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Objective: This experiment was conducted to study the histological characteristics, age-related thickness changes, and expression of HSPs in the skin of yak. Methods: A total of 20 yaks (10 males and 10 females) were used. Different regions of the normal skin of three different ages (newborn, half-year-old and adult) of yaks were harvested for histological study and thickness measurement. Biopsy samples were taken from the scapula regions of the skin from the same five approximately 1-year-old yaks during the hair cycle (telogen, anagen and catagen). RT-PCR, western blot and immunohistochemistry methods using the mRNA and protein levels were used to detect the expression of HSP27, HSP70 and HSP90. RT-PCR method was used to detect the mRNA expression of CGI-58 and KDF1. The IPP6.0 software was used to analyze the immunohistochemistry and measure the thickness of the skin. Results: The general histological structure of hairy yak skin was similar to other domestic mammals. The unique features included prominent cutaneous vascular plexuses, underdeveloped sweat glands, a large number of nasolabial glands in the nasolabial plate, and hair follicle groups composed of one primary follicle and several secondary follicles. The skin, epidermis and dermis thickness did vary significantly between different body regions and different ages. The thickness of the skin, epidermis and dermis increased from newborn to adult in yaks. Yak skin thickness decreased from dorsally to ventrally on the trunk. The skin on the lateral surface was thicker than the skin on the medial surface on the limbs. HSP27, HSP70 and HSP90 showed different expression patterns during the hair cycle using RT-PCR, western blot and immunohistochemistry methods. The expression of HSP27 mRNA and protein in the anagen stage was the highest, followed by the catagen stage, and the expression in the telogen stage was the lowest. The expression of HSP70 mRNA and protein in the telogen stage was the highest, followed by the anagen stage, and the expression in the catagen stage was the lowest. The expression of HSP90 mRNA and protein in the anagen stage was the highest, followed by the telogen stage, and the expression in the catagen stage was the lowest. HSPs were mainly expressed in the outer root sheath of hair follicle during the hair cycle, also expressed in epidermis, sebaceous gland and sweat gland in the skin of Yak. The expression of CGI-58 mRNA in the anagen stage was the highest, followed by the catagen stage, and the expression in the telogen stage was the lowest. The expression of KDF1 mRNA in the telogen stage was the highest, followed by the catagen stage, and the expression in the anagen stage was the lowest. Meaning: In this study, we examined and fully described the histology of normal skin in Yak and measured the skin thickness of different ages and different regions in Yak. These data may be useful to better understand and appreciate the adaptability features of yak skin. Our investigation reports the expression patterns of HSPs in yak skin for the first time. The different expression pattern of HSPs during the hair cycle suggests they may play different roles in yak hair follicle biology. [ABSTRACT FROM AUTHOR]