Integument

Integument: Overview

  • Roles of Integument:

    • Protects from the environment

    • Assists in locomotion

    • Facilitates display

    • Covers all surfaces in contact with the environment:

    • Includes: eyeball (conjunctiva) and eardrum

    • Continuous with the digestive tract

Development of Integument

  • Layers:

    • Multilayered epidermis derived from ectoderm

    • Dermis derived from mesoderm (dermatome)

  • Gland Formation:

    • Glands develop from the epidermis

Structure of Integument

  • Epidermis Structure:

    • Stratified epithelium composed of columnar basal cells that are constantly in mitosis

    • Surface consists of squamous, keratinized cells (which can become hair and feathers)

  • Aquatic Species:

    • Fish and aquatic amphibians develop a mucus layer on the surface:

    • Minimizes water loss and is produced by epidermal glands

    • Evolution of skin types:

    • Skin of amphioxus has 1-layer epidermis

    • Skin of the red eft has a multi-layer epidermis

    • Dermis contains glands supplied by blood capillaries

Epidermis of Lamprey and Fish

  • Characteristics:

    • Abundant epidermal glands

    • Scales located beneath the epidermis

    • Granular gland cells secrete mucus and various chemicals (single-cell glands in fish)

    • Generally possess little or no keratin, with exceptions:

    • E.g., Lamprey features cornified epidermal "teeth"

Epidermis of Tetrapods

  • Layer Composition:

    • Many-layered epidermis allows adaptation to life on land

  • Types of Glands Present:

    • Alveolar sacs and tubular glands

    • Mucus glands

    • Granular glands

    • Sebaceous glands

    • Sudoriferous glands

    • Scent glands

    • Mammary glands

Tetrapod Gland Types

  • Mucus Glands:

    • Generally absent except in mucous membranes

  • Granular Glands:

    • Present in toads and reptiles

    • Absent in mammals and birds

    • Functions include: secretion of toxins and pheromones

    • Example: Parotoid gland in toads secretes a substance that hardens to create spines in lizards

  • Oil Glands:

    • Present in birds (few, with the uropygial gland being the largest, secretes oil for feathers)

    • Present in mammals through sebaceous glands that secrete sebum, accompanying hair follicles

  • Sudoriferous (Scent) Glands:

    • Facilitate cooling evaporation, particularly in mammals

    • Primarily located in the least furry parts of the body (e.g., ears)

    • Absent in marine mammals

  • Scent Glands:

    • Comprise specialized sebaceous and sudoriferous glands that produce pheromones

    • Basis for perfumes (e.g., musk oil)

    • Example: Elephant temporal gland active in males during mating season

Tetrapod Mammary Glands

  • Structure:

    • Described as compound alveolar glands

    • Evolved from sebaceous glands (oil, lipids)

  • Development Process:

    • Mammary epithelia invades the dermis to create alveoli

    • Hormonal changes at puberty lead to growth and branching of glands

    • The number of nipples correlates with the number of offspring

  • Default Structure:

    • Hair follicles typically serve as the default unless signaled to develop into nipples

  • Monotremes:

    • Lack mammary glands or nipples, instead secrete sweat milk

Epidermal Scales: Stratum Corneum

  • Distribution:

    • Found solely in amniotes (not comparable to fish scales), reptiles, mammals, and birds

  • Structural Characteristics:

    • Composed of overlapping folds, with thick keratin on exposed surfaces and thin at the folds

  • Molting Process:

    • Contains 2 layers present continuously; outer layer is shed

    • Features of specialized structures:

    • Scutes are considered thin scales (e.g., belly of snakes, turtle plastrons)

    • Modified stratum corneum evolved into claws, hooves, and nails

Specialized Shapes in Epidermal Scales

  • Includes variations in scales and feathers, contributing to structural diversity

  • Ruffling and Preening Mechanism in Feathers:

    • Barbules interlock with adjacent feathers, ruffling allows the unhooking, and preening rehooks the feathers

  • Dermal Papillae:

    • Present in feathers, hair, placoid scales, and teeth, act as a "small toolkit" during development

Development of Feathers

  • Initiation:

    • Dermal papilla: mesodermal cells indent the undersurface of the epidermis, prompting mitosis and creating feather primordium

  • Formation of Feather Follicle:

    • Follicle develops into a feather sheath

  • Growth Mechanism:

    • Characterized by a mitotically-active growth zone

    • Tall, columnar epidermal cells separate, cornify, and create barbs

  • Pin Feather:

    • A growing feather enveloped by its sheath

  • Sheath Is Split:

    • This leads to the feather fluffing out

Evolution of Feathers

  • Feathers likely did not evolve from scales

  • Characteristics of Emergence:

    • Evolved as tubular structures rather than flat

    • Considered an apomorphy within the reptile clade that led to birds

  • Forms of Structure:

    • Unbranched hollow filaments and cylindrical invagination into epidermis around the papilla

    • Central rachis established through complete fusion of filaments and formed barbs through serial fusion

Characteristics of Hair

  • Functions:

    • Insulation and mechanoreception

  • Development:

    • Hair grows from a follicle with continual mitosis originating from a bulb

    • Hair shaft composed of dense keratin, air vacuoles, and melanin granules

    • Covered by a cuticle of cornified scales

  • Muscle Interaction:

    • Arrector pili muscle contracts, elevating the hair

  • Origin and Growth:

    • Develops from epidermis ingrowth into the dermis (including dermal papilla)

    • Grows deeper to form a bulbous primordium, leading to cornification of cells at the site

Evolution of Hair

  • Linear hair growth pattern suggests an evolution from scales

  • Key Component:

    • Cystein-rich alpha keratin is a major hair constituent, also found in claws of reptiles (such as the Green anole lizard)

  • Inquiry Into Evolutionary Relation:

    • Did hair programming evolve from claw programming? (hypothesized sequence: claws to scales to hair)

Horns and Antlers

  • Bovine Horns and Pronghorns:

    • True horns in Bovidae and pronghorn antelopes feature a core of dermal bone covered in a keratinized horn sheath that is never shed

  • Hair Horns of Rhinoceroses:

    • Comprised of agglutinated keratinized hair-like epidermal fibers forming a solid horn atop the nasal bone

  • Antlers:

    • Composed of dermal bone attached to the frontal bone; covered in highly vascular "velvet"

    • Shed and replaced seasonally; involve a pedicle structure

Evolution of the Dermal Skeleton

  • Phenomena Observed:

    • Plates lost on the tail/trunk region

    • Plates have evolved into facial structures and components forming skull bones and the pectoral girdle

Dermal Plates and Scales

  • Types and Examples:

    • Rhomboid (includes ganoid and cosmoid)

    • Cosmoid Scales:

    • Comprised of dentin and enamel; found in now-extinct Paleozoic sarcopterygian fish

    • Elasmoid Scales:

    • Feature thin, laminar bone; found in teleosts, Amia, Latimeria, and dipnoans

    • Placoid Scales:

    • Composed of lamellar bone, dentin, and enameloid; anchored in the dermis

    • Observed in ancient and modern sharks, skate, and rays

    • Ganoid Scales:

    • Relate to enamel; found in Paleozoic actinopterygian fish and evolved into Elasmoid

    • Similar to modern gars

Modern Elasmoid Scales Types

  • Ctenoid Scales:

    • Characterized by a comb-like border

  • Cycloid Scales:

    • Possess a circular, smooth border

  • General Features:

    • Present in teleosts and other neoactinopterygians and modern sarcopterygians (including dipnoans and Latimeria)

    • Notably flexible and transparent with a thin layer of acellular lamellar bone overlaying dense collagen

    • Overlaid by a thin epidermis and possess an imbricate (overlapping) arrangement allowing for flexibility

Dermal Scales on Land

  • Inheritance from Ancestors:

    • Evolved from cosmoid scales seen in sarcopterygian ancestors

  • Osteoderms:

    • Considered bony scales found in tetrapods

    • Caecilians, crocodiles, and turtle shells are modern examples retaining osteoderms

Dermal Scales in Mammals

  • Armadillos:

    • Unique extant mammals that possess dermal armor

  • Humans:

    • Experience skin conditions pertaining to the integument

Dermal Pigments

  • Chromatophores:

    • Cells that contain pigment granules both in processes and cell bodies

  • Physiological Color Changes:

    • Dispersible pigments are typical (e.g., chameleons)

  • Types of Pigments:

    • Melanophores:

    • Contain melanin granules, typically black or brown

    • Xanthophores:

    • Contain yellow granules

    • Erythrophores:

    • Contain red granules

    • Iridophores:

    • Composed of prismatic guanine, leading to silvery or iridescent effects

Blue Coloration in Animals

  • Teleost Coloration:

    • Blue coloration results from a dispersion effect of black/brown melanin (structure, not pigment-based)

    • This effects structural coloration that contributes to the appearance of blue

Additional Symbolic Example

  • Beipiaosaurus:

    • An observed dinosaur exhibiting broad, single-filament feathers

The Dermis

  • Composition and Function:

    • Composed of collagenous connective tissue that provides structure and tensile strength to the skin

    • Houses: blood vessels, small nerves (including sensory nerves), pigment cells, lymphatics, bases of multicelled glands, as well as bases of hairs and feathers

    • Associated erector muscles

    • Exhibits ancient potentiality for bone formation (in bony fishes)

Ancient Potential for Bone Formation in Dermal Structures

  • Ostracodermii (500-360 mya):

    • Exhibited exoskeleton made of dermal bone combined with a cartilaginous endoskeleton

  • Placodermii (420-360 mya):

    • Demonstrated partially ossified vertebrae and braincases

  • Denticles:

    • Refer to knobby or spiny elevations on the surface of dermal armor

Composition of Ancient Dermal Bone

  • Layered Structure:

    • Four layers identified:

    • Lamellar bone

    • Spongy bone

    • Dentin

    • Enameloid/enamel

Evolution of Dermal Bone to Scales

  • Discussed transformations and adaptations leading to the development of scales from dermal bone structures