The digestive system and fat body of an early-derivative oribatid mite, Archegozetes longisetosus Aoki (Acari: Oribatida, Trhypochthoniidae)
2003 - Volume: 43 Issue: 1-2 pages: 149-219
The digestive system of Archegozetes longisetosus comprises the following components: the preoral cavity in front of the mouth and the digestive tract, which is composed of: 1) a cuticle lined foregut starting with the mouth and continuing with the pharynx and esophagus; 2) a midgut with ventriculus, one pair of preventricular glands, one pair of caeca, a colon, an intercolon, and a postcolon; 3) a cuticle lined hindgut composed of the anal atrium and terminating with the anus. A crop or ingluvies, i.e. a widened posterior portion of the esophagus, is not present. All regions of the digestive tract are provided with muscles that are all of the cross striated type. The pharynx is crescent shape in cross section and is activated by dilator and depressor muscles. In addition there are ventral muscles. The cuticle of the pharynx is specialized in a way that likely has functional implications. The ventral wall of the pharynx is fixed to the ventral wall of the infracapitulum in a peculiar way. A functional model of the pharynx is suggested. The esophagus is a slender tube that passes through the synganglion. It is capable of expansion by means of longitudinal folds in its wall. The esophagus is predominantly provided with ring muscles. However, within the same muscle cell are also myofibrils which run in longitudinal direction. The esophagus projects into the ventriculus with a pronounced esophageal valve. The peculiar termination of the esophageal valve intima is described. The ventriculus is composed of one type of epithelial cell that is mainly characterized by its distinct brush border and concentric inclusion bodies (spherites = type A granules). The ventriculus frequently contains a food bolus that is surrounded by an indistinct layer of secretion forming a premature peritrophic membrane. The preventricular glands are represented by two pouches formed by cells similar to those of the ventriculus, but are densely filled with very dense spherites. These glands approach the lateral parts of the synganglion, which form peculiar lamellated bodies (specialized neurons) that are likely photosensitive. The spherites in the ventriculus and caeca are usually less electron dense. The caeca distinctly show three types (or phases) of epithelial cells: 1) an ER cell, which is very richly provided with rough endoplasmic reticulum; 2) a digestive (= resorptive) cell showing pinocytotic activity and containing numerous vacuoles, and 3) a spherite cell, which contains many spherites. The caeca never contained food and their main functions are likely the secretion of digestive enzymes (ER cells) and resorption (digestive cells). The spherite cells may be end stages of a cell cycle starting with resorption and ending with extrusion of that part of the cell in which spherites had accumulated. It may be that parts of the spherite cells are discarded from the epithelium, as may also be suspected from the ventriculus cells and the cells of the preventricular glands. The colon is rather inconspicuous in contrast to the postcolon, which is mainly characterized by long microvilli. The main function of the postcolon seems to be compaction of the faecal pellet and completion of the peritrophic membrane. Material contributing to this membrane also appears to be added in a short transition zone between the colon and postcolon, called the intercolon. This region is provided with rather conspicuous muscles, suggesting a sphincter function. The region is capable of being distended during the passage of the food due to its peculiar epithelium, which shows deep crypts. Finally, the anal atrium is again provided with a cuticular lining that shows specific differentiations from proximal to distal. At the beginning of the anal atrium, the cuticle seems to function as an elastic cushion that helps to keep the postcolon closed, unless contraction of the postcolon pushes the faecal pellet out. However, there are also very thick sphincter muscles in this region. Since the epithelium of the anal atrium does not show any indicative peculiarities, it seems unlikely that further processing of the faecal pellet occurs in this region. The food bolus or faecal pellet, respectively, always shows some bacteria, fungi, and algae cells, which seem to be undigested and hence may pass through the digestive tract as intact cells. The haemocoel contains peculiar large cells, which are considered to represent a storage tissue and hence are termed fat body cells, as in other mites. These cells are connected to the midgut by small finger like processes that presumably form gap junctions with the midgut epithelial cells. The fat body cells show various supplies of different inclusions and organelles, probably related to the physiological state and/or their location in the mite s body.
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