Structures at the Epidermal -Dermal junction (DEJ) derive their origin from the epidermis and dermis: the basal lamina is primarily of epidermal origin, the anchoring fibrils of dermal origin. The junction serves the following functions: (1) epidermal-dermal adherence, (2) mechanical support for the epidermis, and (3) a barrier to prevent cells and some large molecules form crossing the junction.
The basal lamina or basement membrane (of the skin) is a layer of extracellular matrix secreted by the epithelial cells (of the skin), a stratified squamous epithelium, composed of proliferating basal and differentiated suprabasal keratinocytes. The term “basal lamina” is usually used with electron microscopy, while the term “basement membrane” is usually used with light microscopy. The basal lamina cannot be distinguished under the light microscope, but under an electron microscope, the basal lamina and lamina reticularis are visibly distinct structures. The layers of the basal lamina consist of – lamina lucida / lamina rara interna (containing the glycoprotein laminin), lamina densa (composed of type IV collagen), lamina lucida / lamina rara externa (similar composition to lamina rara interna), lamina reticularis (synthesized by cells from connective tissue fibroblast and contains fibronectin). The basal lamina and lamina reticularis together make up the basement membrane.
Basement membrane includes various protein structures linking the basal layer of keratinocytes to the basement membrane (via a structure called hemidesmosomes) and the basement membrane to the anchoring fibrils of dermis at epidermal-dermal junction. The basement membrane has an important role in making sure the epidermis sticks tightly to the dermis of epidermal-dermal junction.
The lamina densa is the component of the basement membrane zone between the epidermis and dermis of the skin, and is an electron-dense zone between the lamina lucida and dermis, synthesized by the basal cells (basal keratinocytes) of the epidermis, and composed of (1) type IV collagen, (2) anchoring fibrils made of type VII collagen, and (3) dermal microfibrils of the elastic system.
Anchoring fibrils (composed largely of type VII collagen) extend from the basal lamina of epithelial cells and attach to the lamina reticularis (also known as the reticular lamina) by wrapping around the reticular fiber (collagen III) bundles. Although found beneath all basal laminae, they are especially numerous in stratified squamous cells of the skin. Anchoring fibrils are essential to the functional integrity of the dermoepidermal junction.
The dermal-epidermal junction interlocks forming fingerlike projections called rete ridges. Rete ridges are epidermal thickenings that extend downward between dermal papillae (interdigitations of the dermis into the epidermis). Blood vessels in the dermal papillae nourish all hair follicles and bring oxygen and nutrients to the lower layers of epidermal cells. The cells of the epidermis receive their nutrients from the blood vessels in the dermis. The rete ridges increase the surface area of the epidermis that is exposed to these blood vessels and the needed nutrients.
Ultrastructurally, the epidermal-dermal junction is composed of four component areas:(1) the basal cell plasma membrane with its specialized attachment devices or hemidesmosomes, (2) an electron-lucent area, the lamina lucida, (3) the basal lamina, and (4) the sub-basal lamina fibrous components, including anchoring fibrils, dermal microfibril bundles, and collagen fibers.
Scanning electron microscopy studies of human epidermal-dermal basal lamina has demonstrated striking age-related changes. Scanning electron microscopy of mature epidermal-dermal junction and basal lamina showed distinct dermal valleys; tall, dome-shaped dermal papillae; and basal lamina arranged in prominent corrugations that tended to be oriented vertically on papillae and irregularly on interpapillary zones. Aged skin of 7th through 10th decades demonstrated progressive loss of dermal valleys, flattening and widening of dermal papillae, and loss of basal lamina corrugations.
Chronologically aged skin has reduced rete pegs/rete ridges at the Dermal-epidermal Junction that will adversely affect the transport and absorption of nutrients and oxygen from capillary into the epidermal cells.
Collagen IV in lamina densa is an integral constituent of the DE, imparts a structural framework for other molecules and plays a key role in maintaining mechanical stability. Significantly lower levels of collagen IV have been identified at the base of wrinkles. The mechanical stability of the DEJ may be adversely affected by this loss of collagen IV, thereby contributing to wrinkle formation.
Collagen VII is the primary constituent in anchoring fibrils that attach the basement membrane to the dermis. In one study, a significantly lower number of anchoring fibrils were identified in chronically sun‐exposed skin in comparison to that of sun-protected skin. It was believed that wrinkles may form as a result of a weakened bond between the dermis and epidermis, due to anchoring fibril degradation. A more recent study showed such a loss of collagen VII to be more marked in the base of the wrinkle.
Research has shown that some anti-wrinkle ingredients such as Tretionion can increase anchoring fibrils in aged or photodamaged skin. Treatment of photoaged skin with topical tretinoin Increases epidermal-dermal anchoring fibrils in an in vivo study. Skin sites that received topical tretinoin showed double the anchoring fibril density compared with that of control sites. The possible mechanisms by which topical tretinoin increases anchoring fibrils in skin include the drug’s property of inhibiting collagenase, a dermal enzyme that degrades anchoring fibril collagen. Skin collagenase is predominantly localized within the papillary dermis and is capable of degrading anchoring fibril collagen.