Integumentary System

Integumentary System

• Skin is the largest organ of the body.
• It takes up 15-20% of total body mass, covering 1.2-2.3 square meters of body surface.

Functions of Skin

• Protection from physical, chemical, and biological threats.
• Immune response initiation and regulation.
• Sensation through sensory receptors.
• Temperature regulation through sweating and blood vessel dilation/constriction.
• Storage of nutrients and synthesis of vitamin D.
• Excretion of waste products through sweat.
• Absorption of certain substances through pores.

Layers of the Skin

• Epidermis: Most superficial layer of the skin, stratified squamous keratinized epithelium.
• Dermis: Thick layer of connective tissue (CT) located beneath the epidermis, composed of papillary and reticular layers.
• Hypodermis: Deepest layer of the skin, mainly composed of adipose tissue, providing insulation and cushioning.

Epidermis

• Stratum Basale (innermost layer):
  • Single row of columnar/cuboidal keratinocytes connected to the basement membrane with hemidesmosomes.
  • Houses melanocytes, Merkel's cells, and Langerhans' cells.
  • Responsible for the development of new keratinocytes and skin renewal.
• Stratum Spinosum:
  • Several layers of polyhedric keratinocytes, interconnected by desmosomes.
  • Responsible for the synthesis of keratin, forming tonofibrils.
• Stratum Granulosum:
  • Flattened keratinocytes filled with keratohyalin and membrane-coating granules (lamellar bodies).
  • Lamellar bodies release lipids to form the epidermal water barrier.
• Stratum Lucidum:
  • Found only in thick skin, containing flattened dead keratinocytes.
  • Keratohyalin transforms into eleidin.
• Stratum Corneum (horny layer):
  • Outermost layer of dead, keratin-filled cells.
  • Primary barrier against physical, chemical, and mechanical damage.
  • Helps regulate water loss, mineral loss, and protein loss.

Skin Color

• Melanin: Pigment within melanosomes, responsible for skin color.
  • Eumelanin: Usual type of melanin, resulting in brown/black pigmentation.
  • Pheomelanin: Produces blonde and red hair color.
• Tanning: Increased melanin production and transfer of melanosomes to keratinocytes in response to UV radiation.

Nonkeratinocytes

• Langerhans' cells: Star-shaped cells, mainly found in the stratum spinosum.
  • Immune cells that present antigens to T lymphocytes.
  • Contain Birbeck's granules (tennis racket-shaped structures).
• Merkel's cells: Located in the stratum basale.
  • Responsible for fine touch sensation.
  • Connected to nerve fibers and contain neurosecretory granules.

Dermis

• Papillary layer: Superficial layer of the dermis, contains finger-like projections called dermal papillae, increasing surface area for nutrients and waste exchange with the epidermis.
• Reticular layer: Deepest layer of the dermis, responsible for skin's strength and elasticity.

Sweat Glands

• Eccrine sweat glands: Primarily responsible for thermoregulation.
  • Secretory portion contains dark cells, clear cells, and myoepithelial cells.
  • Regulated by acetylcholine (cholinergic).
• Apocrine sweat glands: Located predominantly in the axilla, areola, perianal, and genital areas.
  • Release their product by shedding a portion of cytoplasm (apocrine snouting).
  • Produce pheromones and are regulated by adrenaline (adrenergic).

Sebaceous Glands

• Usually associated with hair follicles, forming the pilosebaceous unit.
• Secrete sebum (oil) to lubricate the skin and hair.
• Holocrine secretion: Secretory cells undergo complete destruction to release sebum.

Changes In Skin With Age, Sun Exposure, Smoking, and Alcohol

• Age: Thinning of epidermis, reduced collagen production, and decreased elastin leading to wrinkles.
• Sun exposure: Increased risk of skin cancer, premature aging, and hyperpigmentation/sunspots.
• Smoking: Premature aging, wrinkles, and increased risk of skin cancer.
• Alcohol: Dehydration, leading to loss of skin elasticity and increased sensitivity.

Layers of the Epidermis

• The epidermis has 5 layers, from deepest to most superficial, which are: Stratum Basale, Stratum Spinosum, Stratum Granulosum, Stratum Lucidum, and Stratum Corneum.
• The Stratum Basale (germinal layer) is composed of a single row of columnar or cuboidal keratinocytes connected to the basement membrane by hemidesmosomes.
• The Stratum Basale also contains melanocytes, Merkel's cells, and dendritic (Langerhans’) cells.
• The Stratum Spinosum is made up of several layers of polyhedric keratinocytes with sharp protrusions called prickle cells.
• The Stratum Spinosum is where keratinocytes synthesize cytoplasmic intermediate filaments called cytokeratins, which accumulate in aggregates called tonofibrils.
• Keratinocytes in the Stratum Granulosum begin to fill with keratohyalin, a substance made of filaggrin and trichohyalin, and have membrane-coating granules.
• The Stratum Lucidum is present only in thick skin and is characterized by a thin layer of flattened dead keratinocytes.
• The Stratum Corneum (horny layer) is comprised of multiple layers of flattened keratinized dead keratinocytes without nuclei or cellular organelles.
• The Stratum Corneum acts as a barrier against physical, chemical, and mechanical damage and helps retain water, minerals, and proteins.
• The Stratum Disjunctum is a layer consisting of desquamating residues of dead keratinocytes present in thick skin.

Cells of the Epidermis

• Cells of the epidermis can be broadly categorized into keratinocytes and nonkeratinocytes.
• Keratinocytes are the most common cell type and originate in the basal epidermal layer.
• Melanocytes are pigment-producing cells derived from the neural crest and produce melanin, a dark brown pigment.
• Melanin is produced and stored in melanosomes, which are then transferred to surrounding keratinocytes.
• Melanin accumulation in the cytoplasm over the nucleus shields the DNA of multiplying cells from UV damage.

Dermal Layers

• The dermis is subdivided into two layers: the papillary dermis and the reticular layer.
• The Papillary Dermis is the superficial layer composed of vascular loose collagenous connective tissue featuring dermal papillae.
• Dermal papillae protrude against the epidermal ridges, increasing the contact area with the epidermis, strengthening the dermoepidermal connection, and facilitating nourishment of the dermis.
• The Reticular Layer is the deeper, thicker layer of the dermis, composed of irregular dense connective tissue containing thick, irregular bundles of collagen and elastic fibers.
• The elastic network in the reticular layer contributes to the skin's elasticity.

Langer's Lines

• Collagen and elastic fibers in the skin form lines of tension called Langer's lines.
• Skin incisions are usually made parallel to Langer's lines to minimize scarring.

Hypodermis

• The hypodermis (subcutaneous tissue) is a layer of loose connective tissue and adipose tissue.
• The thickness of the hypodermis varies among individuals due to the amount of adipose tissue.

Dermal Appendages

• Dermal appendages include sweat glands, sebaceous glands, and hair follicles.
• Sweat glands are responsible for sweating, playing a crucial role in thermoregulation.
• There are two types of sweat glands: eccrine and apocrine.

Eccrine Sweat Glands

• Eccrine glands are regulated by cholinergic pathways and secrete sweat for thermoregulation.
• Eccrine sweat glands contain a secretory portion with three cell types: dark cells, clear cells, and myoepithelial (basket) cells.
• Eccrine gland ducts have a double layer of cuboidal epithelium.

Apocrine Sweat Glands

• Apocrine sweat glands are also merocrine glands and release their product by shedding part of their cytoplasm.
• Apocrine glands are larger and empty onto the superficial regions of hair follicles.
• Influenced by hormones, apocrine glands are found in areas like the axilla, areola, perianal region, and genital area.
• Apocrine glands secrete pheromones and are regulated by adrenergic pathways.

Sebaceous Glands

• Sebaceous glands are typically associated with hair follicles and are simple branched acinar glands.
• They secrete sebum into hair follicles through holocrine secretion, where cells accumulate lipids and undergo degeneration.
• Sebaceous glands are essential for lubricating the skin and hair.
• Free sebaceous glands are also present in areas like the eyelid, external acoustic meatus, and around the anus.

Keratinization

• Desquamatization is the process of shedding dead keratinocytes from the surface of the skin.
• The epidermis undergoes vertical maturation from undifferentiated basal cells to fully differentiated corneocytes in the Stratum Corneum.
• Human skin renews itself in about 47 days, with new keratinocytes generated in the germinal layer.
• Keratinization involves the breakdown of the nucleus and organelles and the thickening of the plasma membrane.
• This process provides the skin with its protective barrier and helps to regulate water loss.

Psoriasis

• Psoriasis is an inflammatory skin condition characterized by a rapid turnover of keratinocytes, resulting in a shorter maturation period (about 8–10 days).

This quiz covers the crucial aspects of the integumentary system, focusing on the skin and its various functions. It includes information on the skin's layers, specific functions, and the overall importance of skin in protecting the body. Test your knowledge on this essential organ!