bone

• Flat bones are thin and flat, such as the skull, ribs, and sternum.
• The connective tissue is one of the four major types of tissues with the following functions: support, compartmentalization, binding together, physical and immunologic protection, and storage.
• Below the region of chondrocyte hypertrophy is the zone of ossification where bony material is deposited on the plates of calcified cartilage matrix.
• The hypertrophied chondrocytes degenerate and form thin plates of calcified cartilage matrix.
• Chondrocytes and lacunae increase in size in the zone of chondrocyte hypertrophy, which is a result of the swelling of the nucleus and cytoplasm.
• The three fundamental components of connective tissues are cells, fibers, and ground substance, also known as extracellular matrix.
• The connective tissues originate from the Embryonic mesenchyme which will later on differentiate into different types of connective tissues.
• There are two types of connective tissue cells: fixed cells and wandering cells.
• Fixed cells are native to the tissues where they are found, including mesenchymal cells, fibroblast cells, reticular cells, and adipose cells/ adipocytes.
• Mesenchymal cells are considered to be the precursor of most connective tissues and are found in the embryonic mesenchyme as a loose network of satellite cells and abundant intercellular matrix.
• Adult mesenchyme usually has a reserve population of adventitial cells.
• Fibroblasts are the predominant cells in the connective tissue and synthesize, secrete, maintain all major components of extracellular matrix.
• Fibroblasts exhibit an elongated cell with cytoplasmic projections, oval nucleus with sparse chromatin, and exhibit either 1 or 2 nucleoli.
• Fibroblasts have two different types: mitotically active (stellate cells with long cytoplasmic processes, large, ovoid, pale-staining nucleus, abundant RER and Golgi complex) and less active (fibrocytes with spindle-shaped body, dark, elongated nucleus, fewer organelles, without cytoplasmic projections, more mature fibrocyte).
• Reticular cells are typically stellate with long cytoplasmic processes and each with a central, pale, irregularly rounded nucleus with distinct nucleolus.
• Organic components/osteoid constitute approximately 50% of bone volume and 25% bone weight, are fibers made up of 90-95% type I collagen, and the ground substance is multi-componential: proteins, carbohydrates, lipids, proteoglycans, and glycosaminoglycans (GAGs).
• Osteoprogenitor cells are stem cells found in the endosteum and periosteum, are spindle-shaped, have ovoid to elongate nuclei, and unremarkable cytoplasm.
• Osteocytes are terminally differentiated bone cells found in cavities of bone matrix, lacunae, and retain synthetic and resorptive capacity.
• Inorganic components make up 50% of bone volume and 75% of bone weight, are mainly Calcium and Phosphate, also some bicarbonates, citrate, magnesium, potassium, trace metals, and needle-like crystals of hydroxyapatite.
• Long bones are compact and spongy (cancellous) bone.
• Intramembranous ossification is the process of bone formation by the replacement of mesenchymal tissue by bone.
• Osteoblasts are major bone-forming cells, are cuboidal in shape, have a round nucleus, and are found on surfaces of new bone formation.
• Endochondral ossification is the replacement of cartilage by bone, a process that occurs in all except membrane bones, and is controlled by growth hormone, thyroid, and sex hormones.
• The basic steps in endochondral ossification are cartilage model, periosteal bone collar formation, proliferation, hypertrophy, calcification, formation of primary bone marrow, and periosteal bud formation.
• Osteoclasts are bone-resorbing cells found lying on bony surfaces in shallow depressions, a.k.a. Howship’s lacunae, are large, multinucleated, and have acidophilic cytoplasm.
• The periosteum is a fibrous layer on the outside of the bone and an osteogenic layer on the inside of the bone, with Sharpey’s fibers that penetrate the bone matrix and anchor the periosteum to the bone.
• Bone tissue consists of two components: cells and matrix.
• The endosteum is the inner, condensed connective tissue of the bone, containing bone and blood cell precursors, and lines marrow cavities and extends into Haversian canals.
• Reticular cells are produced on connective tissues that have reticular fibers and function to phagocytose antigenic materials and cellular debris.
• Adipose cells/ adipocytes are mesenchymal derivatives specialized for lipid storage and are filled with lipid droplets, hence nucleus displaced to the cell periphery, with a narrow rim of cytoplasm and a flattened eccentric nucleus.
• Cartilages are non-vascular, receive their nutrition via diffusion through the extracellular matrix, exhibit strength and support that provides firm structural support for some tissues, and are mainly made up of cells called chondrocytes and chondroblast that synthesize the extensive cellular matrix.
• Dense irregular connective tissue can be found in the dermis of the skin and provides firmer support with more strength required.
• Cartilage is a special form of connective tissue that develops from mesenchyme and is similar to the connective tissue due to the type of cell and extracellular matrix that compose the connective tissue fibers.
• White adipose tissues have a unilocular appearance, are larger than brown adipocytes, exhibit a large single fat droplet, and have a wider distribution than brown adipose tissue.
• Hyaline cartilage is the most common type, serves as a skeletal model in embryos, replaces a bone by a process called endochondral ossification in adults, and is found on the articular surface of the bones.
• Connective tissue contains supporting connective tissue that includes cartilages and bones.
• Fibrous cartilage is a type of cartilage that is found in the rib cage, is characterized by its fibrous nature, and is used for support and protection.
• Bones are the main constituent of the adult skeletal system, provide support, protection, and involvement in the production of hematopoietic tissue.
• Chondrocytes are found inside the lacuna of cartilage, while perichondrium is a vascularized dense irregular connective tissue that surrounds the hyaline and elastic cartilage, with its outer fibrous layer containing a type I collagen and a fibroblast.
• Specialized connective tissue includes reticular tissue, blood, and elastic tissue.