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Nature of lethality of t mutations in embryos.

Mutations in the genes of the T complex in mice can lead to death at various stages of embryogenesis. It has been suggested that the T genes specify cell surface antigens which are essential for normal organisation of certain cells within the embryo. But, there is now evidence that at least some of these T complex mutations are simply generalised cell lethals.

Animals↗

Determination and cellular commitment in the embryonic amphibian mesoderm.

In classical embryology, the term 'determination' was used to describe the process by which a region of the embryo became committed to form a particular part of the body and it was understood that there was a particular stage of development at which determination occurred with respect to each decision. Some authors insisted that only an irreversible commitment should be described as determination, but this leaves undefined the nature of the preceding events involved in its establishment. We show here that there may be several stages of commitment for an organ rudiment that can be distinguished from one another by the use of different methods for altering the pathway of development. We have investigated the region of the amphibian mesoderm that is destined to become somitic muscle in the course of normal development and applied to it a variety of isolation and grafting techniques. The results, which show an extensive period of lability within this region, can be used to explain how pattern formation might occur within the dorsoventral axis of the mesoderm.

Ambystoma↗

Earliest sensory nerve fibres are guided to peripheral targets by attractants other than nerve growth factor.

Recent studies have shown that developing nerve fibres grow directly to their targets and are guided by specific cues, but the nature of these cues and the mechanism of guidance remain unknown. The growth of sympathetic axons towards an artificial source of nerve growth factor (NGF) in vivo and of sensory neurites up a concentration gradient of NGF in vitro has supported the hypothesis that NGF, produced by target tissues, acts as a chemotactic attractant for these nerve fibres during development. Both these studies and those of the influence of NGF or target tissues on neurite growth in vitro were conducted late in development when, following target encounter, the neurones had become dependent on NGF or target for survival. Here we have co-cultured embryonic mouse sensory neurones and their peripheral target tissue at a stage preceding their contact in vivo. Neurites grew directly and exclusively towards their own target but not to regionally inappropriate peripheral tissue. Antiserum to isogeneric NGF did not reduce this outgrowth but did reduce undirected neurite outgrowth which occurred in co-cultures of older neurones with denervated target tissue. These results demonstrate that agents other than NGF guide neurites of NGF-responsive neurones in development.

Animals↗