5.1 Layers of the Skin: Dermis & Hypodermis

Introduction to Skin Layers

  • The dermis is considered the "core" of the integumentary system.

    • Etymology: derma- means "skin".

    • Distinction is made between the three main layers of the skin:

    • Epidermis: epi- derived from "upon" or "over".

    • Dermis: as previously mentioned.

    • Hypodermis: hypo- means "below".

Structure and Function of the Dermis

  • The dermis contains various structures including:

    • Blood and lymph vessels.

    • Nerves.

    • Hair follicles.

    • Sweat glands.

  • Composed of two layers of connective tissue:

    • Interconnected mesh of elastin and collagenous fibers.

    • Produced by fibroblasts.

Diagram and Components of Dermis Layers

  • Figure 5.7: Layers of the Dermis

    • Shows the two components of the dermis.

    • Two primary layers: papillary layer and reticular layer.

Papillary Layer

  • Composition: Loose, areolar connective tissue.

    • Collagen and elastin fibers form a loose mesh.

  • Characteristics:

    • Projects into the stratum basale of the epidermis forming finger-like structures known as dermal papillae.

  • Contains:

    • Fibroblasts: Cells that produce collagen and elastin.

    • Small number of adipocytes (fat cells).

    • Abundance of small blood vessels.

    • Phagocytes: Defensive cells combating infections that breach the skin.

    • Lymphatic capillaries: Vessels that assist in fluid balance in the body.

    • Nerve fibers and Meissner corpuscles: Specialized touch receptors.

Reticular Layer

  • Located beneath the papillary layer; much thicker.

  • Composition: Dense, irregular connective tissue.

    • Well-vascularized with a rich sensory and sympathetic nerve supply.

  • Appearance: Looks reticulated (net-like) due to a tight meshwork of fibers.

  • Functions:

    • Elastin fibers provide elasticity to the skin, enabling movement.

    • Collagen fibers:

    • Offer structure and tensile strength.

    • Extend from reticular layer into the papillary layer and hypodermis.

    • Bind water to maintain skin hydration.

  • Clinical Relevance:

    • Collagen injections and Retin-A creams:

    • Collagen injections introduce collagen externally.

    • Retin-A creams stimulate blood flow and repair of the dermis.

Hypodermis (Subcutaneous Layer)

  • Also referred to as superficial fascia.

  • Functions: Connects skin to the underlying fascia of bones and muscles.

  • Distinction: Not strictly a part of the skin; hard to distinguish the border between hypodermis and dermis.

  • Composition:

    • Well-vascularized loose, areolar connective tissue.

    • Adipose tissue.

Lipid Storage in the Hypodermis

  • The hypodermis is primarily where fat is stored, which is a concern for weight control.

  • Adipose tissue consists of fat-storing cells called adipocytes.

  • Functions of stored fat:

    • Energy reserve.

    • Insulation: Prevents heat loss.

    • Cushioning: Protects underlying structures from trauma.

  • Factors influencing fat distribution:

    • Hormonal influences (testosterone, estrogen, insulin, glucagon, leptin, etc.).

    • Genetic factors.

  • Notable differences in fat accumulation:

    • Men: Commonly accumulate fat in neck, arms, lower back, and abdomen.

    • Women: Commonly accumulate fat in breasts, hips, thighs, and buttocks.

Body Mass Index (BMI)

  • BMI is often used as a measure of fat.

  • Derived from the formula comparing body weight (mass) to height:

    • Mathematical representation: extBMI=racextweightinkg(extheightinmeters)2ext{BMI} = rac{ ext{weight in kg}}{( ext{height in meters})^2}.

  • Limitations:

    • Accuracy can be questioned for individuals who are very physically fit due to muscular mass.

Evolutionary Perspective and Modern Implications

  • In many animals, there is a pattern of storing excess calories as fat for future energy reserves during food scarcity.

  • Issues in developed nations:

    • Lack of exercise and ready availability of high-calorie foods have led to unwanted accumulation of adipose tissue.

    • Historical context: Periodic accumulation of fat once provided an evolutionary advantage during famine.

    • Current context: Chronic accumulation of fat poses a major health threat:

    • A significant percentage of the population is overweight or clinically obese.

    • Implications for individuals and the healthcare system.

  • Solutions:

    • Lifestyle changes: Diet and exercise are essential to control body fat accumulation and reduce the risk of heart disease and diabetes.