Ontogeny of the inflammatory response in the fetal rat.

Fetal rats of 16, 17 and 18 days gestation (dg) were injected with turpentine or Streptococcus faecalis in 1% India ink. Specimens of peritoneal tissues and exudates were collected at 24, 48 and 96 h after injection. Inflammation was present in some of the youngest animals (16 dg) 24 h after injection with bacteria. The response consisted of infiltrates of polymorphonuclear (PMN) and mononuclear (MN) cells in tissues adjacent to the peritoneum. Inflammatory cells were also found within the peritoneal cavity. Phagocytosis of ink particles and bacteria was noted in these cells and in cells of the serosa and adjacent mesenchyme. Free peritoneal cells were not seen in littermate controls. Phagocytosis was also noted in PMNs and MNs in the exudates collected at 48 h. A more marked granulocytic response was seen in animals sacrificed 24 h following bacteria-ink injection at 17 dg. Inflammation was more extensive and consistent by 19 dg. The reaction to turpentine was associated with the variable degree of tissue injury noted in histologic sections. The gestational age at time of injection was a more important determinant of the magnitude and character of response than was the duration of the lesion.