Ultrastructural observations of the mature megagametophyte and the fertilization in wheat (Triticum aestivum)

1985 ◽  
Vol 63 (2) ◽  
pp. 163-178 ◽  
Author(s):  
Ruilin You ◽  
William A. Jensen
Keyword(s):  
Triticum Aestivum ◽  
Pollen Tube ◽  
Cell Walls ◽  
Embryo Sac ◽  
Mature Embryo ◽  
Central Cell ◽  
The Other ◽  
Egg Cell ◽  
Filiform Apparatus ◽  
Egg Apparatus

The mature embryo sac of wheat contains an egg apparatus composed of an egg cell and two synergids at the micropylar end, a central cell with two large polar nuclei in the middle, and a mass of 20 to 30 antipodals at the chalazal end. A comparison was made of the ultrastructural features of the various cells of the embryo sac. The features included the position of the nucleus and vacuoles, the number, structure, and distribution of organelles, and the extent of the cell walls surrounding each cell. The pollen tube enters one synergid through the filiform apparatus from the micropyle. The penetration and discharge of the pollen tube causes the further degeneration of that synergid, which had already undergone changes before pollination. The second synergid does not change further in appearance following the penetration of the first by the pollen-altered tube. Half an hour after pollination at 20–25 °C, two male nuclei are seen in the cytoplasm of the egg and the central cell. At about 1 h after pollination, one sperm has made contact with the egg nucleus, while the other sperm is fusing with one of the polar nuclei.

Early embryogenesis in Arabidopsis thaliana. I. The mature embryo sac

1991 ◽  
Vol 69 (3) ◽  
pp. 447-460 ◽  
Author(s):  
S. G. Mansfield ◽  
L. G. Briarty ◽  
S. Erni
Keyword(s):  
Arabidopsis Thaliana ◽  
Endoplasmic Reticulum ◽  
Embryo Sac ◽  
Mature Embryo ◽  
Central Cell ◽  
Egg Cell ◽  
Cell Cytoplasm ◽  
Intimate Contact ◽  
Egg Apparatus ◽  
Common Wall

Arabidopsis thaliana has a seven-celled eight-nucleate megagametophyte of the Polygonum type; each cell type displays a different form of structural specialization. The egg apparatus cells are highly polarized; the egg has a large micropylar vacuole and chalazally sited nucleus, whereas the opposite is true for the synergids. At the chalazal region of the egg apparatus cells there are no cell wall boundaries, although their plasmalemmas are in intimate contact. The common wall between the two synergids is thin and irregular and contains plasmodesmatal connections. The synergid cytoplasm is rich in organelles; profiles of rough endoplasmic reticulum appear in masses of parallel stacked cisternae, and large accumulations of mitochondria occur adjacent to the filiform apparatus. The egg cell cytoplasm is quiescent; ribosome concentration and frequencies of dictyosomes and endoplasmic reticulum are noticeably lower and plastids are poorly differentiated. The central cell is long and vacuolate with a large diploid nucleus; fusion of the polar nuclei occurs prior to embryo sac maturity. The cytoplasm contains numerous starch-containing plastids accumulated in a shell around the nucleus. A high ribosome concentration and the absence of vacuoles and dictyosomes typifies the antipodal cell cytoplasm. All antipodal cells are interconnected by plasmodesmata as well as being connected to the nucellus and central cell. Key words: Arabidopsis, embryo sac, embryogenesis, cell specializations, stereology.

Cytoskeletal organisation and modification during pollen tube arrival, gamete delivery and fertilisation in Plumbago zeylanica

Zygote ◽  
1993 ◽  
Vol 1 (2) ◽  
pp. 143-154 ◽  
Author(s):  
Bing-Quan Huang ◽  
Elisabeth S. Pierson ◽  
Scott D. Russell ◽  
Antonio Tiezzi ◽  
Mauro Cresti
Keyword(s):  
Pollen Tube ◽  
Embryo Sac ◽  
Central Cell ◽  
Egg Cell ◽  
Target Cells ◽  
Sperm Cells ◽  
Plumbago Zeylanica ◽  
Rhodamine Phalloidin ◽  
Male Gametes ◽  
Pollen Tube Penetration

The cytoskeletal organisation of the isolated embryo sac and egg cells of Plumbago zeylanica was examined before, during and after pollen tube penetration into the embryo sac to determine the potential involvement of microtubules and actin filaments in fertilisation. Material was singly and triply stained using Hoechst 33258 to localise DNA, fluorescein isothiocyanate (FITC)-labelled anti- α-tubulin to detect microtubules and rhodamine-phalloidin to visualise F-actin. Microtubules in the unfertilised egg cell are longitudinally aligned in the micropylar and mid-lateral areas, aggregating into bundles near the filiform apparatus. In the perinuclear cytoplasm of the egg cell, microtubules become more or less randomly aligned. F-actin bundles form a longitudinally aligned mesh in the chalazal cytoplasm of the egg cell. In the central cell, microtubules and F-actin are distributed along transvacuolar strands and are also evident in the perinuclear region and at the periphery of the cell. During pollen tube penetration, sparse microtubule bundles near the pathway of the pollen tube may form an apparent microtubular ‘conduit’ surrounding the male gametes at the delivery site. Actin aggregates become organised near the pathway of the pollen tube and at the delivery site of the sperm cells. Subsequently, actin aggregates form a ‘corona’ structure in the intercellular region between the egg and central cell where gametic fusion occurs. The corona may have a role in maintaining the close proximity of the egg and central cell and helping the two sperm cells move and bind to their target cells. The cytoskeleton may also be involved in causing the two nuclei of the egg and central cell to approach one another at the site of gametic fusion and transporting the two sperm nuclei into alignment with their respective female nucleus. The cytoskeleton is reorganised during early embryogenesis.

The ultrastructure and cytochemistry of the egg apparatus of Brassica campestris

1989 ◽  
Vol 67 (1) ◽  
pp. 177-190 ◽  
Author(s):  
M. J. Sumner ◽  
L. Van Caeseele
Keyword(s):  
Plasma Membrane ◽  
Cell Walls ◽  
Brassica Campestris ◽  
Positive Response ◽  
Uv Light ◽  
Egg Cell ◽  
Filiform Apparatus ◽  
Egg Apparatus ◽  
Large Numbers ◽  
Acidic Polysaccharides

The egg apparatus of Brassica campestris L. cv. Candle (canola-rapeseed) is composed of an egg and two synergids juxtaposed at the extreme micropylar end of the megagametophyte with the egg cell displaced in a chalazal direction. The cell walls of the synergids and egg are uniformly PAS and PA–TCH–SP-positive, but contained β-linked glucans only in the micropylar region. The number and development of the cytoplasmic organelles suggested that the egg cell is relatively inactive metabolically while the synergid cells are active. The synergids contain large numbers of dictyosomes with PA–TCH–SP-positive vesicles at the maturing face. These vesicles appear to fuse with the plasma membrane in the region of the filiform apparatus. The filiform apparatuses of the synergids are micropylar finger-like projections that extend into the cytoplasm of the synergid. These are PAS and PA–TCH–SP-positive, fluoresce in uv light when stained with Calcofluor, and show a positive response for acidic polysaccharides when stained with alcian blue. After treatment with cellulase, fluorescence was not observed. The incipient degenerate synergid was intensely stained by cationic dyes 24–36 h after anthesis.

scholarly journals The ultrastructure of the mature embryo sac in the natural tetraploid of red clover (Trifolium pratense L.): that has a very low rate of seed formation

2014 ◽  
Vol 66 (1) ◽  
pp. 13-20 ◽  
Author(s):  
Gönül Algan ◽  
H. Nurhan Bakar
Keyword(s):  
Endoplasmic Reticulum ◽  
Trifolium Pratense ◽  
Embryo Sac ◽  
Red Clover ◽  
Mature Embryo ◽  
Polar Nucleus ◽  
Central Cell ◽  
Egg Cell ◽  
Ultrastructural Organization ◽  
Seed Formation

In this study, ultrastructural organization of cells in the mature embryo sac of natural tetraploid <em>Trifolium pratense</em> L. was investigated. The mature embryo sac of this plant contains an egg cell with two synergids at the micropylar end, and a central cell with two polar nuclei. The ultrastructure of these cells agrees with what is known for most angiosperms studied with the electron microscope. The egg cell is a large and highly vacuolate cell, partially surrounded by a wall. Much of the cytoplasm is located around the nucleus at the chalazal end and there are few numbers of channel-shaped endoplasmic reticulum, mitochondria, plastids and numerous ribosomes distributed throughout the cytoplasm. Unlike the egg cell, much of the cytoplasm in synergid cells is located at micropylar part of the cell and the synergid cytoplasm contains especially, large numbers of rough endoplasmic reticulum, free ribosomes, mitochondria and plastids. The central cell of <em>T. pratense</em> L. contains two large polar nuclei which lie close to the egg apparatus. Each polar nucleus has a single, large, dense nucleolus that contains several nucleolar vacuoles. Much of the central cell cytoplasm consisting of granular and agranular endoplasmic reticulum, mitochondria, plastids, ribosomes, dictyosomes and lipid bodies are placed around polar nuclei.

Ultrastructure of Synergids in Sugar Beet

2014 ◽  
Vol 926-930 ◽  
pp. 1040-1044
Author(s):  
Wei Li
Keyword(s):  
Cell Wall ◽  
Sugar Beet ◽  
Pollen Tube ◽  
Late Stage ◽  
Pollen Tube Growth ◽  
Embryo Sac ◽  
The Other ◽  
Filiform Apparatus ◽  
Flower Bud ◽  
Cell Degeneration

We use TEM to study the synergids in sugar beet, so as to provide more information for reproductive biology of angiosperm. Results were as follows: two synergids were similar in flower bud stage. Both of them show polarity with developed filiform apparatus (FA) at micropyle end and lacked cell wall at chalazal end. Then electron density in one synergid increased which suggested cell degeneration began. Complete degeneration finished before pollination. Organelles including mitochondrium, plastids and ribosomes gradually increased in the other synergid (persistent synergid). Metabolism of persistent synergid gradually enhanced. It began to degenerate when zygote had alveolate cell wall at the chalazal end, while completely degeneration and disappearance of FA took place at late stage of zygote. The results suggested that degeneration of one synergid in sugar beet must be triggered by other stimulation than pollination and pollen tube growth. As a transfer cell, persistent synergid transported nutrition for the development of embryo sac.

Fertilization in Plumbago zeylanica: entry and discharge of the pollen tube in the embryo sac

1982 ◽  
Vol 60 (11) ◽  
pp. 2219-2230 ◽  
Author(s):  
Scott D. Russell
Keyword(s):  
Cell Wall ◽  
Pollen Tube ◽  
Female Gametophyte ◽  
Embryo Sac ◽  
Central Cell ◽  
Vegetative Nucleus ◽  
Egg Cell ◽  
Ultrastructural Organization ◽  
Plumbago Zeylanica ◽  
Male Gametes

The ultrastructural organization of the megagametophyte of Plumbago zeylanica, which lacks synergids, was examined in chemically and physically fixed ovules after entry of the pollen tube. Similar to angiosperms with conventionally organized megagametophytes, the pollen tube enters the ovule through a micropyle, formed by the inner integument, and approaches the female gametophyte by growing between nucellar cells. Unlike other described female gametophytes, however, continued pollen tube growth results in direct penetration of the base of the egg through cell wall projections forming a filiform apparatus and is completed between the egg and central cell without disrupting either of these cells' plasma membranes. A terminal pollen tube aperture forms when the pollen tube reaches an area of strong curvature near the summit of the egg; this results in the release of two sperm cells, the vegetative nucleus, and a limited amount of pollen cytoplasm. The formerly continuous chalazal egg cell wall is locally disrupted near the tip of the pollen tube and apparently is thus modified for reception of male gametes. Discharged pollen cytoplasm rapidly degenerates between the egg and central cell, but unlike pollen tube discharge in conventionally organized megagametophytes, it is unassociated with the degeneraton of any receptor cell within the female gametophyte. Sperm nuclei are transmitted, one to the egg and the other to the central cell, to effect double fertilization by nuclear fusion with their respective female reproductive nuclei. The vegetative nucleus and discharged pollen cytoplasm degenerate between the developing embryo and endosperm during early embryogenesis. The emerging concept that the egg of Plumbago possesses combined egg and synergid functions is supported by the present study and suggests that the megagametophyte of this plant displays a highly specialized egg apparatus composed exclusively of a single, modified egg cell.

Embryo sac development in soybean: cellularization and egg apparatus expansion

1990 ◽  
Vol 68 (10) ◽  
pp. 2135-2147 ◽  
Author(s):  
M. W. Folsom ◽  
D. D. Cass
Keyword(s):  
Ultrastructural Study ◽  
Cell Walls ◽  
Embryo Sac ◽  
Chain Structure ◽  
Mitotic Division ◽  
The Other ◽  
Synergid Cell ◽  
Egg Apparatus ◽  
Embryo Sac Development

An ultrastructural study of soybean embryo sac development was performed. Prior to the final mitotic division and cellularization a nuclear rearrangement occurs that involves the chalazal movement of one of the two micropylar nuclei. During cellularization this nucleus divides to form the egg and micropylar polar nuclei and produces the wall that separates the central ceil from title space occupied by the egg apparatus. Within this space the other nucleus divides to form the two synergid nuclei and one of the two walls that separate the egg and synergid cells from one another. Egg apparatus cells are initially densely cytoplasmic, each is enclosed by thick, highly dissected walls, and they are all similar with respect to distribution of organelles except that synergid nuclei are micropylar to the egg nucleus. There is a progressive thinning and segmentation of egg apparatus walls during cellular expansion until they resemble the beaded chain structure seen in the mature egg and synergid cell walls. Taken as a whole these observations suggest that the chalazal movement of one of the two micropylar nuclei during the 4-nucleate stage is pivotal in determining future patterns of egg apparatus development.

Ultrastructure of the fertilized embryo sac in the dwarf mistletoe Arceuthobium americanum (Viscaceae) and development of the caecum

2005 ◽  
Vol 83 (5) ◽  
pp. 459-466 ◽  
Author(s):  
Cynthia M Ross ◽  
Michael J Sumner
Keyword(s):  
Cell Walls ◽  
Embryo Sac ◽  
Central Cell ◽  
Filiform Apparatus ◽  
Dwarf Mistletoe ◽  
Primary Endosperm Nucleus ◽  
Complex Cells ◽  
Arceuthobium Americanum ◽  
Fluorescence And Electron Microscopy ◽  
Light Fluorescence

Post-fertilization changes in the seven-celled embryo sac of the parasitic angiosperm Arceuthobium americanum Nutt. ex Engelm. were investigated with light, fluorescence, and electron microscopy. Two embryo sacs, of which only one is fertilized, are found within the reduced ategmic ovule (the placental–nucellar complex). All cells of the fertilized embryo sac developed thickened cellulosic and (or) hemicellulosic bounding walls and, aside from the degenerative synergid, acquired ultrastructure reflective of metabolically active cells. The filiform apparatus became thickened in the persistent synergid, and a similar structure developed in the antipodals. Unlike the changes observed in the zygote and central cell, those in the persistent synergid and antipodals were unusual, as these cells regularly degenerate after fertilization in a typical angiosperm; therefore, in A. americanum, they likely play a role in providing nutrition to the zygote and (or) central cell. A pouch-like outgrowth (caecum) initiated from the central cell near the primary endosperm nucleus and, after vacuolar expansion and intercellular growth, reached the base of the placental–nucellar complex in three days. The observation of mitochondria clusters within the central cell, caecum, and adjacent placental–nucellar complex cells suggests the caecum functions in embryo sac expansion and haustorial nutrient absorption.Key words: Arceuthobium, caecum, cell walls, fertilized embryo sac, mistletoe, ultrastructure.

scholarly journals Ultrastructure of the egg apparatus of Spinacia

2014 ◽  
Vol 50 (1-2) ◽  
pp. 165-168 ◽  
Author(s):  
H. J. Wilms
Keyword(s):  
Endoplasmic Reticulum ◽  
Cell Wall ◽  
Rough Endoplasmic Reticulum ◽  
Embryo Sac ◽  
Spherical Nucleus ◽  
Egg Cell ◽  
Lipid Bodies ◽  
Filiform Apparatus ◽  
Starch Grains ◽  
Egg Apparatus

The egg apparatus of <em>Spinacia</em> was studied from the time the embryo sac reaches its maximal size to just before fertilization, i.e., until about 8-9 hours after pollination. At maturity each synergid has a large elongated nucleus and prominent chalazal vacuoles, Numerous mitochondria, plastids, dictyosomes, free ribosomes, rough endoplasmic reticulum (RER), and lipid bodies are present. The cell wall exists only around the micropylar half of the synergids and each cell has a distinct, striated filiform apparatus. In general, degeneration of one synergid starts after pollination. The egg cell has a spherical nucleus and nucleolus and a large micropylar vacuole. Numerous mitochondria, some plastids with starch grains, dictyosomes, free ribosomes, and HER are present. A continuous cell wall is absent around the chalazal end of the egg cell.

Ultrastructural studies on the embryo sac of Viscum minimum: II. Megagametogenesis

1994 ◽  
Vol 72 (11) ◽  
pp. 1613-1628 ◽  
Author(s):  
M. Zaki ◽  
J. Kuijt
Keyword(s):  
Early Stage ◽  
Light And Electron Microscopy ◽  
Embryo Sac ◽  
Mature Embryo ◽  
Mitotic Division ◽  
Central Cell ◽  
Egg Cell ◽  
Ultrastructural Studies ◽  
Stage Of Development ◽  
Final Length

Embryo sac development of Viscum minimum was investigated using light and electron microscopy. Stages described involve uninucleate, binucleate, four-nucleate, and mature embryo sacs following cellularization. During the early stage of development, prior to mitosis, numerous small vacuoles are initiated in the cytoplasm of a uninucleate functional megaspore. The centrally located nucleus undergoes the first mitotic division and results in formation of two identical nuclei sharing a common cytoplasm. As the vacuole increases rapidly in size, the two nuclei become separated and move to opposite poles where the second mitotic division takes place. A remarkable elongation of the embryo sac is observed between the second and third mitotic division. Eventually, the embryo sac reaches its final length, two-thirds of the length of the ovary, at cellularization. Elongation of the embryo sac is closely related to the increase in vacuole size. Factors involved in vacuole formation and in the elongation of the embryo sac are discussed along with changes accompanying the transition from a sporophytic to a gametophytic pattern of development. Ultrastructural studies on the mature embryo sac, following cellularization, suggest that the egg cell is the least active cell in the megagametophyte. On the other hand, the synergids appear metabolically very active, being rich in plastids, mitochondria, dictyosomes, numerous vesicles, polysomes, and reserves. The central cell is the largest cell in the embryo sac. In a mature embryo sac the central cell has two adjacent nuclei, suggesting that fusion of the nuclei is completed following pollination and fertilization. The antipodals possess a complete set of organelles, numerous free and aggregated ribosomes, and endoplasmic reticulum. It is believed the antipodals play a significant nutritive role during the development of the embryo sac of V. minimum. Modification of the wall between antipodals and central cell and its role in nutrient transportations are discussed. Key words: embryo sac, embryogenesis, gametogenesis, megagametogenesis, mistletoes, ultrastructure.

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