Figure 1. Developmental milestones in zebrafish host–microbe interactions. Schematic depiction of anatomical sites (a) and approximate durations (b) of hematopoietic activity in developing zebrafish. The eye (e), yolk (y) and gastrointestinal tract (gray) are indicated in a. Relocation of definitive hematopoietic stem cells (HSCs) between sites is represented by arrows in b. Primitive erythropoiesis occurs in the intermediate cell mass (ICM, blue) which is active 11–30 hpf [59], whereas primitive myelopoiesis begins in the rostral blood island (RBI) and later the yolk (pink) from 12 to 40 hpf [11]. HSCs appear in the aorta-gonad-mesonephros region (AGM, red) 26 hpf until 3 dpf [[15] and [16]]. These HSCs are mobilized to seed the caudal hematopoietic tissue (PBI/CHT; green) and pronephros (brown) as early as 32 hpf, and the thymus (purple) as early as 48 hpf [[14], [15], [16] and [17]]. Definitive hematopoiesis in the CHT begins de novo 24 hpf [13] and continues until at least 14 dpf [14]. Cells from the CHT contribute to the pronephros and thymus as early as 48 hpf [[14] and [15]]. B cell development initates in the pancreas (orange) starting 4 dpf [21], although the hematopoietic origins of these cells remain unknown. The thymus, pancreas, and pronephros/kidney subsequently serve as sites of definitive hematopoiesis into adult stages [[9] and [10]]. (c) Within this dynamic developmental context, important milestones relevant to zebrafish immunity and microbial interactions are indicated and referenced.