Lamellated (Pacinian) Corpuscles

1. Overview

The hypodermis, also referred to as the subcutaneous layer or superficial fascia, is the deepest layer of the integumentary system. It lies beneath the dermis and serves as a flexible cushion that connects the skin to underlying muscles and bones. The hypodermis is composed primarily of adipose tissue and loose connective tissue, playing critical roles in insulation, energy storage, shock absorption, and skin mobility. It also contains larger blood vessels and nerves that supply the skin above.

2. Location

The hypodermis is located immediately beneath the dermis and forms the interface between the skin and underlying structures such as muscles, bones, and fascia. Its characteristics vary by body region, age, sex, and nutritional status:

• Thicker in areas such as the abdomen, buttocks, breasts, and thighs.

• Thinner in areas like the eyelids, ears, and genitalia.

• Functions as a base layer anchoring the skin to deeper structures while allowing independent movement of the skin.

3. Structure

The hypodermis consists of:

• Adipose tissue:

  • Organized in lobules separated by fibrous septa.

  • Primarily white adipose tissue, involved in energy storage and insulation.

• Loose areolar connective tissue:

  • Contains collagen and elastic fibers that provide flexibility and support.

• Blood vessels and lymphatics:

  • Include larger arteries and veins that branch into the dermis.

• Nerves:

  • Includes sensory and autonomic nerve fibers.

• Specialized structures:

  • May contain portions of sweat glands and the bases of some hair follicles.

4. Function

The hypodermis plays several structural, metabolic, and protective functions:

• Insulation: Adipose tissue conserves body heat and maintains internal temperature.

• Energy storage: Stores triglycerides in adipocytes, serving as a long-term energy reserve.

• Cushioning and shock absorption: Protects internal organs from mechanical trauma.

• Skin mobility: Facilitates movement of the skin over underlying muscles and bones.

• Vascular and neural supply: Serves as a conduit for blood vessels and nerves that extend into the dermis and epidermis.

5. Physiological role(s)

The hypodermis supports vital physiological processes relevant to skin and systemic health:

• Thermoregulation: Adipose insulation helps retain body heat in cold environments and buffer heat loss.

• Endocrine function: Adipocytes secrete hormones like leptin, adiponectin, and inflammatory cytokines that influence appetite, metabolism, and immune response.

• Wound healing: Vascular networks in the hypodermis support tissue repair by delivering oxygen and immune cells.

• Drug absorption: Common site for subcutaneous injection due to its vascular supply and slow absorption rate.

• Fat metabolism and mobilization: Releases free fatty acids during fasting or high energy demand.

6. Clinical Significance

Alterations in hypodermal structure or function are involved in various clinical conditions:

• Obesity:

  • Excessive accumulation of subcutaneous fat can contribute to metabolic syndrome, cardiovascular disease, and insulin resistance.

• Lipodystrophy:

  • Loss of subcutaneous fat, either genetic or acquired, results in abnormal fat distribution and insulin resistance.

• Cellulitis:

  • Bacterial infection of the hypodermis and lower dermis, often requiring antibiotics and supportive care.

• Pressure ulcers (bedsores):

  • Commonly involve the hypodermis and occur when prolonged pressure impairs blood flow to the subcutaneous tissue.

• Hypodermic injections:

  • Many medications (e.g., insulin, vaccines) are administered into the hypodermis due to its slow, sustained absorption characteristics.

• Subcutaneous tumors and cysts:

  • Includes benign lipomas and more serious malignancies that originate from adipocytes or connective tissue components.