Tissue-culture microscope 3.5 is a freeze-fracture electron micrograph of the apical surfaces of epithelial cells lining the lumen of the ileum. For each numbered item naming, or stating the function of a specific structure, choose the ONE best answer.
232. The surface projections indicated by the
(A) microveilli
(B) cilia
(C) basal bodies
(D) desmosomes
(E) flagella
233. If these structures were isolated by cell fractionation and examined by gel electrophoresis, which of the following proteins would most likely be found in large amounts?
(A) dynein
(B) hemoglobin
(C) tubulin
(D) actin
(E) spectrin
234. The membranes covering these surface projections
(A) are held together by numerous desmosomes
(B) do not have a phospholipid bilayer
(C) are continuous with the plasma membranes of the epithelial cells
(D) synthesize the glycoproteins which cover the projections
(E) lack intramembrane particles
235. A transmission electron micrograph of the same projections would be most likely to reveal
(A) numerous microtubule protofilaments
(B) bundles of microfilaments
(C) thin filaments containing actin interdigitating with numerous thick filaments made of myosin.
(D) many secretory vesicles attached to bundles of intermediate filaments of the keratin superfamily.
(E) dense patches on the interior (cytoplasmic) surface of the membranes, to which small bundles of intermediate filaments attach.
236. If the filaments that fill the cores of the projections were traced “downward” Into the apical cytoplasm of the epithelial cells it would be seen that
(A) they insert into a terminal web containing actin microfilaments and a special myosin isotype
(B) all the filaments terminate on dense bodies comprised of the protein ?-actin
(C) the filaments insert into microtubule organizing centers that surround the basal bodies from which the projections radiate
(D) they become continuous with the intermediate filaments that anchor the projections to the Golgi apparatus
(E) the filaments forming the core of the projections are oriented so that adjacent filaments have opposite polarity form one another, thus guaranteeing that filament ends at the tips of the projections are not all of the same polarity
ANSWERS AND TUTORIAL ON ITEMS 232-236
The answers are: 232-A;233-D;234-C;235-B;236-A. Tissue-culture microscope 3.5 show that the apical surfaces of many absorptive epithelial cells contain projections called microvilli; when such microvilli are very numerous-as is the case here – they are referred to as a brush border. Microvilli are surface projections whose principal function is toe provide an expanded membrane surface into which transport proteins can be inserted; the membrane covering them is a typical phospholipids bilayer (with assorted integral and peripheral membrane proteins) and is extended form by a core bundle of actin microfilaments.
All the microfilaments in microvilli are oriented with the same polarity, thus they are all oriented in the “same” polarity as is seen with the insertion of the thin filaments of skeletal muscle cells into the Z-lines of sarcomeres. Microvilli do not appear to functions as motile organelles; they do not contain significant amounts of myosin, nor do they have other cytoskeletal proteins such as tubulins, dyneins, or intermediate filament proteins. The microfilaments of the microvillar cores extend down into the cortical (or terminal) web that fills the apical cytoplasm of the epithelia cells. The terminal web contains other actin-microifilaments, that insert into the tight junctions linking the epithelial cells together.
The cortical web also contains a special myosin that forms small myosin filaments, whose function is still not well understood. Immediately below the cortical web, the cytoplasm contains large numbers of intermediate filaments; these insert into the desmosomes that also help hold adjacent epithelial cells together. Student Low Power Microscopes Teaching Microscopes