3.1 The Skin: Structure and Function

Flashcards covering the structure, layers, cells, accessory structures, and functions of the skin and integumentary system, based on the provided lecture notes. Key terms include definitions for the epidermis, dermis, hypodermis, various cell types (keratinocytes, melanocytes, Langerhans, Merkel), epidermal strata, glands (sebaceous, sudoriferous), hair, and nails.

Integumentary System

The organ system comprising the skin and its accessory structures, which protects the body, senses the world, and regulates temperature.

Skin (Cutaneous Membrane)

The largest and heaviest organ of the body, making up 15% of body weight, composed of two main structural layers: the epidermis and the dermis.

Epidermis

The outermost, avascular (no blood vessel) layer of the skin. It depends on diffusion of nutrients from the underlying dermis. Composed of epithelial tissue, primarily keratinized stratified squamous epithelium. It provides a barrier to infection and prevents water loss. Layers from deep to superficial: basale, spinosum, granulosum, lucidum (thick skin), corneum.

Dermis

The supportive vascular layer beneath the epidermis. Composed of connective tissue rich in collagen with elastic and reticular fibers; contains fibroblasts and macrophages. It is well supplied with blood vessels and houses sweat glands, sebaceous glands, nerve endings, hair follicles, and nail roots. Provides strength and elasticity to the skin, as well as structural support.

Organized in two main layers:

papillary layer (superficial) and reticular layers (deeper and thicker).

Avascular

Lacking blood vessels, a characteristic of the epidermis.

Vascular

Possessing blood vessels, a characteristic of the dermis.

Hypodermis (Subcutaneous Tissue/Layer)

A layer underneath the dermis, not considered part of the skin, consisting of areolar connective tissue and adipose tissue, attaching skin to fascia, storing fat, and housing large blood vessels. It’s function is energy storage (fat), cushioning, and anchoring the skin to underlying fascia.

Keratinocytes

The main cell type found in the epidermis, producing keratin to strengthen the skin and contribute to its water-resistant properties.

Keratin

A structural protein produced by keratinocytes that strengthens the skin and makes it water-resistant.

Melanocytes

Cells found deeper in the epidermis (stratum basale) that produce melanin, the pigment providing skin color and absorbing UV light. Melanin is transfered to keratinocytes via branch projections and surrounds the nucleus to protect DNA.

Melanin

The pigment produced by melanocytes, giving skin its color and protecting DNA from damaging ultraviolet light.

Langerhans Cells (Dendritic Cells, macrophages)

Macrophages found in the stratum spinosum and stratum granulosum of the epidermis that participate in the immune response against pathogens.

Macrophages

Cells capable of phagocytosis, such as Langerhans cells in the epidermis and cells in the dermis, that play a role in the immune response.

Merkel Cells (Tactile Cells)

The least numerous epidermal cells, found in the stratum basale, associated with sensory neurons (Merkel discs) for detecting touch.

Stratum Basale

The DEEPEST layer of the epidermis, containing a single layer of mitotically active keratinocytes, epidermal stem cells, melanocytes, and Merkel cells.

Stratum Spinosum

The layer above the stratum basale, consisting of several layers of keratinocytes connected by desmosomes; thickest layer in most skin. As cells move upward, they synthesize more keratin and flatten; contains Langerhans cells and melanocyte projections that help in immune response and pigmentation.

Desmosomes

Cell junctions that connect keratinocytes in the stratum spinosum.

Stratum Granulosum

Consists of three to five layers of flattened keratinocytes undergoing apoptosis; nuclei/organelles degenerate as they move away from nutrient source; characterized by dark staining keratohyalin granules.

Apoptosis

Programmed cell death, which keratinocytes undergo in the stratum granulosum.

Keratohyalin Granules

Dark staining granules found in the stratum granulosum, characteristic of this layer.

Stratum Lucidum

A clear layer found only in thick skin (palms, soles); 4-6 layers of flattened dead keratinocytes.

Stratum Corneum

The outermost layer of the epidermis, consisting of up to 30 layers of dead, flattened keratinocytes filled with keratin, forming a durable protection against abrasion microbes, and water loss; cells continuously slough off and replaced from beneath. It serves as the primary barrier for the skin.

Papillary Layer

The superficial layer of the dermis, made of loose areolar connective tissue, containing collagen, elastic fibers, and a rich capillary network.

Dermal Papillae

Upward finger-like extensions of the dermis that project into the epidermis, increasing surface area for nutrient exchange and anchoring the layers.

• Some papillae contain tactile corpuscles (Meissner corpuscles) that are sensitive to light touch.

• Other dermal papillae contain free nerve endings that detect temperature, pain, and other stimuli.

Tactile Corpuscles (Meissner Corpuscles)

Nerve endings found in some dermal papillae that are sensitive to touch.

Free Nerve Endings

Nerve endings in dermal papillae that initiate signals for sensations like warmth, coolness, pain, tickling, and itching.

Epidermal Ridges

Patterns on the surfaces of palms and soles (fingerprints) created by the interaction of the stratum basale with dermal papillae, enhancing grip and tactile sensitivity. These ridges develop during fetal growth and are unique to each individual.

Reticular Layer

The thicker, deeper layer of the dermis, composed of dense irregular connective tissue rich in collagen fibers; contains the majority of the skin’s strength and elasticity via collagen and elastin fibers.

Striae (Stretch Marks)

Tears in the collagen fibers of the reticular layer due to overstretching of the skin, leading to scar tissue formation.

Fibroblasts

Cells of connective tissue found in the dermis that produce collagen, elastic, and reticular fibers.

Hair (Pili)

Accessory structures made of dead keratinized cells, serving roles in protection, sensory input, and thermoregulation.

Hair Shaft

The visible portion of the hair above the skin surface.

Hair Root

The portion of the hair embedded within the skin, extending deep into the dermis.

Hair Follicle

A tubular structure that dips into the dermis, houses the hair root from which hair grows.

Hair Bulb

An onion-shaped enlargement at the base of the hair follicle housing the hair matrix where new hair is produced.

Hair Matrix

A germinal layer of actively dividing cells within the hair bulb where new hair is produced.

Arrector Pili Muscle

A small band of smooth muscle attached to the hair follicle that contracts (e.g., when cold or scared) to pull hair upright, creating goosebumps in response to cold or fear.

Hair Root Plexus

Dendrites of sensory neurons surrounding each hair follicle, sensitive to touch. Enables sensation when hair is disturbed even though the hair shaft itself is dead tissure.

Sebaceous Glands (Oil Glands)

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

Sebum

An oily substance secreted by sebaceous glands that lubricates and waterproofs skin and hair, preventing dryness.

Sudoriferous Glands (Sweat Glands)

Glands that release sweat into hair follicles or onto the skin surface through pores.

Eccrine (Merocrine) Sweat Glands

The most numerous type of sweat gland, simple tubular glands releasing watery perspiration for body cooling and thermoregulation.

Apocrine Sweat Glands

Large coiled tubular glands found in specific areas (axilla, groin) that produce a thicker, milkier secretion containing water, electrolytes, lipids, and proteins, active during puberty and influenced by emotional/hormonal factors.

Ceruminous Glands

Specialized sweat glands in the external ear canal that produce cerumen (ear wax). They help protect the ear by trapping dust and debris, and possess antibacterial properties.

Nails

Accessory structures made of hard keratin that protect fingertips and toes, providing support and precision for gripping.

Nail Plate

The visible hard portion of the nail.

Free Edge

The part of the nail plate extending beyond the fingertip, appearing white due to the absence of capillaries.

Nail Root

Embedded part of the nail at the proximal end.

Nail Matrix

Located proximal to the nail root, this is the site of nail growth where cells undergo mitosis to produce new nail cells.

Lunula

The whitish crescent-shaped area at the base of the nail plate, marking the visible portion of the actively growing nail matrix.

Cells in the Epidermis

keratinocytes, melanocytes, Langerhans cells, and Merkel cells

Hair structures

shaft, root, follicle, bulb, matrix, medulla, cortex, cuticles, arrector pili, hair root plexus

Glands

sebaceous (sebum), eccrine (sweat), apocrine (sweat with pheromones), ceruminous (ear wax)

Nails

plate, root, matrix, lunula, free edge