Topics

Topic

design image
Hypodermis of skin

The hypodermis, also known as the subcutaneous tissue or superficial fascia, is the deepest layer of the skin, lying beneath the dermis. It is primarily composed of loose connective tissue and adipose fat lobules, interspersed with fibrous septa that anchor the skin to the underlying muscles and fascia.

This layer acts as a cushion, insulator, and energy reservoir, while also allowing the skin to move freely over deeper structures. Its thickness varies depending on body region, sex, and nutritional status, being thickest in the buttocks, thighs, and abdomen, and thinnest on the eyelids and scalp.

Synonyms

  • Subcutaneous tissue

  • Superficial fascia

  • Subcutis

  • Panniculus adiposus

Structure and Composition

  • Connective tissue: Loose areolar tissue forming a meshwork between dermis and deep fascia.

  • Adipose tissue: Organized into lobules separated by fibrous septa (retinacula cutis).

  • Fibrous septa: Vertical and oblique connective tissue bands connecting dermis to underlying fascia.

  • Vascular plexus: Contains superficial veins and arteries supplying the dermis and skin appendages.

  • Nerves: Richly supplied by cutaneous nerve branches, including sensory and sympathetic fibers.

  • Lymphatics: Dense network draining into superficial lymphatic vessels.

Location and Relations

  • Superficial: Deep to the dermis (reticular layer)

  • Deep: Lies above deep fascia or epimysium of muscles

  • Contents: Fat lobules, blood vessels, lymphatics, sensory nerves, and hair follicle roots

  • Regional variations:

    • Thick and fibrofatty in gluteal and abdominal regions

    • Thin and vascular on eyelids, scalp, and dorsum of hand

    • Specialized adaptations: Fibrous dermal attachments on soles, palms, and scalp prevent excessive sliding

Nerve Supply

  • Cutaneous branches of peripheral nerves (sensory and autonomic fibers)

  • Sympathetic fibers regulate sweat glands, vascular tone, and piloerection

Function

  • Cushioning and protection: Absorbs mechanical shocks and trauma

  • Thermal insulation: Conserves body heat and regulates temperature

  • Energy storage: Fat serves as a metabolic energy reserve

  • Mobility: Permits skin movement over deeper tissues without friction

  • Vascular and neural conduit: Contains vessels and nerves supplying dermis and epidermis

  • Immunologic defense: Houses immune cells such as macrophages and lymphocytes

Clinical Significance

  • Subcutaneous injections: Major site for insulin and vaccine delivery

  • Edema: Fluid accumulation occurs within hypodermal connective tissue

  • Infections: Cellulitis and abscesses often spread along this layer

  • Panniculitis: Inflammation of subcutaneous fat presenting as tender nodules

  • Trauma and surgery: Important in flap design and wound healing

  • Tumors: Lipomas and angiolipomas originate from hypodermal fat

  • Imaging importance: Key layer evaluated in soft-tissue injury, infection, and neoplastic imaging

MRI Appearance

  • T1-weighted images:

    • Fat lobules: Bright (high signal intensity) due to lipid content

    • Fibrous septa: Low signal (dark lines) separating fat compartments

    • Edema or inflammation: Areas of intermediate-to-low signal replacing normal bright fat

    • Vessels and nerves: Appear as low-signal tubular structures within fat

  • T2-weighted images:

    • Fat: Remains bright, slightly less intense than on T1

    • Fibrous septa: Low signal, enhancing contrast between lobules

    • Edema, infection, or inflammation: Bright hyperintense signal due to increased water content

    • Pathology: Abscess or cellulitis shows irregular high signal with surrounding septal thickening

  • STIR:

    • Normal fat: Suppressed (dark)

    • Pathologic regions (edema/inflammation): Bright hyperintense signal

    • Excellent for distinguishing fluid or inflammatory infiltration within fat planes

  • Proton Density Fat-Saturated (PD FS):

    • Fat: Dark (suppressed)

    • Inflammation or edema: Bright hyperintense zones replacing normal dark fat

    • Ideal for evaluating cellulitis, panniculitis, and postoperative changes

  • T1 Fat-Sat Post-Contrast:

    • Normal hypodermis: Minimal enhancement of septa and vessels

    • Infection or inflammation: Septal and diffuse enhancement

    • Abscess formation: Peripheral rim enhancement with central low-signal fluid core

    • Neoplasms (lipoma, angiolipoma): Show variable enhancement depending on vascularity

CT Appearance

Non-Contrast CT:

  • Fat lobules: Low attenuation (-80 to -120 HU)

  • Fibrous septa: Fine, higher-density lines separating fat compartments

  • Vessels: Tubular soft-tissue densities with surrounding fat halo

  • Pathology:

    • Edema/inflammation: Increased attenuation and septal thickening

    • Abscess: Central low density with peripheral rim

    • Lipoma: Well-defined homogeneous low-attenuation mass

    • Panniculitis: Ill-defined increased fat density with blurred septal margins

Post-Contrast CT (standard):

  • Normal hypodermis: Minimal enhancement of septa and vascular structures

  • Inflammation or cellulitis: Enhancing septa and surrounding soft-tissue stranding

  • Abscess or infected collections: Rim enhancement with non-enhancing central core

  • Tumors: Variable enhancement depending on lesion vascularity and type

MRI images

Hypodermis of skin of foot axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000