Chromium Induced Histological Changes in the Body Wall of the Earthworm Eudrillus eugeniae

Abstract

In Sri Lanka, documented evidence indicates that the Chromium (Cr) content in water andsoil is high, reaching upto 0.60 mgl-1 in water and 103 mg kg-1 in soil and sediment. Cr is oneof the most toxic heavy metals, which is capable of inducing adverse effects in exposedorganisms even in trace amounts. Of the two valency states, Cr3+ and Cr6+, the latter ispotentially more toxic than the former. In the present study we examined the histologicalalterations in the body wall of Eudrillus eugeniae, a commonly occurring species ofearthworm, exposed to hexa-valent Cr.

Adult earthworms were collected from a culture facility in Talawakalle, and acclimatized fortwo weeks. Worms with a well-developed clitellum and of 0.29 + 0.009 g weight were usedfor standard chronic exposure trials of 28 days. For the trials, the animals were placed insuitably prepared clay pots containing banana piths soaked in five different concentrations(0.002 – 20 mg l-1) of Cr6+. At the end of the 28 days, the worms were fixed in Zenkerfixative, washed, dehydrated, embedded in paraffin wax and the tissue sections taken at7 μmwere stained using hematoxylin-eosin. Histological alterations in the outer epithelium werethen recorded in detail.

It was evident that Cr6+ induced alterations in the outer epithelium of the earthworm at all testlevels. A reduction in the thickness of the epithelium was noted (Control – 61.6 ± 6.1μm,0.02 mg l-1 – 28.0 ± 1.5 μm). Another observation was the reduction in the number of nuclei(Control – 31.3 ± 2.8, 0.02 mg l-1 - 7.4 ± 0.4) and the increase in the number of pyknoticnuclei in a given area of epithelial tissue (Control - 2.3 ± 0.4, 0.002 mg l-1 - 10.5 ± 1.0).Epithelial cells were neatly compacted with distinct cell margins in earthworms that were notexposed, whilst those exposed showed disintegration of cell margins even at 0.002 mg l-1 ofCr6+. Mucous cells, which are generally apparent, were not distinct in Cr6+ treated worms.Epithelial sloughing was more pronounced in treated earthworms than in the controls. Thespace between the epidermis and muscle layers were greater in treated worms than in thecontrols (Control - 0.7 ± 0.5 μm, 0.02 mg l-1 - 23.6 ± 1.0 μm). The muscle layers of the bodywall of the Cr6+ treated worms showed loss of structural integrity and increased intercellularspaces with the damage being more severe in those exposed to 20 mg l-1. This study providesevidence that histopathological alterations are possible in the earthworm at levels of Crrecorded in Sri Lanka’s natural ecosystems.