Ancylostoma caninum: fine structure of intestinal epithelium

Phagocytosis in cellular defense and nutrition: a food-centered approach to the evolution of macrophages

The uptake of macromolecules and larger energy-rich particles into the cell is known as phagocytosis. Phagocytosed material is enzymatically degraded in membrane-bound vesicles of the endosome/lysosome system (intracellular digestion). Whereas most, if not all, cells of the animal body are equipped with the molecular apparatus for phagocytosis and intracellular digestion, a few cell types are specialized for a highly efficient mode of phagocytosis. These are the ("professional") macrophages, motile cells that seek out and eliminate pathogenic invaders or damaged cells. Macrophages form the backbone of the innate immune system. Developmentally, they derive from specialized compartments within the embryonic mesoderm and early vasculature as part of the process of hematopoiesis. Intensive research has revealed in detail molecular and cellular mechanisms of phagocytosis and intracellular digestion in macrophages. In contrast, little is known about a second type of cell that is "professionally" involved in phagocytosis, namely the "enteric phagocyte." Next to secretory (zymogenic) cells, enteric phagocytes form one of the two major cell types of the intestine of most invertebrate animals. Unlike vertebrates, these invertebrates only partially digest food material in the intestinal lumen. The resulting food particles are absorbed by phagocytosis or pinocytosis and digested intracellularly. In this review, we provide a brief overview of the enteric phagocytes described electron microscopically for diverse invertebrate clades, to then to compare these cells with the "canonical" phagocyte ultrastructure established for macrophages. In addition, we will review observations and speculations associated with the hypothesis that macrophages are evolutionarily derived from enteric phagocytes. This idea was already proposed in the late nineteenth century by Elias Metschnikoff who pioneered the research of phagocytosis for both macrophages and enteric phagocytes. We presume that modern approaches to better understand phagocytosis will be helped by considering the deep evolutionary relationship between the two cell types.

The occurrence of submicrovillar endotube (modified terminal web) and associated cytoskeletal structures in the intestinal epithelia of nematodes

The intestines of 22 genera of nematodes from five different orders were examined for the presence of an endotube, the submicrovillar entity previously described for Haemonchus contortus , a member of the order Strongylida. The endotube can be obtained by blunt dissection as a complex with the microvilli essentially free of the rest of the cytoplasm. Representatives of all three suborders and eight families of the order Strongylida possessed an endotube but of the representatives of the four other parasitic orders and the one free-living group examined only one genus, Strongyloides (Rhabditida) possessed this structure. The thickness of the endotube ranged from about 80 nm in Metastrongylus up to 6 μm in Strongylus . In all samples the filamentous cores of the microvilli, whether formed into an axial bundle (as usual) or dispersed in a net (as in Dictyocaulus ), which extended 0.1-0.5 μm below the base of the microvilli terminated in the luminal surface of the endotube. The basal side of the endotube was usually associated with a layer of microfibrils. The depth and distribution of the microfibrillar layer determined the extent to which the endotube-brush-border complexes could be dissected free from other cytoplasmic components. There was electron microscopic evidence for an endotube-like entity not associated with the microvilli in the intestine of Syphacia (Ascarida). A survey of published electron micrographs of nematode intestines indicated that the true submicrovillar endotube occurred only in members of the order Strongylida and the genus Strongyloides (Rhabditida) in which the structure here described as an endotube has previously been described as terminal web.

Endotube-brush border complexes dissected from the intestines of Haemonchus contortus and Ancylostoma caninum

A procedure for the blunt dissection of the syncytial intestine of Haemonchus contortus to separate the endotube-brush border complex is described. A similar complex can be obtained from the partially cellular intestine of Ancylostoma caninum. The structural features of the complexes from the two sources are compared. The main differences are in the retention of terminal bars by the endotube from A. caninum and in the extent and organization of the microfilaments associated with the basal side of the endotubes.

The fine structure of the alimentary tract of third-stage larvae of Haemonchus placei and Trichostrongylus colubriformis

A comparative study was made of defined regions of the alimentary tract of third-stage larvae of T. colubriformis and H. placei. The general organization of the tract was similar in both nematodes. The mouth, buccal cavity and oesophagus were lined with cuticle. Muscle cells, apical cells and gland cells were present in the oesophagus. In both species the gland cells contained secretory granules of similar morphology. The lumen of the intestine was ill-defined and apparently nonfunctional. Dense bodies observed in the intestinal cells differed in the two species.

We are grateful to Dr D. W. Brocklesby for his help and advice. We would also like to thank Dr D. L. Lee and Dr W. G. MacMillan for helpful discussions.

Studies on gut ultrastructure and digestive physiology in Cyathostoma lari (Nematoda:Strongylida)

Histological, histochemical and ultrastructural methods have been used to study the gut structure and digestive physiology in the parasitic nematode Cyathostomalari E. Banchard, 1849. The nematode is parasitic in the orbito-nasal sinuses of its host and feeds exclusively upon blood.

Three gland cells are present in the oesophagus, one in each sector. The two subventral gland ducts open into the lumen of the oesophagus at the level of the nerve ring, and the dorsal gland duct into the base of the buccal capsule. It is suggested that the latter gland may produce an anticoagulin, histolytic and/or haemolytic secretions, histochemically-inactive in situ. The intestine is syncytial, with no differentiation into secretory components, and its distal surface is provided with a prominent brush border of long microvilli.

A non-specific esterase, thought to originate in the oesophageal gland secretions, causes haemolysis of ingested erythrocytes. The major part of haemoglobin digestion is extracellular, partly by a process of contact digestion on the microvilli, initiated by the esterase, and the protein is then further broken down by an exopeptidase of the ‘leucine aminopeptidase’ type, and the products absorbed into the gastrodermis possibly with the aid of an acid phosphatase present on or in the microvilli. An insoluble iron-containing compound, which is not haematin, results from breakdown of the prosthetic group. A smaller proportion of haemoglobin is taken up by the gastrodermis and digested intracellularly by the action of lysosomal hydrolases. An endopeptidase and an acid phosphatase are present in the gastrodermis, possibly in sacs of GER which are suggested to be primary lysosomes and which fuse with the endocytosed heterophagosomes. The insoluble pigment haematin results from digestion within the heterolysosomes, and this gives rise to the iron-containing gastrodermal pigment granules. The products of extracellular digestion are absorbed by the gastrodermis where they are synthetized into lipid by a lipase, which probably resides in whorls of GER.

This investigation was supported in part by Research Grant AI 06295 of the United States Public Health Service. The final manuscript was prepared during the tenure of a Postdoctoral Fellowship from the British Egg Marketing Board. I wish to extend my gratitude to Dr J. B. Jennings for advice and encouragement throughout this work.