1. Functional morphology of bone
2. Functional morphology of cartilage
Functional morphology of bone
Bone is one of the hardest tissue in the body. Bones ensure the mechanical support and protect soft tissues in human body. Bones form lever system converting the muscle contraction to movement of whole body. They also participate in calcium and phosphorus metabolism. Bone contains extracellular matrix (composed of amorphous and fibrillar components) and cells (osteoblasts, osteocytes and osteoclasts). Two types of bone tissue are distinguished: bundle bone (primary bone) and compact bone (secondary bone).
Bone is covered by collagenous connective tissue called periosteum and its inner cavity is lined by endosteum. Both structures are composed of outer connective tissue layer and inner cellular layer formed by osteoprogenitor cells (they differentiate into the osteoblasts) and preosteoblasts. Endosteum is much thinner than periosteum and ensures the nutrition of bone. It is also source of new osteoblasts which are used for growth and remodeling of bone. Periosteum is formed by dense collagenous connective tissue with numerous fibroblasts, collagen fibers, vessels and nerves. Collagen fibers extending from outer layer of periosteum directly into the bone matrix tightly anchor the periosteum to the bone tissue. These fibers are called Sharpey’s fibres.
Extracellular matrix consists of amorphous and fibrillar components. The amorphous component is formed by proteoglycans (glycosaminoglycan chondroitin sulfate and keratan sulphate), structural glycoproteins (osteonectin, sialoprotein, osteocalcin binding calcium ions). The fibrillar component consists of type I collagen whose fibers represent 95 % of organic bone matrix.
Inorganic component of bone matrix is represented by calcium and phosphorus ions in the form of hydroxyapatite crystals. Inorganic component constitute about 50 % of total weight of bone. Due to the connection of collagen fibers and hydroxyapatite, the dense bone is formed.
Cells of bone tissue
Osteoblasts developed from mesenchyme synthesize the bone matrix (type I collagen, glycosaminoglycans, proteoglycans, glycoproteins). Non-mineralized bone tissue is called osteoid. After the osteoblast is surrounded by extracellular matrix, it converts to osteocyte (doesn’t synthesize bone matrix anymore). Osteoblasts contain organelles typical for synthetically active cells – numerous rER (basophilic cytoplasm) and high activity of alkaline phosphatase. Osteoblasts are located on the surface of bone trabeculae and are arranged in one line resembling simple epithelium. Adjacent osteoblasts are connected together by their processes. When their activity is decreased, they flatten and production of alkaline phosphatase declines.
Osteocytes are osteoblasts surrounded by bone matrix and they occupy spaces called lacunae.
Osteoclasts are multinucleated cells with numerous processes embedded in Howship’s lacunae. They produce collagenase, acid phosphatase and another proteolytic enzymes degrading the bone matrix – they are responsible for resorption of bone tissue. They belong to monocyte-macrophage system.
Classification of bones
Histologically, bone tissue is distinguished into two major types: Primary immature bundle bone (also called woven bone) and secondary mature lamellar bone. The mature lamellar bone is divided into two structural subtypes according to localization and load – the compact bone and trabecular (spongy) bone.
Primary bundle bone
Primary bone contains interlacing collagen fibers. This immature primary bone is present in the developing skeletal system and bone regeneration and is later replaced by lamellar bone tissue. Thus, primary bundle bone is present in fetal development and fracture healing. In adults, the primary bone is replaced by secondary lamellar, except few locations – sutures between flat bones of skull, dental alveoli or near the tendon insertion. Bundle bone differs from lamellar in few aspects:
1) It lacks lamellar arrangement of collagen fibers and no Haversian canals are present
2) It contains more osteocytes
3) Cells are irregularly dispersed
4) It contains less minerals
Secondary lamellar bone
The lamellar bone is more often than bundle bone. Lamellae are formed by parallel collagen fibers surrounded by mineralized amorphous matrix. Concentric lamellae surround the central canal or form parallel lamellar sheath system on the surface of bone. Complex of lamellae surrounding the central canal (Haversian canal) is called osteon or Haversian system. Inner site of Haversian canal is lined by endosteum and contains vessels, nerves and loose collagenous connective tissue. This canal communicate with periosteum and bone marrow. It also communicates with neighbouring Haversian canals via Volkmann’s canals through which the blood vessels and nerves travel from the periosteum to the Haversian canals. Volkmann’s canals are not surrounded by lamellae.
Lamellar bone is distinguished into two types – compact bone and trabecular (spongy) bone. The compact bone is composed by system of osteons and the surface is created by parallel oriented bone lamellae. Lamellar bone composes the bone diaphysis. The trabecular (spongy) bone is formed by trabeculae and it is present in epiphysis and short bones. Flat bones are composed of two layers of compact bone tissue and layer of spongy bone tissue in between. This structure is called the diploe.
Bone develops from existing mesenchymal or cartilaginous tissue by process called ossification. Bone development is classified into two methods:
1) Intramembranous – bone is formed by differentiation of mesenchymal tissue without cartilage precursor.
2) Endochondral – cartilage model serves as precursor and is replaced by bone tissue
The most of bone sis developed by endochondral ossification. The exceptions are some skull bones, sternum, ribs and sesamoid bones – they develop by intramembranous ossification. Osteoblasts and osteoclasts are involved in process of ossification. In both types of ossification appears at first the primary ossification and so the immature bundle bone is formed. The regularly arranged mature lamellar bone is then created by secondary ossification.
Ossification begins in the end of embryonal period (8th week of pregnancy). The condensation of mesenchymal cells within the mesenchymal tissue initiates the process of ossification. These clusters of mesenchymal cells then enlarge, round and differentiate in osteoblasts. Osteoblasts produce osteoid – non-mineralized bone tissue containing type I collagen which surrounds the osteoblasts converting to osteocytes. With time, the matrix become calcified. This is how flat bones of skull, clavicle or mandible are developed.
Also begins with proliferation and aggregation of mesenchymal cells at the sites of future bones but under influence of different growth factors, these mesenchymal cells express type II collagen and they differentiate into chondroblasts which produce cartilage tissue. Thus, a hyaline cartilage model of the future bone is formed. Then, the chondrocytes enlarge and initiate the resorption of surrounding cartilage tissue. Hypertrophic cells synthesize alkaline phosphatase and the cartilage model undergoes calcification (should not be confused with mineralization of bone tissue). Calcified cartilage inhibits diffusion of nutrients causing death of chondrocytes in cartilage model. Osteoprogenitor cells migrate into formed cavities along the vessels and changes into osteoblasts starting production of bone matrix. Thus, the cartilage is not changed into the bone, but replaced by bone tissue.
Functional morphology of cartilage
Cartilage is specialized form of connective tissue composed of cells – chondrocytes, and extensive extracellular matrix. Extracellular matrix consists of fibrillar and amorphous components. Cartilage has some specific characteristics – it is flexible and also strong, avascular without nerve supply. It forms mechanical support of soft tissues and is important for bone development from fetal period until puberty. The surface of cartilage is covered by perichondrium except articular capsule which lacks perichondrium. Cartilage contains 60 % of water and 40 % of organic tissue (about 60 % is attributed to collagen and 40 % to proteoglycans). According to composition of extracellular matrix, three types of cartilage are distinguished: hyaline cartilage, elastic cartilage and fibrocartilage.
There are two types of cartilage growth: interstitial growth and appositional growth (enfolding). Interstitial growth arises from inner portion of cartilage and represents mitotic division of existing chondrocytes. During appositional growth are new cells derived from fibroblasts lining the inner portion of perichondrium which differentiate into chondroblasts. These chondroblasts begin to synthesize extracellular matrix which embeds chondroblasts into newly formed cartilage. Thus, new layer cartilage layer can be formed in the inner surface of perichondrium. The importance of appositional growth is predominant in early stages of cartilage development and with time, its importance decreases. In adults, appositional growth appears chiefly in epiphyseal plates during the elongation of bones. Cartilage regeneration is slow and difficult. In adults, the connective tissue scar is formed during cartilage repair.
Hydration (water content) of cartilages affects their mechanical characteristics e.g. elasticity. Cartilage during load exhibits biphasic behavior resembling the sponge. The water in the basic tissue is freely bonded and during load is water quickly pushed out which causes the change of cartilage shape (e.g. flattening). Subsequently, the higher rigidity of fibrillar component involves which prevents from another change of cartilage shape.
Chondrocytes and extracellular matrix
Chondrocytes are principal cells of cartilage tissue localized in small cavities (called lacunae). They exhibit typical characteristics of cells with high proteosynthetic activity and they produce the extracellular matrix consisting of amorphous and fibrillar components. The amorphous component is represented by glycosaminoglycans = GAG (hyaluronic acid, karatan phosphate, chondroitin phosphate), proteoglycans and structural proteins. With time, the amount of proteoglycans decreases and calcium salts deposits in hyaline cartilage. The fibrillar component is composed of type II collagen. In the case, that the extracellular matrix directly surrounds the cavities with chondrocytes, these matrix is called territorial – it contains more GAG’s and less collagen than interterritorial matrix.
If there are found groups of cells, it is described as isogenic group. In epiphyseal plate are found isogenic lines. These nomenclature is defined by origin of chondrocytes – they origin during mitotic division of one chondrocyte. Chondrocyte exhibits minimal proliferative activity, they are not able to regenerate or proliferate.
Hyaline cartilage is the most often type, it covers articular surfaces, connection between ribs and sternum, walls of trachea and bronchi or temporary skeletal system later replaced by bone. Specifically, it is present in childhood in epiphyseal plate which is responsible for elongation of bones. Its basic tissue consists chiefly of amorphous component. Hyaline cartilage is covered by vascular innervated perichondrium formed by dense collagenous connective tissue and fibrocytes. Due to the high vascularization, the perichondrium is able to ensure the nutrition of cartilage. Articular surfaces are not covered by perichondrium and their nutrition is provided by diffusion from synovial fluid.
Fibrocartilage is present in symphysis or intervertebral discs. It contains less amorphous component – predominates fibrillar component composed of densely arranged irregular collagen connective tissue. Except type II collagen, there is also found type I collagen. Chondrocytes are dispersed – they do not form isogenic groups.
Elastic cartilage is present more rarely than hyaline. It is elastic and flexible, exhibits yellowish color. It is found in auricle, epiglottis and small cartilages of larynx. It consists of chondrocytes embedded in lacunae, type II collagen and numerou elastic fibers. It calcifies with aging.
Subchapter Authors: Lucie Nováková and Martina Šajdíková