Differential target dependence in the developing brain: implications for mental retardation.

Abstract

During development of the brain, many neurons exhibit a dependence on other neuronal populations for their survival and differentiation (target dependence). Evidence suggests that some neural pathways are much more dependent on single target neuronal populations for their survival than are others (differential target dependence). This phenomenon has important implications both for animal models of congenital human brain damage and for ideas concerning the aetiology of behavioural abnormalities associated with human mental retardation. Predictions of the neuronal deficits likely to arise from exposure to cytotoxic agents (e.g. ionizing radiation, hyperthermia, viral infection) at a particular time must take differential target dependence into account. It is known that target dependence affects corticopetal pathways involved with the discriminative senses (e.g. vision), more than monoaminergic and cholinergic corticopetal pathways which are believed to be involved with arousal, selective sensory attention, sleep, memory and cortical vasomotor function. Following prenatal damage to superficial layers of the cerebral cortex, this effect of differential target dependence leads not only to a relative hyperinnervation of the cortex with monoaminergic and cholinergic projections, but a specific deficit in visual pathways. The implications of this combined deficit for the behaviour and rehabilitation of the mentally retarded are considered.