Figure 4

Sequential bone marrow histology of control calf X (upper row) and challenge calf W (lower row) at days 1, 6 and 11. Examples of erythroid clusters are indicated by blue arrows and examples of megakaryocytes by black arrowheads. Left column: Biopsy cores at day 1 from challenge calf (W) and control calf (X) contain similar distributions of erythroid clusters and megakaryocytes. Centre column: By day 6, the numbers of haematopoietic cells (HPC) including megakaryocytes has markedly decreased in the challenge calf, whereas there is an increased density of HPC in the control calf compared with the earlier time point. Erythroid clusters in the challenge calf are composed mainly of cells in the later stages of differentiation. Right column: At necropsy on day 11, trilineage hypoplasia with absence of all HPC clusters is found in the challenge calf whereas there is no change in the control compared with day 6: Bar = 100 μm (applies to all images).