Ontogeny of the chick embryo spleen--a cytological study.

The differentiation of the chick embryo spleen was investigated from E4 till after hatching, using a number of cytological markers. The reticular fiber network, disclosed by silver staining, was a loose array of short fibers until E15. It developed into a complex pattern closely related to the specialized segments of the vascular tree between E15 and E18. While the very first cells belonging to the hemopoietic lineage could be identified as early as E4, steady state hemopoiesis set in by E10. Of the two cell lineages maturing in the organ, erythroid cells disappeared by E15, and granulocytes emigrated within 3 to 4 days around hatching. The process of lymphocyte homing, detected by specific markers, began just before hatching and was completed within the first week of life. Thus differentiation of the stromal anatomy appears as the prerequisite for the transition of embryonic (= hemopoietic) to adult (= secondary lymphoid) functions.