4 - Structures of Skin and Derivatives

What is the epidermis?

• Tissue type

• Vascularity

• General role

• Stratified squamous keratinized epithelium

• Avascular — relies on dermal capillaries for nutrients

• Provides a protective barrier against water loss, pathogens, and mechanical injury

What is the stratum basale?

• Structure

• Types of cells here

• Unique property of cell here

• Deepest epidermal layer, single row of columnar to cuboidal cells

• Keratinocytes (basal cells): stem cell-like, with keratin intermediate filaments (tonofilaments)

• Also contains melanocytes (pigment production) and Merkel cells (light touch)

• Keratinocytes here are mitotically active — they continuously regenerate the epidermis

What is the stratum spinosum?

• Cell shape and features

• Structural proteins and other substances

• Other important cell types here

• Several layers of polyhedral keratinocytes

• Contain tonofilaments and begin producing lamellar bodies (lipid-containing vesicles)

• Also contains Langerhans cells (antigen-presenting immune cells)

What is the stratum granulosum?

• Number of layers

• Cytoplasmic granules

• Function of those granules

• 3–5 layers of flattened keratinocytes

• Contain keratohyalin granules (with profilaggrin and loricrin)

• Profilaggrin is cleaved to filaggrin, which bundles keratin filaments

• Loricrin contributes to the cornified envelope

What is the stratum lucidum?

• Appearance

• Location

• Cell status

• Thin, translucent layer only found in thick skin (palms and soles)

• Contains anucleate, dead keratinocytes

• Organelles have degenerated

What is the stratum corneum?

• Cell condition

• Functional barrier components

• Dynamic process

• Most superficial layer, many layers of dead, flattened, enucleated keratinocytes

• Cells surrounded by lipid lamellae (from lamellar bodies) and enclosed by protein-rich envelope (loricrin)

• Rich in natural moisturizing factor (NMF) from degraded filaggrin

• Undergoes continuous desquamation and renewal

How does the skin form a water barrier?

• In the stratum granulosum, keratinocytes make:

  • Filaggrin (from profilaggrin) → bundles keratin inside the cell

  • Loricrin → forms a strong protein shell under the cell membrane (cornified envelope)

  • Lamellar bodies → release lipids between cells

• In the stratum corneum:

  • Filaggrin breaks down into NMF (natural moisturizing factor) inside dead cells

  • Lipids from lamellar bodies form lipid layers between cells

• Together, these parts make a strong water barrier:

  • NMF holds water inside cells

  • Lipids + loricrin block water between cells

What are keratinocytes and what do they produce?

• Also mention their process of life, and special things they do and where they happen

• Most abundant cells in the epidermis

• Start in the stratum basale and move upward as they mature

• In stratum granulosum, they form keratohyalin granules with:

  • Profilaggrin → becomes filaggrin, bundles keratin

  • Loricrin → builds the tough outer envelope

• In upper stratum spinosum and granulosum, they make lamellar bodies

  • These release lipids between cells to form the skin’s water barrier

• Final result: a strong, waterproof outer layer of dead, flat cells

What are melanocytes and what do they do?

• Location and relation

• Function

• Histo feature

• Key note about skin color

• Found in the stratum basale, but send dendritic processes upward

• Produce melanin in melanosomes and transfer it to keratinocytes

• Appear clear on H&E stain due to pale cytoplasm

• The number of melanocytes is similar across skin tones

• Skin color differences are mostly due to how much melanin is produced and retained, not how many melanocytes there are

What are Langerhans cells and where are they found?

• Immune cells of the epidermis

• Located mainly in the stratum spinosum

• Dendritic antigen-presenting cells → capture and present antigens to T cells

What are Merkel cells, where are they found, and what is their function?

• Found in the stratum basale

• Located near free nerve endings

• Function in mechanoreception → detect light touch or pressure

What are epidermal ridges?

• Downward projections of the epidermis into the dermis

• Interlock with dermal papillae to increase surface area and adhesion

What are friction ridges, and what are they responsible for?

• Raised surface patterns formed by the alignment of epidermal and dermal ridges

• Found in thick skin (palms and soles)

• Contain sweat pores that open at the ridge surface

• Responsible for fingerprint patterns

What is the dermis?

• Connective tissue layer beneath the epidermis

• Vascularized — supplies nutrients to the epidermis

• Provides strength, support, and helps with temperature regulation

What are the two layers of the dermis and how do they differ?

• Papillary dermis:

  • Loose connective tissue

  • Has capillaries, immune cells, and Meissner’s corpuscles

  • Forms dermal papillae that connect to epidermis

• Reticular dermis:

  • Dense irregular connective tissue

  • Contains hair roots, sebaceous/sweat glands, blood vessels, and Ruffini corpuscles(sensory receptors)

  • Provides mechanical strength

What is the hypodermis?

• Also called the subcutaneous tissue

• Made of loose connective tissue, mainly adipose

• Functions:

  • Cushions, insulates, and anchors skin to underlying structures

  • Contains larger blood vessels, nerves, and sometimes hair follicle roots

What normally happens to blood flow in the skin, and how does an arteriovenous anastomosis (AVA) change that?

• Normally, blood flows from arterioles → capillaries → venules, allowing heat exchange and nutrient delivery at the skin

• An AVA is a direct connection between arteriole and venule, bypassing capillaries

• When it’s hot:

  • AVA closes, forcing blood into capillaries → more heat loss

• When it’s cold:

  • AVA opens, bypassing capillaries → heat conserved

What are the main parts of a hair?

• Shaft: visible part above the skin surface

• Root: embedded in the skin, surrounded by the hair follicle

• Hair bulb: expanded base of the root

  • Contains the matrix (mitotically active cells that grow the hair)

  • Sits atop the dermal papilla, which provides blood supply

What are the layers of a hair and its follicle?

• Hair has 3 concentric layers:

  • Medulla (center)

  • Cortex (main mass, pigmented)

  • Cuticle (outermost, protective)

• The hair follicle includes:

  • Epidermal root sheath (internal and external parts)

  • Glassy membrane (basement membrane-like layer)

  • Dermal root sheath (connective tissue layer from dermis)

What are the phases of hair growth?

• Anagen – active growth (longest phase, 4 years)

• Catagen – transition, growth slows, follicle shrinks

• Telogen – resting phase; hair may shed

What is the arrector pili muscle and what does it do?

• Small smooth muscle attached to the hair follicle and dermal–epidermal junction

• Contracts during piloerection (e.g. cold, emotion) to pull hair upright

• Helps expel sebum from the sebaceous gland

What is the structure of a sebaceous gland?

• Simple branched alevolar gland

• In the dermis, usually near a hair follicle and opens into it

What is a pilosebaceous unit?

• Consists of a sebaceous gland, hair + hair follicle, arrector pili

How does a sebaceous gland release sebum, and what is sebum?

• Releases sebum via holocrine secretion → cells rupture to release contents

• Sebum is an oily mixture of lipids and cellular debris

  • Lubricates skin and hair

  • Has mild antibacterial properties

What stimulates sebaceous glands, where are sebaceous glands found, and what are some exceptions to their secretion?

• Sebaceous glands are stimulated by androgens, especially active after puberty

• Found throughout the body except palms and soles

  • Open into hair follicles, but in some areas (e.g., lips, glans penis, labia minora), open directly onto the skin surface

What is the structure and location of sweat glands?

• Simple coiled tubular glands

• Located in the deep dermis or upper hypodermis

• Made of two parts:

  • Secretory portion: coiled, produces sweat

  • Duct: carries sweat to the surface or into a hair follicle

What are eccrine sweat glands?

• Location

• Where they open

• Cell types in secretory portion

• Secretion method

• Function

• Onset of function

• Found all over the body, especially palms, soles, and forehead

• Open directly onto the skin surface

• Secretory portion has:

  • Clear cells (water & electrolytes)

  • Dark cells (glycoproteins)

  • Myoepithelial cells (at basal lamina, contract to release sweat)

• Secretion is merocrine (via exocytosis, clean release)

• Function: thermoregulation and emotional sweating

• Active from birth

What are apocrine sweat glands?

• Location

• Where they open

• Structure

• Secretion method

• Type of secretion

• Stimuli

• Onset of function

Found in axilla, areola, perineum, and ear canal

• Open into the upper portion of a hair follicle

• Secretory portion:

  • One layer of secretory cells

  • Interspersed myoepithelial cells

• Secretion is apocrine (apical membrane pinches off)

• Produces thick, protein-rich fluid

• Responds to emotional stimuli

• Becomes functional at puberty

What are the visible parts of the nail and surrounding skin?

• Nail plate(and 3 related things)

• Surrounding folds

• And special regions(2)

• Nail plate: hard keratinized surface of the nail

  • Divided into:

    • Free edge (extends past finger; oldest cells)

    • Body (visible attached portion)

    • Lunula (whitish crescent at base, part of matrix)

• Surrounding folds:

  • Proximal nail fold (covers root)

  • Lateral nail folds (on the sides)

• Eponychium: aka cuticle, thin layer of skin extending from proximal fold onto plate

• Hyponychium: skin under the free edge; protects nail bed from pathogens

What lies beneath the nail and controls its growth? (3 things)

• Nail root: embedded under the proximal fold

• Nail matrix: deep to the root, contains mitotically active cells that generate new nail

  • Lunula is the visible part of the matrix

• Nail bed: tissue under the nail plate

  • Made of epidermis, only w basale and spinosum

  • Adds support and adherence, not new nail growth