Abstract
In molluscs, internal defence against microorganisms is performed by a single cell type, i.e., the haemocyte or amoebocyte. The origin of these cells in Biomphalaria glabrata was initially thought to be localised within the vasculo-connective tissue. More recently, origin from a single organ, termed the amoebocyte-producing organ (APO), has been postulated based on the occurrence of hyperplasia and mitoses during Schistosoma mansoni infection. The present investigation represents a histological, immuno-histochemical and ultra-structural study of the B. glabrata APO, whereby histological identification was facilitated by means of collecting epithelial basophilic cells. These cells were comprised of single-cell layers that cover a portion of the stroma, which contains many small, round cells and haemolymph sinuses, as well as a small area of the pericardial surface of the reno-pericardial region. On occasion, this epithelial component vaguely resembled the vertebrate juxtaglomerular apparatus, which reinforces its presumed relationship to the kidney. Both in normal and infected molluscs, mitoses were only occasionally found. The present quantitative studies failed to demonstrate the presence of APO cellular hyperplasia, either in normal or schistosome-infected B. glabrata. Conversely, several structural details from the APO region in B. glabrata were found to be consistent with the hypothesis that the APO is a filtration organ, i.e., it is more closely related to the kidney rather than the bone marrow, as has been suggested in the literature.
Highlights
The mollusc Biomphalaria glabrata is an important intermediate host for Schistosoma mansoni, which is the most important vector for human schistosomiasis in Brazil (Paraense 2001)
Our present findings demonstrated no evidence to support the assertion that haemocytes in B. glabrata have a common origin from a central body, i.e., the amoebocyte-producing organ (APO), which has been proposed to be a type of bone marrow that produces the circulating cells responsible for defence in B. glabrata
Current research suggests that haemocytes can be formed at different locations in Pulmonata, such as within the connective tissue and vascular system of snails (Lymnaea stagnalis) (Sminia 1974, Sminia & van der Knaap 1983) or, alternatively, in an organ located in a region anatomically close to the kidney
Summary
The mollusc Biomphalaria glabrata is an important intermediate host for Schistosoma mansoni, which is the most important vector for human schistosomiasis in Brazil (Paraense 2001). The degree of resistance or susceptibility of the mollusc towards the parasite is variable and dependent on several factors, such as the defence cells contained within the mollusc These defence cells belong to a single cell type, i.e., the amoebocyte or haemocyte. These cells circulate within the haemolymph and are present within the interstitial tissues of the mollusc and contribute to phagocytic and scavenging functions. Their site of origin has not yet been established with certainty, initially, the data suggested a multicentric origin for amoebocytes (Wagge 1955). The present paper reports on the structure, nature and significance of the APO in normal and S. mansoni-infected B. glabrata molluscs
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