Human hair follicles (HF) present one of the highest rates of cell division in the human body and mainly engage in aerobic glycolysis and glutaminolysis generating lactate but are also capable of oxidative phosphorylation. HF contains high levels of glycogen, a multibranched polysaccharide of glucose and the main form of energy storage in mammals. The Cori cycle is a metabolic pathway in which excess lactate is converted into glucose through gluconeogenesis which can then be stored as glycogen. Since most glucose metabolised by HF is converted to lactate, we have investigated HF glycogen metabolism and whether the Cori cycle operates in the HF.
PAS staining and enzymatic glycogen assay revealed high levels of glycogen in the outer root sheath (ORS) of anagen HF and which was decreased in catagen and absent in telogen phase of the hair cycle. HF cultured in the absence of Insulin showed significant loss of glycogen. Analysis of main glycogen metabolism enzymes in anagen HF showed Glucose transporter 1 (Glut1) and glycogen phosphorylase (PYGL) in the ORS, mainly co-localized with glycogen, whereas glycogen synthase 1 (GYS1) was expressed in the basal layer of the ORS and IRS. Phosphoglucomutase (PGM1) was expressed in the basal ORS, cuticle and hair matrix. These distinct expression patterns may indicate the metabolic route of glucose. Lactate dehydrogenase and phosphoenolpyruvate carboxykinase (PCK1) an essential enzyme for gluconeogenesis was found in the ORS suggesting HF can synthesise glucose from lactate. Experiments carried out in HepG2 cells as positive control, ORS keratinocytes and whole HFs showed upon glycogen depletion upregulation of glycogen synthase by lactate and glycogen synthesis. 
The presence of key glycogen metabolism enzymes along with other enzymes crucial in glucose metabolism support the idea of ORS being the “metabolic centre” of hair follicles.

Glycogen metabolism and Cori cycle in human hair follicles