The topographical arrangement of the clones of A single, A paired, and A aligned (As, Apr, and Aal) spermatogonia on the basement membrane of seminiferous tubules of the Chinese hamster was studied. It was found that at least some of these clones are not distributed at random as clones of similar cell number were often seen in clusters. Areas were found with few or many As spermatogonia. Also, clusters of Apr spermatogonia were found, indicating that in such an area many As spermatogonia more or less synchronously formed Apr spermatogonia. Since clusters of clones of 16 Aal spermatogonia were observed, it can be concluded that these clusters of Apr spermatogonia may proliferate in at least a roughly synchronous way. It was found that over large areas the densities of undifferentiated spermatogonia may be very low or high in comparison to the mean density in the animal. Whether the ratio of self-renewal and differentiation of the stem cells changed locally in response to the high or low density of undifferentiated spermatogonia in particular areas was investigated. No indications for a regulatory mechanism to keep the density of stem cells and/or the density of undifferentiated spermatogonial clones at a certain level could be detected in the normal Chinese hamster. This lack of regulation was at least partly responsible for the widely different numbers of A1 spermatogonia that were formed in the various areas studied in stage IX.
SUMMARYThe development of highly branched inflorescences is considered. Two main characteristics of these structures are the flowering sequences (acropetal, basipetal or divergent) and the branch production (acrotonic, basitonic or mesotonic), which can be different on various branching orders (paracladia). A widespread pattern, found especially among Compositae, is a basipetal or divergent flowering sequence combined with basitonic or mesotonic branch production. We present quantitative models for this type of inflorescence development, based on the transport of a flower-inducing hormone (florigen) and of a hormone responsible for apical dominance (auxin), together with some additional control factors. The computed structures obtained with these models are compared with observations on the inflorescence development of wall lettuce, Mycelis muralis.
Worldwide, over 26 million patients suffer from heart failure (HF). One strategy aspiring to prevent or even to reverse HF is based on the transplantation of cardiac tissue‐engineered (cTE) constructs. These patient‐specific constructs aim to closely resemble the native myocardium and, upon implantation on the diseased tissue, support and restore cardiac function, thereby preventing the development of HF. However, cTE constructs off‐the‐shelf availability in the clinical arena critically depends on the development of efficient preservation methodologies. Short‐ and long‐term preservation of cTE constructs would enable transportation and direct availability. Herein, currently available methods, from normothermic‐ to hypothermic‐ to cryopreservation, for the preservation of cardiomyocytes, whole‐heart, and regenerative materials are reviewed. A theoretical foundation and recommendations for future research on developing cTE construct specific preservation methods are provided. Current research suggests that vitrification can be a promising procedure to ensure long‐term cryopreservation of cTE constructs, despite the need of high doses of cytotoxic cryoprotective agents. Instead, short‐term cTE construct preservation can be achieved at normothermic or hypothermic temperatures by administration of protective additives. With further tuning of these promising methods, it is anticipated that cTE construct therapy can be brought one step closer to the patient.
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