Chapter 6- Part I Bones and Skeletal Tissues Components of the skeletal system Classification of Bone (bone shapes) Functions of bone Bone structure Microscopic structure of bone and bone cells What are the parts of the skeletal system? Bones of the skeleton The 206 bones of the body are divided into two categories: The axial skeleton and the The appendicular skeleton Cartilage, ligaments, and other connective tissue that stabilize or connect bones The bones and cartilages of the skeleton Growth of Cartilage Appositional cells in the perichondrium secrete matrix against the external face of existing cartilage Interstitial lacunae-bound chondrocytes inside the cartilage divide and secrete new matrix, expanding the cartilage from within Calcification of cartilage occurs During normal bone growth During old age Notes on perichondrium A fibrous membrane covering the external surface of cartilaginous structures The perichondrium nourishes the avascular cartilage, and it also contains mesenchymal cells which can differentiate into chondroblasts. Growth from the perichondrium is appositional growth. perichondrium Bones come in many shapes (a) Long bone (humerus) (c) Flat bone (sternum) chondroblast (b) Irregular bone (vertebra), right lateral view (d) Short bone (talus) 1
What are some of the functions of bone? Support Protection Movement/leverage Storage of minerals/fat Blood cell production Gross anatomy of bones- Bone markings Bone surfaces are rarely smooth- they display projections, depressions and openings WHY? Projections (or elevations) Tuberosity, crest, process, crest, spine, condyle etc, etc. Depressions, grooves, openings in bone Fossae, sulcus, foramen, grooves, canal, etc. These are fixed landmarks Landmarks of the Humerus Parts of the humerus - head, surgical neck, greater tubercle, lesser tubercle, lateral epicondyle, medial epicondyle, trochlea, capitulum, coronoid fossa, olecranon fossa, deltoid tuberosity Compact and Spongy Bone Each bone in the skeleton contains both compact bone and spongy bone. compact bone trabeculae spongy bone Compact bone link Bone structure Let s first take a look at the structure of a typical long bone Then we ll cover/compare the structure of flong bones to short, irregular, and flat bones Structure of long bones Diaphysis- tubular shaft, with wall made of compact bone, and a true marrow cavity inside Epiphysis- each swollen end of the bone. Composed largely of spongy bone, with a thin, outer layer of compact bone The joint surfaces of each epiphysis is covered with hyaline (articular) cartilage Between the epiphysis and diaphysis (in adult bone)is the epiphyseal line 2
Bones have membranes- the periosteum and the endosteum Periosteum (Periosteum) Covers outer surface of all bones, except at joint surfaces Isolates bone from surrounding tissue Route for nerves and blood supply Provides an insertion point for tendons/ligaments Actively participates in bone growth and repair A double membrane with an outer fibrous layer (dense irregular CT) and an inner cellular layer Osteoblasts- bone grows thicker Endosteum (Endosteum) Covers internal bone surfaces Including: trabeculae, marrow cavity and canals within compact bone Contains bone-forming cells, bone destroying cells and stem cells (osteoprogenitor) cells Active in bone growth, repair and remodeling Bone Cells Cells: Osteoblasts bone-forming cells Osteocytes mature bone cells Osteoprogenitor cells stem cells, grandfather cells Osteoclasts large cells that resorb or break down bone matrix More to come Structure of short, irregular, and flat bones Consist of thin plates of compact bone on the outside and spongy bone on the inside Contain bone marrow, but no formal marrow cavity is present In flat bones, the internal layer of spongy bone.. is called diploe Where is the periosteum, endosteum? Where do you find marrow and what is the function? Red marrow, hematopoietic tissue Yellow marrow 3
The microscopic structure of compact bone Osteon- basic functional unit Osteocytes arranged around central canal, between concentric lamellae One small artery, one venule, one nerve, parallel with bone surface Perforating canal Perpendicular to long axis of bone, connect blood supply from periosteum through to marrow cavity Interstitial lamellae Circumferential lamellae Central (Haversian) canal Osteon (Haversian system) Circumferential lamellae (a) Compact bone Lamellae Spongy bone Perforating (Volkmann s) canal Endosteum lining i bony canals and covering trabeculae Perforating (Sharpey s) fibers Periosteal blood vessel Periosteum Structure of compact bone A SEM of compact bone link Functions of compact bone? Canaliculi opening to surface The structure of spongy bone Lamella not in osteons Trabeculae No capillaries within matrix of spongy bone Nutrients reach osteocytes via diffusion through canaliculi that reach surface Trabeculae protect cells of red marrow, yellow marrow Marrow is vascularized Structure of spongy bone Spongy bone (diploë) Compact bone Trabeculae Functions of spongy bone? Link http://depts.washington.edu/bonebio/asbmred/structure.html 4
Composition of Bone (osseous tissue) Remember that in all connective tissues, there are cells, fibers and ground substance Cells Matrix- solid, collagen and Ca2+ deposits Osteoid makes up 1/3 of bone tissue = collagen fibers, proteoglycans and glycoproteins Calcium phosphates account for 2/3 the weight of bone Form hydroxyapatite crystals Crystals lie in, around, and on the collagen fibers. Fibers form framework for crystals & developing osteocytes Cells of the Bone- Four major types (a) Osteogenic cell (b) Osteoblast (c) Osteocyte Stem cell Matrix-synthesizing cell responsible for bone growth Mature bone cell that maintains the bone matrix (d) Osteoclast Bone-resorbing cell Lineage of bone cells Who is missing? Osteogenic (progenitor) cells Stem cells of the bone Produce daughter cells that differentiate into osteoblasts Cigarette smoke? Found in endosteum and periosteum Osteoblasts Osteoblasts line all surfaces of bone Osteoblasts are the builders! Produce new osteoid Differentiate into osteocytes Osteoblasts have receptors for estrogens Effects of estrogen on these cells? osteoid Osteocytes Mature bone cells One osteocyte sits in a lacunae Cytoplasmic cellular extensions reside in canaliculi Gap junctions connect neighboring cells Allow for exchange of nutrients/ions/waste, etc. Functions- maintain protein/mineral content of matrix, and bone repair 5
Osteoclasts Remove/resorb and recycle bone matrix Found largely in endosteum Giant cells, 50 or more nuclei Cousins of macrophage! Do not arise from osteogenic cells Release acids and digestive enzymes onto matrix Important in bone remodeling and regulation of blood Ca 2+ Activity inhibited by estrogen Homeostasis- A balance exists between osteoblast and osteoclast activity If osteoblasts are working faster than osteoclasts, what happens to bone? LINK If osteoclasts are working faster, what happens to bone? Breakdown > building? Blood calcium? 6