jimtrue.com : school : BSC2011 : CH 00: Animal Systems - Integumentary (Skin)

Posted by Jim True on October 28, 2004 6:22 PM. Last Updated October 22, 2006 9:23 PM

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CH 00: Animal Systems - Integumentary (Skin)

Functions

• Protection - Against both physical trauma (abrasions, punctures, etc.) and pathogens.
• Water Retention - Protects against water loss (dehydration or dessication), critical in true terrestrial animals.
• Support - Integument is the main "wrapper" for the body, provides shape and support for underlying tissues.
• Temperature Regulation - Allows removal of excess body heat from metabolism, also absorption of heat from environment.
• Pigments - Protection of tissues from UV radiation, absorption of heat from sun, also display, disguise, communications.
• Sensory Platform - ALL environmental sesors pass through epithelium.
• Waste Removal - In aquatic animals, nitrogenous wastes can diffuse across skin.
• Respiration - Main or auxiliary means of gas exchange.
• Secretions - Regulatory substances such as sweat and oil, also pheromones (attract opposite sex) and other attractant and repellent chemicals.

Structure

• Integumentary structure is highly variable.
• In invertebrates (animals lacking a rigid internal backbone), typically a single layer, the epidermis, derived from ectoderm.
• In many cases, the epidermis secretes an acellular outer layer which may be formed by complex molecules and may include mineral salts for added strength, support, rigidity and protection.
1. P.Porifera - The outer covering is formed by flattened cells, the pinacocytes.
   • These may be reinforced and provide additional protection from spicules, secreted "needles" of SiO2 or CaCO3 which may protrude from intercellular spaces.
   • The pinacocytes overlie a layer of acellular material called the mesohyl.
   • This is NOT epidermis in the Porifera; as the Porifera have no systems. This is merely their outer coverings.
2. P.Cnidaria - The epidermis secretes a thin acellular veneer of glycoproteins called the glycocalyx.
• Beneath the epidermis is an acellular layer, the mesoglea.
3. P.Platyhelminthes - In the free living class Turbellaria, the epidermis is ciliated. the epidermis also secretes mucus so that the cilia of the epidermis can generate a gliding movement.
• In the classes Cestoidea and Trematoda, a structure known as the tegument is formed.
• The tegument is a syncytium, a multinucleate cellular layer.
• This protects endoparasites against host body fluids and pH levels and ectoparasites against abrasions from host movements & activities.
4. P.Nematoda - The cells of the epidermis secrete a thick, complex acellular cuticle.
• The cuticle serves to protect the worm from physical harm, whether free living or parasitic.
• The epidermis may be syncytial (a multinucleate sheet) or multicellular, depending on the species. In nematodes and certain rotifers there is a fixed number of cells in the body.
5. P. Mollusca - In the class Bivalvia, most of the class Gastropoda and nautiloid cephalopods, the epidermis of the mantle secretes CaCo₃ to produce a protective shell.
• In shell-less mollusks such as slugs and nudibranchs, the body epidermis may secrete mucus for protection, and the foot of most gastropods secretes mucus to allow for ease of movement.
6. P.Annelida - A thin cuticle is secreted by the epidermis. Provides some physical protection, but is thin enough for gas exchange.
7. P.Arthropoda - The epidermis secretes a two region complex cuticle.
• The inner cuticle is the procuticle, which is formed in two layer sof chitin and proteins and may be impregnated with CaCO3 (as in most crustaceans). The outer part of the procuticle is absent at joints, allowing flexibility.
• The outer cuticle is the epicuticle, which includes proteins plus wax and so is waterproof. Essential for terrestrial arthropods to prevent water loss from the body.
• The epicuticle may be absent in some regions (as over the gills of aquatic forms) which allows gas and liquid exchanges.
8. P. Echinodermata - These possess a very thin epidermis, which may be ciliated, over the entire body.
• In some (class Holothuroidea), a thin cuticle may also be secreted by the epidermis.
9. P.Chordata, S.p. Urochordata - Possess and epidermis covered by a thick cuticle referred to as a tunic. Includes glycoproteins plus a special form of CELLULOSE!
10. P.Chordata, S.p. Cephalochordata - Primitive two layered integument.
• Outer layer is a thin, nonciliated epidermis without a cuticle.
• Beneath this lies a touch, connective tissue layer, the dermis. Provides additional strength and support to the body wall. First time we've seen a two-layered integument.
11. P.Chordata, S.p. Vertebrata - A two layered integument formed by the epidermis and the dermis.
• The epidermis is a thin layer of stacked epithelial cells formed from the ectoderm.
• The outermost cells are often dead and toughened by the impregnation of a fbrous protein, keratin.
• This forms a protective layer where there is a higher level of friction, ie, a callus.
• The epidermis may be highly modified.
• Epidermal Modifications in different vertebrate classes include:
• Scales - Reptilia, Aves, Mammalia
• Feathers - Aves, some ancient Reptilia.
• Hair - unique to Mammalia
• Scutes - The outer, platelike structures on the shell of turtles.
• Claws, Nails - Reptilia, Aves, Mammalia.
• No blood vessels or nerves in epidermis.
• The dermis (sometimes referred as "true skin") is a much thicker layer, is derived from mesoderm, and contains most of the pigment cells, blood vessels, and nerves.
• Epidermal modifications such as hair and feathers are embedded deeply into the dermis (to anchor them).
• Like the epidermis, there are many dermal modifications amongst the different vertebrates.
• Scales - In chondrichthyes, placoid scales are teethlike in structure, possess a living pulp layer covered with enamel.
• In Osteichthyes, scales (cycloid or ctenoid) are thin non living bony sheets.
• Horns - In Mammalia, true horns are thick growths of dermal bone overlaid with a thin sheet of modified epidermal material.
• Antlers - In mammalia, highly branched dermal bone produced and lost annually.
• Plastron (ventral) and Carapace (dorsal) - the heavy bone layers beneath the scutes in turtles.

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