To investigate further whether removal of apoptotic cells is impaired in Ptdsr -/- mice, we stained immunohistochemically for activated caspase 3 (aCasp3) and analyzed additional organs and tissues where apoptosis plays a crucial role in tissue remodeling during development. Starting at E12.5, we analyzed and compared the number and distribution of aCasp3-positive cells in over 140 serial sections of three wild-type and six Ptdsr -/- embryos in consecutive and corresponding sections. The sagittal sections were separated by 5 μm, allowing a detailed analysis of apoptosis in several organs and tissues. Tissue restructuring by programmed cell death occurred most notably within the ventral part of the neural tube (Figure 6b,f) and in the developing paravertebral ganglia (Figure 6d,h) with many apoptotic cells being present. In these tissues Ptdsr is highly expressed at E12.5 (Figure 2c) but we observed no difference in the number or distribution of apoptotic cells in Ptdsr+/+ and Ptdsr -/- embryos. The same was true for the developing kidney: apoptotic cells were present in Ptdsr+/+ and Ptdsr -/- embryos, in limited numbers, but we failed to detect any differences in the number of apoptotic cells between the genotypes (Figure 6c,6g). Furthermore, when we continued our analysis of apoptotic cell clearance in vivo at E16.5, E17.5 and E18.5 of embryonic development as well as in neonatal mice, the number and distribution of apoptotic cells was similar in both genotypes. As already observed at E12.5, analysis of aCasp3-stained sections of the developing thymus, heart, diaphragm, genital ridge, eyes and retina convincingly showed that there was no impairment in apoptotic cell removal in Ptdsr -/- mice. Moreover, because Li and colleagues [31] reported impaired clearance of dead cells during lung development in Ptdsr-deficient mice, we examined the rate of apoptosis induction and cell clearance in our Ptdsr-knockout mice in the lung. Analysis of aCasp3-stained lung tissue from Ptdsr+/+ and Ptdsr -/- mice at E17.5 and P0 demonstrated that apoptosis was an extremely rare event during lung morphogenesis at this stage. In addition, there were no differences in the number or distribution of apoptotic cells in Ptdsr -/- and Ptdsr +/+ mice. Furthermore, we were unable to detect any evidence of tissue necrosis in lungs from Ptdsr-deficient mice. In contrast to the report of Li et al. [31], we never observed recruitment of neutrophils or other signs of pulmonary inflammation at any stage of development in our Ptdsr-deficient mice.