Figure 1. Cross-section of mouse femoral bone marrow. CAP, capillary; EB, erythroblasts; G, granulocyte precursor; L, lymphocytes; MK, megakaryocyte. H & E; original magnification ×200.

Figure 2. Paracrine and juxtacrine regulation of bone marrow function. GF, growth factor; N, nucleus.

Figure 3. Haematopoiesis in mice. Long-term repopulating haematopoietic stem cells belong to a population of cells which bears none of the markers present on mature blood cells (lineage negative) and is characterized by the following markers Thy 1.1^lo, Sca1⁺, Kit⁺, Rho^–/lo; about 10 percent of cells in such a population will, when injected into lethally irradiated mice, continue to produce mature multilineage progeny for at least 11 months. The turnover time of the population containing long-term repopulating stem cells, as inferred from labelling studies with bromodeoxyuridine or the proportion of cells in S/G₂/M phases of the cell cycle, is about 20 days. Short-term repopulating stem cells belong to a population bearing, in addition to the markers indicated for long-term repopulating stem cells, Mac-1^lo and CD4^–/lo. When injected into irradiated mice, cells of this type are able to produce multilineage descendants for 8–16 weeks. The population containing short-term stem cells has a turnover time of 5–2 days, as judged by the proportion of cells in S/G₂/M, depending on whether they are CD4^– or CD4^lo. Common lymphocyte progenitors are Li^–, IL7Ra⁺, Thy1.1^–, Sca1^lo*, Kit⁺, S+G₂+M = 22%. (*Sca1^lo means Sca1 fluorescence less than that of long-term repopulating stem cells.) T, T cell; B, B cell; NK, natural killer cell; G, granulocyte; M, monocyte; E, erythrocyte; MK, megakaryocyte; Ma, macrophage.

Figure 4. B lymphopoiesis in mice. In this figure and Figures 5 and 6, kinetic analysis of [ ³H]TdR-labelled cells has been carried out following the rules governing concatenated cell compartments in steady state. Compartments of cells are represented by circles. In steady state, the number of cells in each compartment is invariant and input = output. The number of cells in a compartment, n₂, equals the input of cells into the compartment multiplied by either the cycle time (T_c) for proliferating cells, or the transit time (T_t) for nonproliferating cells. The input into n₂ is consequently equal to either n₂/T_c or n₂/T_t. For proliferating compartments in which there is no cell loss the input into compartments, I_n2 = o₁ × 2; when there is loss I_n2 = o₁ × 2(1–Lf) where Lf = loss factor, fraction of the output which is lost and does not proceed into the next compartment. For nonproliferating compartments I_n2 = o₁, or I_n2 = o₁(1–Lf).

Blue triangle, B-220 marker; purple circle, CD19 marker; Ig, immunoglobulin; H, heavy; L, light; C, cytoplasmic chain; Y, surface IgM; TdT, terminal deoxynucleotide transferase; G, immunoglobulin genes in germline configuration; R, rearranged genes; E, early; I, intermediate.

Figure 5. Granulocytopoiesis in humans. Cell cycle times (T_c) are estimated on the basis of published labelling indices and measured duration of S phase (11 hours as estimated from the [ ³H]TdR by fraction-labelled mitosis curve analysis). It is assumed that myelocytes go through two cycles, as the cycle time of 1.4 days and the published transit time (T_t) of 2.9 days, do not agree. The input into promyelocytes is estimated to be equal to the total number of promyelocytes divided by promyelocyte T_c. Cell loss is postulated to occur in the actively dividing myelocyte and promyelocyte compartments, as there appears to be no amplification in the system. The total cell contents and transit times in the nondividing compartments show that no loss occurs in these compartments.

Figure 6. Erythropoiesis in humans. Haemoglobin (Hb) content was measured in cells in mitosis; cycle time from [ ³H]TdR labelling studies. Hb, increase in Hb content per cell cycle; T_c, cycle time; T_t, transit time.