5.5 Summary of Section 5
When oestradiol combines with its receptor inside neurons, the cell produces proteins which protect it from cell death. As a consequence, the male brain, which has oestradiol in its neurons in early life, becomes different from the female brain, which does not have oestradiol in its neurons. Retinoic acid is needed in high concentrations to produce those proteins associated with posterior structures of the embryo, whilst retinoic acid is needed in low concentrations to produce those proteins associated with anterior structures. This is an example of one signal, i.e. one transcription factor, whose effect differs with concentration. Transcription factors are the gatekeepers of the genome, through which environmental factors exert their effects. Profound effects on the physical development of the organism result from interference with transcription factors.