Avian permeability barrier function reflects mode of sequestration and organization of stratum corneum lipids: reevaluation utilizing ruthenium tetroxide staining and lipase cytochemistry

Water relations of tetrapod integument

The vertebrate integument represents an evolutionary compromise between the needs for mechanical protection and those of sensing the environment and regulating the exchange of materials and energy. Fibrous keratins evolved as a means of strengthening the integument while simultaneously providing a structural support for lipids, which comprise the principal barrier to cutaneous water efflux in terrestrial taxa. Whereas lipids are of fundamental importance to water barriers, the efficacy of these barriers depends in many cases on structural features that enhance or maintain the integrity of function. Amphibians are exceptional among tetrapods in having very little keratin and a thin stratum corneum. Thus, effective lipid barriers that are present in some specialized anurans living in xeric habitats are external to the epidermis, whereas lipid barriers of amniotes exist as a lipid-keratin complex within the stratum corneum. Amphibians prevent desiccation of the epidermis and underlying tissues either by evaporating water from a superficial aqueous film, which must be replenished, or by shielding the stratum corneum with superficial lipids. Water barrier function in vertebrates generally appears to be relatively fixed, although various species have`plasticity' to adjust the barrier effectiveness facultatively. While it is clear that both phenotypic plasticity and genetic adaptation can account for covariation between environment and skin resistance to water efflux, studies of the relative importance of these two phenomena are few. Fundamental mechanisms for adjusting the skin water barrier include changes in barrier thickness, composition and physicochemical properties of cutaneous lipids,and/or geometry of the barrier within the epidermis. While cutaneous lipids have been studied extensively in the contexts of disease and cosmetics,relatively little is known about the processes of permeability barrier ontogenesis related to adaptation and environment. Advances in such knowledge have didactic significance for understanding vertebrate evolution as well as practical application to clinical dermatology.

Immunocytochemical and autoradiographic studies on the process of keratinization in avian epidermis suggests absence of keratohyalin

The process of keratinization in apteric avian epidermis and in scutate scales of some avian species has been studied by autoradiography for histidine and immunohistochemistry for keratins and other epidermal proteins. Acidic or basic alpha-keratins are present in basal, spinosus, and transitional layers, but are not seen in the corneous layer. Keratinization-specific alpha-keratins (AE2-positive) are observed in the corneous layer of apteric epidermis but not in that of scutate scales, which contain mainly beta-keratin. Alpha-keratin bundles accumulate along the plasma membrane of transitional cells of apteric epidermis. In contrast to the situation in scutate scales, in the transitional layer and in the lowermost part of the corneous layer of apteric epidermis, filaggrin-like, loricrin-like, and transglutaminase immunoreactivities are present. The lack of isopeptide bond immunoreactivity suggests that undetectable isopeptide bonds are present in avian keratinocytes. Using immunogold ultrastructural immunocytochemistry a low but localized loricrin-like and, less, filaggrin-like labeling is seen over round-oval granules or vesicles among keratin bundles of upper spinosus and transitional keratinocytes of apteric epidermis. Filaggrin-and loricrin-labeling are absent in alpha-keratin bundles localized along the plasma membrane and in the corneous layer, formerly considered keratohyalin. Using ultrastructural autoradiography for tritiated histidine, occasional trace grains are seen among these alpha-keratin bundles. A different mechanism of redistribution of matrix and corneous cell envelope proteins probably operates in avian keratinocytes as compared to that of mammals. Keratin bundles are compacted around the lipid-core of apteric epidermis keratinocytes, which do not form complex chemico/mechanical-resistant corneous cell envelopes as in mammalian keratinocytes. These observations suggest that low amounts of matrix proteins are present among keratin bundles of avian keratinocytes and that keratohyalin granules are absent.

Local cutaneous water barrier in cold- and heat-acclimated pigeons (Columba livia) in relation to cutaneous water evaporation

The thermoregulatory function of the skin differs in adult cold-acclimated and heat-acclimated rock pigeons (Columba livia). In general, the cutaneous evaporative cooling mechanism is not activated by appropriate stimuli in cold-acclimated pigeons in contrast to heat-acclimated pigeons. We studied with electron microscopy whether the differences in the function of the skin are reflected in the structure of the epidermal water barrier of these two extreme acclimation states. The epidermis of cold-acclimated pigeons is attenuated, and the underlying dermis lacks any intimate vascularization. Both the extracellular and the intracellular domains in the stratum corneum contain organized lamellar lipids. At the stratum transitivum-stratum corneum interface, multigranular body secretion is indicated by the highly convoluted cell membranes and membraneous sacculae enclosing the multigranular bodies. Alternatively, multigranular bodies retain in the corneocytes, and the lipoid material originated from them is reprocessed to broad lamellae. The keratohyalin (KH) granules are spotlike and oriented as cortical bands beneath the plasma membrane. In heat-acclimated pigeons, the epidermis displays modified patches side by side with basic structural type of epidermis. The modified areas are characterized by hypertrophy and abundance of dermal capillaries adjacent to the hypertrophied patch. No lamellar lipids are discerned in the dilated extracellular space. The structure of multigranular bodies is abnormal, and the numbers of lipid droplets in the outer viable epidermis and stratum corneum are decreased. The transitional cells contain stellate KH granules, which form a network throughout the cell. It is concluded that cold-acclimated pigeons have a lamellar, extracellular water barrier, the cutaneous water evaporation is minimized, and heat is stored in the body core. Acclimation to heat leads to formation of structurally heterogeneous skin. The structurally modified skin patches show disruption of the barrier-forming machinery in the multigranular bodies and marked reorganization of fibrillar proteins and electron-dense KH masses in the transitional layer. Thus water barrier adjustments in cold- and heat-acclimated pigeons manifest the dynamic function of avian skin as a thermoregulatory organ.

Inhibiting ventilatory evaporation produces an adaptive increase in cutaneous evaporation in mourning doves Zenaida macroura

We tested the hypothesis that birds can rapidly change the conductance of water vapor at the skin surface in response to a changing need for evaporative heat loss. Mourning doves (Zenaida macroura) were placed in a two-compartment chamber separating the head from the rest of the body. The rate of cutaneous evaporation was measured in response to dry ventilatory inflow at three ambient temperatures and in response to vapor-saturated ventilatory inflow at two ambient temperatures. At 35 degrees C, cutaneous evaporation increased by 72 % when evaporative water loss from the mouth was prevented, but no increase was observed at 45 degrees C. For both dry and vapor-saturated treatments, cutaneous evaporation increased significantly with increased ambient temperature. Changes in skin temperature made only a minor contribution to any observed increase in cutaneous evaporation. This indicates that Z. macroura can effect rapid adjustment of evaporative conductance at the skin in response to acute change in thermoregulatory demand.

Microwave incubation improves lipolytic enzyme preservation for ultrastructural cytochemistry

Standard methods for the ultrastructural detection of lipase and sphingomyelinase activities in the skin result in considerable loss of structural preservation, often interfering with accurate delineation of enzyme localization in association with specific organelles. Moreover, poor preservation occurs, even after extensive aldehyde prefixation, owing to the prolonged incubation times needed to detect residual enzyme activity, which often require non-physiological conditions. A modified incubation protocol is described here, which uses microwave irradiation in conjunction with drastically shortened incubation times, resulting in both superior ultrastructural preservation and excellent localization in mammalian epidermis. This method should be useful generally not only for the study of lipase localization in skin, but also in conjunction with the cytochemical detection of a variety of enzymes in various types of tissue.

Morphology of lipid alterations in the epidermis: a review

An overview of the morphological correlates of mammalian epidermal lipids and the cutaneous permeability barrier is provided. The following features are discussed: 1) ultrastructural characterization of the progressive lipid alterations accompanying normal stratum corneum (SC) formation; 2) effects of drugs/inhibitors that interfere with specific aspects of epidermal lipid metabolism; 3) lipid alterations in certain human skin disorders; 4) effects of selected topical emollients on SC lipid organization; and 5) potential pitfalls in interpretation of ultrastructural data in terms of cutaneous function and dysfunction.

Ultrastructural organization of avian stratum corneum lipids as the basis for facultative cutaneous waterproofing

The ultrastructure of naked neck epidermis from the ostrich (Struthio camelus) and ventral apterium from watered, and water-deprived, Zebra finches (Taeniopygia [Poephila] guttata castanotis) is presented. The form and distribution of the fully differentiated products of the lipid-enriched multigranular bodies are compared in biopsies post-fixed with osmium tetroxide or ruthenium tetroxide. The fine structure of ostrich epidermis suggests it is a relatively poor barrier to cutaneous water loss (CWL). The fine structure from watered, and 16-hr water-deprived Zebra finches, considered in conjunction with measurements of CWL, confirms previous reports of "facultative waterproofing," and emphasizes the rapidity of tissue response to dehydration. The seemingly counterintuitive facts that one xerophilic avian species, the ostrich, lacks a "good barrier" to CWL, whereas another, the Zebra finch, is capable of forming a good barrier, but does not always express this capability, are discussed. An explanation of these data in comparison to mammals centers on the dual roles of the integument of homeotherms in thermoregulation and conserving body water. It is concluded that birds, whose homeothermic control depends so much on CWL, cannot possess a permanent "good barrier," as such would compromise the heat loss mechanism. Facultative waterproofing (also documented in lizards) protects the organism against sudden reductions in water availability. In birds, and probably in snakes and lizards, facultative waterproofing involves qualitative changes in epidermal cell differentiation. Possible control mechanisms are discussed.