10. bone histology and ossification .pptx

Histology

1. Bone Nino Buishvili

2. Bone tissue - functions Bone is a highly specialized support tissue which is characterized by its rigidity and hardness. Its four main functions are: to provide mechanical support (e.g. ribs), to permit locomotion (e.g. long bones), to provide protection (e.g. skull) to act as a metabolic reservoir of mineral salts. Bone is composed of: • Support cells (osteoblasts and osteocytes) • A non-mineral matrix of collagen and glycosaminoglycans (osteoid) • Inorganic mineral salts deposited within the matrix • Remodelling cells (osteoclasts).

3. Most bones have a basic architecture composed of: An outer cortical or compact zone An inner trabecular or spongy zone. Bone architecture: cortical and trabecular bone. (a) Low-power scanning electron micrograph showing the architecture of bone (cortical and trabecular) and its relationship to bone marrow; A typical long bone shaft, schematic view.

4. Basic Features of bone Note bone cells, their usual locations; and the typical lamellar organization of bone. Osteoblasts secrete the matrix that then hardens by calcification, trapping the differentiating cells now called osteocytes in individual lacunae. Osteocytes maintain the calcified matrix and receive nutrients from microvasculature in the central canals of the osteons via very small channels called canaliculi that interconnect the lacunae. Osteoclasts are monocyte-derived cells in bone required for bone remodeling. The periosteum consists of dense connective tissue, with a primarily fibrous layer covering a more cellular layer. Bone is vascularized by small vessels that penetrate the matrix from the periosteum. Endosteum covers all trabeculae around the marrow cavities.

5. Bone cells: osteoblasts (continued on next slide) Osteoprogenitor cells. Micrograph of toluidine blue-stained epoxy resin section showing numerous plump spindle-shaped osteoprogenitor (Op) cells in the developing skull bone of a 15-week human fetus. Derived from primitive mesenchymal cells, they are transformed into osteoblasts (Ob), which are larger and more cuboidal. The osteoblasts have begun to deposit osteoid collagen (C).

6. Note osteoblasts (OB) actively depositing new osteoid on a bone surface. When active, the osteoblasts Ob are large, broad, spindle-shaped or cuboidal cells with abundant basophilic cytoplasm containing much rough endoplasmic reticulum and a large Golgi apparatus. These features reflect a high rate of protein (type I collagen) and proteoglycan synthesis. In micrograph (b), the mineralised bone (blue) can easily be distinguished from the new osteoid (red) which is being produced by the row of cuboidal osteoblasts. There is always a short delay between osteoid production and its mineralisation. When inactive, osteoblasts are narrow, attenuated, spindleshaped cells lying on the bone surface. Oc – osteocytes.

7. Osteoblasts synthesize the organic component of the bone matrix (osteoid) Micrograph (left) of a toluidine blue-stained thin epoxy resin section of actively growing bone, with a row of cuboidal osteoblasts (Ob) actively synthesizing and secreting organic matrix (osteoid, OS); the cells have bulky basophilic cytoplasm owing to their protein-synthesizing rough endoplasmic reticulum. Some of the osteoid is lightly mineralized (mOS). Low-power electron micrograph (right) showing an osteoblast (Ob) at the face of newly forming bone. Note the zone of unmineralized osteoid (OS), which has recently formed between the active osteoblast and the older mineralized bone (mB).

8. Mineralization of Osteoid (a) The events believed to occur in a newly formed osteoid. ( b) High-power electron micrograph showing early mineral deposition in a zone of recently formed osteoid from fetal bone. Note the early crystalline pattern of mineralization of a matrix vesicle (arrows) in zone B, enlargement of foci of mineralization in zone C, becoming confluent in zone D. Zones A and E are not shown.

9. Paget’s disease; Paget disease is a localized disorder of bone remodeling that typically begins with excessive bone resorption followed by an increase in bone formation.This osteoclastic overactivity followed by compensatory osteoblastic activity leads to a structurally disorganized mosaic of bone (woven bone), which is mechanically weaker, larger, less compact, more vascular, and more susceptible to fracture than normal adult lamellar bone.

10. osteomalacia; osteoporosis

11. Osteocytes are inactive osteoblasts trapped in mineralized bone Once trapped in calcified matrix, osteoblasts are differentiated into osteocytes; osteocytes and their processes reside inside spaces called lacunae and canaliculi, respectively; These cells maintain the bony matrix, and their death is followed by rapid matrix resorption.

12. osteoclasts (a) single osteoclast (Oc), which has alighted on an irregular spur of bone prior to reabsorption. (b )active resorption of bone on one face, with a multinucleate osteoclast (Oc) lying in a Howship’s lacuna. There is active osteoblast deposition of new osteoid on the other face (arrow), in a bone that is undergoing active modelling; Diagram (lower) showing an osteoclast’s circumferential zone where integrins tightly bind the matrix and surround a ruffled border of cytoplasmic projections close to this matrix. The sealed space between the cell and the matrix is acidified to ~pH 4.5 by a proton pump located in the osteoclast membrane and receives hydrolytic enzymes secreted from lysosomes by exocytosis. Acidification of this confined space facilitates the dissolution of calcium apatite from bone and creates the optimal pH for activity of the lysosomal hydrolases. Bone matrix is thus resorbed, with products of matrix digestion released for reuse and calcium, carbonate, and other ions released from uptake by the blood.

13. The two main patterns of bone are called woven and lamellar Bone exists in two main forms, woven bone W and lamellar bone L . Woven bone is an immature form with randomly arranged collagen fibres in the osteoid. Lamellar bone is composed of regular parallel bands of collagen arranged in sheets. Woven bone is produced when osteoblasts synthesise osteoid rapidly, as in fetal bone development.

14. Bone remodeling

15. Intramembranous ossification selected centrally located mesenchymal cells cluster and differentiate into osteoblasts, forming an ossification center; osteoblasts begin to secrete osteoid, which calcifies in a few days; Trapped osteoblasts become osteocytes .

16. Intramembranous ossification Accumulating osteoid is laid down between embryonic blood vessels in a manner that results in a network of trabeculae(woven bone); Vascularized mesenchyme condenses on the external face of the woven bone and becomes periosteum. Trabeculae just deep to the periosteum thicken. Mature lamellar bone replaces them, forming compact bone plates. Spongy bone ( diploe) consisting of distinct trabeculae, persists internally and its vascular tissue becomes red marrow.

17. Endochondral ossification

18. Use the mnemonic to remember the names of epiphyseal growth zones : “ Real People Have Career Options“- Resting, Proliferative, Hypertrophic, Calcified, Ossification Note the difference between open and closed growth plates on X-ray

19. Bone reparation

20. Metabolic role of bone The skeleton serves as the calcium reservoir, containing 99% of the body’s total calcium; The principal mechanism for raising blood calcium levels is their mobilization from bone; this process is regulated by two hormones: PTH and Calcitonin.

21. JOINTS

22. Synovial Joints the relationship between the articular surfaces of bone and the joint capsule. The bone end is cortical bone C , covered by a cap of articular cartilage AC . This protrudes into the joint cavity JC . The joint cavity is contained by a dense collagenous fibrous capsule Cp which is lined internally by a layer of synovium S . The synovial lining cells secrete serous fluid which lubricates the articulation of the joint. The collagen fibres of the joint capsule merge with those of the periosteum P over the shaft of the bone.

23. synovium The inner surface of the capsule of synovial joints and tendon sheaths is lined by a specialised collagenous tissue, the synovium , which is responsible for the elaboration of the synovial fluid that lubricates the movement of articular surfaces. In the normal joint, the synovial fluid is little more than a thin film covering the articular surfaces. In that the articular space is not demarcated from the synovium by an epithelium, the synovial fluid represents a highly specialised fluid form of synovial extracellular matrix rather than a secretion in the usual sense. Its major constituents are hyaluronic acid and associated glycoproteins which are secreted by the type B synoviocytes . Its fluid component is a transudate from the synovial capillaries. This arrangement facilitates the continuous exchange of oxygen, carbon dioxide and metabolites between blood and synovial fluid, which is the major source of metabolic support for articular cartilage. Normal synovial fluid also contains a small number of leucocytes ( < 100/mL), predominantly monocytes.

24. Articular cartilage

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