Cell kinetic studies of the proliferation of the neural epithelium during the embryonic development of the rat brain are described.Pregnant rats received a single injection of "-thymidine PH-TdR) between the tenth and twenty-first day of pregnancy. Autoradiographs of the brains of the 25-day-old offspring were prepared. For a number of different cell types (table 1 , 11) the percentage of labeled neurons as a function of time of prenatal :'H-TdR injection was determined.The end of the proliferating period of the neural epithelial cells of certain cell types can be derived from these percentages of labeled cells (table 1 , V). Furthermore, i t can be demonstrated that the percentage of labeled neurons is equal to the labeling index of the neural epithelial cells of these cell types at the time of prenatal "H-TdR injection.Based on the labeling index of the neural epithelial cells and their S phase, cycle times can be calculated. This way it is possible for the first time to determine the cycle times of those neural epithelial cells that later on differentiate into a certain type of neuron (table 1, IV).The calculated cycle times show that up to the sixteenth prenatal day the neural epithelial cells behave like a homogeneous cell population which proliferates with an approximately constant cycle time of about half a day. The cycle time seems to increase with increasing fetal age.From the total number of a certain type of neuron the number of mitotic divisions or cycle times respectively can be derived which is necessary to produce the number of neural epithelial cells providing these neurons. With regard to the mean cycle time of the neural epithelial cells i t can be concluded that the beginning of proliferation of the neural epithelial cells coincides with the formation of the neural plate on the ninth day of embryonic development or starts even shortly before this time.Prenatal labeling with :3H-thymidine ("H-TdR) of the proliferating neural epithelial cells of the embryonic brain and the observation of labeled neurons in the young or adult animal has proved a useful tool in studying the development of the brain. Labeling of neurons can only be achieved if :jH-TdR is applied prenatally, prior to differentiation of the neural epithelial cells into neuroblasts (cells produced by the last division of neural epithelial cells that do not divide any more but differentiate into the corresponding neurons) and neurons. With this method the prenatal "time of origin" or "date of birth" of many types of neurons in the chick, the mouse and the rat has been studied.

The cycle time of the proliferating glial cells outside the subependymal layer of the lateral ventricle as well as that of endothelial cells was studied autoradiographically in the brains of adult and untreated mice. To determine the mean cycle time two independent methods were used. A mean cycle time of about 20 hours was obtained for glial and endothelial cells from the decrease of the mean grain number/nucleus as a function of time after tritiated thymidine (3H-TdR) injection. Another group of experiments utilized the "method of labeled S phases". With this method the passage of labeled cells through successive S phases is observed. Passing through S phase following 3H-TdR injection the 3H-labeled cells are double labeled by an additional 14C-TdR injection. This method again resulted in a cycle time of 20 hours for glial and endothelial cells. From the present work and a former study (Korr et al., '73) the following cell cycle parameters were derived: Cycle time 20 hours; S phase 9.4 hours; G2 less than three hours; (G2+M) five hours; G1 five hours. The growth fraction of glial cells related to all glial cells is only 0.004. Furthermore, the present experiments show that in the case of glial cells 17% of the daughter cells after mitosis become pyknotic and are eliminated from the glial cell population. Apart from this cell loss, after mitosis about one-fourth of the daughter cells do not enter the next S phase. These cells leave the growth fraction and are replaced by a corresponding number of non-proliferating glial cells. There is a relatively extensive permanent exchange of cells between the growth fraction and non-growth fraction of glial cell.

The proliferation of glial cells outside the subependymal layer of the lateral ventricle as well as of endothelial cells was studied autoradiographically in the brain of the adult and untreated mouse. The double labeling method with 3H-and 14C-thymidine was applied in order to show experimentally the existence of a DNA synthesis phase (S phase) and to measure its duration.Adult mice received a first injection of i4C-thymidine, two or four hours later a second injection of 3H-thymidine and were sacrificed one hour after the last injection by perfusion fixation. Double layer autoradiographs were made from serial sections of the region from the corpus callosum/commissura anterior up to the corpus callosum/commissura fornicis ventralis in order to register purely 3H-, doubly 3H-and "C-, and purely 14C-labeled nuclei. From the ratio of all 3H-labeled cells with and without 14C to the purely 3H-labeled cells a DNA synthesis phase of 9.4 _t 0.5 hours for glial cells and one of 11.0 * 2.2 hours for endothelial cells was obtained. Based on the first appearance of labeled mitoses and labeled pairs of glial cells after injections of labeled thymidine the G2 phase was estimated to be < three hours and Ge + M about five hours. The duration of the measured S phase as well as the appearance of labeled mitoses about three hours after application of labeled thymidine are very similar to these cycle parameters in many other somatic cells in different kinds of animals. This has led to the conclusion that a well-defined DNA synthesis phase with doubling of the DNA content and a successive mitosis also exists in dial and endothelial cells of the adult mouse brain.

In the context of existent programmes of environmental monitoring which have been established as efficient tools for permanently observing environmental changes, a concept for a genetic monitoring in forests was recently elaborated by a German forest geneticist working group. Genetic monitoring is assumed to contribute essentially to the estimation and valuation of the effect of factors influencing the genetic system of trees in the forests, thus making it an early warning and controlling system for ecosystemic changes. The “Concept of a Genetic Monitoring for Forest Tree Species in the Federal Republic of Germany” gives scientifically-based guidelines for monitoring the current state and dynamics of genetic systems in forest stands of diverse tree species in an extensively, harmonized manner. Both objectives and the realization of the genetic monitoring concept are presented here. The status of the genetic systems of forest tree populations is assessed on the basis of criteria, indicators and verifiers. For this purpose the genetic as well as the phenological and physiological levels are taken into consideration in order to follow temporal developments and to estimate influencing factors. The results of a pioneer study concerning the tree species Fagus sylvatica and Prunus avium are reported

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