In an attempt to understand the factors involved in morphogenesis of a complex cell like a scale or bristle, the fine structure of the normal development of bristle cells in Drosophila melanogaster (Oregon R ) has been studied and compared with that of the mutants sn3 and Sb. In the development of the normal bristle rounded bundles of longitudinally oriented fibrils lie just beneath the cell surface at regularly spaced intervals. Fiber bundles constitute about 20% of the cross sectional area. The cytoplasmic surface between these bundles is active in enveloping the nerve fiber associated with the bristle and in sending out cytoplasmic processes associated with which the longitudinally oriented bristle ridges form. Singed bristles are bent and twisted and the fiber bundles are present as flattened bands constituting only about 5% of the cross-sectional area. In Sb mutants the total cross-sectional area of fiber bundle material is the same as that in Oregon R, but fiber bundles are smaller and more numerous, being distributed over the larger surface of this thicker and shorter bristle. They constitute only 7% of the cross-sectional area of the bristle. In Sn3Sb mutants characteristics of each gene are exaggerated and an extremely short, wide, and irregular bristle is formed.A close description of the fine structure of developing scales and bristles (Paweletz and Schlote, '64; Overton, '66; Locke, '66; Lawrence, '66) suggests that the orientation and grouping of microtubules and microfibrils are important in the morphogenesis of these cells. It is difficult to interfere experimentally in a system of this kind to confirm this suggestion. Although shape changes in scales have been produced by heat treatment (Kohler and Feldotto, '37), irradiation (Lobbecke and Muller, '60), and administration of DNA base analogues (Caspari, Muth and Pohley, '65) none of these changes are such that they could be easily analyzed at the fine structure level. Attempts to analyze factors involved i n the development of cell shape have also been made by considering the effects of genetic interference (Shoup, '67; Waddington, '62). This approach has been utilized here. Singed mutants which have bent and twisted bristles and stubble mutants which have regularly shaped but short and thick bristles were compared with wild type bristles and with singed stubble when present together. Although mutations which eliminate certain microtubules in other systems are known (Warr et al., '66) in the bristle mutants studied here there was no evident effect on the microtubules. However, the fiber bundles which are associated with the cell surface of the developing bristle rudiment were markedly different in their size, shape, and number in the mutants. Thus a difference i n cell shape can be correlated with differences in the amount and distribution of this fibrous material. Microtubules may, of course, also be involved in cell elongation but the present work provides no critical test of this. MATERIALS AND METHODSMutants studied were sn3 (1-21.0),...

Cell surfaces in the duodenal mucosa have been studied in maturing tissue of the chick from incubation until hatching. Changes in the distribution of mitoses in this tissue give an indication of its rate of maturation. This rate is paralleled in developmental changes in microvilli and junctional complexes. Concentration of mitotic figures towards the base of villous folds is gradual from day 9 to day 16, then rapid to day 19, after which the mature pattern is acquired. By day 11, microvilli appear in a regular pattern which does not alter through hatching. Their height remains the same to day 16 when it increases gradually to day 19, then very sharply. The angle formed between the microvilli and the cell surface increases gradually to day 16, giving evidence of advancing internal structure. Changes in cell adhesion also occur at day 16. Thereafter, following trypsin treatment cells are held together in patches by the tight junctions of the terminal bar, although the desmosomes are separated. The timing of these morphological changes is compared with that of alkaline phosphatase accumulation in this tissue as reported by Moog (13). Increase in the surface area of the microvilli parallels the increase in the activity of the enzyme.

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The development of scale cells in insects has been studied from the appearance of the first cytoplasmic projection which forms the scale rudiment. This rudiment contains numerous longitudinally oriented microtubules throughout. Immediately under its outer surface lie a series of adjacent but distinct bundles of longitudinally oriented circa 60-A fibrils with a circa 120-A center-to-center spacing. As the rudiment broadens, the microtubules become distributed near the surface. The rudiment finally becomes extremely broad and flattened. Fibril bundles are now widely separated and equally spaced. They still lie immediately below the cell surface. Then the cytoplasm protrudes midway between each fibril bundle to form longitudinal ridges and the major shape changes of the scale have been achieved. The final pattern can thus be related to the cytoplasmic organization of the rudiment. The main cytoplasmic elements which seem important in scale morphogenesis, on the basis of frequency, orientation and grouping, are 60-A fibrils and microtubules.The difficulties in understanding how regional differences in mature cells arise have been stressed by Waddington (23). In the development of an insect scale, there is a radical modification of cell size and shape resulting in an intricate surface pattern. One may ask if the development of the final pattern is foreshadowed in any way by the cytoplasmic organization of the cell in the earliest stages of the change.

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