The critical point drying method of preparing samples for scanning electron microscopy is associated with a variable amount of specimen shrinkage. We studied the causes of this phenomenon is isolated mouse hepatocyte nuclei and in human erythrocytes and found that the critical point drying process itself caused most of the shrinkage that we observed (a 25-30% reduction in diameter in both specimens). Glutaraldehyde fixation and ethanol dehydration caused only minimal size reduction, prior to critical point drying. Substitution of an inert (ethylene glycol-ethylene glycol monethyl ether) dehydration technique did not alter the final result. Previous studies in our laboratory using high resolution SEM and correlative transmission microscopy of isolated nuclei have demonstrated that the shrinkage represents a miniaturization of the organelles in which all structural components retain their usual relationships.

We have used high resolution scanning electron microscopy (SEM) to study the nuclear envelope components of isolated mouse liver nuclei. The surfaces of intact nuclei are covered by closely packed ribosomes which are distinguishable by SEM from nuclear pore complexes. After removal of nuclear membranes with the nonionic detergent Triton X-100, the pore complexes remain attached to an underlying, peripheral nuclear lamina, as described by others. The surface of this dense lamina is composed of particulate granules, 75-150 A in diameter, which are contiguous over the entire periphery. We did not observe the pore-topore fibril network suggested by other investigators, but such a structure might be the framework upon which the dense lamina is formed. Morphometric analysis of pores and pore complexes shows their size, structure, and density to be similar to that of other mammalian cells. In addition, several types of pore complexassociated structures, not previously reported by other electron microscope (EM) techniques, are observed by SEM. Our studies suggest that the major role of the dense lamina is associated with the distribution, stability, and perhaps, biogenesis of nuclear pore complexes. Treatment of isolated nuclei with a combination of Triton X-100 and sodium deoxycholate removes membranes, dense lamina, and nuclear pore complexes. The resulting "chromatin nuclei" retain their integrity despite the absence of any limiting peripheral structures.Despite the attention devoted to the nuclear envelope, our knowledge of the organization, function, and biogenesis of this complex membrane system is still incomplete. There is sufficient experimental evidence to suggest its role in nuclear-cytoplasmic exchange (39,40,45) and in the organization of interphase chromatin (see references 12, 14, 17, 25, 44, and 50 for recent reviews) but the mechanisms involved are poorly understood. In part, this is due to a lack of certainty about the structure and interrelationships of the various envelope components to each other and to the underlying chromatin and associated nucleoplasmic substances. For example, current methods of exploring the morphological relationship of nuclear pores to outer membrane ribosomes require extrapolation of data from either multiple thin sections (13, 24) or from freeze-etched sections (49) to reconstruct surface topography. Unfortunately, freeze-cleaving produces uncertain fracture planes, and heavy 118

Objective. To examine risk factors for sudden infant death syndrome (SIDS) with the goal of reducing SIDS mortality among blacks, which continues to affect this group at twice the rate of whites. Methods. We analyzed data from a population-based case-control study of 260 SIDS deaths that occurred in Chicago between 1993 and 1996 and an equal number of matched living controls to determine the association between SIDS and factors in the sleep environment and other variables related to infant care. Results. The racial/ethnic composition of the study groups was 75.0% black; 13.1% Hispanic white; and 11.9% non-Hispanic white. Several factors related to the sleep environment during last sleep were associated with higher risk of SIDS: placement in the prone position (unadjusted odds ratio [OR]: 2.4; 95% confidence interval [CI]: 1.7–3.4), soft surface (OR: 5.1; 95% CI: 3.1–8.3), pillow use (OR: 2.5; 95% CI: 1.5–4.2), face and/or head covered with bedding (OR: 2.5; 95% CI: 1.3–4.6), bed sharing overall (OR: 2.7; 95% CI: 1.8–4.2), bed sharing with parent(s) alone (OR: 1.9; 95% CI: 1.2–3.1), and bed sharing in other combinations (OR: 5.4; 95% CI: 2.8–10.2). Pacifier use was associated with decreased risk (unadjusted OR: 0.3; 95% CI: 0.2–0.5), as was breastfeeding either ever (OR: 0.2; 95% CI: 0.1–0.3) or currently (OR: 0.2; 95% CI: 0.1–0.4). In a multivariate model, several factors remained significant: prone sleep position, soft surface, pillow use, bed sharing other than with parent(s) alone, and not using a pacifier. Conclusions. To lower further the SIDS rate among black and other racial/ethnic groups, prone sleeping, the use of soft bedding and pillows, and some types of bed sharing should be reduced.

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