Development of hematological and immunological characteristics in neonatal rats

- Differential blood counts are still dominated by precursor cells.
- Spleen cell proliferation was low with mitogens showing stimulatory effects.
- BAL displayed increasing cell numbers, with cell composition similar to adults.
- Immune phenotyping showed pronounced maturational processes.
- Thymus architecture displayed lymphoblast migration and epithelial structure appearance.

As major immunological and hematological parameters evolve during the early period of life, laboratory data must be interpreted in relation to developmental changes. Wistar (WU) rats were sacrificed on PND2, 4, 7, 10, 14, 17 and 21. Peripheral blood, bone marrow, thymus samples and spleen cells were collected and a bronchoalveolar lavage (BAL) performed. Parameters of blood counts changed considerably between time points. IgM and IgG levels steadily increased. Spontaneous spleen cell proliferation was low before PND21, although mitogens had stimulatory effects above baseline. In the spleen, T-lymphocyte counts tripled by PND17 (mainly attributed to CD8+ cytotoxic T-cells and CD4+ T-helper cells). In peripheral blood an increase in B-lymphocytes to about 60% of the cell number was observed. In BAL fluid, macrophages represented 95-98% of the cells. In thymus architecture, lymphoblast migration was seen and epithelial structures appeared. The data presented will help to distinguish between maturational changes and treatment-related effects.