WJEC Biology for A2: Student Bk

YOU SHOULD KNOW › › › ››› The structure and locations of hyaline, fibrous and elastic cartilage ››› The structure of compact bone ››› The roles of osteoblasts and osteoclasts Key Terms Connective tissue : Tissue that connects and supports organs or other tissues. It has cells embedded in a matrix with fibres of collagen and elastic tissue. Cartilage : Hard, flexible connective tissue in the respiratory airways, at the ends of bones, at the anterior end of the ribs, at the nose and the outer ear. Skeletal tissues The early embryos of most animals have cells arranged in germ layers. A germ layer contains the cells that eventually become the tissues of the body. In nine-day-old human embryos, cells form three germ layers, the ectoderm, mesoderm and endoderm. Connective tissue , including the skeletal tissues bone and cartilage, develops from the mesoderm. Cartilage Cartilage is a hard, flexible connective tissue. It is more compressible and flexible than bone, but less so than muscle. ▪ Being somewhat rigid it can hold open tubes, such as the trachea and bronchi, the Eustachian tube, between the middle ear and the throat, the nostrils and the pinna. ▪ Being flexible, it permits movement of the ribcage. ▪ It returns to its original shape after bending and has a role in load-bearing joints such as the hips and knees. The cells in cartilage are chondrocytes . They secrete an extra-cellular matrix made of a transparent protein called chondroitin. It may contain fibres of collagen and of elastic material. Chondrocytes are in spaces in the matrix, called lacunae. Cartilage has no blood vessels so nutrient acquisition and gas exchange rely on diffusion. Because diffusion is slow, cartilage has a very slow turnover rate and healing is very slow. Cartilage has no nerves. There are three types of cartilage, defined by their density and type of fibres they contain: ▪ Hyaline cartilage has a high proportion of collagen but is the weakest type of cartilage. It becomes bone in the foetus, i.e. it ossifies, but is retained in the adult as the articular cartilage at the ends of bones, joining the ribs to the sternum, in the nose, larynx, trachea and bronchi. If it is damaged, it is replaced by fibrocartilage scar tissue. A fibrous coat of connective tissue, called the perichondrium, surrounds hyaline cartilage. ▪ White fibrous cartilage (fibrocartilage) is the strongest cartilage, and makes the intervertebral discs and ligaments. Its collagen is organised in dense fibres so it has greater tensile strength than other types of cartilage. The fibres are arranged in the direction of stress. Diagram to show structure of fibrocartilage lacuna chondrocyte extra-cellular matrix containing collagen Hyaline cartilage in the light microscope lacuna collagen bres matrix chondrocyte 279 Option B: Human musculoskeletal anatomy