Hypodermis (Subcutaneous Layer): Anatomy, Functions & Clinical Importance

The hypodermis (also subcutaneous tissue or superficial fascia) is the deepest layer of the integumentary system, lying beneath the dermis.

Structure

• Adipose tissue: Adipocytes organized in lobules separated by fibrous septa (Colles' fascia superficially, Scarpa's fascia deeply in abdomen).
• Vascular supply: Perforating arteries from deep fascia → subdermal plexus; veins parallel.
• Nerves: Cutaneous branches; Pacinian corpuscles (deep pressure/vibration).
• Appendages: Deep hair follicles, sweat glands, sebaceous glands extend into hypodermis.

Functions

Function	Mechanism
Thermal insulation	Adipose low conductivity; reduces heat loss.
Mechanical cushioning	Protects bone, muscle, nerves from trauma.
Energy storage	Triglycerides → lipolysis → free fatty acids.
Endocrine	Adipokines (leptin, adiponectin, resistin, cytokines).
Skin mobility	Loose areolar tissue allows gliding over deep fascia.
Drug delivery	Subcutaneous injection depot (insulin, heparin, biologics).

Clinical Relevance

• Obesity: Hypertrophic/hyperplastic adipose → inflammation, insulin resistance, mechanical stress.
• Lipodystrophy: Loss of adipose → metabolic syndrome, ectopic fat.
• Pressure injury: Bony prominences with thin hypodermis (sacrum, heels) at high risk.
• Subcutaneous injections: Site rotation prevents lipohypertrophy/lipoatrophy.
• Flap surgery: Vascular territories (angiosomes) guide reconstructive design.
• Imaging: Ultrasound/MRI assess thickness, edema, masses (lipoma, sarcoma).

The hypodermis is metabolically active, not merely "fat under the skin."