Immunohistochemical and ultrastructural properties of the larval ciliary band-associated strand in the sea urchin Hemicentrotus pulcherrimus.
BACKGROUND
The swimming activity of sea urchin larvae is dependent on the ciliary band (CB) on the larval surface and is regulated by several neurotransmitters, including serotonin (5HT), dopamine, and γ-aminobutyric acid (GABA). However, the CB signal transmission mechanism remains unknown. The present study investigated the structural relationship between the CB and external signal receptors by immunohistochemical and transmission electron microscopic analyses of sea urchin, Hemicentrotus pulcherrimus, larvae.


RESULTS
Glutamate decarboxylase (GAD; GABA synthetase) was detected in a strand of multiple cells along the circumoral CB in 6-arm plutei. The GAD-expressing strand was closely associated with the CB on the oral ectoderm side. The ciliary band-associated strand (CBAS) also expressed the 5HT receptor (5HThpr) and encephalopsin (ECPN) throughout the cytoplasm and comprised 1- to 2-μm diameter axon-like long stretched regions and sporadic 6- to 7-μm diameter bulbous nucleated regions (perikarya) that protruded into the oral ectoderm side. Besides the laterally polarized morphology of the CBAS cells, Epith-2, which is the epithelial lateral cell surface-specific protein of the sea urchin embryo and larva, was expressed exclusively by perikarya but not by the axon-like regions. The CBAS exposed its narrow apical surface on the larval epithelium between the CB and squamous cells and formed adherens junctions (AJs) on the apical side between them. Despite the presence of the CBAS axon-like regions, tubulins, such as α-, β-, and acetylated α-tubulins, were not detected. However, the neuroendocrine cell marker protein synaptophysin was detected in the axon-like regions and in bouton-like protrusions that contained numerous small ultrastructural vesicles.


CONCLUSIONS
The unique morphology of the CBAS in the sea urchin larva epithelium had not been reported. The CBAS expresses a remarkable number of receptors to environmental stimuli and proteins that are probably involved in signal transmission to the CB. The properties of the CBAS explain previous reports that larval swimming is triggered by environmental stimuli and suggest crosstalk among receptors and potential plural sensory functions of the CBAS.
Research Center for Marine Biology, Tohoku University, Asamushi, Aomori, Aomori 039-3501 Japan ; Center of Research Instruments, Institute of Development, Aging and Cancer, Tohoku University, Sendai, 980-8575 Japan.
Hideki
H
Katow
Research Center for Marine Biology, Tohoku University, Asamushi, Aomori, Aomori 039-3501 Japan.
Tomoko
T
Katow
Center of Research Instruments, Institute of Development, Aging and Cancer, Tohoku University, Sendai, 980-8575 Japan.
Hiromi
H
Yoshida
Center of Research Instruments, Institute of Development, Aging and Cancer, Tohoku University, Sendai, 980-8575 Japan ; Marine and Coastal Research Center, Ochanomizu University, Tateyama, Chiba 294-0301 Japan.
Masato
M
Kiyomoto
Division of Biological Sciences, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397 Japan.
Isao
I
Uemura
5HThpr; Adherens junctions; Encephalopsin; Epith-2; GAD; Sea urchin pluteus; Synaptophysin