Ultrastructure of the nerves and sense receptors of Polystomoides renschi Rohde and P. malayi Rohde (Monogenea: Polystomatidae)

Workshop no. 4. Biology of monogeneans

The oncomiracidia larvae of monogeneans are small, ciliated, relatively slow-moving organisms with a limited free-swimming life. For example, the oncomiracidium of the skin parasiteEntobdello, soleaeis 250 µm in length, swims at a speed of 5 mm/sec and lives for about 24 h (Kearn, 1967a). Oncomiracidial survival is dependent on the larvae finding and attaching themselves to their specific fish hosts and selection pressure has been exerted on the timing of hatching of oncomiracidia and on their behaviour during their short freeswimming lives to enhance the chances of making this important contact.

The ultrastructure of the pseudohaptoral squamodiscs of Diplectanum aequans (Monogenea)

The ultrastructure of the pseudohaptoral squamodiscs of the monogenean Diplectanum aequans is described. The most prominent feature of the squamodisc is the presence of epidermally-embedded spines which are covered by the outer epidermal membrane. These spines, which are ‘hollow’, are composed of a moderately electron-dense material with denser fibrils embedded within it. The squamodisc epidermis is similar in appearance to the haptor epidermis, with the exception of the epidermis ‘inside’ the spines. Here the epidermis contains free ribosomes and stacks of alternating electron-lucent/electron-dense, membrane-bound sacs. The arrangement of the musculature of the spines is described. The free, prehensile, edges of the squamodiscs contain numerous radially orientated nerves, some of which are assumed to be mechano-receptors.

Role of regulatory peptides in parasitic platyhelminths and their vertebrate hosts: possible novel factors in host-parasite interactions

The study of regulatory peptides has its origins in the classical work of Bayliss & Starling (1902). Their pioneering work on the presence of a factor in intestinal extracts which, when injected into the bloodstream of experimental animals, elicited pancreatic secretion, led to the genesis of the concept of the hormone, i.e. a chemical messenger which is released from one part of the body in response to a stimulus to travel in the bloodstream to a distant target tissue where it would elicit a physiological response appropriate to the original stimulus. In keeping with accepted scientific tradition, this concept had its critics. Pavlov, who had been studying secretory stimulation from a different perspective, concluded from his work on salivation in dogs, that this was mediated via neural pathways. With hindsight, and the benefits of knowledge obtained from nearly a century of scientific research, we now know that these pioneers were in actual fact studying different aspects of the same process and that both theories were complementary. In fact, it is becoming increasingly difficult to ascribe secretory control to either circulating or neuronal factors as both appear to be intimately involved in regulation.

Neuropeptides in platyhelminths

The neuropeptide story began in 1928 with the description by Ernst Scharrer of gland-like nerve cells in the hypothalamus of the minnow,Phoxinus laevis.Because these nerve cells were overwhelmingly specialized for secretory activity, overshadowing other neuronal properties, Scharrer termed them ‘neurosecretory neurons’. What was even more remarkable about the cells was that their products were released into the bloodstream to act as hormones, specifically neurohormones. Neurosecretory cells were identified largely on morphological grounds. That is, they could be stained with special techniques, such as chrome-haematoxylin and paraldehyde-fuchsin, although the techniques are far from specific, staining non-neurosecretory cells as well. However, the basis for the ‘special’ neurosecretory techniques is the demonstration of sulphur-containing proteins – so they are indicative of peptide-producing neurones. An alternative characteristic of neurosecretory cells is the presence of large (> 100 nm), dense-cored vesicles at the electron microscope level; these are the so-called elementary granules of neurosecretion, or ENGs. However, implicit in the concept of neurosecretion is that the prime function of the neurosecretory cell is in endocrine regulation, exerting a hormone-like control over some aspect of the organism's metabolism, by controlling endocrine glands and other effector organs. To satisfy this criterion, evidence had to be obtained of cycles of secretory activity within the cell that could be correlated with a change in the physiological condition of the organism.

Fine structural observations on the ciliary receptors in the epidermis of three otoplanid species (Turbellaria proseriata)

In Notocaryoturbella bigermaria, Otoplana truncaspina and Paroto-planella heterorhabditica three types of epidermal receptors are recognized. Type I: with a single cilium running in a duct, piercing the distal dendrite process of the receptor. The internal wall of the dendrite process has eight ridges with longitudinal filaments lying inside them. The ciliary basal body lacks a longitudinal rootlet but is encircled by a thin annular formation. Type II: with a single (A) or several (B) cilia which protrude from the outer epithelial surface and are provided with a large and striped rootlet. Both types are considered as mechanoreceptors. Type III: with two or more short and stumpy cilia devoid of rootlets and displaying the usual 9 + 2 pattern in the proximal part only. They are considered as chemoreceptors.