Light and electron microscopic study of omental milky spots in New Zealand Black mice, with special reference to the extramedullary hematopoiesis

Abstract

Omental milky spots are especially large and numerous in New Zealand Black (NZB) mice, which are known to develop spontaneous autoimmune diseases. We investigated omental milky spots in NZB mice by light and electron microscopy. The milky spots were composed of abundant lymphocytes/plasma cells with macrophages, neutrophils, eosinophils, megakaryocytes, and various stromal cells. In addition, clustered neutrophils in various maturation stages with occasional mitotic figures were frequently present in the milky spots: apparent neutrophilic myelopoiesis was present. The presence of megakaryocytes was sporadic. Considering the giant size of megakaryocytes, their direct migration into the milky spots from the bone marrow or spleen seems improbable. Thus, the presence of megakaryocytes was interpreted as probable megakaryopoiesis. Erythroblasts were not contained in the milky spots. These findings seem to indicate that the milky spots in NZB mice represent a special type of lymphoid tissue with active neutrophilic myelopoiesis and probable megakaryopoiesis. Reticulum cells in the milky spots in NZB mice had well-developed dense bodies consisting of clustered parallel tubules that showed a hexagonal array. However, the biological significance of these cells remains unknown.

Myelopoiesis in the omentum of normal mice and during abdominal inflammatory processes

“…Omentum has been shown to produce several myeloid lineages, and it is able to respond to specific local demands (Cranshaw and Leak 1990;Hirai et al 1992;Takemori et al 1994). Milky spots are considered to be the source of peritoneal macrophages under normal conditions, and in regeneration after macrophage depletion (Biewenga et al 1995;Zhu et al 1997).…”

Leukocyte antiadhesive actions of annexin 1: ALXR- and FPR-related anti-inflammatory mechanisms

“…Besides cell rolling, adhesion, and transendothelial passage (diapedesis), an extravasating leukocyte has to move in the subendothelial matrix tissue prior to emigration through the mesothelium; finally, cell detachment from the connective tissue into the peritoneal cavity is required. In addition, white blood cell movement not from the mesentery but through the omentum (or milky spots) 35,36 seems to be the most common path taken by blood-borne granulocytes during peritonitis. For all these reasons we focused the present study on intravital microscopy of the mouse mesentery to identify the receptor mechanism(s) mediating the inhibitory effect of peptide Ac2-26 and, by analogy, full-length annexin 1 on the initial stages of the leukocyte extravasation process.…”

Fat-Associated Lymphoid Clusters in Inflammation and Immunity

“…Interestingly, some authors have divided MS in two categories: vascularized and non-vascularized MS. Vascularized MS are supplied with blood vessels and are found in the omental fat region. On the other hand, non-vascularized MS do not have blood supply and are located in the omental membrane (45). Whether these features that translate into functional differences remain to be investigated.…”