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The subcutaneous tissue (from Latin subcutaneous 'beneath the skin'), also called the hypodermis, hypoderm (from Greek 'beneath the skin'), subcutis, or superficial fascia, [2] is the lowermost layer of the integumentary system in vertebrates. [3] The types of cells found in the layer are fibroblasts, adipose cells, and macrophages.
The skin weighs an average of four kilograms, covers an area of two square metres, and is made of three distinct layers: the epidermis, dermis, and subcutaneous tissue. [1] The two main types of human skin are: glabrous skin, the hairless skin on the palms and soles (also referred to as the "palmoplantar" surfaces), and hair-bearing skin. [3]
The epidermis, "epi" coming from the Greek language meaning "over" or "upon", is the outermost layer of the skin. It forms the waterproof, protective wrap over the body's surface, which also serves as a barrier to infection and is made up of stratified squamous epithelium with an underlying basal lamina.
Skin secretions originate from glands that in dermal layer of the epidermis. Sweat, a physiological aid to body temperature regulation, is secreted by eccrine glands. Sebaceous glands secrete the skin lubricant sebum. Sebum is secreted onto the hair shaft and it prevents the hair from splitting. It consists mostly of lipids.
The dermis is the layer of skin between the epidermis and subcutaneous tissue, and comprises two sections, the papillary dermis and the reticular dermis. [24] The superficial papillary dermis interdigitates with the overlying rete ridges of the epidermis, between which the two layers interact through the basement membrane zone. [24]
The basal layer is a stem cell layer and through asymmetrical divisions, becomes the source of skin cells throughout life. It is maintained as a stem cell layer through an autocrine signal, TGF alpha , and through paracrine signaling from FGF7 ( keratinocyte growth factor ) produced by the dermis below the basal cells.
Between the integument and the deep body musculature there is a transitional subcutaneous zone made up of very loose connective and adipose tissue, the hypodermis. Substantial collagen bundles anchor the dermis to the hypodermis in a way that permits most areas of the skin to move freely over the deeper tissue layers. [4]
The panniculus adiposus is the fatty layer of the subcutaneous tissues, superficial to a deeper vestigial layer of muscle, the panniculus carnosus. [1] It includes structures that are considered fascia by some sources but not by others. Some examples include the fascia of Camper and the superficial cervical fascia. [2]