The Integumentary System: Skin Layers, Cells, and Pigmentation

The Integumentary System

General Layers of the Skin

• The skin is composed of two major layers:

  • Epidermis: The outer, thinner layer, consisting of keratinized stratified squamous epithelium (KSSET).

  • Dermis: The inner, thicker layer, consisting of connective tissue (C.T.).

• Subcutaneous (subQ) Layer / Hypodermis:

  • Located underneath the dermis, it is not considered part of the skin, but is associated with it.

  • Composed of loose areolar and adipose connective tissue (C.T.).

  • Its primary function is to attach the skin to underlying tissues and organs.

Diagrammatic Overview (from Sectional View of Skin and Subcutaneous Layer)

• Epidermis: The most superficial layer, showing structures like sweat pores and epidermal ridges.

• Dermis: Located beneath the epidermis, further divided into:

  • Papillary Region: Contains dermal papillae and capillary loops.

  • Reticular Region: Deeper part of the dermis, housing structures like sebaceous (oil) glands, arrector pili muscles, hair follicles, hair roots, eccrine sweat glands, apocrine sweat glands, and various sensory receptors.

• Subcutaneous Layer: Deepest layer shown, primarily composed of adipose tissue, and containing larger blood vessels (veins and arteries) that form the cutaneous plexus.

• Sensory Receptors: Included are the Corpuscle of touch (Meissner corpuscle) in the papillary region and the Lamellated (Pacinian) corpuscle and free nerve endings deeper within the dermis and subcutaneous layer.

• Blood Vessels: Papillary plexus (in the papillary region of the dermis) and cutaneous plexus (in the subcutaneous layer/reticular dermis border).

The Epidermis

• Composition: The epidermis is primarily composed of keratinized stratified squamous epithelium.

• Four Major Types of Cells: The epidermis contains four principal cell types:

  • Keratinocytes

  • Melanocytes

  • Langerhans cells

  • Merkel cells

Detailed Cell Types of the Epidermis

• Keratinocytes:

  • Constitute approximately $90 \%$ of the epidermal cells.

  • Produce keratin, a tough, fibrous protein that provides protection to the skin.

  • Also produce lamellar granules, which release a water-repellent sealant, contributing to the waterproofing property of the skin.

• Melanocytes:

  • Constitute approximately $8 \%$ of the epidermal cells.

  • Produce the pigment melanin.

  • Melanin plays a crucial role in protecting the skin against damage caused by ultraviolet (UV) radiation from the sun.

• Langerhans Cells:

  • Are a type of macrophage (phagocytic immune cell).

  • Originate in the red bone marrow and migrate to the epidermis.

  • Involved in immune responses within the skin, helping to detect and fight off microbes.

• Merkel Cells:

  • Function in the sensation of touch.

  • They are associated with adjacent tactile discs (Merkel discs), which are specialized nerve endings that act as receptors for touch.

Specific Layers of the Epidermis

• The epidermis is organized into either four layers (in thin skin) or five layers (in thick skin).

• These layers, from deepest to most superficial, are:

  • Stratum Basale (or Stratum Germinativum):

    • Always the bottom (deepest) layer of the epidermis.

    • Consists primarily of a single row of cuboidal or columnar shaped cells.

    • This layer is responsible for continuous cell division (mitosis), producing all the other epidermal layers above it.

    • Melanocytes and Merkel cells are also found in this layer.

  • Stratum Spinosum:

    • Located superficial to the stratum basale.

    • Comprises $8-10$ layers of keratinocytes.

    • These cells appear