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The early days of electron microscopy provided the first views of cell ultrastructure. This electron micrograph shows the plasma membranes of two closely apposed glial cells from the central nervous system of the annelid Aphrodite (commonly known as a Sea mouse) obtained from a thin section. EM images of cell membranes, such as this one, suggested a tri-laminar structure to the membrane, detected as two thin dark lines separated by a layer of lighter intensity. The ability to visualize previously unseen ultrastructural elements of the cell, like cell membranes, fueled debate about their composition, organization, and biological function. The distance of the intercellular cleft between the two membranes is 15nm. This is Figure 1 from Chapter 1 (Cell Surface) of 'The Cell, 2nd Ed.' by Don W. Fawcett M.D. A PDF copy of the accompanying chapter is available on the ASCB's BioEDUCATE website.

Biological Sources
NCBI Organism Classification
Cell Type
glial cell (sensu Nematoda and Protostomia)
Cellular Component
plasma membrane
extracellular region part
Biological Context
Biological Process
plasma membrane organization
Don W. Fawcett
ISBN 0721635849
The Cell, 2nd Ed. by D.W. Fawcett
Digital Object Identifier (DOI)
Archival Resource Key (ARK)
Grouping This image is part of a group.
Image Type
transmission electron microscopy (TEM)
Image Mode
illumination by electrons
detection of electrons
Parameters Imaged
electron density
Source of Contrast
stain with broad specificity
Visualization Methods
osmium tetroxide
lead salt
Data Qualifiers
processed data
Sample Preparation
osmium tetroxide fixed tissue
Relation To Intact Cell
microtome-sectioned tissue
Spatial Axis Image Size Pixel Size
X 1004px ——
Y 1293px ——