Ultrastructural response of pulmonary intravascular macrophages to exogenous oestrogen in the bovine lung: translocation of the surface-coat and enhanced cell membrane plasticity and angiogenesis

The pulmonary intravascular macrophages (PIMs) of domestic ungulates are re cognised by their specific surface coat, consisting of linearly arranged gl obules along the external leaf of the plasma membrane. The coat is sensitiv e to in vitro digestion with lipolytic lipase (LPL), intravenous heparin an d clinical exposure to halothane anaesthesia. The sensitivity to these expe rimental manipulations suggests that the globules of the coat are predomina ntly composed of lipoproteins (LDL). The present administration of oestradi ol proprionate in castrated male calves potentiated the translocation of th e surface coat into the endocytotic pathway of the PIMs. Concurrently with mobilisation of the coat, the plasma membrane was thrown into prominent arr ays of lamellipodial extensions. The sprawling macrophages made extensive a dhesive contacts with the lining endothelium of the capillaries. Consequent ly, the endothelial cells were highly attenuated and precariously maintaine d the integrity of the vascular wall. At some focal points, the vascular wa ll was penetrated by the filopodial processes of PIMs, which protruded into the perivascular space. Furthermore, there were signs of neovascularisatio n in the form of overt mitotic changes, sprouting and precursor capillary f ormation. It is conceivable that the evolving profile of angiogenesis is du e to the vascular endothelial growth factor (VEGF) paracrine function of PI Ms. Endothelial cell specificity has been considered an important advantage of VEGF for neovascularisation. It allows pleotrophic response of endothel ial cells to proliferate and to assemble into endothelial tubes.