sebaceous glands secrete lipids onto the skin surface
greatest concentration on the face, followed by the back and the chest; none on the palms, soles and back of the feet
sebaceous gland activity is stimulated by androgens and adrenal corticosteroids
high sebum activity has a direct correlation with SD and acne, and an inverse relationship with
atopic eczema (AD)
skin surface lipid film is composed of both sebocyte and keratinocyte derived lipids
keratinocyte lipids are incorporated within the layers of the stratum corneum, while sebocyte lipids are secreted onto the skin surface
squalene is present only in sebum lipids and is used as a marker to differentiate sebaceous from keratinocyte lipids
certain bacteria may also contribute to the pathogenesis of SD by their ability to hydrolyse sebum and supply nutrients that promote the growth of Malassezia
Staphylococcus species and M. restricta were associated with an increased incidence of scalp disease
Propionobacterium species and M. globosa were associated with a normal scalp
the balance of M. restricta with other bacterial and fungal organisms was found to be important in the development of SD
the Malassezia fungus (a lipophilic budding yeast) colonizes areas that are covered with lipids
lipase is secreted by Malassezia, resulting in the generation of free fatty acids (FFA) and lipid peroxides that activate the inflammatory response
Malassezia uses saturated fatty acids, leaving behind irritating unsaturated fatty acids such as oleic acids which are believed to be the main trigger for SD
if oleic acid is applied topically, patients with SD experience more extensive skin desquamation than non-SD subjects
the immune system then generates cytokines, such as IL-1α, IL-1β, IL-2, IL-4, IL-6, IL-8, IL-10, IL-12 and TNF-α.
this stimulates keratinocyte proliferation and differentiation.
this results in skin barrier disruption with resulting clinically evident erythema, pruritus and scaling
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the quantity of yeasts directly correlates with the severity of disease in those predisposed to SD
M. restricta and M. globosa are likely the most virulent subspecies, producing large quantities of irritating oleic acids, leading to IL-8 and IL-17 activation
virulence factors include:
high cell wall lipid content, which provides mechanical stability, promotes resistance to osmosis and also protects it from phagocytosis
increased expression of lipase genes
on the scalp and forehead, M. restricta is the most common while M. globosa is the most common species found on the chest and back - this is due to to different lipid content at different body sites
M. globosa seems to be more common in individuals younger than 14 years, and M. sympodialis seems to be more common in older subjects.
adults between 21-30 have been shown to have the highest positive culture rate, with the chest having the highest and the thighs having the lowest positive culture rates of all evaluated body parts
IL-17 and IL-4 might play a big role in pathogenesis but needs further research
The complex interplay of Malassezia, keratinocytes and the immune response against an altered lipid composition in the skin plays a crucial role in SD pathogenesis.
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HIV / AIDS, SD is thought to be secondary to a combination of immune dysregulation and disruption in skin microbiota with unhindered Malassezia proliferation.
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Parkinson's disease, SD is most likely secondary to parasympathetic hyperactivity with increased sebum production as well as facial immobility which leads to sebum accumulation