Integument In Vertebrates – Structure, Functions and Derivatives.

What is Integument?

The integument refers to the outermost covering of the body in animals, which includes the skin and its associated structures. The skin is the largest organ of the body and serves as a protective barrier between the internal organs and the external environment.

In addition to the skin, the integument includes other specialized structures such as the conjunctiva of the eyeballs and the external surface of the eardrums. These structures are essential for protecting the delicate tissues of the eyes and ears.

Furthermore, the integument is continuous with the mucous epithelial lining of various body openings, such as the mouth, rectum, nostrils, eyelids, and urinogenital duct. This continuity allows for a seamless transition between the external and internal surfaces of these openings and ensures their protection and proper functioning.

The primary function of the integument is to provide a physical barrier that protects the body from injury, pathogens, and harmful environmental factors. It also helps in regulating body temperature, preventing water loss, and providing sensory information about the external environment.

The skin itself is composed of multiple layers, including the epidermis, dermis, and subcutaneous tissue. The epidermis is the outermost layer and consists of several layers of cells that constantly renew and provide protection. The dermis is located beneath the epidermis and contains blood vessels, nerves, hair follicles, sweat glands, and sebaceous glands. The subcutaneous tissue, also known as the hypodermis, is the deepest layer and consists of fat cells that provide insulation and cushioning.

Overall, the integument is a vital part of the body that not only acts as a protective barrier but also performs various other functions essential for maintaining the health and well-being of animals.

Structure of integument in vertebrates

The skin of all vertebrates is multicellular and having 2 layers.

1. An outer epidermis developed from ectoderm,
2. An inner dermis derived from the mesoderm.

1. Epidermis

The epidermis is a stratified epithelium that forms the outermost layer of the skin. It consists of two main regions:

• Stratum Corneum: The outermost region of the epidermis is composed of many layers of dead, flattened, and squamous cells. These cells form a tough, resistant covering known as the stratum corneum, which is visible on the surface of the skin. The cells in this layer accumulate a protein called keratin, which provides strength and durability. Over time, these cells gradually die and are sloughed off in the form of scurf or dandruff. The keratinized stratum corneum plays a crucial role in protecting the underlying tissues from mechanical injuries, fungal and bacterial attacks, and excessive loss of body moisture. The presence of keratin makes this layer tough and insoluble in water, enhancing its protective function.
• Stratum Germinativum (Malpighian layer): The innermost region of the epidermis is known as the stratum germinativum or Malpighian layer. It consists of a single layer of living columnar cells that are in contact with the basement membrane. These cells are actively dividing and continually replace the worn-out cells of the stratum corneum. The Malpighian layer is responsible for the regeneration of the epidermis and plays a vital role in the growth and maintenance of the skin. It is from this layer that various structures such as scales, feathers, hairs, nails, claws, horns, and enamel of teeth are derived in land vertebrates. The thickness of the epidermis varies among different vertebrates, with aquatic vertebrates having a thinner epidermis rich in mucous glands, while land vertebrates have a thicker epidermis.

In summary, the epidermis is a stratified epithelium consisting of two distinct regions: the outermost stratum corneum, which provides protection against mechanical injuries and moisture loss, and the innermost stratum germinativum or Malpighian layer, responsible for cell regeneration and the formation of various structures found in land vertebrates.

2. Dermis or corium

• The dermis, also known as the corium, is the inner layer of the skin located beneath the epidermis. It is primarily composed of fibrous connective tissue and contains various other structures that contribute to the overall function of the skin.
• The dermis is a highly vascularized layer, meaning it contains numerous blood capillaries that supply nutrients and oxygen to the skin cells. It also contains lymph vessels, which are responsible for draining excess fluid and waste products from the skin tissue.
• Within the dermis, there are muscle fibers that are associated with hair follicles. These muscles, known as arrector pili muscles, contract in response to stimuli such as cold or fear, causing the hairs to stand up and creating “goosebumps.”
• Nerve fibers are also present in the dermis, providing sensory information to the brain and enabling the perception of touch, pressure, temperature, and pain. Specialized sense organs, such as Meissner’s corpuscles for touch and Pacinian corpuscles for pressure, are embedded within the dermis, allowing for a heightened sense of touch in certain areas of the body.
• Elastic fibers are another important component of the dermis. They provide the skin with elasticity, allowing it to stretch and recoil, while maintaining its original shape. This elasticity is essential for the skin to accommodate movements and changes in body size without tearing or losing its integrity.
• Melanin pigments and pigment granules are also found within the dermis. Melanocytes, specialized cells in the epidermis, produce melanin, which is responsible for skin and hair color. The distribution and concentration of melanin in the dermis contribute to variations in skin color among individuals.
• Furthermore, the dermis contains adipocytes, which are fat-storing cells. These cells can accumulate fat as a reserve energy source, and their presence in the deeper parts of the dermis and subcutaneous tissue contributes to insulation and protection of underlying structures.
• In summary, the dermis is the inner layer of the skin composed of fibrous connective tissue. It houses blood vessels, lymph vessels, muscle fibers, nerve fibers, sense organs, elastic fibers, melanin pigments, pigment granules, and adipocytes. These components work together to provide structural support, elasticity, sensory perception, thermoregulation, and other essential functions of the skin.

Derivatives of integument in vertebrates

In vertebrates, the integument gives rise to various derivatives that serve specific functions. These derivatives can be classified into two categories: epidermal derivatives and dermal derivatives.

A. Epidermal Derivatives:

1. Epidermal Glands: These are glands derived from the epidermis and serve various purposes. Examples include:
   • Mucous glands in fish and amphibians that secrete mucus to keep the skin moist and slippery.
   • Sebaceous glands in mammals that produce oily secretions (sebum) to lubricate the skin and hair.
   • Mammary glands in mammals that produce milk for nourishing the young.
   • Scent glands that secrete odorous substances to attract mates or mark territories.
2. Hard Horny Structures: These structures are derived from the epidermis and form the outer protective layer or exoskeleton of the animal. Examples include:
   • Epidermal scales found in reptiles and fish, which provide protection and aid in locomotion.
   • Cuticles found in arthropods, which serve as a tough outer covering.
   • Beaks, horns, claws, nails, hooves, feathers, and hairs found in various vertebrates. These structures have diverse functions such as defense, prey capture, insulation, and flight.

B. Dermal Derivatives: Dermal derivatives are derived from the dermis, the inner layer of the skin. They include:

1. Bony or Dermal Scales: Found in certain fish and reptiles, these scales are composed of bony tissue and provide protection and support.
2. Plates or Scutes: These are thickened, often bony, structures found in reptiles such as turtles and crocodiles. They provide armor-like protection and contribute to the animal’s overall body structure.
3. Fin Rays: Present in fish, fin rays are thin, elongated structures that support and control the movement of fins, enabling swimming and maneuverability in water.
4. Antlers: Antlers are bony structures found in certain species of deer, such as moose and reindeer. They are unique to males and are shed and regrown annually. Antlers serve primarily as weapons for male-male competition during mating season.

These derivatives of the integument play crucial roles in locomotion, protection, thermoregulation, sensory perception, and species-specific behaviors. They are specialized structures that have evolved to meet the specific needs of different vertebrate groups.

A. Epidermal derivatives

1. Epidermal glands

Epidermal glands are specialized structures found in the epidermis, the outermost layer of the skin. These glands can be unicellular or multicellular and serve various functions. Here are the different types of epidermal glands:

• Mucous glands: These glands secrete a substance called mucin, which forms a slimy or sticky mucous layer when it comes in contact with water. Mucous glands are abundant in the skin of amphibians. The mucous layer helps keep the skin moist and slimy, providing protection against harmful bacteria and fungi.
• Poison glands: Poison glands are larger and fewer in number compared to mucous glands. They are modified multicellular cutaneous glands. For example, the parotid glands located behind the head of toads are aggregations of poison glands. The secretion of poison glands may be bitter, irritating, or even dangerous to predators, serving as a defense mechanism.
• Luminescence glands or photophores: Certain deep-sea teleost fish have multicellular epidermal glands known as photophores. These specialized glands serve as light-emitting organs, producing bioluminescent light. This luminescence helps the fish attract prey, communicate, or camouflage themselves in their dark environment.
• Femoral glands: Found in lizards on the ventral side of both thighs, femoral glands secrete a sticky substance that hardens in the air, forming temporary tiny spines. These spines aid in holding the female during copulation.
• Sweat glands: Sweat glands are abundant in most mammalian skin. They are slender, coiled tubes embedded deep in the dermis, with long ducts that open on the skin surface. Sweat glands play a crucial role in regulating body temperature by producing sweat, which cools the body through evaporation. However, sweat glands are absent in scaly anteaters and marine mammals. In some animals, they occur in unique locations such as the soles of the feet (cats) or lips (rabbits). Some mammals, like the male giant kangaroo and hippopotamus, secrete red-colored sweat. Modified sweat glands can also be found in the eyelashes and the long margins of eyelids.
• Sebaceous glands: These branched alveolar glands open into the hair follicles of mammals. They secrete an oily substance called sebum or grease. Sebum helps keep the skin and hair soft, greasy, waterproof, and shiny. In some mammals, such as whales, sebaceous glands are practically absent, as these animals are largely devoid of hair. Additionally, ceruminous glands in the external ear canals are modified sebaceous glands. They secrete a greasy substance called cerumen, which helps protect the ear from dust particles and trap insects. Meibomian glands found in the eyelids are also modified sebaceous glands.
• Scent glands: Scent glands are modified sebaceous glands found in mammals. They secrete odoriferous substances that serve to repel foes or attract the opposite sex. Scent glands may occur between the toes on the feet of animals like goats, rhinos, and horses, or on the naval area of the abdomen in musk deer, among other locations.
• Mammary glands: Mammary glands are a defining characteristic of mammals. These compound tubular glands produce milk during the lactation period, providing nourishment to the young offspring. Usually, mammary glands are present only in females, but in monotremes (such as the platypus), primates, and some other mammals, they are also present in males.

2. Epidermal scales and scutes

Epidermal scales and scutes are structures formed by the accumulation of a protein called keratin. These structures are found in various vertebrates and contribute to the development of a horny and hard exoskeleton. Here is some information about epidermal scales and scutes in different animals:

Reptiles: Reptiles have a continuous outer covering of horny epidermal scales, which serve multiple purposes. These scales act as a protective barrier and help prevent water loss through the skin surface. In snakes, the scales are overlapping and enlarged on the head, forming a shield, and on the ventral surface, they are called scutes. These scales and scutes aid in locomotion by providing traction and reducing friction against the ground. Crocodiles and turtles also possess scutes, which are hardened scales that provide protection and support.

Birds: Although birds are known for their feathers, they also have small scales present on their long legs, feet, and beaks. These scales provide additional protection to these areas, which are subjected to constant stress and contact with the environment.

Mammals: While epidermal scales are less common in mammals, there are a few exceptions. For example, the scaly anteater (pangolin) exhibits large scales on its body, which provide defense against predators. In armadillos, the large body scales become fused into plates and bands, forming a protective armor-like structure.

Overall, the presence of epidermal scales and scutes in different vertebrates demonstrates the versatility and adaptability of keratin as a structural protein. These scales and scutes serve various functions, including protection, water regulation, and locomotion, depending on the specific needs of each animal species.

B. Dermal derivatives

Dermal derivatives are

1. dermal scales and scutes,
2. disital cornification,
3. horns,
4. feathers and
5. hairs.

1. Dermal scales and scutes

Dermal scales and scutes are bony structures that develop within the dermis, originating from the mesoderm. They serve different functions in various organisms. Here is some information about dermal scales and scutes in different groups of animals:

a. Dermal scales in fishes: In fishes, scales form the exoskeleton and provide protection. There are five types of scales based on their structure:

• i. Cosmoid scales (extinct): These scales were characteristic of extinct fish species and had a thick, bony base covered by an enamel-like layer.
• ii. Placoid scales: Placoid scales are characteristic of elasmobranchs (cartilaginous fish such as sharks and rays). These scales have a flattened, tooth-like structure with a bony base covered by a layer of dentine and enamel.
• iii. Ganoid scales: Ganoid scales are found in ganoid fish, which include chondrosteans (sturgeons) and holosteans (gar and bowfin). These scales are thick and have a shiny, enamel-like surface.
• iv. Cycloid scales: Cycloid scales are characteristic of modern teleosts (bony fish). These scales are thin, round or oval in shape, and have a smooth surface.
• v. Ctenoid scales: Ctenoid scales are also found in modern teleosts. They are similar to cycloid scales in shape but have small comb-like projections on the posterior edge.

b. Dermal scales and scutes in tetrapods: Some reptiles exhibit small dermal scales. Crocodiles and alligators have many oval bony plates embedded in their dermis, providing protection. Turtles possess large bony plates called scutes, which form their shell.

Among mammals, bony plates known as osteoderms are present in armadillos. These osteoderms provide protection and form a tough outer covering. In whales, bony plates can also be found.

c. Dermal fin rays: Fin rays are long, flexible structures embedded in the dermis that support the fins of fishes. They are composed of bone or cartilage and provide structural support and flexibility to the fins, aiding in swimming and maneuvering.

Overall, dermal scales, scutes, and fin rays demonstrate the adaptation and diversification of skeletal structures in different groups of animals, serving purposes such as protection, support, and locomotion.

2. Digital cornification

Digital cornification refers to the modification of the stratum corneum, the outermost layer of the epidermis, at the tips of the digits. This process results in the formation of specialized structures such as claws, nails, and hoofs in different animal groups. Here is some information about digital cornification and its manifestations:

• Claws: Claws are formed by hard, pointed, narrow, and curved dorsal plates. They are found in various animals, including reptiles, birds, and mammals. Claws serve different purposes depending on the species. In reptiles, claws are used for climbing, digging, and capturing prey. In birds, claws, also known as talons, are used for grasping and catching prey. Mammals with claws, such as cats and dogs, use them for various activities like climbing, hunting, and self-defense.
• Nails: Nails are modified versions of claws and are characteristic of primates, which are a group of mammals. Nails have a flattened shape and a harder and less curved structure compared to claws. Nails serve a variety of functions for primates, including manipulation, grooming, and fine motor skills. They also protect the sensitive tips of the fingers and provide better dexterity.
• Hoofs: Hoofs are characteristic of ungulates, which are hoofed mammals. Hoofs are thick, hard, and usually keratinized structures that cover the distal ends of the digits. They serve as a protective covering for the underlying structures and provide support and stability during locomotion. Hoofed mammals include horses, cows, deer, and many other ungulate species.

Digital cornification demonstrates the adaptability of the stratum corneum to form specialized structures at the tips of the digits in different animal groups. Claws, nails, and hoofs are all variations of these cornified structures, each serving specific functions suited to the lifestyles and ecological niches of the respective species.

3. Horns

Horns are specialized structures found in hoofed mammals and serve as organs of defense and offense. There are various types of horns recognized, although not all of them are true horns, meaning they are not solely composed of the stratum corneum. Here are some types of horns and their characteristics:

• True horns: True horns, also known as hollow horns, are found in both sexes of animals such as goats, cattle, and sheep. They are unbranched, cylindrical, and tapering structures. True horns are permanent and continue to grow throughout the animal’s life without shedding. They are composed of a bony core surrounded by a sheath made of keratinized tissue.
• Prong horns: Prong horns are considered true horns, but they have a horny sheath that bears one to three branches. The horny sheath sheds annually, while the permanent bony core serves as the base for the development of a new horn.
• Antlers: Antlers are characteristic of the deer family and are found only on males. Unlike true horns, antlers are annual growths that are shed and regrown each year. Antlers are composed of bone and are covered in velvet, which is a soft, vascular skin. The velvet provides nourishment during growth but is shed when the antlers are fully developed.
• Giraffe horns: Giraffes possess stunted, unbranched horns that are present in both males and females. These horns consist of a short bony dermal core and are covered with simple, unmodified skin or velvet. Unlike antlers, giraffe horns are permanent structures that are not shed.
• Hair horns or fiber horns: Hair horns, also known as fiber horns, are found in both sexes of rhinoceros. They are located on a roughened area of the nasal bones. Hair horns are composed entirely of thick, keratinized epidermal fibers fused together. These horns are permanent structures, and if broken, they can regrow.

The diverse types of horns seen in different hoofed mammals demonstrate the adaptations and evolutionary strategies developed for defense, offense, and species-specific functions. Horns provide protection, aid in territorial disputes, and play a role in mating rituals and social hierarchies within these animal groups.

4. Feathers

Feathers are unique structures found exclusively in birds. They are dry, nonliving, and cornified products derived from the stratum corneum of the epidermis. Feathers possess several remarkable characteristics that contribute to the avian lifestyle:

1. Lightweight and Strong: Feathers are incredibly lightweight, allowing birds to achieve flight efficiently. Despite their lightness, feathers are also strong and provide structural integrity to the wings, enabling birds to maneuver in the air.
2. Waterproof: Feathers are equipped with a complex structure that repels water, keeping birds dry even in wet conditions. This is essential for maintaining the insulating properties of the feathers and preventing the bird’s body from getting soaked.
3. Coloration: Feathers exhibit a wide range of colors due to the presence of pigments. These pigments, such as melanin and carotenoids, contribute to the vibrant hues seen in bird plumage. Feather colors serve various purposes, including species recognition, camouflage, mate attraction, and signaling dominance.
4. Protection: Feathers play a crucial role in protecting the bird’s body. Contour feathers, the most prominent type, cover the entire bird and provide insulation, shielding it from temperature fluctuations and environmental elements.
5. Flight Surfaces: The broad surfaces of wings and tail feathers are essential for flight. These specialized feathers, known as flight feathers or remiges, have a streamlined shape and precise arrangement that generates lift and allows for controlled flight.

Feathers can be categorized into three main types:

a. Contour Feathers: Contour feathers are the outermost feathers that cover the bird’s body. They provide the characteristic shape and coloration of the bird and contribute to its aerodynamic capabilities.

b. Down Feathers (Plumules): Down feathers are soft and fluffy feathers found beneath the contour feathers. They serve as insulation, trapping air and providing thermal regulation to keep the bird warm.

c. Filoplumes: Filoplumes are hair-like feathers with a fine shaft and a few small barbs at the tip. They are less conspicuous and are thought to have a sensory function, providing feedback on feather position and movement.

Feathers represent a remarkable adaptation in birds, enabling them to conquer the skies and fulfill their ecological roles. Their unique structure, strength, insulation, and coloration make feathers essential for bird survival and their remarkable display of beauty in the natural world.

5. Hairs

Hairs are a distinctive feature of mammals. They can either cover the entire body of an animal, giving it a furred appearance, or be reduced to patches in certain mammals like humans, or even scattered in species like whales. Collectively, all the hairs on a mammal’s body are referred to as the pelage. Hairs are periodically shed and replaced through a process called moulting.

The primary function of hairs is to provide insulation to the body. The layer of fur acts as a barrier, helping to regulate body temperature by trapping air and reducing heat loss. Additionally, hairs can also serve as sensitive tactile organs, with specialized types of hairs called vibrissae or whiskers. Vibrissae are highly sensitive and aid in the detection of objects and movements in the environment, helping mammals navigate and locate prey or avoid obstacles.

Hairs exhibit various modifications and can take on different forms depending on the mammal species:

1. Bristles: Bristles are stiff, specialized hairs found in certain mammals. They can serve various purposes, such as aiding in sensory perception or defense mechanisms. For example, the whiskers of a cat are a type of bristle hair.
2. Quills or Spines: Some mammals, such as porcupines and hedgehogs, have modified hairs known as quills or spines. These hairs are sharp and stiff, acting as a defensive mechanism to deter predators.
3. Scales: Certain mammals, like the pangolin, possess modified hairs known as scales. These scales provide protection and form a tough outer layer for the animal.
4. Horns: In some mammals, hair is modified to form horns. Horns are bony structures covered with a sheath made of keratinized hair. They serve various functions, including defense, territorial displays, and mating rituals.

Hairs have significant economic value and find various uses in industries. They are utilized in the production of textiles, such as wool and fur, as well as for making brushes, wigs, and other products.

Overall, hairs in mammals have diverse adaptations and functions. They provide insulation, serve as tactile organs, and can undergo modifications to fulfill specific purposes. The wide range of hair types and their industrial applications highlight the importance of hairs in both the natural world and human society.

Functions of Integument in Vertebrates

The integument, which includes the skin and its associated structures, performs several important functions in vertebrates:

• Protection: The skin serves as a protective barrier, safeguarding the body from mechanical and chemical injuries caused by pressure, friction, harmful gases, and fluids. Various protective adaptations such as scales, feathers, hairs, claws, nails, hooves, and horns reduce the force of injury, prevent excessive loss of body moisture, and prevent the entry of harmful bacteria, fungi, and other foreign bodies. Additionally, protective coloration and camouflage help vertebrates escape detection by enemies, while skin pigments provide protection against solar radiation. Certain glands, such as uropygial glands in birds, secrete oil for preening feathers, enhancing their protective function.
• Locomotion: The integument contributes to locomotion in vertebrates. Dermal fin rays in fish fins and skin webs in the feet of frogs, turtles, and aquatic birds aid in swimming. Adhesive pads and claws on digits assist in climbing, while feathers on wings and short tails of birds, as well as cutaneous patagia or wings in bats, flying lizards, and squirrels, facilitate flying.
• Dermal endoskeleton: The skin can contribute to the formation of bony dermal armor, such as in crocodiles and turtles. This dermal endoskeleton protects the brain and sense organs.
• Secretion: Various glands within the skin secrete substances that serve several purposes. Mucous glands in aquatic vertebrates keep the skin moist and slippery, while poisonous, bitter, or offensive secretions from certain glands deter potential enemies. Sebaceous glands in mammals produce oil that lubricates the skin and hair. Mammary glands manufacture milk for the nourishment of young offspring. Scent glands release odors that attract the opposite sex. Lacrimal glands produce tears that wash the conjunctiva of mammalian eyeballs, and glands in the auditory meatus secrete earwax (cerumen) to lubricate eardrums.
• Food storage: Thick and fatty layers of blubber in whales and seals serve as insulation and reserve food. Some animals, such as those preparing for hibernation or migration, accumulate subcutaneous fat as a food reserve.
• Temperature control: The integument plays a role in temperature regulation. In warm-blooded animals, fur, feathers, and scales provide insulation and help conserve body heat in cold climates. Sweat glands in mammals aid in cooling through evaporation, allowing for the elimination of excess heat in the summer.
• Excretion: Sweat glands also contribute to the excretion of excess water, salts, and urea. Additionally, shedding of the skin during ecdysis (molting) helps eliminate some metabolic wastes.
• Sensation: The integument contains cutaneous nerve endings and other sensory organs that respond to touch, pain, changes in pressure and moisture, extremes of heat and cold, and chemicals, enabling vertebrates to perceive their environment.
• Sexual selection: The integument can play a role in sexual selection. Brilliantly colored skins, antlers of male deer, long tails, and coverts of peacocks are examples of adaptations that lead to sexual dimorphism and serve to attract females for mating.
• Miscellaneous functions: The skin also performs various other functions. It synthesizes vitamin D in mammalian skin from sebum of sebaceous glands when exposed to UV light. Brood pouches under the skin of some fishes and amphibians protect unhatched eggs. Nasal glands in tetrapods help keep nostrils free of water and dirt. Amphibians rely on their richly vascular skin for considerable respiration. The skin can selectively absorb substances such as oil ointments, iodine, and beneficial sunrays.

In summary, the integument in vertebrates serves multiple functions, including protection, locomotion, secretion, food storage, temperature control, excretion, sensation, sexual selection, and various other specialized roles. It is a dynamic and vital organ system that contributes significantly to the overall health and survival of vertebrate organisms.

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