Continuity of intercellular space and endoplasmic reticulum of keratinocytes

Is the Golgi apparatus the obligatory final step for lipoprotein secretion by intestinal cells?

It is currently admitted that the synthesis and excretion of triglyceride-rich lipoproteins (chylomicrons and 'small chylomicrons') by intestinal epithelial cells involves the Golgi apparatus as an obligatory final step before exocytosis. The cells of the proximal intestine of the trout are an excellent model for investigating functional compartmentalization in the course of lipid absorption. Using this model, our data invalidate morphological data which were the basis for considering the Golgi apparatus as the mandatory final stage for their secretion. In particular, we show that triglyceride-rich particles can be transported directly from the endoplasmic reticulum to the intercellular space. Two pathways of intestinal lipoprotein excretion appear to coexist. One follows the classical export route, the second functions in a manner that bypasses the Golgi apparatus. The arguments used to affirm the requirement for the Golgi apparatus as a final step (glycosylation of apoprotein B, membrane vehicle for exocytosis) are discussed.

A histochemical investigation on the percutaneous absorption of vitamin D synthesized into the mammal epidermis

The vitamin D transepidermis absorption was studied by means of a histochemical technique suitable to detect this vitamin and to discriminate it from cholesterol and its esters. Such technique shows vitamin D inside the mast cell granules. As the mast cell granules contain metachromatic substances its own histochemical reactivity must be previously blocked by methylation. After this treatment the mast cell granules do not stain by toluidine blue and do not react to the peracetic acid-toluidine blue reaction. However, the granules remains reactive to alkaline permanganate-toluidine blue and to alkaline permanganate-Schiff reactions. These results show that the mast cell granules do not contain cholesterol but they contain vitamin D. The lack of cholesterol suggests that vitamin D is not synthesized inside the granules. As the mast cells may appears within the epidermis or in close relationship with the epidermis, although it is placed into the superficial dermis, it was admitted that the mast cells uptake vitamin D contained inside the epidermis intercellular compartment. In such instances, the vitamin D synthesized by the keratinocytes enter the intercellular compartment, where its synthesis accomplishes, and migrate towards the basement membrane. At the basal epidermis layer or after passing through the basement membrane the vitamin D is taken up by mast cells, where it is stored inside its granules.

A histochemical study on the vitamin D synthesis into the epidermis

Using a suitable histochemical method vitamins D and 7-dehydrocholesterol could be shown into the epidermis of several mammal species. As the histochemical method used is able to discriminate vitamins D and 7-dehydrocholesterol from cholesterol and its esters, the sites where these vitamins were synthesized within the epidermis layers could be established. Vitamins D and 7-dehydrocholesterol were found into the epidermis in the same sites where cholesterol and its esters take place, such as: the keratinizing cell thick membrane and the stratum spinosum and stratum granulosum keratinocytes cytoplasm. Inside the keratinocyte cytoplasm vitamin D shows a granular pattern and appears weakly bound to proteins. The reactivity of such granules seems to be partially blocked as could be shown by an hydrolysis accomplished previously. After the hydrolysis reactive vitamin D was also found inside the epidermis intercellular space. The results suggest that vitamin D is synthesized into the cytoplasm of stratum spinosum and stratum granulosum keratinocytes, where it appears weakly bound to proteins. Afterwards it reaches the intercellular space, where its synthesis is accomplished and it becomes firmly protein-bound losing its histochemical reactivity. However, after a suitable hydrolysis the histochemical reactivity could be recovered. From the intercellular spaces vitamin D could take 2 fates: It was partially incorporated on the keratinizing cell thick membrane out surface and eliminated by means of the epidermis exfoliation. It was partially absorbed after passing across the basement membrane. On the other hand, the vitamin D placed inside the stratum spinosum and stratum granulosum keratinocytes cytoplasm become incorporated on the inner surface of the keratinizing cell thick membrane. The relationship between vitamin D biosynthesis and the epidermis lamellar bodies was discussed.

Histochemical aspects concerning the synthesis and the fate of cholesterol into the epidermis

By means of a suitable histochemical method free cholesterol and its esters could be detected into the epidermis layers. The results show that in the stratum spinosum keratinocytes free cholesterol appears as an amorphous or granular structure apparently protein unbound; into the stratum granulosum keratinocytes the cholesterol becomes protein-bound and its most part undergoes esterified. The extracellular compartment nearly the stratum granulosum contains a little amount of cholesterol esters loosely bound to proteins. The results suggest that free cholesterol after being synthesized into the cytoplasm of the stratum spinosum and granulosum keratinocytes, it is partially esterified and becomes protein-bound, appearing as fine granules within the cytoplasm of the granulous cells. From this site it takes to fates:1. Its most part remains into the granulous cell cytoplasm and at the same time the granulous cell develop to the horny cell it is placed on the thick cell membrane inner surface contributing to its thickness; 2. Another part after reaching the extracellular compartment it is spread over the thick membrane out surface. Inside the thick cell membrane, into the horny layer, free cholesterol is continuously esterified since the keratinizing cell migrate to the periphery; however even at the most peripheral layers the free cholesterol predominates. Either free cholesterol or its esters, contained into the keratinizing cell thick membrane, were excreted throughout the horny layer exfoliation. The keratinizing cell cytoplasm does not contain neither free cholesterol nor its esters.

Ultrastructural studies of allergic contact dermatitis in man. Epidermal cell changes at 3, 6, and 12 h after application of DNCB

Early epidermal lesions of allergic contact dermatitis were examined by electron microscopy. Normal human volunteers were sensitized to DNCB, and contact reactions were elicited sequentially. Epidermal cell changes at 3 h included: the occurrence of small vacuoles with or without membrane, focal dilatation of intercellular spaces, and the alteration of tonofilaments into short, aggregated bundles. Non-sensitized skin to which DNCB was applied also showed small vacuoles with or without membrane. Monocytes infiltrated into the intercellular spaces which were not dilated, and the neighboring tonofilaments of keratinocytes remained normal.

The ultrastructure of the human epidermis following ultracentrifugation

Ultracentrifugation of the human epidermis was undertaken to uncover the forces responsible for the establishment and maintenance of the supranuclear melanin cap in basal cells. The wide spacing and elasticity of the tonofibrils caused a selective trapping of melanosomes in this region. At the same time the experiments produced pictures resembling dedifferentiation of the plasma membrane as in the fetal state, and differentiation of the tonofibrillar mass resembling the granular and horny layers. Once the tonofibrils had retracted and massed together, the order of intracellular sedimentation was: melanosome, chromatin, mitochondrion, tonofibril and ribosome.

Trends in electron microscopy of skin

Some trends in electron microscopy of skin have emerged and should be pursued in the future. The fine structure and some basic cellular reaction patterns of epidermal cells are discussed to illustrate the interplay of morphologic, cytochemical, and tracer studies. Intracytoplasmic membranes and secretory granules, lysosomes and endocytic mechanisms, cytomembranes and cell surface specialization are discussed to show how these can be used to arrive at a more meaningful interpretation of structure. Despite all advances, however, a great deal more needs to be done before the details of skin structure are completely elucidated.

Immunochemical localization of in vivo bound immunoglobulins in pemphigus vulgaris epidermis. Employment of a peroxidase-antiperoxidase multistep technique for light and electron microscopy

A multi-step immunocytochemical method utilizing horseradish peroxidase (HRP) as an immunological marker was employed to demonstrate, at the ultrastructural level, IgG antiepithelial autoantibodies bound in vivo to pemphigus epidermis, and circulating IgG antibodies, using monkey oesophagus as substrate. To demonstrate IgG the following antisera were employed in sequential steps: Goat antihuman IgG; rabbit antigoat IgG; goat-anti-HRP-serum. After incubating with the final antigen, HRP, the latter was visualized with a cytochemical method. Results paralleled those obtained by immunofluorescence, the antiepithelial antibody being demonstrated at the sites of the intercellular spaces of the epidermis. Electron microscopic cytochemistry showed IgG in the surface coat of epidermal cells and in the intercellular space, both in the desmosomal and the interdesmosomal areas. These results confirm the results of a previous study but the localization of the antibody is more exact than that obtained with HRP-conjugates. The present method is more versatile and specific than HRP methods that utilize HRP-conjugated antibodies.

Rapid regeneration of the dermal-epidermal junction after partial separation by vacuum: an electron microscopic study

A process of rapid repair of dermal-epidermal adherence, as found in experiments with interrupted suction, was investigated electron microscopically. Biopsies in different stages of the process of suction blister formation and of the repair process were studied. Results show that suction blister formation occurs by successive detachment of hemidesmosomes from the basement membrane, and that, after partial separation of the epidermis from the dermis, a rapid regeneration of the dermal-epidermal junction takes place. This regeneration process apparently consists of two steps: realignment of basal cells to the basement membrane accompanied by autophagocytosis of detached hemidesmosomes, and de novo formation of hemidesmosomes. The time required for this structural regeneration corresponds well with the speed of the functional repair of dermal-epidermal adherence measured with the technique of interrupted suction.