Structure of the Skin

Integumentary System

Overview

  • The integumentary system is the first of 11 systems to be covered.

  • Consists of:

    • Skin

    • Hair

    • Sweat and oil glands

    • Nails

    • Sensory receptors

  • Plays a vital role in the body.

  • Maintains body temperature at 3737 degrees Celsius.

  • Provides sensory information for interaction with the outside world.

  • Helps to maintain homeostasis.

  • Converts vitamin D into its active form, aiding in skeletal system growth and development.

Homeostasis and the Skin

  • Homeostasis is a central concept.

  • Skin and hair provide a barrier against invaders, protecting us from the external world.

  • The integumentary system is the first line of defense.

  • Contamination vs. Infection:

    • Contamination: Pathogen comes into contact with the body.

    • Infection: Pathogen enters the body and alters physiological functions.

  • The skin prevents pathogens from progressing from contamination to infection.

  • Temperature control:

    • When hot, sweat (water) cools the body.

    • When cold, vasoconstriction occurs in the skin to push blood to deeper tissues, keeping the core body warm.

Structure of the Skin

  • Two major layers:

    • Epidermis (superficial layer)

    • Dermis (underneath the epidermis)

  • Hypodermis (subcutaneous layer):

    • Located deep under the dermis.

    • Made of areolar and adipose tissues.

    • Functions as a protective pillow/buffer.

  • Epidermal ridges connect the epidermis to the dermal papillae (upper part of the dermis).

  • Epidermis:

    • Consists of thick layers of stratified cells.

    • Avascular (no blood vessels).

    • Contains nerve sensations (sensory and pain).

    • Paper cut example: if there's pain but no blood, the cut is only in the epidermis.

  • Dermis:

    • Vascularized (capillaries, arterioles, venules).

    • Many nerve terminations (touch, pain).

    • Attachment site for accessory structures of the integumentary system.

    • Hair root.

    • Glands: eccrine (water), apocrine.

    • Sebaceous glands (oil/grease).

    • Arrector pili muscles (cause goosebumps).

      • Ancestral function: warming, scaring predators making the hair appear larger.

  • Hypodermis:

    • Adipose tissue (cushioning).

    • Terminal nerves.

Microscopic Structure and Cell Types

  • Epidermis: Layers of stratified cells, including dead cells on the surface.

  • Dermis: Contains hair roots and follicles.

  • Zoomed view of a thumb shows epidermal ridges and sweat pores.

Cells of the Epidermis:

  • Epithelial tissue makes up most of the epidermis.

    • Keratinocytes: Produce keratin, the main component of skin, hair, and nails.

    • Melanocytes: Produce melanin, a pigment that protects the skin from UV radiation.

    • Intraepidermal macrophages: Immune cells that ingest and digest invaders.

    • Tactile epithelial cells: Sense and record pressure; stimulate tactile neurons to send signals to the central nervous system.

Types of Skin:

  • Thin Skin: Covers most of the body; has hair.

  • Thick Skin: Covers palms and soles; hairless; more calluses.

Layers of the Epidermis

  • Stratum basale (basal layer)

  • Stratum spinosum (spiny layer)

  • Stratum granulosum (granular layer)

  • Stratum corneum (outer layer)

  • In thick skin, the stratum lucidum (translucent layer) is present between the stratum granulosum and stratum corneum.

Stratum Basale:

  • Attached to the basal membrane via hemidesmosomes.

  • Cells undergo constant mitosis, pushing older cells upwards.

  • Contains tactile epithelial cells with sensory neurons.

Stratum Spinosum:

  • Contains melanocytes that produce melanin.

  • Pigmentation takes time because melanin is produced in the deepest part and gradually moves to the surface.

  • Macrophages are present to phagocytose invaders.

  • This is the thickest layer of the epidermis.

Stratum Granulosum:

  • Cells are squeezed, less hydrated, and older.

  • Cells begin to degenerate.

  • Rich in keratin.

Stratum Corneum:

  • Made of dead cells (keratinocytes) held together by keratin.

Stratum Lucidum:

  • Found only in thick skin.

  • Composed of dead cells with a high keratin content.

Review of Epidermal Layers

  • Stratum Basale:

    • Deepest layer.

    • Cuboidal or columnar keratinocytes.

    • Active cell division.

  • Stratum Spinosum:

    • 8-10 rows of keratinocytes.

    • Contains melanocytes and macrophages.

  • Stratum Granulosum:

    • Thin layer (3-5 rows).

    • Cells are beginning to degenerate.

    • Rich in keratin.

  • Stratum Lucidum:

    • Present only in palms and soles.

    • Dead keratinocytes with a high keratin content.

  • Stratum Corneum:

    • Outermost layer.

    • Dead cells filled with keratin.

    • Cells are constantly shed off, preventing infection.

Dermis

  • Papillary region matches the ridges of the epidermis.

  • Deeper reticular portion.

  • Made of connective tissues with elastic fibers.

  • Contains hair follicles, nerves, and glands.

  • Vascularized and innervated.

Pigments

  • Melanin:

    • Produced by melanocytes.

    • Protects against UV radiation.

  • Other Pigments:

    • Hemoglobin (red blood).

    • Carotene (orange/yellow) in stratum corneum and adipose cells.

Clinical Connections

  • Albinism:

    • Congenital disorder.

    • Absence of pigmentation.

    • Melanin is not produced in hair, skin, or eyes.

    • Increases risk of cancer (e.g., melanoma) due to UV radiation exposure.

  • Vitiligo:

    • Chronic disorder.

    • Patches of white skin due to lack of melanin.

    • Melanin is not released, or melanocytes are destroyed.

    • Autoimmune disease of unknown cause.

Hypodermis

  • Richly vascularized.

  • Contains adipose tissues.

  • Provides a protective cushion for internal organs.

  • Protects internal organs.