Sample Preparation and Fixation for Histology and Pathology

The basic goal of all histological techniques is the identification of tissue components, both normal and pathologic. In the case of any immunodetection technique we expect: specific detection of the antigen with a mark strong enough to be easily detected; and a clear preservation of histological and cytological details of the environment. To achieve these expectations the technical procedures employed must be clearly understood so we can control the previous methodological steps, avoiding any failure and/or complication that could seriously affect the results of our study. In this chapter, we review the basic concepts related to representability of the sample, fixation process, and all procedures carried out before performing the immunohistochemical technique on paraffin sections, which constitute what we know as sample preparation. We describe and analyze the key events so the pathologist and/or the researcher can control the variables in order to obtain the best results in an immunodetection procedure.

Non-specific esterase-positive dendritic cells in epithelia of the frog Rana pipiens

Langerhans cells are antigen-presenting cells located in epithelia and have a dendritic outline, a convoluted nucleus surrounded by an electron lucent cytoplasm with sparse organelles and occasionally containing the characteristic Birbeck granule; their membrane contains class II molecules of the major histocompatibility complex and a strong membrane reactivity for both ATPase and non-specific esterase. Despite increasing knowledge about mammalian Langerhans cells, only a few studies have examined the possible occurrence of Langerhans-like cells in lower vertebrates. Our group has previously demonstrated the presence of dendritic cells in different epithelial membranes co-expressing a strong membrane ATPase reactivity and class II molecules of the major histocompatibility complex in the frog Rana pipiens. Adding another criterion in the characterization of Langerhans-like cells in amphibians, we now report evidence for the expression of membrane non-specific esterase reactivity in dendritic cells located in the epidermis, nictitant membrane and cornea with topographical and light and electron microscopical characteristics identical to those previously described for dendritic cells positive for ATPase and major histocompatibility complex class II in Rana pipiens. We postulate that, taking all this data together, these dendritic intraepithelial cells constitute the amphibian counterpart of mammalian Langerhans cells.

ATPase and MHC class II molecules co-expression in Rana pipiens dendritic cells

Mammalian Langerhans cells are antigen-presenting cells located in different epithelia. These cells have a characteristic ultrastructural pattern, present a plasmatic membrane ATPase activity and constitutively express class II molecules of the major histocompatibility complex. ATPase-positive dendritic cells that are morphologically similar to Langerhans cells have also been found in amphibian epidermis. In order to demonstrate that ATPase-positive dendritic cells of amphibian epidermis express class II molecules and are present in other stratified epithelia, histochemical and immunohistochemical as well as ultrastructural analysis were performed. ATPase-positive dendritic cells and class II-positive dendritic cells were observed in epidermis, nictitant membrane and cornea. In epidermis the number of ATPase-positive dendritic cells was 656+/-186/mm2 while class II-positive dendritic cells was 119+/-45/mm2. Some ATPase-positive dendritic cells showed co-expression of class II molecules. These results suggest the existence of dendritic cell subsets in amphibians as is clearly demonstrated in mammals.