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An endogenous β-galactoside-specific lectin has previously been isolated from rabbit bone marrow. The quantification of extracted lectin now indicates that approximately 75% of the lectin is intracellular in marrow. Indirect immunofluorescence studies show the extracellular lectin is associated with the erythroblast cell surface and is also found in some acellular areas of the marrow stroma. At enucleation, lectin surrounds the extruded nucleus while some residual lectin is observed in the cytoplasm of circulating reticulocytes and erythrocytes.

The tissue concentration of an endogenous β-galactoside-specific lectin is developmentally regulated in rabbit tissues during foetal and neonatal development. Immunoassay data and localization studies both indicate that the lectin is particularly associated with late embryonic and early postnatal development. No lectin could be detected in embryos prior to 21 days of gestation, whereas many tissues showed an increase in lectin around the time of birth. It is likely that in most tissues the high concentrations of lectin are largely due to increased synthesis by fibroblasts, which are particularly abundant and active in connective tissue during the extensive tissue reorganization taking place at this time. The lectin appears to be synthesized by other differentiated cell types also, notably myoblasts, alveolar cells and erythroblasts. The peak of lectin concentration seen in foetal liver probably reflects lectin associated with foetal erythropoiesis. The rabbit lectin has a low specific activity but its tissue concentration is correspondingly higher than lectin concentrations in other mammals. This conservation of total lectin activity suggests a fundamental role for the lectin dependent upon its saccharide binding activity. This and other indirect evidence suggests that these lectins are involved in the synthesis, secretion or organization of extracellular matrix components.

Using indirect immunofluorescence we have localized an endogenous P-galactoside-specific lectin in resin-embedded rabbit tissue sections. The pattern of lectin distribution correlates well with biochemical estimations of lectin levels, being abundant in intestine, lung and heart tissue and relatively less abundant in skeletal muscle, liver and kidney. In all tissues lectin is found in connective tissue associated with fibroblasts and the extracellular matrix, and at the periphery of morphologically recognizable smooth muscle cells. The lectin is abundant in skin, intestine and blood vessels, where connective tissue forms the tissue architecture. It is also abundant in heart, where it is particularly associated with the capillaries and lung, where it is also found in alveolar cells. Discrete localization of lectin occurs in areas of connective tissue where epithelial elements are differentiating, such as the crypts of Lieberkuhns in the small intestine and hair follicles in the skin. From these observations we suggest that in cells of mesenchymal origin these endogenous lectins may play a role in the elaboration or organization of the extracellular matrix that regulates tissue differentiation in a number of embryonic and adult tissues.

Soluble β-galactoside-specific lectins have been detected in many rabbit tissues and are relatively abundant in certain organs. These lectins are apparently identical to the lectin in rabbit bone marrow, which may be involved in inter-erythroblast associations during differentiation, and are very similar in structure and activity to lectins described in a number of species both in adult and embryonic tissues. The rabbit lectin is a monomer with a relative molecular mass of approximately 12000 and has a low specific activity in the haemagglutination of trypsinized rabbit erythrocytes relative to lectins from other species, which are predominantly dimeric. In addition to the major antigen two minor isoforms of the rabbit galaptin were detected in all tissues. Since these lectins are widely distributed, it seems probable that they are involved in cellular functions that are prominent during development and also maintained in certain differentiated tissues. Experimental evidence currently available suggests that these lectins may be involved in the organization of extracellular matrix components.