Fine structures of the human fetal cochlea were observed by scanning electron microscopy. The study aimed to make the knowledge on the developmental microanatomy of the cochlea more accurate. The temporal bones of fetuses were removed and the membranous portions of the cochleas were prepared for observation.
In a 14-week 6-day old fetus, the hair cells were arranged mainly in four rows. The sensory hairs differed in appearance from those in the adults. The innermost or first row hair cells corresponded to the inner hair cells and revealed certain characteristic structures. From the second to the third or outermost row, a rather uniform hair pattern with a bud-like appearance was noted. Neither W nor V letter formation of sensory hairs was found. One strong kinocilium was identified with its thick base; it was located lateral to the outermost stereocilia.
With older fetuses, 22 weeks 2 days and 24 weeks 0 day of age, the hair patterns closely resembled those of the adult with the exception of the presence of a short kinocilium on the outer hair cells and a slim kinocilium on the inner hair cells.
The present study indicates that structures including the sensory hairs and their attachment to the tectorial membrane, the Reissner's membrane and the stria vascularis essentially complete their morphological development in 6 months after gestation.

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Effects of pregnancy and lactation on lymphocytes in the bone marrow of the mouse were quantitatively examined by electron microscopy.
Pregnancy induces a decrease in the frequency of marrow lymphocytes, and delivery causes further depletion of marrow lymphocytes. Then the frequency of lymphocytes gradually increases to normal 20 days after delivery. The frequency is restored to the normal level earlier in non-lactating than in lactating mothers.
Marrow small lymphocytes, which constitute the majority of marrow lymphocytes, are distinguishable by electron microscopy into dark and light types. During pregnancy, the two types of small lymphocytes are equally decreased in proportion. Just after delivery dark small lymphocytes decrease markedly and thereafter increase gradually during lactation. Light small lymphocytes exhibit a gradual increase in proportion during lactation and reach the normal level at 20 days postpartum.
The results are discussed as compared with the effect of pregnancy and lactation on the lymphatic tissues, central and peripheral.

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Cell junction of the myelinated nerve mesaxon in the sciatic nerve and spinal posterior funiculus of rabbits and rats was examined by the freeze-fracture method. In replicas of fractured specimens, the cell junctions were the tight junction which was composed of parallel arrays of membrane particles. The rows of particles were straight or gently undulating along the long axis of the fibers. Occasional interconnections were formed among these rows. In the external mesaxon, the tight junctional particles in the central nerve were arranged in 3 to 6 lines and well developed more than those of the peripheral nerve consisting of 1 to 2 linear arrangement of particles. “Intramyelinic” tight junctions were well developed as well as some arrays of linear arrangement of particles in the central nerve. However, the junctions were poor and fragmental in the peripheral nerve.
Functional meanings of the difference of the mesaxonal and intramyelinic tight junctions between the central and peripheral myelinated nerve are discussed.

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The axonal initial segment of the Mauthner cell of tadpoles of Rana catesbeiana and its surrounding neuropil, the axon cap, were examined by electron microscopy.
The initial segment is almost completely covered by numerous synaptic terminals with profuse clear spherical synaptic vesicles. These synaptic terminals are of one uniform type and are found to be extensions of preterminal swellings, which are filled with accumulated mitochondria and glycogen granules in addition to the synaptic vesicles. Some swellings were shown to have two or more terminals. The synaptic terminals as well as the preterminal swelling are occasionally linked by small synaptic junctions. The significance of this structural organization in the inhibitory control of the Mauthner cell activation was discussed.

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The liver of the crucian, Carassius carassius (teleost) was studied by electron microscopy with special reference to the sinusoidal wall.
1. The sinusoidal endothelium partly comprises double layered endothelial sheets.
2. The fat-storing cell (FSC) or Ito cell is the only resident in the Disse's space. Containing several lipid droplets, it shows the same morphological characteristics as mammalian FSC.
3. Perisinusoidal space containing abundant hepatocytic microvilli lacks collagen fibrils and fibers (type III collagen). Instead, sparse fine fuzzy filaments (possibly type IV collagen) occur among microvilli. They increased in number and formed bundles or felt-like accumulations in random places in the ample vitamin A-given crucian.
4. After the same vitamin A administration, FSC showed dilated cisternae of its RER filled with a flocculent material. This finding seems to support that FSC may be responsible for production of the fuzzy filaments in the Disse's space.
5. The microfilaments and microtubules occur in the cytoplasm of the crucian FSC. They are increased after administration of large doses of vitamin A.
6. The most striking feature of the crucian FSC is the existence of characteristic, membrane-bound, electron-dense bodies mingling with lipid droplets. They consist of densely packed, parallel microtubules and, in untreated crucian, may show intimate topographical relationships with the lipid droplets.
7. After large doses of vitamin A, the crucian FSC contains, in conformity with an increase of small lipid droplets, fewer opaque dense bodies composed of loosely packed parallel microtubules and small vacuoles containing a bundle of short microtubules or microfilaments. These membrane-bound microtubular structures may possibly be precursors of the dense bodies. Though the nature of the dense bodies is obscure, cytochemically they are distinguished from lysosomes.
8. Desmosomal junctions frequently occur between an FSC, either cell body or cytoplasmic processes, and several hepatocytes. They unite several hepatocytes into one set and may presumably serve to maintain the hepatic parenchymal organization. Such desmosomal junctions between two cell types of different developmental origin have for the first time been recognized in the crucian.
9. In the normal untreated and vitamin A-stimulated crucian liver, Kupffer cells are completely absent.

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Fine structural characteristics of hepatic sinusoidal walls of the goldfish were described. Kupffer's stellate cells (macrophages) are few in number in the sinusoidal endothelial lining of this fish. Very rarely macrophages, whose small cytoplasmic protrusion is sometimes inserted between two endothelial cells, are localized in the sinusoidal lumen in contact with the endothelial cell.
Typical desmosomes are frequently seen between the two endothelial cells, between the endothelial cell and the Ito cell (perisinusoidal fat storing cell and fibroblast-like cell), between the endothelial cell and the hepatocyte, between two Ito cells, and between the Ito cell and the hepatocyte. Numerous microfilaments running almost parallel to the cell surface are gathered near the desmosomal structure of these cells.

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The synaptic junctions between glomus cells and nerve endings in the rabbit carotid body were studied by electron microscopy. Many of them were morphologically of afferent type, glomus cells being presynaptic to nerve endings; occurrence of efferent synapses was less common than that of afferent synapses. In addition, reciprocal synapses in which two or more junctions of opposite polarity were lying side by side were frequently seen particularly between large nerve endings and glomus cells. Some single synapses were discovered to be part of reciprocal synapses through examination of serial sections. The results support possible participation of dopamine in a modulatory feedback loop through reciprocal synapses in the rabbit carotid body.

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Just prior to the emergence of the tooth into the oral cavity, a small circular crater forms in the gingival epithelium immediately above the crown. The craters appear to expand in diameter during eruption to accommodate the emerging crown of the tooth. The observations support the concept that the crown of the tooth does not actively push through and rupture the gingival epithelium, but rather that the epithelium degenerates and then the tip of the crown emerges into the formed defect.

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