Bone Tissue PDF

Bone tissue • Main constituent of the skeleton • It supports soft tissues • Protects vital organs • It serves as support and lever to the muscles and together with them forms the locomotor system, which allows body movements • Reservoir of minerals, contributes to the maintenance of homeostasis •...

Bone tissue • Main constituent of the skeleton • It supports soft tissues • Protects vital organs • It serves as support and lever to the muscles and together with them forms the locomotor system, which allows body movements • Reservoir of minerals, contributes to the maintenance of homeostasis • It houses the bone marrow, a hematopoietic organ • It is a specialized connective tissue made of cells and matrix. • Its matrix is mineralized and is totally rigid. • Despite being one of the hardest substances in the body, it is a dynamic tissue that constantly changes its shape depending on the forces that act on it. Endosteum Periosteum Bone marrow • The bones have a central cavity that is occupied by the bone marrow. • Its external surface is covered by the periosteum, which consists of an outer layer of dense connective tissue and an inner cellular layer containing osteogenic cells and osteoblasts. • The medullary cavity is covered by the endosteum, which is a specialized thin connective tissue containing osteogenic cells and osteoblasts. Bone matrix • The bone matrix contains both organic and inorganic components and both are responsible for bone consistency and hardness. Inorganic component of the matrix • Constitutes around 65% of its dry weight • It is made of minerals, mainly calcium and phosphorus, but also small amounts of bicarbonate, citrate, magnesium, sodium and potassium. • Calcium and phosphorus are found mainly forming hydroxyapatite crystals. These crystals are arranged in an orderly way along the collagen fibers and are also surrounded by an amorphous ground substance. • The association of collagen with hydroxyapatite is what gives hardness to the bone. If we decalcify it so that only the fibers remain, the bone will maintain its shape but will be flexible and rubbery. Organic component of the matrix • It constitutes approximately 35% of the dry weight of the bone. • It consists mainly of type I collagen that will form large beams. • There will also be ground substance with proteoglycans and adhesion glycoproteins. • If we eliminate this organic part of the matrix, we will only have the minerals. The bone will maintain its shape but will be fragile and break easily. Bone cells Osteogenic cell Osteoblast Osteocyte Osteoclast Osteogenic or osteoprogenitor cells are found in the periosteum and endosteum. Derived from mesenchymal stem cells, they can divide by mitosis and can differentiate into osteoblasts. They have an elongated shape and an oval nucleus. Osteoblasts They come from osteoprogenitor cells and are also located on the surfaces of bones. They make and release the organic components of the matrix (osteoid). They have a cubic or cylindrical shape, but when they are inactive, they appear flattened and are called bone covering cells. When the cell is completely surrounded by matrix, it will stop producing matrix and the cell will become an osteocyte that is enclosed in a space called lacuna. Osteoblasts have Parathormone (PTH) receptors in their membranes. This hormone makes osteoblasts to activate osteoclasts to degrade the bone. Osteoblasts Osteocytes Osteocytes They are the mature bone cells, derived from the osteoblasts that were trapped in their lacunae. They are elongated cells that present numerous cytoplasmic processes. These extensions will cross the matrix thanks to very thin channels that we know as calcoforous canaliculi. Thanks to them, the osteocytes establish contact with other cells and can also reach the bone surfaces to be nourished. Within the canaliculi there is also extracellular fluid that will facilitate the transport of nutrients to the osteocytes. They secrete substances necessary for the maintenance of the matrix. Osteoclasts Trabeculae being reabsorbed Osteoclasts They are very large, mobile and multinucleated cells that come from precursors in the bone marrow. They are responsible for bone resorption. In the active osteoclast we distinguish 4 regions: - Basal zone: the furthest from the surface of the bone that contains the nuclei and most organelles. - Brush border: microvilli oriented towards the bone surface. It is the part that actively participates in the reabsorption. - Clear zone: it is the zone that surrounds the periphery of the brush border. It has no organelles, but many filaments of actin that help to join the osteoclast to the bone, also sealing the area of the brush border. - Vesicular zone: between the basal zone and the brush border we find numerous vesicles of endocytosis and exocytosis that will transport the materials of the bone resorption. Bone resorption Bone resorption is the process by which osteoclasts break down the tissue in bones and release the minerals, resulting in a transfer of calcium from bone tissue to the blood. • When PTH stimulates the osteoblasts, they activate the osteoclasts that migrate to the bone surfaces and then present their 4 zones. • The osteoclast secretes protons (H+) into the subosteoclastic space and acidifies the medium. This acid medium will dissolve the inorganic components of the matrix. The minerals released will enter the osteoclasts and from there they will pass to nearby capillaries to be used by the organism. • In addition, the osteoclast secretes enzymes that will degrade the organic component. The products of this degradation are endocytosed by osteoclasts, which may use them or release them into the bloodstream. Ossification • Bone formation during the embryonic development can be carried out in two ways: – Intramembranous – Endochondral • In either case, the bone obtained is identical and we call it primary bone. • The primary bone is reabsorbed and replaced by secondary bone, which will continue to be reabsorbed throughout life, but at a slower rate. Intramembranous ossification Most flat bones are formed by this method. It happens in highly vascularized mesenchymal tissue. Mesenchymal cells differentiate into osteoblasts and begin to secrete bone matrix forming a network of trabeculae. The region where the bone begins to form is called the primary ossification center. The collagen fibers that appear in this primary bone are disordered and mineralize rapidly. The osteoblasts that are trapped in the matrix will become osteocytes. Connective tissue trapped between the trabeculae will be transformed into bone marrow. Endochondral ossification Most long and short bones are formed by this method: - First a model of hyaline cartilage is formed and it will grow for a while. The chondrocytes in the center of this cartilage become hypertrophied, widening their gaps and decreasing the matrix partitions, which will calcify. - The perichondrium in the middle zone of the bone is vascularized and the chondrogenic cells become osteoprogenitor cells, they differentiate into osteoblasts and begin to secrete bone matrix. - A bone collar is formed on the surface of the cartilage that prevents nutrients from reaching the hypertrophied chondrocytes that will die by apoptosis. The empty gaps remain inside the cartilage. - The osteoclasts will open cavities in the bone collar and communicate with the cavities that have remained inside. The blood vessels and osteoprogenitor cells can reach the inside of the bone. - Osteoprogenitor cells within the bone will differentiate into osteoblasts and begin to secrete bone matrix onto the cartilage septa. - Osteoclasts will degrade cartilage remnants and expand the medullary cavity. - The bone collar will grow, causing this process to extend along the entire length of the bone. - At the ends of the bone appear the secondary ossification centers, which will follow a process similar to that of the primary center, but there is no bone collar. - When this process advances, all the cartilage is replaced by bone with the exception of the epiphyseal plate and the articular surface. Bone growth Chondrocytes of the epiphyseal plate proliferate to maintain the chondrocyte population, while some of them will suffer hypertrophy and ossification of that cartilage zone will occur. As long as cartilage remains, the bone may grow in length. At around 20 years of age, this cartilage stops proliferating and this longitudinal growth stops. The growth in thickness of the bone will occur apositionally from the periosteum. Bone remodeling • During growth, bone development overcomes resorption. Once the required growth is achieved, the rate of formation and resorption are equalized under normal conditions. • The internal structure of an adult's bone varies continuously to adapt to the forces acting on it, such as weight changes, postural changes or microfractures. • Bone remodeling also occurs to maintain calcium homeostasis. Calcium homeostasis • 99% of the body's calcium in the bones • 1% remaining circulating in the plasma, fundamental: – Muscle contraction – Transmission of the nervous impulse – Blood clotting • Lack of calcium in blood (hypocalcemia): parathyroid secretes PTH, stimulates bone resorption releasing calcium into the bloodstream. • Excess calcium in blood (hypercalcemia): thyroid secretes calcitonin that inhibits osteoclasts. Morphological classification of bones • Long bones • Short bones • Flat bones • Irregular bones • Sesamoid bones Long bones Spongy bone Bone Marrow Compact bone Epiphyseal plate • In all types of bones we can see a denser part on the outer surface that we call compact bone and another more porous area covering the marrow that we call spongy bone. • The axis of the long bones is called the diaphysis and the articular ends are the epiphysis. Between them there is the epiphyseal plate. • All the bone is covered by periosteum except the articular surface and the place where tendons and ligaments are inserted. Spongy bone It appears mainly in the interior of the flat, short, irregular bones and in the epiphyses of long bones. Made of trabeculae where the matrix contains bundles of collagen fibers in parallel. The osteocytes appear enclosed in the matrix, and on the surface of the trabeculae we will see endosteum. The spaces between the trabeculae contain the bone marrow. Compact bone It appears mostly on the surfaces of bones and diaphysis of long bones. The collagen fibers are organized in concentric sheets around a channel that carries vessels and nerves called the Havers channel. The cylinder that forms the matrix sheets, with their osteocytes and the Havers channel is what is called an osteone or Havers System. Communicating the Havers channels there are other channels called Volkmann's ducts and with both duct systems we ensure the blood supply to the entire bone. Bone repair A bone fracture causes destruction of the matrix, cell death, endosteal and periosteal tears and sometimes the displacement of the ends of the fracture. The blood vessels that rupture near the fracture cause a hemorrhage and then clots will form. The area that does not receive blood supply is widened due to the accumulation of dead cells. The clot will soon be invaded by small capillaries that will bring osteoprogenitor cells. These cells differentiate into osteoblasts and begin to form a bone collar. Some osteoprogenitor cells will evolve to chondrogenic cells and then to chondroblasts, which will form cartilage around the bone collar. In the end the cartilage will be replaced by bone, so there is ossification both intramembranous and endochondral. And once the ends of the fracture have been joined, the primary bone will be replaced by secondary bone. Articulations According to the degree of movement • Synarthrosis: without movement or with minimal movement – Synostosis: bone and bone join directly (skull) – Synchondrosis: between the bones there is hyaline cartilage (ribs-sternum) – Syndesmosis: between the bones there is dense conjunctive (tibia-peroneum) • Diarthrosis: articulate with a wide range of movements Diarthrosis Most limb articulations are of this type and have very different degrees and types of movements. In all of them, the bones are covered by hyaline cartilage, the articular cartilage, which has no perichondrium. Between them there is a capsule, filled with synovial fluid and sealed by a fibrous layer (dense connective that is continuous with the periosteum) and a synovial layer (cell membrane). Synovial fluid contains a large amount of hyaluronic acid and is formed by a plasma filtrate. It nourishes the chondrocytes of the articular cartilage and also lubricates the articulation.