The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis provides tensile strength and elasticityto the skin through an extracellular matrix composed of collagen fibrils, microfibrils, and elastic fibers, embedded in proteoglycans.
It harbors many Mechanoreceptors (nerve endings) that provide the sense of touch and heat. It also contains the hair follicles, sweat glands, sebaceous glands, apocrine glands, lymphatic vessels and blood vessels. The blood vessels in the dermis provide nourishment and waste removal from its own cells as well as for the epidermis.
The dermis is tightly connected to the epidermis through a basement membrane and is structurally divided into two areas: a superficial area adjacent to the epidermis, called the papillary region, and a deep thicker area known as the reticular region.
The epidermis is composed of 4 or 5 layers depending on the region of skin being considered. Those layers in descending order are:
– cornified layer (stratum corneum)
Composed of 10 to 30 layers of polyhedral, anucleated corneocytes (final step ofkeratinocyte differentiation), with the palms and soles having the most layers.Corneocytes are surrounded by a protein envelope (cornified envelope proteins), filled with water-retaining keratin proteins, attached together through corneodesmosomesand surrounded in the extracellular space by stacked layers of lipids. Most of the barrier functions of the epidermis localize to this layer.
– clear/translucent layer (stratum lucidum, only in palms and soles)
– granular layer (stratum granulosum)
Keratinocytes lose their nuclei and their cytoplasm appears granular. Lipids, contained into those keratinocytes within lamellar bodies, are released into theextracellular space through exocytosis to form a lipid barrier. Those polar lipids are then converted into non-polar lipids and arranged parallel to the cell surface. For example glycosphingolipids become ceramides and phospholipids become free fatty acids.
– spinous layer (stratum spinosum)
Keratinocytes become connected through desmosomes and start produce lamellar bodies, from within the Golgi, enriched in polar lipids, glycosphingolipids, free sterols, phospholipids and catabolic enzymes. Langerhans cells, immunologically activecells, are located in the middle of this layer.
– basal/germinal layer (stratum basale/germinativum)
Composed mainly of proliferating and non-proliferating keratinocytes, attached to the basement membrane byhemidesmosomes. Melanocytes are present, connected to numerous keratinocytes in this and other strata through dendrites.Merkel cells are also found in the stratum basale with large numbers in touch-sensitive sites such as the fingertips and lips. They are closely associated with cutaneous nerves and seem to be involved in light touch sensation.
The term Malpighian layer (stratum malpighi) is usually defined as both the stratum basale and stratum spinosum.
The epidermis is separated from the dermis, its underlying tissue, by a basement membrane.