Polarization Microscopy - Biomedical Optical Phase Microscopy and Nanoscopy - page 301

CHAPTER 15
Polarization Microscopy
Rudolf Oldenbourg
Marine Biological Laboratory, Woods Hole, MA and Physics Department,
Brown University, Providence, RI
Editor: Lisa L. Satterwhite
15.1 Introduction
The polarized light microscope is used to analyze the anisotropy of a specimen's optical
properties such as refraction and absorption. Optical anisotropy is a consequence of
molecular order, which renders material properties such as absorption, refraction, and
scattering dependent on the polarization of light. Polarized light microscopy exploits this
dependency and provides a sensitive tool to analyze the alignment of molecular bonds or
fine structural form in a specimen.
Most biological structures exhibit some degree of anisotropy that is characteristic of their
molecular architecture, such as membranes and filament arrays. A membrane, for example,
is composed of lipid molecules in which proteins are embedded, all of which maintain some
degree of orientation with respect to the plane of the membrane. Hence, tissues, cells,
and organelles that include extensive membranous structures, such as mitochondria,
photoreceptors, and the retina, can exhibit birefringence (anisotropy of the refractive index)
and dichroism (anisotropy of the absorption coefficient) that are characteristic of their
molecular architecture.
In addition to membranes, biological structures commonly include filaments that are in
themselves anisotropic, such as collagen fibrils, stress fibers made of filamentous actin and
myosin, and microtubules (MTs) that form the mitotic spindle. Double-stranded DNA is a
polymer with high intrinsic anisotropy due to the alignment of the aromatic rings that are
part of the uniformly stacked base pairs. Figure 15.1 shows the birefringence observed in
a sperm head in which the packing arrangement of DNA results in a series of birefringent
domains inside each chromosome [1] . Figure 15.2 shows a series of birefringent figures that
appear in the center of a freshly fertilized sea urchin egg observed with a polarizing