Granulosa cells: characteristics and functions of these neurons

What are granulosa cells? Let us look at the characteristics of this cell type.

Granulosa cells can be found in various brain structures, such as the cerebellum, the olfactory bulb or the dentate gyrus.such as the cerebellum, the olfactory bulb or the dentate gyrus of the hippocampus, among others.

This group of neurons share a unique characteristic, and that is their particular smallness. The different functions they perform vary depending on the brain area in which they are located, and they are involved in auditory and olfactory processes, memory and motor learning.

In this article we explain what granulosa cells are, where they are located, what their structure is, and what kind of functions they perform.

Granulosa cells: definition and anatomical location

The term granulosa cell is used to define different types of neurons, whose only common characteristic is that they all have very small cell bodies.whose only common characteristic is that they all have very small cell bodies. Granule neurons can be found within the granular layer of the cerebellum, in the dentate gyrus of the hippocampus, in the superficial layer of the dorsal cochlear nucleus, in the olfactory bulb, and in the cerebral cortex.

The vast majority of neurons in the brain are granulosa cells and practically half of the cells of the nervous system are part of the cerebellum.. Cerebellar granulosa cells receive excitatory inputs (using glutamate as a neurotransmitter) from mossy fibers originating in the pontine nuclei, located in the ventral pons and related to motor skill activity and learning.

In turn, the granulosa cells of the cerebellum also send parallel fibers up through the Purkinje layer to the molecular layer where they branch and extend by branching to the dendrites of the cells named after that layer, the Purkinje cells, large neurons that function via the neurotransmitter GABA and whose dendrites are able to release endocannabinoids that reduce the excitatory or inhibitory potential of the synapses.

On the other hand, the axons of the granulosa cells of the hippocampal dentate gyrus are transformed into mossy fibers that connect exclusively with the foot of the hippocampus. In addition, studies have observed that hippocampal granule neurons are among the few cells capable of regenerating throughout the life cycle, mainly in certain mammals (although whether this is the case is under study). (although whether the same is true in humans is under study).

Structure

Granulosa cells in different brain regions are functionally and anatomically diverse. are functionally and anatomically diverse. As mentioned at the beginning of this article, the only thing they have in common is their small size. For example, granule neurons in the olfactory bulb function with the neurotransmitter GABA and have no axons, whereas the same cells in the dentate gyrus of the hippocampus have projection axons that function with glutamate.

The granule cell nuclei of the olfactory bulb and dentate gyrus are the only groups of neurons that undergo adult neurogenesis, unlike those in the cerebellum and cortices. On the other hand, all granulosa cells (with the exception of the olfactory bulb) have a typical structure consisting of a neuron with dendrites, a soma or cell body and an axon..

The granulosa cells of the cerebellum have a tightly packed round nucleus and synaptic glomeruli formed by granulosa neurons, golgi cells and mossy fibers (one of the main inputs to the cerebellum from the cerebral cortex and other regions). Those located in the dentate gyrus of the hippocampus, on the other hand, have an elliptical soma and their dendrites project towards the molecular layer.

In the dorsal cochlear nucleus we can find small granule cells with two or three short, claw-shaped dendrites, which act as inhibitory interneurons. These form glomeruli through which the mossy fibers pass, similar to what occurs in the cerebellum.

As for the structure of the granule neurons of the olfactory bulb, it should be noted that they lack a main axon (and accessory axon), and each of the granule neurons of the olfactory bulb has its own axon. (and accessory), and each cell has several short dendrites in its central part and a single long dendrite with a tip at the end. The branches project into the outer plexiform layer of the olfactory tract.

Functions

Granulosa cells have different functions depending on the structure in which they are located.

1. Granulosa cells in the cerebellum

It has been suggested that granulosa cells located in the cerebellar cortex receive a few excitatory inputs from mossy fibers, whose function would be to encode different combinations of inputs from the latter cells. Another type of fibers, the climbing fibers, would be responsible for sending specific signals to the Purkinje cells to modify the strength of the synaptic connections of the parallel fibers.

This last explanation is part of the well-known theory of the cerebellum by neuroscientist David Marrknown among other things for his work on computational theories of the cerebellum, neocortex and hippocampus. However, these are claims that have not been corroborated, so more research is still needed in this regard.

2. Granulosa cells of the hippocampal dentate gyrus

As is well known, the hippocampal dentate gyrus is involved in processes involved in the formation and consolidation of episodic memory, navigation and spatial memory. Research suggests that granule cells in this brain area would play an important role in the formation of spatial memories.

Furthermore, it appears that granule cells born in adults would be highly active during the first weeks after functional integration into the neural network. What animal studies have been able to prove is that, as adult granulosa cells age, their function changes from being specialized in pattern separation (formation of different memories of similar episodes by generating different representations of the temporal and spatial relationships of events) to rapid completion of those same patterns. from being specialized in pattern separation (formation of different memories of similar episodes by generating different representations of the temporal and spatial relationships of events), to the rapid completion of those same patterns.

3. Granulosa cells of the dorsal cochlear nucleus

The cochlear nuclei are the first relay of the primary auditory pathway and receive axons from the ganglion cells of the auditory nerve, whose function is to decode auditory information (duration, intensity and frequency). (duration, intensity and frequency).

The granule cells in the ventral area of the cochlear nucleus receive projections from the primary auditory cortex and the signals received by these neurons contain information on parameters such as head position, which allows correct auditory orientation to occur. The granulosa cells of this brain structure would also be involved in the perception and orientation response to environmental sound stimuli.

4. Granulosa cells of the olfactory bulb

The granulosa cells of the olfactory bulb receive inputs from deep areas of the brain involved in memory formation and cognition, and are responsible for inhibiting the neurons that receive the sensory inputs. In this way, granulosa cells allow the brain to interpret and shape olfactory experiences..

In addition, granule neurons located in the olfactory bulb would also have an essential role in the formation of memories, as well as in the selection and discrimination of the most significant odors, discarding the less important ones so that the brain focuses only on the most salient part of the olfactory stimulus.

Bibliographical references:

• Ambrogini, R., Lattanzi, D., Ciuffoli, S., Agostini, D.,Bertini, L., Stocchi, V., Santi, S., et al. (2004). Morpho-functional characterization of neuronal cells at different stages of maturation in granule cell layer of adult rat dentate gyrus. Brain Res. 1017: 21 - 31.
• Balu, R., Pressler, R. T., & Strowbridge, B. W. (2007). Multiple modes of synaptic excitation of olfactory bulb granule cells. Journal of Neuroscience, 27(21), 5621 - 5632.
• Weedman, D. L., & Ryugo, D. K. (1996). Projections from auditory cortex to the cochlear nucleus in rats: synapses on granule cell dendrites. Journal of Comparative Neurology, 371(2), 311-324.