Ontogeny of ramified CD45 cells in chicken embryo and their contribution to bursal secretory dendritic cells

Abstract

Embryonic tissues contain highly ramified stellate-shaped cells expressing CD45 and MHC II antigens but their origin and immunophenotype are unknown. Using staged avian embryos and cell-type-specific antibodies, we establish a detailed spatiotemporal ontogeny of cells that express CD45, the earliest marker of hematopoietic stem cells in the chick. CD45 immunostaining marks three distinct embryonic cell populations: round, ramified and amoeboid cells. The round and ramified CD45+ cells appear first in yolk-sac blood islands before the onset of circulation. A subpopulation of round cells co-expresses the thrombocyte-specific CD51/CD61 antigen. Amoeboid cells express macrophage-specific antigens and frequently occur in regions of apoptosis. Ramified cells are distributed uniformly in the embryonic mesenchyme, colonize lymphoid and non-lymphoid organs and later express MHC II. To study the origin of CD45+ cells, 2-day-old chick embryos were ablated from the yolk sac before the establishment of circulation and incubated for 2-5 days. Large numbers of CD45+MHC II+ ramified cells differentiated in the yolk sac. Yolk-sac chimeras were generated by grafting embryos into GFP-expressing de-embryonated yolk sacs. GFP/CD45 co-expressing ramified and amoeboid cells colonized all organ primordia in the donor embryo. We also recombined GFP+ yolk sac with the bursa of Fabricius and found ramified GFP+CD45+ cells in the bursa where they differentiated into dendritic cells. Thus, CD45 cells are first present in the yolk sac during primitive hematopoiesis and then migrate from the extra-embryonic yolk sac to give rise to cells throughout all organ primordia, including dendritic cells in the bursa of Fabricius.

Development of innate immunity in chicken embryos and newly hatched chicks: a disease control perspective

“…These cells arise from single pluripotent haematopoietic stem cells (HSCs) from the yolk sac, indicating an extra-embryonic origin of haematopoiesis (Melchers, 2010). In chicken embryos, the presence of a large number of CD2+ cells in the peripheral blood as well as CD45+ cells in the yolk sac of 2-day-old embryos prior to liver and thymus development implies yolk sac origin of immune cells (Zhou et al, 2008;Dóra et al, 2017). Furthermore, the detection of pan-leukocyte antigen (CD45) and αIIb integrin subunit (CD41) on the cells of aortic clusters (and para-aortic foci) suggests the embryo proper as the origin of definitive HSCs (Melchers, 2010).…”

Intraganglionic macrophages: a new population of cells in the enteric ganglia

“…While characterizing the distribution and ontogeny of CD45 + hematopoietic cells in the chick embryo, we noted the presence of a ramified CD45 + cell type characteristic of macrophages within the gut mesenchyme (Dóra et al. ). Similar to mammals, chicken CD45 is a transmembrane tyrosine phosphatase protein expressed by all cells of hematopoietic lineage, excluding erythrocytes.…”

In and Out of the Bursa—The Role of CXCR4 in Chicken B Cell Development

“…Follicle formation and immigration of B cells are preceded by the immigration of CD45 + /CSF1R + precursors of bursal dendritic cells into follicle buds (14,37). CD45 staining at E12 revealed that CXCR4 + cells are also positive for CD45 and hence of hematopoietic origin.…”

“…Then in a second migration step, those earliest bursal B cells have to migrate from the bursal mesenchyme into developing follicle buds. Formation of these follicle buds is triggered by immigrating precursors of bursal secretory dendritic cells (BSDC), which are of hematopoietic origin and cross the basement membrane under the surface epithelium to induce a so-called dendro-epithelial tissue (13,14). In a third migration step, B cells migrate to the follicle border, when the formerly homogeneous follicles separate into cortex and medulla.…”