Skeletal II

Intramembranous Ossification

Bone formed directly from mesenchyme

Early development: conception → 2 months

Steps: Mesenchymal cells → osteoblasts → spongy bone → compact bone

Ex: flat bones of skull and clavicle

Step A: Development of the Ossificatinon Center

Mesenchymal cells → Osteoprogenitor cells

Osteoprogenitor cells → Osteoblasts

Ossification center

Secrete Osteoid

Step B: Calcification and Formation of Early Bone martix

Calcium Salts

Osteoblasts trapped > Osteocyte in lacunae

Osteoid forms spicules around ossification center

• Trabeculae

Form delicate trabecular network

Step C: Formation of Trabeculae and Early Spongy Bone

Spicule Fusion

• Full Trabecular Network

Blood Vessels

Mesenchymal Tissue condensation

• Forms periosteum

Step D : Formation of Compact Bone and Red Marrow

Remodeling Spongy Bone → Compact bone

• At Surface

Inner Spongy bone remains

• Houses red marrow

Periosteum remains active

• Appositional Growth

The mature structure now contains outer compact bone, inner spongy bone, and red marrow within the trabecular spaces

Summary table of Intramembranous Ossification

Endochondral Ossification Overview

Vast majority of bone in the body

• All bones below skull expect for Clavicle

Primary form of ossification after 2 months gestation

Hyaline Cartialge

• The Blueprint

Step A: Formation of the Cartilage Model

Mesenchymal cells → Chondroblasts

Hyaline Cartilage Matrix

Perichondrium

Interstitial Growth - Division of Chondroblasts and chondrosign

Appositional Growth - From new layer around hyaline cartialge

Step B: Growth and calcification of the Cartialge Model

Chondrocyte Hypertrophy

Calcification

• Nutrient Impermeability

Chondrocyte Death

• Lacuna (cavities left behind)

Perichondrium Response

• Forms Periosteum

• Osteoblasts Form

Step C: Invasion by Blood Vessels and Formation of the primary Ossification Center

Nutrient Artery in Diaphysis

• Brings in osteoprogenitors and osteoblasts

Primary Ossification Center

Osteoblasts Secrete Osteoid

• Form Spongy bone

Periosteum Thickens

• Periosteal Bone Collar

Step D : Formation of the Medullary Cavity

Osteoclasts form Medullary Cavity

Bone Grows in length and diameter

Blood vessels grow throughout

At this stage, the bone has a bony diaphysis surrounding a medullary cavity, while the epiphyses (ends) are still mostly cartilage

Step E: Formation of secondary Ossification Centers

We are now around the time of birth

Secondary ossification centers in bone epiphyses

Blood vessels enter the epiphyses

Bone tissue replaces cartialge

• No medullary cavity formation

Articular cartilage remains

Cartilage remains between epiphysis and diaphysis

Step F: Formation of Articular Cartilage and Epiphyseal Plate

Articular cartilage still remains

Epiphyseal plate

• Growth plate

• Contains actively dividing chondrocytes

Longitudinal bone growth

Epiphyseal line

• When bone growth stops

Summary table of Endochondral Ossification

Indentify the following as Endochondral or Intramembranous

Which statement correctly describes the starting material for endochondral ossification?

A: Dense fibrous connective tissue membrane

B: Hyaline cartilage model of the future bone

C: Sheets of mesenchymal cells without cartilage

D: Calcified Periosteum surrounding Osteoblasts

B: Hyaline cartilage model for future bone

Put the Following events in the correct order for endochondral ossification:

1: Formation of secondary ossification centers

2: Formation of cartilage model

3: Formation of primary ossification center

4: Perichondrium becomes periosteum

5: Formation of Medullary cavity

2: Formation of cartilage model

4: Perichondrium becomes periosteum

3: Formation of primary ossification center

5: Formation of medullary cavity

1: Formation of secondary ossification center

Epiphyseal Plate Zones

Resting (Reserve): Storage Zone

Proliferative: Chondrocytes rapid division drive elongaton

Hypertrophic: Chondrocytes Elarge

Calcification: Mineralization chondrocyte death

Ossification: Calvilary: Osteogenitor/ Osteoblaste

Longitudinal Growth

Epiphyseal Plate Closure

During puberty RoCD = RoO

RoO > RoCD

Formation of Epiphyseal line

• Longitudinal growth ceases

For studying

Appositional (Width) Growth

Osteoblast activity in periosteum vs Osteoclast activity in Endosteum

Allow bone to grow in diameter and remain lightweight 

Mechanical Stress Wolff’s Law

Bone adapts to stress (exercise, Weight-bearing)

Hormonal Regulation

Growth Hormone

Thyroid Hormone

Sex Hormone

• Testosterone

• Estrogen - tell considy stop growth