The ultrastructure of the metachromatic mucous cells of some British lumbricids (Annelida)
Authors: Richards K. Sylvia 
Published: August 2009

The ultrastructure of the metachromatic cells is described. The secretory globules arising from the polarized Golgi apparatus do not show taxonomic variation as do those of the orthochromatic mucous cells.

The globular material is not uniform, three main variations being described and interpreted as a reflection of differing hydration states. Gradations and coexistence of more than one form within a cell or single globule occur but, in general, the majority of globules within a cell present the same ultrastructural pattern and size.

A relationship exists between the globule size and the three main ultrastructural expressions of the mucus, a fourth variation occurring in cells with very large globules, and a correlation is described between the globule size and the state of the granular endoplasmic reticulum and Golgi apparatus. As a cell ages it gives rise to larger globules containing material of a different hydration level. Young replacement cells are described and their microanatomical differences detailed.

The fibrous nature of the mucus is the same as that forming the supracuticular mucoid coat and similar to the acid mucus described in other annelids and other animal groups. Such similarities support the idea of a generalised function and it is suggested that the metachromatic mucus provides the medium through which the respiratory exchanges occur. The necessity for constant secretion leads to speculation concerning the life‐span of the metachromatic mucous cell.

The metachromatic mucous cells of 11 British lumbricids, from six genera, are characterized by the fibrous substructure of the secretory globules lying within the secretory area. Ultrastructurally the globule material does not show species variation as does the orthochromatic mucus but, nevertheless, is not consistently uniform in appearance. Variations are recorded between cells within a section, between globules within a cell and even within areas of an individual globule.

Three main types of variation are described: finely fibrous, the fibrils exhibiting a random arrangement; stranded, where strands of greater electron density are frequently disposed parallel to one another; densely whorled, where the material assumes an exaggerated pattern. Since gradations between these categories exist and more than one form can be present within a cell or even a single globule, it is presumed that the variations are reflecting differing hydration states of the secretory product. In general, the majority of globules within a cell present the same ultrastructural form and size.

The globules arise from the mature face of the basally situated, polarized Golgi apparatus and are extruded from the cell through the apical pore which has no occlusor organelles, though mucous pore microvilli are present.

A correlation exists between the size of the globules and the fine structural appearance of the secretory material. The densely whorled form occurs only in globules smaller than 0.9 μm; the stranded type in medium sized globules 0.9 μm‐1.3 μm; the finely fibrous variation is only found in cells where the globule size exceeds 1.5 μm but is less than 2 μm. Occasionally cells occur containing very large globules, in excess of 2 μm; and here the secretory material is eccentrically condensed within the globule limit in species other than D. hibernica. This species presented a substructure, within the very large globules, consisting of a finely fibrous matrix in which there were several electron dense areas and for which no taxonomic or eco‐physiological explanation can be advanced at present.

A further correlation is apparent between the globule size and the state of the basally situated secretory apparatus. Cells with small or medium sized globules possess an active granular endoplasmic reticulum, the cisternae of which are filled with medium electron dense material. In those cells with large globules the rough endoplasmic reticulum is frequently bloated, distorted and invariably empty. In cells with very large globules the Golgi system also appears disorganised and the nucleus lacks contrast and shows a distended perinuclear space. Such cells are regarded as being senescent and it is suggested that as a secretory cell ages its ultrastructure alters and the secretory apparatus gives rise to large globules of material of a differing hydration level.

Young replacement cells are described and these differ from the normal metachromatic cell in having a nucleus at mid‐epithelial level and granular endoplasmic reticulum and Golgi apparatus not restricted to the basal area of the cell.

The fibrous ultrastructural nature of the metachromatic mucus is similar to that forming the supracuticular mucoid coat trapped by the epicuticular projections. It also accords well with descriptions of acid mucus in other annelids and other animal groups. A very striking similarity exists between the present results and those for a histochemically comparable mucus in a turbellarian platyhelminth where globule size and ultrastructural variation has been described.

Such comparisons support the idea that the metachromatic mucus has a generalized function and its presence covering the cuticle, combined with blood capillary loops extending to within 1 μm of the cuticle, leads to the suggestion that it is the metachromatic mucus in lumbricids which provides the medium through which the primary gaseous exchanges take place. In providing such a constant respiratory surface, the metachromatic mucous cells are presumably permanently active during their life‐span which might therefore be expected to be shorter than that of a less permanently secretory cell type. The presence or both young and ageing metachromatic cells within the tissue pieces, such stages not being observed for the other secretory cell types, would appear to support such a hypothesis.