Dermis
Attachment of epidermis to dermis is enhanced by an increased interface between the two tissues.
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 der- mis. If the plane of section is parallel to the surface of the epi- dermis 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 der- mal papillae that project into the epidermis. At sites where in- creased 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 demon- strated in histologic sections that show both palmar and dor- sal 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 individ- ual 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 epi- dermis 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 sub- jacent basal lamina. A series of hemidesmosomes link the intermediate filaments of the cytoskeleton into the basal lam- ina. In addition, focal adhesions that anchor actin fila- ments 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. This delicate collagen network contains predominately type I and type III collagen molecules. Similarly, the elastic fibers are threadlike and form an irregular network. The papillary layer is relatively thin and includes the substance of the der- mal papillae and dermal ridges. It contains blood vessels that serve but do not enter the epidermis. It also contains nerve processes that either terminate in the dermis or penetrate the basal lamina to enter the epithelial compartment. Because the blood vessels and sensory nerve endings are concentrated in this layer, they are particularly apparent in the dermal papillae.
• The reticular layer lies deep to the papillary layer. Although its thickness varies in different parts

Hiatt
Dermis
-Located deep to the epidermis, and separated from it by a basement membrane, the dermis is derived from mesoderm and is composed mostly of dense irregular collagenous connec- tive tissue. It contains capillaries, nerves, sensory organs, hair follicles, sweat and sebaceous glands, as well as arrector pili muscles It is divided into two layers: a superficial papillary layer and a deeper ret

1. Papillary layer
(Thick skin): Is comprised of loose connective tissue containing capillary loops and terminals of mechanoreceptors. These dermal papillae interdigitate with the epidermal ridges of the epidermis. These interdigitations are very prominent in thick skin.

(Thin skin): The papillary layer is comprised of the same loose connective tissue as in thick skin. However, its volume is much reduced. The depth of the dermal/epidermal interdigitations is also greatly reduced.

2. Reticular layer
(Thick skin): Is composed of dense irregular collagenous connective tissue containing the usual array of connective tissue elements, including cells, blood, and lymphatic vessels.
Sweat glands and cutaneous nerves are also present and their branches extend into the papillary layer and into the epidermis.

(Thin skin): Same as in thick skin with the addition of.
Sebaceous glands and hair follicles along with their arrector pili muscles are observed.

----The dermis of the skin, lying directly deep to the epi- dermis, is derived from mesoderm. It is composed of dense, irregular collagenous connective tissue containing mostly type I collagen and numerous elastic fibers that assist in securing the skin to the underlying hypodermis.
• The dermis is subdivided into a loosely woven papil- lary layer (composed of primary and secondary dermal ridges), a superficial region that interdigitates with the epidermal ridges (and interpapillary pegs) of the epi- dermis, and
• a deeper, coarser, and denser reticular layer. The interface between the papillary and reticular layers is indistinct.
• Dermal ridges (as well as secondary dermal ridges) display encapsulated nerve endings, such as Meissner’s corpuscles, as well as capillary loops that bring nour- ishment to the avascular epidermis.

Moore
Dermis
The dermis develops from mesenchyme, which is derived from the mesoderm underlying the surface ectoderm (see Fig. 19-1A and B). Most of the mesenchyme that differentiates into the connective tissue of the dermis originates from the somatic layer of lateral mesoderm; however, some of it is derived from the dermatomes of the somites (see Chapter 14). By 11 weeks, the mesenchymal cells have begun to produce collagenous and elastic connective tissue fibers (see Fig. 19-1D). As the epidermal ridges form, the dermis projects into the epidermis, forming dermal ridges that interdigitate with the epidermal ridges (see Fig. 19-2). Capillary loops (endothelial tubes) develop in some of these ridges and provide nourishment for the epidermis. Sensory nerve endings form in others. The developing afferent nerve fibers apparently play an important role in the spatial and temporal sequence of dermal ridge formation. The development of the dermatomal pattern of innervation of the skin is described in Chapter 16.
The blood vessels in the dermis begin as simple, endothelium-lined structures that differentiate from mesenchyme. As the skin grows, new capillaries grow out from the primordial vessels (angiogenesis). Such capillary-like vessels have been observed in the dermis at the end of the fifth week. Some capillaries acquire muscular coats through differentiation of myoblasts developing in the surrounding mesenchyme and become arterioles and arteries. Other capillaries, through which a return flow of blood is established, acquire muscular coats and become venules and veins. As new blood vessels form, some transitory ones disappear. By the end of the first trimester, the major vascular organization of the fetal dermis is established.

Langman's

The dermis is derived from the mesenchyme. During the 3rd and 4th months this tissue, the corium, forms many irregular papillary structures, the dermal papillae, which project upward into the epidermis. These papillae usually contain a small capillary or a sensory nerve end organ. The deeper layer of the dermis, the subcorium, contains large amounts if fatty tissue.