2009 年 40 巻 3 号 p. 24-33
The vomeronasal organ (VNO), a chemosensory organ, is present in most terrestrial vertebrates. Snakes and rodents possess highly developed VNOs in which their sensory cells called vomeronasal receptor cells (VRCs) contain an extremely well-developed smooth endoplasmic reticulum (SER). However, the functional roles of SER have not been elucidated. With the goal of understanding these roles, this study aimed to analyze the SER of the rodent species rat using a conventional transmission electron microscopic (TEM) technique, TEM tomography, post-embedding immunogold electron microscopy (EM), and a molecular biological technique. Adult (8-week-old) Sprague-Dawley rats of both sexes were fixed by transcardial perfusion with a mixture of paraformaldehyde and glutaraldehyde diluted in phosphate buffer. VNOs and testes were dissected and processed by epon-embedding and LR White resin-embedding. Conventional TEM showed that the SER in VRCs is composed of thin cisternae with minute arc- and circle-like units structures that are not present in the SER of Leydig cells in testes, which are known to possess a well-developed SER. TEM tomography showed that the arc- and circle-like units formed a complex looped structure. By post-embedding immunogold EM, the immunoreactivity for 17β-hydroxysteroid dehydrogenase type 1 (17β-HSD-1), a steroid hormone-synthesizing enzyme, was detected on the SER cisternal membrane. Western blot analysis confirmed the presence of 17β-HSD-1 in VNO. Furthermore, RT-PCR analysis indicated that VNOs expressed at least eight different types of 17β-HSD. Real-time PCR focusing on the mRNA expression of 17β-HSD-1 and -2 demonstrated that VNOs expressed both enzymes and contained even higher amounts of 17β-HSD-2 compared with that in testes and ovaries. The present study provides the first morphological evidence of the unique features of SER in VRCs, and the first immunocytochemical evidence for the presence of steroidogenic enzymes in sensory cells.
