Developmental expression of neuronal and endocrine markers in the parathyroid glands of the rat

The parathyroid glands of the adult rat harbor a number of neuroendocrine markers, biologically active peptides and ”classical” neuromessengers in addition to parathyroid hormone (PTH). Their appearance during parathyroid development is, however, not known. In the present study we have examined several neuroendocrine markers and neuromessengers in the parathyroid glands of the developing rat [embryonic stage 21(E21), newborn, 1, 2, 3, 4 week old, and adult rats] using immunocytochemistry. Chromogranin A- and PTH-mRNA were also examined by in situ hybridization and the mRNA levels were quantitated by computerized image analysis. Protein gene product 9.5- and synaptophysin-containing nerve fibers appeared already before birth and then gradually increased in number postnatally, and at the age of 4 weeks the nerve fibers were moderate in number to numerous. Nerve fibers containing calcitonin gene-related peptide, neuropeptide Y and vasoactive intestinal polypeptide also increased gradually in number, while galanin- , substance P- and tyrosine hydroxylase-containing fibers remained few throughout development. The glandular cells expressed chromogranin A, pancreastatin and PTH already before birth. The levels of chromogranin A- and PTH-mRNA were low at E21 and increased markedly at birth; chromogranin A mRNA levels had increased even more at 1 week postnatally. Three to 4 weeks after birth the levels of PTH- and chromogranin A mRNA again increased, then stabilized at a slightly lower level in the adult rat. Our findings demonstrate that the parathyroid glands of rat are already innervated and express PTH and chromogranin A before birth and that the density of peptide-containing nerve fibers changes during development. The stepwise increases of PTH- and chromogranin A mRNAs during development indicate marked changes in parathyroid activity occurring at birth and at weaning.