A morphological anomaly of the dorsal lateral geniculate nucleus in Macaca fascicularis

In the present report we describe a morphological anomaly of the thalamus. In three macaque monkeys (Macaca fascicularis), we observed up to five finger-like protrusions that emanated from the posterior pole of the dorsal lateral geniculate nucleus (LGN) and extended posteriorly between the lateral pulvinar and reticular nucleus of the thalamus. These anomalous fingers measured up to 1.7 mm in length and contained dense accumulations of neurons and glia. The fingers received a direct retinal input from the contralateral eye indicating that they were part of the LGN rather than of other adjacent thalamic nuclei. In order to determine with which subcompartment(s) of the LGN the fingers were associated (parvocellular, magnocellular, or intercalated layers), we examined the immunochemical properties and size of neurons in the fingers and LGN subcompartments. We concluded that the fingers were not associated with the intercalated layers, since neurons in the fingers did not stain with an antibody to calbindin-D28k, whereas intercalated neurons stained intensely with this antibody. In addition, neurons located in the fingers were significantly smaller than those found in the magnocellular layers but were not significantly different in size from neurons in the parvocellular layers. We therefore consider that the fingers are an anomaly of the parvocellular subcompartment of the LGN. Interestingly, in two of the three cases with anomalous fingers, we also observed subsidiary parvocellular laminae, suggesting that these two anomalies were related. In five additional animals, however, we observed subsidiary parvocellular laminae without anomalous fingers. Thus, if there are common mechanisms underlying the development of both anomalous fingers and subsidiary layers, our data indicate that they do not always result in the concomitant expression of both anomalies.