Skeletal System: Bone Anatomy and Function

• Skeletal system functions include support, protection, anchorage for muscles, mineral homeostasis (mostly calcium phosphate), blood cell formation/hemopoiesis (red marrow), and triglyceride storage (yellow/fatty marrow).

Bone Anatomy

• Bones are classified according to their shapes (long, short, flat, and irregular). Compact bone and spongy bone are types of bone that can be seen with the naked eye.
• Long bones are hollow and contain compact bone (dense/hard) surrounding the medullary cavity. The ends of long bones, the proximal and distal epiphyses, contain spongy bone filled with red marrow and are covered in articular cartilage (mostly hyaline). The periosteum is the outer membrane and is a protective lining which allows blood vessels through. The endosteum is the inner membrane and is a protective inner lining.

Microscopic Anatomy

• Cell types involved include Osteogenic/Osteoprogenitor (stem cells that mature into osteoblasts), Osteoblasts (form calcified matrix and eventually become trapped in lacunae, maturing into osteocytes), Osteocytes (maintain bone tissue and calcium homeostasis), and Osteoclasts (degrade/resorb bone).
• Osteons are the functional units of compact bone and are weight-bearing pillars.
• A single osteon contains a central/Haversian canal (allows blood vessels and nerve fibers through), lamellae (concentric rings around central canals), osteocytes in lacunae (mature cells that maintain bone tissue), and canaliculi (microscopic cracks within lamellae that allow nutrients to diffuse).
• Trabeculae align at the bone's stress lines and prevent hollow bone from collapsing.
• Trabeculae contain lamellae (irregularly arranged layers), osteocytes in lacunae, and canaliculi, similar to compact bone.

Bone Development and Homeostasis

• Ossification is the process of bone formation.
• In the first 8 weeks, the embryo's skeleton is hyaline cartilage. After 9 weeks gestation, the developing fetus will convert from cartilage to bone
• Prenatal ossification occurs in two ways: intramembranous ossification and endochondral ossification. Intramembranous ossification is for ossifying flat bones. An ossification center forms and contains osteoblasts. Matrix is formed and trabeculae develop into spongy bone, and periosteum develops. Endochondral ossification is for ossifying long bones. A primary ossification center forms in the diaphysis and the medullary cavity hollows out, also a secondary ossification center forms in the epiphyses.
• Postnatal ossification, which forms in the epiphyses from infancy to early adulthood. Interstitial ossification is when bone lengthens. Chondrocytes within lacunae gradually die and become calcified.
• The epiphyseal plate contains five zones of ossification: a quiescent or resting zone (normal hyaline cartilage), a proliferation or growth zone (chondrocytes undego mitosis that lengthens bone), a hypertrophic zone (lacunae start to swell and chondrocytes start to die), a calcification zone (lacunae rupture, chondrocytes die & calcium invades), and an ossification zone (brand new bone is formed).

Appositional Ossification

• Appositional ossification thickens bone
• Osteoblasts deposit bone on the outer surface to thicken bone
• Osteoclasts widen the medullary cavity from within
• Osteoblasts outpace osteoclasts
• Results in thicker, stronger but still hollow & light bones

Remodeling and Repair

• Throughout life, bones are remodeled and repaired
• The ability to remodel bones helps maintain bone homeostasis
• Remodeling and homeostasis are primarily maintained by osteoblasts and osteoclasts
• Bone homeostasis maintains blood calcium levels by monitoring blood calcium levels
• Calcium is important for skeletal, nervous, and muscular system functions
• Too much or too little calcium is detrimental to the body.
• A negative feedback mechanism ensures normal blood calcium levels. if there is hypercalcemia, or elevated calcium, the thyroid gland detects elevated calcium, secretes calcitonin to build bone, suppressing the osteoclasts activity, and calcium levels go back down to normal.
• If there is hypocalcemia, or low blood calcium, the parathyroid glands detect low blood calcium. It secretes parathyroid hormone (PTH), which stimulates osteoclasts to degrade bone and trigger the kidneys to conserve calcium, with a result that calcium levels go back up to normal.
• Bones respond to mechanical stress. Wolff’s Law states that bone grows or remodels in response to demands placed on it.

Bone Repair

• Bone repair involves internal bleeding. A hematoma forms, and blood will clot at the fracture site. A soft callus forms where the clot was, then a bony callus forms as the soft callus is ossified. Finally, bone remodeling occurs to reshape it.

Homeostatic Imbalances

• Osteomalacia is the softening of bones, and rickets causes bowed legs and other bone deformities in children. In rickets, epiphyseal plates do not calcify and widen instead.
• Osteoporosis involves bone degradation outpacing bone formation, causing breakdown of bone to happen rapidly.
• Fractures can be simple/closed or compound/open (breaks the skin, bone popping out of the skin). An impacted fracture is when the bone stays within the skin (bones jammed in with each other). A comminuted fracture is when the bone is shattered or fragmented. A compression fracture is when bones are crushed / applying pressure. Compression fractures occur in vertebrae.
• A greenstick fracture is a partial fracture. Epiphyseal fractures occur an epiphyseal plates, which is cartilage/soft bone, and are common in children. A depressed fracture is a caving end of a hallow skull. A spiral fracture is a twisting force. Fractures are common in young athletes.