Fumihiko Iwaku

Occlusal disharmony induces spatial memory impairment and hippocampal neuron degeneration via stress in SAMP8 mice

We examined the effect of occlusal disharmony in senescence-accelerated (SAMP8) mice on plasma corticosterone levels, hippocampal neuron number, and spatial performance in the water maze. The bite-raised condition was associated with an accelerated age-related decline in spatial memory, increased plasma corticosterone levels, and a decreased number of neurons in the hippocampal CA3 region. The findings suggest that the bite-raised condition in aged SAMP8 mice induces hippocampal neuron loss, thereby leading to senile memory deficits.

Molarless-induced changes of spines in hippocampal region of SAMP8 mice.

We examined the effect of the molarless condition on the dendritic spines of hippocampal pyramidal cells in SAMP8 mice in comparison to its effect on learning ability in a maze test. The molarless condition caused a decrease in the number of the spines of CA1 pyramidal cells only in the aged mice showing a reduced learning ability. The results suggest the involvement of the molarless condition in an attenuation of input activities in the hippocampal synapses.

Occlusal disharmony attenuates glucocorticoid negative feedback in aged SAMP8 mice.

To evaluate the mechanism underlying impaired cognitive function due to occlusal disharmony, we examined the effect of the bite-raised condition on spatial performance and hippocampal expression of glucocorticoid receptors (GR) and glucocorticoid receptor messenger ribonucleic acid (GRmRNA) using behavioral, immunohistochemical, and in situ hybridization techniques. Learning ability in the water maze test was significantly impaired in aged bite-raised mice compared with age-matched control mice. There was no difference between control and bite-raised young and middle-aged mice. Also, immunohistochemical and in situ hybridization analysis showed that the bite-raised condition enhanced the age-related decrease in GR and GRmRNA expression in the hippocampus. In particular, GR and GRmRNA expressions were significantly decreased in aged bite-raised mice compared to age-matched control mice. These findings suggest that the bite-raised condition in aged SAMP8 mice decreases GR and GRmRNA, which impairs the hypothalamic-pituitary-adrenal feedback inhibition, thereby leading to memory deficits.

Structure and direction of jaw adductor muscles as herbivorous adaptations in Neotoma mexicana (Muridae, Rodentia)

The herbivorous adaptations of the jaw adductor muscles in Neotoma mexicana were clarified by a comparative study with an unspecialized relative, Peromyscus maniculatus. In P. maniculatus, the anterior part of the deep masseter arises entirely from the lateral side of an aponeurosis, i.e., superior zygomatic plate aponeurosis, whereas N. mexicana has an additional aponeurosis for this part of the muscle, and the fibers attach on both sides of the superior zygomatic plate aponeurosis. Although the structure of the temporalis muscle is nearly identical in the two genera, a clear aponeurosis of origin occurs only in N. mexicana. These characteristics allow fibrous tissues to be processed with a large occlusal force. The deep masseter, internal pterygoid, and external pterygoid muscles of N. mexicana incline more anterodorsally than those of P. maniculatus. The transverse force component of these muscles relative to whole muscle force is smaller in N. mexicana than in P. maniculatus, with the exception of the internal pterygoid. The anterior part of the temporalis muscle of N. mexicana is specialized to produce occlusal pressure. These findings suggest that in N. mexicana a large anterior force is required to move the heavy mandible, due to the hypsodont molars, against frictional force from food, and that the posterior pull of the temporalis, which adjusts the forward force by the other jaw adductor muscles to a suitable level, need not be large for the mandibular movement.

Masticatory muscle architecture in a murine murid, Rattus rattus, and its functional significance

ABSTRACT The internal architecture of the masticatory muscles in a murine murid, Rattus rattus, was examined and compared with our previous description of Peromyscus maniculatus, which seems to have primitive jaw muscles for muroid rodents. Remarkable differences were observed mainly in the masseter muscle and corresponding aponeurosis. The anterior and posterior mandibular aponeuroses for insertion of the masseter deep layer are united as a single sheet in Rattus. In Peromyscus, both the origin and insertion of the anterior part of the masseter deep layer lie on the aponeurosis. In Rattus, however, the deep fibers of the anterior part form a muscular attachment to the bone surface at either end. This characteristic suggests that the maximum angle of gape is increased in murine murids, which allows them to ingest various-sized foods.

Microvasculature of Bone and Bone Marrow

The blood vascular architecture of bone, until lately, has mainly been investigated by various methods using long bones. These methods have been the India-ink injection method, the latex injection replica technique, angiography, and others. Many features of the blood circulation and bone vascular architecture have been clarified, but the fine three-dimensional vascular architecture remains inadequately understood [1–12].

Foramen magnum angle and its effect on visual field in two Apodemus murids

Abstract In rodents, an adaptation for biting off hard materials, accompanied by extension of the masseter origin anterior to the orbit, might interfere with vision in the anteroventral direction. In arboreal rodents, the anatomy must be modified somewhat to allow for the clear anteroventral vision necessary to safely descend from, or move about on trees. In the present study, we tested this hypothesis by comparing the craniometric data between two Apodemus murids, semiarboreal A. argenteus and terrestrial A. speciosus. Based on the correlation between the index of relative encephalization (IRE; cube root of the brain case volume divided by the basicranial length) and the angle of the foramen magnum, the head in A. argenteus is situated so that the snout is more depressed than in A. speciosus, which has the same IRE value. The head posture of A. argenteus does not seem to be due to the relative brain size, but rather to a semiarboreal adaptation that ensures clear anteroventral vision by lowering the anterior portion of the masseter to a greater degree than the eyeball.

Effect of tube feeding on hippocampal-dependent memory in SAMP1 mice

Abstract This study examined the effect of tube feeding on hippocampal Fos induction and spatial performance in a water maze task in senescenceaccelerated mice (SAMP1). Tube feeding accelerated the age-related decline in spatial memory and decreased Fos induction in the hippocampal CA1 region in aged SAMP1 mice. The results suggest that tube feeding in aged SAMP1 mice reduces input activity in the hippocampus, thereby leading to senile memory deficits.

The Ganoine Membrane of the Scales of Polypterus senegalus Includes a Labyrinth-Like Structure

The ganoine membrane is a basement membrane-like structure appearing between the maturation stage inner ganoine epithelium (IGE) and mineralized ganoine of polypterids and lepisosteids. In the present study, further ultrastructural investigation of the ganoine membrane of Polypterus senegalus was made and it was confirmed that it possessed a specialized labyrinth-like structure. The complete ganoine membrane was composed of six distinct zones: (1) a filamentous zone facing the distal plasmalemma of IGE, (2) lamella zone I, (3) vertical striation zone, (4) lamella zone II, (5) electron-dense zone, (6) electron-lucent zone directly facing the maturing ganoine surface. The ganoine membrane sometimes partially lacked some of these zones, but the electron-dense zone was usually present. Oblique or horizontal sections of the membrane showed that the vertical striation zone was made up of a typical labyrinth-like structure. Certain sulfated glycoconjugates were localized at the filamentous zone, especially adjacent to the lamella zone I and over the electron-dense zones.