Turnover of accessory cells

The dendritic cells found in T cell areas of secondary lymphoid tissues are termed interdigitating cells. These cells are continually renewed by the influx into lymph nodes of veiled cells via the afferent lymph. [3H]TdR labeling suggests they are replaced from dividing precursors (in the bone marrow) at a rate of about 10% per day. This means that the average lifespan is around 5 days and the whole pool is exchanged within 3 weeks.

Dendritic cells found in B cell follicles are termed follicular dendritic cells (FDC). These cells appear to be extremely long-lived as befits their role in long-term antigen retention. No thymidine uptake has been noted in these cells following long-term infusion. Bone marrow chimeras made between H-2 disparate mice show no replacement of the host FDC population with donor FDC one year after construction, suggesting that FDC are not bone marrow derived and are not of the myeloid lineage (as interdigitating cells are).

The precursors of macrophages, monocytes, are produced in large numbers from the bone marrow. The transit time of these cells in the circulation is between 1 and 2 days, after which time they extrava-sate. Tissue macrophages do not seem to divide except at sites of inflammation and can persist for long periods (50-70 days). However, there are many different populations of tissue macrophages which may have differing lifespans. For instance, splenic red pulp macrophages turnover completely in bone marrow chimeras within one month, whereas macrophages in the adjacent marginal zone are replaced much more slowly (25% in one month).

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