Biology 322 Human Anatomy Tissues of the Skeletal System Recall: SYSTEMS are composed of one or more organs, all serving a common function ORGANS are composed of one or more types
of tissues, all serving a common function TISSUES are composed of one or more types of cells and their products, all serving a common function Organs of skeletal system = Bones (e.g. femur, ulna, vertebra, mandible) Like all organs, these contain four types of tissue: Epithelium, Connective tissue,
Nervous tissue, Muscle tissue However: Two types of specialized connective tissues predominate: Cartilage Tissue & Bone Tissue each of which is surrounded by dense irregular connective tissue: Perichondrium and Periosteum Cartilage: Function = Flexible Support
Skeletal System: Costal cartilage Intervertebral disk Pubic symphysis Meniscus in a joint Other Systems: External ear External nose Larynx Trachea & bronchi
Cartilage Begins forming at about 5 weeks within mesoderm Forms embryonic "model" for many bones In 7th week, ossification centers begin to develop Cartilage: a) Remains on ends of long bones throughout life b) Remains at growth plates (epiphyseal plates) of immature growing bones.
Cartilage: Young cells = chondroblasts Actively forming new extracellular matrix Mature cells = chondrocytes Maintain and repair extracellular matrix Extracellular Matrix Ground substance = very
firm gel Fibers = collagen, elastic Chondrocytes located in spaces called lacunae Cartilage: Three types: Hyaline Elastic
Fibrous All three types: Cells are the same Differences due to fibers Hyaline Cartilage: Extracellular matrix appears smooth, No fibers evident by light microscopy Cells (in lacunae) form isogenous groups or nests
Extracellular matrix often stains more darkly around cells Elastic Cartilage: Many dark elastic fibers visible in extracellular matrix Cells (in lacunae) form isogenous groups or nests Extracellular matrix often stains more darkly around cells Fibrous Cartilage: Many large collagen fibers visible in extracellular matrix Cells (in lacunae) usually individual (no isogenous groups)
Extracellular matrix usually stains more darkly around cells Bone Tissue: Young cells = osteoblasts Actively forming new extracellular matrix Mature cells = osteocytes Maintain and repair extracellular matrix Located in lacunae Third cell type = osteoclast
Reabsorb extracellular matrix Bone tissue: Extracellular matrix: Fibers: Collagen Ground substance contains precipitated calcium phosphate called calcium hydroxyapatite Bone tissue constantly remodeling itself:
Old extracellular matrix removed by osteclasts New extracellular matrix produced by osteocytes Two forms of bone tissue: Trabecular ("woven", "spongy") Shelves or ridges ("trabeculae") of extracellular bone matrix with osteocytes within
their lacunae. Lamellar ("compact", "Haversian") Concentric rings ("lamellae") of extracellular matrix and osteocytes, surrounding a central canal containing nerves and capillaries Lamellar Bone:
Lamellar Bone: Osteocytes in lacunae, between lamellae Both trabecular and lamellar bone: Osteocytes have lost ability to divide
so Osteoblasts at edges Growth can occur only by differentiation of new osteoblasts, which then form extracellular matrix around themselves As the osteoblasts become trapped in their matrix, they mature into osteocytes
located in lacunae Both trabecular and lamellar bone: Osteocytes have long cellular extensions to retain connections with each other. These extensions of
osteocytes run through little canals called canaliculi in the rigid extracellular matrix Two types of bone formation ("osteogenesis") for growth, remodeling, and repair: - Endochondral osteogenesis: Model of cartilage formed first. Cartilage then replaced by bone
. - Intramembranous osteogenesis: Bone formed within pre-existing ordinary connective tissue, often at edge of existing bone. Bones (organs) grow by both intramembranous and endochondral osteogenesis: Average age (years) at completion of ossification:
Bones repair fractures by both intramembranous and endochondral osteogenesis: Bone tissue is constantly remodeling itself to adjust to different physical and biochemical demands placed upon it. Old bone is destroyed by osteoclasts and new bone is produced by osteoblasts and osteocytes. Example: If you have a cast on your arm, the bones will become
smaller since they are not being stressed and more bone tissue is removed than is synthesized. But: When the cast is removed and the bones are again subjected to stress, they will become larger again. Bone remodeling regulated by many hormones: Growth hormone stimulated osteoblast activity & collagen synthesis Thyroid hormone stimulates osteoblast activity & collagen
synthesis; stimulates formation of ossification centers Testosterone stimulates osteoblast activity & bone growth Progesterone stimulates osteoclast activity & bone loss Estrogen stimulates osteoblast activity & bone growth Two hormones primarily responsible for day-to-day remodeling of bone to regulate concentration of calcium in blood: Calcitonin Secreted by thyroid gland Stimulates osteoblasts to produce more matrix
Inhibits osteoclasts from breaking down matrix Thus: Calcium removed from blood & stored in bone Parathyroid Hormone Secreted by parathyroid glands Inhibits osteoblasts from producing more matrix Stimulates osteoclasts to break down matrix Thus: Calcium released from bone into blood
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