Passive and active reactions of embryonic tissues to the action of dosed mechanical forces

With the help of a suction manometric device, the relation between the deformation of Xenonus laevis embryo at the gastrula and neurula stages and the value of the applied force has been studied. Stiffness modules of embryonic tissues were in the order of several dozens of Pascal and they were inversely proportional during deformation from 40 to 20%. At the gastrula stage, a uniform or an increasing rate of expansion of the embryo body in the suction capillary with the diameter of approximately half that of the embryo was observed for 30 min after the action of the suction forces. The length of the stretched portion of the embryo correlates with the value of its deformation at the first minute. As a result of the expansion, the total body surface area of the deformed embryo increases more than twice compared to intact embryos. After expelling the embryo from the capillary, its surface reduced and the deformation became smoothened within 5 min, which indicates the existence of tensional force in the expanded embryo. These data confirm that, at the embryo gastrula stage, external mechanical forces do not only passively deform the embryo but also initiate the active expansion of the embryo which takes place at zero external force and overcomes the tensional resistance of tissues. The mechanism of active expansion and its link with the processes of normal morphogenesis are discussed.     

Haversian bone remodelling in human fetus

Haversian bone remodelling used to be considered an attribute of adult bone. In the present paper, typical haversian remodelling was observed in the humerus, ulna, radius, femur, tibia, fibula and 6th and 7th ribs as soon as the 24th week. Its aspect suggests an already ancient occurrence. Remodelling was found in the same bone pieces of the neonate as well as in the clavicle and the other ribs. Mechanical factors may be responsible for starting the remodelling but their effects could be modulated by metabolic needs. Other still unknown factors might play a role too.     

INTERACTION OF IONIZING AND VISIBLE RADIATION IN MUTATION INDUCTION IN NEUROSPORA CRASSA

Klein , Richard M. (New York Bot. Gdn., New York, N.Y.), and Deana T. Klein. Interaction of ionizing and visible radiation in mutation induction in Neurospora crassa. Amer. Jour. Bot. 49(8): 870–874. 1962.—Conidia of the purple adenineless strain of N. crassa were irradiated with 25 kr of X rays and then exposed to far-red or red radiations or to far-red followed by red radiation. Far-red light, without effect on un-irradiated conidia, augmented the genetic damage caused by X rays as measured by survival (colony count), back mutation to adenine prototrophy, and the induction of mutants affecting colony morphology. Post-X-irradiation with red light ameliorated the severity of X-radiation as measured by survival and back mutation. The potentiation of X-ray-induced genetic damage by far-red light could be completely negated by subsequent exposure to red light.