Chapter 15: Integumentary System & Skin

The integumentary system is the largest organ of the body, encompassing the skin and its specialized derivatives, such as hair follicles, nails, and glands, all performing essential functions including protection against external agents (acting as a mechanical, chemical, and ultraviolet barrier), temperature regulation, fluid homeostasis, endocrine activities (such as Vitamin D3 synthesis), and conveying sensory information. The skin is structurally composed of the superficial, ectoderm-derived epidermis, which is a keratinized stratified squamous epithelium, and the deeper, mesoderm-derived dermis, a dense irregular connective tissue providing strength and support; underlying these layers is the hypodermis or subcutaneous fascia, rich in insulating adipose tissue. The epidermis is organized into four main layers—stratum basale, stratum spinosum, stratum granulosum, and stratum corneum—with the stratum lucidum observed only in thick skin like the palms and soles, which lack hair follicles. Keratinocytes, the dominant cell type, originate in the basale layer and terminally differentiate (a specialized form of apoptosis) as they migrate superficially, forming keratohyalin granules and lamellar bodies which release lipids to establish the crucial epidermal water barrier. This barrier involves the internal cell envelope (rich in loricrin) and the external lipid envelope (rich in ceramides). Other epidermal cells include melanocytes, which produce and donate eumelanin or pheomelanin pigment to keratinocytes to protect nuclear DNA from ultraviolet damage, Langerhans cells (dendritic antigen-presenting macrophages containing Birbeck granules for immunosurveillance), and Merkel cells (mechanoreceptors forming corpuscles with afferent nerve endings). The dermis is divided into the superficial papillary layer and the thicker reticular layer, the latter characterized by Type I collagen bundles that form tension lines known as Langer lines. The dermis harbors immune cells, including dermal dendritic cells, resident dermal macrophages which are notable for engulfing tattoo ink particles in an endless capture–release–recapture cycle leading to tattoo aging, and T-resident memory (Trm) cells essential for robust immune responses. Sensory input is processed by numerous free nerve endings in the epidermis, or encapsulated receptors in the dermis and hypodermis, such as Meissner corpuscles (light touch) and Pacinian corpuscles (deep pressure/vibration). The epidermal appendages include hair follicles, which possess epidermal stem cells within the follicular bulge that contribute to hair regeneration and wound repair; sebaceous glands secrete oily sebum via holocrine secretion into the pilosebaceous unit; and two types of coiled sweat glands exist: eccrine glands regulate body temperature by secreting hypotonic sweat, while apocrine glands, restricted to areas like the axilla, secrete a protein-rich fluid potentially containing pheromones, with both gland types utilizing myoepithelial cell contraction for secretion. Damage to these layers can result in conditions ranging from skin aging (marked by keratinocyte atrophy and collagen loss) to cancers like basal cell carcinoma and malignant melanoma, which often follow the ABCD warning signs.