Cytological and ultrastructural study of the lymphoid-like and X cell populations in mouse bone marrow during the neonatal period.

During the recovery period following a sublethal irradiation, lymphoid-like cells accumulate in the bone marrow of the mouse. Among these cells, a peculiar element called 'X cell' has been previously described. Its morphological aspects are different from the precursors of the various hemopoietic lines. The 'X cell' could be an ancestral element normally repressed in the adult marrow. The aim of the present work is to determine whether this peculiar element can be detected in the bone marrow of young mice during the first month of life. The myelograms showed that on the day of birth, the marrow was almost exclusively composed of erythrocytic cells. During the postnatal period up to day 20, there was a progressive increase of granulocytic cells. The percentage of lymphoid-like cells increased from 5 per cent at birth to 28 per cent on day 20. The absolute number of these elements, calculated from the percentages obtained on the myelograms and from the number of nucleated cells per two femurs, increased from 20,000 at birth to 7,000,000 at day 30. The ultrastructural analysis demonstrated that the lymphoid-like population was composed of small lymphocytes, lymphoblasts and X cells, the proportions of which varied in relation to the age of the animals. X cells and lymphoblasts were found to be especially numerous around the 4th or 5th day, whereas the small lymphocytes were scarce.