Tissue distribution of the lipocalin alpha-1 microglobulin in the developing human fetus

Abstract

Alpha-1 microglobulin (α1m), a lipocalin, is an evolutionarily conserved immunomodulatory plasma protein. In all species studied, α1m is synthesized by hepatocytes and catabolized in the renal proximal tubular cells. α1m deficiency has not been reported in any species, suggesting that its absence is lethal and indicating an important physiological role for this protein to clarify its functional role, tissue distribution studies are crucial. Such studies in humans have been restricted largely to adult fresh/frozen tissue. Formalin-fixed, paraffin-embedded multi-organ block tissue from aborted fetuses (gestational age range 7-22 weeks) was immunohistochemically examined for α1m reactivity. Moderate to strong reactivity was seen at all ages in hepatocytes, renal proximal tubule cells, and a subset of pancreatic islet cells. Muscle (cardiac, skeletal, or smooth), adrenal cortex, a scattered subset of intestinal mucosal cells, tips of small intestinal villi, and Leydig cells showed weaker and/or variable levels of reactivity. Connective tissue stained with variable location and intensity. The following cells/sites were consistently negative: thymus, spleen, hematopoietic cells, lung parenchyma, glomeruli, exocrine pancreas, epidermis, cartilage/bone, ovary, seminiferous tubules, epididymis, thyroid, and parathyroid. The results underscore the dominant role of liver and kidney in fetal α1m metabolism and provide a framework for understanding the functional role of this immunoregulatory protein.