Origin , morphology , histochemistry and function of the mucosal mast cell and the globule leukocyte . A review

Parasites invading mucous membranes elicit an i n f l a m m a t o r y response f r o m the host. W i t h appropriate f ixat ion and staining methods, cells w i t h intracytoplasmic granules may be observed. Closer examinat i o n may reveal several types of granular cells, the eosinophi l ic granulocytes being the most c o m m o n l y identi fied i n parasitic infections, but also observable are peculiar mast cells and globule leukocytes whose funct ions are not yet understood. This review describes the most important facts about the mucosal mast cell and the globule leukocyte relevant to their significance i n parasitic infections.


Introduction
Mast cells (= MC) belong to a heterogeneous group of granular cells (Michels, 1938).Their phenotypes may be regulated by factors in the microenvironment.Local growth factors may influence their differentation, maturation, and development, and thus the function of the cells, (Selye, 1965, Dvorak & Galli, 1985, Befus & al., 1986, Enerback & Norrby, 1989).Accordingly, when the microenvironment changes, the phenotypic expression of MC also may change.For example, MC in the subserosal tissue of young rats became after 8-12 weeks positive for berbe¬ rine sulfate and safranin, indicative of maturation of heparin (Arizono & Nakao, 1988).There are at least two specific MC-types; those in the connective tissue called connective tissue mast cells (= CTMC) and those at mucosal and serosal surfaces called mucosal mast cells ( = MMC) (Enerback & al., 1986).
Globule leukocytes (= GL) are granular cells in the epithelium that resemble mast cells.Al- Rangifer,11 (1): 3-11 though comparison of the properties of MMC and GL has led to the assumption that GL are derived from MMC (Miller & al., 1968), there are several indications that GL are an independent cell type (Ruitenberg & Elgersma, 1976, 1979, Ruitenberg & al., 1979, 1982).This question is reviewed in the light of facts on the morphology, histogenesis, histochemistry, and function of MMC and GL.

Occurrence of MMC and GL
MMC and GL are probably found in most vertebrates, but they are not equally distributed in different tissues.MMC are mainly located in lamina propria and also in the epithelium of the alimentary and respiratory tracts.GL occur only in the epithelium of those systems and also intraepithelially in the urinary and reproductive tracts.MMC are more frequent than GL in normal tissues.Parasitic infections influence the number of both cell types, but the number of GL is especially increased during parasitic infections (Table I).
MC are particularly abundant beneath the epithelial surfaces, in the vicinity of peripheral nerves and blood and lymphatic vessles; in certain animal species, they are common within peritoneal and pleural cavities (Selye, 1965, Gal¬ li & al., 1984, 1987, Enerback & Norrby, 1989).Maximow (1905) probably was the first to observe that lamina propria of the intestine of the rat contained MC which differed from the classical CTMC.An aberrant granular celle type first observed by Heidenhein (1888) was described by Weill as a «Schollenleukozyt» in 1919.The name «Schollenleukozyt» was translated into «globule leukocyte» by Keasbey (1923).Weill (1919) described GL in the epithelia of the alimentary tract of the dog, cat, pig, mouse, rabbit, guinea-pig, and man.
Normally GL form a very small proportion of the cells of the epithelium: 0.001 % in the urinary tract of the rat.In rats fed a Mg-deficient diet, however, GL increased to 5 % of the epithelial cell population in the renal pelvis, uterus, and urinary bladder (Cantin & Veilleux, 1972).

Structure of MMC and GL
The size and number of MMC-granules vary according to the size of the cell.The globules of GL are usually bigger and fewer in number than in MMC, causing indentations on the nucleus.Mitotic figures and cytoplasmic projections have been observed in both MMC and GL (Table I).
The fixation method is of extreme importance for the preservation of the structures of MMC and GL (Enerback, 1966, 1974, 1981, 1987, Enerback & Lundin, 1974, Ruitenberg & al., 1982, Enerback & al, 1986).It influences the preservation and visualization of all types of MC.In tissue sections fixed in alcohol, no MC could be found by Maximow (1905).On the other hand, in biopsies of human duodenum fixed in both standard formalin and a formalinacetic acid (FA) fixative, MC were equally well preserved, but an additional population of MC with MMC staining characteristics could be visualized when the FA-fixative was used (Guy-Grand & al., 1984, Enerback & Norrby, 1989).
GL occur as solitary cells in the epithelium.Junctions between GL and epithelial cells have occasionally been observed.GL are able to migrate with the use of pseudopods through the epithelium (Toner, 1965).Accordingly, GL have been found freely in the tracheal lumen (Vandenberghe & Baert, 1981).
The nucleus of GL is usually eccentric, ovoid or spherical, and commonly indented by the cytoplasmatic globules (Gregory, 1979).The nuclear membrane is distinct and the cartwheel distribution of the chromatin is common (Akpavie & Pirie, 1985).Binucleated cells have also been observed (Keasbey, 1923, Akpavie, 1985).
The cytoplasm of GL is filled with refractile, acidophilic and spherical globules (Keasbey, 1923, Kent, 1952).The number of globules in a cell section ranges from 5 to 40 (Keasbey, 1923, Kent, 1952, Kellas, 1961, Akpavie, 1985).The largest globules are seen in cells with few globules.GL with only two large irregularly formed globules were observed in the main bile duct of a goat infected with liver flukes (Rahko, 1972).

Histochemistry of MMC and GL
The electron-dense metachromatic granules of MMC and globules of GL are made up of proteoglycans which can be distinguished histochemically.Low sulphated mucins in MMC differentiate them from CTMC with highly sulphated compounds of heparin.In GL the sulphation degree is histochemically even lower than in MMC.
Ehrlich in 1878 pointed out that the specific feature of MC was the metachromatic granules in the cytoplasm (cited by Michels, 1938).A proteoglycan of MMC, a chondroitin sulphate, does not show fluorescent berberine binding.It stains preferentially with alcian blue in a staining sequence with safranin (Enerback & al., 1986).The cells possess IgE receptors (Ishizaka & Ishizaka, 1984) which respond wery rapidly (Enerback, 1987).Conjugated avidin reacts with the granules of rodent and human MC (Tharp & al., 1985).

Function of MMC and GL
MC play an important role in many biological processes.They probably interact with the cells of many tissues, organs, and body cavities.The cells can elaborate and release a variety of biologically active compounds.Obviously, this hold true specifically for CTMC.MMC and GL are, on the other hand, involved in the ex-pulsion of parasites, the «self-cure» phenomenon.GL is thus possibly associated with the immune response to parasitic infections (Table I).
Numerous stimuli cause MC to release biologically active mediators (Selye, 1965).Morphological studies have documented structural changes in MC during different inflammatory, immunological, reparative, metabolic, and neoplastic responses but the role of MMC has remained unclarified.
Hyperplasia of instestinal MMC occurs in the rat after an infection with Nippostrongylus brasiliensis.The reaction in N. brasiliensis «seif cure» expulsion is considered to be of an anaphylactic type.(Miller, 1971).However, there was no documentable change in the permeability during the expulsion of N. brasiliensis in the mast-celldeficient mice (Crowle & Reed, 1981).Nevertheless it is suggested that biogenic substances released from MMC are responsible for mucosa lysis and that this causes the increase in mucosal permeability which is known to occur during worm expulsion (Miller & al., 1968).
The function of GL was originally connected with the assimilation of food substances (Weill, 1919).The observation that GL contained hemoglobin led to the theory that they had the same function as the erythrocytes (Keasbey, 1923).Later it was supposed that GL had a nutritive function for the surrounding tissue (Heine & Schaeg, 1977).
The first to suggest that GL could be involved in parasitic infections was Hole in 1937Hole in . hr 1939 Taliaferro & Sarles showed that GL were associated with parasitic infections and mast cell proliferation.Others also have observed a correlation between parasites and GL (Kirkman, 1947, Sommerville, 1956, Ahlqvist & Kohonen,

Supposed precursors for MMC Evidences of relationship
1. Hemopoietic tissue MMC are thymus-independent cells of bone marrow?

Lymphocytes
Lymphocytes are known to be migrating cells with capability to transform.

Basophilic granulocytes
Analogous composition of the granules in both basophilis and MMC.
Table III.Theories and arguments on the origin and histogenesis of GL Supposed precursors for GL Evidences for relationship 1. Hemopoietic tissue Thymus-dependent cell of bone marrow with intracytoplasmic granules and short life span (2-3 months)?

Lymphocytes
A complete morphological series from lymphocyte to GL is demonstrable.

Mast cell
Analogous ultrastructure and composition of granules of MMC and globules of GL.

Large granular lymphocyte
Intraepithelial granular cell with similar nuclear structure to GL.

Russel body cell
Globules of GL resemble the Russel bodyinclusions with the same fluorescence, being surrounded by pyroninophilic cytoplasm like in plasma cells.

Eosinophilic granulocyte
Analogous acidophilic granules in GL and eosinophils, both associated with parasitic infections.
A Trichinella-'mduced proliferation of MMC occurred in the intestine of thymus-bearing and not in athymic mice.The enhanced behaviour was both antigen-and thymus dependent but only observed for the stromal intestinal MMC and not for the intraepithelially located GL.In congenitally mast cell-deficient mice infected with Trichinella, no proliferation of GL was observed.In contrast, an increasing number of GL was observed intraepithelially in the intestine of normal mice.A positive correlation between the infiltration of GL and the rapidity of T. spiralis expulsion from the intestine was suggested, (Kamiya & al., 1985).

Origin of MMC and GL
There is evidences that MMC are derived from the hemopoietic tissue, possibly having the same precursors as other types of MC.The differentation of MC precursors may be locally regulated.The histogenesis of GL, on the contrary, is doubtful.Erythrocytes, plasma cells, Russel body cells, and other types of leukocytes and mast cells have been proposed as precursors for GL.However, GL seem to be an independent migrating cell type of nonepithelial origin and the histogenesis of GL-like cells may be different in different vertebrate classes.Furthermore, cytochemical dissimilarities between tracheal and intestinal GL might indicate that they represent different phenotypes or even different types of GL (Tables II and III).
Practically pure populations of one specific MC phenotype exist in the mucosa and of another phenotype in connective tissue (Enerback, 1981, 1987, Kitamura & al., 1983).Peritoneal MC themselves might give rise to MC phenotypically similar to either CTMC or MMC (Nakano & al., 1985).
There is evidence that MC derived from cultured hemopoietic tissues are MMC (Jarrett & Haigh, 1984).The cells are regarded as an independent cell population.Their histogenesis is thymus-independent, but the response to Trichi¬ nella infections is mainly thymus-dependent (Ruitenberg & al, 1979, Galli & al, 1984).
Similarities between GL-globules and erythrocytes led to the opinion that the globules are derived from phagocytosed erythrocytes (Gregory, 1979).Duran-Jorda (1945) considered that GL secreted erythrocytes, then becoming epithelial lymphocytes.The morphology and staining properties also contributed to the hypothesis that GL belong to the erythrocytic series.Dawson (1943) proposed that GL occupy an intermediate position between erythrocytes and leukocytes.Kent in 1952 andToner in 1965 suggested a lymphocyte origin on the basis of finding a complete series, with transitional forms, from the small lymphocytes to the GL.Kitagawa & al. (1979) reported that GL in chicken originated from the thymus lymphocytes, being stainable with anti T-lymphocyte serum.Baert & Frederix (1985) considered large granular lymphocytes as a possible precursors for GL.Other origins have also been suggested, including eosinophils, plasma cells, and mast cells (Kent, 1952).The inclusions of Russel body cells and the globules fluoresced similarly, and the cytoplasm in both cells was pyroninophilic (Dobson, 1966, Carr, 1967, Whur & Johnston, 1967).But Gl differ from plasma and Russel body cells because they lack immunoglobulins (Whur & White, 1970).
In 1977 Heine and Schaeg stated that their results indicate that the GL may originate both from M(t and from eosinophilic granulocytes.GL were considered to be MMC which have migrated to the epithelium and partly discharged their granules; the conclusion was based on their similarities both ultrastructurally and histochemically with MMC (Jarrett & al., 1967, 1968, Miller & al., 1968, Murray, al., 1968, Miller & Jarrett, 1971).Huntley & al. (1984) showed transitional cells between MC and GL in the intestine of sheep infected with nematodes.The size of the globules changed from small to large and proteoglycans, serine esterase, dopamine, and immunoglobulins were demonstrated in all cell types.When the globules incresed in size, their amine content decreased.Histochemical analyses thus supported the view that MMC and GL have a common lineage (Huntley & al., 1984).
GL has been proposed to be a specific cell type of mesenchymal origin (Kirkman, 1950).There are several indications that GL probably are an independent cell type.This fact is supported by the findings of a neoplastic behaviour of GL in the intestine of the cat (Finn & Schwartz, 1972).Furthermore, GL undergo mitosis during Trichinella infection.Ruitenberg & Elgersma (1979) did not totally discard the MMC origin of GL.Some other data suggest a common source for MMC in intestinal mucosae and support the idea that MMC and GL are two independent cell populations (Parmentier & al., 1982).

Table I . Comparision of different characteristics of MMC and GL MMC Occurrence lamina propria and epithelium of different mucous membranes GL within mucosal epithelium Number of cells variable, no absolute counts made normally 1-2 GL/1000 epithelial cells, experimentally max. 32/1000 epithelial cells Function of cells Nuclear form Structure of chromatin release biological active unknown associate with MC-proliferation in mediators, take part in the expul- parasitic infections and take part in sion of parasites and facilitate tran- the «self cure» by local immediate sport of antibodies into the lumen of hypersensitivity reaction intestine spherical ovoid, spherical, often indented lymphocytic-type distribution cartwheel-type distribution Cytomembrane with pseudopodia, no desmosoms with pseudopodia, no desmosoms Size and ultra- structure of granules and globules small
, 1-2 fx, electron-dense, usually large, usually 2-5 electron-dense, unevenly usually diffusely