Biology: Integumentary System

The dermis is made up of 2 layers:

(1) a superficial papillary layer and (2) a deeper reticular layer.

epidermal layers (superficial to deep)

1. stratum corneum 2. stratum lucidum* 3. stratum granulosum 4. stratum spinosum 5. stratum basale *the stratum lucidum is only present in the palms of the hands and the soles of the feet

the accessory structures are also known as _______

During embryonic development, these structures originate from the epidermis, so they are also known as epidermal derivatives. Although located in the dermis, they project to the surface of the skin.

accessory structures → glands

Exocrine glands are composed of specialized cells that create and secrete a product through a duct to an external surface, such as the skin or gastrointestinal tract (which can be considered an "external" tube from the mouth to the anus). There are several types of exocrine glands. The ones associated with the integumentary system are Sudoriferous (sweat) glands & Mammary glands

2 major parts of integumentary system

cutaneous membrane (skin) and accessory structures

dermal layer → reticular layer

deep to the papillary dermis, and it gets its name from its reticular fibers However, collagen and elastic fibers are also present within this dense irregular connective tissue → why the dermis is so strong and elastic blood vessels, sensory receptors, hair follicles, nail beds, and glands are contained within this fibrous network

What is responsible for friction ridges (fingerprints)?

dermal papillae projections in the papillary layer of the dermis

in deep dermis we find Meissner's corpuscles:

detect light touch and sensitivity

in deep dermis we find Thermoreceptors:

heat detection (warmth and cold)

epidermal layer → stratum corneum → insensible + sensible perspiration

insensible perspiration · water from interstitial fluids slowly diffuses to the surface and evaporates into the surrounding air · you are unable to see or feel (sense) the water loss sensible perspiration · produced by active sweat glands. · you are usually very aware of it

in deep dermis we find Ruffini corpuscles:

mechanoreceptors that detect stretching and pressure

in deep dermis we find Pacinian corpuscles:

mechanoreceptors that perceive touch, pressure, and vibrations

Hypodermis (subcutaneous layer)

not a part of the integument important in stabilizing position of skin in relation to underlying tissues, such as skeletal muscles or other organs, while permitting independent movement. consists of connective tissue which binds the skin to the underlying muscle contains larger nerves & blood vessels than those in dermis - the venous circulation of this region contains a a lot of blood, and most of it will shift to general circulation if these veins constrict - often describe the skin as a blood reservoir its main components are loose connective tissue and adipose (fat) tissue. In fact, its main function is fat storage. It helps conserve body heat, and it helps prevent injury by acting as a shock absorber.

in deep dermis we find Nociceptors:

painperception • Myelinated nerve endings that branch freely in dermis!

accessory structures → sebaceous glands

secrete oily and waxy products (sebum) into the hair follicle, which lubricates and waterproofs the hair and surrounding skin. They are located throughout the entire body surface with the exception of the palms of the hands and soles of the feet. Overactive sebaceous glands can contribute to acne.

accessory structures → sudoriferous (sweat) glands

sudoriferous (sweat) glands come in two types: eccrine and apocrine, which differ in their location and product Eccrine glands are the main sweat glands of the body. While they are located throughout the entire body surface, they are the most abundant in the axilla (armpit), palms, and soles of feet. Eccrine glands are important for thermoregulation (regulation of body temperature). Secretion from apocrine glands is more oily and viscous. Apocrine glands secrete their products into the hair follicle and are present in specific locations (armpit, nipple, ear canal, eyelid, nostril, and parts of the pubic area). Their function depends on their location and cell specialization. · Ceruminous glands are specialized apocrine glands. They produce ear wax (cerumen), which is secreted into the external auditory canal. · Mammary glands are also specialized apocrine glands. They produce milk, which is secreted to the nipple areola.

stratum spinosum (spiny or prickle layer)

· Provides skin with strength and flexibility bc of high prevalence of desmosomes holding keratinocytes together · keratinocytes have "spiny" appearance here bc desmosomes process them w/ chemicals that shrink the cytoplasm but left the cytoskeletal elements and desmosomes intact. · highest prevalence of Langerhans cells (dendritic cells of the epidermis) - integral to communication between innate and adaptive immune systems. Langerhans cells are present in all layers of the epidermis, but they are most abundant in the stratum spinosum.

epidermal layer → stratum corneum (horny layer)

· composed of dead, fully differentiated keratinocytes (corneocytes: flattened cells of keratin) · The dead cells in each layer of the stratum corneum remain tightly interconnected by desmosomes. · This layer provides protection against infection, dehydration, and physical harm. · water resistant, but not waterproof

epidermal layer → stratum lucidum* (clear layer)

· contains dead keratinocytes that are not fully differentiated into corneocytes · These cells are densely packed with eleiden, a clear protein rich in lipids, derived from keratohyalin, which gives these cells their transparent (i.e., lucid) appearance and provides a water barrier. *Remember, this layer is only present in the palms of the hands and the soles of the feet

Dermis

· derived from mesoderm · network of dense irregular connective tissue that functions to cushion the body from injury and provide a home for functional structures of the skin, such as vessels, glands, nails, hair.

2 components of the cutaneous membrane

· epidermis (superficial epithelium) · dermis (connective tissues)

integumentary system supporting structures are:

· extensive network of blood vessels branches through the dermis. · Nerve fiber endings and sensory receptors monitor touch, pressure, temperature, and pain, providing valuable information to the central nervous system about the state of the body

functions of integumentary system

· first line of defense against a hostile environment · physical protection from environmental hazards · thermoregulation · excretion · synthesis of vitamin D₃ · storage of lipids · insulation · sensory information · coordination of immune response to pathogens and skin cancer

the accessory structures are

· hair · hair follicles · exocrine glands · nails They are embedded in the dermis and project up to or above the surface of the epidermis.

accessory structure → hair

· hair = keratin protein filament that protects mammals from UV radiation and hypothermia · hair follicles generate hair and keep it attached to the body surface · each hair follicle opens onto epidermal surface but extends deep into dermis and usually into hypodermis · deep to epidermis, each follicle is wrapped in a dense connective tissue sheath. · a root hair plexus of sensory nerves surrounds the base of each hair follicle. · a bundle of smooth muscle cells forms the arrector pili muscle, also called arrector muscle of hair - from papillary layer to the connective tissue sheath surrounding the hair follicle. · when stimulated, the arrector pili muscle contracts, pulling on the follicle and forcing the hair to stand erect. This results in "goosebumps," and in some mammals it plays a role in temperature regulation and scaring off predators.

epidermal cells

· keratinocytes - produce keratin · melanocytes - produce melanin · langerhans cells - dendritic, immune cells · merkel cells - tactile, sensory cells

integumentary system → cutaneous membrane → epidermis

· most superficial layer of the skin · derived from ectoderm · "above the dermis" · thin and avascular · relies on diffusion of O₂ and nutrients from neighboring dermis · composed of stratified squamous cells · 4-5 layers of cells - mostly keratinocytes

epidermal layer → stratum granulosum (grainy layer)

· mostly keratinocytes that have migrated from stratum spinosum. · secrete lamellar bodies which form a lipid-containing, hydrophobic membrane that helps create the skin's water barrier. · keratinocytes make a protein, keratohyalin, that forms dense cytoplasmic granules that promote dehydration of cell and aggregation & cross-linking of keratin fibers. The nuclei and other organelles disintegrate, and the cells die. · most cells stop dividing and make large amts of keratin: as keratin fibers accumulate, cells grow thinner and flatter & their plasma membranes thicken and become less permeable

dermal layer → papillary layer

· of areolar tissue · contains capillaries, lymphatic vessels, and sensory · nerve fibers that supply skin surface (epidermis) gets its name from the dermal papillae that project between the epidermal ridges · these projections create our fingerprints (friction ridges), and increase the surface area between the dermis and epidermis · this increases capacity for delivery of oxygen and nutrients to the epidermis, which relies on the blood vessels of the papillary dermis for nourishment and waste removal · its high surface area also increases the strength of the junction between the dermis and epidermis

epidermal layer → stratum basale or stratum germinativum

· provides continuous renewal of epidermal keratinocytes, and houses other specialized cell types, like Merkel cells and melanocytes. · dominated by basal cells (germinative cells) - stem cells that divide to replace superficial keratinocytes shed at the epithelial surface aka Keratinocyte stem cells · melanocytes - generates melanin pigment that's responsible for skin's pigmentation & protection from UV radiation · Merkel cells (tactile cells) are specialized mechanoreceptor cells - respond to light touch sensations so it's extremely abundant in the fingertips · each Merkel cell synapses with a somatosensory (somato = body) afferent nerve fiber, which conveys the message to CNS · Hemidesmosomes attach the cells of this layer to the basement membrane that separates the epidermis from the areolar tissue of the dermis · forms epidermal ridges, which extend into the dermis and are adjacent to dermal projections called dermal papillae (singular, papilla; a nipple-shaped mound) that extend into the epidermis

the third layer deep to the dermis is

· subcutaneous layer (hypodermis) · loose connective tissue