Lesson 6 Lesson Outline: General Features of the Integument Embryonic Origins of the Epidermis Specializations of the Epidermis o Glands o Keratin and Stratum Corneum Objectives: At the end of this lesson you should be able to: Describe the basic structure and function of the epidermis Describe the embryonic origin of the different layers of the integument Describe the phylogenetic trends seen in the development of the epidermis. References: Chapter 6: 105-120 Reading for Next Lesson: Chapter 6: 120-127
Integument General Features Skin is a composite organ - composed of and outer layer or layers of epithelium (the epidermis), and a much thicker inner layer (the dermis) composed of closely packed fibrous connective tissue. The epidermis arises from the single cell layer of the ectoderm while the dermis arises firm the mesoderm of the dermatome. Variations are found in the integument of craniates involving: - the relative number and complexity of skin glands - the extent of differentiation and specialization of the epidermis - the extent to which bone develops in the dermis Basic Functions Protection -mechanical barrier - prevents entrance of pathogens - cryptic/warning or camouflage colouration - protection from UV light Exchange Surface -water -wastes - ions - gases - heat Sensory Reception Epidermis Embryonic Origin The single layer of ectoderm proliferates to become a multilayered epidermis in most vertebrates (but not all). It produces an inner layer called the basal lamina. Cells in the basal lamina are constantly undergoing mitosis, pushing older cells outward and replacing those lost at the surface. Epidermal cells may also undergo the process of keratinization to form a keratinized or cornified layer. Keratin is a class of proteins which are produced in specific epidermal cells just before they die to form a nonliving tough, water proof layer the stratum corneum. This layer can be of varying degrees of thickness and can be used to protect the soles of the feet to the roof of the mouth. It may also differentiate into hair, hooves, horn and other specialized cornified structures.
Keratinizing systems are systems where an elaborate interaction between the epidermis and dermis gives rise to cornified structures such as teeth, denticles and scales in most fish. Invagination of the surface epidermis forms a variety of skin glands. These are called exocrine glands if the retain a duct to the surface and endocrine glands if they separate from the surface and secrete their products into the vascular system. One of the most common types of glands is the mucus secreting glands. The dermis contains connective tissue, blood vessels, lymphatic vessels, nerves and pigment cells. It sometimes adheres closely to the underlying muscle but is usually separated from it by an intervening layer of loose connective tissue and adipose tissue - the hypodermis. Specializations of the Epidermis Two kinds of nonliving coverings overlie the living epidermis of vertebrates. These coverings are produced by the epidermis. In fishes and aquatic amphibians it is a thin coat of mucus that is continually being replaced. In terrestrial vertebrates it is the stratum corneum (dead keratinized cells). The epidermis is glandular to some extent in all vertebrates - more so in fish and amphibians. Multicellular epidermal glands invade the dermis where they are surrounded by blood capillaries that provide them with nutrients, and in the case of endocrine glands, transport their products. Epidermal Glands Fish - most are single cells - goblet cells secrete only mucus - granule cells secrete mucus and other substances - mucus is used for: reducing friction avoiding predation cocoons for estivation and sleeping nutritious mucus for feeding young in some species - slime glands in hagfish and lamprey are used for protection and for suffocating prey - granule cells also produce some toxins and alkaloids - some deep sea teleosts have multicellular glands that have been adapted to serve as light emitting organs or photophores. The basal layer of cells in the gland emit light as a product of an enzymatic reaction. The mucus cells above them act as a magnifying lens.
Amphibians - most epidermal glands are multicellular - leydig cells (antiviral and antibacterial substances) - mucus glands for moist skin, - poisons glands for defense (poison dart frog, bufotenin in toads etc.) Tetrapods - epidermal glands are multicellular - they can be tubular (rare except in mammals) or saccular (alveolar) - They vary in they way in which they liberate their products. - merocrine glands secrete their products across the cell membrane and the cell remains intact. (eg. sweat glands) - in holocrine glands, the cells themselves are secreted (eg. sebaceous and oil glands) - in apocrine glands the secretion accumulates in the apical portion of the cell which is then pinched off along with some cytoplasm. The cell then repairs itself. (eg. mammary glands) Mucous Glands Mucus secreting epidermal glands have all but disappeared in terrestrial tetrapods except in mammals where they are confined to areas where lubrication is essential. Granular Glands Found only in amphibians and reptiles. They are defensive and secrete toxins or alkaloids to ward of predators. They also secrete pheromones which are chemical cues important in reproductive behaviour. Femoral glands in lizards secrete a substance on the inner thighs of males that hardens to form temporary spines to restrain females during copulation. Avian Oil Glands Uropygial glands near the rump secrete a water repellent oil that is transfered to the feathers during preening. It is largest in aquatic birds. Sebaceous Glands These are the mammalian equivalent of the avian oil gland. They are present wherever there is hair and secrete oil to water proof hair and fur. They are found independent of hair on the lips, glans penis, labia minora and skin surrounding the nipples where they serve a lubricating function. Sudoriferous Glands These are sweat glands found primarily in mammals which serve a thermoregulatory role. In furry mammals they are confined to the least furry regions(feet, lips, etc.). They are most abundant in hairless mammals (such as humans).
Scent Glands Sebaceous and sudoriferous glands also produce a variety of scents, most of which are pheromones. These can be found on any part of the body and can be used to drive away enemies or attract mates. Mammary Glands Found only in mammals, they are used for nutrition. The glands themselves consist of many lobules that are each composed of many alveoli (same term as in the lung) that secrete the milk. The glands may open directly to the surface (monotremes) or through a raised epidermal papilla (nipple - multiple openings) or into a common chamber in an enlarged epidermal papilla (teat). The glands usually proliferate and become enlarged under hormonal stimulation just before females give birth to their young. (no need to produce milk when it is not required) Adipose tissue can build up beneath the mammary gland (in the hypodermis) to form a breast. Breasts and mammary glands are not synonymous. The adipose tissue does not secrete milk. Breasts are primarily for sexual display and courtship. In most mammals they only develop during mating season. Humans are one of the few mammals where they are permanently fully developed after puberty. Keratin and Stratum Corneum Fish and Amphibians - keratin is not found very often in the epidermis of fish or amphibians (it is found in the dermis). - Some salamanders have a thin stratum corneum - Lampreys and hagfish have cornified epidermal spines that act as teeth for grasping food or rasping prey. Tetrapods - Keratinization becomes highly developed in the epidermis of tetrapods. It provides protection against desiccation on land. Keratinization and formation of a stratum corneum also occur where friction or direct mechanical abrasion insult the epithelium. For example, the epidermis in the oral cavity of aquatic and terrestrial vertebrates often exhibits a keratinized layer, especially if the food eaten is unusually sharp or abrasive. In areas of the body where friction is common, such as the soles of the feet or palms of the hands, the cornified layer may form a thick protective layer, or callus, to prevent mechanical damage (figure 6.6). The stratum corneum may be differentiated into hair, hooves, horn sheathes, or other specialized cornified structures. With the acquisition of an amnion, which liberated the amniotes from water, the stratum corneum became increasingly specialized in various regions of the body for protection against abrasion, for offense and defense (scales, claws and horny protuberances), and as an adjunct for thermoregulation (hair and feathers). Epidermal Scales: Scales are present as thickenings of the stratum corneum in amniotes. In snakes and lizards they form as overlapping folds of the epidermis. Despite appearances, the epidermis is continuous. Many reptiles are able to shed and replace large portions of their
skin at one time in the process of moulting. What is moulted is the epidermis. The inner of the three layers reproduces the outer two layers (one living and one dead) from below and white blood cells invade the space between the new and old skin causing it to slough off. If the scale is large and plate like it is called a scute. These occur on the bellies of snakes and turtles (The scutes in turtles are distinct from the shell itself which is of dermal and skeletal origin). They may be modified into crests, spines or hornlike processes. The epidermis in birds has the same basic structure as that of reptiles. Epidermal scales remain on legs and feet and in the facial areas of some species. In mammals, they are also usually confined to the legs and tail but in armadillos they occur all over the body. Claws, Hoofs and Nails Plates of tightly compacted, cornified epithelial cells produced through keratinization. Continuously produced at the base and worn down at the tips. Serve for protection, gripping, grasping, tearing, and for cushioning and shock absorption Horns and Antlers While some reptiles have horns that are pointed epidermal scales, the terms horns and antlers really only apply to mammals. True horns have a keratinized surface. Antlers do not. Horns occur in bovids (oxen, cows, sheep, goats, antelopes) and pronghorn antelopes (not true antelopes). They have a core of dermal bone covered by a sheath of horn. They occur in both sexes and are usually present year-round. The epidermal component can often be far more extensive than the core of dermal bone. The horns of the rhinoceros are only made of compact keratinized fibers of epidermis (hair). Antlers appear in cervids (deer family), usually only in males. Throughout much of their growth they are covered in living skin (called velvet) which is eventually shed to reveal the dead bone beneath. They are shed and replaced annually. This means that the dermal bone must break off and this is a hormonally controlled process. The horns of giraffes are ossified cartilage that fuse to the top of the skull. Baleen Keratinized plates that form from the epidermis in the mouths of many whales. Dermal papillae extend and lengthen, carrying the overlying epidermis which forms cornified frills which act as strainers to extract small invertebrates from the water they gulp. They can then strain out the water and only swallow the food. Differences in the arrangement and configuration of the sheets are correlated with feeding habits. Baleen is continually worn away and replaced.