The brain structure of selected trypanorhynch tapeworms.

The brain architecture in four species of tapeworms from the order Trypanorhyncha has been studied. In all species, the brain consists of paired anterior and lateral lobes, and an unpaired central lobe. The anterior lobes connect by dorsal and ventral semicircular commissures; the central and lateral lobes connect by a median and an X-shaped crisscross commissure. In the center of the brain, five well-developed compact neuropils are present. The brain occupies a medial position in the scolex pars bothrialis. The ventral excretory vessels are situated outside the lateral lobes of the brain; the dorsal excretory vessels are located inside the brain and dorsal to the median commissure. The brain gives rize four anterior proboscis nerves and four posterior bulbar nerves with myelinated giant axons (GAs). The cell bodies of the GAs are located within the X-commissure and in the bulbar nerves. Highly developed serotonergic neuropils are present in the anterior and lateral lobes; numerous 5-HT neurons are found in the brain lobes including the central unpaired lobe. The X-cross commissure consists of the α-tub-immunoreactive and 5-HT-IR neurites. Eight ultrastructural types of neurons were found in the brain of the three species investigated. In addition, different types of synapses were present in the neuropils. Glial cells ensheath the brain lobes, the neuropils, the GAs, and the bulbar nerves. Glia cell processes form complex branching patterns of thin cytoplasmic sheets sandwiched between adjacent neural processes and filling the space between neurons. Multilayer myelin-like envelopes and a mesaxon-like structure have been found in Trypanorhyncha nervous system. We compared the brain architecture of Trypanorhyncha with that of an early basal cestode taxon, that is, Diphyllobothriidea, and present a hypothesis about the homology of the anterior brain lobes in order Trypanorhyncha; and the lateral lobes and median commissure are homologous brain structures within Eucestoda.

Development of piriform cortex interhemispheric connections via the anterior commissure: progressive and regressive strategies.

The anterior commissure (AC) is a phylogenetically conserved inter-hemispheric connection found among vertebrates with bilateral symmetry. The AC connects predominantly olfactory areas but many aspects of its development and structure are unknown. To fill this gap, we investigated the embryonic and postnatal development of the AC by tracing axons with DiI and the piggyback transposon multicolor system. With this strategy, we show that axon growth during establishment of the AC follows a strictly regulated timeline of events that include waiting periods ("regressive strategies") as well as periods of active axon outgrowth ("progressive strategies"). We also provide evidence that these processes may be regulated in the midline via overexpression of chondroitin sulfate proteoglycans. Additionally, we demonstrate that the ipsi- and contralateral innervation of piriform cortex occurs simultaneously. Morphologically, we found that 20% of axons were myelinated by postnatal day (P) 22, in a process that occurred fundamentally around P14. By immunohistochemistry, we described the presence of glial cells and two new subtypes of neurons: one expressing a calretinin (CR)-/MAP2+ phenotype, distributed homogeneously inside the AC; and the other expressing a CR+/MAP2+ phenotype that lies beneath the bed nucleus of the stria terminalis. Our results are consistent with the notion that the AC follows a strictly regulated program during the embryonic and postnatal development similarly to other distal targeting axonal tracts.