<P> However, in terms of glial cells, adults had far larger estimates than those in newborns; 36.3 million on average in adult brains, compared to 10.6 million in the newborn samples . The structure of the brain is thought to change when degeneration and deafferentation occur in postnatal situations, although these phenomena have not been observed in some studies . In the case of development, neurons which are in the process of loss via programmed cell death are unlikely to be re-used, but rather replaced by new neuronal structures or synaptic structures, and have been found to occur alongside the structural change in the sub-cortical gray matter . </P> <P> Synaptic pruning is classified separately from the regressive events seen during older ages . While developmental pruning is experience dependent, the deteriorating connections that are synonymous with old age are not . The stereotyped pruning can be related to the chiseling and molding of stone into a statue . Once the statue is complete, the weather will begin to erode the statue and this represents the experience independent deletion of connections . </P> <P> The three models explaining synaptic pruning are axon degeneration, axon retraction, and axon shedding . In all cases, the synapses are formed by a transient axon terminal, and synapse elimination is caused by the axon pruning . Each model offers a different method in which the axon is removed to delete the synapse . In small - scale axon arbor pruning, neural activity is thought to be an important regulator, but the molecular mechanism remains unclear . Hormones and trophic factors are thought to be the main extrinsic factors regulating large - scale stereotyped axon pruning . </P> <P> In Drosophila, there are extensive changes made to the nervous system during metamorphosis . Metamorphosis is triggered by ecdysone, and during this period, extensive pruning and reorganization of the neural network occurs . Therefore, it is theorized that pruning in Drosophila is triggered by the activation of ecdysone receptors . Denervation studies at the neuromuscular junction of vertebrates have shown that the axon removal mechanism closely resembles Wallerian degeneration . However, the global and simultaneous pruning seen in Drosophilia differs from the mammalian nervous system pruning, which occurs locally and over multiple stages of development . </P>

Why is pruning essential part of brain development