In this review, we will first briefly overview the basic principles of the organization and development of the human CNS and highlight certain developmental features that differ from commonly studied mammals. Particular emphasis will be given to studies of the cerebral neocortex, also called the isocortex and neopallium, both because it has been the focus of many developmental and comparative studies and because of its importance in higher-order cognition, emotional regulation, and complex behaviors. We will next detail the current understanding of the transcriptional, epigenomic, and regulatory landscapes of the developing human neocortex and other regions of the CNS, noting the essential conserved features, as well as clade and species-specific differences, revealed by these studies. For a more comprehensive description of the state of knowledge on the evolution of the human CNS, we recommend a number of recent reviews (Clowry et al., 2010,Dehay et al., 2015, Geschwind and Rakic, 2013, Lui et al., 2011, Marín-Padilla, 2014, Molnár and Pollen, 2014, Sherwood et al., 2012).
The Human CNS in Numbers. How Many Neurons and Synapses Are there?
The human CNS contains approximately 86.1 billion neurons on average in 50- to 70-year-old males (Herculano-Houzel, 2015). There are approximately 16.34 billion neurons in the cerebral cortex, which includes as many as 2.58 billion neurons in the cortical white matter (Azevedo et al., 2009, Herculano-Houzel, 2009; Sigaard et al., 2014). However, it should be noted that published estimates of the number of cerebral neocortical neurons varies by as much as a factor of 2 (between 14.7 to 32.0 billion neurons) (Pakkenberg and Gundersen, 1997, Pakkenberg et al., 2003). It was estimated that there are 164 trillion synapses in the adult human cerebral neocortex (Tang et al., 2001). The published estimates for the number of synapses an individual neocortical neuron receives also vary between approximately 7,200 (Pakkenberg et al., 2003, Tang et al., 2001), 29,642 (DeFelipe et al., 2002), and 80,000 (Huttenlocher, 1979) synapses. The present authors did not find calculations for other human CNS regions. However, the average number of synapses per neuron certainly varies tremendously in other mammals and brain regions. In the rat brain, the number of synapses associated with a neuron ranges from an average of 2,186 for a calretinin-positive hippocampal interneuron (Gulyás et al., 1999), 31,700 for a hippocampal CA1 pyramidal neuron, to 175,000 for a cerebellar Purkinje neuron (Napper and Harvey, 1988). Thus, if we take the lower estimate for human neocortical neurons (i.e., 7,200) and assume that this establishes a lower boundary for a typical CNS neuron, there may be around 620 trillion synapses in the entire adult CNS. Remarkably, if we take the higher of these estimates for neocortical neurons (i.e., 29,642 or 80,000), then there may be as many as several quadrillion synapses in the entire adult CNS.
Source: Neuron 2016