Consecutive appearance of coagulation factor XIII subunit A in macrophages, megakaryocytes, and liver cells during early human development.

Thirty embryonic and fetal samples were investigated to study the appearance and characteristics of factor XIII subunit A (FXIIIA)-containing cells in the course of human development. Samples were either vacuum-embedded in paraffin for staining FXIIIA by a sensitive biotin-streptavidin system or snap-frozen for double-labeling studies to characterize FXIIIA-containing cells. FXIIIA appeared as early as the fifth gestational week in yolk sac samples in stellate-shaped cells. Nonparenchymal cells in liver samples showed intense labeling for FXIIIA from the sixth week of gestation. The relative amount of FXIIIA-containing cells rapidly diminished up to the 13th gestational week. When characterized, the majority of these cells proved to be KiM7-positive macrophages, while GPIb (CD42b)-labeled cells accounted for less than 10% of FXIIIA-positive cells. Liver cells did not show any staining for FXIIIA in first trimester samples. The earliest liver specimen showing FXIIIA was at the 20th week, when FXIIIA appeared in some liver cells, particularly in those surrounding the central veins. In bone marrow smears, FXIIIA-positive cells started to appear at week 10 in the clavicles and increased in number in subsequent stages of development. Intracellular FXIIIA was distributed between GPIb-, RFD7-, and KiM7-positive cells. The results indicate that, apart from liver cells, at least three different cell populations (KiM7+ RFD7+ GPIb-, KiM7- RFD7- GPIb-, and KiM7- RFD7- GPIb+) contain FXIIIA in the early phase of human development. We conclude that FXIIIA appears very early during human development and is detectable in both extra- and intraembryonic hematopoietic organs. Intracellular FXIIIA in early human development is distributed between different macrophages and megakaryocytes, and by week 20, it appears in liver cells as well.