As was true for loose connective tissue, the dense and skeletal connective tissues involve more than just cells; it is the non-living extracellular substances which determine the nature and function of the tissue. In the matrix, we find the same materials as before: (1) amorphous substances (glycosaminoglycans, proteoglycans) and (2) fibrous elements (collagen and elastin).
Dense connective tissue
This tissue is classified into two categories based on the arrangement of the fibrous elements of the extracellular matrix:
Dense irregular connective tissue in which collagen and elastic fibers are found running in all different directions and planes. Within the matrix the fibroblast is the major cell type and is responsible for the repair and maintenance of the matrix (Dense Irregular CT).
Dense regular connective tissue in which the extracellular fibers all run in the same direction and plane. It is further classified relative to function and by the type of fibers present. If the matrix consist mainly of collagen, it provides great tensile strength and can withstand tremendous pull in the direction that the fibers run without stretching. This type of dense regular connective tissue is found in the tendon, which connects muscles to bones or cartilage (Tendon 1). Between bundles of eosinophilic collagen, fibroblasts are readily observed (Tendon 2). Ligaments connect bones to bones in the movable joints. Since bony tissue has no flexibility, the connective tissue must provide it and therefore elastic fibers are now found in the matrix; collagen is also present to provide tensile strength (Ligament 1). Histologically, ligaments consist of bundles of eosinophilic collagen, fibroblast, and yellowish elastin fibers (often with a "ribbon candy" morphology) (Ligament 2).
Note the lack of blood vessels within these tissue; all are considered avascular, with oxygen and nutrients diffusing in to the cells from blood vessels outside the tissue.
Mature cartilage is a relatively solid, non-vascularized tissue. During the development of the fetus it plays a major role, since most of axial skeleton is laid down as cartilage in the embryo. This cartilage is replaced by bone. After post-natal growth ceases, cartilage is found in only two general areas of the body: (1) extra-skeletal (nose, ear, epiglottis, trachea) and (2) covering the caps of bones in movable joints. The types of cartilage found in these areas are classified on the basis of differences in the type and amount of fibers present in the matrix. All types also contain the amorphous matrix materials.
Hyaline cartilage or pure cartilage possesses a matrix consisting of collagenous fibers and glycosaminoglycans, which are not visible in ordinary preparations and give the matrix a light blue color in routine preparations. Basically a piece of cartilage has at its center the most mature cells, surrounded by immature cells and a chondrogenic layer (Hyaline Cartilage 1). The mature cells of cartilage, chondrocytes, surround themselves with matrix living in pits called lacunae. Since these cells can still proliferate, one can find up to four cells in a lacunae. This organization is referred to as a cell nest. This morphology is found in the central portions of cartilage. As one moves toward the periphery, the chondrocytes are smaller and are usually individually arranged. These cells, the chondroblasts, represent newly formed cells, actively synthesizing matrix to establish lacunae and cell nests. Surrounding cartilage is a multicellular layer of flattened cells called the perichondrium. The inner layers possess chondrogenic properties, thereby permitting the appositional growth of cartilage; the outer layer represents a covering of dense fibrous connective tissue (Hyaline Cartilage 2 and Hyaline Cartilage 3).
Elastic cartilage has the same basic organization as hyaline cartilage, containing chondrocytes in lacunae, cell nests, and perichondrial layer. The major differences between these two tissues is that the matrix of elastic cartilage contains large amounts of elastin (Elastic Cartilage 1). These fibers are often present in such high concentration to mask the straining of other matrix components (glycosaminoglycans, collagen), give this type of cartilage a great deal of flexibility (Elastic Cartilage 2).
Fibrocartilage is a transitional tissue between dense regular connective tissue and cartilage or bone. It consists of rows or lines of rounded chondrocytes in lacunae between parallel bundles of collagenous fibers (Fibrocartilage 1). The collagen fibers stain pink and the amorphous matrix materials around the chondrocytes stains basophilic (blue-purple) (Fibrocartilage 2). This cartilage type lacks a perichondrium.
Bone composes most of the adult skeleton. Its rigidity is produced by the decrease of amorphous matrix components, an increase in collagenous fibers and the calcification of the matrix. Two types of bone, based on the density or amount of space between calcified tissues, are found in the body: compact (dense) bone and spongy (cancellous) bone. The two basic preparations used are to study bone histology: (a) dry ground bone, cut with a saw; and (b) decalcified bone, in which the mineral content is removed by acid treatment. In the dry ground bone, the organic material is destroyed and relative position of cells indicated in the sample by staining with India ink. In decalcified samples, the relationship between cells and the organic matrix is maintained.
Compact or dense bone is a highly vascularized tissue with no cell greater than 0.2mm from a blood vessel. The calcified tissue is built up in concentric lamellae around a central blood vessel producing a Haversian System or Osteon. In dry ground specimens, osteons are identified by the central blood vessel running in the Haversian Canal. The mature bone cells (osteocytes) live in lacunae as single cells connected to one another by cell processes housed in tiny channels in the calcified matrix call canaliculi (Dry Bone 1). Blood vessels running between Haversian systems travel in canals known as Volkmann canals and are usually visualized running at right angles from the center of a Haversian system (Dry Bone 2). In decalcified sections, the bone organic matrix, rich in collagen, albumin, and glycosaminoglycans stains a pink-purple and a surrounding layer, the periosteum is observed (Decalcified Bone 1). The individual osteocytes, Haversian canals, and Volkmann canals are all easily identified (Decalcified Bone 2).
Cancellous or Spongy Bone is found in two places: (1) during bone development and (2) with the marrow cavities of the long bones. It is characterized by the presence of irregular "bars or beams" of calcified tissue (Developing Bone 1). Each piece consists of osteocytes in lacunae connected by canaliculi, surrounded by a layer of flatten (osteogenic) or columnar (osteoblastic) cells, and areas of calcified and organic (osteoid) matrix (Developing Bone 2).
Since bone serves as a store for the body's calcium, it is constantly being remodeled. Osteoclasts, the cells which resorb bone, are often found associated with cancellous bone spicules with the marrow cavity (Osteoclast 1). These cells are very large, usually multinucleate, and are found in pits (resorption pits) within bone (Osteoclast 2).
Bone develops by two different processes in the body. It can form directly from embryonic
mesoderm (intramembranous ossification), primarily in the skull, or from cartilage
(endochondral ossification), by which the axial skeleton forms.
Intramembranous ossification occurs when embryonic mesenchyme organizes into groups of cells forming an ossification center. Similar to the appositional growth of cartilage, the central cells of will begin to synthesize organic matrix, establish canaliculi with their neighbors, and begin to calcify the matrix becoming osteocytes. Those cells on the outside will proliferate as osteoblasts, producing a new layer before maturing as osteocytes. The tissue first becomes organized as cancellous bone with the spaces filling in to produce the osteons of compact bone.
Endochondral ossification occurs within cartilage. During embryonic development, the axial skeleton is initially formed by cartilage. By the process of endochondral ossification, the cartilage is replaced by bone as the long bones grow. The diaphysis (shaft) of the bone consists of compact bone with cancellous bone in the developing marrow cavity. The epiphysis (end) of the bone is composed of cartilage. At the interface with the developing bone (transition zone), 4 different zones are seen in the epiphyseal cartilage: (1) resting zone, (2) proliferating zone, (3) mature zone, and (4) calcifying zone (Epiphysis 1).
The synovial or free moving joints of the body are composed of a slippery surface (articular cartilage), a lubricating fluid (synovial fluid), and are held together by the joint capsule (Joint 1). The articular cartilage lacks a perichondrium and appears as a thin layer on the top of the bone (Joint 2). The joint capsule is lined by the synovial membrane which consists of an inner most epithelial layer, lying on top of a layer of adipose. This membrane produces the lubricating synovial fluid. The outer most layers of the joint capsule consist of dense connective tissue and ligaments (Joint 3).
See if you can identify the following types of connective tissue:
Scope 1 Scope 2 Scope 3 Scope 4 Scope 5 Scope 6 Scope 7 Scope 8 Scope 9 Scope 10 Answers