Abstract
The angular bundle is a white matter fiber fascicle, which runs longitudinally along the parahippocampal gyrus. It is best known for carrying fibers from the entorhinal cortex (EC) to the hippocampus through the perforant and alvear pathways, as well as for carrying hippocampal output to the neocortex, and distributing fibers to polysensory cortex. The angular bundle is already present prenatally at the beginning of the fetal period. Connections between the EC and the hippocampus are established by the 20th gestational week (gw). In the postnatal period, it shows increasing myelination. The angular bundle, as well as other white matter portions of gyral surfaces in the brain, presents interstitial neurons, a remnant of subplate neurons. Those interstitial neurons show neurochemical phenotypes both prenatally and postnatally, among which, neuropeptide Y (NPY) and Somatostatin-28 (SOM-28) peptidergic populations are noticeable, and accompany the fiber connections in the maturation of the hippocampal formation. We sought to investigate the topography of the postnatal distribution and relative density of neurons immunoreactive for NPY or SOM in the angular bundle along the rostrocaudal axis of the hippocampus. The study was carried out in 15 cases, ranging from 35 gws, up to 14 year old. All cases showed positive neurons showing a polygonal or spindle shaped morphology for both peptides, scattered throughout the angular bundle. The highest number of positive neurons appeared around birth and the ensuing weeks. Up to one and a half years, the density of both peptidergic populations decreased slightly. However, cases older than 2 years of age showed a substantial decrease in density of immunolabeled neurons, density that did not showed a minor decrease in density of positive neurons in cases older than 2 years. In addition, a topography from caudal to rostral levels of the angular bundle was detected at all ages. The functional significance of interstitial cells is unknown, but the existence of SOM and NPY peptidergic neurons, presumably inhibitory, in the white matter of the angular bundle, could contribute to the basic wiring of the hippocampal formation, through which autobiographical and spatial memories can begin to be stored in the infant brain.
Highlights
The presence of neuropeptides in the white matter of the human brain has been known since 40 years
We aim in this study at giving a more detailed account of white matter interneurons immunoreactive for two neuropeptides (NPY and SOM-28) in the angular bundle, as well as its distribution in the rostrocaudal extent of the angular bundle, a fiber tract that serves as pathway for important interconnections of memory related centers, such as the hippocampal formation and parahippocampal region, and Witter (2002) for that of parahippocampal region
The boundaries change along the rostrocaudal axis: rostrally, the upper limit is the amygdaloid complex; layer VI of the entorhinal cortex (EC) is the medial boundary, and the white matter lining the cortex of the medial bank of the collateral sulcus is the lateral boundary
Summary
The presence of neuropeptides in the white matter of the human brain has been known since 40 years. The subplate disappears postnatally, notwithstanding many cells remain, in particular in the white matter of the gyral crown, and in association cortices, as it is the case of prefrontal cortex (Kostovic et al, 2011) and the parahippocampal gyrus (Insausti and Amaral, 2012). We aim in this study at giving a more detailed account of white matter interneurons immunoreactive for two neuropeptides (NPY and SOM-28) in the angular bundle, as well as its distribution in the rostrocaudal extent of the angular bundle, a fiber tract that serves as pathway for important interconnections of memory related centers, such as the hippocampal formation and parahippocampal region (temporopolar, perirhinal and posterior parahippocampal cortices; see Insausti and Amaral, 2012 for concept of hippocampal formation), and Witter (2002) for that of parahippocampal region
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