Pigment granules accumulate in human cells for various reasons. The granules may be stored in the organelles that produce and/or transport them, or they may be cytoplasmic inclusion bodies that result from aging or disease. Organelles are found in the cytoplasm of the cell, as opposed to the nucleus, which contains the genetic material.
Melanin is a pigment that gives the skin, hair, and eyes their color and protects the body from ultraviolet rays (e.g., sunlight). It is produced by specialized cells called melanocytes (also found as retinal pigment epithelial cells in the eye,) which produce and contain an organelle called the melanosome. They also share melanosomes with keratinocytes for hair and skin pigmentation. Wasmeier et al. published a review of melanosome structure and function in the Journal of Cell Science in 2008.
UV irradiation stimulates the production of black eumelanin, one of two types of melanin produced in mammals. Both eumelanin (brown-black) and pheomelanin (blond-red) occur in hair, but the presence of both in the cytoplasm of skin cells was not known until 1991, when Thody et al. published such a study in Investigative Dermatology.
Melanin is also found in the brain. The substantia nigra is so-named due to the black pigment found in the cytoplasm of its neurons. The precursor of melanin is also found in the brain – DOPA (dihydroxyphenylalanine), as melanin is a byproduct of dopamine production. As described by the University of Wisconsin’s Online Neuroscience Resource, degradation of the melanin-containing cells of the pars compacta in the substantia nigra is seen in Parkinson’s Disease.
Melanin levels in the cytoplasm can change with age or disease – resulting in grey hair, age spots, and even being associated with skin cancer (melanoma). The age-related changes in pigmentation were reviewed in the Annals of the NY Academy of Sciences in 2002.
Lipofuscin is also called the aging pigment because it accumulates with age. The brownish granules are a byproduct of the breakdown of damaged red blood cells and accumulate in the subsarcolemmal region of the muscle fibers in the heart and smooth muscles. As shown by Washington University in St. Louis, lipofuscin stains with acid phosphatase and esterase, as well as a number of other visualization methods.
Lipofuscin granules are found in cytoplasmic vacuoles near the nucleus. They increase with age due to normal accumulation in non-dividing cells like nerves and muscle, and in some diseases – lipid and centronuclear myopathies (muscle disease, including acromegaly), denervation atrophy, and chronic obstructive pulmonary disease (COPD). In active myopathies, such as Duchenne’s muscular dystrophy, lipofuscin is usually reduced.
Ceroid pigments are associated with “liver spots,” or aging spots, darker splotches that occur on the skin of the elderly (which are often attributed to lipofuscin.) Less is known about these pigments compared to melanin and lipofuscin. Ceroids are waxy, lipid-protein polymers thought to be the result of lipid peroxidation and an indication of oxidative stress and Vitamin E deficiency, as indicated by research in 1971 and 2002. A number of definitions for the pigment exist that do not necessarily agree because not much is known, including in which cells the pigment accumulates – the liver, muscles, and nerves all potentially suffer disease upon ceroid accumulation. Ceroid pigment is considered by some researchers to be oxidized lipofuscin.