Population biology of infectious diseases: Part I.

If the host population is taken to be a dynamic variable (rather than constant, as conventionally assumed), a wider understanding of the population biology of infectious diseases emerges. In this first part of a two-part article, mathematical models are developed, shown to fit data from laboratory experiments, and used to explore the evolutionary relations among transmission parameters. In the second part of the article, to be published in next week's issue, the models are extended to include indirectly transmitted infections, and the general implications for infectious diseases are considered.     

The organizer factors Chordin and Noggin are required for mouse forebrain development

In mice, there is evidence suggesting that the development of head and trunk structures is organized by distinctly separated cell populations. The head organizer is located in the anterior visceral endoderm (AVE) and the trunk organizer in the node and anterior primitive streak. In amphibians, Spemann's organizer, which is homologous to the node, partially overlaps with anterior endoderm cells expressing homologues of the AVE markers cerberus, Hex and Hesx1. For mice, this raises the question of whether the AVE and node are independent of each other, as suggested by their anatomical separation, or functionally interdependent as is the case in amphibians. Chordin and Noggin are secreted bone morphogenetic protein (BMP) antagonists expressed in the mouse node, but not in the AVE. Here we show that mice double-homozygous mutants that are for chordin and noggin display severe defects in the development of the prosencephalon. The results show that BMP antagonists in the node and its derivatives are required for head development.