional growth

2. proliferation zone

hyaline cartilage under goes rapid, sequential, and continuous interstitial growth. chondrocytes line up in rows

3. hypertrophy zone

chondrocytes enlarge in size as proliferation continues

4. calcification zone

mineral deposits in the ECM of cartilage signals the eminent death of the chondrocytes. the color of the ECM on the slide changes

5. ossification zone

osteoblasts begin making osseous connective tissue on top of mineralized cartilage ECM. Osseous ECM stains a different color than calcified cartilage ECM and can be differentiated on the slide.

Periosteal bud and red marrow

ossification zone. What is the area circled blue? What about the red dots?

avascular

one reason cartilage is thin

Periosteum

Sharpy’s fibers

Haversion canals

One osteon showing lamellae

Osteocytes

Endosteum

covering small amount of spongy bone just inside the innermost layer of compact bone

Volkman’s Canal

collagen fibers

______ _____ in each lamella of the osteon change direction in order to add strength to resist compression from top to bottom of long bones

intramembranous bone formation

pink = newly formed osteoid

purple = calcified bone

fibrocartilage

blue = numerous collagen fibers

chondrocytes in lacunae

elastic cartilage

ear? (keratinized)

chondrocytes in lacunae

perichondrium (layer of fibrous connective tissue covering cartilage)

hyaline cartilage

chondrocytes in lacunae

perichondrium (layer of fibrous connective tissue covering cartilage)

osteoclast

larger than osteoblasts & has multiple nucleii

developing long bone

de-calcified spongy bone

osteocytes in lacunae

osteoblasts line surfaces of trabeculae

endosteum = layer around white circles

decalcified compact bone (cross section)

osteocytes in lacunae

central canal & longitudinal section

periosteum

big white dots = harversion canal

ground bone compact (cross section)

osteons

osteocytes in lacunae

canaliculi

haversion canals