Toward a predictive model of Ca2+ puffs.

We investigate the key characteristics of Ca(2+) puffs in deterministic and stochastic frameworks that all incorporate the cellular morphology of IP(3) receptor channel clusters. In the first step, we numerically study the Ca(2+) liberation in a three-dimensional representation of a cluster environment with reaction-diffusion dynamics in both the cytosol and the lumen. These simulations reveal that Ca(2+) concentrations at a releasing cluster range from 80 to 170 microM and equilibrate almost instantaneously on the time scale of the release duration. These highly elevated Ca(2+) concentrations eliminate Ca(2+) oscillations in a deterministic model of an IP(3)R channel cluster at physiological parameter values as revealed by a linear stability analysis. The reason lies in the saturation of all feedback processes in the IP(3)R gating dynamics, so that only fluctuations can restore experimentally observed Ca(2+) oscillations. In this spirit, we derive master equations that allow us to analytically quantify the onset of Ca(2+) puffs and hence the stochastic time scale of intracellular Ca(2+) dynamics. Moving up the spatial scale, we suggest to formulate cellular dynamics in terms of waiting time distribution functions. This approach prevents the state space explosion that is typical for the description of cellular dynamics based on channel states and still contains information on molecular fluctuations. We illustrate this method by studying global Ca(2+) oscillations.

A technique for correlative scanning and transmission electron microscopy of individual human placental villi: an example demonstrating syncytial sprouts in early gestation.

Correlating the surface appearances of certain features with their internal structure is made particularly difficult in the human placenta by the complex three-dimensional branching pattern of the villous tree. This places a possible limitation on the use of the scanning electron microscope in this field, both for basic research purposes and as a tool in pathological diagnosis. To help overcome this problem, a technique for handling individual placental villi has been devised. By attaching single villi to glass pipette tips it has proved possible to scan the villi, embed them in resin and then section them in a known pre-determined orientation. Exact correlations between the surface appearances and the internal structure, as seen with either the light or transmission electron microscope, can then be drawn. This paper describes the technique and, using an example based on syncytial sprouts in early pregnancy, illustrates the precision afforded by the method.

Nasal polyps. Scanning electron microscopy and artifact.

Nasal polyps are grossly oedematous tissue which has undergone considerable shrinkage when processed for scanning electron microscopy (SEM), converting the normally smooth surface into one with undulations. This shrinkage resulted in variable degrees of loss of the surface epithelium and thus gave rise to artifact. In its severest form, the entire surface epithelium was removed and the basement membrane exposed; lesser damage resulted in exposure of the basal cells, giving the previously described 'cobblestone' appearance. When the epithelium was intact, it was typical respiratory in type.

Scanning electron microscope studies of the endometrium of the cyclic mare.

Endometrial biopsies obtained from mares at different stages of the oestrous cycle, during anoestrus and in various abnormal conditions were examined with the scanning electron microscope. Preliminary observations suggest that the patterns of secretory and ciliary activity in the uterine epithelium are similar to those observed by electron microscopical techniques in laboratory and other large domestic animals. The response of the epithelial cells to hormonal variations and infections is compared with that of the endometrium as seen with the light microscope.

Ultrastructural studies on the surface membrane of the mouse egg.

Fertilized and unfertilized mouse eggs were examined by scanning and transmission electron microscopy for evidence of mosaicism in the organization and concanavalin A-binding properties of their surface membranes. No obvious quantitative mosaicism in concanavalin A binding was noted. The egg membrane was microvillous over most of its surface, but was smooth in the region overlying the 2nd metaphase spindle of the unfertilized egg and on the polar body of the fertilized egg.