BIO 2030 - Integumentary System

Integumentary system

The boundary between the external environment and internal structures

Consists of 3 major regions: Epidermis, dermis, and hypodermis

Cells of the epidermis

• Keratinocytes

• Melanocytes

• Langerhans cells

• Merkel cells

Keratinocytes

Cells that produce fibrous protein keratin (within themselves in their cytoplasm)

Superficial cells are dead, the more deep ones are alive and are stem cells that constantly divide

Melanocytes

Cells that produce the brown pigment melanin, which is then taken internally by keratinocytes

Langerhans cells

Epidermal macrophages that help activate the immune system as the 1st line of immune defense

Merkel cells

Cells that function as touch receptors in association with sensory nerve endings

Layers of the epidermis (deep to superficial)

• Stratum basale

• Stratum spinosum

• Stratum granulosum

• Stratum lucidum

• Stratum corneum

Stratum basale

The deepest epidermal layer, consisting of a single row of the youngest keratinocytes and is firmly attached to the dermis

Its cells undergo rapid division, hence its alternate name stratum germinativum

Stratum spinosum

The epidermal layer where melanin granules and Langerhans’ cells are abundant, as well as the cells in this layer contain a web-like system of intermediate filaments attached to desmosomes

Stratum granulosum

The epidermal layer consisting of 3-5 cell layers in which drastic changes in keratinocyte appearance occurs, as well as keratohyaline and lamellated granules accumulate in the cells of this layer

Stratum lucidum

The epidermal layer that’s thin and translucent, found in only thick skin areas (ex. palms of the hand, soles of the feet), containing 3-5 layers of dead keratinocytes

Stratum corneum

The outermost epidermal layer consisting of keratinized cells, accounting for 3/4 of the epidermal thickness

Functions in waterproofing, protection from abrasion and penetration, making the body relatively insensitive to biological, chemical, and physical assaults

Dermis

The 2nd major skin region containing strong, flexible connective tissue, including cells such as fibroblasts, macrophages, and occasionally mast cells and WBCs

Composed of 2 layers: Papillary and reticular

Papillary layer

The layer of the dermis that consists of areolar connective tissue with collagen and elastic fibers, along with peg-like projections dermal papillae, also connects with the epidermis basement membrane

Reticular layer

The layer of the dermis that accounts for ~80% of the thickness of the skin, consisting of collagen fibers that add strength and resilience to the skin and elastin fibers to provide stretch-recoil properties

Dermal papillae

Peg-like projections found in the papillary layer of the dermis, containing capillary loops, Meissner’s corpuscles, and free nerve endings

Meissner’s corpuscles

Mechanoreceptors for light touch

Hypodermis

The 3rd major skin region, subcutaneous (“under the skin”), consisting of adipose and areolar connective tissue

Skin color and the 3 main pigments

Due to the density of the pigments

• Melanin

• Carotene

• Hemoglobin

Melanin

Yellow to reddish-brown to black pigment, responsible for dark skin colors

Local accumulations of this pigment result in freckles and pigmented moles

Carotene

Yellow to orange pigment, most obvious in the palms and soles of the feet (stratum corneum and thick skin)

Hemoglobin

Reddish pigment responsible for the pinkish hue of the skin

Types of sweat glands

• Eccrine

• Apocrine

• Ceruminous

• Mammary

Eccrine sweat glands

Sweat glands that are the most common on the body, mainly secreting water and salt, expels oil directly onto the skin and all over the body

Found in the palms, soles of the feet, and forehead

Apocrine sweat glands

Sweat glands developed around puberty that are responsible for the “funky” smell (due to bacteria metabolizing the glands’ release of lipids), expels into hair follicles

Found in axillary, anogenital areas, and around nipples

Ceruminous glands

Modified apocrine glands in the external ear canal that secrete cerumen (ear wax)

Mammary glands

Specialized sweat glands that secrete milk

Sebaceous glands

Simple alveolar glands that are found all over the body, secreting sebum (oil) to soften skin when stimulated by hormones

Hair

Filamentous strands of dead keratinized cells (which are more durable than the soft keratin of the skin) produced by hair follicles, consisting of the medulla (core), cortex, and outermost cuticle

Made up of the shaft projecting from the skin and the root embedded in the skin (dermis), and pigmented by melanocytes at the base of the hair

Distributed over the entire skin surface except palms, soles, lips, nipples, and portions of the external genitalia

Hair functions

• Help maintain warmth

• Alerting the body to presence of insects on the skin

• Guarding the scalp against physical trauma, heat loss, and sunlight

Hair follicle

The root sheath extending from the epidermal surface into the dermis, with the deep end expanded to form the hair bulb, which contains a root hair plexus (a knot of sensory nerve endings) that wrap around each hair bulb

Bending a hair stimulates these endings (explaining how hair acts as sensitive touch receptors)

Goosebumps

Sensation created by arrector pili muscles contracting, causing the hair to stand upright, and the skin bunching up

Hair papilla

A small, cone-shaped structure located at the base of the hair follicle, made up of specialized connective tissue cells an a dense network of capillaries (tiny blood vessels)

2 types of hair

• Vellus

• Terminal

Vellus hair

Pale, fine body hair found in children and adult females

Terminal hair

Coarse, long hair found in the eyebrows, scalp, axillary, and pubic regions

Alopecia

Hair thinning, found in both sexes

True/frank baldness

The genetically determined and sex-influenced hair condition, caused by follicular response to DHT (dihydrotestosterone) and results in male pattern baldness

Autoimmune diseases related to baldness

• Alopecia areata (“circle”)

• Alopecia totalis (“entire region”)

• Alopecia universalis ("entire body”)

Fetal developmental aspects of the integument

The epidermis develops from ectoderm, and the dermis and hypodermis develop from mesoderm

Lanugo

The downy coat of delicate hairs covering the fetus, which is covered by vernix caseosa (a wax that protects the fetus skin from amniotic fluid)

Vernix caseosa

The substance produced by sebaceous glands that protects the skin of the fetus in the amnion; The waterproof protective coating formed from dead skin cells and oily sebum