Bone Growth and Formation

Differences from Compact Bone: Trabecular bone has a distinct nutritional pathway compared to compact bone previously discussed.
Nutrient Delivery: Unlike contact bone, which relies on a vasculature system through canals feeding into central canals and then osteocytes via canaliculi, trabecular bone is nourished differently.
Canaliculae: These are small holes located along the outer layer, near the endosteum, which is the inner layer of the osteons. These canaliculi facilitate nutrient distribution.
Osteons in Trabecular Bone: The osteons in trabecular bone lack central canals and neurovascular structures. They consist primarily of lamellae and osteocytes.
Direct Diffusion: Nutrients perfuse directly into the trabecular bone, bypassing the complex canal system of compact bone.

Periosteum: The outer membrane covering bone.
Endosteum: The inner membrane lining the medullary cavity and the inner surfaces of osteons.
Lamellae Types:
- Concentric Lamellae: Layers within an individual osteon.
- Circumferential Lamellae: The outer layer of bone that extends and grows in a similar manner to individual osteons, primarily contributing to appositional growth.
Interstitial Lamellae: These are remnants of old osteons that have been replaced, appearing as irregularly shaped lamellae located between complete osteons. They retain their lamellar structure.
Canal System (Perforating Canals/Haversian Canals):
- Blood vessels from the periosteum perforate through the bone.
- These perforating canals (also called Haversian canals) run perpendicular to the long axis, establishing communication between the inner medullary cavity and the exterior.
- They feed along each osteon on the long axis.

Appositional Growth:
- Responsible for the increase in diameter of a bone.
- Mechanism: The inner layers of the periosteum produce daughter cells that differentiate into osteoblasts. These osteoblasts lay down bony matrix on the surface, forming successive layers of circumferential lamellae.
- Osteoblasts trapped within the lamellae differentiate into osteocytes.
- This process is analogous to how a single osteon lays down concentric lamellae, but here it applies to the overall growth of the bone's diameter via the periosteum and endosteum.
- Location: Occurs across each osteon within compact bone and specifically by adding concentric layers within the periosteum, sequentially increasing bone width.
- Medullary Cavity Remodeling: As appositional growth adds layers externally, osteoclasts along the endosteum (and trabecular bone) continuously remodel the internal medullary cavity. This process maintains a proper ratio of medullary cavity size to bone shaft thickness, preventing bones from becoming excessively thick, heavy, and stronger than necessary.
Interstitial Growth:
- Responsible for the increase in length of a bone.
- This will be discussed further, particularly in endochondral ossification.
Key Distinction: Appositional = Diameter; Interstitial = Length. This is a fundamental concept for understanding bone development.

Joint Cavities: An important exception to the rule that compact bone is wrapped by a periosteum. The superficial layer of compact bone within joint cavities is not covered by a periosteum.

Attachment Method: These fibers serve as a critical method of attachment for the periosteum to the underlying circumferential lamellae.
Anchoring: They are anchored into the bone layer, and as the bone hardens, the surrounding environment solidifies around the fibers, creating a strong attachment.

Function: Osteoclasts are large, multinucleated cells rich in enzymes, often referred to as