Histology of Bones Dr.Amjad Shatarat MD, PhD The University of Jordan 4/23/2019

(osteo= bone, genic= forming). Connective tissue Cells Ground substances 1- Osteogenic (osteo= bone, genic= forming). 2- Osteoblasts (blast= seed). 3-Osteocytes (cyte= cell). 4-Osteoclasts (clast = broken). Fibers Mostly of Calcium and Phosphorus in the form of Hydroxyapatite crystals, responsible about Hardness of bone Collagen Dr.Amjad Shatarat MD, PhD The University of Jordan 4/23/2019

Bone cells 1- Osteoprogenitor cells Found in the inner cellular layer of periosteum, the endosteum. Have the ability to begin mitotic division. Can differentiate into Osteoblasts. Most active during bone growth and repair. Dr.Amjad Shatarat MD, PhD The University of Jordan 4/23/2019

2- Osteoblasts Located on surface of bone. Synthesize collagen fibers and can initiate calcification, called Bone-building cells. Imprisoned inside the matrix they secrete inside spaces called Lacunae and become Osteocytes. Dr.Amjad Shatarat MD, PhD The University of Jordan 4/23/2019

Do not undergo cell division. 3- Osteocytes Mature bone cells derived from osteoblasts and housed in lacunae inside calcified matrix. Do not undergo cell division. Adjacent osteocytes contacted via cytoplasmic processes inside canaliculi that radiate in all directions. Actively involved in Maintenance of matrix. Dr.Amjad Shatarat MD, PhD The University of Jordan 4/23/2019

Dr.Amjad Shatarat MD, PhD The University of Jordan 4/23/2019

Dr.Amjad Shatarat MD, PhD The University of Jordan 4/23/2019

4-Osteoclasts Multinucleated giant cells that originates from BM and involved in the resorption and remodeling of bone tissue. Concentrated in the endosteum. They releases lysosomal enzymes and acids that dissolve the underlying matrix. Dr.Amjad Shatarat MD, PhD The University of Jordan 4/23/2019

Bone Tissue Components: Endosteum: Periosteum: Osseous tissue. A membrane that covers external surface of bone. Endosteum: A membrane that lines cavities within bone. Red and yellow bone marrow. Dr.Amjad Shatarat MD, PhD The University of Jordan 4/23/2019

Endosteum Thin membrane that lines bone cavities. Contains monolayer of osteogenic cells. Functions: Nutrition of bone. Provider of a continuous supply of osteoblasts for bone growth or repair. Dr.Amjad Shatarat MD, PhD The University of Jordan 4/23/2019

fibrous dense irregular connective tissue Periosteum Periosteum: double-layered membrane on external surface of bones Outer layer: Inner layer fibrous dense irregular connective tissue osteogenic stem cells that differentiate (specialize) into bone cells like osteoblasts (bone forming) or osteoclasts (bone dissolving) cells protective

Organization of bone tissue compact spongy Dr.Amjad Shatarat MD, PhD The University of Jordan 4/23/2019

1. Compact bone About 80% of the whole skeleton. Form external layer of all bones. The basic unit of compact bone arranged in units called Osteons (Haversian system). Their central canal runs longitudinal & lines by endosteum. A transverse Volkmann's canals interconnects the osteons, periosteum and endosteum together. Osteons are aligned in the direction along lines of stress. Dr.Amjad Shatarat MD, PhD The University of Jordan 4/23/2019

2- Spongy bone About 20% of the whole skeleton. Consist of irregular network of trabeculae (= little beams) with numerous spaces in between filled with bone marrow. No Haversian system. Contain osteocytes within lacunae that nourished by diffusion from bone marrow. Trabeculae are precisely oriented along lines of stress and located where stresses applied from many directions. Dr.Amjad Shatarat MD, PhD The University of Jordan 4/23/2019

Parts of a long bone Diaphysis-shaft Epiphyses- ends Come together at metaphyses Articular cartilage protects epiphyses Periosteum covers bone Medullary cavity contains marrow; is lined by endosteum 4/23/2019 Dr.Amjad Shatarat MD, PhD The University of Jordan

Dr.Amjad Shatarat MD, PhD The University of Jordan 4/23/2019

Ossification (osteogenesis) 1-Intramembranous Bone develops from fibrous membrane For example, Flat bones of skull and mandible 2-Osteocytes secrete calcium Compact and spongy bone form Dr.Amjad Shatarat MD, PhD The University of Jordan 4/23/2019

Endochondral ossification Bone develops from hyaline cartilage Most bones are formed this way Calcification is triggered when chondrocytes die Vascularization triggers ossification Dr.Amjad Shatarat MD, PhD The University of Jordan 4/23/2019

Bone growth in length (epiphyseal plate) Growth is on epiphyseal side Resting cartilage secures Dr.Amjad Shatarat MD, PhD The University of Jordan 4/23/2019

Bones get thicker? Periosteum Dr.Amjad Shatarat MD, PhD The University of Jordan 4/23/2019

Bone remodeling replaces old tissue (destroyed and reformed) Bone is Dynamic Bone remodeling replaces old tissue (destroyed and reformed) Coupled process between: Bone deposition (by osteoblasts) Bone destruction/resorption (by osteoclasts) 5-7% of bone mass recycled weekly All spongy bone replaced every 3-4 years. All compact bone replaced every 10 years. Dr.Amjad Shatarat MD, PhD The University of Jordan 4/23/2019

Wolff’s law Life style and bone formation Why is exercise good for bones? Wolff’s law Bones respond to muscles pulling on them (mechanical stress) and to gravity by keeping the bones strong where they are being stressed. Therefore, an active life style will enhance the bone formation Dr.Amjad Shatarat MD, PhD The University of Jordan 4/23/2019

How does aging affect bones? Loss of bone mass due to calcium loss Decreased production of collagen 4/23/2019 Dr.Amjad Shatarat MD, PhD The University of Jordan