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The integument is made up of two layers: the epidermis (an example of stratified squamous epithelium, recall) and the dermis (an example of dense irregular connective tissue). The epidermis is derived from ectoderm while the dermis is derived mainly from mesoderm. While many animals have the same basic integument structure, the epidermis has becomes specialized in a number of ways, as outlined in the chart below:
| Amphibians | - | mucous and poison glands |
| Reptiles | - | scales and claws |
| Birds | - | feather follicles and claws |
| Mammals | - | hair follicles and sebaceous glands, nails, claws, hooves, horns, sweat glands and mammary glands |
| NOTE | : | The scales of fish are dermal derivatives of bone and are of mesodermal, not ectodermal, origin. |
This page shows micrograph sections of the integument of the frog and the mammal.
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| In the frog, the dermis is subdivided into two layers: the stratum spongiosum and the stratum compactum. These names refer to the appearance of the connective tissue within these layers. In the top picture, note the large poison glands. These produce substances to that can be anything from mildly irritating to very toxic, depending on the species, which serves to ward off predators. The epithelium of poison glands is anything from simple squamous to low columnar. It is difficult to appreciate here, but each gland is surrounded by connective tissue that contains smooth muscle cells. These contract when stimulated to squeeze the poison out of the gland lumen and onto the surface of the frog's skin. Mucous glands, seen in the lower picture, are smaller than poison glands and serve to produce mucous to keep the frog's skin moist and permiable for gas exchange. The epithelium of this gland is generally cuboidal. Both poison and mucous glands form as invaginations from the epidermis which migrate down into the dermis. In the developed animal, the bulk of the gland is in the dermis, with only a duct to the surface travelling through the epidermis. Finally, the very dark staining "spots", seen especially well on the lower picture, are chromatophores. These cells have a neural crest origin and produce pigment which is picked by the epithelial cells neighbouring it. |
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| The two slides above show the palmar skin of a monkey. This is an example of "thick" skin. The most striking feature of think skin is the thick layer of superficial keratin (the bright pinkish-purple stuff!). Keratin is the collective name for a family of tough proteins which are found in a number of structures. Keratin is found in hair, feathers, nails, claws, hooves and reptile scales. The keratinized part of the epidermis is known as the stratum corneum. Keratinization occurs when underlying epidermal cells produce and accumulate keratin within their cytoplasm. As they are being pushed towards the surface by the production of new epidermal cells from beneath (only the layer of epidermal cells immediately next to the dermis divide), their organelles and nucleus disappear and they become flattened. The keratin then becomes external from the broken down cells, forming the acellular layer of keratin seen on the surface of mammalian epithelium. As the outer keratinized cells are worn off, they are replaced by the cells coming up from below. The keratin layer serves to form an impervious barrier between the external body and the environment, as well as protecting the skin from damage. In the lower image, there are ducts visible travelling through the stratum corneum. These are the ducts of the sweat glands found lower down in the dermis (see the upper image). Sweat glands are coiled tubular glands which produce a watery secretion and serve an important function in thermoregulation for mammals | |
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Special thanks to Hans Christoffersen for the preparation of this page.
Send comments to:
Sandra K. Ackerley (),
Department of Zoology, University of Guelph, Guelph, Ontario,
Canada N1G 2W1.