Cross references: Red Nucleus Red Nucleus Iron
Red Nucleus Cerebellum Red Nucleus Evolution
Heme Globin Myoglobin Myoglobin Evolution
Up until 11-28-13, the Wikipedia page for the red nucleus said: "It is pale pink in color; the color is believed to be due to a form of lipofuscin ." But this didn't make sense. Lipofuscin is yellow-brown.
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Micrograph showing lipofuscin, in brown/yellow, in a liver biopsy with ground glass hepatocytes ; H&E stain
Lipofuscin is the name given to finely granular yellow-brown pigment granules  composed of lipid -containing residues of lysosomal digestion. It is considered to be one of the aging or "wear-and-tear" pigments, found in the liver , kidney , heart muscle, retina, adrenals , nerve cells, and ganglion cells. It is specifically arranged around the nucleus, and is a type of lipochrome .
1. The above micrograph is NOT of a red nucleus. It is a micrograph of a LIVER biopsy.
2. The micrograph has been stained, so the pink color is not natural. It is from the stain.
So. What colors the red nucleus red?
The answer seems to be iron, which is present in the red nucleus in at least two different forms; hemoglobin and ferritin. Abstracting from Red Nucleus Iron :
Although none of the articles I've seen so far mention hemoglobin by name, many refer to the red nucleus' high vascularity. This suggests hemoglobin.
Hemoblobin " is the iron -containing oxygen -transport metalloprotein in the red blood cells of all vertebrates  "
Although not referred to as frequently as high vascularity, high iron content is sometimes also mentioned as a characteristic of the red nucleus. "Brain iron is stored in the protein ferritin ( Hallgren and Sourander, 1958 ; Griffiths et al., 1999 ), which is present at a high concentration within some of the subcortical nuclei, specifically the globus pallidus, substantia nigra and red nucleus."
"Ferritin is a ubiquitous intracellular protein that stores iron and releases it in a controlled fashion."
Wikipedia does not specify ferritin's color, but the Sigma Aldrich catalog says:
Ferritin from equine spleen Type I, saline solution
"color - red to brown"
"a form of lipofuscin ."
" iron, which is present in the red nucleus in at least two different forms; hemoglobin and ferritin."
Wikipedia Red Nucleus Revision
131203 - 1623
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