Hypodermis
Tela subcutanea
Also known as: subcutaneous tissue, subcutis, superficial fascia, panniculus adiposus
The hypodermis (subcutaneous tissue) is the layer of loose connective and adipose tissue beneath the dermis that anchors the skin, stores energy, insulates and cushions underlying structures.
Overview
Though not strictly part of the skin, the hypodermis is inseparable from it functionally. It consists of lobules of white adipose tissue separated by fibrous septa (retinacula cutis) that tether the dermis to the deep fascia; where retinacula are dense (palms, soles, scalp) the skin is firmly fixed, and where they are sparse (dorsum of the hand, eyelids) it is mobile. Its thickness varies greatly with region, sex and nutrition: it is absent from the eyelids, penis and scrotum (where smooth dartos muscle replaces it), and distribution is influenced by sex hormones (gluteofemoral "gynoid" versus abdominal "android" patterns).
The hypodermis carries the larger cutaneous arteries, superficial veins, collecting lymphatics and cutaneous nerves, the deep parts of hair follicles and sweat glands, and Pacinian corpuscles. Adipocytes also have endocrine roles (leptin, adiponectin, aromatisation of androgens to oestrogens).
- Location
Between the dermis and the deep fascia covering muscles; continuous over the whole body except where specialised (e.g. dartos fascia).
Parts & subdivisions
- Unilocular white adipocytes in lobules supplied by small arterioles.
- Retinacula cutisFibrous septa anchoring dermis to deep fascia; determine skin mobility.
- Membranous layerIn the lower anterior abdominal wall and perineum the subcutaneous tissue has a deep membranous layer (Scarpa and Colles fascia) that confines extravasated urine.
Anatomy
- Arterial supply
Perforating arteries from deep fascia and muscle; give rise to the deep cutaneous plexus.
- Venous drainage
Superficial (subcutaneous) veins such as the great and small saphenous, cephalic and basilic veins.
- Lymphatic drainage
Collecting lymphatic vessels accompany superficial veins to regional nodes.
Function
- Thermal insulation
- Energy storage (triglyceride)
- Mechanical cushioning and shock absorption
- Anchors skin while allowing mobility over deeper structures
- Conduit for cutaneous vessels and nerves; site of Pacinian corpuscles
- Endocrine functions of adipocytes (leptin, adiponectin, oestrogen synthesis)
Clinical significance
Subcutaneous injection
Insulin, low-molecular-weight heparin and some vaccines are injected into the hypodermis, from which absorption is slower and steadier than from muscle. Repeated injection at one site causes lipohypertrophy, which impairs insulin absorption.
Cellulitis and necrotising fasciitis
Cellulitis is a spreading bacterial infection (mostly Streptococcus pyogenes, Staphylococcus aureus) of the deep dermis and subcutaneous tissue with ill-defined erythema, unlike erysipelas, which involves the upper dermis and superficial lymphatics with a raised, sharply demarcated edge. Necrotising fasciitis spreads along the fascial planes with pain out of proportion to signs and is a surgical emergency.
Lipoma
Common benign tumour of mature adipocytes: soft, mobile, painless subcutaneous lump.
Compiled from standard anatomy and physiology references — see the Integumentary System sources. For education only; not medical advice.