Integumentary System: Skin Structure and Layers

• The integumentary system, or skin, and its derivatives have two main regions: the epidermis and the dermis

Epidermis

• The outermost layer
• Composed of epithelium
• It is avascular
• Protects underlying layers

Dermis

• Composed of the underlying layers
• Vascular and innervated
• Provides strength and resilience
• Composed of connective tissue
• Contains smooth muscles associated with hair follicles and nerve fibers to detect and monitor sensory input

Hypodermis

• A subcutaneous layer
• Composed of areolar and adipose connective tissue
• Not technically considered part of the skin
• Anchors the skin to underlying structures
• Acts as a shock absorber and insulator

Epidermis layers

• Consists of keratinized stratified squamous epithelium
• The three innermost layers consist of living cells, while the two outermost layers are dead cells
• Progressing from inner to outermost, the layers go from Stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum

Stratum Basale

• Referred to as the basal layer
• Deepest layer
• Attached to a basement membrane that separates it from the dermis
• Epidermal ridges increase grip
• Consists of a single layer of cuboidal to low columnar cells
• Mostly composed of keratinocytes and a small population of melanocytes

Epidermis primary cells

• Keratinocytes are the most abundant
• Keratinocytes produce keratin (fibrous protein that forms intermediate filaments) that provides the protective properties of the skin
• They are tightly connected to one another by desmosomes
• Melanocytes have long, branching spiderlike processes
• They will synthesize and store melanin in response to UV light
• Melanin granules are taken up by keratinocytes and accumulate on the superficial side of the nucleus to protect the nucleus from UV light

Tactile Discs and Free Nerve Endings

• Tactile discs include Merkel cells (tactile cells), which are mechanoreceptors located in the stratum basale
• They have small receptive fields and are responsive to tactile stimulation to help determine the shape and texture of objects including fine touch and pressure
• Free nerve endings are responsible for pain; trigger tickling (light touch) and itching (noxious stimuli)

Stratum Spinosum

• Overlies the stratum basale
• Several layers thick
• Consists of keratinocytes that differentiate into non-dividing, specialized keratinocytes
• Cells connected by desmosomes result in spiny appearance

Epidermal Dendritic Cells

• Dry surface inhospitable to micro-organismal growth
• Epidermal dendritic (Langerhans') cells are found within the stratum spinosum and granulosum
• These are phagocytic cells capable of stimulating an immune response

Stratum Granulosum

• Three to five layers thick
• Keratinization occurs here
• Keratinocytes produce keratohyalin (precursor to keratin) granules
• The cells' nucleus and organelles begin to disintegrate, and the cells begin to die due to the lack of organelles and being further from the source of nutrients
• Keratinization won't be complete until after keratinocytes rise to the superficial layer

Stratum Lucidum

• A thin, clear layer of about two to three cell layers thick
• Protects against friction and found only in thick skin on palms of hands and soles of feet
• Cells become flattened and featureless
• Filled with eleidin (intermediate protein formed by keratohyalin) during keratin maturation, which is an intermediate protein and protects the skin from UV light

Stratum Corneum

• Most superficial layer
• Individual keratinocytes migrate from stratum basale to strata corneum and exist for about four weeks
• Consists of 15 to 30 layers of dead, scaly, interlocked keratinized cells that are anucleated and tightly packed together containing large amounts of keratin
• Durable and protective overcoat
• Thickened plasma membrane enhanced by special glycoproteins waterproofs the strata corneum, making it relatively insensitive to biological, chemical, and physical assault

Skin Thickness

• Thickness is determined by the number of layers in the epidermis
• Thick skin has 5 layers, including the stratum lucidum
• Found on the palms of hands and soles of feet, it contains sweat glands, but lacks hair follicles and sebaceous glands
• Thin skin has 4 layers since it lacks the stratum lucidum
• It covers most of the body and contains sweat glands, hair follicles, and sebaceous glands

Melanin

• Only pigment made in the skin
• Delivered to keratinocytes within melanosomes
• Has a variety of colors, including eumelanin (black-brown pigment) and pheomelanin (red-yellow pigment)
• Amount of melanin produced and retained gives varying skin colors
• Freckles and pigmented moles are local accumulations of melanin

Carotene

• A yellow/orange pigment found in certain plants
• Accumulates in the stratum corneum and fatty tissues of the hypodermis

Dermis Layers

• Cells found are typical of those found in connective tissue proper
• It is richly innervated and vascularized and possesses lymphatic vessels
• Composed of two layers: the papillary layer and the reticular layer

Papillary Layer

• Located near the superior surface
• Composed of areolar connective tissue and dermal papillae (small projections indenting into epidermis)
• Contains capillary loops
• Contains free nerve endings, which act as pain receptors and touch receptors
• Interdigits with epidermal ridges to increase area of contact/interlock the two layers

Reticular Layer

• It is a dense irregular connective tissue underlying the papillary layer
• Composed mostly of collagen fibers running parallel to the skin surface, creating cleavage lines
• Contains elastin providing elasticity
• Highly vascularized and innervated, in which the nerves distinguish different types of sensory stimuli
• Blood vessels supply nutrients for dermis and epidermis and play a role in temperature regulation
• Possesses flexure lines, which are dermal folds where dermis is tightly secured to deeper structures

Vitamin D

• Vitamin D3 is produced in the skin on exposure to UV light from a cholesterol precursor; may also be absorbed in diet
• Vitamin D3 is converted to its active form by two enzymatic reactions (one in liver then one in kidney)
• Vitamin D increases intestinal reabsorption of Ca2+ and PO43-

Other Integument structures

• Nails are scalelike modifications of the stratum corneum comprised of hard keratin that are used for protection and grasping
• Hair is more flexible strands of dead keratinized cells produced by the hair follicle that is stronger and more durable

Integumentary Glands

• Sudoriferous Glands (aka sweat glands) are scattered over the whole body excluding the nipples and genitals
• Includes Merocrine (Eccrine) sweat glands and Apocrine sweat glands

Explore the structure and layers of the integumentary system. Learn about the epidermis, dermis, and hypodermis, including their composition and functions. Discover the different layers of the epidermis, from the stratum basale to the stratum corneum.