Fine structure of the tapetum cellulosum of the grey seal (Halichoerus grypus)

The morphology of the tapetum lucidum of the grey seal (Halichoerus grypus) has been studied by light and electron microscopy. The reflective layer in this species is a tapetum cellulosum situated in the choroid and covering the entire effective fundus. Posteriorly the tapetum is composed of 30-35 layers of flattened polygonal cells. This number gradually declines to 15-20 layers in the extreme periphery. Near the retinal epithelial layer the tapetal cells are larger and more regular (brick-like) in arrangement whereas further from the retina the tapetal cells become more irregular in outline and more widely separated by collagen fibrils and connective tissue cells. In this outer region the tapetal cells are gradually replaced by melanocytes of the choroid. Within the tapetal cells a few mitochondria and profiles of smooth endoplasmic reticulum are scattered peripherally while the majority of the cell organelles are clustered near the centrally located vesicular nucleus. The dominant feature of the tapetal cells is, however, an accumulation of numerous electron-dense rodlets of presumed zinc cysteine. These rodlets are the reflective material of the tapetum and are arranged with their long axes perpendicular to the incoming light. The orientation of these rodlets is usually uniform within each tapetal cell but varies between adjacent cells. The diameter (0.10 micron) and spacing (0.15 micron) of these rodlets is consistent with the principles of constructive interference. Blood vessels penetrate the tapetum at right angles to supply the choriocapillaris which is indented into the amelanotic retinal epithelium to give a flat reflective surface to the tapetum.     

Electron microscopic study of the occlusible tapetum lucidum of the southern fiddler ray (Trygonorhina fasciata).

The choroidally located tapetum lucidum of the southern fiddler ray (Trygonorhina fasciata) has been examined by light and electron microscopy in both light- and dark-adaptation. In this species, the tapetum consists of a single layer of overlapping cells oriented at an angle of about 30 degrees to the incoming light. These are situated immediately external to the choriocapillaris. These tapetal cells alternate with and are separated from one another by melanocytes which have an inner extension that curves and intervenes between the tapetal cells and the choriocapillaris. The tapetal cells and the melanocytes are flattened cells with their widest dimension facing the retina. Internally the tapetal cells display a peripherally-located, vesicular nucleus with most of the cell organelles in a paranuclear location. The bulk of the cell is packed with regularly-spaced crystals reported to be guanine. The size and spacing of these reflective crystals is commensurate with constructive interference. In light-adaptation the small melanosomes of the melanocytes are widely dispersed and fill the portion of the cell intervening between the tapetal cells and the incoming light. This effectively occludes the tapetum as light is unable to reach the reflective material. In dark-adaptation the melanosomes withdraw from this location, exposing the tapetum to light and allowing it to act as a reflective layer. The retinal epithelium overlying the tapetal area is totally unpigmented so as not to interfere with the passage of light.     

Abnormal Vacuolization of the Tapetum During the Tetrad Stage is Associated with Male Sterility in the Recessive Genic Male Sterile <Emphasis Type="Italic">Brassica napus</Emphasis> L. Line 9012A

In the recessive genic male sterile line 9012A of Brassica napus, pollen development is affected during the tetrad stage. According to the light and electron microscopy analysis of tapetal cells and tetrads, the sterile tapetal cells swelled with expanded vacuoles at the early tetrad stage and finally filled the center of the locules where a majority of tetrads encased with the thick callose wall collapsed and degraded. We suggested that an absence of callase, which is a wall-degrading enzyme stored in the vacuoles of tapetal cells before secretion, resulted in the failure of tetrad separation. Moreover, transmission electron microscopy analysis showed that the secretory tapetal cells were not observed in sterile anthers, which indicated that the transition of the tapetum from the parietal type to the secretory type was probably aberrant. In plants, degeneration of the tapetum is thought to be the result of programmed cell death (PCD). PCD of tapetal cells was investigated by terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling assay and signals indicative of deoxyribonucleic acid fragmentation were detected much earlier in sterile anther than in fertile anther. This suggests that tapetal breakdown does not occur by the normal procession of PCD and might be following an alternative mechanism of unscheduled apoptosis in line 9012A. This research supports the hypothesis that premature PCD is associated with male sterility in B. napus.     

Development of the retinal tapetum lucidum of the walleye (Stizostedion vitreum vitreum)

The development of the retinal tapetum lucidum within the cells of the retinal pigment epithelium (RPE) has been investigated by both light and electron microscopy in the walleye (Stizostedion vitreum vitreum) in specimens ranging in total length from 25-140 mm. In addition changes in the arrangement of the photoreceptors (both rods and cones) in both light and dark-adaptation have also been studied. At 25 mm no evidence of a tapetum is present. At about 30 mm it makes its initial appearance as granular bodies formed within the apical smooth endoplasmic reticulum (SER) cisternae of the RPE cells in the superior temporal fundus. The developing tapetum then spreads peripherally and continues to thicken in existing areas. By 90 mm it is well established throughout the fundus but always appears better developed in the superior fundus. By 125-140 mm it is essentially adult in appearance. At 60-70 mm the rods and cones begin to form bundles producing macroreceptors of 20-30 photoreceptors. In dark-adaptation the rod bundles are retracted and have one or more cone cells centrally located in each bundle, with the bundles separated from one another by melanosomes. Initially when no tapetal material is present, post-larval walleye are positively phototactic and feed on zooplankton. In the adult condition when a tapetum lucidum and large macroreceptors are present, the walleye is negatively phototactic and feeds almost exclusively on larger organisms such as other fish.     

Abnormal Development of Tapetum and Microspores Induced by Chemical Hybridization Agent SQ-1 in Wheat

Chemical hybridization agent (CHA)-induced male sterility is an important tool in crop heterosis. To demonstrate that CHA-SQ-1-induced male sterility is associated with abnormal tapetal and microspore development, the cytology of CHA-SQ-1-treated plant anthers at various developmental stages was studied by light microscopy, scanning and transmission electron microscopy, in situ terminal deoxynucleotidyl transferasemediated dUTP nick end-labelling (TUNEL) assay and DAPI staining. The results indicated that the SQ-1-treated plants underwent premature tapetal programmed cell death (PCD), which was initiated at the early-uninucleate stage of microspore development and continued until the tapetal cells were completely degraded; the process of microspore development was then blocked. Microspores with low-viability (fluorescein diacetate staining) were aborted. The study suggests that premature tapetal PCD is the main cause of pollen abortion. Furthermore, it determines the starting period and a key factor in CHA-SQ-1-induced male sterility at the cell level, and provides cytological evidence to further study the mechanism between PCD and male sterility.     

THE CYTOLOGY OF POLLEN ABORTION IN C-CYTOPLASMIC MALE-STERILE CORN ANTHERS

Anther development of the C-cytoplasmic male-sterile (cms C) and the normal cytoplasm version (N) in the W182BN corn inbred was studied by light and electron microscopy. Deviation from normal pollen development was first observed in the tapetal cells at the tetrad stage of development. Two types of tapetal abnormalities were observed in plants with C cytoplasm. The first behaved like the N anther until the tetrad stage, when numerous small vacuoles appeared in the tapetal cells. Inner and radial tapetal cell walls broke down normally, but irregular Ubisch body deposition was observed, and exine development was inhibited and delayed. The tapetum and microspores disintegrated at the intermediate microspore stage. The second type of tapetum was highly vacuolated at the early tetrad stage, with dense inner and radial cell walls that remained intact and enlarged when the tetrads aborted. No organellar abnormalities, such as the mitochondrial changes observed in cms T, were observed in C anthers.     

A comparative light and electron microscopic analysis of microspore and tapetum development in fertile and cytoplasmic male sterile radish

To gain further insight into the abortive stages and ultrastructural changes leading to pollen degeneration of a novel cytoplasmic male sterile radish 805A, we compared differences of cellular and subcellular structure of sterile anther with fertile anther by light and electron microscopy analysis. Two types of locule degeneration in sterile anther were detected, of which the time of degeneration occurred and completed was different. In type I, abnormality of pollen mother cells (PMCs) and tapetal cells, including condensation of cytoplasm and large vacuoles within tapetal cells, was shown at PMC stage. In type II, meiosis and early tetrad stage progressed normally except for large vacuoles that appeared in tapetal cells. Ultrastructural alterations of the cellular organization were observed in the type II locules, such as chromatin condensation at the periphery of the nucleus and degeneration of the karyotheca, compared with normal pollen development. The results suggested that the cytoplasmic male sterility anther degeneration was probably caused by dysfunctions of tapetum and vacuolation of tapetum, PMCs, and microspores. Thus, the identical factors, which induced CMS in the same cytoplasmic and nuclear genetic background, might affect development of tapetum and microspore at different stages during the cytoplasmic male sterile 805A anther development.     

Isolation,sorting, and characterization of uni- and binucleate tapetal protoplasts from anthers of normal and Texas cytoplasmic male-sterileZea mays L.

Summary A cytological study of Texas cytoplasmic male sterile (Tcms) and normal (N) anther tapetal protoplasts ofZea mays was undertaken to determine whether there were any differences prior to Tcms male cell abortion not noted in previous published studies. Squash preparations, tapetal protoplast separation via flow cytometry, image analysis, and electron microscopy were utilized. Chemically preserved tapetal protoplasts from both lines were prominently angular in shape and typically smaller than any other cell type in the anthers. The tapetum from both lines consisted of a mixture of uninucleate and binucleate protoplasts. The Tcms tapetum consistently had a higher proportion of binucleate protoplasts during all stages of microsporogenesis prior to abortion. The size of Tcms uniand binucleate tapetal protoplasts was more variable than the N tapetal protoplasts and was largest during the microspore stage when male cells abort. Tapetal nuclear size in both lines was less variable. Uni- and binucleate tapetal protoplasts from each line could be separated from the other anther cells and from each other by filtration and then by flow cytometry, based on intensity of nuclear fluorescence. These results suggest that Tcms uninucleate tapetal protoplasts have a higher level of DNA than N uninucleate tapetal protoplasts. Both fluorescence microscopy and electron microscopy confirmed pure populations of intact uni- and binucleate tapetal protoplasts using flow cytometry. The results from this study indicate that the methodology presented here could be used for a variety of further studies to better understand the cellular and molecular basis of male sterility in maize, and in other taxa, where the tapetum is the primary target that leads to male sterility.Abbreviations AO acridine orange - Bi binucleate protoplast - D dyad - DAPI 4,6-diamidino-2-phenylindole - FC flow cytometry - M meiocyte - MI microspore - MMC mithramycin - N normal anther tapetal protoplast - PI propidium iodide - PS protoplast sorting - RT room temperature - SM sporogenous mass - Tems Texas cytoplasmic male sterile anther tapetal protoplast - Uni uninucleate protoplast     

Microsporogenesis of the feathers (fertile branches derived from annual buds) in Vitis vinifera,cv Picolit giallo

Abstract

Using light and electron microscopy, we have studied the microsporogenesis and tapetal development of the feathers in two different low producing clones of Picolit giallo (sp. Vitis vinifera). In these clones while the productivity of the main branches (fertile branches originated from buds, formed in the previous year, that remained silent during the winter) is very low, that of the feathers (fertile branches derived from annual buds) is always normal.

The microsporogenesis and tapetal development proceed normally in almost all the examined anthers; it is remarkable that at the tetrad stage the tapetal cells appear well structured without any degeneration symptom, unlike what observed for the main branches. Moreover in most of the mature anthers the pollen grains are numerous, pleinty of organelles and show sometimes thickenings in the callose layer under their wall. The tapetal cells of these anthers have disappeared. Only in few anthers we observed the presence of collapsed pollen grains and tapetal cells with anomalous development, that are still present when the pollen grains are mature. This rare situation for the feathers is on the contrary frequent for the main branches.     

Microsporogenesis and tapetal development in fertile and cytoplasmic male-sterile sugar beet (Beta vulgaris L.)

Summary The development of sporogenous and tapetal cells in the anthers of male-fertile and cytoplasmic male-sterile sugar beet (Beta vulgaris L.) plants was studied using light and transmission electron microscopy. In general, male-sterile anthers showed a much greater variability in developmental pattern than male-fertile anthers. The earliest deviation from normal anther development was observed to occur in sterile anthers at meiotic early prophase: there was a degeneration or irregular proliferation of the tapetal cells. Other early aberrant events were the occurrence of numerous small vesicles in the microspore mother cells (MMC) and a disorganized chromatin condensation. Deviations that occurred in sterile anthers at later developmental stages included: (1) less distinct inner structures in the mitochondria of both MMC and tapetal cells from middle prophase onwards. (2) dilated ER and nuclear membranes at MMC prophase, in some cases associated with the formation of protein bodies. (3) breakdown of cell walls in MMCs and tapetal cells at late meiotic prophase. (4) no massive increase in tapetal ER at the tetrad stage. (5) a general dissolution of membranes, first in the MMC, then in the tapetum. (6) abortion of microspores and the occurrence of a plasmodial tapetum in anthers reaching the microspore stage. (7) no distinct degeneration of tapetal cells after microspore formation. Thus, it seems that the factors that lead to abortive microsporogenesis are structurally expressed at widely different times during anther development. Aberrant patterns are not restricted to the tetrad stage but occur at early prophase.     

Cytological and ultra-structural study on microsporogenesis of cytoplasmic male sterility in <Emphasis Type="Italic">Raphanus sativus</Emphasis>

The cytological development of microspores and tapetum in cytoplasmic male sterile (CMS) line A₁₄ and its maintainer B₁₄ in radish were studied using light- and transmission electron microscopy (LM and TEM). The microspores of the CMS line began to abort soon after they were released from tetrads in pollen sacs with light microscopy investigation, while abnormal behavior of pollen mother cells (PMC) were observed during its meiotic stage in its ultra-structural study, including degeneration of organelles and irregularity of nuclear membrane. At the same time, development of tapetal cells was similar to that of the maintainer. With further development of the anther, the tapetal cells of CMS line showed an abnormal increase in size and other appearances, such as fewer organelles and indistinct cytoplasm. The microspores of the CMS line were always distinguishable from the maintainer line with irregular structure, more osphilic deposits and abnormal exine. It is inferred that abortion of microspores is attributed to mutation of genes controlling male sterility, which further leads to hypertrophy of tapetum and destruction of ultra-structure.     

Hereditary abnormality in tapetum lucidum of the Siamese cats

Summary Gross examination showed a weaker reflection (less shining) of the tapetum lucidum of the Siamese cats compared with common cats. Toluidine blue sections revealed that many tapetal cells were weakly stained and giving vacuolated appearance under high magnification. Further examination with electron microscope showed that those weakly stained cells were filled with disrupted tapetal rods. In these affected cells, the arrangement of the tapetal rods was no longer regular. The membranes of the tapetal rods were either enlarged or disrupted. Some of them appeared to be myelin-like structures. The cores of the tapetal rods were either empty or filled with electron-dense materials which may be the remnant of the original cores. The severity of this type of abnormality or degeneration in the tapetum varied from lavers to layers. Those layers closer to the retina showed a greater number of cells with degeneration. Quantitative analysis of histochemical detection of zinc showed a significantly smaller amount of zinc in tapetal rods of the Siamese cats as compared with common cats. Less zinc and disruption of the regular arrangement of the tapetal rods may result in weaker reflection of light by Siamese cat tapetum. In four of the nine Siamese cats studied, this type of abnormality was observed. It suggests that it is a hereditary disorder of relatively high frequency.     

Localization and release of allergens from tapetum and pollen grains ofBetula pendula

Summary Although intact pollen grains are assumed to be the primary carrier of pollen allergens, specific immunoreactive components have been found in other aerosol fractions, e.g., starch grains and remains of tapetal cells Cryo-scanning-electron-microscopy results demonstrate the presence of a clear network of strands connecting the tapetum with the microspores. The distribution of protein in tapetal orbicules, pollen wall, and pollen cytoplasm was tested by histochemical stains for light microscopy and transmission electron microscopy. The protein is mainly localized at the apertures and starch grains in the cytoplasm of pollen and in the core and on the surface of tapetal orbicules. Monoclonal antibodies Bv-10, BIP3, and BIP4 have been used to locate the cellular sites of pollen and tapetal allergens inBetula pendula (syn.B. verrucosa). The application of rapid-freeze fixation prevented relocation of allergens from their native sites. The allergens are predominantly found in the starch grains and to lesser extent in the exine. We also tested interactions between mature birch pollen and human fluids: saliva, nostrils fluid, and eyes solution. The aim was to mimic more closely the in vivo situation during allergenic response. In all cases we observed several pollen grains that were burst and had released their cytoplasmic contents. In the nose the allergens are released from the pollen within minutes. In rhinitis, nasal pH is increased from the normal pH 6.0 to 8.0. When we used nasal fluid at pH 8.0, the number of ruptured pollen grains increased. The mechanism that might induce formation of small allergen-bearing particles from living plant cells is discussed.     

Chemical agents that inhibit pollen development: effects of the phenyl-cinnoline carboxylates SC-1058 and SC-1271 on the ultrastructure of developing wheat anthers (Triticum aestivum L. var Yecora rojò)

Summary Phenylcinnoline carboxylate compounds SC-1058 and SC-1271 cause complete male sterility in wheat when applied at suitable dosages at the pre-meiotic stage of anther development. Anthers from treated and untreated plants were compared using light and electron microscopy from the pre-meiotic stage through the formation of nearly mature pollen. Overall anther development is gradually slowed in treated plants and pollen development is generally arrested in the late prevacuolate or early vacuolate microspore stage, although the first pollen mitosis does sometimes occur. The sporopollenin-containing exine walls are thinner, and show abnormally developed foot and tectum layers with sparse connecting baculi. Microspore cytoplasm degenerates and the cells eventually collapse. At the early, prevacuolate, free microspore stage treated tapetal cells hypertrophy, expanding into the locule. They contain abnormally large vacuoles that appear to form from the fusion of secretory vesicles, and some vacuoles contain electrondense deposits. The sporopollenin-containing orbicular wall and Ubisch bodies are retarded in their development and are structurally deformed. Acetolysis of whole anthers and of thick sections shows that the sporopollen-in-containing structures of treated materials are greatly reduced in thickness and are less rigid than in the control. We conclude that application of these compounds causes interference with the secretory function of tapetal cells which supplies sporopollenin cell-wall polymers to the exine of the microspores and to the tapetal orbicular wall and associated Ubisch bodies. Interference with the tapetal secretion of other nutrients required for microspore development is strongly suggested.     

Fine structure of the retinal epithelium and tapetum lucidum of the ranch mink Mustela vison

The morphology of the retinal pigment epithelium (RPE), Bruch's membrane (complexus basalis), choriocapillaris and tapetum lucidum has been studied in the eye of the ranch mink (Mustela vison) by light and electron microscopy. The RPE is composed of a single layer of cells joined laterally by apically located junctional complexes. Basally (sclerally) these cells display numerous infoldings whereas apically (vitreally) two types of processes are associated with rod and cone outer segments. Smooth endoplasmic reticulum and mitochondria are abundant in these cells whereas rough endoplasmic reticulum and polysomes, although present, are not plentiful. An occasional wandering phagocyte is noted at the RPE-photoreceptor interface. In the posterosuperior part of the fundus, a degenerative tapetum lucidum is present. The presence of only a few layers of tapetal cells containing but little reflective material and the haphazard arrangement of this material makes it very unlikely that this area functions as an effective tapetum lucidum. The RPE over the aberrant tapetum, however, shows the morphology that is seen when a functioning tapetum cellulosum is present, namely the absence of melanosomes and an indented choriocapillaris. Bruch's membrane in non-tapetal areas is pentalaminate but, over the tapetum and where it is associated with capillary profiles, it is reduced to a single, thickened basal lamina. The choriocapillary endothelium is highly fenestrated and in nontapetal areas these capillaries are not indented into the epithelial layer.     

Ultrastructural aspects and programmed cell death in the tapetal cells of Lathyrus undulatus Boiss

Programmed cell death (PCD) in the tapetum of Lathyrus undulatus L. was analyzed based on light, fluorescence and electron microscopy to characterize its spatial and temporal occurrence. Development and processes of PCD in secretory tapetal cells of Lathyrus undulatus L. were correlated with the sporogenous cells and pollen grains. At early stages of development the tapetal cells appeared similar to pollen mother cells, structurally. Concurrent with meiosis, tapetum expanded both tangentially and radially as vacuoles increased in size. Tapetal cells most fully developed at young microspore stage. However, tapetum underwent substantial changes in cell organization including nucleus morphology monitored by DAPI. The TUNEL staining confirmed the occurrence of intra-nucleosomal DNA cleavage. In addition to nuclear degeneration which is the first hallmark of PCD other diagnostic features were observed at vacuolated microspore stage intensely; such as chromatin condensation at the periphery of the nucleus, nuclear membrane degeneration, chromatin release to the cytoplasm, vacuole collapse according to tonoplast rupture, shrinkage of the cytoplasm, the increase and enlargement of the endoplasmic reticulum cisternae and disruption of the plasma membrane. After vacuole collapse due to possible release of hydrolytic enzymes the cell components degraded. Tapetal cells completely degenerated at bicellular pollen stage.     

Two new male-sterile mutants of Zea mays (Poaceae) with abnormal tapetal cell morphology

Two new recessive male-sterile mutants of Zea mays (Poaceae), or maize, were studied to identify the timing of pollen abortion and to examine the involvement of anther wall cell layers. The results of test crosses indicated that these mutants were not allelic with any known male-sterile mutants of maize. Light and transmission electron microscopy were used to compare pollen development in homozygous male-sterile mutants to that in fertile heterozygous siblings. In both mutants, microspores abort soon after release from the meiotic tetrad. However, the two mutations have strikingly different phenotypes. Large lipid bodies accumulate in the tapetal cells as the microspores vacuolate and die in the mutant ms25. Large vacuoles appear in both the tapetal cells and the young microspores as they begin to disintegrate in the mutant ms26. Because abnormal tapetal cell morphology is detected in both mutants, it is possible that both of these mutations affect the expression of genes in tapetal cells.     

POLLEN DEVELOPMENT IN OENOTHERA BIENNIS (ONAGRACEAE)

Development of the exine and viscin threads in Oenothera was studied by a combination of transmission electron microscopy and field emission scanning electron microscopy. Exine formation is initiated in the early tetrad stage by plate-like structures of preexine formed evenly around the microspore within a callosic wall. In the late tetrad stage, an endexine accumulates on white-lined lamellae underneath the preexine. After dissolution of the callosic wall, the preexine develops into beaded ektexine which is differentiated into an undulate tectum-like layer and columellae-like components. Interwoven fibrous strings connect the developing ektexine and the surface of the tapetal cells, and later develop into the viscin threads. These developmental processes imply that the columellae-like components are different in structure from the columellae of other angiosperms and that the formation of viscin threads is associated with the tapetal cells.     

Retinal epithelial fine structure in the southern fiddler ray (Trygonorhina fasciata).

The morphology of the retinal epithelium (RPE), choriocapillaris and Bruch's membrane (complexus basalis) has been investigated by light and electron microscopy in an elasmobranch, the southern fiddler ray or guitarfish (Trygonorhina fasciata). The RPE consists of a single layer of cuboidal cells which display basal (scleral) infoldings as well as numerous apical (vitreal) finger-like processes which interdigitate with the photoreceptor outer segments. The lateral cell borders are relatively smooth and are joined in the mid-region by a series of tight junctions. Internally the RPE nucleus is large, vesicular and centrally located. Smooth endoplasmic reticulum (SER) is abundant while rough endoplasmic reticulum (RER) is scarce. Polysomes are however widespread and mitochondria are plentiful. Two unusual organelles are also noted. One consists of a membrane bound array of tubules while the other is a membrane bound structure consisting of a granular matrix with again an internal tubular array. This species possesses a choroidally located tapetum lucidum in the superior fundus and over this tapetal area, melanosomes are absent from the RPE cells. In non-tapetal locations a few melanosomes are present that do not appear to undergo photomechanical movements. Bruch's membrane is a pentalaminate structure with an almost continuous central elastic layer (lamina densa). The choriocapillaris forms a single layer of capillaries with a thin but only minimally fenestrated endothelium facing Bruch's membrane.     

Ultrastructure of tapetal cell ontogeny inBeta

Summary Tapetal cell development and degeneration in anthers ofBeta vulgaris L. were studied with the electron microscope. Tapetal cells become differentiated from sporogenous cells early in anther ontogeny. The tapetal nuclei divide mitotically; binucleate tapetal cells contain relatively little endoplasmic reticulum and otherwise resemble meristematic cells of higher plants. There follows an increase in endoplasmic reticulum and by the time the sporogenous tissue has entered meiotic prophase, the tapetal cells have differentiated the usual characteristics of secretory cells. Degenerative changes begin to appear in tapetal cells after meiosis of the sporogenous tissue. Such changes include loss of inner tangential and anticlinal walls, degeneration of tapetal nuclear envelopes, disruption of the plasmalemma, and changes in the cytoplasmic organelles. Coated tubules are associated with tapetal nucleoli during degenerative stages and the tubules persist after tapetal nuclei have degenerated. Tapetal cell cytoplasm disappears completely by the stage of microspore mitosis.