Short Answer: Answer 2/3 or all 3 for extra credit (5 pt each)Answers are to be typed and handed in on 11/14/05 or must be emailed to me at teachbiochem@aol.com by 8:00 PM on 11/14/05.

1. Compare and contrast the four primary types of tissue in the human body on the basis of their major functions, location and structural characteristics. Explain how the ratio of cells to extracellular matrix differs in the tissue types and how the characteristics of the extracellular matrix differs in the tissue types.

2. Briefly describe the 4 stages of wound healing. Label the inflammatory phase, the proliferative phase, and the remodeling phase.Which would heal faster, cartilage or bone? Stratified squamous or simple columnar? Why?

3. Describe the three types of intercellular junctions – include their structure, function and location.

1. The four primary types of tissue in the human body are epithelial, connective, muscle, and nervous tissue. They are differentiated by their function, cell-to-matrix density, and intercellular matrix composition.

Epithelial tissue mainly functions to absorb and secrete various substances (mucus, serous fluid) as well as acting as barrier surfaces (skin, blood vessels, alveoli, kidneys, uterus, intestines, etc.); epithelial cells are densely packed, with little IC matrix (including no blood vessels), and rest on a basal lamina (basement membrane) made of connective tissue that also provides nutrition.

Connective tissue functions as mechanical support/protection material, transport organs (blood), energy storage (adipocytes) and heat production, and connecting tissue for organs. Connective tissue is very loosely distributed, despite being overall most common and varied tissue type, so tissue-to-matrix ratio is very low. The IC matrix is made of glycosaminoglycans (GAGs), involved in water and electrolyte balance since they attract sodium ions and water molecules; proteoglycans (protein GAG composites) that bond with other cells or macromolecules; and glycoproteins (protein-carbohydrate composites) that bind plasma with collagen or proteoglycans and also demarcate cell migration routes.

Muscle tissue is involved with movement – movement of food down the intestines, movement of blood through the body, movement of bones, etc. Muscles are tightly packed and need a lot of energy, so they have many blood vessels going through them. Nerve tissue lie in a bed of glial cells (also nerve tissue, I guess) and blood vessels. Glial cells outnumber neurons by several orders of magnitude.

2. The four stages of healing are the hemostatic stage, inflammatory stage, proliferative state, and remodeling state. The hemostatic stage involves the constriction of blood vessels (to contain blood), platelet aggregation and release of clotting factors. Shortly after, the inflammatory stage starts with the release of histamine from mast cells which actually dilate blood vessels (increasing permeability as well as blood flow) so that antibodies and leukocytes can flow in for pathogenic “damage control.” Many people don’t make the distinction between the hemostatic and inflammatory stages since they are mostly simultaneous, occurring over 2 to 5 days. The next stage, the proliferative stage (lasting 2 days to 3 weeks) involves the formation of collagen fibers by fibroblasts (clot)

and repair of damaged blood vessels; the wound edges contract, local temperature rises from higher metabolic activity, and epithelial cells start forming scar tissue over the still moist and swollen wound surface. The last stage is the remodeling stage which involves 3 weeks to 2 years of slow reorganization and strengthening of collagen scar tissue so that wound partially returns to original appearance.

Bone heals faster than cartilage because cartilage has no blood vessels coursing through them – it must rely on diffusion of the necessary substances from nearby connective tissue, rather than a direct blood source. Simple columnar tissue heals faster than stratified squamous tissue for the same reason, as stratified squamous tissue is often not in direct contact with a blood supply.

3. There are three types of intercellular junctions: tight junctions, gap junctions, and desmosomes. Tight junctions are self-explanatory, and the cells are connected by ridges and grooves sort of like a jigsaw puzzle – tight junctions are impermeable, and totally encircle the cells, helping form a barrier surface that suits them for forming the wall linings of the GI and urinary tracts. Desmosomes are similar to tight junctions but do not totally encircle the cell and have connecting filaments that end in a protein plaque (mass) – they are a little more elastic, and tougher too, and are found in the uterus, heart, and epidermis. Gap junctions are interesting: they are also known as communicating junctions because they are a ring of 6 proteins (transmembrane i.e. spanning membranes) that form a water channel for the passage of electrochemical signals – useful for peristalsis (and so found in smooth muscle), coordinated muscle contractions (like in the heart), and more general cell communication (in the embryo).