skin

1. Definition of the Integumentary System

• Skin (cutis, integument) and its derivatives together form the integumentary system.

• The skin:

  • Forms the external covering of the body

  • Is the largest organ

  • Accounts for 15–20% of total body mass

2. Layers of the Skin

The skin has two main layers:

1⃣ Epidermis

• Histology

  • Keratinized stratified squamous epithelium

• Growth

  • Grows continuously

  • Maintains normal thickness by desquamation

• Embryologic origin

  • Derived from ectoderm

2⃣ Dermis

• Histology

  • Dense connective tissue

• Functions

  • Provides:

    • Mechanical support

    • Strength

    • Thickness to the skin

• Embryologic origin

  • Derived from mesoderm

3. Hypodermis (Subcutaneous Layer)

• Contains variable amounts of adipose tissue

• Adipose tissue is arranged in lobules

• Lobules are separated by connective tissue septa

• Location:

  • Deep to the dermis

• Equivalent to subcutaneous fascia in gross anatomy

• Clinical variation:

  • Well-nourished individuals and people in cold climates → adipose tissue can be quite thick

4. Epidermal Derivatives (Skin Appendages)

Also called epithelial skin appendages or integumentary products

Includes:

• Hair follicles

• Hair

• Sweat (sudoriferous) glands

• Sebaceous glands

• Nails

• Mammary glands

5. Functions of the Integumentary System

The skin performs essential functions related to its external surface location.

Skin and its derivatives contain many different cell types that work together to help the body cope with the external environment.

Major Functions

1⃣ Barrier Function

Protects against:

• Physical agents

• Chemical agents

• Biologic agents

Types of barriers:

• Mechanical barrier

• Permeability barrier

• Ultraviolet barrier

2⃣ Immunologic Function

• Provides immunologic information

• Information obtained during antigen processing

• Delivered to appropriate effector cells in lymphatic tissue

3⃣ Homeostasis

Participates in homeostasis by regulating:

• Body temperature

• Water loss

4⃣ Sensory Function

• Conveys sensory information about the external environment

• Information transmitted to the nervous system

5⃣ Endocrine Function

• Skin performs endocrine functions by:

  • Secreting hormones

  • Secreting cytokines

  • Secreting growth factors

• Converts precursor molecules into hormonally active molecules:

  • Example: Vitamin D₃

6⃣ Excretory Function

• Functions in excretion via exocrine secretion of:

  • Sweat glands

  • Sebaceous glands

  • Apocrine glands

6. Classification of Skin

Skin is categorized as:

• Thick skin

• Thin skin

This classification reflects thickness and location.

8. Thick Skin

Found at:

• Palms of the hands

• Soles of the feet

Characteristics:

• Areas subject to most abrasion

• Hairless

• Have a much thicker epidermal layer than other skin regions

This hairless skin is called thick skin.

9. Thin Skin

• Found everywhere else

• Has a much thinner epidermis

• Contains hair follicles in almost all locations

10. Histologic Terminology Note

In histology:

• The terms thick skin and thin skin actually refer only to the thickness of the epidermal layer, not the whole skin.

Epidermis

1. Epidermis: General Histology

• The epidermis is composed of stratified squamous epithelium.

• Four layers are normally identified.

• In thick skin, a fifth layer is present.

Epidermal layers (deep → superficial)

1. Stratum basale (stratum germinativum)

2. Stratum spinosum

3. Stratum granulosum

4. Stratum lucidum (only in thick skin)

5. Stratum corneum

2. Epidermal Cell Differentiation

• Differentiation of epithelial cells is a specialized form of apoptosis.

Process

• Begins with cell division in the stratum basale.

• As cells migrate upward:

  • Cells in stratum granulosum show typical apoptotic nuclear morphology.

  • Includes DNA fragmentation.

• However:

  • Typical apoptotic cellular fragmentation does not occur.

• Instead:

  • Cells become filled with keratin filaments.

  • Eventually sloughed from the skin surface.

3. Stratum Basale (Stratum Germinativum)

Structure

• Single layer of cells

• Rest on the basal lamina

Cell Characteristics

• Contain stem cells

• Produce keratinocytes via mitotic division

Cell morphology

• Cells are:

  • Small

  • Cuboidal to low columnar

• Have less cytoplasm than cells of upper layers

• Nuclei closely spaced

Staining property

• Basophilic cytoplasm

• Closely spaced nuclei + basophilic cytoplasm → noticeable basophilia

Melanin

• Basal cells contain melanin in cytoplasm

• Melanin is transferred from melanocytes

• Melanocytes are interspersed in this layer

Cell Junctions

• Cells connected to each other and keratinocytes by desmosomes

• Cells connected to basal lamina by hemidesmosomes

Cell Migration

• New keratinocytes formed by mitosis

• Cells move upward into the next layer

• Migration continues until cells become mature keratinized cells

• Finally sloughed off at skin surface

4. Stratum Spinosum

Thickness

• Several cells thick

Cell Characteristics

• Keratinocytes larger than in stratum basale

Spiny appearance

• Cells exhibit numerous cytoplasmic processes (spines)

Cell junctions

• Processes attach to adjacent cells by desmosomes

Light microscopy feature

• Desmosome site appears as slight thickening

• Called the node of Bizzozero

Cause of visible spines

• During tissue preparation:

  • Cells shrink

  • Intercellular spaces expand

• Makes spines more visible

Alternate name

• Cells often called prickle cells

Maturation changes

As cells move toward the surface:

• Increase in size

• Become flattened parallel to surface

• In superficial cells:

  • Nuclei elongate

  • Change from ovoid → elongated

  • Reflect acquired squamous shape

5. Stratum Granulosum

Position

• Most superficial layer of the nonkeratinized portion of epidermis

Thickness

• 1–3 cells thick

Major feature

• Keratinocytes contain numerous keratohyalin granules

Keratohyalin granules

Contain:

• Cysteine-rich proteins

• Histidine-rich proteins

These proteins are precursors of filaggrin.

Filaggrin function

• Aggregates keratin filaments

• Present in cornified cells of stratum corneum

Granule characteristics

• Irregular shape

• Variable size

Staining

• Intensely basophilic

• Easily seen in routine histologic sections

6. Stratum Corneum

Cell type

• Anucleate squamous cells

Cell contents

• Cells filled largely with keratin filaments

Differentiation

• Cells are the most differentiated cells in the skin

Transition from granulosum

• Abrupt transition between:

  • Nucleated cells of stratum granulosum

  • Flattened anucleate cells of stratum corneum

Cellular changes

Cells:

• Lose nucleus

• Lose cytoplasmic organelles

• Become filled with keratin filaments

Plasma membrane

• Thick plasma membrane

Lipid layer

• External surface coated with extracellular lipids

• Lipids form major component of the epidermal water barrier

Thickness

• Layer varies most in thickness

• Thickest in thick skin

Role in thick vs thin skin

• Thickness of this layer is the principal difference between thick skin and thin skin

Adaptation to friction

• Becomes thicker at sites of unusual friction

Example:

• Calluses

• Palms

• Fingertips

7. Stratum Lucidum

Occurrence

• Normally visible only in thick skin

Histologic appearance

• Appears refractile under light microscopy

• May stain poorly

Cell characteristics

• Contains eosinophilic cells

Keratinization stage

• Keratinization well advanced

Cellular degeneration

During keratinization:

• Nucleus disappears

• Cytoplasmic organelles disappear

• Cell fills with keratin

Cells of the Epidermis :

2. Keratinocytes

• Predominant epidermal cell type

• Origin: Stratum basale

Two main activities

1. Production of keratins (cytokeratins)

2. Participation in formation of the epidermal water barrier

3. Keratin Proteins

• Keratins = major heteropolymeric structural proteins of the epidermis

• Keratins form intermediate filaments

• They constitute almost 85% of fully differentiated keratinocytes

4. Ultrastructure of Basal Keratinocytes

Basal keratinocytes contain:

• Numerous free ribosomes

• 7–9 nm intermediate (keratin) filaments

• Small Golgi apparatus

• Mitochondria

• Rough endoplasmic reticulum (rER)

Cytoplasmic staining

• Cytoplasm appears basophilic due to large number of free ribosomes

5. Keratin Filament Formation

• Ribosomes synthesize keratin proteins.

• Keratin proteins assemble into keratin filaments.

Terminology

• Intermediate filaments → tonofilaments

In the stratum spinosum

• Tonofilaments aggregate into bundles.

These bundles are called:

Tonofibrils

Cytoplasmic change

• Cytoplasm becomes eosinophilic

• Due to accumulation of tonofibrils.

6. Keratohyalin Granules

Keratohyalin granules contain intermediate filament–associated proteins that help aggregate keratin filaments.

Major proteins

• Filaggrin

• Trichohyalin

Synthesis

• Keratinocytes in the upper stratum spinosum begin synthesizing keratohyalin granules.

Clinical relevance

• Expression of filaggrin can be used as a clinical marker for initiation of the final stage of apoptosis.

7. Keratinization

When keratohyalin granules increase:

• Granule contents released into cytoplasm

• Filaggrin + trichohyalin promote aggregation of keratin filaments into tonofibrils

This converts:

Granular cells → Cornified cells

Process name

Keratinization

Timing

• Occurs in 2–6 hours

Keratin produced

• Called soft keratin

(Contrast: hard keratin of hair and nails)

8. Cellular Changes During Keratinization

Transformation of a granular cell into a keratinized cell involves:

• Breakdown of nucleus

• Breakdown of cytoplasmic organelles

• Thickening of plasma membrane

pH change

• pH decreases:

  • ~7.17 in stratum granulosum

  • pH 4.5–6.0 at surface of stratum corneum

9. Desquamation of Surface Keratinocytes

Surface keratinocytes are continuously exfoliated.

Regulation

Desquamation is regulated by:

Proteolytic degradation of desmosomes

10. Lamellar Bodies

Keratinocytes in stratum spinosum producing keratohyalin granules also produce:

Lamellar bodies (membrane-coating granules)

Characteristics

• Membrane-bound organelles

• Tubular or ovoid shaped

• Unique to mammalian epidermis

Lipid components synthesized

• Glycosphingolipids

• Phospholipids

• Ceramides

Enzymes present

• Acid sphingomyelinase

• Secretory phospholipase A₂

Also contain

Proteases including:

• SC chymotryptic enzyme

• Cathepsin D

• Acid phosphatase

• Glycosidases

• Protease inhibitors

11. Secretion of Lamellar Bodies

Lamellar body contents are:

• Released by exocytosis

• Into intercellular spaces

• Between stratum granulosum and stratum corneum

Function

Forms intercellular lipid lamellae

These lamellae are responsible for formation of the:

Epidermal water barrier

12. Epidermal Water Barrier

Essential for mammalian “dry” epithelia.

Main role

• Maintains body homeostasis

Formed by two factors

1. Insoluble proteins deposited on inner plasma membrane

2. Lipid layer attached to outer membrane surface

13. Structure of the Epidermal Barrier

A. Cell Envelope (CE)

Thickness

• ~15 nm

Location

• Inner surface of plasma membrane

Function

• Provides mechanical strength

Thickness increases in areas with high mechanical stress

Examples:

• Lip

• Palm of hand

• Sole of foot

Formation

CE formed by cross-linking:

• Small proline-rich (SPR) proteins

• Larger structural proteins

Major proteins

• Cystatin

• Desmoplakin

• Elafin

• Envoplakin

• Filaggrin

• Involucrin

• Keratin chains

• Loricrin

Important fact

Loricrin

• Major structural CE protein

• Accounts for ~80% of total CE protein mass

Additional feature

• 26 kDa insoluble protein

• Has highest glycine content of any known protein

B. Lipid Envelope

Thickness

• ~5 nm

Attachment

• Attached to cell surface via ester bonds

Major components

• Ceramides

• Cholesterol

• Free fatty acids

Key molecule

Acylglucosylceramide

Function:

• Forms monomolecular “Teflon-like” coating on cell surface.

Additional roles of ceramides

• Participate in cell signaling

• Induce cell differentiation

• Trigger apoptosis

• Reduce cell proliferation

14. Maintenance of the Epidermal Barrier

• Barrier continuously maintained by keratinocytes undergoing terminal differentiation.

Lamellar lipid arrangement

Lamellae may appear as:

• Recognizable discs

• Broad sheets or layers in intercellular space

• Dermis –

1. Epidermal–Dermal Junction

• Appears:

  • Uneven boundary in LM (except thinnest skin)

• Structure:

  • Dermal papillae

    • Finger-like connective tissue projections

    • Project into epidermis

  • Epidermal (rete) ridges

    • Downward projections of epidermis into dermis

2. Sectional Appearance (Exam Trap)

• Perpendicular section:

  • Shows dermal papillae clearly

• Parallel section:

  • Epidermis appears continuous

  • Contains circular islands of connective tissue

  • These = cross-sections of dermal papillae

3. Functional Significance

• Increased interface:

  • Enhances attachment between epidermis and dermis

4. Dermal Ridges

• Arrangement:

  • Parallel

  • Dermal papillae lie between ridges

• Function:

  • Form surface patterns → epidermal grooves & ridges

• Clinical relevance:

  • Basis of:

    • Dermatoglyphics (fingerprints, footprints)

  • Genetically unique

5. Distribution of Ridges & Papillae

• Most prominent in:

  • Thick skin

  • Palmar & plantar surfaces

6. Thick Skin Characteristics (Related Concept)

• Basal surface:

  • Much larger than free surface

• Result:

  • More cells enter stratum corneum per unit time

  • Leads to:

    • Thicker cornified layer

7. Attachment Mechanisms

Hemidesmosomes

• Function:

  • Attach epidermis to basal lamina

• Mechanism:

  • Link:

    • Intermediate filaments → basal lamina

Focal Adhesions

• Function:

  • Anchor:

    • Actin filaments → basal lamina

Additional Feature (TEM)

• Basal epidermal cells:

  • Show irregular cytoplasmic protrusions

  • Increase attachment surface area

8. Layers of Dermis

A. Papillary Layer

• Location:

  • Superficial dermis

• Tissue:

  • Loose connective tissue

• Fibers:

  • Thin collagen fibers

  • Types:

    • Type I and Type III collagen

• Elastic fibers:

  • Thread-like, irregular network

• Contains:

  • Dermal papillae

  • Blood vessels

    • Do NOT enter epidermis

  • Nerve processes

    • Either terminate in dermis

    • Or penetrate basal lamina → epidermis

• Note:

  • Sensory endings concentrated → prominent in papillae

B. Reticular Layer

• Location:

  • Deep to papillary layer

• Characteristics:

  • Thicker

  • Less cellular

• Thickness:

  • Varies by body region

9. Special Features of Reticular Layer

Smooth Muscle

• Found in:

  • Areola, penis, scrotum, perineum

• Arrangement:

  • Loose plexus in deep reticular layer

• Function:

  • Causes skin puckering (especially erectile organs)

Arrector Pili Muscle

• Origin:

  • From dermal smooth muscle

• Attachment:

  • Connects:

    • Hair follicle → superficial dermis

• Function:

  • Contraction causes:

    • Hair erection

    • Goose flesh

10. Hypodermis (Subcutaneous Layer)

Structure

• Located:

  • Deep to reticular layer

• Components:

  • Adipose tissue (panniculus adiposus)

  • Loose connective tissue

Functions

• Energy storage

• Insulation

• Thickness:

  • Variable

  • Increased in cold climates

Terminology

• Also called:

  • Hypodermis

  • Subcutaneous fascia

11. Muscle in Hypodermis

Striated Muscle

• Present in some areas

• Special layer:

  • Panniculus carnosus

    • Prominent in animals

    • Vestigial in humans

Human Remnants

• Present in:

  • Neck, face, scalp

• Forms:

  • Platysma

  • Muscles of facial expression

Melanocytes

1. Origin

• Neural crest–derived cells

2. Migration (Embryology)

• Melanocyte precursors:

  • Migrate from neural crest

  • Enter developing epidermis

3. Location in Skin

• Found in:

  • Stratum basale

• Distribution:

  • Scattered among basal keratinocytes

4. Epidermal–Melanin Unit

• Definition:

  • Functional association between:

    • 1 melanocyte + keratinocytes

• In humans:

  • ≈ 1 melanocyte : 36 keratinocytes

• Ratio range:

  • 1:4 → 1:40 (or higher)

• Notes:

  • Constant in all races

  • Influenced by:

    • Age

    • Environmental factors (sun exposure)

5. Adult Stem Cell Pool

• Location:

  • Hair follicle – follicular bulge

• Cell type:

  • Undifferentiated melanocyte stem cells

6. Molecular Regulation

• Key gene:

  • Pax3 gene (PAX transcription factor family)

• Pax3 function:

  • Activates:

    • MITF (microphthalmia transcription factor)

• MITF role:

  • Essential for:

    • Melanocyte development

    • Melanocyte differentiation (melanogenesis)

7. Proliferation

• Melanocytes:

  • Replicate throughout life

  • Slower rate than keratinocytes

8. Cell Type & Morphology

• Type:

  • Dendritic cells

• Cell body:

  • Located in stratum basale

• Processes:

  • Extend between keratinocytes into:

    • Stratum spinosum

9. Cell Junctions

• Do NOT form desmosomes with keratinocytes

• Near basal lamina:

  • Have structures resembling:

    • Hemidesmosomes

12. Main Function

• Produce and secrete:

  • Melanin

• Role:

  • Protection against:

    • Nonionizing ultraviolet radiation

13. Melanin Synthesis (Melanogenesis)

• Substrate:

  • Tyrosine

• Intermediate:

  • 3,4-dihydroxyphenylalanine (DOPA)

• Final product:

  • Melanin

• Enzyme:

  • Tyrosinase

14. Site of Melanin Production

• Occurs in:

  • Premelanosomes

• Characteristics:

  • Membrane-bound

  • Lysosome-related organelles

  • Derived from:

    • Golgi apparatus

15. Regulation of Melanin Production

• Hormone:

  • Melanocyte-stimulating hormone (MSH)

• Source:

  • Anterior pituitary

• Receptor:

  • Melanocortin 1 receptor (MC1R)

• Mechanism:

  • G-protein signaling cascade

  • ↑ Tyrosinase activity

  • ↑ Melanin synthesis

16. Melanosome Development Stages

1. Premelanosomes / Early melanosomes

   • Low melanin content

   • Organized internal structure (visible on TEM)

2. Progressive melanin deposition:

   • Structure becomes obscured

3. Mature melanosomes

   • Electron-opaque granules

17. Intracellular Distribution

• Premelanosomes:

  • Near Golgi apparatus

• Nearly mature melanosomes:

  • At base of cell processes

• Mature melanosomes:

  • At ends of processes

18. Transfer to Keratinocytes

• Mechanism:

  • Pigment donation

• Process:

  • Keratinocytes phagocytose:

    • Tips of melanocyte processes

• Type:

  • Cytocrine secretion

  • Includes:

    • Transfer of some cytoplasm

19. Fate of Melanosomes

• Degradation:

  • Via macroautophagy

• Variation:

  • Darker skin:

    • Slow degradation

    • Melanosomes remain discrete

  • Lighter skin:

    • Faster degradation

Langerhans’ Cells

1. Definition

• Antigen-presenting cells (APCs) of the epidermis

• Dendritic-appearing cells

2. Origin

• Derived from:

  • Common lymphoid progenitor (CLP) cells in bone marrow

• Pathway:

  • Bone marrow → bloodstream → epidermis

• Differentiate into:

  • Immunocompetent cells

3. Location

• Found in:

  • Epidermis

  • Readily seen in:

    • Stratum spinosum (with special staining)

4. Function

• Immunosurveillance of epidermis

• Process:

  1. Encounter antigens entering through skin

  2. Phagocytose and process antigen

  3. Display antigen on cell surface

  4. Migrate to regional lymph node

  5. Interact with T lymphocytes

• Part of:

  • Mononuclear phagocyte system (MPS)

5. Cell Ratio

• Ratio in normal epidermis:

  • 1 Langerhans cell : 53 other epidermal cells

7. Special Staining / Identification

• Identified using:

  • Gold chloride impregnation

  • Immunostaining for CD1a molecules

8. Morphology

• Dendritic processes

  • Similar to melanocytes

• Cell junctions:

  • No desmosomes with keratinocytes

10. Surface Markers / Receptors

• Express:

  • MHC I molecules

  • MHC II molecules

  • Fc receptors for IgG

  • Complement C3b receptors

  • CD1a molecules (variable amounts)

Langerhans’ Cells –

1. Definition

• Antigen-presenting cells (APCs) of the epidermis

• Dendritic-appearing cells

2. Origin

• Derived from:

  • Common lymphoid progenitor (CLP) cells in bone marrow

• Pathway:

  • Bone marrow → bloodstream → epidermis

• Differentiate into:

  • Immunocompetent cells

3. Location

• Found in:

  • Epidermis

  • Readily seen in:

    • Stratum spinosum (with special staining)

4. Function

• Immunosurveillance of epidermis

• Process:

  1. Encounter antigens entering through skin

  2. Phagocytose and process antigen

  3. Display antigen on cell surface

  4. Migrate to regional lymph node

  5. Interact with T lymphocytes

• Part of:

  • Mononuclear phagocyte system (MPS)

5. Cell Ratio

• Ratio in normal epidermis:

  • 1 Langerhans cell : 53 other epidermal cells

6. Light Microscopy (H&E)

• Cannot be reliably identified in routine H&E sections

• Features (when noted):

  • Nucleus:

    • Darkly stained (hematoxylin)

  • Cytoplasm:

    • Clear

7. Special Staining / Identification

• Identified using:

  • Gold chloride impregnation

  • Immunostaining for CD1a molecules

8. Morphology

• Dendritic processes

  • Similar to melanocytes

• Cell junctions:

  • No desmosomes with keratinocytes

9. Electron Microscopy (TEM) Features

• Nucleus:

  • Irregular, indented (uneven nuclear profile)

• Cytoplasmic organelles:

  • Birbeck granules

    • Shape:

      • Tennis racquet–shaped

    • Appearance:

      • Rod with bulbous expansion at one end

10. Surface Markers / Receptors

• Express:

  • MHC I molecules

  • MHC II molecules

  • Fc receptors for IgG

  • Complement C3b receptors

  • CD1a molecules (variable amounts)

Merkel’s Cells –

1. Definition

• Epidermal cells involved in cutaneous sensation

2. Location

• Found in:

  • Stratum basale

  • 4. General Characteristics

• Dendritic cells

• Possess:

  • Antigenic markers of both epidermal and neural type

5. Distribution

• Most abundant in:

  • Areas of acute sensory perception

  • Example:

    • Fingertips

6. Cell Junctions

• Connected to keratinocytes by:

  • Desmosomes

7. Cytoskeleton

• Contain:

  • Intermediate (keratin) filaments in cytoplasm

8. Light Microscopy Features

• Nucleus:

  • Lobed

• Cytoplasm:

  • Denser than melanocytes and Langerhans’ cells

9. Cytoplasmic Contents

• May contain:

  • Melanosomes

• Characteristic feature:

  • 80-nm dense-cored neurosecretory granules

    • Resemble granules of:

      • Adrenal medulla

      • Carotid body

10. Association with Nerve Fibers

• Closely associated with:

  • Expanded terminal bulb of afferent myelinated nerve fibers

• Nerve fiber features:

  • Loses Schwann cell covering

  • Penetrates basal lamina

  • Expands into:

    • Plate-like ending (disc receptor)

11. Merkel’s Corpuscle

• Formed by:

  • Merkel cell + nerve terminal

• Function:

  • Sensitive mechanoreceptor

12. Key Exam Points

• Located in stratum basale

• Desmosomes present (contrast with melanocytes & Langerhans cells)

• Neurosecretory granules (80 nm) = hallmark

• Associated with afferent nerve ending

• Forms Merkel’s corpuscle → mechanoreception

Skin – Nerve Supply

1. General Organization

• Skin contains:

  • Sensory receptors

    • Peripheral terminals of sensory nerves

  • Motor nerve endings to:

    • Blood vessels

    • Arrector pili muscles

    • Sweat glands

2. Free Nerve Endings

Definition

• Most numerous receptors in epidermis

Location

• Terminate in:

  • Stratum granulosum

Structure

• “Free” because:

  • No connective tissue capsule

  • No Schwann cell covering

Function

• Detect:

  • Fine touch

  • Heat

  • Cold

  • Pain

• Note:

  • No distinct morphology for each modality

Association with Hair

• Networks surround:

  • Hair follicles

• Attach to:

  • External root sheath

• Function:

  • Act as mechanoreceptors (hair movement)

Specialization

• Around tactile hairs (vibrissae):

  • Highly specialized

  • Each hair has specific cortical representation

3. Encapsulated Nerve Endings

General Feature

• Surrounded by:

  • Connective tissue capsule

Types

1. Pacinian corpuscles

2. Meissner’s corpuscles

3. Ruffini’s corpuscles

4. Pacinian Corpuscles

Function

• Detect:

  • Pressure

  • Vibration

• Type:

  • Deep pressure mechanoreceptors

Location

• Deep dermis

• Hypodermis

• Also found in:

  • Connective tissue

  • Joints

  • Periosteum

  • Internal organs

• Especially:

  • Fingertips

Structure

• Central:

  • Myelinated nerve fiber

• Entry:

  • Enters capsule with myelin intact

• Inside:

  • Myelin lost after 1–2 nodes

  • Axon becomes unmyelinated

Core (Inner Core)

• Formed by:

  • Flattened Schwann cell lamellae

Outer Core (Capsule)

• Concentric lamellae

• Between lamellae:

  • Fluid-filled spaces

  • Sparse collagen fibrils

  • Occasional capillaries

Mechanism

• Pressure → displacement of lamellae

• → Axon depolarization

5. Meissner’s Corpuscles

Function

• Touch receptors

• Sensitive to:

  • Low-frequency stimuli

Location

• Dermal papillae

• Just beneath:

  • Epidermal basal lamina

Distribution

• Hairless skin

  • Lips

  • Palmar surfaces

  • Volar surfaces of fingers and toes

Structure

• Contains:

  • 1–2 unmyelinated nerve endings

    • Derived from myelinated fibers

• Arrangement:

  • Spiral course within corpuscle

Cellular Component

• Flattened Schwann cells

• Form:

  • Irregular lamellae

6. Ruffini’s Corpuscles

Function

• Detect:

  • Stretch

  • Torque

• Stimulus:

  • Mechanical displacement of collagen fibers

Type

• Simplest encapsulated mechanoreceptors

Structure

Capsule

• Thin:

  • Connective tissue capsule

• Contains:

  • Fluid-filled space

Collagen Fibers

• Pass through capsule from surrounding CT

Neural Component

• Single myelinated fiber enters

• Loses myelin inside capsule

• Branches into:

  • Dense arborization of axonal endings

• Each ending:

  • Knob-like bulb

Arrangement

• Axonal endings:

  • Dispersed and intertwined

Mechanism

• Respond to:

  • Displacement of collagen fibers

  • Due to:

    • Sustained mechanical stress

Physiological Type

• Rapidly adapting (phasic receptors)

• Generate:

  • Action potentials at:

    • Beginning and end of stimulus

Hair Follicles and Hair –

1. Definition

• Hair follicle = invagination of epidermis

• Site of:

  • Hair formation

2. Distribution

• Present:

  • Almost entire body

• Absent from:

  • Palmar surfaces (hands)

  • Plantar surfaces (feet)

  • Sides of hands & feet

  • Lips

  • Region around urogenital orifices

3. Hormonal Influence

• Hair distribution influenced by:

  • Sex hormones

• At puberty:

  • Male:

    • Thick pigmented facial hair

  • Both sexes:

    • Pubic & axillary hair

• Aging:

  • Male → hairline recedes

  • Both sexes → scalp hair thins

    • Due to ↓ estrogen & estrogen-like hormones

4. Hair Follicle Function

• Responsible for:

  • Production and growth of hair

5. Hair Color

• Determined by:

  • Type and amount of melanin

6. Follicle Variation

• Depends on:

  • Growing vs resting phase

• Growing follicle:

  • Most elaborate (exam focus)

7. Regions of Hair Follicle

1. Infundibulum

• Extends:

  • Surface opening → sebaceous gland opening

• Part of:

  • Pilosebaceous canal

• Function:

  • Route for:

    • Sebum discharge

2. Isthmus

• Extends:

  • Infundibulum → arrector pili insertion

3. Follicular Bulge

• Location:

  • Near arrector pili insertion

• Contains:

  • Epidermal stem cells

4. Inferior Segment

• Present in:

  • Growing follicle

• Shape:

  • Uniform diameter except base → expands into:

    • Hair bulb

8. Hair Bulb & Dermal Papilla

Hair Bulb

• Expanded base of follicle

• Contains:

  • Hair matrix

Dermal Papilla

• Composition:

  • Vascularized loose connective tissue

• Invaginates into:

  • Hair bulb

Hair Matrix

• Composed of:

  • Matrix cells

• Location:

  • Adjacent to dermal papilla

• Function:

  • Rapid division → hair growth

• Origin:

  • Cells migrated from:

    • Follicular bulge (stem cells)

Melanocytes (Matrix)

• Present in:

  • Germinative layer

• Function:

  • Transfer melanosomes → developing hair cells

Differentiation

• Matrix cells → form:

  • Hair shaft cells

  • Internal root sheath

9. Root Sheaths

A. Internal Root Sheath (IRS)

• Function:

  • Surrounds deep part of hair

• Structure:

  • Multilayered

Layers (must remember)

1. Henle’s layer

   • Outer layer

   • Single layer of cuboidal cells

2. Huxley’s layer

   • Middle layer

   • Single or double layer of flattened cells

   • Forms:

     • Middle plate

3. Cuticle of IRS

   • Squamous cells

   • Faces:

     • Hair shaft

B. External Root Sheath (ERS)

• Represents:

  • Down-growth of epidermis

• Continuous with:

  • Epidermal layers

10. Key Relationships

• Henle’s layer:

  • In contact with:

    • External root sheath

• IRS cuticle:

  • Faces:

    • Hair shaft

Hair Follicle & Hair –

1. Follicular Bulge (Stem Cell Niche)

• Located in external root sheath at level of arrector pili muscle insertion

• Contains relatively undifferentiated epithelial cells

• Called follicular bulge

Function

• Acts as a niche of epidermal stem (ES) cells

• ES cells:

  • Self-renew

  • Differentiate into specific lineages

Role under normal conditions

• Provide stem cells for:

  • Hair follicle growth

    • Hair matrix

    • Internal root sheath

    • Cortex

    • Medulla

  • Sebaceous glands

Important exam point

• Do NOT contribute to basal stem cells of epidermis normally

• Activated in injury → help in skin regeneration

2. External Root Sheath – Key Relations

• Contains:

  • Insertion of arrector pili muscle

  • Origin of sebaceous duct and gland

• Nerve endings surround it at this level

3. Keratinization of Hair

• Occurs in keratogenous zone

  • Located in lower third of follicle

Process

• Cells:

  • Differentiate

  • Lose organelles

  • Become packed with keratin intermediate filaments

• Final product:

  • Fully keratinized hard keratin (hair shaft)

Internal Root Sheath

• Contains soft keratin

• Does NOT emerge with hair

• Breaks down at isthmus level

4. Supporting Structures

• Glassy membrane

  • Thick basal lamina

  • Separates follicle from dermis

• Surrounding tissue:

  • Dense irregular connective tissue

• Arrector pili muscle

  • Attached near follicular bulge

  • Landmark for stem cell niche

5. Hair Structure (Hair Shaft Layers)

A. Medulla

• Central core

• Large, loosely connected keratinized cells

• Contains soft keratin

• Present only in thick hairs

B. Cortex

• Largest layer (~80% of hair mass)

• Located outside medulla

• Contains:

  • Hard keratin intermediate filaments

  • Keratin-associated proteins (KAPs)

Function

• Provides:

  • Strength (disulfide cross-linking)

  • Texture

  • Elasticity

  • Color

Pigment

• Melanin produced by melanocytes in hair bulb germinative layer

C. Cuticle of Hair Shaft

• Outermost layer

• Made of overlapping keratinized squamous cells

• Cells resemble:

  • Fish scales / roof tiles

• Free edges point away from follicle

Function

• Protects against:

  • Physical damage

  • Chemical damage

• Determines hair porosity