Cortical neurons

The cerebral cortex is the brain’s outer layer of neural tissue in humans and other mammals. The cerebral cortex plays a vital role in memory, attention, perception, awareness and consciousness (Figure 1). It’s folded and has a much greater surface area in the confined volume of the skull. The fold or ridge is termed as gyrus and the groove or fissure is termed as sulcus. In humans, more than 2/3 of the cerebral cortex is buried in the sulci.

The neocortex

Most of the cerebral cortex is neocortex. However, phylogenetically there are older areas of cortex termed the allocortex. These more primitive areas are located in the medial temporal lobes and are involved with olfaction (smell) and survival functions such as visceral and emotional reactions. In turn, the allocortex has two components: the paleocortex and archicortex. The paleocortex includes the piriform lobe, specialized for olfaction, and the entorhinal cortex. The archicortex consists of the hippocampus, which is a three-layered cortex dealing with encoding declarative memory and spatial functions. The neocortex represents the great majority of the cerebral cortex. It has six layers and contains between 10 and 14 billion neurons.

Six cell layers of the cerebral cortex

Table adapted from https://sisu.ut.ee/histology/cerebral-cortex

Functionally, the layers of the cerebral cortex can be divided into three parts. The supragranular layers consist of layers I to III. The supragranular layers are the primary origin and termination of intracortical connections, which are either associational (i.e., with other areas of the same hemisphere), or commissural (i.e., connections to the opposite hemisphere, primarily through the corpus callosum). The supragranular portion of the cortex is highly developed in humans and permits communication between one portion of the cortex and other regions. The internal granular layer, layer IV, receives thalamocortical connections, especially from the specific thalamic nuclei. This is most prominent in the primary sensory cortices. The infragranular layers, layers V and VI, primarily connect the cerebral cortex with subcortical regions. These layers are most developed in motor cortical areas. The motor areas have extremely small or non-existent granular layers and are often called 'agranular cortex'. Layer V gives rise to all of the principal cortical efferent projections to basal ganglia, brain stem and spinal cord. Layer VI, the multiform or fusiform layer, projects primarily to the thalamus.

In vitro applications of cortical neurons in research

In vitro cortical cell culture systems (primary cell cultures) are considered to be the most valuable tool for studying Neuronal Plasticity and Neurophotonics. In addition, their role in Down’s syndrome, immunocytochemistry, in-silico models, drug screening studies, gene transfer technology, and advanced cell culture models have been extensively studied. Lonza cells and media have been used by different research groups for a better understanding of these applications.

• Neuronal plasticity: Neuronal plasticity refers to the adaptation of neural function and structure. Cortical neurons have been used in understanding the mechanism of neuronal plasticity. When neurons are lost due to stroke, for example, a group of new cortical neurons will begin to perform the functions of the original group. Clinical examples of this phenomenon include taking control over speech and swallowing. 
• Neurophotonics: Cortical neurons have been used to study the connectivity and function of neuronal circuits. Photonics and optical tools offer possibilities to assess how signals are integrated in cells, how cells are interconnected to form circuits, and how neuronal activity relates to behavior. Two basic approaches are often taken to decipher how motor cortex relates to movements: 1. stimulate or silence motor cortex and measure the resulting effect on skeletal muscle activity and body movements and 2. record motor cortex activity during motor behavior and assess how this correlates with movement parameters. With neurophotonics and optogenetic methods, these two basic types of approaches are being pursued at even high levels of specificity and spatiotemporal precision.
• Drug screening: Microelectrode array (MEA) technology enables drug screening and “disease-in-a-dish” modeling by measuring the electrical activity of cultured networks of neural or cardiac cells. Human stem cell technologies, advancements in genetic models, and regulatory initiatives for drug screening have increased the demand for MEA-based assays. A multiwell MEA platform enhanced by optogenic stimulation would enable selective excitation and inhibition of targeted cell types. The system enables finely graded selective control of light delivery during simultaneous recording of network activity in each well. This technique can be used for applications including cardiac safety screening, neural toxicity assessment, and advanced characterization of complex neuronal diseases like Alzheimer’s disease. Lonza cells isolated from neural cortex of rat have been used by certain groups and have been cited in the publications.
• Gene transfer technology (transfection): Efficient gene transfer is an important tool for the study of neuronal function and biology. This has proved difficult and inefficient with traditional transfection strategies, which can also be fairly toxic. Even viral mediated gene transfer, although highly efficient, can be time consuming. Nucleofector technology, based on optimized electroporation in a cell type–specific solution, enables direct delivery of DNA and/or RNA into the cell nucleus. This strategy results in reproducible, rapid, and efficient transfection of a broad range of cells, including primary neurons. Lonza has developed nucleofection kit for mouse and rat neuronal cells which have been used and reported by various groups.