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Fig. 1 | EvoDevo

Fig. 1

From: Amphioxus, motion detection, and the evolutionary origin of the vertebrate retinotectal map

Fig. 1

The amphioxus photoreceptors relevant to this account. The photoreceptors of the frontal eye (FE) and lamellar body (LMB), shown in blue, are ciliary, whereas the Joseph cells (JCs, in green) are rhabdomeric. a Lateral view of the anterior nerve cord from the frontal eye and neuropore (np) to the end of somite 1, a composite diagram combining features from multiple developmental stages. Data on the frontal eye come from TEM reconstructions of pathways in young Branchiostoma floridae larvae, which document a projection from one putative retinal neuron (4L, yellow) to the primary synaptic zone (psz) near the center of the dien-mesencephalon as defined by molecular markers; there 4L synapses to dendrites belonging to the large paired neurons of the primary motor center (PMC). External sensory inputs to the psz are principally from fibers in the rostral nerve (rn), the anterodorsal nerve (dn), and the anterior group of dorsal bipolar neurons, included here as the anterior-most cells of the anterolateral group of migrated neurons (alm), which enlarges as the larvae mature. The Joseph cells are features of older larvae and the adult, and eventually extend forward to the front of the lamellar body according to data from B. lanceolatum. The assumption here is that output from the Joseph cells is directed to the cells and neuropile immediately beneath, and is local, so a moving light or passing shadow (large arrow) would be converted into a traveling wave of neural activity in adjacent neurons and neuropile (small arrows). The lamellar body could also play a role here, although, as a putative pineal homolog [10], its involvement in functions related to vision seems less likely, and its output appears to be predominantly to more ventral parts of the neuropile. b Cross section of the nerve cord of a juvenile stage at the level of the caudal dien-mesencephalon, showing the position of the lamellar body and Joseph cells, and the much larger zone of neuropile (npl) in the juvenile cord compared with young larvae. c TEM image of two adjacent Joseph cells, showing the microvillar array of one. Scale bar = 5 μm

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