Neutrophil aggregation during migration in vivo.

Stacks of six membrane filters (8-mu pore size) were placed over the mouths of plastic tubes. The tubes were filled with casein (test) or NaCl (control) solution and implanted subcutaneously in white rats; each animal received one tube with casein and one control tube. After 12, 18, 24, and 36 h, the tubes were removed and the filters stained and examined microscopically. Immigrant neutrophils were found either individually or in spherical aggregations within the filter meshwork. Crucial factors in the formation of aggregates included the frequency of individual cells, the chemotactic milieu (casein or NaCl), the duration of exposure, and the location within the filter stacks. The size of the aggregates depended on the duration of exposure. The phenomenon of "neutrophil aggregation" is thought to participate in the formation of the granulocyte wall around a focus of inflammation.