2.4. D-Galactose-Induced Rat Skin Aging Model

Thirty SD rats (10 weeks old) were used in this experiment, randomly divided into five groups of six rats each. The six rats of the normal group were subcutaneously injected (S.C.) with saline for 8 weeks and then orally given distilled water for 4 weeks. The remaining twenty-four rats were designated to the D-galactose-induced skin aging group. The induction procedure was modified using the previous literature [36]. Briefly, rats were treated with D-galactose (100 mg/kg/day, s.c.) in normal saline solution for 8 weeks. D-galactose-induced skin aging rats were then randomly divided into four groups of 6 rats each. The six rats of the D-galactose-induced skin aging group (control group) were orally treated with distilled water daily for 4 weeks after 8 weeks of D-galactose treatment. The remaining eighteen rats were designated to the PCE treatment groups. These groups were divided into PCE-L, PCE-M, and PCE-H groups, in which the rats were orally given either PCE-L (1 mg/kg), PCE-M (3 mg/kg), or PCE-H (6 mg/kg) after 8 weeks of D-galactose treatment. After 4 weeks of distilled water or PCE administration, all rats were sacrificed to collect skin tissues from the rat’s back. The rat’s left back skin tissues were used for hematoxylin and eosin (HE) or immunohistochemical (IHC) staining. The skin tissues from the rat’s right back were used for assaying type I collagen protein expression and the hyaluronic acid content.