Lymphopoiesis - The Process of Lymphopoiesis

The Process of Lymphopoiesis

Lymphopoiesis can be viewed in a mathematical sense as a recursive process of cell division and also as a process of differentiation, measured by changes to the properties of cells.

• Given that lymphocytes arise from specific types of limited stem cells - which we can call P (for Progenitor) cells - such cells can divide in several ways. These are general principles of limited stem cells.

Considering the P as the ‘mother’ cell, but not a true stem cell, it may divide into two new cells, which are themselves identical, but differ to some degree from the mother. Or the mother cell P may divide unequally into two new daughter cells both of which differ from each other and also from the mother.

Any daughter cell will usually have new specialized abilities and if it is able to divide it will form a new sub-lineage. The difference of a daughter cell from the mother may be great, but it could also be much less, even subtle. What the P mother cell does not do is divide into two new P mother cells or a mother and a daughter; this is a matter of observation as such limited progenitor cells are known to not self-renew.

• There is a sort of exception when daughter cells at some level of the lineage may divide several times to form more seemingly identical cells, but then further differentiation and division will inevitably occur, until a final stage is reached in which no further division can occur and the cell type lineage is finally mature. An example of maturity is a plasma cell, from the B cell lineage, which produces copious antibody, but cannot divide and eventually dies after a few days or weeks.

• The progenitor CLP of the mouse or the progenitor MLP of the human differentiates into lymphocytes by first becoming a lymphoblast (Medical Immunology, p. 10). It then divides several more times to become a prolymphocyte that has specific cell-surface markers unique to either a (1) T cell or (2) B cell. The progenitor can also differentiate into (3) natural killer cells (NK) and (4) dendritic cells.

• T Cells, B Cells and NK Cells are unique to the lymphocyte family, but dendritic cells are not. Dendritic cells of identical appearance but different markers are spread throughout the body, and come from either lymphoid and myeloid lineages, but these cells may have somewhat different tasks and may take up lodging preferentially in different locations. (Revise in light of new research) This is now an open question; also, the different dendritic cell lineages may have different ‘tasks’ and stay in different ‘locations.’

T and B lymphocytes are indistinguishable histologically（that is, under a light microscope they cannot be told apart.）Indeed, the inactive B and T cells are so featureless with few cytoplasmic organelles and mostly inactive chromatin that until the 1960s textbooks could describe these cells, now the central focus of immunology, as having no known function!!

However T and B lymphocytes are very distinct cell lineages and they ‘grow up’ in different places in the body. They perform quite different (although co-operative) functions in the body. No evidence has ever been found that T and B cells can ever interconvert. T and B cells are biochemically distinct and this is reflected in the differing markers and receptors they possess on their cell surfaces. This seems to be true in all vertebrates, although there are many differences in the details between the species.

• Regardless of whether the CLP (mouse) or MLP or a small closely related set of progenitor cells take credit for generating the profusion of lymphocytes, it remains an interesting observation that the same lymphoid progenitors can still opt to generate some cells that are clearly identifiably myeloid.