Skeletal Structure and Histology

Skeletal System

Primary functions

1. Movement - acts as lever system, directs forces generated by muscles across joints
2. Support - weight (load) bearing, framework for soft tissue attachment
3. Protection - surrounds vital organs, provides low friction surfaces
4. Storage- Ca⁺² & PO₄, reserve lipid in yellow marrow
5. Blood cell production - RBC, WBC & platelets produced in red marrow

What are the requirements of a system that is capable of the above functions?

Cartilage Tissue

Characteristics

Stiff/rigid but still flexible/elastic/resilient, not particularly strong, avascular

Functions

• Protection - areas of reduced friction at joints
• Support - fetal skeleton preformed in cartilage, support for some soft tissues

Tissue Components (compact connective tissue)

1. Chondrocytes - oval cells occur singly or in groups
   1. located in small spaces called lacunae surrounded by matrix
   2. dependent on diffusion for nutrients and gases
2. Matrix - firm gel-like ground substance with network of fibers
   1. proteoglycans containing chondroitin sulfate
      1. proteoglycans trap water (~75%) to produce stiffness, compression slowly forces water out and deforms cartilage, at load release there is a rapid elastic recovery and slower rehydration
   2. type and amount of fibers determines mechanical properties, collagen fibers provide strength
   3. mostly avascular, secretes antiangiogenesis factor
3. Perichondrium - outer dense fibrous C.T. layer + inner layer of chondroblasts*

Growth and Replacement

1. Interstitial growth - occurs within the matrix
   1. chondrocytes divide and secrete new matrix
   2. occurs in embryonic tissues
2. Appositional growth - adds new layers on outside
   1. chondroblasts* divide and secrete new matrix
   2. occurs throughout adolescence but little in adult
3. In adults cartilage seldom grows and heals slowly

Types of cartilage

Distinguished on basis of the amount of matrix and the fiber type present

1. Elastic - very flexible
   1. irregularly arranged branching elastic fibers for increased flexibility
   2. scattered cells in lacunae with less matrix, which maintains shape
   3. ex. external ear, auditory tube, epiglottis, some laryngeal cartilages
2. Hyaline - stiff but flexible
   1. homogenous hydrated matrix resists compressive forces, constantly recycled, secrete smaller molecules with age
   2. collagenous fibers (15-20%) network resists tension and maintains shape
   3. ex. articular cartilage at joints, costal cart of ribs, trachea and bronchi, nose end
3. Fibrous - very strong and durable, low flexibility
   1. dense interwoven bundles of collagenous fibers provide tensile strength (prevent exploding*) and resist tension
   2. ex. intervertebral discs*, pubic symphysis, insertion of tendons or ligaments to bone

Bone Tissue

Bone cells:

1. Osteochondrial progenitor (osteoprogenitor) cells - unspecialized mesenchymal (stem) cells
   1. mitotic cells, can produce osteoblasts and chondroblasts
2. Osteoblasts - bone-forming cells located at edges of bones
   1. secrete organic matrix including collagenous fibers
   2. establish conditions favorable for calcification
   3. nonmitotic, convert to osteocytes when surrounded by calcified matrix
   4. also strips off organic matrix on outside of bone for osteoclasts to resorb bone tissue
3. Osteocytes - mature bone cells in lacunae surrounded by calcified matrix
   1. processes extend through canaliculi to form gap junctions with other processes, allows movement of nutrients and gases
   2. do not form organic matrix but do recycle inorganic matrix
4. Osteoclasts - large motile multinucleated cells at bone edges that break down bone
   1. ruffled border secrete acids and enzymes that dissolve inorganic and organic matrix
   2. derived from common precursor of monocytes and macrophages

Bone matrix:

1. Organic matrix - strong and tough/flexible
   1. collagen fibers in parallel arrays in lamellar bone, random orientation in woven bone
   2. amorphous proteoglycan ground substance including chondroitin sulfate
2. Inorganic matrix - hard/stiff and brittle
   1. calcium phosphate crystals called hydroxyapatite {Ca₁₀(PO₄)₆(OH)₂}
   2. deposited to reflect organization of organic matrix particularly collagen fibers
3. Final properties of bone tissue result from proportions of organic to inorganic matrix
   1. ex. nonweight-bearing bones can have more inorganic matrix (genetic)

Bone density:

Differs in amount of matrix and the amount of marrow spaces.

1. Woven bone - collagen fibers are randomly oriented, initially formed in fetus or at fractures, remodeling converts it into compact or cancellous bone
2. Compact bone - densely packed bony substance arranged in regular lamellae in osteons
   • Compact bone is thickest where stresses are most aligned.  In the diaphysis both axial compressive forces and tension (stretch) occur along the longitudinal axis; osteons run parallel to long axis.
   • Osteon or Haversian system (figure in class)
     • Central canal -  contains longitudinal running vessels & nerves
     • Concentric lamellae - circular
     • Osteocytes in lacunae
     • Canaliculi - thin channels connect lacunae with central canal and each other, cytoplasmic processes of osteocyte extend through
     • Perforating canals (Volkmann's canal) - horizontal canals between adjacent central canals
     • Interstitial lamellae - fill gaps between osteons
     • Circumferential lamellae - follow outer contours of bone, added by appositional growth
3. Cancellous bone or spongy bone - lamellae with osteocytes but no osteons, bone arranged in plates or struts called trabeculae with many large irregular marrow spaces
   • Trabeculae arranged in latticework and oriented along stress lines, cross-bracing prevents buckling during compression, withstand stresses from many directions (bending). Trabeculae withstand large loads without excessive amounts of bone tissue, which reduces weight, spaces contain red marrow

Bones classified by shape:

1. Long bones - longer than wide, shaft with two ends, predominately compact bone
2. Short bones - length equals width, primarily spongy bone, ex. carpals and tarsals
3. Flat bones - thin and flat, layer of spongy bone sandwiched between two thin layers of compact bone, ex. cranium, sternum, ribs, and scapula
4. Irregular bones - complex shapes, ex. vertebrae & middle ear bones
5. Sutural (Wormian) bones -  small variable bones formed along cranial sutures, individual variability
6. Sesamoid bones - small bones formed in tendons, variable in number, ex. patella

Long bone structure

(figure in class)

1. Diaphysis - shaft, mostly compact bone
2. Epiphysis - ends, mostly cancellous bone, contains red bone marrow
3. Articular cartilage - hyaline cartilage on joint surfaces to reduce friction
4. Periosteum - outer dense fibrous c.t. covering bone surface except articular cart, continuous with tendons via perforating (Sharpey's) fibers (collagen fibers embedded in bone lamellae)
   • inner osteogenic layer contains vessels, osteoprogenitor cells, osteoblasts, and osteoclasts
5. Medullary cavity - large central cavity containing yellow bone marrow (lipid reserve) in adult bones, in fetal bones RBC are also produced
6. Endosteum - connective tissue layer lining medullary cavity, containing osteoprogenitor cells, osteoblasts and osteoclasts
7. Nutrient foramen - opening allowing vessels to enter bone
8. Epiphyseal plate/line - area where diaphysis connects to epiphysis
9. Metaphysis - columns of spongy bone that unites epiphyseal plate to diaphysis