Histological analysis of the pigeon crop sac. (A) Junction of the non-'lactating' and 'lactating' walls of the lateral lobe of the pigeon crop. There is evidence of early hyperplasia in the non-'lactating' region (black arrowhead) with the formation of small rete pegs. These rete pegs are more pronounced as the hyperplasia becomes more obvious (black arrows), and proliferation of the germinal layer into the epithelium is also apparent (white arrows). *: lamina propria. Haematoxylin and eosin. (B) Same as A but stained with Masson's trichrome. The vascularized lamina propria is stained green. (C) 'Lactating' epithelium of the lateral lobe. There is extensive proliferation and folding of the basal layer of the epithelium. Branches of lamina propria extend into the epithelium (white arrows), and become progressively more attenuated (thick black arrows). These vascularized branches are narrowest near the luminal surface of the crop (thin black arrows). Pigeon 'milk' rice-shaped pellets, composed of lipid-laden, parakeratinised cells, are embedded closely in the epithelium (black arrowheads). Haematoxylin and eosin. (D) Same as C, but stained with Masson's trichrome. The green branches of the lamina propria are seen more readily with this stain. (E) High magnification of the epithelial wall of the lateral lobe of the lactating crop. Small vascularized branches of the lamina propria (thick black arrows) originate from a major branch (white arrow). One, two or three layers of slightly basophilic basal cells line these branches (thin black arrows), but, more removed from the blood supply, the cells have an eosinophilic, ground-glass appearance (black arrowhead). Haematoxylin and eosin. (F) Same as E, but stained with Masson's trichrome. The vascularized branches of the lamina propria can be seen quite clearly, as can the regions of protein-rich cells. (G) Lactating epithelium stained with Oil Red O (X5). Lipid can be seen consistently throughout the epithelium and is not present in the lamina propria or basement membrane. (H) Magnified view (X20) of lipid droplets in the epithelia of 'lactating' pigeon crop.