Integumentary System: Skin Structure and Function

• The integumentary system is an organ system comprising the skin and associated structures like glands, hair, nails, scales, hooves, and feathers.

Functions of the Integumentary System

• Protection and support
• Sensory reception
• Respiratory exchange
• Thermoregulation
• Locomotion
• Water balance
• Protective coloring behavior
• Vitamin D synthesis

Structure of Skin

• Epidermis: Outer, avascular covering made of keratinized stratified squamous epithelium derived from the ectoderm that produces the basal lamina.
• The Epidermis provides waterproofing, serves as a barrier to infection, and produces structures like hair, feathers, baleen, claws, nails, horns, beaks, and some scales.

Principal Cells of the Epidermis

• Keratinocytes: Produce keratin, protecting against heat, microbes, and chemicals.
• Melanocytes: Produce melanin for skin color and UV light absorption.
• Langerhans Cells: Derived from bone marrow; involved in immune response.
• Merkel Cells: Contact tactile (Merkel) discs for touch sensation.
• Thin Skin: Covers most of the body and has 4 layers of keratinocytes.
• Thick Skin: Covers palms and soles and has 5 layers of keratinocytes.

Structure of the Epidermis (Layers)

• Stratum Germinativum/Basale: Innermost layer, actively dividing cells, contains melanin.
• Stratum Spinosum: Superficial to stratum basale.
• Stratum Granulosum: Keratinocyte cytoplasm appears granular.
• Stratum Lucidum: Found in palms and soles; dead keratinocytes.
• Stratum Corneum: Outermost layer; mostly dead cells filled with keratin.

Keratin

• Keratinization: Forms keratin, a tough, insoluble protein substance.
• Tetrapod Feature: Keratin is the chief structural constituent of hair, nails, horns, and hooves.
• Keratinocytes: Specific epidermal cells participating in keratinization.

Epidermis in Sauropsids

• Two types of keratinocytes are produced: one with alpha (soft) keratin and one with beta (hard) keratin.
• Alpha Keratin (soft): Present in flexible epidermal layers for shape changes.
• Beta Keratin (hard): Common in specializations like hard scales, claws, beaks, and feathers.
• Synapsids: Only alpha-keratins are present.
• Photophores: Light-producing organs in marine fishes emit light from specialized structures or symbiotic luminescent bacteria.

Epidermal Derivatives - Keratin Structures

• Epidermal Scales: Continuous layer of repetitious stratum corneum thickenings.
• Claws and Talons: Curved, laterally compressed keratinized projections from digit tips.
• Hooves
• Nails
• Horns
• Baleen
• Beaks
• Feathers
• Hair

Dermis

• Deepest layer made of dense connective tissue proper (irregular).
• Derived from the mesoderm and produces the reticular lamina.
• Location for skin appendages.
• Collagen and elastic fibers are arranged in a diffusedly bundled and woven pattern.
• Dermal bones and osteoderms found in reptiles.

Two Layers of the Dermis

• Papillary Layer: Arranged into microscopic ridges, forming fingerprints.
• Reticular Layer: Beneath the papillary layer; white fibrous tissue supporting blood vessels.

Hypodermis

• Transitional subcutaneous region between the integument and deep body musculature, made of loose connective and adipose tissues.
• Referred to as superficial fascia in gross anatomical dissection.
• Subcutaneous Tissue: Loose connective tissue proper and adipose tissues.
• Supports, nourishes, insulates, and cushions the skin.
• Also called the basement membrane

Integument of Fishes

• Structure: Epidermis is avascular and contains mucous cells; the dermis contains blood vessels, nerves, sense organs, connective tissue, and pigment cells.
• Function: Osmotic balance, physical protection, site of coloration, respiration (for some).
• Location: Skin
• Similarity: Both provide immense protection to the body.
• Difference: Human skin does not help with respiration, but fish skin can.

Skin of Jawless Fishes

• Relatively thick skin.
• Epidermal glandular cells secrete a protective cuticle.
• Multicellular slime glands secrete large amounts of mucous for protection.

Skin of Cartilaginous Fishes

• Multilayered with mucus and sensory cells.
• The dermis contains bones in the form of placoid scales called denticles.
• Denticles contain blood vessels and nerves, similar to vertebrate teeth.

Skin of Bony Fishes

• They contain scales.
• A thin layer of dermal tissue overlies the superficial epidermis.
• Scales grow at the margins and over the lower surface because they are not shed.
• Their skin is permeable and used in gas exchange.
• Mucus helps fight off bacterial and fungal infections.

External Anatomy

• Ostracoderms and Placoderms: Had dermal scales for armor.
• Hagfishes and Lampreys: Smooth and without dermal scales.
• Hagfishes: Skin contains thread cells.
• Most fishes are covered with scales.

Function of Scales

• Protection
• Reflecting light
• Hydrodynamics

Types of Scales

• Placoid Scale: Found only in cartilaginous fish (Chondrichthyes); ancestor to teeth; denticle points decrease friction while swimming.
• Ganoid Scale: Found only on some bony fishes, including gars, paddlefishes, sturgeons, bowfin, and bichirs.
• Cycloid Scale: Most common on fish with soft fin rays, such as salmon, trout, and carp.
• Ctenoid Scale: Found on fishes with spiny fin rays, such as perch-like fishes.
• Cosmoid Scale: Rare and found only on ancient lobe-finned fishes, including some of the earliest lungfishes.

Modified Scales

• Lateral Line: Present on most fishes.
• External Spines: On porcupine fishes.
• Fused and modified placoid scales: Dorsal fin spines of dogfish sharks and chimaeras, stinging tail spines of stingrays, and "saw" teeth of sawsharks

Fish Colors

• Many bony fishes show brilliant colors.
• Caused by chromatophores (in the dermis) and iridocytes or guanophores.
• Chromatophores: Contain pigments for color production and variation.
• Iridocytes or Guanophores: Reflecting cells containing guanine crystals but no pigment.

Integument of Amphibians

• Structure: Multilayered epidermis; stratum corneum has flattened, highly keratinized cells and is cast off in fragments or as a whole.
• Two-layered dermis: Outer and looser stratum spongiosum and inner stratum compactum.
• Function: Vital respiratory organ, supplied with blood vessels and lymph spaces
• Location: Skin
• Similarity: Both systems act as a blood reservoir
• Difference: Amphibians use skin as a vital respiratory organ.
• Amphibians are transitional between aquatic and terrestrial vertebrates.
• Consists of stratified epidermis and a dermis containing mucus and serous glands plus pigmentation cells.
• The earliest amphibians were covered by dermal bone scales.

Integument of Reptiles

• Structure: Beta keratin and pliable alpha keratin.
• Function: Protection, shedding and growing of new scales, heat exchange.
• Location: Skin
• Their skin reflects their greater commitment to a terrestrial existence.
• The outer layer of the epidermis (stratum corneum) is thick, lacks glands, and is modified into keratinized scales, scutes, plaques, and beaks.
• The thick keratinized layer resists abrasion, inhibits dehydration, and protects like a suit of armor.

Integument of Birds

• Structure: Epidermis is thin, delicate, and clothed in feathers; thin epidermis is mostly a network of connective tissue fibers and muscle fibers.
• Function: Barrier, thermal insulation, sensation, pigments for protection, converts sunlight into vitamin D.
• Location: Skin, feathers, claws, beak
• Similarity: Both provide protection for the body.
• Difference: Humans do not have feathers.
• Over most of the birds’ bodies, the epidermis is usually thin and only two or three layers thick.
• The most prominent part of the epidermis are the feathers (collectively known as plumage).
• Feathers are derived from the scales of reptilian ancestors and are most complex of all derivatives of the vertebrate stratum corneum.
• Feathers provides the strong yet lightweight surface area needed for powered, aerodynamic flight.
• They also serve as insulation, trapping pockets of air to help birds conserve their body heat.
• The varied patterns, colors, textures, and shapes of feathers help birds to signal their age, sex, social status, and species identity to one another.
• Some birds have plumage that blends in with their surroundings to provide camouflage, helping these birds escape notice by their predators.

Integument of Mammals

• Structure: Notably hairy; a variety of epidermal glands than in other vertebrate class; a highly stratified cornified epidermis; a dermis many times thicker than the epidermis.
• Location: Skin, nails, hair, glands, nerves
• Function: It forms a barrier that helps prevent harmful microorganisms and chemicals from entering the body.
• It also prevents the loss of life-sustaining body fluids.
• It protects the vital structures inside the body from injury and potentially damaging ultraviolet rays of the sun.
• Function: The skin also helps regulate body temperature, excretes some waste products, and is an important sensory organ.
• It contains various types of specialized nerve cells responsible for the sense of touch.
• Glands: Sudoriferous Glands (Sweat Glands) and Sebaceous Glands (Oil Glands)
• Appendages: Hair (arrector pili) and Horny derivatives