The junction between the dermis and epidermis is seen in the light microscope as an uneven boundary except in the thinnest skin. Sections of skin cut perpendicular to the surface reveal numerous fingerlike connective tissue protrusions, dermal papillae, that project into the undersurface of the epidermis (see Figs. 15.1 and 15.2). The papillae are complemented by what appear to be similar epidermal protrusions, called epidermal ridges or rete ridges, that project into the dermis. If the plane of section is parallel to the surface of the epidermis and passes at a level that includes the dermal papillae, however, the epidermal tissue appears as a continuous sheet of epithelium, containing circular islands of connective tissue within it. The islands are cross sections of true fingerlike dermal papillae that project into the epidermis. At sites where increased mechanical stress is placed on the skin, the epidermal ridges are much deeper (the epithelium is thicker), and the dermal papillae are much longer and more closely spaced, creating a more extensive interface between the dermis and epidermis. This phenomenon is particularly well demonstrated in histologic sections that show both palmar and dorsal surfaces of the hand, as in a section of a finger.
True dermal ridges are present in thick skin in addition to dermal papillae.
Dermal ridges tend to have a parallel arrangement, with the dermal papillae located between them. These ridges form a distinctive pattern that is genetically unique to each individual and is reflected in the appearance of epidermal grooves and ridges on the surface of the skin. These patterns are the basis of the science of dermatoglyphics, or fingerprint and footprint identification.
The dermal ridges and papillae are most prominent in the thick skin of the palmar and plantar surfaces. Here, the basal surface of the epidermis greatly exceeds its free surface. The germinal layer is thus spread over a large area; assuming a near-constant rate of mitosis in the stratum germinativum, more cells per unit time enter the stratum corneum in thick skin than in thin skin. These additional cells are thought to account for the greater thickness of the cornified layer in thick skin.
Hemidesmosomes strengthen the attachment of the epidermis to the underlying connective tissue.
When studied with the transmission electron microscope (TEM), the basal surface of the basal epidermal cells exhibits a pattern of irregular cytoplasmic protrusions that increase the attachment surface between the epithelial cell and its subjacent basal lamina. A series of hemidesmosomes link the intermediate filaments of the cytoskeleton into the basal lamina. In addition, focal adhesions that anchor actin filaments into the basal lamina are also present. These specialized anchoring junctions are discussed on page 144.
The dermis is composed of two layers: The papillary layer and the reticular layer.
Examination of the full thickness of the dermis at the light microscope level reveals two structurally distinct layers.  The papillary layer, the more superficial layer,  consists of loose connective tissue immediately  beneath the epidermis (Plate 43, page 517). The collagen fibers located in this part of the dermis are not as thick as those in the deeper portion.
I and type III collagen molecules. Similarly, the elastic fibers are threadlike and form an irregular network. The papillary