Skin and its derivatives and appendages form the integumentary system. In humans, skin derivatives include nails, hair, and several types of sweat and sebaceous glands. Skin, orintegument,consists of two distinct regions, the superﬁcial epidermis and a deep dermis. The superﬁcialepidermis is nonvascular and lined by keratinized stratiﬁed squamous epithelium with distinct cell types and cell layers. Inferior to the epidermis is the vascular dermis, which is characterized by dense irregular connective tissue. Beneath the dermis is the hypodermis, or the subcutaneous layer of connective tissue and adipose tissue that forms the fascia seen in gross anatomy.
EPIDERMIS: Thick Versus Thin Skin
The basic histology of skin is similar in different regions of the body, except regarding the thickness of the epidermis .Palms and soles are constantly exposed to increased wear, tear, and abrasion. As a result, the epidermis in these regions, especially the outermost stratiﬁed keratinized layer, is thick. Skin in these regions is called thick skin. Thick skin also contains numerouss we a t glands,but it lacks hair follicles, sebaceous glands, and smooth muscle ﬁbers. The remainder of the body is covered skin called thin skin. In these regions, the epidermis is thinner and its cellular composition simpler than that of thick skin. Thin skin containshair follicles, sebaceous glands,and sweat glands.Attached to the connective tissue sheath of hair follicles and the connective tissue of the dermis are smooth muscle ﬁbers, called arrector pili. Also associated with the hair follicles are numerous sebaceous glands (see the overview ﬁgure).
DERMIS: Papillary and Reticular Layers
Dermis is the connective tissue layer that binds to the epidermis. The junction of the dermis with the epidermis is irregular. The superﬁcial layer of the dermis forms numerous raised projections, called dermal papillae, that interdigitate with evaginations of epidermis,...