Histology,histochemistry and ultrastructure of the nasopharyngeal tonsil of the buffalo (Bubalus bubalis)

The light microscopic appearance and ultrastructure of the nasopharyngeal tonsil (tonsilla pharyngea), collected from 12 adult buffaloes of local mixed breed, were explored for the distribution of different types of epithelia, lymphoid tissue and high endothelial venules. The tonsillar mucosa was lined by pseudostratified columnar ciliated epithelium having goblet cells. The respiratory epithelium associated with the underlying lymphoid tissue formed the lymphoepithelium. The epithelium was further modified into follicle‐associated epithelium (FAE) characterized by reduced epithelial height, presence of a few dome‐shaped cuboidal cells equivalent of the M‐cells and absence of goblet and ciliated cells. The lymphoid tissue was distributed in the form of isolated lymphoid cells, diffuse lymphoid tissue and lymphoid follicles, mainly distributed within the propria‐submucosa along with the sero‐mucous glandular tissue. The goblet cells of the respiratory epithelium and the acinar cells contained different mucopolysaccharides. Scanning electron microscopy of the surface mucosa demonstrated a dense mat of cilia, island‐like arrangement of microvillus cells, M‐cells and a few brush‐like cells. The transmission electron microscopy revealed the different cell organelles of the respiratory epithelium and the FAE. Lymphocyte migration via the high endothelial venules in the propria‐submucosa was also observed.     

Conjunctiva-associated lymphoid tissue (CALT) in normal and Bordetella avium-infected turkeys

Conjunctiva-associated lymphoid tissue (CALT) was characterized in normal and Bordetella avium-infected turkey poults during the first 5 weeks of life. At 1, 5, 12, 19, 25, and 33 days post-hatching (DPH), upper and lower eyelids were examined by gross, histologic, and electron microscopic techniques. CALT was confined to the proximal part of the lower eyelid near the conjunctival fornix; it appeared by 5 DPH as individual lymphoid nodules and as dense masses by 19 DPH. In the upper eyelid, CALT was present only as isolated nodules. Histologically, CALT was composed of dense lymphocyte infiltrates within subepithelial connective tissue, intraepithelial lymphocytes, and flattened lymphoid-associated epithelium that lacked goblet cells. Germinal centers were in CALT by 19 DPH. By scanning electron microscopy, epithelial cells over lymphoid areas were flat and had short, irregular microvilli; non-lymphoid areas were covered by cells with tall, regular microvilli. Transmission electron microscopy revealed that with increasing age of birds, the epithelium over conjunctival lymphoid infiltrates became progressively flattened and infiltrated by lymphocytes. Some blood vessels in CALT had high endothelial cells; lymphocytes were in the lumen and between or beneath endothelial cells. In B. avium-infected poults, CALT was increased, developed earlier, and contained more germinal centers than in normal poults. We conclude that CALT of turkeys closely resembles other mucosal lymphoid tissues and may serve as a site for local antigen uptake.(ABSTRACT TRUNCATED AT 250 WORDS)     

Electron microscopic studies on conjunctiva associated lymphoid tissue (CALT) in Angora goats

The purpose of this study was to investigate the electron microscopic structure of conjunctiva associated lymphoid tissue (CALT) and to determine the uptake of macromolecules from follicle associated epithelium (FAE) of Angora goats. The sample tissues taken from lower and upper eyelids of ten 5-6 month-old healthy Angora goats were used in this study. The conjunctiva associated lymphoid tissue of Angora goats was formed by solitary or aggregate lymphoid follicle and follicle associated epithelium which cover these follicle. FAE was formed by flattened epithelial cells, lymphocytes, monocytes and polymorph nuclear leukocytes but no goblet cells. Ferritin particles were seen on the apical surface, in the invaginations, vesicles, vacuoles of flattened epithelial cells. It was concluded that these epithelial cells were specialized to uptake macromolecules.     

Particle uptake by conjunctiva-associated lymphoid tissue (CALT) in turkeys

Uptake of tracer particles was assessed in lower eyelid conjunctiva-associated lymphoid tissue (CALT) of 3-week-old turkeys. Tracer particle suspensions, including carbon, iron oxide, or three sizes of latex beads (0.81 micron, 1.7 microns, and 2.9 microns), were placed into the experimentally sealed conjunctival space. After 5, 15, or 30 minutes, eyelids were removed and CALT was examined by light microscopy. Uptake was confirmed for all tracers and occurred within the lymphoepithelium of CALT. The uptake of latex beads was not as frequent as for carbon and iron. Tracers were increasingly evident in lymphoepithelium and subepithelial macrophage clusters as contact time increased. These findings provide additional evidence that CALT is capable of antigen uptake and may play a role in paraocular and upper respiratory immunity in turkeys.     

Morphologic characterization of conjunctiva-associated lymphoid tissue in chickens.

Conjunctiva-associated lymphoid tissue (CALT) in the eyelids of chickens was studied by gross, histologic, and electron microscopic techniques. Structural features were characterized at 1 day of age and at posthatching week (PHW) 1, 2, 3, 4, 6, 8, 12, and 16. Beginning at PHW 1, prominent lymphoid nodules containing a heterogenous population of lymphocytes, lymphoblasts, and macrophages were first observed within conjunctival folds and fissures of the lower eyelid. Nodules contained germinal centers by PHW 2 and plasma cells by PHW 4. The epithelium associated with these nodules was flat, had short, irregular microvilli, contained intraepithelial lymphocytes, and lacked goblet cells. High endothelial venules were located at the base of lymphoid nodules and contained lymphocytes within and below the cuboidal endothelium. In the upper eyelid, CALT was morphologically similar to lymphoid tissue in the lower eyelid, but nodules were smaller and more random, lacked association with epithelial folds and fissures, and were clustered around the opening of the nasolacrimal duct. By PHW 12, CALT was characterized by basal germinal centers outlined by collagenous stroma, suprafollicular plasma cells, columnar epithelium with goblet cells, and fewer intraepithelial lymphocytes. On the basis of these features, CALT in chickens has morphologic characteristics similar to other components of the mucosal immune system and, therefore, may have a role in mucosal immunity.     

Mucosa-associated lymphoid tissue lymphoma of the third eyelid conjunctiva in a dog

A 4-year-old, neutered female Cocker Spaniel was presented to the veterinary clinic for protrusion of the left third eyelid. When the third eyelids from both eyes were everted, lobulated masses were present on the bulbar surface. The left third eyelid had a larger protrusion. There was no apparent associated ocular or systemic involvement. The tumor of left third eyelid was removed and referred for histological examination. Histologically, there were proliferations of lymphoid follicles surrounded by lymphoid cells forming a marginal zone. Those lymphoid cells occasionally infiltrated into conjunctival epithelium. A few apoptotic bodies with karyopyknotic and karyorrhexic nuclei were observed in the germinal center of lymphoid follicles. Mitotic figures were rare. On immunohistochemistry, tumor cells expressed CD79a but not CD3. A diagnosis of extranodal marginal zone B-cell lymphoma of mucosa-associated lymphoid tissue (MALT) of the third eyelid was established based on the histological and immunophenotypical features. At the 1-year follow-up, there was no evidence of recurrence of the mass at the area of excision of the left third eyelid and the remaining tumor of the right third eyelid was still a similar size. The dog still showed no significant findings, except those of the tumor, and no evidence of systemic involvement. To the authors' knowledge, this is the first reported case of MALT lymphoma of the third eyelid in a dog.     

Quantification of particle uptake by conjunctiva-associated lymphoid tissue (CALT) in chickens

Tracer particle uptake by conjunctiva-associated lymphoid tissue (CALT) was quantified in the lower eyelids of 1-, 3-, 5-, and 7-week-old broiler chickens. CALT was measured histologically by computerized image analysis in all birds and in additional 1-day-old and 16-week-old chickens not subjected to uptake assessment. Suspensions of carbon or iron oxide were placed in contact with CALT for 5, 15, or 30 minutes (contact time). After eyelid removal, tracer uptake was scored by light microscopy, CALT was measured, and a mathematically derived uptake index was evaluated statistically. At each age examined, computer-generated measurements showed a significant increase in the proportion of CALT lymphoepithelium within proximal eyelids. Within the conjunctival sites evaluated, tracer uptake was significantly greater in lymphoepithelium than in non-lymphoepithelium at all ages and at all contact times. Uptake increased significantly between 3 and 5 weeks of age and between 5 and 15 minutes of tracer contact. Based on these uptake data for CALT in chickens, a minimal age-specific maturity is suggested that may influence function in mucosal immunity.     

Incidence and significance of isolation of Mycoplasma felis from conjunctival swabs of cats

Conjunctival swabs taken from 40 cats with conjunctivitis and 65 cats without conjunctivitis were examined for the presence of Mycoplasma felis. The incidence of M. felis in cats with conjunctivitis was 25%. M. felis was not isolated from clinically normal cats. Inoculation of two isolates into the conjunctival sacs of healthy cats induced conjunctival hyperemia, starting 2-3 days after inoculation and disappearing without treatment within 7 days. It is concluded that M. felis plays a role in feline conjunctivitis.     

Histology, immunohistochemistry and ultrastructure of the equine tubal tonsil

The tubal tonsil of the horse surrounds the pharyngeal opening of the eustachian tube and is lined by pseudostratified columnar ciliated epithelium interspersed with areas of follicle-associated epithelium (FAE) heavily infiltrated by lymphocytes but devoid of goblet and ciliated cells. Scanning and transmission electron microscopy revealed microvillous cells and cells with features characteristic of M cells such as reduced microvilli or depressed bare surface, more numerous mitochondria, small vesicles and lysosomes, as well as vimentin filaments and epitopes specific for GS 1-B4 as previously seen in the nasopharyngeal tonsil. M cells were also identified in areas of respiratory epithelium not associated with lymphoid follicles and appeared to be the nasal mucosal counterparts of recently described intestinal villous M cells in the mouse. The underlying lymphoid tissue of the FAE was generally organized as solitary lymphoid follicles without germinal centres in contrast to the diffuse and large amount of organized lymphoid follicles with germinal centres that characterize the nasopharyngeal tonsil. CD8+ T and B-lymphocytes were much fewer than in the nasopharyngeal tonsil. High endothelial venules were mainly oriented towards the parafollicular area and contained much fewer endothelial pores and vesiculo-vacuolar organelles. Finally, scattered small clusters of mucus acini and striated muscles were other features that differentiated the tubal and nasopharyngeal tonsils.     

Gut-associated lymphoid tissue in the large intestine of calves. II. Electron microscopy

Scanning electron microscopy of lymphoid tissue in the large intestine of three germfree calves (age 3, 6, and 7 days) revealed two different units: propria nodules and lymphoglandular complexes (LGC). Propria nodules had lymphoid tissue predominantly in lamina propria and were covered by distinct follicle-associated epithelium which lacked goblet cells; nodules were surrounded by wide crypts, which were also lined by follicle-associated epithelium towards the luminal side. Lymphoglandular complexes had lymphoid follicles in the tunica submucosa; epithelial diverticulae extended through the muscularis mucosae branching into the lymphoid nodule. In centers of lymphoglandular complexes, protrusions of lymphoid tissue were covered with distinct follicle-associated epithelium. By transmission electron microscopy cells compatible with M cells in the small intestine of calves and cells with characteristics of both enteroabsorptive and M cells were found. Follicle-associated epithelium of propria nodules and lymphoglandular complexes differed only in the relative frequency of cell types.     

The histological characteristics of the aggregated lymphoid nodules area in abomasum of Bactrian camels (Camelus bactrianus) of different ages

The aggregated lymphoid nodules area (ALNA) in abomasum of Bactrian camels is a special immune structure discovered only in Bactrian camels in recent years (2003). The anatomy research found that there was a close relationship between degree of development, anatomical characteristics and age. To further establish the relationship between histological characteristics of this special structure and animal age, 24 Alashan Bactrian camels of the following four age groups were studied: young (1-2 years), pubertal (3-5 years), middle-aged (6-16 years) and old (17-20 years). Mucosal-associated lymphoid tissue (MALT) of ALNA in abomasum was particularly observed and analyzed by histology, histochemistry and statistical methods. The results showed that the average number of lymphoid nodules in reticular mucosal folds region of ALNA in abomasum from young group to old group was in order of 26.8, 32.7, 17.6 and 7.8, and in longitudinal mucosal folds region was 20.1, 26.0, 10.3 and 5.1. The number of lymphoid nodules in the four experimental groups first increased and then decreased with increasing age (P<0.01). In young and pubertal camels lymphoid nodules were distributed evenly on both sides of the axis of mucosal folds and mostly displayed round, oval or wedge shape. The number of lymphoid nodules, follicle-associated epithelium (FAE), reticular fibers and plasmocytes in mucosal folds gradually increased from 1 to 2 years and peaked at puberty. There were up to 37 visible lymphoid nodules in a mucosal fold. However, ALNA of middle-aged and old camels gradually degenerated as aging. Lymphoid nodules were unevenly distributed on both sides of the axis of mucosal folds, which mostly displayed oval or irregular shape. Lymphoid tissue in old camels mostly existed as diffuse form. Although germinal centers of the lymphoid nodules were still obvious, the number of reticular fiber and plasmocyte and lymphoid nodules gradually decreased. The results indicated that in accord with the anatomical results, there was a close relationship between histology characteristics of lymphoid tissue of ALNA in abomasum and animal age. In summary, the lymphoid tissue of ALNA in abomasums gradually increased from young to pubertal groups with increasing age, peaked in 3-5 year-old camels, and subsequently declined with age and when 17-20 years old this immunity structure had severely atrophied.     

Local immunity of the respiratory mucosal system in chickens and turkeys

This review article presents fundamental mechanisms of the local mucosal immunity in selected regions of the respiratory tract in healthy birds and in some pathological conditions. The respiratory system, whose mucosa come into direct contact with microorganisms contaminating inhaled air, has some associated structures, such as Harderian gland (HG), conjunctive-associated lymphoid tissue (CALT) and paranasal glands (PG), whose participation in local mechanisms of the mucosal immunity has been corroborated by numerous scientific studies. The nasal mucosa, with structured clusters of lymphoid tissue (NALT - nasal-associated lymphoid tissue) is the first to come into contact with microorganisms which contaminate inhaled air. Lymphoid nodules, made up of B cells with frequently developed germinal centres (GC), surrounded by a coat of CD4+ cells, are the major NALT structures in chickens, whereas CD8+ cells are situated in the epithelium and in the lamina propria of the nasal cavity mucosa. Studies into respiratory system infections (e.g. Mycoplasma gallisepticum) have shown the reactivity of the tracheal mucosa to infection, despite a lack of essential lymphoid tissue. Bronchus-associated lymphoid tissue (BALT) takes part in bronchial immune processes and its structure, topography and ability to perform defensive function in birds is largely age-dependent. Mature BALT is covered by a delicate layer of epithelial cells, called follicle-associated epithelium (FAE). Germinal centres (GC), surrounded by CD4+ cells are developed in most mature BALT nodules, while CD8+ lymphocytes are dispersed among lymphoid nodules and in the epithelium, and they are rarely present in GC. Macrophages make up the first line of defence mechanisms through which the host rapidly responds to microorganisms and their products in the respiratory mucosal system. Another very important element are polymorphonuclear cells, with heterophils being the most important of them. Phagocytic cells obtained from lung lavages in birds are referred to as FARM (free avian respiratory macrophages). Their number in chickens and turkeys is estimated to be 20 times lower than that in mice and rats, which indicates a deficit in the first-line of defence in the birds' respiratory system. There are numerous B cells and antibody secreting cells (ASC) present throughout the respiratory system in birds. Their role comes down to perform antigen-specific protection by producing antibodies (IgM, IgY or IgA class) as a result of contact with pathogenic factors.     

Characterization of the diffuse mucosal associated lymphoid tissue of feline small intestine

Characterization of the feline intestinal mucosal associated lymphoid tissue (MALT) will facilitate investigation of intestinal disease in the cat and promote the cat as an animal model for a range of human diseases which involve the intestinal lymphoid tissue. This includes inflammatory bowel disease, viral and non-viral associated intestinal lymphomas and immunodeficiency associated syndromes. Morphologic and phenotypic characterization of the normal small intestinal diffuse MALT in 22 SPF cats was performed using flow cytometry and cytology on isolated intestinal leukocytes from the intra-epithelial and lamina proprial compartments, as well as immunohistology on tissues from the feline duodenum, jejunum and ileum. The intra-epithelial compartment (IEC) was dominated by lymphocytes (>85%) which frequently contained intracytoplasmic granules. The most striking findings in the IEC were the elevated percentages of CD8 alpha+ lymphocytes (40%), presumed to express CD8 alpha alpha chains, and CD4-/CD8- (double negative) lymphocytes (44%), and the consistent presence of a minor subpopulation of CD3+/CD11d+ IELs (6%). Small percentages of CD4+ lymphocytes (10%) were observed such that the IEL CD4:CD8 ratio (0.25) was low. The LPC also contained a majority of T cells and few plasma cells. However, this compartment had reduced percentages of CD8 alpha+ lymphocytes (28%) and increased percentages of CD4+ lymphocytes (27%) relative to the IEC. However, the LPL CD4:CD8 ratio (1.0) remained low compared with the ratio in peripheral blood. In feline MALT, MHC class II expression was lower than in other peripheral lymphoid compartments. The results of this study provide important reference values for future investigations involving feline intestinal lymphocytes and demonstrates that the leukocyte distribution and phenotypic characteristics of the feline diffuse MALT appear largely similar to the murine, rat and human counterparts.     

Lymphoid nodules in skin biopsies from dogs, cats, and horses with nonneoplastic dermatoses

In a retrospective histopathologic study of nonneoplastic dermatoses, lymphoid nodules were found in 0.3% of 3,408 canine, 5.1% of 469 feline, and 4.5% of 325 equine skin biopsies. In all 3 species, the majority of cases wherein lymphoid nodules were found were diseases of presumed immune-mediated nature. In cats and horses, the majority of cases were also diseases characterized by tissue eosinophilia.     

Flow cytometric detection of alpha-1-acid glycoprotein on feline circulating leucocytes

To assess whether alpha‐1‐acid glycoprotein (AGP) can be detected on the membrane of feline circulating leucocytes. Design The presence of AGP on circulating leucocytes was investigated in both clinically healthy cats and cats with different diseases. A group of feline coronavirus (FCoV)‐positive cats, comprising cats with feline infectious peritonitis (FIP) and cats not affected by FIP but seropositive for FCoV, were included in this study because the serum concentration of AGP increases during FCoV infection. Procedure Flow cytometry (using an anti‐feline AGP antibody), serum protein electrophoresis, routine haematology and measurement of the serum AGP concentration were performed using blood samples from 32 healthy cats (19 FCoV‐seropositive), 13 cats with FIP and 12 with other diseases (6 FCoV‐seropositive). The proportion of cats with AGP‐positive leucocytes in the different groups (e.g. controls vs sick; FIP vs other diseases, etc.) or in cats with different intensities of inflammatory response was compared using a Chi‐square test. Results AGP‐positive leucocytes were found in 23% of cats. Compared with controls, the proportion of patients with positive granulocytes and monocytes was higher among sick cats (especially cats with diseases other than FIP) and cats with high serum AGP concentration, but not in cats with leucocytosis or that were FCoV‐seropositive. Conclusion AGP‐positive leucocytes can be found in feline blood, especially during inflammation. Conversely, no association between AGP‐positive leucocytes and FIP was found. Further studies are needed to elucidate the mechanism responsible for this finding and its diagnostic role in cats with inflammation.     

切除结膜结合淋巴组织鸡对新城疫疫苗点眼免疫反应 Ⅱ.组织学与免疫组织化学研究

60只 1日龄健康雏鸡随机分为 A、B、C三组。A组于 1日龄手术切除下眼睑结膜。A、B两组于 12日龄时结扎鼻泪管 ,用新城疫克隆 30苗点眼。C组为对照组。 2 3日龄分别采取其 CAL T和哈德氏腺 ,比较观察其组织细胞。结果 ,1日龄手术切除下眼睑结膜后造成鸡 CAL T缺失 ,点眼免疫后 ,哈氏腺的免疫细胞也较正常免疫鸡少 ,而正常接种 ND克隆 30 (B组 )后 ,CAL T发生早 ,生长快 ,淋巴细胞数量多 ,故本实验证实 CAL T对哈氏腺免疫细胞的数量与成分有一定的调控作用。 CAL T缺失鸡是研究黏膜免疫系统比较理想的模型     

Distinctive peripheral lymph node hyperplasia of young cats

Peripheral lymph node enlargement was found in 14 of a series of 132 feline lymph node biopsy specimens. Six of nine cats tested had antibodies for feline leukemia virus (FeLV). Half of the cats were clinically normal while the remainder had fever, lethargy, anorexia, and hepatosplenomegaly. There was severe distortion of lymph nodal architecture with variable loss of discernible follicles and sinuses. Histiocytes, lymphocytes, immunoblasts, and plasma cells were present in expanded paracortical regions which encroached on, and occasionally effaced, lymphoid follicles. Postcapillary venules were numerous and prominent throughout the paracortex. The lymphadenopathy was most commonly transient (86% of cases) with subsequent development of lymphoma in one cat. Lymph nodes from seven kittens with experimental FeLV infection were compared with spontaneously enlarged lymph nodes; four of seven had B and T lymphocyte hyperplasia with normal nodal architecture. Three had partial loss of nodal architecture as a result of expanded paracortical regions populated largely by histiocytes and lymphocytes. Proliferation of postcapillary venules was not prominent in nodes from FeLV-infected cats. The cause of spontaneous lymph node hyperplasia of young cats was not determined. However, the similarity of lesions to those of kittens with experimental FeLV infection and the association with FeLV by serologic tests in six of nine cats suggest that this retrovirus may be involved in the pathogenesis of the lesion.     

Evaluation of different sampling methods and results of real-time PCR for detection of feline herpes virus-1, Chlamydophila felis and Mycoplasma felis in cats

Objective To investigate how different sampling techniques affect detection of DNA from feline herpes virus Type 1 (FHV-1), Chlamydophila felis and Mycoplasma felis and to study the correlation between positive test results and clinical signs in cats. Animals Fifty-one cats; 24 with ocular signs and 27 healthy control cats. Procedures Samples were collected from all cats using cotton swabs, conjunctival and corneal biopsies, and corneal scrapings. Samples were analyzed for presence of FHV-1, C. felis, M. felis, and feline DNA, defined by 28S rDNA, by using real-time PCR. Results In affected cats, FHV-1 was detected in only one cat; C. felis and M. felis were not detected in any affected cats. None of the three organisms was detected in any control cats. Feline DNA was demonstrated in all conjunctival samples, in 82% of corneal swabs, 92% of corneal scrapings, and 100% of keratectomy samples. Conclusions Because of the generally low detection rate for FHV-1, C. felis, and M. felis DNA in this study, differences regarding sampling technique could not be determined and correlation between positive test results and degree of clinical signs could not be made. Detection of feline DNA in most samples irrespective of sampling technique, suggests a low prevalence of FHV-1, C. felis and M. felis in this population of cats.     

切除结膜结合淋巴组织鸡对新城疫疫苗的免疫反应Ⅰ.局部与循环抗体检测

实验将 60只 1日龄健康雏鸡随机分为A、B、C 3组。A组为CALT缺失组 ,B组为免疫组 ,C组为非免疫对照组。A组于 1日龄手术切除下眼睑结膜 ,A、B两组于 1 2日龄结扎鼻泪管后进行点眼免疫接种新城疫克隆 30疫苗。2 3日龄时 ,收集A、B、C各实验组鸡的血清和泪液 ,做红细胞凝集抑制试验 (HI试验 )、酶联免疫吸附试验(ELISA)。比较观察各实验组雏鸡血清和泪液抗体HI效价与IgA、IgG、IgM含量的变化特点。结果 ,1日龄手术切除下眼睑结膜后造成鸡CALT缺失 ,CALT缺失鸡点眼免疫后 ,泪液抗体HI效价与IgA、IgG、IgM含量比免疫组雏鸡低。但对雏鸡的抗体生成的规律并不产影响。故实验证实了CALT对眼区局部抗体的产生及其种类有重要的调控作用 ,协同哈氏腺完成局部免疫的功能     

The Epithelium Covering Peyer’s Patches in Young Milk-Fed Calves: An Ultrastructural and Enzyme Histochemical Investigation

Between the ordinary villi over Peyer’s patches there are small domes or “pseudovilli” caused by bulges in the lymphoid tissue. These “pseudovilli” were studied in 5 healthy milk-fed, about 3-week-old, pre-ruminant calves. Scanning electron microscopy revealed that the “pseudovilli” were covered by a specialized follicle associated epithelium (FAE). The FAE had poorly developed microvilli and often extensive folding of the cell surface close to the cell borders. By transmission electron microscopy the tips of the marginal folds of the FAE seemed to fuse, probably in the process of enfolding bulk material from the lumen. The FAE apical cytoplasm contained numerous thick-walled and bristle-coated invaginations, tubules and vesicles indicative of micropinocytosis. Multivesicular bodies and large vacuoles were frequent. Indications of extracellular unloading of residual bodies were found. Intraepithelial lymphocytes tended to group together, and some were rich in rough endoplasmic reticulum. Enzyme histochemistry showed weak reactions of adenosine triphosphate splitting enzyme and aminopeptidase in the FAE luminal cell border. Cytoplasmic acid phosrphatase showed a marked basal-apical decrease along the “pseudovillus” probably caused by the onset of endocytosis. The results of this study appear compatible with the concept that the FAE takes up macromolecules from the lumen by pinocytosis and sensitizes lymphocytes.