Abstract Coelomic cavities are relatively isolated from the systemic circulation of blood cells. Resident cell populations have a proper phenotype and kinetics, main- taining their steady-state populations and their respon- siveness to local inflammatory reactions, in which the number and quality of coelomic cells can be greatly in- creased and modified. We have addressed the question of whether the increase in cell infiltrate in the inflamed ab- dominal cavity is sustained by the proliferation of my- eloid cells in the omentum, and if so what are the charac- teristics of the progenitor cells involved and how the omentum controls their proliferation and differentiation. In the omentum under normal conditions and with in- flammation due to schistosomal infection we found that pluripotent early myeloid progenitors were capable of giving rise to all the myeloid lineages in clonogenic as- says, but not to the totipotent blood stem cells. Besides the major haemopoietins (GM-CSF, M-CSF, G-CSF, IL-5), the omentum stroma constitutively expressed SDF-1α, the chemokine which elicits homing of circu- lating early haemopoietic progenitors. While normal omentum stroma produced LIF, its expression was sub- stituted by SCF in inflamed tissues. In the first situation a slow steady-state renewal of progenitors is potentially favoured, while their intense expansion may be predomi- nant in the latter one. We propose that the increase in cells in the abdominal cavity in inflammatory reactions is due to the enhanced input and expansion of early my- eloid progenitors sustaining the in situ production of ab- dominal cell populations, rather than to the input of sys- temic circulating inflammatory cells. Keywords Omentum · Milky spots · Mesenterium · Schistosomiasis · Myelopoiesis · Mouse (C3H/HeN) Introduction The peritoneal cavity is formed early in embryogenesis, and it remains relatively isolated from the systemic blood cell circulation throughout life. Intraperitoneal res- ident cells have their own phenotypic characteristics and proliferation patterns, which maintain their steady-state populations, but they may also circulate through the main lymphatics, and reach extraperitoneal sites under normal conditions. In inflammatory reactions of the ab- dominal cavity, rapid and extensive modulation of the peritoneal cell populations can be due either to the ex- pansion of resident peritoneal cells or to the entry of in- flammatory cells from the mesenteric blood and lym- phatic vessels (Tsilibary and Wissig 1987; Wassilev et al. 1998). Under normal conditions, peritoneal cells originate from the mesenteric tissues, mainly from the mesenteric milky spots and from omentum (Carr 1967). These coelom-specific structures have been characterised as the “coelom-associated lymphomyeloid tissue” (CALT) (Lenzi et al. 1996). Omentum is the major abdominal A publication of the Millennium Institute of Tissue Bioengineer- ing. Supported by PRONEX and CNPq grants of the Brazilian Ministry of Science and Technology, and a FAPERJ grant from the Rio de Janeiro State Government M. de F. B. Pinho Departamento de Imunobiologia, Instituto de Biologia, Universidade Federal Fluminense, Niterói, RJ, Brazil S.P. Hurtado · M.C. El-Cheikh · R. Borojevic ( ✉ ) Departamento de Histologia e Embriologia, Instituto de Ciências Biomédicas, Universidade Federal do Rio de Janeiro, Rio de Janeiro, RJ, Brazil e-mail: radovan@iq.ufrj.br Tel.: +55-21-25626484 M.I.D. Rossi Departamento de Patologia, Hospital Universitário Antônio Pedro, Universidade Federal Fluminense, Niterói, RJ, Brazil M.I.D. Rossi · H.S. Dutra · R. Borojevic Programa Avançado de Biologia Celular Aplicada à Medicina, Hospital Universitário Clementino Fraga Filho, Universidade Federal do Rio de Janeiro, Rio de Janeiro, RJ, Brazil R. Borojevic Caixa Postal 68021, 21941-970 Cidade Universitária, Rio de Janeiro, RJ, Brazil Cell Tissue Res (2002) 308:87–96 DOI 10.1007/s00441-002-0550-y REGULAR ARTICLE Maria de Fátima B. Pinho · Sandra P. Hurtado Márcia C. El-Cheikh · Maria Isabel D. Rossi Hélio S. Dutra · Radovan Borojevic Myelopoiesis in the omentum of normal mice and during abdominal inflammatory processes Received: 26 November 2001 / Accepted: 20 February 2002 / Published online: 4 April 2002 © Springer-Verlag 2002