A MORPHOLOGICAL ANOMALY OF THE DORSAL LATERAL GENICULATE-NUCLEUS IN MACACA-FASCICULARIS

In the present report we describe a morphological anomaly of the thala mus. 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. Th ese anomalous fingers measured up to 1.7 mm in length and contained de nse accumulations of neurons and glia. The fingers received a direct r etinal 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 as sociated (parvocellular, magnocellular, or intercalated layers), we ex amined the immunochemical properties and size of neurons in the finger s and LGN subcompartments. We concluded that the fingers were not asso ciated with the intercalated layers, since neurons in the fingers did not stain with an antibody to calbindin-D28k, whereas intercalated neu rons stained intensely with this antibody. In addition, neurons locate d in the fingers were significantly smaller than those found in the ma gnocellular layers but were not significantly different in size from n eurons in the parvocellular layers. We therefore consider that the fin gers are an anomaly of the parvocellular subcompartment of the LGN. In terestingly, 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 observ ed subsidiary parvocellular laminae without anomalous fingers. Thus, i f there are common mechanisms underlying the development of both anoma lous fingers and subsidiary layers, our data indicate that they do not always result in the concomitant expression of both anomalies.