A maior rede de estudos do Brasil

Grátis
11 pág.
Cap 6

Pré-visualização | Página 4 de 5

(after Mulisch et al., 1981). Note 
the single transverse microtubule at the right end of the transverse ribbon (T) of the anterior kinetosome and the 
 variable arrangement of transverse microtubules (T) associated with the posterior kinetosome. Kd, kinetodesmal fibril 
homologue ; Pc, postciliary microtubular ribbon . B Somatic cortex of Blepharisma with postciliodesmata composed 
of overlapping ribbons in the ribbon + 1 arrangement. (Redrawn after Ishida et al., 1992.)
136 6. Subphylum 1. POSTCILIODESMATOPHORA: Class 2. HETEROTRICHEA – Once Close to the Top
the oral polykinetids may be covered by somatic 
kineties in some forms (e.g., Stentor ) or not in oth-
ers (e.g., Condylostoma ). 
 The oral polykinetids , termed paramembranelles 
(de Puytorac & Grain, 1976), are minimally 
composed of a row of dikinetids, whose anterior 
kinetosomes bear a transverse ribbon and poste-
rior kinetosomes bear a divergent postciliary rib-
bon. Additional complete or incomplete rows of 
 kinetosomes may be added to these first two “rows”, 
depending upon the species and upon the position in 
the adoral zone at which the oral polykinetid lies. 
Kinetosomes on the right border of the second and 
third rows may also have postciliary micro tubules. 
For example, in Stentor , those polykinetids furthest 
from the cytostome have only two to four kinetosomes 
in the third row while closer to the cytostome this 
number increases to twenty, close to the number of 
kinetosomes in the first two rows (Jacobson & Lynn, 
1992). This is also the case for other genera (Mulisch 
& Hausmann, 1984; de Puytorac & Grain, 1976; Da 
Silva Neto, 1993b, 1994b). However, the number 
of kinetosomes in each oral polykinetid may be 
reduced as the cell size of the heterotrich decreases 
(Jacobson & Lynn, 1992). 
 It is in the structure of the paroral that most 
variation is seen. This is presumably because the 
conservation of the paroral structure is less critical 
to feeding function in upstream filter feeders like 
the heterotrichs . De Puytorac and Grain (1976) have 
provided definitions for a variety of terms applied 
to describe the structure of the heterotrich paroral 
(see also the Glossary ; Mulisch & Hausmann, 
1984; Da Silva Neto, 1994b). These include paro-
ral in pairs (e.g., Climacostomum ), stichodyad 
(e.g., Blepharisma , Stentor ), stichomonad (e.g., 
Spirostomum ), and polystichomonad (e.g., 
Condylostoma ). Our knowledge of this diversity is 
increased with each new description (e.g., Fabrea , 
Da Silva Neto, 1993b; Condylostomides , Da Silva 
Neto, 1994b). It is further complicated by vari-
ations within the same species along the course 
of the paroral from near the cytostome to out 
onto the body surface (e.g., Stentor , Bernard & 
Bohatier, 1981; Climacostomum , Fischer-Defoy & 
Hausmann, 1981; Condylostomides , Da Silva Neto, 
1994b). We conclude that it is futile to capture this 
diversity by establishing new terms for each new 
paroral structure described, and we recommend 
that descriptions preface the character of the paro-
ral with the taxonomic name of the group (e.g., 
Stentor paroral). 
 Tuffrau (1968) diagrammed the variety of fibril-
lar structures that support the cell surface of the 
oral region and the cytopharynx. Ultrastructural 
studies have demonstrated these to be kinetosomal 
postciliary ribbons and nematodesmata that extend 
from the bases of the oral polykinetids and paroral 
kinetosomes (Grain, 1984). The nematodesmata 
from adjacent polykinetids join to form overlapping 
microtubular rootlets, which may serve a cytoskeletal 
function for the oral region and may also be involved 
in re-extension of the oral region, especially in 
 folliculinids with their elongated peristomial wings 
(Mulisch & Hausmann, 1984). The cytopharynx 
may be supported by paroral postciliary microtu-
bules ( Climacostomum , Fischer-Defoy & Hausmann, 
1981) or oral polykinetid postciliary microtubules 
(Eufolliculina , Mulisch & Hausmann, 1984). 
 Filamentous structures are often observed in the 
cytostomal region. As for other ciliates, Mulisch and 
Hausmann (1984) have proposed that these function 
to facilitate the pinching off of the forming food vac-
uole. This hypothesis has received support in experi-
ments using cytochalasins , “anti-actin” drugs, which 
inhibit phagocytosis in Spirostomum (Zackroff & 
Hufnagel, 1998). Once food is ingested, food vacu-
oles and primary lysosomes fuse, and eventually 
the food vacuole fragments into smaller vesicles, 
which may distribute digesting materials (Fischer-
Defoy & Hausmann, 1982). The defecation vacuole 
ultimately fuses with the plasma membrane at the 
 cytoproct and its membrane is presumably recycled 
by the cell (Fischer-Defoy & Hausmann, 1992). 
 6.5 Division and Morphogenesis 
 Heterotrichs typically divide while swimming 
freely, and are characterized as having parakinetal 
stomatogenesis (Foissner, 1996b). The early studies 
of Fauré-Fremiet (1932) and Villeneuve-Brachon 
(1940) demonstrated that kinetosomal replication in 
one to several subequatorial kineties produced the 
initial anarchic field of the oral primordium. This 
often occurs in the zone of stripe contrast, a region 
where there is a marked contrast in the width of the 
pigment stripes or interkinetal separation (Frankel, 
1989; Tartar, 1961). This anarchic field may be 
in the ventral region in the long axis of the body 
 posterior to the proter oral region (e.g., Blepharisma ) 
(Fig. 6.3) or might even occur on the dor-
sal surface (e.g., Fabrea ). Initially, the anarchic 
field is composed of unoriented kinetosomal pairs 
(Bernard & Bohatier, 1981; Mulisch & Hausmann, 
1988). Eventually, the anarchic field divides lon-
gitudinally and paroral structures differentiate on 
its right side while adoral structures differentiate 
on its left side. Oral polykinetids initially differ-
entiate in the centre of the oral primordium as 
dikinetids assemble from the right towards the left. 
Additional oral polykinetids join these central ones 
Fig. 6.3. Stomatogenesis of the heterotrich Blepharisma . ( a ) The process begins with proliferation of kinetosomes 
and dikinetids along a ventral postoral kinety. ( b ) The oral polykinetids begin to differentiate as dikinetids align 
beginning in the middle of the anlage and extending towards each end. ( c ) Differentiation continues towards the 
ends, visible at this stage by the addition of a third row to oral polykinetids initially in the middle of the anlage. The 
paroral begins to differentiate in the posterior right region ( d ) The paroral continues its differentiation as the adoral 
zone begins to curve towards and right in preparation for invagination of the opisthe’s oral cavity. Note that there is 
some dedifferentiation and redifferentiation of the oral structures of the proter. (From Aescht & Foissner, 1998.)
6.5 Division and Morphogenesis 137
138 6. Subphylum 1. POSTCILIODESMATOPHORA: Class 2. HETEROTRICHEA – Once Close to the Top
as the adoral field develops from the center towards 
the two ends of the primordium (Fig. 6.3). This 
latter feature is considered a strong apomorphy for 
the class (Aescht & Foissner, 1998; see also Shao 
et al., 2006). 
 Foissner (1996b) has described and defined 
the variations in heterotrich stomatogenesis: it is 
 parakinetal when one kinety is involved, polypara-
kinetal when more than one kinety is involved, and 
 amphiparakinetal when the oral primordium curves 
to intersect somatic kineties at both its anterior and 
posterior ends. These intersected somatic kineties 
become the peristomial field kineties in Stentor and 
Fabrea , but are resorbed in folliculinids. 
 The parental or proter oral apparatus may be only 
slightly dedifferentiated during cell division (e.g., 
Blepharisma , Condylostoma , Stentor ) or entirely 
dedifferentiated and regenerated (e.g., Spirostomum ). 
In folliculinids, the entire proter

Crie agora seu perfil grátis para visualizar sem restrições.