Regulation of interkinetic nuclear migration by cell cycle-coupled active and passive mechanisms in the developing brainIn proliferating neural epithelia, cells undergo interkinetic nuclear migration: stereotyped cell cycle-dependent movements in the apico-basal plane. The microtubule-binding protein Tpx2 is here shown to regulate the G2-phase basal-to-apical migration, while passive displacement effects are responsible for basally directed movements.

Intermediate filaments (IFs) are involved in many important physiological functions, such as the distribution of organelles, signal transduction, cell polarity and gene regulation. However, little information exists on the structure of the IF networks performing these functions. We have clarified the existence of seven kinds of IF networks in the cytoplasm of diverse polarized cells: an apex network just under the terminal web, a peripheral network lying just beneath the cell membrane, a granule-associated network surrounding a mass of secretory granules, a Golgi-associated network surrounding the Golgi apparatus, a radial network locating from the perinuclear region to the specific area of the cell membrane, a juxtanuclear network surrounding the nucleus, and an entire cytoplasmic network. In this review, we describe these seven kinds of IF networks and discuss their biological roles.

In rabbit intestinal epithelium, vimentin intermediate filaments are selectively expressed in the M cells of follicle-associated epithelium (FAE). To find intestinal epithelial cells belonging to the M cell lineage, vimentin was detected immunohistochemically in the rabbit small and large intestines. Vimentin-positive columnar cells were scattered throughout the villus epithelium of the small intestine. In their cytoplasm, vimentin was located from the perinuclear region to the cell membrane touching intraepithelial lymphocytes. These cells had microvilli shorter than those of absorptive cells, and the alkaline phosphatase activity of the microvilli was markedly weaker than that of absorptive cell microvilli. Glycoconjugates on the surface of the microvilli were alcian blue positive and periodic acid-Schiff negative. The morphological and histochemical features of these vimentin-positive villus epithelial cells differed from those of adjacent absorptive cells and closely resembled those of the M cells in FAE covering Peyer's patches and solitary lymphatic nodules. These results suggest that the vimentin-positive cells in the villus epithelium belong to the M cell lineage.

The livers of 12-day mouse embryos contain many hemopoietic cells in the hepatic cords and macrophages which are laden with large heterophagosomes in the sinusoids. Macrophages, as scavengers, engulfed circulating primitive erythroblasts from the yolk sac, as well as nuclei expelled from erythroblasts. At the 14th day of gestation, scavenger macrophages in mitosis were seen, extending long and thin cytoplasmic projections from their cell surface. At the 13-14 day of gestation, macrophages migrated from sinusoids into hepatic cords, and erythroblasts gathered around them, thus forming cell clusters designated as erythroblastic islets. Central macrophages formed a cell-socket-like structure on their cell surface for surrounding erythroblasts, reflecting the close association between macrophages and erythroblasts. In the erythroblastic islands, contained in late fetal and neonatal livers, erythroblasts have dissociated from the central macrophages having a starfish-shaped cell profile. The macrophage-erythroblastic association in the islands became less marked rapidly after birth.

No abstract

The expression of intermediate filaments is sensitively reflected in cell function. To examine the involvement of keratin in a secretory function, 15 kinds of keratin (keratin-2, 3, 4, 5, 6, 7, 8, 10, 13, 14, 16, 17, 18, 19, 20) were detected immunohistochemically and immunoelectron microscopically in the rabbit duodenum. Four types of secretory cells existed in the rabbit duodenum: enteroendocrine cells and goblet cells in the epithelium and mucous and serous cells in the duodenal glands. Among the 15 kinds of keratin, keratin 20 was selectively expressed in all these secretory cells. However, localization of keratin 20 in the endocrine cells differed from that in three types of exocrine cells. In the enteroendocrine cells, keratin 20-containing filaments formed a juxtanuclear network from which they extended to the apical cell membrane. These filaments may play a role in intracellular signal transduction, since the apical cell membrane contains some receptors for binding a specific extracellular signal. In the exocrine cells, on the other hand, keratin 20-containing filaments existed just beneath the cell membrane. These filaments may play some role in maintaining cell shape, which is remarkably changed during the secretory cycle.

A few neurons of the adult rabbit spinal ganglion express keratin. To examine the characters of these keratin-positive neurons, six kinds of intermediate filament proteins, namely keratin 8, keratin 14, nestin, vimentin, neurofilament 68 (NF-L) and glial fibrillary acidic protein (GFAP), were investigated immunohistochemically in developing and adult rabbit spinal ganglia. At 15 days of gestation, the spinal ganglion increased rapidly in volume and mainly consisted of three kinds of cells: small cells expressing vimentin, spindle-shaped cells co-expressing vimentin and nestin, and ovoid cells with an eccentric nucleus expressing nestin. Since some ovoid cells coexpressed nestin with either NF-L or GFAP, the ovoid cell may be considered to be an embryonic neural stem cell of the ganglion. In addition, a few keratin-positive polymorphic cells could be observed among these three kinds of cells. These polymorphic cells expressed five kinds of intermediate filament proteins, namely keratin 8, keratin 14, nestin, NF-L and GFAP. These cells were also detected in newborn and adult ganglia. A few neurons in the adult ganglion also expressed these five kinds of proteins as a Golgi-associated network. However, neurons expressing these proteins could not be detected in embryonic and newborn ganglia. Therefore, it may be considered that the keratin-positive polymorphic cell is a postnatal neural stem cell of the ganglion and that neurons transiently express keratin when polymorphic cells differentiate into neurons.

Accumulation and cell death of neutrophils were studied by light and electron microscopy in neonatal mouse bone marrow. At the beginning of bone marrow hematopoiesis, the marrow cavity contained a large number of polymorphonuclear leukocytes. Polymorphs comprised approximately 75% of the total nucleated cells in the hematopoietic compartment of the newborn marrow, the majority being neutrophils. Mature neutrophils were sometimes crossing the endothelium of the marrow blood sinus. Neutrophils in neonatal marrow show features typical of apoptosis, e.g. formation of nuclear pockets and blebs, margination of compact nuclear chromatin to form sharply circumscribed masses, condensation of cytoplasm, and convolution of cell outlines. Dying neutrophils were devoured and digested by phagocytes. The occurrence of large-scale neutrophil death and removal of neutrophils by phagocytes in neonatal bone marrow are discussed in relation to programmed cell death in development of the fetal hematopoietic system.