apocrine sweat glands dermatology dermis eccrine sweat glands eponychium hair human skin color inner root sheath integumentary system keratinocyte lamina densa lamina lucida medical subject headings nail plate outer root sheath panniculus adiposus panniculus carnosus papillary dermis paronychium reticular dermis sebaceous gland skin appendage skin disease stratum corneum stratum germinativum stratum granulosum stratum lucidum stratum spinosum subcutaneous tissue