B-lymphocyte subpopulations in the mouse. Organ distribution and ontogeny of immunoglobulin-synthesizing and of mitogen-sensitive cells.

The first fetal cells synthesizing Ig can be demonstrated at day 10 of gesta tion, well before cells with detectable surface-bound Ig have been found. These early Ig-synthesizing cells are exclusively large cells. The first small cells synthesizing Ig appear at day 15-16 of gestation, a time when cells. with detectable surface-bound Ig have been found in fetal liver. Ig synthesis of liver cells declines 2 days after birth, presumably reflecting a change in cellular composition of the liver, whereas spleen cells, on a per cell basis, synthesize Ig at a constant rate from birth to adult life. At the time that the first small Ig-synthesizing cells are detected and surface-bound Ig has been found on fetal liver cells, such small fetal liver cells can be stimulated by LPS in serum-containing media to generate 19S IgM secreting PFC. Three to four days after birth, cells develop in the small fraction of spleen which can be stimulated in serum-free media by either LPS or PPD to PFC development. Thus we can distinguish three types of B-lymphocytes as they appear sequentially during ontogeny: I. Large, Ig-producing cells, not susceptible to polyclonal activation to PFC development by either LPS or PPD. II. Small, Ig-producing cells, susceptible to LPS-stimulation in serum-containing media; and III. Small, Ig-producing cells, susceptible to LPC- and PPD-stimulation in serum-free media.