TRANSFORMATION OF ELEMENTS OF CYTO-SKELETON IN RECEPTOR CELLS OF OLFACTIC ANALYZER IN FISHES AT DIFFERENT STAGES OF LIFE CYCLE

N.S. Kositsyn, I.V. Klimenkov
Presented By Academician G.I. Galazii 24.12.93

Translated and published per authors´ request.
Source: "DOKLADY AKADEMII NAUK", 1994, Vol., 336, No 2, pp 261-263

At present, various approaches are employed for the investigation of sensitive cells of olfactic analyzer [1,2]. In particular, one of those is based upon elucidation of adaptive capacities of peripheral segment of olfactic system in connection with its special role in providing chemical communication between animals during sexual behavior [3].

Attempts undertaken towards the establishment of correlation between the structure of receptive neuron, its sensitivity and implementation of adaptive behavior did not yield unambiguous results as yet [4].

In this connection, it appeared important to dwell on certain aspects of the yet not fully understood phenomenon of morphologic differentiation of olfactic cells, discovered by us in fishes during their spawn season [5], while we concentrated the focus of our attention on the investigation of tubulin-containing elements of cyto-skeleton, which, according to available information, participated both in the processes of intracellular transport and provided for manifold forms of non-muscular mobility [6]. Those poly-functional properties of cyto-skeleton could be of no small importance in the course of transformation of receptor cell.

In order to solve that neurobiological problem, a natural model of changing of olfactic sensitivity and ultra-structure of chemoreceptor cells in yellow-fin gobies (Cottocomephorus grewingki, Dyb.), species endemic to Lake Baikal, at different stages of their life cycle, was employed by us.

For morphological analysis, olfactic lining was sampled from males during pre-spawning period, during the phase of increasing sensitivity to sexual pheromones (active spawning), and during the stage of transition to parental behavior (protection of fecundated set) in males when chemical communication between individuals of different sexes deteriorated sharply. Olfactic rosette, fixed by standard procedure, was examined by means of transmission and scanning electron microscopy.

It should be mentioned that a relatively clean water at individual sites of Lake Baikal, free of antropogenic loads, created an opportunity to employ the lake as a unique test-ground for obtaining experimental materials that could virtually be treated as the basic-level data, characteristic of living systems in the absence of unfavorable ecological factors peculiar for other natural water-bodies.

Electron microscopy studies demonstrated that, during different phases of life cycle of Lake Baikal yellow-fin gobies, the level of structural organization of elements of cyto-skeleton in olfactic cells would vary. Hence, a fragment of distal section of olfactic cell during a pre-spawning period is represented in Fig. 1a.(Hover mouse to enlarge the picture.)

olfactic receptor cell

Figure 1.a- General view of olfactic sensitive cell of olfactic analizer in fishes; a fragment of apical section of receptor cell with flagella during pre-spawning period is separated out, 15000x; b- distal section of flagellate receptor cell during active spawning; cytoplasmic microtubules and flagella of the clava are visible, 15000x; c- apical section of flagellate receptor cell during the post-spawning period; flagellum with a well-preserved cyto-skeleton is located inside olfactic apex; 15000x: a,b,c,- pointer marks basal bodies.

It is easily observable that micro-tubes are missing from cytoplasm; while infrequent mitochondria, channels of smooth endoplasmic reticulum andvarious vesicular structures predominate. During the period of ripening and spawning of reproductive products in fishes, in parallel with activation of nuclear-cytoplasmic interaction and increase in the density of ribosomes, a very distinct structurization was attained by microtubules.

They were located throughout the entire length of peripheral cell appendage in the form of regular parallel bundles that, uninterruptedly, terminated in the matrix of olfactic clava (Fig. 1b).


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It is our impression that tubulin would attain such polymerized form exclusively during the period of functional mobilization of sensitive cells, when the behavior of the animal testified of the predominance of pheromone exitator. Naturally, the question would emerge: what is the purpose of such restructuring of cyto-skeleton components? It followed from behavioral experiments that fishes, by the commencement of reproduction, would lose olfactic perception of nutritional stimuli, concurrently demonstrating highly differentiated sensitivity to sexual pheromones [5]. It is likely that during that specific transitional stage, the olfactic analyzer would tune in for distinctively new biologically meaningful agents that would trigger sexual behavior.

As noted earlier, certain morphological characteristics of metabolic activation were observed in peri-nuclear zone ofchemoreceptor cells. It is not inconceivable that certain restructuring should as well occur on the level of the surface membrane of sensitive cell, where, as is known, the primary processes of perception would occur.

It would be natural to suppose that such adaptive changes should affect, in particular, the density of membrane receptors, their specificity, as well as their provision with corresponding ferment systems. Reasoning from that, it could be supposed, on the basis of our cytological data, that the system of cytoplasmic microtubules revealed in cells could serve as means necessary for emergency transportation of information of molecular compounds from the place of their synthesis in peri-nuclear zone towards apical section of the cell. .

There, they were incorporated into the surface membrane and directly participated in the transformation of non-cellular signals (messages). Consequently, such regulated structure of cyto-skeleton in olfactic cells presented an important link in the transition of peripheral branch of animal olfactic system into the condition of increased specific sensitivity. It is likely to refer to sexual pheromones as well as to other substances significant for signal (message) action. It should be emphasized that such communication between the body of the cell and apical zone of dendrite would obviously function for a limited time. As became evident, the microtubes would be disassembled again when the motivation towards search and selection of sexual partner deteriorated sharply and males turned to protection of eggs.

It was demonstrated earlier that, during the initial phase of parental behavior, sensitive cells with high content of secretor vesicles would lose their flagella and microvilli. Detailed studies of series of ultra-thin mounts helped to reveal that, in the course of incorporation of vesicle into surface membrane, flagellum would gradually sink inside the olfactic apex, losing its covering membrane with receptor components located in it (Fig. 1b; Fig. 2). It is likely that, as a result of such ´´fouling´´ (´´ encrustation´´), it would lose motive and chemoreceptive ability characteristic of it. (Hover mouse to enlarge the picture.)

olfactic receptor cell

The pattern of restructuring of apical section of olfactic receptor cell at different stages of life cycle of yellow-fin goby; 1- microtubules of olfactic flagella; 2- basal bodies; 3- secretor vesicles; 4- cytoplasmic microtubules; left- period of activation of receptor function of a cell (spawning period), right- a cell during a phase of disengagement of receptor function (parental behavior).

In comparison with cytoplasmic microtubules of peripheral appendage that are disaggregated during the phase of parental behavior, microtubular apparatus of cilium complete with basal bodies would demonstrate a considerably greater structural stability. Moving into the cytoplasm of olfactic clava, microtubules of flagellum would not immediately dissociate into components.

They would maintain structural connection with each other, preserving the interrelated positioning pattern ((9 x2)+2) characteristic of them. In all likelihood, such morphological stability of axoneme was achieved due to auxiliary proteins that, as is known, would tie the microtubes throughout their length, forming a complex regular structure.

It is difficult to predict further development of that complicated microtubular complex on the basis of available material. It is possible that it would disintegrate with time. However, the possibility should not be dismissed that the unusual localization of cilium in the cytoplasm of the apex of the receptor cell could be a peculiar means of preserving cyto-skeleton in assembled state, ready to be used to form flagellate antenna, should the necessity arise. In that case, such ready-for-use block of microtubules, after its disengagement from the matrix of olfactic apex would be able to restore chemoreceptive, as well as motional features characteristic of cilia.

Thus, the performed investigation demonstrated that tubulin-containing elements of cyto-skeleton in olfactic cells could be represented at least in two ways: as cytoplasmic microtubules that provide for transportation processes, and as regulated aggregates in flagella. They are different in both the level of over-molecular organization and by the degree of stability, which laid structural foundation for different modes of functioning of chemoreceptor cells in the processes of adaptive variation of sensitivity of olfactic system during the life cycle of an animal.

Figure 2. The pattern of restructuring of apical section of olfactic receptor cell at different stages of life cycle of yellow-fin goby; 1- microtubules of olfactic flagella; 2- basal bodies; 3- secretor vesicles; 4- cytoplasmic microtubules; left- period of activation of receptor function of a cell (spawning period), right- a cell during a phase of disengagement of receptor function (parental behavior).

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