Postnatal Development of NPY and Somatostatin-28 Peptidergic Populations in the Human Angular Bundle

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.

Keywords: angular bundle; human development; neuropeptide Y; somatostatin-28; white matter.

Figure 1

Microphotographs of neuropeptide Y (NPY)-immunoreactive neurons and fibers at different ages. Panels (A–C) are the youngest cases (35–40 weeks), which show maximal amount of neuron and fiber immunoreaction. Most NPY-positive neurons are polygonal in shape. Panel (D) shows NPY-immunoreactive fibers, and panel (F) shows spindle shaped neurons at 14 years (730 weeks). In relation to the topography of the hippocampus, (A,D,F) are from rostral levels, (C) is midlevel, and (B,E) are at caudal levels. Scale bar is 50 μm.

Figure 2

Density of NPY- (in red) and somatostatin-28 (SOM-28)- (in blue) immunoreactive neurons expressed as units per square millimeter across different ages as determined at midlevel of the angular bundle.

Figure 3

Line-drawings (A–F) of the angular bundle white matter at midlevel of the hippocampus across ages in which NPY-positive neurons have been plotted. Individual neurons are represented by single dots. Other labeled neurons outside the angular bundle have not been represented. The age is indicated on each drawing. Note the decrease in the density of NPY-positive neurons with age. Scale bar is 2 mm.

Figure 4

SOM-28-positive neurons and fibers in the angular bundle white matter. Note the polygonal (panel D, newborn, case 5), spindle-shaped neurons (panel E, case 7, 2 weeks), or mixed morphology population (A,B,F). Fibers containing SOM-28 are presented in panel C. Note that the youngest ages show the most detailed morphology. Microphotographs (A,E) correspond to rostral levels of the hippocampus; Pictures (B–D,F) are at midlevel of the hippocampus. Scale bar is 50 μm.

Figure 5

Low power photomicrographs of the distribution of SOM-28-immunoreactive neurons in the angular bundle at increasing ages. Panel (A) is a 37 gestational week (gw) case showing multipolar, neurons, dendrites as well as immunostained fibers deep in the angular bundle. Panel (B) shows the concentration of stained neurons at the border with the entorhinal cortex (EC; to the right in the panel). Panel (C) shows that the density of labeled neurons is still high at 5 months, while panel (D) shows that, by 18 months, the apparent density is clearly lower. Topographically, panels (B,C) are at the rostral portion of the angular bundle; panels (A,D) correspond to caudal levels of the hippocampus. Scale bar is 200 μm.

Figure 6

Line drawings with representation of a series of sections of the angular bundle at midlevel in which SOM-28 neurons have been plotted. Only white matter neurons were charted. Panels (A,B) shows high density of SOM-28-immunolabeled neurons, which decrease by 23 months (E) and continues into 14 years (F). Other conventions as in Figure 3. Scale bar is 2 mm.

Figure 7

Histograms showing the density of NPY and SOM-immunoreactive neurons per square millimeter distributed at three levels of the angular bundle: rostral, mid and caudal. Note the different scale for the number of labeled neurons used in NPY and SOM neuropeptides, respectively.

Figure 8

The variation in density of both NPY and SOM-28 neurons at birth is depicted at three rostrocaudal levels of the angular bundle studied. Note the overall high, homogeneous density of labeled neurons for both neuropeptides in the angular bundle at the three levels. Other conventions as in Figure 3. Scale bar is 2 mm.

Figure 9

Line drawings of charted NPY-positive neurons at three different rostrocaudal levels of the angular bundle at one and a half years (18 months). While the rostral (A) and mid-level (B) of the series contains a substantial number of labeled neurons, the caudal section (C) shows fewer labeled neurons. Other conventions as in Figure 3. Scale bar is 2 mm.

Figure 10

Distribution of SOM-28-positive neurons at three rostrocaudal levels of the angular bundle at 18 months. Note the relative higher density rostral at midlevel. Conventions as in Figure 3. Scale bar is 2 mm.

Figure 11

Line drawings with the distribution of NPY- and SOM-28-immunoreactive neurons in the angular bundle of a 5 years old case at rostral (A,B), mid (C,D) and caudal (E,F) levels. Note the decrease in density at all levels compared to younger ages. Other conventions as in Figure 3. Scale bar is 2 mm.

Figure 12

Representation of the NPY-positive labeling in a 14-year-old case at the level of the head (A) body (B) and tail (C) of the angular bundle. The decrease in density at the three topographical levels of the angular bundle is noticeable, in particular compared to ages younger than 23 months. Other conventions as in Figure 3. Scale bar is 2 mm.

Figure 13

Plots of SOM-28 positive neurons in the angular bundle at rostral (A) middle (B) and caudal (C) levels of the hippocampus at 14 years. Note the decrease in density at the three levels compared to younger ages, at the same time that the remaining neurons localize preferentially near the border of the gray matter with layer VI of the overlying cortex (entorhinal or parahippocampal cortices). Other conventions as in Figure 3. Scale bar is 2 mm.