Anatomy of the Skin
The skin is the largest human organ representing 10% of total body weight, with thickness varying from 0.05 to 2 mm depending on anatomical region, and is divided into three main layers: epidermis, dermis, and hypodermis, each with distinct cellular composition and functions. 1
Epidermal Layer
The epidermis is the outermost layer of the skin and can be subdivided into four distinct strata:
- Stratum Corneum: The most superficial layer composed of dead cells called corneocytes. This layer provides the greatest barrier to drug diffusion through the skin. 1
- Stratum Granulosum: Contains keratinocytes with decreasing water content as they progress upward.
- Stratum Spinosum: Composed of irregular polyhedral keratinocytes with extensions that connect to adjacent cells through specialized junctions called desmosomes, creating the characteristic "spiny" appearance. This layer is particularly important as the site where spongiosis (intercellular edema) occurs in many inflammatory skin conditions. 2
- Stratum Basale: The deepest epidermal layer where keratinocytes are created before migrating upward in a continuous cellular renewal process. 1
The primary cell type in the epidermis is the keratinocyte, which undergoes progressive differentiation as it migrates from the basal layer to the surface, with decreasing water content throughout this journey. 1
Dermal Layer
The dermis lies directly beneath the epidermis and has several key characteristics:
- It is a vascularized and innervated layer 1
- Composed primarily of a network of proteins, mainly elastin and collagen, providing notable elastic properties 1
- Contains fibroblasts, macrophages, and leukocytes 1
- The vascular network in this layer provides nutrients to both dermal and epidermal cells while removing metabolites 1
- This vasculature is crucial for systemic drug absorption, as substances reaching the dermal microcirculation can be absorbed into the bloodstream 1
Hypodermal Layer
The hypodermis is the deepest skin structure:
- Often not considered part of the skin proper
- Consists of a matrix of fatty and connective tissues
- Connects the skin to underlying tissues and organs
- Its composition and extent vary based on factors such as gender, age, anatomical site, nutritional status, and endocrine function 1
Skin Appendages
Several important structures are associated with the skin:
- Sweat glands and sebaceous glands
- Hair and hair follicles
- These structures are often targets for local therapies (e.g., for acne and alopecia) 1
- Hair follicles, though occupying only approximately 0.1% of total skin surface, can significantly impact transdermal drug diffusion by offering a thinner barrier between the external environment and dermal microcirculation 1
Functional Significance
The skin performs several critical functions:
- Provides a robust barrier against external xenobiotics, allergens, and microorganisms
- Maintains homeostasis by preventing body dehydration
- Protects against harmful ultraviolet radiation
- Contains various receptors sensitive to pressure, pain, and temperature, facilitating interaction with the environment 1
Understanding the complex structure of the skin is essential for developing effective treatments for various dermatological conditions and for optimizing drug delivery through this important barrier.