Cell kinetics in the large Lewis carcinoma growing in mice subcutaneously was characterized by the following parameters: at the tumor periphery -tc=11.3 hr, tG2=1.4 hr, tS=8.8hr, tG1=1.1 hr, LI=47%, Pc=61.8%; at the tumour center - 20.7 hr, 2.9 hr, 16.0 hr, 1.8 hr,31.2%, resp. Injection of cyclophosphamide induced degeneration of all the cells in slowly growing central part of the tumour, and only of part of cells on the fast proliferating tumour periphery. It is suggested that in the central part of the tumour there are no stem cells, and that this region regenerates when the transition of cells from the tumour periphery is arrested by cyclophosphamide.

Two-thirds of the liver was removed from (CBA X C57BL/6j) F1 female mice. A significant increase of the number of endogenous colonies count in the spleen of partially hepatectomized mice was observed on the 5-th day after the operation. This increase was not associated with the changes in the number of stem cells in the bone marrow as partial hepatectomy at different times after the operation exerted no effect on the number of colony-forming units (CF1) in the bone marrow.

A study of the saffron ontogenesis has been carried out, with special reference to the morphophysiological state of cells and nuclei in the stem apices. 4 periods were established in the development of stem apices: (1) formation of apical meristem (November), (2) slow development coinciding with the intensive plant vegetation (December--February), (3) transition to generative development when the vegetative organs begin to dry off (March) and (4) differentiation of generative organs at the time when the corm is underground and vegetative organs almost fully absent (summer). The development of apex is accompanied by changes in the nuclear structure and fluctuations of mitotic activity. The critical period of transition to the phase of generative development takes place in March, long before the formation of visible generative organs and flowering.

Various methods of commitment regulation have been analysed on the basis of the mathematical model of the haemopoietic stem cell number dynamics. The starting point is the requirement on the system stability. The basic hypothesis is that of the intrinsic nature of the population commitment regulation. The analysis states that the commitment specific velocity increases with stem cell number. This way of regulation may be performed in the case the commitment takes place at Go phase of the cell cycle. The role of the committed precursor pool in providing the haemopoietic system reliability is discussed.

The effect of inhibitors of DNA-dependent RNA synthesis (sibiromycin and actinomycin D) and radio toxic doses of 3H-uridine on the bone marrow stem hemopoietic cells in adult mice was studied by means of spleen colonies. The short-term in vitro preincubation of the bone marrow cell suspension with the antibiotics and 3H-uridine resulted in the practically complete inhibition of colony formation. The bone marrow stem hemopoietic cells incorporated 3H-uridine intensively. In the normal adult animals the number of stem cells with the high level of RNA synthesis attained 40% whereas the rest 60% of cells were capable to activate the ribonucleic metabolism upon their in vitro preincubation.

The in vitro treatment of the mouse spleen cells immunized by the ram erythrocytes with the rabbit and mouse sera against the thermoaggregated mouse immunoglobulins resulted in the inhibition of antigen binding receptors of rosette forming cells. The mouse serum, unlike the rabbit one, induced the inactivation of receptors in rosette forming lymphocytes both in the non-immune and immune mice on the 8th day after the antigenic stimulation. The treatment of bone marrow cells from the intact mice with these sera increased insignificantly the number of hemopoietic colonies in the spleens of lethally irradiated syngenic recipients and stimulated markedly the migration of spleen cells. This may be due both to the direct effect of these sera and to their mediated (through the humoral factor) influence. The inactivation of antigen binding receptors in the spleen rosette forming cells suggests the presence of immunoglobulins on the membrane of B-lymphocytes in the aggregated state or in the form of antigen--antibody complexes.

In the system of non-syngeneic transfer of stem hemopoietic cells, the preliminary incubation of the cells of bone marrow or embryonic liver of the C57BL mice with different temperature RNA fractions isolated from the spleen of (CBAXC57BL) F1 was shown to lead to the complete or partial restoration of the colony forming ability of the donor cells. The 63 degrees RNA fraction was shown to have the greatest restorative effect.

The direction of differentiation of the stem cells with respect to the physiological activity of thymus determined by the age of an animal was studied by means of histological analysis of hemopoietic colonies in the spleen of lethally irradiated mice. The immaturity of thymus of its involution are characterized by the inhibition of differentiation of the stem cell along the granuloid path. An analysis of the data on differentiation of the stem cells in mice of different age, as well as in thymectomized mice allows to draw a conclusion that the process of differentiation of the hemopoietic stem cells is thymus-dependent.

The effect of the thymus cells of the C57BL/6 mice on the colony forming ability of the stem hemopoietic cells of the embryonic liver and bone marrow of young (3 months) and old (2 years) mice was studied their joint transplantation into the mice (CBAXXC57BL/6) F1. The stimulating effect of the thymus cells on the colony forming ability of the stem hemopoietic cells of different age depends both on the dose of the stem hemopoietic cells of embryonic liver and the dose of T-lymphocytes. A suggestion is put forward that the stimulating effect of the thymus cells on the colony formation is due to their interaction with the stem cells in the G2 phase of the mitotic cycle.

The model of heterotopic transplantation of the mixture of bone marrow and thymus fragments was used to study the interaction of hemopoietic and lymphoid tissues under their direct contact. The bone marrow and thymus fragments of adult mice F1 (CBAXXC57BL) were transplanted separately or in the mixture under the kidney capsule of mice of the same strain. During the whole period of observation (from 10 days up to 14 months), the development of bone marrow and thymus fragments in the joint transplants proceeded independently, no "mixed" stroma appeared, and the stroma of each organ ensured the differentiation characteristic of its organ. The development of joint transplants somewhat differs from that of isolated transplants: on the 10th day a greater amount of hemopoietic tissues was noted in the former; the bone marrow component increases continuously up to 6 months (vs. 1--2 months in the isolated transplants); the bone and hemopoietic tissues predominate in the joint transplants by 14 months, the amount of thymic tissue markedly decreases but it does not disappear completely.