Slide #DMS 033 [Thick Skin]. Glabrous "thick skin" is found in limited areas of the bodu, such as the soled of the feet and palms of the hands.

Identify the following epidermal strata: basale (germinativum), spinosum, granulosum. lucidum (often difficult to discern), and corneum. In which one of these layers would you expect to find mitotic figures? Study the spinosum at high mag. Note the so-called "intercellular bridges" (spines) between the keratinocytes from which this layer derives its name. Recall that this appearance reflects a shrinkage artifact as the cells pull away from each other during dehydration, and remain connected only by their desmosomal junctions. Note the keratohyaline granules in the stratum granulosum. What is their composition/function? At what strata do the keratinocytes become so heavily keratinized that they die and become cornified plates (squames)? Identify the papillary and reticular layers of the dermis. Find dermal papillae. What important sensory organs are found within the papillae? Now define the hypodermis. Note the abundance of blood vessels, nerves, and glands. What specific kind of glands? Find their secretory units and ducts. What type of epithelium lines the ducts. Try to follow the ducts through the epidermis proper to their openings at the surface of the stratum corneum. Look at the skin of your finger tip using an inverted ocular as a hand lens. Where are the openings of the ducts located?

A distinct basement membrane separates the epidermis from the underlying connective tissue dermis. The interface between these two strata, the dermoepidermal junction, varies over the body from relatively smooth in thin skin to highly corrugated in thick skin. Based on the organization of the various connective tissue fibers, especially collagen, the dermis can be divided into two layers: an outer or superficial papillary layer and an inner or deep reticular layer. The former tends to be loose, irregular fibrous connective tissue, the latter dense, irregular fibrous connective tissue.

Beneath the dermis is a loose connective tissue layer of variable thickness and composition but usually containing considerable white adipose tissue. Its boundary with the overlying dermis is often indistinct. This subcutaneous layer, which corresponds to the superficial fascia of gross anatomy, is called the hypodermis.

This is a very low power view through a section of thick skin, described as such because of the very thick epithelium or epidermis forming its surface. Beneath the epidermis is the connective tissue of the dermis, and then the fat-laden hypodermis or subcutis (the superficial fascia of gross anatomy).

This is a low power view of the epidermis and a bit of the underlying dermis with which it interdigitates. The epidermis is a keratinized stratified squamous epithelium, which, in thick skin, has a very thick keratinized layer known as the stratum corneum. A few layers of darkly-stained cells constitute the stratum granulosum, beneath which are several cell layers of the stratum spinosum. A single layer of basally-positioned cells forms the stratum basale (germanitivum).

At higher power, one can appreciate that the stratum corneum consists of the remnant scales of skin cells (keratinocytes) that have undergone complete keratinization. Cells of the stratum granulosum contain numerous keratohyalin granules that contribute to the keratinization process. The cells of the stratum spinosum are distinguished by the artifactual intercellular spaces surrounding them and which are spanned by desmosomal connections that may appear as spines or prickles on the cells. The stratum basale is the single layer of cells at the base of the epithelium. Note the intimate association of the dermal blood vessels with the avascular epithelium.

In this low power view through thick skin, one can define epidermis, dermis and the hypodermis in which lobules of adipose tissue are seen. The dermis may be subdivided into an outer, more delicate papillary layer and an inner reticular layer characterized by rather course bundles of collagen fibers. In the dermis and hypodermis, one may find various skin appendages such as the eccrine sweat glands and associated ducts seen here.

At higher power, one may see the numerous profiles made through the secretory portion of the simple coiled eccrine sweat gland located here at the border between dermis and hypodermis. Likewise, several sections through the rather tortuous ducts of this or other sweat glands are seen as they progress from the secretory elements of the gland up to the surface of the epidermis.

This is a high power view comparing the morphology of the secretory component of an eccrine sweat gland with the duct which drains it. The larger secretory cells form a simple cuboidal epithelium that is surrounded by an eosinophilic layer of contractile myoepithelial cells. The ducts draining these glands are distinguished by their stratified epithelium, often containing cells with rather heterochromatic nuclei.

Several neural elements may be seen including small nerve trunks and Meissner and Pacinian corpuscles.

This is a low power view of the dermo-epidermal junction which is characterized by an interdigitation of the papillary layer of the dermis and its dermal papillae with extensions of the epidermis known as the rete ridges. Fine-touch sensors known as Meissner's corpuscles may be found in the dermal papillae.

A high power view of the dermo-epidermal junction shows the characteristic architecture of a Meissner's corpuscle occupying a portion of a dermal papilla. The distinctive sheath of modified Schwann cells surrounds one or two nerve fibers poised to detect fine touch of the overlying epidermis.

This very low power view of the skin and associated hypodermis allows for an appreciation of the location and large size of another sensory receptor, the Pacinian corpuscle. Understand that Meissner's corpuscles are significantly smaller and located in the dermal papillae.

A medium power view through the hypodermis shows a Pacinian corpuscle, a sensory receptor for deep pressure and vibration. A unmyelinated nerve fiber is surrounded by multiple lamellae of modified Schwann cells, arranged much like the layers of an onion.