Mitogen-activated protein kinase (MAPK) cascades consist of members of three families of protein kinases: the MAPK family, the MAPK kinase family, and the MAPK kinase kinase (MAPKKK) family. Some of these cascades have been shown to play central roles in the transmission of signals that control various cellular processes including cell proliferation. Protein kinase NPK1 is a structural and functional tobacco homologue of MAPKKK, but its physiological function is yet unknown. In the present study, we have investigated sites of expression of the NPK1 gene in a tobacco plant and developmental and physiological controls of this expression. After germination, expression of NPK1 was first detected in tips of a radicle and cotyledons, then in shoot and root apical meristems, surrounding tissues of the apical meristems, primordia of lateral roots, and young developing organs. No expression was, however, observed in mature organs. Incubation of discs from mature leaves of tobacco with both auxin and cytokinin induced NPK1 expression before the division of cells. It was also induced at early stages of the development of primordia of lateral roots and adventitious roots. Thus, NPK1 expression appears to be tightly correlated with cell division or division competence. Even when an inhibitor of DNA synthesis was added during the germination or the induction of lateral roots by auxin, NPK1 expression was detected. These results showed that the NPK1 expression precedes DNA replication. We propose that NPK1 participates in a process involving the division of plant cells.
In the medaka, movement of spermatozoa and changes in the egg micropyles during fertilization were observed through a video camera and recorded with a video recorder to analyse sperm movement. Movement of spermatozoa as they entered micropyles in both intact and isolated chorions was compared before and after fertilization of the eggs. The inner one third of the micropyle was completely closed and the micropylar vestibule became shallow by 5 min after sperm attachment. Spermatozoa did not increase in swimming speed in the vicinity of the micropyle and were not attracted to it. The majority of spermatozoa entering the micropyle were rotating at a n average frequency of 8 Hz in the right-hand direction. The rotation direction did not correspond to the left-hand spiral structure of the micropylar wall, though a small amplitude of the beating sperm flagellum corresponded with the narrow micropylar canal. The frequency of sperm entry into the micropyle decreased significantly in intact eggs and isolated chorions following fertilization, independent of artificial occlusion of the micropylar canal. Moreover, in isolated chorions before fertilization, spermatozoa rarely entered the micropylar canal from its inner aperture. The present data suggest the existence of some substance in the micropyle which guides spermatozoa into the micropylar canal, although the perimicropylar depression might also play a role in the guidance of spermatozoa. o 1993 Wiley-Liss, Inc.
Ultrastructural changes in differentiating micropylar cells and in the micropyle occur during oogenesis in the medaka, Oryzias latipes. A micropylar cell is not detectable in previtellogenic ovarian follicles. In the early vitellogenic phase, the micropylar cell becomes differentiated from neighboring granulosa cells by its electrondense cytoplasm. The micropylar cell in this phase characteristically displays an increase in rough endoplasmic reticulum, Golgi complexes, and tonofilaments around the nucleus. By the late vitellogenic phase, the enlarged micropylar cell extends a broad cytoplasmic process to the oocyte surface. A conspicuous feature of the process is a large bundle of microtubules oriented perpendicular to the oocyte surface. The inner surface of the micropylar canal has a spiral structure and is covered with the outermost layer of the chorion. In the postvitellogenic phase, the main cell body possesses many tonofilaments, mitochondria, Golgi complexes, and rough endoplasmic reticulum, and the winding cytoplasmic process contains a twisted large bundle of microtubules with a bundle of tonofilaments as its core. The spiral structure of the micropylar vestibule and the micropylar canal reflects the twisting associated with elongation of fibrous bundles of the micropylar cell anchored on the chorion at the animal pole of the oocyte.
The relationship between spiral patterns in the inner wall of the micropyle and attaching and non-attaching filaments on the chorion was investigated by electron microscopy in eggs of the medaka, Oryzias latipes. There were both right-and left-handed spiral patterns of attaching filaments that arose from the chorion at the vegetal pole area. In the majority of eggs examined, however, the prominent folds in the inner wall of the micropylar vestibule descended toward the micropylar canal in a left-handed spiral. This spiral pattern in the micropyle may reflect surface indentations in the cytoplasmic extension of the micropylar cell during formation of the micropyle apparatus. The present observations suggest that the patterns of these spiral structures of the chorion are caused by the rotation of the oocyte due to the movement of follicular cells during oogenesis. On the other hand, there is no unique structure located at the vegetal pole in most teleosts. In the medaka egg, however, a cluster of many long attaching filaments identify the chorion overlying the vegetal pole (Hart et al., '84; Iwamatsu, '75; Kubo, '35). It has been found that the long attaching filaments are wound spirally in one direction on the axis between the animal and vegetal poles of the egg (Iwamatsu, '75, '92a). Medaka eggs exhibit spherical symmetry around the animal-vegetal axis that can be drawn between the micropyle and the center of the cluster of the attaching filaments. The relationship between the right-and left-handed orientation of the spiral folds in the inner wall of the 0 1993 WILEY-LISS, INC.micropyle and the attaching filaments is very interesting because of its association with formation of the chorion and especially with determination of egg polarity during oogenesis.In order to gain a clue as to the mechanism for determination of egg polarity, the morphology of the attaching filaments and micropyles of mature eggs was investigated using scanning and transmission electron microscopy. The results suggest that the spiral orientation of such chorionic structures as the inner wall of the micropyle and the attaching and non-attaching filaments reflects the dynamic development of the oocyte within the basement membrane during oogenesis. MATERIALS AND METHODSThe medakas, Oryzias latipes (orange-red variety) used in the present study, were purchased from a fish farm (Yamato-koriyama, Nara Pref.) and kept for more than 2 weeks under conditions suitable for reproduction (light 14 hr, darkness 10 hr; temperature 26-28OC). After the females' brains were pithed, the ovaries were removed from their body cavities and placed in saline by a routine procedure. Unfertilized mature eggs were released from the ovarian lumen, and the long attaching filaments on each egg were cut short to facilitate observation.For observations by electron microscopy, eggs were prefixed for more than 6 hr with buffered 4%
Ultrastructural changes in the maturing oocyte of the sea urchin Hemicentrotus pulcherrimus were observed, with special reference to the behavior of centrioles and chromosomes, using oocytes that had spontaneously started the maturation division process in vitro after dissection from ovaries. The proportion of oocytes entering the maturation process differed from batch to batch. In those eggs that accomplished the maturation division, it took ~4.5-5 h from the beginning of germinal vesicle breakdown to the formation of a second polar body. Serial sections revealed that a young oocyte before germinal vesicle breakdown had a pair of centrioles with procentrioles, located between the presumed animal pole and the germinal vesicle and accompanied by amorphous aggregates of moderately dense material and dense granules (granular aggregate). Just before germinal vesicle breakdown, a pair of fully grown centrioles located in the granular aggregate, which is present until this stage and then disappears, had already separated from another pair of centrioles. In meiosis I, each division pole had two centrioles, whereas in meiosis II each had only one. The two centrioles in the secondary oocyte separated into single units and formed the mitotic figure of meiosis II. The first polar body had two centrioles and the second had only one. The two centrioles in the first polar body did not form the mitotic figure nor did they separate at the time of meiosis II. These results indicate that, in sea urchins, duplication of the centrioles does not occur during the two successive meiotic divisions and the egg inherits only one centriole from the primary oocyte, confirming the results previously reported for starfish oocytes.
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