Abstract
In this study we investigated the developmental expression of histidine decarboxylase (HDC) mRNA and the distribution of histamine-immunoreactive (histamine-ir) cells in the rat embryonic tissues. We applied Northern blot analysis, in situ hybridization with synthetic oligonucleotide probes complementary to the rat HDC cDNA, and indirect histamine immunocytochemistry. Northern blot analysis revealed the appearance of a major (2.6 KB) HDC mRNA species in liver on embryonic Day 14. Its hybridization level peaked on Day E18, when two minor (1.6 and 3.5 KB) mRNA species were also present. During the periparturition period, a rapid decrease in HDC RNA was apparent, as the 2.6 KB mRNA species was expressed at a low level on postnatal Day P1. The embryonic liver expressed HDC on days E14-E20. On days E18 and E20, the periosteum and the epiphyseal growth plates of the endochondrally ossificating bones, and some striated muscle cells, showed hybridization signal for HDC. Histamine immunoreactivity was detected in many epithelial and neuronal cell types during embryogenesis. An intense histamine immunoreaction appeared first in essentially all cells of the liver parenchyma on day E12. This parenchymal histamine immunoreactivity disappeared by birth, after which this immunofluorescence in liver was restricted to a few scattered mast cells until adulthood. Some neurons in the peripheral sensory, sympathetic and cranial nerve ganglia were histamine-immunoreactive from day E16 to birth. In addition, many immunoreactive nerve fibers were detected in the gastrointestinal muscularis externa, mesentery, salivary glands, kidney, lung, and muscle tissue. We conclude that during rat embryogenesis histamine is produced and stored transiently by cells in liver, developing bone, and a few striated muscle cells, in addition to previously reported neurons in rat brain. Many peripheral neurons, epithelial cells, and mast cells display histamine immunoreactivity during rat embryogenesis but are devoid of detectable HDC mRNA with the current method. It remains possible that histamine is formed by another enzyme or is taken up from the extracellular space. The results support the concept that a significant proportion of histamine is formed and stored by embryonic cells other than mast cells.