Histology of developing bone part I

mesenchymal cells

• the cells of connective tissue, such as chondroblasts, osteoblasts and
• fibroblasts.

Intramembranous Ossification 

Endochondral Ossification

produce flat bone

long bone

produce some kind of collagen

derive from mesenchyme

develops around the growing cartilage.  This will contain the blood supply and stem cells for the cartilage.

type II collagen and cartilage-specific matrix components.

type I collagen produce by fibroblast in one direction 

resist shear force

shear force vs compression force

type I collagen 

found in ligament, osteoblast in bone

type I collagen (Strength) and mineralized matrix (hardness)

3 layers: outer table, Diploe and inner table

1. Mesenchymal cells condense into a vascularized sheet of cells, and differentiate into osteoblasts.

2. osteoblast begin secreting and calcifying osteoid

3. Some osteoblasts become completely surrounded by osteoid, and differentiate into osteocytes.  This process produces islands of woven bone.

4. Trabecular bone forms, a rudimentary periosteum forms from mesenchyme, blood vessels grow into bone islands.

5. Woven bone is remodeled to compact bone around the periphery to create inner, outer circumferential lamellae, which are represented by the outer and inner tables

maintain bones

one direction

The process of forming the bones of the skull is not complete by birth. 

As a result, the incompletely ossified areas between the bones are covered only by periosteum and primitive connective tissue. 

• We know these areas as “soft spots” or fontanelles.
•  
• Rationale: child birth and cerebral growth

unmineralized bone matrix

cavity where bone marrow reside 

trabecular bone

• Development of a long bone – requires
• that the articular surface remain largely unchanged throughout process, plus subject must comply with birth canal size restrictions, but still have bones capable of being used and capable of
• increasing in length

1. The process of forming a long bone begins with the construction of a template made from cartilage.

2. Osteoblasts differentiate from cells in the perichondrium, secrete osteoid around the center of the cartilage, forming a bone collar.

3. The bone collar blocks nutrients from reaching chondrocytes located within the cartilage. These chondrocytes swell up and degenerate.  They secrete alkaline phosphatase (a bone matrix mineralization enzyme) and enlarge their lacunae.

• 4. Blood vessels from  the perichondrium
• infiltrate this tissue, they bring osteoprogenitor cells (these cells differentiate into osteoblasts).

• 5. .  Osteoblasts adhere to the mineralized cartilaginous matrix and secrete osteoid,
• which is transformed into woven bone.

child birth: right shape but not big size 

continue to development

• will form between the metaphysis and
• the epiphysis. 

• –This process is initiated by the growth of blood vessels into the epiphysis, which
• brings a supply of  osteoprogenitor
• cells into the center of the epiphyseal cartilage.

• –The cartilage in the epiphysis will be gradually replaced with bone marrow in a
• similar manner as the primary ossification center.

–The articular cartilage will be spared from ossification.

Located in the metaphysis

Site at which chondrocytes proliferate, degenerate, and are replaced by bone resulting in increases in bone length during adolescence and young adulthood.

–1  Resting Zone

–2  Proliferating Zone: chondroblast 

–3  Hypertrophic and Degenerative Zone: dying cell and enlarger lacunnae 

–4  Calcification and Ossification Zone

form after birth

vertical: maintain trabecullae integrity

Horizontal: got compressed and dense

small nuclei, 

equal spacing

inactive metabolically

isogenic group

evidence of growth

unequal pacing

vertical and horizontal septa

hypertrophic and dying cell

compressed trabecullae