The optic tectum (OT) and its mammalian equivalent, the superior colliculus (SC), is a key processing centre for sensory information. The OT receives the majority of its inputs from the retina and constructs an image of the physical surroundings. Through connections with several other regions of the brain, it integrates visually acquired information with motor inputs and outputs to initiate appropriate behavioural responses.
The main input of the OT comes from the retina in the form of the topographically mapped retinotectal projections of the retinal ganglion cells (RGCs), but it also has bilateral connections with the pretectum, dorsal thalamus (which relays the tectal signals to the telencephalon), dorsal tegmentum, nucleus isthmi, reticular formation and controlateral tectum. The OT also receives input from the torus longitudinalis (TL), a structure that serves as a relay of telencephalic inputs. Most of the afferents from these different sources terminate in different, characteristic layers of the OT, a feature important for its coordinating and integrating roles.
During embryogenesis, the tectum develops from the simple neuroepithelium of the mesencephalic alar plate into a complex, multilayered structure, one of the most conserved sections of the vertebrate brain.
In most vertebrates three tectal layers can be distinguished: the superficial and central zones, where the tectal afferents end, and the periventricular zone (SPV), where the majority of the tectal cell bodies reside. The superficial and central zones can be subdivided into further layers, but the number of these varies among phyolgenetic groups. In fish the superficial zone consists of the stratum marginale (SM) and stratum opticum (SO); the central zone consists of the stratum fibrosum et griseum superficiale (SFGS), stratum griseum centrale (SGC) and stratum album centrale (SAC).
In a typical teleost, the OT contains between 11-15 morphologically distinct cell types are distinguishable, with the most abundant cells (over 90% of the total) being piriform neurons. These observations based on classical staining procedures, however, most likely understate the true diversity of the tectal neurons, as morphologically similar neurons can express very different transcription factor combinations.
The OT/SC coordinates the saccadic/microsaccadic eye movements of vertebrates and through the integration of topographic optic- and somatosensory information it regulates the motor functions of prey-catching and avoidance behaviours in frogs, and possibly humans. In teleosts tectal size and complexity varies depending on the behaviour and ecological niche of the fish: species that process more visual information have larger tecta.
Fish continuously grow throughout their life, thus unlike in mammals, where the neurogenic capacity of the SC diminishes postnatally, the neural stem cell niche of the OT in teleosts is preserved and the proliferation of tectal progenitor cells continues well into adulthood and most likely throughout the life of the animals.
The progenitor cells of the OT in zebrafish reside at the mediocaudal edge of the tectal hemispheres, adjacent to the tectal ventricles. Similarly to the stem cells located in the zebrafish cerebellum, and unlike the neural progenitors of the mammalian brain, these cells are neuroepithelial in their character and do not express glial markers.
The retinotectal pathway: formation and maintenance of topographic visual maps
One of the hallmark features of the vertebrate visual system is the topographic organization of the retinotectal projections. Retinal ganglion cells (RGCs) from the retina project through the optic chiasm to the controlateral tectal hemisphere, and through a seemingly simple matching rule Cartesian coordinates of the eye are projected on the OT. RGCs located in the nasal part of the retina project project to the caudal part of the controlateral OT, whereas temporal RGCs project rostrally. Similarly, the medial/dorsal parts of the controlateral tectal hemisphere receive projections from ventral RGCs, while the lateral/ventral tectal neurons are innervated by the dorsal retina. Intermediate RGCs terminate in the apporpriate intermediate position of the OT.
A further layer of complexity of retintotectal projections is added by the presence of multiple RGC subtypes in the retina. Although the axons of these ganglion cells traverse the tectum together through the SO, at their appropriate termination sites they will project to subtype-characteristic layers of the OT/SC.
Authors: Kara Cerveny and Máté Varga.
is part of: midbrain
Transgenic Lines/Antibodies that label this brain region
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