Mononuclear fraction in the blood system comprises the cells released from the bone marrow or peripheral blood through the separation from erythrocytes, platelets and granulocytes by a density gradient.





Monocytopoiesis originates from the pluripotent precursor-cell of the progenitor of myelopoiesis followed by the development into a monocyte shoot (II class), and continues with the development into a unipotent precursor-cell of the progenitor of monocytes (III class). However, the cellular elements included in the II and III classes are not differentiable morphologically. Morphological recognition begins with a blast cell of the monocyte shoot (IV class) - monoblast, which turns into a monocyte via the stage of promonocyte.


In contrast to other cell lineages whose cycle of maturation ends in the red bone marrow, the cells of a monocyte shoot finally mature only in the tissues where a promonocyte and monocyte transform into a macrophage.


Morphology of cells:


If a monoblast differentiates into a promonocyte and monocyte, the cell undergoes a series of morphological and functional changes.





Fig. 7. The lineage of the monocyte cells: A - monoblasts, B – promonocytes, C - monocytes.


In the red bone marrow


1. A monoblast has a diameter of 12-20 microns. Normally it is difficult to distinguish it from a myeloblast or a non-differentiated blast, and it is also not always possible to distinguish it from a lymphoblast. Only marked outlines of the nucleus and the broader light basophilic cytoplasm may indicate that this “blast” is developing to a monocyte cell. The nucleus of the fine structure contains 1-2 nucleoli of blue colour. The cytoplasm is blue; dust-like azurophilic granules may be present in it.


2. A promonocyte has a diameter of 15-20 microns. Normally it has a bean-shaped light violet nucleus of a promyelocyte. The chromatin is fine and wide-meshed. There are 1-2 nucleoli in the nucleus. The cytoplasm is grey-blue and smoke-grey with small azurophil granules.


A promonocyte as a precursor of a monocyte runs through two consecutive cycles of division before turning into a monocyte; the duration of the mitotic cycle is 30 hours. A promonocyte is capable of pinocytosis and phagocytosis, although to a lesser extent than a monocyte or macrophage.


3. A monocyte has a diameter of 16-18 microns. The nature and intensity of its nucleus and cytoplasm staining are morphologically various. The nuclei form may be closer to rounded and bean-shaped ones. A monocyte can be categorised as a promonocyte, given the fine structure of its nucleus and the presence of nucleoli (or their remnants). The cytoplasm is grey or pale blue in colour; it may contain numerous dust-like azurophilic granules.


Differentiation of a monoblast into a monocyte occurs in the red bone marrow during a period of 5 days. A monocyte stay in bone marrow averages 3 days (the minimum stay is 9 hours), then it divides and without forming a bone marrow reserve it goes out into the peripheral blood.


In blood


A monocyte is the largest blood cell. In blood it matures, the nucleus transforming from a round shape to bean-shaped then palmate; and the structure of chromatin changes. A different level of differentiation of monocytes is found in the peripheral blood, with more mature monocytes dominating in the blood of healthy people. There are remnants of nucleoli in an immature monocyte and in the cytoplasm the enzymes change.


In the blood the monocytes split into parietal and circulating pools exchanging with each other, and their quantitative proportions can vary. In humans, in normal conditions the circulating pool of monocytes is of 18x10 raised to the power of 6 cells/kg of body weight, and the marginal pool, which currently does not participate in the circulation, adhering to the inner wall of a microvessel, is 3.5 times bigger (63x10 raised to the power of 6 cells/kg). In general, the aggregate pool of monocytes in the peripheral blood ranges from 1 to 10% of all leukocytes (80-600 x 109 units/litre).


Monocytes circulate in the blood from 36 to 104 hours (1.5 - 4.5 days) and then leave it according to the stochastic (objective) principle, interacting with specialised adhesive molecules in the endothelial cells. The migration of a mononuclear cell from the bloodstream to the focus of inflammation takes place through the micro vascular stream, which has endothelium of the second type – these are post capillaries and venules.


In tissue


Van Furth R. (1988): After getting into the tissue, the blood mononuclear cells transform into macrophages, which in turn adapt to the microenvironment of their future habitation. The heterogeneity of the population of macrophages “horizontally” – is microenvironment in which they operate.


Coming out of the bloodstream, a blood monocyte becomes a tissue one and is no longer able to return to the circulation. A tissue monocyte transforms into an organ-specific and tissue-specific macrophage according to the following stages of transition: macrophage blast, pro-macrophage, macrophage. A macrophage can be formed from a haematopoietic stem cell and from a promonocyte, according to the same stages of transition.


4. Macrophage - a heterogeneous specialised cellular population of the organism’s defence system (Fig. 8). Its diameter is 15-80 microns, the shape of the cell is irregular, and the nucleus is oval or oblong. The life expectancy is calculated in months and years.


Macrophages can be formed: in the connective tissue (histiocytes), lung (alveolar macrophages), liver (Kupffer’s cells), spleen, lymph nodes, bone tissue (osteoclasts), nervous tissue (microglial cells), skin (Langerhans cells) in pleural effusion and in ascites etc.




Fig. 8. Macrophages.


Macrophages are active: in non-specific defence against pathogenic micro organisms; in the processes of repair; in initiation of specific immune response; in the metabolism of lipids and iron; in regulation of haematopoiesis; in haemostasis and in the secretion of cytokines and other biologically active substances which regulate proliferation, differentiation and functional activity of different cells.


A variety of phenotypic characteristics of macrophages allows us to imagine the possibilities during the transformation of a promonocyte and monocyte and the influence of the microenvironment. A macrophage by its functional capabilities significantly prevails versus a promonocyte and monocyte. So in the process of transformation, a macrophage acquires the features that were absent in a promonocyte and monocyte.


In pathological conditions human promonocytes, monocytes and macrophages are able to proliferate to a limited extent. A promonocyte of this series is the most proliferating cell and it can, bypassing the monocyte stage, transform into a macrophage in the tissues.





Given that mononuclear cells are elements of the immunocompetent system, it is quite justifiable to expect a change in their quantitative and qualitative state during various pathological processes. In particular this is evident in the case of chronic inflammation, when the number of monocytes increases. This increase is observed in the red bone marrow, peripheral blood, spleen, lymph nodes and peritoneal cavities. At the same time in the peripheral blood a diverse level of differentiation of monocytes is found, and also the less mature cells - promonocytes may appear and dominate here.


An accelerated version of the proliferation of bone marrow mononuclear cells takes a minimum of 9 hours in the red bone marrow and 12 hours in the bloodstream, totalling 21 hours. For the maturation of a monocyte from a promonocyte two divisions of 30 hours each are necessary, i.e. in the case of urgent demand for macrophages, taking into account the absence of the bone marrow reserve, it is promonocytes and immature monocytes which go into the bloodstream.


In the process of differentiation the morphologically recognisable cells during the monocytopoiesis undergo 7-8 mitoses from monoblast to macrophage. In the case of inflammatory processes the number of macrophages and their activity especially increase. The inflammatory infiltrate can be maintained only in the case of constant renewal of mononuclear phagocytes in the focus, on account of the blood monocytes. In turn, a pool of monocytes is recovered from the red bone marrow. Consequently, the focus of chronic inflammation obtains persistent connections with the bone marrow and with the centre of the fresh monocytes’ generation through the mononuclear phagocytes. The focus will progress with the excess of the monocytopoiesis and vice versa. At the same time, activated macrophages take a live part in stimulation of monocytopoiesis.


Thus, in the system of haematopoiesis, the promonocyte and monocyte are the only intermediate simplified universal cells, which in the organs and tissues transform into organ-specific and tissue-specific macrophages.