HHP:1100 Human Anatomy - Microscopic Anatomy of Bone

Microscopic Anatomy of Bone

Bone Cells

  • Osteoprogenitor cells: These are mesenchymal stem cells that develop into osteoblasts.

  • Osteoblasts:

    • Function: Secrete osteoid, which forms the bone matrix.

    • Location: Found on the surface of bone tissue where new bone is being formed.

  • Osteocytes:

    • Function: Maintain the bone matrix. They are mature bone cells.

    • Development: Some osteoblasts differentiate into osteocytes once entrapped within the bone matrix they have formed.

    • Location: Reside in small spaces called lacunae within the bone matrix.

  • Osteoclasts:

    • Function: Bone resorption, breaking down bone tissue. Characterized by a ruffled border and lysosomes to digest bone matrix.

    • Development: Formed from the fusion of bone marrow cells.

    • Location: Often found in resorption lacunae (Howship's lacunae) on the bone surface.

Compact and Spongy Bone

Compact Bone (Cortical Bone)

  • Basic Unit: The osteon, also known as a Haversian system.

  • Structure:

    • Osteons: Cylindrical structures that run parallel to the diaphysis (shaft) of a long bone.

    • Central Canal (Haversian Canal): A hollow canal at the center of each osteon, containing blood vessels and nerves.

    • Concentric Lamellae: Rings of bone matrix that surround the central canal.

    • Osteocytes: Located within lacunae between the lamellae.

    • Canaliculi: Tiny canals that connect lacunae to each other and to the central canal, allowing for nutrient and waste exchange.

    • Perforating Canals (Volkmann's Canals): Canals that run perpendicular to the central canals, connecting adjacent osteons and blood vessels of the periosteum and medullary cavity.

    • External Circumferential Lamellae: Layers of bone matrix that encircle the entire outer surface of compact bone, just deep to the periosteum.

    • Interstitial Lamellae: Incomplete lamellae located between intact osteons, often remnants of old osteons.

  • Location: Forms the dense outer layer of bones.

Spongy Bone (Cancellous Bone)

  • Structure:

    • Lack Osteons: Unlike compact bone, spongy bone does not contain osteons.

    • Trabeculae: An open lattice of narrow rods and plates of bone.

    • Parallel Lamellae: Trabeculae contain parallel lamellae, along with osteocytes in lacunae and canaliculi.

  • Function: Distributes stress from mechanical forces across its framework, providing strength without excessive weight. It also houses red bone marrow.

  • Location: Found in the interior of bones, especially in the epiphyses of long bones and within flat bones.

Ossification (Osteogenesis)

Overview

  • Definition: Ossification is the process of forming bone connective tissue.

  • Two Patterns:

    1. Intramembranous ossification

    2. Endochondral ossification

Intramembranous Ossification

  • Bones Formed: Produces flat bones of the skull, some facial bones, the mandible, and the central portion of the clavicle.

  • Process: Bones develop directly from a mesenchyme membrane (undifferentiated connective tissue).

    • Step 1: Mesenchymal cells aggregate and differentiate into osteoblasts within the membrane.

    • Step 2: Osteoblasts secrete osteoid, which then calcifies. This calcification entraps osteoblasts within lacunae in the bone matrix.

    • Step 3: Entrapped osteoblasts become osteocytes, maintaining the new bone matrix. Woven bone (immature bone) and surrounding periosteum form.

    • Step 4: Lamellar bone (mature bone) replaces woven bone, as compact and spongy bone structures develop.

Endochondral Ossification

  • Bones Formed: Accounts for the formation of most bones of the skeleton, including upper and lower limbs, pelvis, vertebrae, and the ends of clavicles.

  • Process: Bones develop from a hyaline cartilage model.

    • Step 1: Fetal Hyaline Cartilage Model Develops: Chondroblasts secrete cartilage matrix, creating a hyaline cartilage model of the future bone, encapsulated by a perichondrium.

    • Step 2: Cartilage Calcifies, and a Periosteal Bone Collar Forms: Chondrocytes in the center of the model enlarge and eventually die, leading to calcification of the cartilage matrix. Blood vessels penetrate the perichondrium, stimulating mesenchymal cells to differentiate into osteoblasts, forming a periosteal bone collar around the diaphysis.

    • Step 3: Primary Ossification Center Forms in the Diaphysis: A periosteal bud (composed of blood vessels, nerves, and osteoprogenitor cells) invades the degenerating cartilage. Osteoblasts begin to lay down woven bone, forming the primary ossification center in the diaphysis. The medullary cavity (marrow cavity) begins to form.

    • Step 4: Secondary Ossification Centers Form in the Epiphyses: Around the time of birth, secondary ossification centers appear in the epiphyses (ends of the bone). This process is similar to the primary center but no medullary cavity is formed in the epiphysis.

    • Step 5: Bone Replaces Cartilage (Except Articular Cartilage and Epiphyseal Plates): Bone tissue continues to replace the cartilage model. However, two areas of hyaline cartilage remain: the articular cartilage (covering the joint surfaces) and the epiphyseal plates (growth plates).

    • Step 6: Epiphyseal Plates Ossify and Form Epiphyseal Lines: During late adolescence or early adulthood, the rate of osteoblast activity exceeds chondrocyte proliferation at the epiphyseal plate. The cartilage in the epiphyseal plate is completely replaced by bone, forming a bony structure called the epiphyseal line, indicating that longitudinal bone growth has ceased.

Long Bone Growth and Remodeling

  • Growth in Length: A long bone's growth in length is due to interstitial growth occurring at the epiphyseal plates.

  • Growth in Diameter: Growth in a bone's diameter (thickness) is due to appositional growth, which involves the addition of new bone tissue to the outer surface of the bone by osteoblasts in the periosteum.

Epiphyseal Plate Morphology

  • Site of Interstitial Growth: The epiphyseal plate is a cartilaginous layer within the metaphysis of a growing long bone that functions as the site of interstitial growth, allowing the bone to lengthen.

  • Microscopic Zones: It consists of five distinct microscopic zones:

    • Zone of Resting Cartilage: Nearest the epiphysis, small, scattered chondrocytes anchor the epiphyseal plate to the epiphysis.

    • Zone of Proliferating Cartilage: Chondrocytes divide rapidly and arrange into longitudinal columns of flattened lacunae.

    • Zone of Hypertrophic Cartilage: Chondrocytes enlarge and mature.

    • Zone of Calcified Cartilage: Chondrocytes die, and the surrounding cartilage matrix calcifies.

    • Zone of Ossification: Calcified cartilage is invaded by osteoblasts and capillaries, and new bone tissue is laid down.

Effects of Exercise

  • Exercise, particularly activities that place mechanical stress on bones, can lead to bone remodeling and increased bone density (hypertrophy). A study by Jones et al. (1977) on humeral hypertrophy in response to exercise provides evidence for this phenomenon.

Learning Outcomes (You Should Be Able To…)

  • List and describe the different types of bone cells (osteoprogenitor cells, osteoblasts, osteocytes, osteoclasts) with respect to their location and function.

  • Describe the structural organization of osteons, and identify osteon locations and functions in bones.

  • Compare and contrast the structures and locations of compact bone versus spongy bone.

  • Compare and contrast intramembranous and endochondral bone formation; provide examples of bones that form through each process.

  • Identify the general location of the epiphyseal plates in long bones and outline the sequence of events associated with long bone growth in these regions.

  • Differentiate between interstitial growth and appositional growth processes in long bones.