Trypanosoma lewisi: electron microscopy of the infected spleen

The spleens of Lewis rats, both normal and infected with Trypanosoma lewisi were examined by electron microscopy. Special attention was directed to clusters of splenic cells which occur in the course of the infection. The reticular cells first showed alterations of their structure by the second day of infection, with considerable surface membrane activity. By the fourth day and thereafter various cells were found gathered around the reticular cells. These cell clusters mainly contained lymphocytes, plasma cells, and erythropoietic elements in many stages of differentiation. It was not unusual that several cell types were found adjacent to the same central reticular cell. These arrays, similar in geometry to the erythropoietic island of the bone marrow, became more predominantly “plasma cell” islands as the infection progressed. Parasites were recognizable within the reticular cells, and were noted to be in regions where the cellular membranes of adjacent cells demonstrated vesiculations resembling rhopheocytosis. A further observation was the pinching off of neighboring plasma cell cytoplasm into the reticular cells.     

Isolated cilia and crystalloid inclusions in murine bone marrow stromal cells

Bone marrow stromal cells, especially reticular cells and macrophages, are thought to have important functions in the hematopoietic inductive microenvironment (HIM). In order to define the details of their fine structure, the femoral bone marrow of 50 C 57 BL mice was examined by electron microscopy after fixation by vascular perfusion. Almost all the structures of the murine bone marrow stromal cells were similar to those previously reported. Murine bone marrow reticular cells had some isolated cilia (with a frequency of 6.0%) of 9 + 0 pattern in their axoneme. The murine bone marrow macrophages had crystalloid inclusions (CI) that revealed characteristic periodic substructure. These periodic structures could be classified into four types (types A-D). Type A inclusions had a rather amorphous structure and were the most common (with a frequency of 61.9%). Type B inclusions had a periodic lamellar structure oblique to the long axis of the CI itself at intervals of about 3.5 nm. Type C inclusions had a periodic lattice structure at intervals of about 3.7 nm, and were the least frequent (with a frequency of 6.0%). Type D inclusions had periodic laminated structure parallel to the long axis of CI at intervals of about 3.5 nm. Although the true significance of isolated cilia of murine bone marrow reticular cells and the periodic internal structures of CI in murine bone marrow macrophages remains unknown, they must not be ignored as only incidental findings: they may be useful to the understanding of the relationship between function and structure in future research on HIM.     

The argentophil reticular cells of the mammalian spleen. II. The argentophil reticular cell arrangement inside the red pulp and its relationship with the endothelial lining cells of the splenic sinuses

The red pulp's argentophil reticular cell network of the spleen is composed by 3 types of fixed cells: 1. the primitive reticular cell, slightly argentophil; 2. the small reticular cell; 3. the larger reticular cell, strongly argentophil and phagocytic. This latter shows the classical morphological characteristics attributed to the reticular cells of the spleen. The large argentophil reticular cell may become free, constituting a 4th cell type, the free macrophage. A 5th reticular cell type is the dendritic cell found into the lymphatic follicles of the white pulp. The argentophil reticular cells of the red pulp assemble together to form the reticular cells' network, that occurs inside the red pulp cords. The primitive and the small reticular cell form the fundamental network on which the large cells are apposed. The reticular cells of this network maitain relationship with the arterial terminal vessels of the red pulp, being responsible by the ellipsoid structure. In those arteriolar segments without ellipsoid and in those mammalian species devoid of ellipsoid, the white pulp reticular cells, that surround the blood vessel as a part of the lymphoid periarteriolar sheath, mix with the red pulp's reticular cells and both can hardly be discriminated. The ellipsoids are formed by large argentophil cells arranged in concentrical layers around its lumen that sometimes appear devoid of endothelial lining cells. The red pulp's argentophil reticular cells, either the small or the large ones, contributed to the structure of the splenic sinuses' wall; its thin processes surround the sinus wall outside the endothelial lining cell as fibrillar structures that cross the back side of the lining cells. Two or more argentophil reticular cells send fibrillar processes to a single sinus. The perisinusal reticular cells may send a process between adjacent endothelial lining, cells that insinuate and attain the sinus lumen; this process becomes thick and eventually, the reticular cell enter the sinus lumen as a free macrophage. The argentophil reticular cells of the red pulp make connection between the capsule or the trabeculae and the reticular cell network. The endothelial lining cells of the splenic sinuses are not argentophil.     

Three-Dimensional Reconstruction of Hematopoietic Organ of Bombyx mori Larva

Three‐dimensional reconstructions of a hematopoietic organ (HPO) from Bombyx mori larva were undertaken using light and electron microscopy. Each compact islet varied in sizes, but in the central area of the HPO their size became smaller. Compact islets and loose islets were made up of prohemocytes, plasmatocytes, and reticular cells, but there were differences in the proportions of these cells. Within the cytoplasm of reticular cells and within their cell projection, vacuoles were observed. Cell proliferation occurs primarily in the compact islets, and differentiation associated with the reticular cells occurs primarily in the loose islets. It can be inferred that reticular cells have a significant influence on proliferation and differentiation associated with hematopoiesis. According to the results of the 3‐D reconstruction, one reticular cell is in contact with eight or nine hemocytes. Each reticular cell is presumably of approximately ten hemocytes. Movies relevant to Figs. 3, 4 and 5 can be found at http://biotech.korea.ac.kr/ cellbio/pages/ 3D‐results.html.     

Immunohistochemical characteristics of chicken spleen ellipsoids using newly established monoclonal antibodies

Ellipsoids, the extra-vasculature sites surrounding penicilliary capillaries of the chicken spleen, play critical roles in the immune response and also in the clearance of pathogens or other particles. The meshwork of ellipsoids is formed by fibroblastic reticular cells. To characterize ellipsoidal reticular cells, a series of monoclonal antibodies against the chicken spleen have been developed. Of these antibodies, CSA-1 antibody reacts with fibroblastic reticular cells in ellipsoids and with endothelial cells. The reticular nature of positive cells in ellipsoids is indicated by immuno-electron microscopy, and by double-staining with anti-heat-shock protein 47 kDa (hsp47) antibody. The reaction of CSA-1 with reticular cells is limited in ellipsoids; CSA-1 does not react with reticular cells in other lymphoid organs. These findings indicate that ellipsoidal reticular cells share the antigen with endothelial cells. Ontogenic studies reveal that, on embryonic day 18, the development of ellipsoids is completed, penicilliary capillaries become fenestrated, and CSA-1 expression in ellipsoids begins. These findings suggest that CSA-1 is expressed on the cell surface of ellipsoidal reticular cells once they are exposed to blood flow.     

Innervation of the guinea pig spleen studied by electron microscopy

The innervation of the guinea pig spleen was investigated by electron microscopy. Unmyelinated nerve fibers in the capsulotrabecular and arterial systems were found to contain large and small granular and small agranular synaptic vesicles in their terminals and are thought to be sympathetic adrenergic in nature. They influence the contraction of the smooth muscle cells by diffusion innervation in these systems. These nerve terminals were also scattered in both the red and the white pulp. Pulp nerves wrapped by Schwann cells were further enclosed by myofibroblastic reticular cells. This condition revealed that the pulp nerves pass through the connective-tissue spaces of the reticular fibers, which contain elastic fibers, collagenous fibrils, and lamina densa-like materials of the usual basement laminae. One of the target cells for the pulp nerves is considered to be the myofibroblastic reticular cell in the reticular meshwork. Neurotransmitter substances released from the naked adrenergic nerve terminals travel through the reticular fibers and may play a role, by both close association innervation and diffusion innervation, in the contraction of reticular cells to expose the reticular fibers. At the exposed sides, connective-tissue elements of the reticular fibers are bathed with blood plasma, and the included naked nerve terminals, devoid of Schwann cells but with basement laminae of these cells, face free cells at some distance or are in close association with free cells, especially lymphocytes, macrophages, and plasma cells. The close ultrastructural relationship between the naked adrenergic nerve terminals and immunocytes strongly suggests that there is an intimate relationship between the immune system and the sympathetic nervous system through both close association innervation and diffusion innervation. Thus splenic adrenergic nerves of the guinea pig may play a triple role in 1) contraction of smooth muscle cells to regulate blood flow in the organ, 2) induction of the exposure of reticular fibers by contraction of the reticular cells in order to form a close relationship of the nerve terminals with the immunocytes, and 3) subsequent neuroimmunomodulation of the immunocytes.     

Scanning- and transmission electron-microscopic study of lymphatic vessels in the splenic white pulp of the macaque monkey

Summary The fine structure of the lymphatic vessels in splenic white pulp of the macaque monkey was studied by scanning and transmission electron microscopy.Lymphatic vessels were slit-like or widened channels which extended along central arteries and their large branches. The walls of the vessels were very thin in comparison with those of nearby arteries. They were composed only of a layer of endothelium supported by underlying reticular cells. Endothelial cells were mostly ribbon-like and extended along the long axis of the vessels. Perikarya of the endothelial cells were slightly protruded into the lumen. The thin peripheral cytoplasm showed smooth surfaces, except for some tiny processes, especially at boundaries between adjacent cells. The basal surface of the endothelial cells was attached to the lattice of reticular cell processes forming the framework of the white pulp. Basal laminae in strands were intercalated between endothelial cells and reticular cells. Perforations were often seen through the endothelial cell cytoplasm. Lymphocytes or processes of macrophages seen in the perforations were considered to be in migration. Large patent openings through the endothelium were not observed. The wall structure of the lymphatic vessels in the splenic white pulp suggests that lymphocytes in the white pulp may move directly into the lymph flow, in addition to moving into the blood flow via the vascular sinuses.Supported by Research Grant-in Aid from the Ministry of Education, Japan (Grant NO. 56480081).     

Subcellular compartmentalization of saccharide moieties in cultured normal and malignant cells

We studied subcellular localization of saccharide moieties in cultured normal and malignant cells fixed in paraformaldehyde and treated with a nonionic detergent, using lectins specific for various surgar residues as probes in fluorescence microscopy. In normal cells, concanavalin A and Lens culinaris agglutinin, specific for mannose-rich carbohydrate cores in glycoproteins, labeled the endoplasmic reticulum as a wide perinuclear region. Other lectins, on the other hand, stained the Golgi apparatus as a juxtanuclear reticular structure. A similar compartmentalization was also seen in all malignant cells studied, although the Golgi apparatus in these cells was distinctly vesicular in appearance. Our results indicate that saccharide moieties in both normal and malignant cells are similarly compartmentalized, and thus speak in favor of a unidirectional subcellular flow for both membrane and secreted glycoconjugates.     

Tissue nature of the lining of the lymph node sinuses]

The lymphatic nodes of intact albino rats were investigated electron microscopically. It was shown that the lymphatic sinuses were restricted by a layer of flattened cells; the basal membrane was absent. Certain distinctions in the structure of the cell lining sinuses and the reticular cells comprising the reticular base of the lymphoid tissue of the lymphatic node were found. The structure of the "sinus network" strands is shown. The structure of the cells of the sinuses lining is shown to be identical to the structure of cells of the vascular endothelium. It suggests the endothelial nature of the lining of the lymphatic node sinuses.     

Association of mesenchyme with attenuated basement membranes during morphogenetic stages of newt limb regeneration

The interface between epithelium and mesenchyme may be involved in inductive interactions which occur during development. This interface within the growth bud, or blastema, of a regenerating limb has been examined to determine whether changes in basement-membrane structures are visible in regions of putative epithelial-mesenchymal inductive interaction. Regenerating forelimbs of adult newts were fixed by perfusion with osmotically balanced aldehydes. Late-bulb to early-digit stage regenerates were collected and processed either for light and transmission electron microscopy or for scanning electron microscopy. Light microscopy confirmed that regions characterized by increased numbers of subepithelial mesenchymal cells were covered by a diffusely stained basement membrane. Transmission electron microscopy of these regions revealed two structural components of the basement membrane. The thin basal lamina was continuous in all regions of all stages examined, but it was attenuated apically in areas of mesenchymal cell accumulation. The thicker underlying reticular lamina was markedly attenuated in these regions near the blastemal apex. Scanning electron microscopy of de-epithelialized blastemas revealed that, apically, the reticular lamina formed only a delicate lacelike network. On the base of the blastema, it formed a dense fibrillar meshwork which was further organized into a geometric pattern in the adjacent stump skin. Cumulatively, these observations suggest that physical contact between epithelial and mesenchymal cells is not essential at these stages, but that regions of putative epithelial-mesenchymal interaction are characterized by a distinctly diminished reticular lamina. Structural changes in basement-membrane components may be related to termination of local inductive events.     

Structure and function of the reticular cell in the planarian Dugesia dorotocephala

Structural and functional characteristics of the reticular cell in the planarian Dugesia dorotocephala were studied by light and electron microscopy. Since the reticular cells have numerous glycogen granules, lipid droplets and some lysosomes in their cytoplasm, they are easily distinguishable from other cell types. They migrate into the injured tissue, cover the injured mesenchyme, and also phagocytize debris of degenerating cells. The reticular cells also recognize foreign invaders such as bacteria. The larger aggregates of killed bacteria are encapsulated by reticular cells and eliminated into the intestine, whereas small aggregates are phagocytized by reticular cells. When cell wall extract of bacteria was inserted into the planarian body before insertion of killed bacteria, reticular cells were found to respond more quickly and vigorously to subsequent insertion of killed bacteria, indicating that the reticular cell has an immune response memory. When planarians were treated with 0.3 ppm cadmium sulfate and 0.01 ppm TPA, reticuloma tumors were induced in 76% of exposed planarians, indicating the similarity to blood cell diseases in mammals such as leukemia or lymphoma which are also induced by TPA. When these tumors were transplanted into normal hosts, the tumor cells were attacked by host reticular cells. These observations indicate that planarian reticular cells are primitive blood cells, playing important roles in nutrient transportation, homeostatic control of cells, and in defence and immune surveillance systems.     

The axolotl thymus: cell types of the microenvironment

Summary The three-dimensional structure of the axolotl (urodele amphibian) thymus was studied by combined scanning and transmission electron microscopy. The epithelial cell is the major component of the microenvironment forming the meshwork where thymocytes differentiate. Three different types of epithelial cells could be defined by their intracytoplasmic organelles and their localization in the subcapsular or deeper part of the organ. These epithelial cells participate in various types of lymphostromal interactions. Other stromal elements, such as interdigitating reticular cells, macrophages, eosinophil granulocytes and epithelial cysts were also defined. The absence of a true cortico-medullary differentiation in the axolotl thymus, the presence of different stromal elements and the physiological significance of these various microenvironments are discussed.     

Reticular meshwork of the spleen in rats studied by electron microscopy

The reticular meshwork of the rat spleen, which consists of both fibrous and cellular reticula, was investigated by transmission electron microscopy. The fibrous reticulum of the splenic pulp is composed of reticular fibers and basement membranes of the sinuses. These reticular fibers and basement membranes are continuous with each other. The reticular fibers are enfolded by reticular cells and are composed of two basic elements: 1) peripheral basal laminae of the reticular cells, and 2) central connective tissue spaces in which microfibrils, collagenous fibrils, elastic fibers, and unmyelinated adrenergic nerve fibers are present. The basement membranes of the sinuses are sandwiched between reticular cells and sinus endothelial cells and are composed of lamina-densalike material, microfibrils, collagenous fibrils, and elastic fibers. The presence of these connective tissue fibrous components indicates that there are connective tissue spaces in these basement membranes. The basement membrane is divided into three parts: the basal lamina of the reticular cell, the connective tissue space, and the basal lamina of the sinus endothelial cell. When the connective tissue space is very small or absent, the two basal laminae may fuse to form a single, thick basement membrane of the splenic sinus wall. The fibrous reticulum having these structures is responsible for support (collagenous fibrils) and rebounding (elastic fibers). The cells of the cellular reticulum--reticular cells and their cytoplasmic processes, which possess abundant contractile microfilaments, dense bodies, hemidesmosomes, basal laminae, and a well-developed, rough-surfaced endoplasmic reticulum, and Golgi complexes, which are characteristic of both fibroblasts and smooth muscle cells--are considered to be myofibroblasts. They may play roles in splenic contraction and in fibrogenesis of the fibrous reticulum. The contractile ability may be influenced by the unmyelinated adrenergic nerve fibers that pass through the reticular fibers. The three-dimensional reticular meshwork of the spleen consists of sustentacular fibrous reticulum and contractile myofibroblastic cellular reticulum. This meshwork not only supports the organ but also contributes to a contractile mechanism in circulation regulation, in collaboration with major contractile elements in the capsulo-trabecular system.     

The development of porcine zona pellucida using monoclonal antibodies: II. Electron microscopy

Following our previous study on the immunohistochemistry of porcine zonae pellucidae (ZP), we undertook the present study to localize the components of the ZP with immunoelectron microscopy, using three types of anti-porcine-ZP monoclonal antibodies (Mabs), named STA-1, STA-2, and STA-3. Some organelles of the oocyte were seen to react with STA-2 and STA-3 prior to ZP formation. As soon as a follicle began to mature, STA-2 and STA-3 reacted with the perinuclear space and the endoplasmic reticular membrane of the oocyte. The follicle first reacted with STA-1 at the secondary follicle stage. At this stage, the positive reaction involved the follicular cell layer as well as the oocyte and ZP. Positive reaction was scattered within and limited to the interfollicular cell space and was never found in the cytoplasm of follicular cells. At the antral follicle stage, the oocyte was surrounded by a thick, electron-dense ZP. A strong reaction was observed in the outer layer, but no significant reaction occurred in the inner layer. The convex and ragged outer margin of the ZP was characterized by the strongest reaction.     

Studies on the ultrastructure of the lymph nodes

Summary The reticular network of the lymph nodes has been studied with a variety of methods aimed to elucidate its chemical composition, submicroscopic structure and relationship with the cells.To study the chemical composition, the reticular network has first been isolated by means of sonic disruption and differential centrifugation, avoiding the use of enzymes or chemical extractions. The material prepared in this way has been studied with the polarized light and electron microscopes and has been subjected to X-ray diffraction and chemical analysis, including chromatographic separation of the collagenous proteins.The results of this work show that the reticular network of lymph nodes has a chemical composition similar to that of loose connective tissues: the fibrils are made of collagenous proteins and the concept of reticulin as a chemical or morphological entity does not appear to be justified.In addition the structure of the reticular network and its relationships with the different cells have been studied with the electron microscope on ultrathin sections of lymph nodes from dogs, cats, rabbits and rats: the reticular fibres appeared to be made of bundles of collagen fibrils with the typical banded structure; the fibres have close contacts with the reticular cells and also with the lymphoid elements of the nodes.In conclusion no elements exist that point towards some specific properties of the reticular connective tissue: the reticular network of the lymph nodes appears not to be basically different from the usual loose connective tissue from which it is distinct only because it harbours a considerable number of immunologically competent cells of the lymphoid series; functional interrelationships between the reticular and lymphoid cells in the nodes are, of course, not excluded.To Prof. F. E. Lehmann on his sixtieth birth day.Supported by a grant of C.N.R.     

Direct visualization of an IgM directed against a membrane antigen involved in both cell-matrix interactions and microfilament organization

Dental mesenchymal cells were cultured in the presence of a monoclonal antibody. MC16A16, consisting of IgM and directed against a hydrophobic 165 kDa protein. Since the epitope recognized by MC16A16 was found to be at the outer cell surface, a direct visualization of IgM antibodies was used to localize the 165 kDa antigen by transmission electron microscopy. The present results demonstrate that the 165 kDa antigen is a membrane protein.     

Ultrastructural and histochemical features of the thymus glands of the adult lungless salamander, Plethodon glutinosus (Caudata: Plethodontidae).

The thymus glands of adult slimy salamanders (Plethodon glutinosus) were examined by light and electron microscopy with the objective of describing the populations of epithelial cells believed to be secretory. The results of various histochemical procedures designed to demonstrate nucleic acids, proteins, lipids, and mucosubstances were evaluated by light microscopy. Each thymus is incompletely subdivided into a variable number of interconnected lobules by trabeculae extending inward from a thin capsule composed of connective tissues. The thymic parenchyma lacks distinct cortical and medullary regions, although developing lymphocytes and plasma cells tend to accumulate in larger numbers in the outermost portions of the glands. Basophils are found regularly in the capsule and trabeculae, but only very rarely within the thymic parenchyma. The epithelial cells of the thymus can be classified into five categories: epithelial reticular cells; three varieties of granulated cells (types I, II, and III), and myoid cells. Epithelial reticular cells form a three-dimensional network which extends throughout all portions of the thymus. Type I and type II granulated cells can be distinguished from one another by various morphological criteria at the ultrastructural level, but only small differences in the composition of their inclusions can be demonstrated histochemically. Both types of granules are composed principally of a proteinaceous material containing an abundance of primary amino and guanidyl groups. In addition, most type I inclusions possess a lipid component that cannot be demonstrated in type II granules. Type III granulated cells possess very small cytoplasmic inclusions resembling those of gastroenteric endocrine cells. Myoid cells contain concentrically arranged myofibrils composed of sarcomeres. In favorably oriented material, small cysts can be identified whose walls are composed of mixtures of type I cells, type II cells, and epithelial reticular cells. Groups of degenerating epithelial cells form lamellated structures corresponding to Hassall's (thymic) corpuscles.     

Zur positiven Reaktivität der Retikulumfasern des Lymphknotens bei der Perjodat-Leukofuchsinreaktion (PAS-Reaktion)

Zusammenfassung Die positive Perjodatreaktion der Retikulumfasern des Lymphknotens findet sich nur an der Grenzschicht der Retikulumfasern zu den Retikulumzellen. Dies entspricht weitgehend der Lokalisation der Argyrophilie. Diese perjodatpositive Oberschicht der Retikulumfasern geht an der Kapsel und an den Trabekeln direkt in die perjodatpositive Basalmembran des Sinusendothels über, so daß die oberflächliche Schicht der Retikulumfasern ebenso als Basalmembran gelten kann.

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Summary The periodic acid-Schiff reaction of the reticular fibers is examined in the cervical lymph node of a dog. The reticular fibers possess PAS reactivity especially at their surface layer bounding the reticular cells. This greatly resembles the site of their argyrophilic staining. The surface layer of the reticular fibers runs directly into the PAS positive basement membrane of the endothelial cells of lymphatic sinuses at the inner surface of capsule and trabeculae. Accordingly the PAS reactive layer of the reticular fibers can be regarded — as far as light microscopy is concerned — as a basement membrane.

     

Ontogeny of reticular cells in the ileal Peyer's patch of sheep and goats.

The ontogeny of reticular cells in the ileal Peyer's patch of sheep from 70 days gestational age was studied by light and electron microscopy and by enzyme histochemistry. Small to medium-sized lymphocytes were seen in the lamina propria at 97 days, when the stroma was essentially still mesenchymal. By 110 days, the stromal cells in the dome/follicle primordia had differentiated into reticular fibroblasts, whose processes and fibers were seen to surround groups of lymphocytes. With advancing age the number and size of primordia increased, and proliferation was obvious among the lymphocytes. Processes of reticular cells increased in number and penetrated between individual lymphocytes of the groups. Coarser desmosome-like contacts were seen between the reticular cells from 115 days onwards. A central light area in the follicle was apparent from 130 days onwards. The fine structure of the stromal cells in this light follicle center developed towards but never became similar to that of follicular dendritic cells in a typical germinal center. The fine interdigitating end branches of the stromal cells were less numerous, and the dense homogeneous material present in between the end branches was not observed in the ileal Peyer's patch follicle. Instead, small particles and vesicles were seen between the various cell types of the light center and were not restricted to the intercellular spaces between the stromal cells. In the dark peripheral zone of the follicle, the stromal cells retained more immature features. The follicle became bordered by a capsule at an early stage. This capsule was formed by multiple layers of flattened fibroblasts separated by small amounts of intercellular material only. The alkaline phosphatase, Mg(2+)-dependent adenosine triphosphatase and 5' nucleotidase reactivities of the follicular dendritic cells in the ileal Peyer's patch were similar to those of early prenatal primary follicles of sheep lymph nodes. This study indicates that the stromal cells of the ileal Peyer's patch are mesenchymal in nature and different from those of germinal centers and the epithelial stromal cells of bursa Fabricii of birds.     

Phagocytic response of planarian reticular cells to heat-killed bacteria

The defence mechanism of the planarian Dugesia dorotocephala (Woodworth) against invasion of foreign material was studied by inserting heat-killed bacteria into an incision and then examining the tissues around the incision by light and electron microscopy. The incision was made behind the right eye of each planarian, and a small aggregate of heat-killed Mycobacterium tuberculosis H37Ra was inserted into it using a fine needle. Samples of tissues were collected at 2, 4, 6, 8, 10, 12, 24, and 48 h after insertion of bacteria. As early as 10 h after insertion, bacteria were found in phagosomes of reticular cells, which are mesenchymal cells similar to fixed parenchymal cells and known previously to phagocytize degenerate tissues. By 12 h, aggregates of bacteria were found encapsulated in extensions of reticular cells, and by 24 h encapsulated bacteria were found between epithelial cells of the intestinal wall. These findings indicate that foreign material can be phagocytized or encapsulated by reticular cells and expelled into the intestinal cavity; it is thus conceivable that reticular cells could act as an immune surveillance system against foreign invaders.