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Microanatomy of Thalamic Radiations

BACKGROUND: Thalamic radiations also known as thalamocortical pathways are reciprocal myelinated nerve fibers, arranged in a fanning pattern, grouped into tracts or fasciculi; and connecting the thalamus to the cerebral cortex. Detailed in vitro study of these tracts is seldom reported in the literature. OBJECTIVE: We sought to describe the microanatomy of thalamic radiations by means of the fiber-dissection technique to discuss challenges in dissection techniques and anatomic nomenclature, and follow through with a literature review. METHODS: Twenty formalin-fixed normal human hemispheres were dissected according to Klingler’s fiber-dissection technique under operative microscope. RESULTS: Thalamic radiations are reciprocal myelinated nerve fibers connecting the thalamus to the cerebral cortex and are referred to as corticothalamic and thalamocortical tracts. They are the most medial fibers of the internal capsule and consist of anterior (thalamofrontal), superior (thalamo-fronto-parietal or thalamoparietal), posterior (thalamooccipital) and inferior (thalamotemporal) thalamic fasciculi. CONCLUSION: From the cerebral cortex, thalamic radiation fibers fan out into the thalamus and are the most medial fibers of the internal capsule. There is a great deal of controversy surrounding the distinction between anterior and superior thalamic radiations, sub-ependymal stratum and the fronto-occipital fasciculus. DOI : 10.14302/issn.2577-2279.ijha-17-1719 Corresponding Author: : Dominique N’dri Oka, UFR des Sciences médicales d’Abidjan/Cocody Côte d’Ivoire Email :ndriokad@gmail.com


Introduction
Detailed information regarding in vitro study of thalamic radiations is seldom found in the extant medical literature. Déjerine [9] (1895), Crosby [7] (1962) and Nieuwenhyus [20] (1988) successively demonstrated the existence of a superior occipital bundle. In 1909, Curran described for the first time the occipitofrontal fasciculus without mention of the superior occipitofrontal fasciculus. Tϋre demonstrated through his seminal work on brain dissection the absence of a superior occipital fasciculus [28] . Recent advances in diffusion-tensor MRI study of the brain white matter have shown the existence of other bundles such as an association tract connecting Broca and Wernicke areas, other than the superior longitudinal fasciculus [4] and the inferior occipitofrontal fasciculus [3,19,30] . It would be interested to investigate the inferior occipitofrontal bundle using the Klingler dissection technique. We sought to describe the microanatomy of thalamic radiations by means of the fiber-dissection technique to discuss challenges in dissection techniques and anatomic nomenclature, and follow through with a literature review

Anatomic Specimen Preparation
Twenty normal human brain hemispheres were dissected according to Klingler's fiber-dissection technique [18] regardless of gender. Specimens were taken from adult cadavers with no intracranial pathology nor history of such pathology in their medical history.
Brain hemispheres were removed 36 hours after death and immediately immersed in water, then fixed in a 5% formalin solution in which specimens were suspended by a string attached to the basilar artery. This helped preserve the morphology of specimens by restricting their contact with the bottom of the crystallizer. Formalin solutions were renewed on a weekly basis for 3 months until the brain parenchyma became firm but not hard.
After cleaning with running water, specimens were frozen at -15°C for 3 to 5 days. Afterwards they were allowed to thaw in water at room temperature, then dissected and stored back into 5% formalin solution.
The effect of this process is to loosen up gray matter so that individual white matter nerve bundles delineate clearly [18] .
The cortex was resected using curets and underlying white matter fasciculi could be followed progressively by peeling them off under the operating microscope (Zeiss OPMI 9FC Oberkechen Germany).
Stepwise dissection of fasciculi was documented with a digital camera and 2D images were then obtained.

Dissection technique
Dissections started with the identification of a  19,20] . But according to the results of the diffusion-tensor MRI [3,19,30] , the bundle of fibers uniting the frontal and parietal lobes could be referred to as the SOFF.
The stepwise dissection started from the lateral surface of the cerebral hemisphere, then moved to the medial surface using diffusion-tensor MRI tractography as a dissection guide.
At the lateral surface of the cerebral hemisphere, with or without operative microscope, we resected the cortex from the top of gyri to the deepest sulci using curets and wooden blunt-ended spatulas. As a result, U-shaped or arcuate association fibers connecting adjacent gyri were exposed. The gradual zabscission of these fibers from frontal, parietal and temporal opercula exposed the insula which cortex was ablated to discover the extreme capsule.
At the medial surface of the cerebral hemisphere, we resected the rostrum, genu of the corpus callosum and the cingulum respectively. The excision of the ependyma allowed us to expose the thalamus and the head of the caudate nucleus which, when ablated led us to the fibers of the superior occipitofrontal fasciculus as described by the diffusiontensor MRI. The SOFF was removed completely to expose, in front, the thalamic radiations which are the most medial fibers of the internal capsule, and behind, the tapetum of the corpus callosum which is a sub-

Results
Thalamic radiations or thalamocortical pathways are reciprocal myelinated nerve fibers, arranged in a fanning pattern, grouped into tracts or fasciculi; and connecting the thalamus to the cerebral cortex (Fig 2,3,4).  Posterior and inferiorly, the thalamotemporal fasciculus unites the posterior nucleus and the lateral geniculate body to the temporal cortex.

Tracts
Our dissections failed to identify the occipitofrontal fasciculus connecting the frontal and occipital lobes which was different from the subependymal stratum and anterior and superior thalamic radiations. The following is our stepwise dissection as anatomic structures are exposed in the order they appear. The removal of the external capsule and claustrum exposed the lateral surface of the putamen.
The internal capsule was exposed after the excision of the putamen and globus pallidus which were perfectly identifiable. The abscission of the outermost fibers of the internal capsule situated approximately 26 mm from the cerebral cortex and the head of the caudate nucleus exposed thalamic radiations (Figures 3).
The abscission of the frontal, parietal and temporal opercula exposed the superior longitudinal fasciculus (SLF) which was the first hemispheric and the largest association bundle to be seen and is located medial to the insular cortex. The SLF is an arcuate fasciculus which wraps around the insula and connects the frontal, parietal, occipital and temporal lobes together (Figures 1, 3). It has a superior and an inferior surface, anterior or frontal end, posterior extremity and the lateral and medial margins. The lateral opercular border is flattened and more spread across.
Its fibers project to the straight gyrus which was      The sub-ependymal stratum also known as the subcallosal stratum was made of a fiber-free layer of white matter. This layer of white matter was exposed after excision of the ependymal layer lining the ventricular system. It is located between the caudate nucleus and callosal radiations (Figures 1, 2, 4). Under Klingler [18] and Ebeling are the only authors which works on the pyramidal tract [11] stood out.
These technical challenges paved the way for the development in recent years of the diffusion-tensor MRI tractography which should be given credit as a new technology. This new technology was partially recognised in the seminal work of Kier on the uncinate fasciculus [15] . Questions remain on the credibility of his results given the fact that the dissection focused on a region of easy access and there are unanswered questions on the superposition in-vivo and ex-vivo tractography data.
The principle of tractography is to conduct a three-dimensional study of the heterogeneity of water molecules' speed. The brain white matter is highly anisotropic [14,19,22] . Tractography is based on the measurement of the degree of anisotropy of white matter fibers which allow the study of their microstructural organization [6,14,19,22] .
Our anatomic findings lead us to address common issues related to both methods. They both allow a three-dimensional study of white matter bundles. comparability of data and of correcting artefacts. This is true when analysing origins, junctions and the termination of fasciculi [14] . But tractography is noninvasive and can be done in-vivo or during surgery [1,21] .
Our anatomic region-of-interest identified as the superior occipitofrontal fasciculus presented dissection challenges in diffusion-tensor MRI tractography and in the Klinger technique alike, because it is located at the junction between callosal fibers and the internal capsule.

Difficulty of the Anatomic Nomenclature
Generally speaking, there is a great deal of controversy regarding the anatomic nomenclature of white matter fiber bundles in the extant medical literature probably due to the difficulty in their study.
Déjerine (1895) mentioned the occipitofrontal fasciculus which matched the superior occipitofrontal fasciculus described by the diffusion-tensor MRI tractography [3,19,30] . He did not mention the superior occipitofrontal fasciculus and failed to distinguish the inferior occipitofrontal fasciculus from the uncinate fasciculus. It is a long fasciculus located between the cingulum and the superior longitudinal fasciculus of Burdach [9] . This fasciculus is separated from the cingulum by the entire thickness of the corpus callosum and the superior longitudinal fasciculus by the foot of the corona radiata.

It looks like an open curve forward and downward, and
it is similar to the one described by the diffusion-tensor MRI. In 1909 Curran described in detail the inferior occipitofrontal fasciculus and paradoxically did not mention its superior counterpart. In 1956 Klingler described a subcallosal fasciculus connecting the frontal and occipital lobes which is attached laterally to the caudate nucleus, and did not named it the superior occipitofrontal fasciculus [18] . This fasciculus might had been described by Muratoff but there is no evidence of his work in the literature. Crosby (1962), in his treatise drew this fasciculus but failed to provide photographic evidence from his dissections [7] . He described the superior occipitofrontal fasciculus as a bundle of white matter fibers connecting the occipital and temporal cortex to the frontal and insular cortex [7] . Crosby and  [22] . Others think, it is a single fasciculus with two branches, an anterior branch and a posterior branch [18,21,29] . Diffusion-tensor MRI identified a fronto-parietal fasciculus quite different from the superior occipitofrontal fasciculus [3,19,30] . But that description is still close to that of Crosby [7] and Nieuwenhuys [20] . The superior occipitofrontal fasciculus is located under callosal radiations, hence the name subcallosal fasciculus [3] ; and between the cingulum and the superior longitudinal fasciculus [9] . Tractography data of this fasciculus showed a fasciculus different from the superior occipitofrontal fasciculus [3,19,30] . These provided that data interpretation is robust and reliable.