Microanatomy of Bone PDF - Spring 2024 - Ross University

Microanatomy of Bone Dr Georgios Paraschou DVM, DACVP, DipRCPath, MRCVS Slides courtesy of Dr Heather Fenton modified by Dr Spring 2024 Paraschou Disclaimer: Images from this presentation come...

Microanatomy of Bone Dr Georgios Paraschou DVM, DACVP, DipRCPath, MRCVS Slides courtesy of Dr Heather Fenton modified by Dr Spring 2024 Paraschou Disclaimer: Images from this presentation come from different sources as indicated: McGavin’s Pathologic Basis of Veterinary Disease, Tuft’s veterinary gross pathology image collection, Noah’s Arkive, web images, author’s own images or images from colleagues, etc. This presentation is intended to be for teaching purposes only, please do not distribute.  Bone – Is alive and constantly being remodeled! Osteoid = 90% collagen and Inorganic (mineral) components – Hydroxyapatite [Ca10(PO4)6(OH)2] Hydroxyapatite inorganic mineralpresentinhuman bone teeth role structuralstrength ofboneandinbone regeneration  skeletalsystem Bones are organs in that they are a major part of the skeletal system; protect hematopoeticcells Function(s): bones protect other organs (brain), produce hematopoietic cells, store minerals, provide structure and support for the body. They are lightweight, but strong and hard yet have internal flexibility because of spongy bone (spicules with spaces between) spongybonecancellousbones porouslightwrappingseverallargespacesprovidingthe characteristicspongyorhoneycombedappearance makeupinnercavitiesofbone Trabecular bone (T) T also referred to as cancellous or spongy bone large surface area; bone marrow C D Cortical bone (C) also referred to as compact bone and more dense than trabecular bone T E – epiphysis D - diaphysis M AC M – metaphysis P P – physis = growth plate AC – articular cartilage E BIRDS BONES?? PNEUMATIC (RAREFIED TRABECULAR BONE WITH BROADER SPACES BONES as ORGANS  in Mature lamellar bone - hard Wovenan bone - soft 4 affidize Polarized light Collagen in parallel layers resulting in Collagen disorganized, weak, new bone formation strong mature bone; in trabeculae and cortical bone Locations = fracture repair, inflammation, neoplasia  d Mature lamellar bone - hard Woven bone - soft 90 0 HE Polarized light Collagen in parallel layers resulting in Collagen disorganized, weak, new bone formation strong mature bone; in trabeculae and cortical bone Locations = fracture repair, inflammation, neoplasia Lamellar bone – strong mature, forms slowly collagen fibers in matrix arranged in layers (lamellae) resulting in strong mature bone (osteoid) Woven bone organized into osteons (Haversian systems) in mature can be re- cortical and trabecular bone modelled into lamellar bone Woven bone – weak immature, produced quickly disorganized arrangement of collagen fibers present at sites of rapid bone formation (e.g. fracture repair, inflammation, neoplasia) E. g. periosteal new bone or reactive bone  Margareta Osteon, the functional unit of lamellar bone located in circles around the central canal (Haversian) and in the interstitial bone between the circles of bone India ink has been used to demonstrate the canaliculi and lacunae. TheHaversiancanalisseenat center of each osteon. Haversiancanal Ground(undecalcified)section,700x Osteons are in mature bone = lamellar; not woven bone Located in cortex, trabeculae of bones Volkman’s canals are canals running in bone and connecting Haversian (central) canals blue line Haversian System Haversian Thin channels running between lacunae Osteon – circles of lamellar bone around central canal. Small holes are lacunae with osteocytes Alfonso Lopez Cellular elements of bone tissue Osteoblasts Derived from Osteocytes osteoprogenitor cells Bone lining cells Osteoclasts Monocytic origin Alfonso Lopez  Osteoblasts Produce osteoid (the organic component of bone matrix) and initiate its mineralization Some osteoblasts (ob) become surrounded by osteocytes osteoid and embedded in the bone (osteocytes) osteoblasts  Most abundant cells Reside in lacunae surrounded by mineralized matrix (long filipods to increase their cell area) moreflat slenderthinlegs Parathyroid hormone (PTH) stimulates activity Bone resorption absorbCa't Reside in shallow pits (Howship’s lacunae) Acid demineralizes bone and enhances activity of acid hydrolases released from osteoclasts (extracellular digestion)  Bone Cells Osteoblasts are rounded, form a single row of cells along trabeculae. Osteocytes are thinner and located within lacunae embedded in bone. Osteoclasts are multinucleated cells located along trabeculae and will resorb bone under stimulation of cytokines secreted by osteoblasts or inflammatory cells  Cellular Elements of Bone Bone Lining Osteoblasts Osteocytes Osteoclasts Cells (inactive osteoblasts) Flat, elongated Osteoid formation Originate from Digest bone and cells Secrete Type I osteoblasts play an important Cover endosteal collagen Maintain bone role in Ca surfaces of inactive Regulate matrix homeostasis. bone – bone that is mineralization Occupy lacunae They are large multi- undergoing neither inside bone Positioned external nucleated cells formation nor to osteoid matrix Extend filopodia Exhibit ruffled resorption. through canaliculi that Differentiate to Mobilize Ca++ border and clear become osteocytes zone R’ for PTH Origin stem cells of bone marrow monocyte origin  Parathyroid hormone (PTH)– promotes bone resorption 1,25-dihydroxyvitamin D3 – required fornormal bone growth and mineralization; also stimulates bone resorption Calcitonin – inhibits bone resorption Glucocorticoids – inhibit formation and stimulate resorption Estrogen – inhibits bone resorption Insulin, growth hormone, androgens and thyroid hormones are also involved in bone metabolism  Osteoid (organic component) contains: Type I collagen – main component of osteoid (~90%) Osteonectin – involved in mineralization Osteocalcin – vitamin K-dependent; also involved in mineralization Proteoglycans – less abundant than in cartilage matrix Growth factors – e. g. BMPs, IGFs, TGFs, FGFs Hydroxyapatite [Ca10(PO4)6(OH)2] (inorganic component) – needle-like crystals – Mineral Unmineralized bone (osteoid) is blue, produced by osteoblasts (ob). Mineralized bone is black in this photo (special prep). Osteoclasts (oc) only remove mineralized bone. The process of formation and resorption are coupled. When the processes are uncoupled a disease is produced. oc With conventional preps and stains (H&E) osteoid is pink and you cannot tell if it is mineralized. With special stains (von Kossa) osteoid that is mineralized is black and unmineralized osteoid blue (left) In bone, osteonectin and osteocalcin adsorb to type I collagen and bind Ca2+ Osteoid is not mineralized immediately after deposition (delay of 5-10 days) – failure to mineralize bone produces Rickets = osteomalacia Which can result in soft bones and fractures vertebrae etc.  BONE DEVELOPMENT Intramembranous ossification – adult Bone formed from mesenchymal tissue in periosteum the Occurs in flat bones of skull and pelvis and on whatcovers bone defn all periosteal surfaces throughout life Responsible for the growth of bones in width NO cartilage needed first Endochondral ossification – young Bone formed from hyaline cartilage precursors in PHYSES Most bones develop by this method (appendicular, axial and base of skull) At physes responsible for growth of bones in length Hyaline cartilage precedes and is scaffold Endochondral ossification – Intramembranous – LENGTH WIDTH Physis = growth plate at ends Periosteum at sides No cartilage needed last ayercontaini hfhffh.io osteoblasts layer mesynchmeor pey Ffessor Metaphysis (M), physis (arrow) and epiphysis (E) of the distal femur of a healthy newborn calf (some undulations of the physis are considered normal) M E  Growth plate = Physis (P) Endochondral bone formation is preceded by hyaline cartilage. It occurs in physes when cartilage differentiates, matures, mineralizes, dies and is scaffold that osteoblasts reside on. Osteoblasts produce osteoid on top of the spicules of dark blue cartilage (arrow). This zone is called Primary Spongiosa. Osteoid is pink, it is unmineralized at this stage, it is soft. Cartilage is blue. Bone produced in the primary spongiosa is gradually remodeled and mineralized into mature lamellar bone E AC P M Epiphysis Primary spongiosa Physis Metaphysis Articular cartilage need to know this Bone Blood supply Blood enters the marrow cavity of bones through nutrient, metaphyseal and epiphyseal arteries. In young animals, the cortex is supplied mainly from the endosteal surface and exits via periosteal veins (i. e. blood flows centrifugally). With age, the cortex becomes increasingly dependent on periosteal arteries. Bone Blood Supply  Terms to Know - Bone Osteocytes Osteoclasts Osteoblasts PTH Calcitonin Osteoid Spongy/cancellous/trabecular bone Compact/cortical bone Epiphysis, Diaphysis, Metaphysis, Physis Periosteum, Endosteum Lamellar bone vs woven bone Osteon Haversian canal Hydroxyapatite Howship’s lacunae Endochondral vs Intramembranous bone formation Nutrient, epiphyseal, metaphyseal arteries Bone blood supply