Introduction - components The Bones - about 206 –Bone tissue, or osseous tissue, is the major component of the skeletal system. Bones are dynamic organs made up of several tissues types.
Introduction - Functions The skeletal system has a variety of functions: –Support: bones are the body’s infrastructure –Storage of minerals: calcium salts; 98% of the body’s calcium is in the bones and adipose –Hematopoiesis: the bone marrow produces new blood cells –Protection: many delicate organs are surrounded by bone –Leverage: muscles pull on bone to produce movement
Structure of Bone Bones: –Support connective tissues: Cells Solid matrix containing calcium salts –hydroxyapaptite –Outer covering called periosteum: Continuous with the deep fascia –Inner cellular lining is called the endosteum.
The Histological Organization of Mature Bone The matrix: –2/3 of bone weight is calcium phosphate: Hydroxyapatite crystals: –Very resistant to compression Collagen fibers: –1/3 of the bone matrix: Very resistant to stretch Collagen and hydroxyapatite make bone tissue extremely strong. Cells account for about 2–3% of bone tissue.
Cells of Mature Bone Osteocytes = mature cells: –Maintain bone tissue Osteoblasts immature, active cells: –Found on inner and outer surfaces of a bone. –Osteoblasts produce osteoid. –The process of making new bone is called osteogenesis. Osteoprogenitor cells: –Found on inner and outer surfaces of a bone. –Divide and differentiate to form new osteoblasts. Osteoclasts are giant multinucleated cells: –Perform osteolysis
Functional Differences between Compact and Spongy Bone Figure 5.3a Anatomy of a Representative Bone Epiphyses, or ends The diaphysis, or shaft The metaphysis: –Connecting region between the epiphyses and diaphysis
Figure 5.4 The Periosteum and Endosteum The Periosteum and Endosteum
Bone Development and Growth Before six weeks of development the skeleton is cartilage. Osteogenesis is bone formation. –Ossification is bone replacing existing tissue. Calcification is the process of depositing calcium salts into tissues.
Factors Regulating Bone Growth Ions: –Calcium, phosphate, magnesium, citrate, carbonate, sodium Vitamins: –Vitamins A and C –Vitamin D derivatives
Factors Regulating Bone Growth Parathyroid hormone (PTH) acts to increase overall availability of calcium ions in the blood. –Increased osteoclast activity is the direct result of PTH levels. Calcitonin is the antagonist of PTH. Growth hormone and thyroxine increase osteoblast activity leading to bone growth. Sex hormones increase bone growth dramatically during puberty.
Hormonal Control of Bone Tissue Sex Hormones: (estrogen, progesterone, testosterone) Calcitriol: Human Growth Hormone (hGH): Thyroxine: Calcitonin: Parathyroid Hormone: (Thyroid gland) (Parathyroid gland) (Skin and kidneys) (Ovaries and Testes) (Pituitary gland) Stimulate Osteoblasts Inhibits Osteoclasts Stimulates Osteoclasts Increases Ca 2+ absorption from intestine.
Bone disorders Ricketts Osteoporosis Osteomyelitis Osteomalacia Pagets Disease Osgood-Schlatter
Figure 5.8 Epiphyseal Cartilages and Lines Endochondral Ossification
STEPS OF ENDOCHONDRAL OSSIFICATION 1. The perichondrium covering the hyaline cartilage “bone” is infiltrated with blood vessels. 2. Osteoblasts secrete osteoid against the hyaline cartilage diaphysis, encasing it in a bony collar.
STEPS OF ENDOCHONDRAL OSSIFICATION 3. Chondrocytes within the diaphysis hypertrophy and the surrounding cartilage matrix starts to be calcified. 4. The chondrocytes, however, die and the matrix begins to deteriorate.
STEPS OF ENDOCHONDRAL OSSIFICATION 5. In month 3, the forming cavities are invaded by a collection of elements called the periosteal bud. 6. The entering osteoclasts partially erode the calcified cartilage matrix.
7. Osteoblasts secrete osteoid around the remaining fragments of hyaline cartilage forming trabeculae of spongy bone.
STEPS OF ENDOCHONDRAL OSSIFICATION 8. As the primary ossification center enlarges, osteoclasts break down the newly formed spongy bone and open up a medullary cavity in the center of the diaphysis.
STEPS OF ENDOCHONDRAL OSSIFICATION 9. The epiphyses remain formed of cartilage until shortly before or after birth. 10. Secondary ossification centers form in the epiphyses. The events of ossification are like the events of the diaphysis, except, that spongy bone mains in the internal and no medullary cavity forms.