Lecture 16

• Skeletal System


  • Supports the body and organs – framework of the body
  • Protection – encloses the vital organs – the brain, spinal cord, heart and lungs
  • Movement – force placed on the skeleton by the skeletal muscles provides for locomotion
  • Blood formation – location of red bone marrow that is the source of both red and white blood cells
  • Reservoir for metabolic calcium – stores excess nutritional calcium and releases calcium into blood when needed
• Skeletal Anatomy


  • Axial Skeleton – Cranium, vertebral column, thorax, hyoid bone
  • Appendicular Skeleton – Pectoral girdle – scapula and clavicle
  • Pelvic girdle – innominate (pelvic) bones
  • Upper extremities – humerus, radius, ulna, carpals, metacarpals, phalanges
  • Lower extremities – femur, tibia, fibula, tarsals, metatarsals, phalanges
• Bone Types


  • Flat bones – cranial bones, facial bones, costae, sternum, scapula, ilium, ischium and pubis. Laminar outer later, cancellous center
  • Short bones – laminar outer cortex, thick cancellous medulla
  • Long bones – humerus, radius, ulna, metacarpals, femur, tibia, fibula, metatarsals, phalanges. Cancellous heads, Haversian (compact) shafts, medullary cavity
  • Irregular bones – mandible, vertebrae. Combinations of the above-listed types
  • Sesamoid bone – found imbedded in tendon. The largest is the patella
• Cancellous Bone Tissue


  Spongy bone with large open, marrow-filled spaces. Made up of spicules and trabeculae. Highly vascularized. Marrow might be either red (hemopoietic) or yellow (adipose)
• Haversian (Compact) Bone


  Dense matrix of calcium hydroxyapatite, laid down on concentric circles around Haversian canals. Haversian (longitudinal) and Volkmann (transverse) canals contain a nerve, artery and vein, which supply osteocytes. Osteocytes isolated within matrix in small lacunae
• Histology of Haversian Bone
• Histology of Osteon
• Anatomy of Long Bone
• Composition of Osseous ECM

  65% of matrix by weight consists of the inorganic salt calcium hydroxyapatite (calcium, phosphate, hydroxyl and other trace minerals). The remaining 35% collagen Type I, proteoglycans and several other bone-specific proteins that link the collagen to the salt.
• Bone Marrow


  • Red Bone Marrow – hemopoietic (blood-forming) tissue. In children it is scattered throughout most spongy bone. In adults it is primarily concentrated in the sternum and iliac crest of the ilium.
  • Yellow bone marrow – fills the medullary cavity of long bones and replaces red bone marrow in most of the cancellous bones of adults
• Other Bone Types
• Intramembranous Ossification


  1. Mesenchyme (embryonic connective stem cells) condense and form trabeculae (bone sheets)

  2. Osteoblasts (osteogenic cells) adhere to the surface and lay down osteoid tissue. Inner layers become calcified, trapping some osteocytes. The outer layer (periosteum) remains uncalcified.

  3. The outer layer immediately under the periosteum remains as a compact layer (laminar bone), while the inner layers are acted on by osteoclasts to produce large spaces, forming cancellous bone
• Intramembranous Ossification
• Growth of Long Bone - Endochondral Ossification

  1. A layer of chondroblasts first lay down hyaline cartilage surrounded by a perichondrium. The perichondrium produces new cartilage populated by chondrocytes, increasing the thickness of the hyaline cartilage layer.

  2. Eventually osteoblasts replace the chondroblasts and the cartilage is slowly replaced by bone.

  3. Blood vessels invade the deteriorating calcified cartilage, osteoblasts form an osteoid laying down true bone and osteoclasts create a marrow cavity.

  4. This process continues during the first 16-18 years of life where long bone elongation results in most of the height gain observed in children.

  5. Around age 12 and continuing through age 18, the zone of chondrocyte proliferation disappears in each of the long bones, the cartilaginous epiphyseal plate disappears and elongation stops. The hands and feet are the last to stop growing.
• Zone of the Epiphyseal Plate
• Hormonal Control of Bone Elongation

  • The rate of cell reproduction within the epiphyseal plate is determined by the concentration of growth hormone (GH or HGH) released by the pituitary into the blood.
  • During puberty the sex hormones testosterone (males) and estrogen (females) first stimulates elongation (adolescent growth spurt), but as the levels rise both inhibit cell reproduction and further elongation.
  • At the end of puberty, the epiphyseal plate becomes calcified and can no longer elongate.
• Epiphyseal Plates in Adolescent