Skin Cells, Layers, and Associated Structures: Study Notes for ANP College Students

Skin Cells & Layers

Keratinocytes

Keratinocytes are the most abundant cell type in the epidermis, responsible for producing the protein keratin, which provides protection and waterproofing.

• Function: Produce protective keratin

• Location: Throughout the epidermis

• Tissue Type: Epithelial (stratified squamous)

• Example: Keratinocytes form the bulk of the skin's outer layer, shielding underlying tissues from environmental damage.

Merkel Cells

Merkel cells are specialized epithelial cells that function as touch receptors, contributing to the sensation of light touch.

• Function: Detect touch sensations

• Location: Epidermal-dermal junction layer

• Tissue Type: Epithelial

• Example: Merkel cells are concentrated in areas of the skin sensitive to touch, such as fingertips.

Melanocytes

Melanocytes are pigment-producing cells found in the deepest layer of the epidermis. They synthesize melanin, which determines skin color and protects against UV radiation.

• Function: Produce pigment melanin

• Location: Stratum basale layer

• Tissue Type: Epithelial

• Example: Increased melanin production leads to tanning after sun exposure.

Meissner’s Corpuscles

Meissner’s corpuscles are sensory nerve endings located in the dermal papillae, responsible for detecting light touch.

• Function: Detect light touch

• Location: Dermal papillae layer

• Tissue Type: Nervous (within connective tissue)

• Example: Meissner’s corpuscles are abundant in fingertips and lips.

Dendritic Cells

Dendritic cells (Langerhans cells) are immune cells within the epidermis that help defend against pathogens.

• Function: Activate immune responses

• Location: Stratum spinosum layer

• Tissue Type: Connective/immune

• Example: Dendritic cells capture antigens and present them to lymphocytes.

Cornified Keratinocytes

Cornified keratinocytes are dead, flattened cells in the outermost layer of the epidermis, providing a waterproof barrier.

• Function: Provide waterproof protection

• Location: Stratum corneum layer

• Tissue Type: Epithelial (keratinized)

• Example: These cells are continuously shed and replaced.

Epidermal Layers

Stratum Corneum

The stratum corneum is the outermost layer of the epidermis, composed of dead, keratinized cells that are regularly shed.

• Function: Sheds dead cells

• Location: Outermost epidermis layer

• Tissue Type: Epithelial

Stratum Granulosum

The stratum granulosum is a middle layer where cells begin to die and form granules containing keratohyalin and lamellar granules.

• Function: Cells form granules

• Location: Middle epidermis layer

• Tissue Type: Epithelial

Stratum Spinosum

The stratum spinosum provides structural strength to the epidermis, containing several layers of keratinocytes and dendritic cells.

• Function: Provides structural strength

• Location: Above stratum basale

• Tissue Type: Epithelial

Stratum Basale

The stratum basale is the deepest layer of the epidermis, where new keratinocytes are generated through cell division.

• Function: Divides new cells

• Location: Deepest epidermis layer

• Tissue Type: Epithelial

Glands

Sudoriferous Glands

Sudoriferous (sweat) glands are responsible for producing sweat, which helps regulate body temperature and excrete waste.

• Function: Produce sweat liquid

• Location: Deep dermis layer

• Tissue Type: Epithelial (glandular)

• Example: Eccrine glands are a type of sudoriferous gland found throughout the skin.

Sebaceous Glands

Sebaceous glands secrete sebum, an oily substance that lubricates and waterproofs the skin and hair.

• Function: Secrete oily sebum

• Location: Dermis near hair

• Tissue Type: Epithelial (glandular)

• Example: Overactivity can lead to acne.

Ceruminous Glands

Ceruminous glands are specialized sweat glands in the external ear canal that produce cerumen (earwax).

• Function: Produce earwax protection

• Location: External ear canal

• Tissue Type: Epithelial (glandular)

Multicellular Exocrine Glands

Multicellular exocrine glands release their secretions onto epithelial surfaces via ducts.

• Function: Release external secretions

• Location: Various epithelial surfaces

• Tissue Type: Epithelial

Conditions

Jaundice

Jaundice is a condition characterized by yellowing of the skin and eyes, often indicating liver dysfunction.

• Function: Signals liver dysfunction

• Location: Skin and eyes

• Tissue Type: Epithelial & connective

• Example: Jaundice is commonly seen in hepatitis or bile duct obstruction.

Addison’s Disease

Addison’s disease is an endocrine disorder that causes skin bronzing due to increased melanin production.

• Function: Causes skin bronzing

• Location: Adrenal cortex origin

• Tissue Type: Glandular epithelial (endocrine)

Cyanosis

Cyanosis is a bluish discoloration of the skin and nails, indicating low oxygen levels in the blood.

• Function: Signals low oxygen

• Location: Skin and nails

• Tissue Type: Epithelial & connective

Dermis & Associated Structures

Dermal Ridges

Dermal ridges are patterns in the superficial dermis that create fingerprints and enhance grip.

• Function: Create fingerprint grip

• Location: Superficial dermis layer

• Tissue Type: Connective

Hypodermis

The hypodermis is a layer beneath the dermis, primarily composed of adipose tissue, providing insulation and energy storage.

• Function: Stores fat insulation

• Location: Beneath the dermis

• Tissue Type: Adipose connective

Arrector Pili Muscle

The arrector pili muscle is a small smooth muscle attached to hair follicles, responsible for causing hair to stand upright (goosebumps).

• Function: Pulls hair upright

• Location: Dermis hair follicles

• Tissue Type: Smooth muscle

External Root Sheath

The external root sheath is an epithelial layer surrounding the hair root, providing protection and support.

• Function: Protects hair follicle

• Location: Surrounding hair root

• Tissue Type: Epithelial

Layers of Dermis

The dermis consists of two main layers: the papillary layer (superficial) and the reticular layer (deep), both supporting epidermal structures.

• Function: Support epidermal structures

• Location: Beneath the epidermis

• Tissue Type: Connective

• Example: The papillary layer contains capillaries and nerve endings; the reticular layer contains dense irregular connective tissue.

Membranes & Linings

Serous Membranes

Serous membranes line closed body cavities and secrete a lubricating fluid to reduce friction between organs.

• Function: Secrete lubricating fluid

• Location: Closed body cavities

• Tissue Type: Epithelial + connective

• Example: The pericardium, pleura, and peritoneum are serous membranes.

Mucous Membranes

Mucous membranes line open body cavities and secrete mucus for protection and lubrication.

• Function: Secretes protective mucus

• Location: Open body cavities

• Tissue Type: Epithelial + connective

• Example: The lining of the respiratory and digestive tracts.

Endothelium

Endothelium is a simple squamous epithelial tissue lining the interior of blood vessels and the heart.

• Function: Lines blood vessels

• Location: Cardiovascular system interior

• Tissue Type: Epithelial (simple squamous)

Connective Tissue Types

Chondroblasts

Chondroblasts are immature cartilage cells that produce the cartilage matrix during growth and repair.

• Function: Form cartilage matrix

• Location: Growing cartilage tissue

• Tissue Type: Connective

Elastic Cartilage

Elastic cartilage provides flexible support and maintains shape in structures such as the external ear.

• Function: Maintains flexible shape

• Location: External ear structure

• Tissue Type: Connective

Hyaline Cartilage

Hyaline cartilage is the most common type, reducing friction and providing support at joints.

• Function: Reduces joint friction

• Location: Ends of bones

• Tissue Type: Connective

Fibrocartilage

Fibrocartilage is tough and absorbs compressive shock, found in intervertebral discs and pubic symphysis.

• Function: Absorbs compressive shock

• Location: Intervertebral disc spaces

• Tissue Type: Connective

Osseous Tissue

Osseous (bone) tissue supports body weight and protects organs, forming the skeletal system.

• Function: Supports body weight

• Location: Entire skeletal system

• Tissue Type: Connective

Reticular Tissue

Reticular tissue forms a supportive framework for lymphoid organs such as lymph nodes and spleen.

• Function: Forms supportive framework

• Location: Lymph nodes, spleen

• Tissue Type: Connective

Mesenchyme

Mesenchyme is an embryonic connective tissue that gives rise to all other connective tissues.

• Function: Forms connective tissues

• Location: Developing human embryo

• Tissue Type: Embryonic connective

Collagen Fibers

Collagen fibers are strong, flexible protein fibers providing tensile strength to tissues such as bone and ligaments.

• Function: Provide tensile strength

• Location: Bone and ligaments

• Tissue Type: Connective

Matrix of Connective Tissue

The matrix is the extracellular material in connective tissue, consisting of fibers and ground substance.

• Function: Fills extracellular space

• Location: Between tissue cells

• Tissue Type: Connective

Components of Connective Tissue

Connective tissue is composed of cells, fibers (collagen, elastic, reticular), and ground substance, providing structural support throughout the body.

• Function: Provide structural support

• Location: Throughout the body

• Tissue Type: Connective

Epithelial Specializations

Microvilli

Microvilli are finger-like projections on epithelial cells that increase surface area for absorption.

• Function: Increase absorption area

• Location: Small intestine lining

• Tissue Type: Epithelial (simple columnar)

• Example: Microvilli are abundant in the small intestine to maximize nutrient absorption.

Pseudostratified Ciliated Columnar Epithelium

This epithelial tissue appears layered but is actually a single layer, with cilia that move mucus along the respiratory tract.

• Function: Propels respiratory mucus

• Location: Trachea lining wall

• Tissue Type: Epithelial

Goblet Cells

Goblet cells are specialized epithelial cells that secrete mucus for lubrication and protection in the respiratory and gastrointestinal tracts.

• Function: Secrete lubricating mucus

• Location: Respiratory, GI tracts

• Tissue Type: Epithelial

Simple Squamous Epithelium

Simple squamous epithelium consists of a single layer of flat cells, allowing rapid diffusion of gases and nutrients.

• Function: Allows rapid diffusion

• Location: Lung alveoli sacs

• Tissue Type: Epithelial

Stratified Squamous Epithelium

Stratified squamous epithelium is composed of multiple layers of cells, providing protection against abrasion.

• Function: Protects underlying tissues

• Location: Outer skin epidermis

• Tissue Type: Epithelial

Other

Endocrine vs Exocrine Glands

Glands are classified based on their method of secretion: endocrine glands release hormones into the bloodstream, while exocrine glands secrete fluids via ducts to external surfaces.

• Function: Secrete hormones/fluids

• Location: Bloodstreams versus ducts

• Tissue Type: Epithelial (glandular)

Type	Secretion	Destination
Endocrine	Hormones	Bloodstream
Exocrine	Fluids (e.g., sweat, sebum)	Ducts to surface

Muscular Tissue

Muscular tissue is specialized for contraction, enabling movement of the skeleton, heart, and internal organs.

• Function: Generates body movement

• Location: Skeleton, heart, organs

• Tissue Type: Muscle

• Example: Skeletal muscle moves limbs; cardiac muscle pumps blood; smooth muscle controls organ function.