Apoptosis and regression of embryonic mesenchyme in the development of the middle ear spaces.

Apoptosis was studied using temporal bones from three fetuses representing different times of gestation and from three neonates. Paraffin-embedded sections 20-microm thick were studied using the terminal deoxynucleotide transferase-mediated dUTP nick-end labeling method based on 3'-end-labeling of fragmented DNA. Phenotyping of the immune cells was performed using regular monoclonal antibodies. In the bone marrow the granulocyte series dominated and the number of cells in the macrophage series was noticeably fewer, with apoptotic cells occurring in both. In the embryonic mesenchyme, solitary apoptotic cells occurred in all locations in both the fetuses and neonates. Apoptosis is a basic factor in the regression of embryonal mesenchyme, but may not be preprogrammed. Basic scientific data obtained from modified tissue cultures show that mechanical forces cause cells to switch between different genetic programs. It is suggested that the act of swallowing causes periodic changes in the amniotic fluid pressure and provides the necessary force for regression of the mesenchyme by apoptosis.