[Human fetal mast cells under development of the skin and airways]

We made histochemical and ultrastructural studies of mast cells of the skin and airways in human fetuses (7 to 40 weeks' gestation) and newborn infants. Epithelial differentiation of the skin and bronchial airways was assessed of immunohistochemical characteristics, especially on their basal cells. Histological phenotype and the characteristics to anti-total keratin antibody became identical with the phenotype in the adult by the 11th and 36th week of gestation, respectively. Interstitial fibroblastoid cells appeared oval in shape and remained relatively immature to the termination. Mast cells were first recognized by electron microscopy in the fetal skin of 18-week-old fetuses and histochemically detected in both skin and airways of 24-week-old fetuses. Mast cell counts under high power magnification (40X optical lens) light microscopy gradually increased and made a relatively dramatic rise in the early postnatal periods. The cytoplasm of mast cells showed irregular pseudopod-like elongations which gradually developed microvillous protrusions. It contained immature granules composed of varieties of morphology such as amorphous dense bodies, vesicles with dense cores, particulates and their compound forms. They were located in a corner of the cytoplasm surrounded by lamellated rough-surfaced endoplasmic reticulum. These granules became scattered widely in the cytoplasm showing distinct particulate type. Granules typical of the scroll type became dominant in the process of maturation, and the transformation of crystal-substructures occasionally took place. Prior to birth, mast cell granules fully developed as mature as those in the adult. This process of granule maturation is indicative of a reversed proceeding of slow degranulation, which is commonly seen in the so-called mucosal mast cells.