Anatomy, physiology, and pathology of the skin
Skin is an intricate organ, providing a protective outer layer that supplies insulation, mechanical protection, defense from pathogens, and temperature regulation. This chapter reviews skin anatomy, embryology, and physiology, as well as age-related changes focusing on photoaging.
The earliest stage of skin development is known as the specification stage, which occurs by the 4th week of gestation. During specification, ectoderm lateral to the neural plate is committed to become epidermis, and subsets of mesenchymal and neural crest cells are committed to form the dermis. Initially, a single layer of ectodermal cells covers the embryo’s surface. In the 2nd month of gestation, this layer divides to form a superficial protective layer of simple, flattened squamous epithelial cells, the periderm. The cells of the periderm layer then undergo keratinization and begin to slough superficially to be replaced by cells arising from the basal layer. The framework of future layers and specialized structures of skin are now formed. Next, during the morphogenesis stage, these tissues begin to take their particular form: The dermis and hypodermis are delineated; vascular formation occurs; epidermal stratification and epidermal appendage formation also take place. By the 11th week, the basal layer (stratum germinativum) forms an intermediate skin layer. By the end of the 4th month, all the epithelial layers of the epidermis have acquired their definitive arrangement. Also by the 4th month, the mesoderm underlying the ectoderm has differentiated into the dermal layer, which begins to produce collagen and elastic fibers; blood vessel formation also takes place in this dermal layer. Hair follicle and sebaceous gland growth initiate in the growing fetus by the 5th month. During the first 3 months of development, neural crest cells infiltrate the epidermis and differentiate into pigment-producing melanocytes.