Cytoplasmic inclusions

Inclusions are cytoplasmic accumulations of metabolic products or stored substances, usually not bounded by a lipid bilayer. Glycogen forms 20–30 nm β-particles (assembled into larger α-rosettes in hepatocytes), abundant in liver and muscle; it stains magenta with PAS and is removed by diastase. Lipid droplets store triacylglycerols and cholesteryl esters inside a phospholipid monolayer coated with perilipins; they are dissolved by routine paraffin processing, leaving empty vacuoles, but can be stained with Oil Red O or Sudan dyes on frozen sections.

Pigments include lipofuscin — a yellow-brown "wear-and-tear" pigment of oxidised lipid and protein in residual bodies, accumulating with age in neurons, cardiac myocytes and hepatocytes; melanin in melanocytes, keratinocytes, retinal pigment epithelium and (as neuromelanin) the substantia nigra; and haemosiderin, golden-brown aggregates of ferritin-derived iron in macrophages, stained blue by Perls' Prussian blue. Crystals occur in a few cells, e.g. the Reinke crystals of Leydig cells.