Microglia are perhaps the most underestimated cell type of our immune system. Not only were immunologists unaware of their capabilities until recently, but also, some neuroscientists denied their actual existence until the late 20th century. Nowadays, their presence is confirmed extensively, as demonstrated by numerous reports describing their involvement in virtually all neuropathologies. However, despite distinct approaches, their origin remains a point of controversy. Although many agree about their myeloid-monocytic ancestry, the precise progenitor cells and the differentiation mechanisms, which give rise to microglia in the different developmental stages of the CNS, are not unraveled yet. Mostly, this can be attributed to their versatile phenotype. Indeed, microglia show a high morphological plasticity, which is related to their functional state. This review about microglia aims to introduce the reader extensively into their ontogeny, cell biology, and involvement in different neuropathologies.

CAM expression was investigated immunohistochemically in tissue sections and in pure cultures of human proximal and distal tubular cells. In the fetal kidney, N-CAM immunoreactivity was detected in the non-induced and condensing metanephrogenic mesenchyme, and in all stages until the S-shaped bodies. A-CAM (N-cadherin) first appeared in the non-induced mesenchyme and remained present thereafter. Its expression became exclusively associated with the lower limb of the S-shaped bodies and the developing proximal tubule. In contrast, L-CAM (E-cadherin; uvomorulin) staining was observed in the fetal collecting duct, the upper limb of the S-shaped bodies, and the developing distal tubule. This segment-specific expression of A-CAM and L-CAM in the early developing nephron was maintained in the adult kidney: A-CAM staining was restricted to adherens junctions in the proximal tubule and thin limb, whereas L-CAM was expressed in Bowman's capsule and in all tubular segments except the proximal convoluted and straight tubule. Also after in vitro culture, A-CAM expression was an exclusive property of proximal tubular cells, while L-CAM was confined to distal tubular cells. In conclusion, each major subdivision of the fetal and adult nephron displays a characteristic combination of L-CAM and A-CAM, suggesting that they may be the basis of segmental differentiation and border formation between adjacent nephron segments.

We report on the use of several proximal tubular cell (PTC) surface markets and corresponding antibodies in fluorescence-activated cell sorting (FACS), and their ability to identify and flow sort cells of defined proximal tubular origin (S1S2S3) or of defined proximal subsegmental origin (S1S2 only/S3 only). We tested monoclonal/polyclonal antibodies directed against five different surface peptidases [leucine aminopeptidase (LAP), neutral endopeptidase 24.11 (NEP), dipeptidyl peptidase IV (DPPIV), aminopeptidase A (APA) and gamma-glutamyl transferase (gamma-GT)], the S3 segment-specific marker intestinal type alkaline phosphatase (iAP) and an S1S2 marker (TN20-antigen), originally proposed as a surface marker for interstitial fibroblasts. Segmental (proximal tubular vs. distal tubular) and proximal subsegmental (S1S2 vs. S3) expression of all five surface peptidases and TN20 antigen were first assessed by comparing immunohistochemical staining on normal human kidney tissue with staining for well-known segment-specific differentiation markers (intestinal type alkaline phosphatase, Tamm-Horsfall protein) on adjacent sections. All five peptidases were found to be expressed to a certain degree in all subsegments (S1 S2 and S3) of the proximal nephron, whereas expression was never seen in the more distal parts of the nephron. Flow cytometry was performed on cells obtained following gradient purification of collagenase-digested human renal tissue. Labeling cells for expression of LAP, NEP or DPPIV resulted in high yields of specifically labeled PTC (S1S2S3 origin). Labeling with anti-LAP resulted in the clearest distinction between positive and negative cell subpopulations, and therefore LAP was considered the best PTC marker for use in FACS. iAP histochemical staining on sorted cells showed that flow sorting with monoclonal antibody (moAb) 250 (anti-intestinal type alkaline phosphatase) allowed sorting of S3 cells with > 90% purity. Likewise, moAb TN20 enabled us to obtain a highly purified S1S2 population as confirmed by the absence of iAP on sorted cells.

The recovery from gentamicin-induced phospholipidosis in the rat kidney cortex was characterized both morphologically and biochemically after a single 12-hr drug infusion. Total dosages administered were 10, 60, or 140 mg/kg, achieving constant serum concentrations of 3, 11, and 27 micrograms/ml, respectively. At the end of the 12-hr infusion, the cortical drug concentrations corresponding to the three dosages were 124, 450, and 993 micrograms/g of wet tissue. At the low dose (10 mg/kg), myeloid bodies were seen inside lysosomes of proximal tubular cells, along with a modest decrease of lysosomal sphingomyelinase activity. The cortical drug level declined steadily following first-order kinetics along with a disappearance of myeloid bodies and return of sphingomyelinase activity to control levels. At the high dose (140 mg/kg), we observed a sustained loss of sphingomyelinase activity (37% of controls), a subsequent increase of phospholipid concentration in the kidney cortex (up to 117% of controls 2 days after) and a prominent accumulation of myeloid bodies inside the lysosomes of proximal tubular cells (up to 4% of cell volume). Tubular regeneration and interstitial infiltration became detectable by histology and the increase of DNA synthesis as from day 1, along with an apparent reduction of the phospholipidosis at days 3 and 4. Drug cortical concentrations showed a sharp decline 2 days after infusion. An intermediate behavior was observed at 60 mg/kg. It is concluded that the proximal tubular cells behave in a fundamentally different way after gentamicin loading with low and high doses. At the low dose there is a regression of the drug-induced changes in the absence of any sign of necrosis-regeneration. Above a threshold in cortical drug concentration there is further development of the alterations leading to cell death-regeneration.