Partitioning of bone marrow into stem cell regulatory domains

Ontogenic emergence of the hematon, a morphogenetic stromal unit that supports multipotential hematopoietic progenitors in mouse bone marrow

Development of the full repertoire of hematopoietic-lymphopoietic cells from a single stem cell requires specific contacts with stromal cells. The spatio-temporal organization of these cell associations in the bone marrow in ontogeny is, however, not well understood. In the adult, 10% of marrow cells form a cohort of compact aggregates, the hematon. In the hematon mesenchymal cells (Stro-1+), perivascular lipocytes (desmin+), endothelial cells (CD34+, Flk-1+, Sca-1+), and macrophages amalgamate with the hematopoietic progenitors long-term culture-initiating cells (LTC-IC), cobblestone area–forming cell (CAFC), high-proliferative-potential colony-forming unit (HPP-CFU), granulocyte-macrophage (GM)–CFU, and burst-forming unit–erythroid (BFU-E). During endochondral ossification of the femur, GM-CFU and day 7 CAFC numbers increased progressively from day 17 of gestation, but primitive, day 35 LTC-IC appeared from postnatal day 2. Unexpectedly, bone marrow (BM) taken between embryonic day 17 and day 5 was unable to support myeloid cell production in long-term cultures or to support day 35 LTC-IC growth. However, a gain in stromal cell competence occurred between days 7 and 10, which was correlated with the emergence of hematon in the BM. Thus, acquisition of hematopoietic competence by BM lags behind for approximately 10 days after the initial hematopoietic cell influx. In the adult, the hematon fraction was 3.7-fold enriched in day 35 LTC-IC over the buffy coat. It produced more GM-CFU and HPP-CFU in myeloid culture and more B cells in lymphopoietic “switch” cultures. It is reported that stromal hematopoietic units named hematons are specific morphogenetic structures that emerge at a well-defined postnatal stage of development in long bones, delineate discrete territories for hematopoietic stem cell seeding and development, embody the most productive hematogenous compartment in the BM, and probably enclose a morphogenetic organizer.

Ontogenic emergence of the hematon, a morphogenetic stromal unit that supports multipotential hematopoietic progenitors in mouse bone marrow

Development of the full repertoire of hematopoietic-lymphopoietic cells from a single stem cell requires specific contacts with stromal cells. The spatio-temporal organization of these cell associations in the bone marrow in ontogeny is, however, not well understood. In the adult, 10% of marrow cells form a cohort of compact aggregates, the hematon. In the hematon mesenchymal cells (Stro-1+), perivascular lipocytes (desmin+), endothelial cells (CD34+, Flk-1+, Sca-1+), and macrophages amalgamate with the hematopoietic progenitors long-term culture-initiating cells (LTC-IC), cobblestone area–forming cell (CAFC), high-proliferative-potential colony-forming unit (HPP-CFU), granulocyte-macrophage (GM)–CFU, and burst-forming unit–erythroid (BFU-E). During endochondral ossification of the femur, GM-CFU and day 7 CAFC numbers increased progressively from day 17 of gestation, but primitive, day 35 LTC-IC appeared from postnatal day 2. Unexpectedly, bone marrow (BM) taken between embryonic day 17 and day 5 was unable to support myeloid cell production in long-term cultures or to support day 35 LTC-IC growth. However, a gain in stromal cell competence occurred between days 7 and 10, which was correlated with the emergence of hematon in the BM. Thus, acquisition of hematopoietic competence by BM lags behind for approximately 10 days after the initial hematopoietic cell influx. In the adult, the hematon fraction was 3.7-fold enriched in day 35 LTC-IC over the buffy coat. It produced more GM-CFU and HPP-CFU in myeloid culture and more B cells in lymphopoietic “switch” cultures. It is reported that stromal hematopoietic units named hematons are specific morphogenetic structures that emerge at a well-defined postnatal stage of development in long bones, delineate discrete territories for hematopoietic stem cell seeding and development, embody the most productive hematogenous compartment in the BM, and probably enclose a morphogenetic organizer.

The architecture of bone marrow cell populations

Marrow is a loosely bound tissue in which hemopoiesis has frequently been considered to be randomly distributed. The case is presented, however, for an organized and structured marrow in which close relationships exist between hemopoietic tissue and a regulatory microenvironment. Distributions of myeloid cells in the mouse femur are described, and a dynamic picture of their movement, with differentiation and maturation from the endosteal surface of the bone to their release via the central venous sinus, is painted. It is also shown that this structure is established within three weeks of birth. By contrast, mature lymphoid cells (but not their progenitors) are uniformly distributed. Regulatory stromal elements in the marrow are also structured and their localization is found to correspond closely to the properties of the progenitor populations. Such structure has potential practical importance, particularly in the field of medical, industrial or accidental radiation exposure where bone may introduce non-uniform dose distributions in the marrow.

Development of arteries in embryonic chick bone marrow with special reference to the appearance of periarterial granulopoietic sheaths

Arterial development was studied in embryonic chick bone marrow starting on day 11 of incubation and continuing until day 18. Arteries first appear on day 11 as vessels with two components to their wall structure, namely a thin, complete endothelium and a nearly complete pericyte layer. With advancing age, three distinct layers--an endothelium, a complete muscularis, and adventitia--appear. Long, narrow endothelial cells form cellular junctions at their basal surfaces. Smooth muscle cells remain relatively undifferentiated and do not consist of more than three layers even in the largest arteries. The adventitia consists of a hypocellular zone containing fibroblasts and a collagenous extracellular matrix. Unmyelinated nerves are located at the peripheral margin of the adventitia. All of the larger arteries are surrounded by a sheath of heterophilic granulopoietic cells. These cells do not intrude into the adventitia, but basophilic or mast cells often do intrude. The stromal cells of the periarterial granulopoietic sheaths remain as either fibroblastic cells or as multilocular preadipocytes. They, unlike those stromal cells beyond the sheath, never become unilocular adipocytes. By day 18, granulopoiesis is restricted to the periarterial sheaths and to the endosteal regions of the marrow. These studies indicate that fibroblastic stromal cells and preadipocytes, but not fully developed adipocytes, support granulopoiesis in the chick marrow. The stromal cells in the periarterial sheaths either represent a subpopulation of cells that do not develop into adipocytes or represent cells whose development into adipocytes is locally inhibited.

Studies of hematopoietic stem cells spared by 5-fluorouracil

Mouse marrow cells were exposed to 5-fluorouracil (FU) either in vivo or in vitro and the effects on the hematopoietic stem cell compartment were studied. The drug was highly toxic to bone marrow cells including the spleen colony-forming unit (CFU-S) population. The small population of stem cells surviving FU, however, caused a different pattern of spleen colony growth when injected into lethally irradiated mice. Whereas numbers of spleen colonies caused by normal marrow cells remained constant during an 8-14 d period after transplantation, spleen colonies derived from FU-treated marrow cells increased by as much as 100-fold during this time. This effect on stem cells was dose dependent both in vitro and in vivo. When FU was given in vivo, the day 14/day 8 ratio of colonies was greatest 1 d after injection and, over the next 7 d, returned to a near-normal value, that is, unity. A number of studies have shown that the stem cell compartment is heterogeneous with respect to self-replicative capacity and developmental potential. An age structure for the stem cell compartment has been proposed wherein cells with a short mitotic history are more likely to self-replicate than they are to differentiate; hence they are more primitive. 'Older' stem cells with a longer mitotic history are, according to the hypothesis, more likely to differentiate. 5-fluorouracil may be toxic to the older stem cells and selectively spare the more primitive subpopulation. Although the surviving cells may not themselves be able to form spleen colonies, they may give rise to an older cohort of cells more likely to differentiate and form spleen colonies. It is the requisite developmental maturation within the stem cell compartment that may be responsible for the delay in appearance of spleen colonies derived from FU-treated marrow. Our results support this explanation and identify the locus of at least part of this activity as the bone marrow. We found that the FU-treated marrow did not cause an increase in spleen colony numbers between 8 and 14 d in hosts with a long-standing marrow aplasia, due to the incorporation of 89Sr into bone. I propose that the delayed spleen colony appearance in normal hosts is the result of developmental maturation of the primitive stem cell compartment that survives FU and is responsible for spleen colonies arising around day 14. This maturation, at least initially, occurs in the marrow and leads to the replenishment of the more differentiated CFU-S subsets ablated by FU, which are normally responsible for spleen colonies appearing earlier after transplantation.

Decrease in hematopoietic stem cell domains as a delayed effect of x-irradiation

Although the hematopoietic integrity of locally X-irradiated sites can be restored for a time even after fairly large doses, a secondary aplasia often occurs some months later. To gain further insight into this delayed effect within the framework of the stem cell regulatory domain hypothesis, we characterized the growth kinetics of spleen colony forming units (CFU-S) in WBB6FI-+/+ bone marrow transplanted into WBB6FI-W/WV mice in which one leg had been exposed to 10-30 Gy of X rays 4-5 months previously. Compared to unirradiated contralateral marrow, fewer CFU-S either reached the previously irradiated marrow or were seeded into sites that could support growth. The initial exponential growth of effectively seeded CFU-S was unchanged, but growth deceleration (inflection point) occurred at a lower level of CFU-S in marrow previously irradiated with 20-30 Gy. This change in the inflection point indicates a radiation dose-dependent decrease consistent with the decrease in bone marrow cellularity. The decrease in effective stem cell domains after 20 Gy was calculated to be about 35%. We interpret these results to reflect the highly localized nature of delayed radiation damage to the marrow microenvironment.

Special proliferative sites are not needed for seeding and proliferation of transfused bone marrow cells in normal syngeneic mice

The widely held view that transfused bone marrow cells will not proliferate in normal mice, not exposed to irradiation or other forms of bone marrow ablation, was reinvestigated. Forty million bone marrow cells from male donors were given to female recipients on each of 5 consecutive days, 5 to 10 times the number customarily used in the past. When the recipients were examined 2-13 weeks after the last transfusion, donor cells were found to average 16-25% of total marrow cells. Similar percentages of donor cells were found when variants of the enzyme phosphoglycerate kinase determined electrophoretically were used for identification of donor and recipient cells. Evidence is presented that the proportion of donor cells is compatible with a linear dependence on the number of cells transfused over the range tested--i.e., 20-200 million bone marrow cells injected intravenously. Special proliferative sites thus do not appear to be required.