Ultrastructural aspects of the first postmeiotic mitosis and cytokinesis in the pollen grain of allium cepa l

This report deals with research work on first pollen grain mitosis and the formation of vegetative and generative cells in Allium cepa L.The haploid nucleus of young microspores has a long interphase period which starts when the microspores begin to free themselves from the surrounding special callose wall and finishes when a central vacuole is formed in their cytoplasm. The pollen wall (Exine and Intine) is formed during this period.Before the mitosis, a large vacuole is formed in the cytoplasm. The microspore nucleus that previously occupied a central position in the cell can now be found in a lateral position generally on the opposite side of the porus. At this moment the nucleus enters prophase and the mitotic division takes place. This is characterized by the formation of an asymmetric spindle and as a result two unequal daughter cells are formed (Fig. 1) : the vegetative cell is larger and its nucleus remains near the centre of the pollen grain, whilst the generative cell is smaller and at the beginning lies close to the pollen wall (Fig. 2, 3).

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Fine Structure Study of Pollen Development in Haemanthus katherinae Baker

Journal of Cell Science ◽

10.1242/jcs.8.2.303 ◽

1971 ◽

Vol 8 (2) ◽

pp. 303-315

Author(s):

JEAN M. SANGER ◽

W. T. JACKSON

Keyword(s):

Endoplasmic Reticulum ◽

Fine Structure ◽

Pollen Development ◽

Generative Cell ◽

Structure Study ◽

Pollen Grain ◽

Pollen Wall ◽

Spheroidal Shape ◽

Oriented System

The newly formed generative cell of the pollen grain of the African blood lily is spheroidal after its detachment from the pollen wall. Plastids are almost always excluded from the generative cell, while dictyosomes, mitochondria, ribosomes, endoplasmic reticulum, and microtubules are present throughout development of the cell. During the time that the generative cell is spheroidal, microtubules are found scattered randomly throughout the cytoplasm. The cell subsequently elongates and concurrently an oriented system of microtubules appears along the wall of the cell. The microtubules are aligned with their long axes parallel to the long axis of the cell. This system of microtubules persists in the generative cell throughout its development. The microtubules can be destroyed by exposure to isopropyl N-phenylcarbamate or colchicine and as a result the generative cell reverts to a near spheroidal shape.

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The Ultrastructure and Ontogeny of Pollen in Helleborus Foetidus L

Journal of Cell Science ◽

10.1242/jcs.11.1.111 ◽

1972 ◽

Vol 11 (1) ◽

pp. 111-129

Author(s):

P. ECHLIN

Keyword(s):

Endoplasmic Reticulum ◽

Cell Wall ◽

Vegetative Cell ◽

Generative Cell ◽

Ultrastructural Level ◽

Mitotic Division ◽

Tube Formation ◽

Pollen Grain ◽

Granular Body ◽

Development Proceeds

The final stages of Helleborus pollen-grain ontogeny, which culminate in maturation and germination of the grain, have been investigated at the ultrastructural level. Following the deposition of primary and secondary exine, and during the early stages of intine formation, the microspore passes through a vacuolate phase, in which the cytoplasm appears devoid of most organelles other than the prominent nucleus. The formation of the vacuole results in the displacement of the nucleus to one side of the pollen grain. The vacuole quickly disappears and a number of organelles reappear in the cytoplasm, in particular the dictyosomes and strands of endoplasmic reticulum, with associated grey bodies. Following mitotic division of the pollen grain, the first signs of the generative cell wall appear as a pair of tightly appressed unit membranes in the narrow strand of cytoplasm separating the two newly formed generative and vegetative nuclei. As development proceeds, the space between the two membranes gradually fills with an electron-transparent material similar to the substance found in the numerous dictyosome-derived vesicles which, together with the endoplasmic reticulum, are both closely associated with the developing cell wall. The generative cell wall fuses with the cellulosic intine, which has gradually increased in amount during these stages, and the cell division is complete. The smaller generative cell contains a prominent nucleus and a small amount of cytoplasm devoid of plastids and most other organelles. The larger vegetative cell also contains a prominent nucleus and a large amount of cytoplasm containing amyloplasts, mitochondria, dictyosomes and endoplasmic reticulum, and abundant ribosomes, many of which are in a polysome configuration. The final stages in development are characterized by a progressive decrease in the amount of starch in the vegetative cell and an increase in the size of grey bodies, many of which are invested in multilayered shrouds of endoplasmic reticulum. The generative cell wall disappears and a multivesicular/granular body gradually appears at the periphery of the pollen grain. The granular-vesicular material, which is formed from the dictyosomes and/or the degenerating plastids, is thought to represent metabolic reserves necessary for pollen-tube formation. One or more pollen tubes emerge from the apertural sectors of the pollen grain, and maturation of the grain is complete.

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Pollen ultrastructure in anther cultures of Datura innoxia. III. Incomplete microspore division

Journal of Cell Science ◽

10.1242/jcs.22.3.493 ◽

1976 ◽

Vol 22 (3) ◽

pp. 493-501

Author(s):

J.M. Dunwell ◽

N. Sunderland

Keyword(s):

Mitotic Spindle ◽

Generative Cell ◽

Marked Change ◽

Pollen Grain ◽

Wall Material ◽

Pollen Embryogenesis ◽

Vegetative Nucleus ◽

Central Position ◽

Datura Innoxia ◽

Anther Cultures

During the microspore division in Datura innoxia, the mitotic spindle is oriented in planes both perpendicular (PE) and oblique (OB) to the spore wall against which the nucleus is situated. However, irrespective of polarity, the usual type of hemispherical wall is laid down at cytokinesis and isolates the generative cell from the rest of the pollen grain (type A). In PE spores the vegetative nucleus initially occupies a central position in the pollen grain, whereas in OB spores the vegetative nucleus lies at the periphery of the grain close to the generative cell. In anther cultures initiated just before the microspore division is due to take place, no marked change can be observed in either orientation or symmetry of the mitotic spindle when the spores divide. In some, however, cytokinesis is disrupted and deposition of the hemispherical wall arrested. In the absence of a complete wall, differentiation of the generative cell cannot take place and binucleate pollen grains are formed having 2 vegetative-type nuclei (type B). The 2 nuclei in the B pollens are always situated against the pollen-grain wall, suggesting that the disruption phenomenon is related to the OB spores. The incomplete wall always makes contact with the intine on the intine-side of the spindle. Wall material may be represented merely as short stubs projecting out from the intine into the cytoplasm, in which event the 2 nuclei lie close to each other and are separated by only a narrow zone of cytoplasm. In other grains the wall is partially developed between the nuclei and terminates at varying distances from the tonoplast; in these, the nuclei are separated by a wider zone of cytoplasm. The significance of these binucleate grains in pollen embryogenesis is discussed.

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Optical and ultrastructural study of the pollen grain development in hermaphrodite papaya tree (Carica papaya L.)

Brazilian Archives of Biology and Technology ◽

10.1590/s1516-89132008000300013 ◽

2008 ◽

Vol 51 (3) ◽

pp. 539-545 ◽

Cited By ~ 3

Author(s):

Lídia Márcia Silva Santos ◽

Telma Nair Santana Pereira ◽

Margarete Magalhães de Souza ◽

Pedro Correa Damasceno Junior ◽

Fabiane Rabelo da Costa ◽

...

Keyword(s):

Carica Papaya ◽

Generative Cell ◽

Pollen Grains ◽

Male Gamete ◽

Mitotic Division ◽

Pollen Grain ◽

Grain Development ◽

Normal Pattern ◽

Papaya Tree ◽

Mother Cells

The objective of this study was to describe the pollen grain development in hermaphrodite papaya tree. The flower buds were collected at different stages of the development and the anthers were treated chemically for observation under optical and electronic transmission microscopes. The pollen grain development followed the normal pattern described for the Angiosperms. The pollen grain development was described from meiocyte to the mature pollen grain. In the microsporogenesis, the microspore mother cells or the meiocytes underwent meiosis giving rise to the tetrads that were enclosed by the calose. Later, the tetrads were released by the dissolution of the calose by calase activity and microspores underwent mitosis. Microgametogenesis was characterized by asymmetrical mitotic division of each microspore giving rise to bi-nucleate pollen grains. The structures similar to the plastids were found in the cytoplasm and close to the nucleus of the generative cell. Gradual degeneration was observed in the tapetum during the male gamete development.

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Fine Structure Study of Pollen Development in Haemanthus katherinae Baker

Journal of Cell Science ◽

10.1242/jcs.8.2.289 ◽

1971 ◽

Vol 8 (2) ◽

pp. 289-301

Author(s):

JEAN M. SANGER ◽

W. T. JACKSON

Keyword(s):

Fine Structure ◽

Vegetative Cell ◽

Daughter Nucleus ◽

Generative Cell ◽

Pollen Grains ◽

Cell Plate ◽

Structure Study ◽

Pollen Grain ◽

Pollen Wall ◽

Fibrillar Material

When microspores of the African blood lily divide, they form pollen grains which consist of 2 cells of unequal size. This is accomplished when the microspore nucleus is displaced from the centre of the grain prior to division. The displacement is always towards the side of the grain opposite the furrow, and large vacuoles form in the cytoplasm between the furrow and the nucleus. During cell division the cell plate curves around one daughter nucleus and fuses with the pollen wall to enclose the generative cell. The cell-plate attachment always occurs with the wall that is opposite the furrow of the grain. Most of the microspore's organelles become incorporated in the larger vegetative cell, whereas the generative cell has few, if any, plastids and only a small number of other organelles. The wall around the generative cell is composed of finely fibrillar material enclosed within 2 unit membranes. The generative cell eventually becomes detached from the pollen wall, becomes spheroidal, and moves to a position near the centre of the pollen grain. At the same time, the large vacuoles disappear from the vegetative cell and the number of organelles increases substantially.

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