Items 237-242

Tissue-culture microscope 3.6 is a scanning electron micrograph of the apical surface on the epithelial cells lining the trachea. For each numbered item of identity or functions, choose the ONE best answer.

237. The structures labeled A are

(A) cilia
(B) flagella
(C) microvilli
(D) microtubules
(E) collagen fibers

238. The structures labeled B are

(A) cilia
(B) flagella
(C) microvilli
(D) microfilaments
(E) collagen fibers

For Items 239 through 242 use the following answers:

(A) Structures labelled A only
(B) Structures labelled B only
(C) Both structure A nor B
(D) Neither structure A nor B

239. Formed by coordinated polymerization of keratins and vimentins.

240. Elongate structures whose core contains an elaborate array of microtubules

241. These structures would be expected to “collapse” upon treatment with cytochalasin B.

242. These structures would be expected to bind rhodamine-labelled phalloidin very strongly.

ANSWERS AND TUTORIAL ON ITEMS 237-242

The answers are: 237-C; 238-A; 239-A; 240-B; 241-A; 242-A. The scaning electron micrograph in Tissue-culture microscope 3.6 shows that the apical surfaces of the epithelial cells lining the trachea may have two district types of apical projections. The shorter ones (A) are microvilli; although not as numerous as the microvilli in the brush border of intestinal epithelial cells, these microvilli have a similar structure and function. They are maintained as elongate extensions of the plasma membrane by a core of actin mircofilaments and serve to provide extra membrane surface to house a variety of proteins that play a role in transfer of substances across the apical surface of the epithelium.

The longer projections (B) are cilia; they contain a core complex of microtubules called the axoneme and serve, by their coordinated “beating” to move fluids over the surface of the epithelium, in this case in a direction towards the lumen of the oral cavity. Because they contain a bundle of microfilaments, microvilli can be proved by against specific for actin. They will bind the fungal toxin phalloidin and, if a fluorescent derivative (rhodamine-pholloidin) of that toxin is applied, the microfilaments (and thus the microvilli) will be strongly and selectively labeled. Another probe that is nearly, but not absolutely, specific for actin is the family of drugs called cytocholasins. Cytoshalasins ( the most specific of which is cytochalasin D) bind to actins in such a way as to bring about their breakdown, hence causing large scale shortening (collapse) of the microvilli. Toolmakers Microscopes