Lesson 7 - Connective Tissue Types - Specialized (Modified) Connective Tissues
Student Performance Objectives1. List 3 types of cartilage and two locations where each is found. 2. Explain why cartilage heals so slowly.3. How is the matrix of bone different from that of all other connective tissues?4. Explain how adipocytes and adipose tissue form. 5. List three locations for adipose tissue accumulation and give three functions for this tissue.
Lesson OutlineA. Cartilage - cartilage begins in the embryo as stem cells aggregate roughly in the shape of the structure to be formed and secrete typical connective tissue matrix. Stem cells are called chondroblasts when they begin secreting the substances that give cartilage matrix its typical consistency and the cells are enclosed in fluid-filled spaces (lacunae) within the matrix. The toughness of the matrix makes it difficult for the in-growth of blood vessels, hence cartilage is avascular and derives its nourishment by diffusion from adjacent tissues (see below, A, section d). Due to its limited nutrition supply it heals slowly when injured. The 3 following types of cartilage found in adults are classified on the basis of their proportion of fibers in the ground substance of the matrix. a. Fibrocartilage - the dense aggregates of collagen fibers embedded in the cartilage matrix give fibrocartilage great strength in withstanding compression. It is found between the minimally movable skull bones and between the vertebrae as the intervertebral discs. b. Elastic cartilage - the predominant fiber type here is elastin that gives the cartilage matrix a yellowish color and flexibility. It is found in the cartilages of the larynx that flex to move the vocal cords, in the epiglottis, the external ear, and in the eustachian tube linking the middle ear with the nasopharynx (for air pressure equalization). c. Hyaline cartilage - both collagen and elastin fibers are present in the ground substance of this most common of all the cartilages in the human body. Its color is a translucent bluish-white and it is found composing the early embryo prior to bone formation, as articular cartilage on the joint-surfaces of bones, as the costal cartilages joining ribs to the sternum, and in the rings that hold open the respiratory passages from the trachea through the finest branches of the bronchi. d. Special note - with one exception, all cartilage tissue is enclosed within a fibrous connective tissue called the perichondrium (analogous to the periosteum covering bone surfaces). Cartilage is nourished through the blood supply within the perichondrium and, most importantly, the perichondrium contains stem cells required for cartilage repair. The one cartilage location lacking a perichondrium is the articular surfaces of bones in joints. Why do you suppose this is so and what conclusions may be drawn from this?B. Bone - Many details will be given about bone as a tissue and bones as organs and structural elements of the human body, but not in this section. We observe bone to be a greatly modified connective tissue in that the matrix is ossified by the presence of apatite salts (inorganic complexes of calcium, phosphate and other minerals) combined with the typical collagen fibers of connective tissue. Its hardness and ability to "give" slightly under stress makes it exceedingly powerful under tests of both tensile and compressional strength.C. Fat (adipose tissue) - interestingly, it is the cells, rather than the fiber type or consistency of the ground substance, that become the most important feature in the modified connective tissue we call fat. In addition to storing energy, fatty tissue cushions other tissues from damage and contours the body through its distribution pattern. 1. Adipose tissue typically develops within the framework of surrounding, loose, irregular connective tissue (areolar). 2. The stem cells in the region that will become adipose tissue begin accumulating the typically human semi-liquid lipid droplets (other animals, like the cattle we use for our food supply, accumulate more solid fatty deposits). 3. The droplets merge, fat deposition continues until the cells are filled with a single large fat droplet, and the cytoplasm and cell nucleus become jammed to the sides of the cell against the cell membrane. 4. Mature fat cells, or adipocytes, cluster together in groups and are restrained in movement by a surrounding layer of reticular connective tissue fibers. These fibers are embedded in areolar connective tissue (see C, section 1, above). 5. The whitish or yellowish fat lobules visible to the naked eye during mammalian dissection are groups of adipocytes held together by reticular fibers. Squeezing them ruptures the adipocytes and give the tissue its fatty, greasy feel. 6. Adipose tissue accumulates, according to a genetically determined pattern, under the skin (subcutaneous fat), in the membranes of the abdominal cavity including the greater omentum and mesentery, and around organs such as the kidneys and heart. 7. Fat also accumulates in special, shock-absorbing fat pads in the feet, behind the eyeball and in joints where it cushions and protects other tissues.