Growth and differentiation of fetal rat small intestinal epithelium in tissue culture. Relationship to fetal age

Development and characterization of 2-dimensional culture for buffalo intestinal cells

Small intestinal epithelial cells (IEC) play a major role in the absorption of nutrients and toxins. Due to the similarity of genome-wide single copy protein orthologues between cattle and human, establishment of ruminant's primary small IEC culture could be a valuable tool for toxicity studies. Therefore, the current study focused on the development and characterization of buffalo IEC culture, as cattle slaughter is banned in India. The buffalo jejunum fragments were washed consecutively several times in saline, warm phosphate buffered saline (PBS), PBS with 5 mM dithiothreitol, digesting solution and 2% sorbitol in PBS. The cells were cultured on 17 µg/cm² collagen coated plates and transwell plates with serum (2% Fetal bovine serum (FBS) and 10% FBS) and serum-free culture conditions. The cells were differentiated into typical epithelial cobblestone morphology from day 5 onwards in 50% successful cultures. The cultured IEC were characterized by gene expression of epithelial cell markers, cytokeratin and vimentin, and enterocyte markers like villin, zonula occluden (ZO1), fatty acid binding protein 2 (FABP2) and small intestinal peptidase (IP). Based on the morphology and gene expression profile, 10% FBS has been recommended for culturing primary buffalo IEC on collagen coated plates for 10 days. However, 50% of the successful cultures could not show epithelial phenotype on 10% FBS culture conditions even on collagen coated plates. Interestingly, undifferentiated IEC showed an increasing expression of FABP2, IP and ZO1 transcripts compared to differentiated intestinal cells with 10% FBS on collagen plates. Therefore, future studies are needed to understand the role of FABP2, IP and ZO1 in differentiation of buffalo IEC.

Establishment of fetal bovine intestinal epithelial cell cultures susceptible to bovine rotavirus infection

Mucosal epithelial cells are infected by a wide variety of pathogens and determining their response to infection is critical for understanding disease pathogenesis. A protocol was developed for culturing primary epithelial cells from fetal bovine intestine and the cultured cells were evaluated for susceptibility to an enteric viral infection. Immunohistochemical staining for cytokeratin confirmed that 60–75% of cultured cells were epithelial cells. Furthermore, following infection with bovine rotavirus (BRV) over 80% of cells in the ileal and jejunal cultures contained viral protein at 16 h post-infection. The intestinal epithelial cell cultures also contained fibroblasts so a jejunal fibroblast culture was established and infected with BRV. Viral protein was detected in jejunal fibroblasts but viral-induced cytopathology was delayed in fibroblast cultures when compared to epithelial cell cultures. This study describes an effective protocol for culturing bovine epithelial cells from fetal intestine and confirmed that the epithelial cells were susceptible to BRV infection. Ileal and jejunal cultures displayed limited growth following continuous passage but early passage epithelial cells provide competent target cells for studying host cell responses to an enteric viral pathogen.

Intestinal epithelial stem cells and progenitors

The adult intestinal epithelium contains a relatively simple, highly organized, and readily accessible stem cell system. Excellent methods exist for the isolation of intestinal epithelium from adults, and as a result collecting large quantities of intestinal stem and progenitor cells for study or culture and subsequent clinical applications should be routine. It is not, however, for two reasons: (1) adult intestinal epithelial cells rapidly initiate apoptosis on detachment from the basement membrane, and (2) in vitro conditions necessary for survival, proliferation, and differentiation are poorly understood. Thus to date the study of intestinal stem and progenitor cells has been largely dependent on in vivo approaches. We discuss existing in vivo assays for stem and progenitor cell behavior as well as current methods for isolating and culturing the intestinal epithelium.

Immunochemical, biomolecular and biochemical characterization of bovine epithelial intestinal primocultures

BACKGROUND

Cultures of enterocytes and colonocytes represent valuable tools to study growth and differentiation of epithelial cells. In vitro models may be used to evaluate passage or toxicity of drugs, interactions of enteropathogenes bacteria strains with intestinal epithelium and other physiologic or pathologic phenomenon involving the digestive tract.

RESULTS

Cultures of bovine colonocytes and jejunocytes were obtained from organoid-enriched preparations, using a combination of enzymatic and mechanical disruption of the intestine epithelium, followed by an isopicnic centrifugation discarding most single cells. Confluent cell monolayers arising from plated organoids exhibited epithelium typical features, such as the pavement-like structure, the presence of apical microvilli and tight junctions. Accordingly, cells expressed several markers of enterocyte brush border (i.e. maltase, alkaline phosphatase and fatty acid binding protein) as well as an epithelial cytoskeleton component (cytokeratin 18). However, enterocyte primocultures were also positive for the vimentin immunostaining (mesenchyme marker). Vimentin expression studies showed that this gene is constitutively expressed in bovine enterocytes. Comparison of the vimentin expression profile with the pattern of brush border enzymes activities, suggested that the decrease of cell differentiation level observed during the enterocyte isolation procedure and early passages of the primoculture could result from a post-transcriptional de-repression of vimentin synthesis. The low differentiation level of bovine enterocytes in vitro could partly be counteracted adding butyrate (1-2 mM) or using a glucose-deprived culture medium.

CONCLUSION

The present study describes several complementary approaches to characterize bovine primary cultures of intestinal cells. Cultured cells kept their morphologic and functional characteristics during several generations.

Progress on isolation and short-term ex-vivo culture of highly purified non-apoptotic human intestinal epithelial cells (IEC)

Intestinal epithelial cells (IEC) form the largest surface of the human body and are of pivotal importance to digest and absorb nutrients. Furthermore these cells play a critical role shielding the organism against microorganisms and toxins present in the intestinal lumen. It is therefore not surprising that a large group of researchers take great interest in the study of these cells. However, to date it is a challenge to purify viable primary human intestinal epithelial cells and it has been even more fastidious to maintain IEC in culture ex-vivo as IEC undergo apoptosis within hours due to loss of cell anchorage ('anoikis') following the isolation process. Over recent years the authors aimed to continuously improve the isolation technique for primary IEC, allowing a simple, effective and rapid isolation of highly purified non-apoptotic human IEC. In this study the newly improved method is presented and applied to establish ex-vivo cultures of highly purified, fully viable primary IEC displaying important functional properties, making these cells amenable for ex-vivo research on primary human intestinal epithelial cells.

Mammalian intestinal epithelial cells in primary culture: a mini-review

Epithelial cells lining the digestive tract represent a highly organized system built up by multipotent stem cells. A process of asymmetric mitosis produces a population of proliferative cells that are rapidly renewed and migrate along the crypt-villus axis, differentiating into functional mature cells before dying and exfoliating into the intestinal lumen. Isolated crypts or epithelial cells retaining high viability can be prepared within a few h after tissue sampling. After cells are cultured in serum-free media, short-term studies (16-48 h) can be conducted for endocrinology, energy metabolism, or programmed cell death. However, long-term primary culture of intestinal cells (up to 10 d) is still difficult despite progress in isolation methodologies and manipulation of the cell microenvironment. The main problem in developing primary culture is the lack of structural markers specific to the stem cell compartment. The design of a microscopic multidimensional analytic system to record the expression profiles of biomarkers all along the living intestinal crypt should improve basic knowledge of the survival and growth of adult crypt stem cells, and the selection of totipotent embryonic stem cells capable of differentiating into intestinal tissues should facilitate studies of the genomic basis of endodermal tissue differentiation.

Mammalian intestinal epithelial cells in primary culture: a mini-review

Epithelial cells lining the digestive tract represent a highly organized system built up by multipotent stem cells. A process of asymmetric mitosis produces a population of proliferative cells that are rapidly renewed and migrate along the crypt-villus axis, differentiating into functional mature cells before dying and exfoliating into the intestinal lumen. Isolated crypts or epithelial cells retaining high viability can be prepared within a few h after tissue sampling. After cells are cultured in serum-free media, short-term studies (16-48 h) can be conducted for endocrinology, energy metabolism, or programmed cell death. However, long-term primary culture of intestinal cells (up to 10 d) is still difficult despite progress in isolation methodologies and manipulation of the cell microenvironment. The main problem in developing primary culture is the lack of structural markers specific to the stem cell compartment. The design of a microscopic multidimensional analytic system to record the expression profiles of biomarkers all along the living intestinal crypt should improve basic knowledge of the survival and growth of adult crypt stem cells, and the selection of totipotent embryonic stem cells capable of differentiating into intestinal tissues should facilitate studies of the genomic basis of endodermal tissue differentiation.

Primary cell cultures of bovine colon epithelium: isolation and cell culture of colonocytes

Epithelial cells from bovine colon were isolated by mechanical preparation combined with an enzymatic digestion from colon specimens derived from freshly slaughtered animals. After digestion with collagenase I, the isolated tissue was centrifuged on a 2% D-sorbitol gradient to separate epithelial crypts which were seeded in collagen I-coated culture flasks. By using colon crypts and omitting the seeding of single cells a contamination by fibroblasts was prevented. The cells proliferated under the chosen culture conditions and formed monolayer cultures which were maintained for several weeks, including subcultivation steps. A population doubling time of about 21 hr was estimated in the log phase of the corresponding growth curve. During the culture period the cells were characterized morphologically and enzymatically. By using antibodies against cytokeratine 7 and 13 the isolated cells were identified as cells of epithelial origin. Antibodies against vimentin served as negative control. Morphological features such as microvilli, desmosomes and tight junctions, which demonstrated the ability of the cultured cells to restore an epithelial like monolayer, were shown by ultrastructural investigations. The preservation of the secretory function of the cultured cells was demonstrated by mucine cytochemistry with alcian blue staining. A stable expression of enzyme activities over a period of 6 days in culture occurred for gamma-glutamyltranspeptidase, acid phosphatase and NADH-dehydrogenase activity under the chosen culture conditions. Activity of alkaline phosphatase decreased to about 50% of basal value after 6 days in culture. Preliminary estimations of the metabolic competence of these cells revealed cytochrome P450 1A1-associated EROD activity in freshly isolated cells which was stable over 5 days in cultured cells. Then activity decreased completely. This culture system with primary epithelial cells from the colon will be used further as a model for the colon epithelium in toxicological studies in vitro.

Isolating and maintaining highly polarized primary epithelial cells from normal human duodenum for growth as spheroid-like vesicles

A method is described for the three-dimensional (3-D) in vitro culture of nontransformed gastrointestinal epithelial cells from the human duodenal mucosa. Biopsies obtained from human duodenum were finely minced. The tissue fragments were suspended in culture medium supplemented with 5% fetal calf serum and the appropriate antibiotics. The suspended mucosal fragments generated spheroid-like multicellular vesicles consisting of highly prismatic absorptive and goblet cells retaining most of the histological features of the tissue in vivo. We performed immunocytochemical studies to determine the origin of the vesicles using monoclonal antibodies against EP4. The histochemistry of the vesicles showed alkaline phosphatase activity. Ultrastructural studies revealed that these cells exhibit characteristics of normal duodenal cells in vivo: apical microvilli, glycocalyx, tight junctions and desmosomes, lateral membrane interdigitations, mucous droplets, and a well-developed Golgi apparatus. An overgrowth of the vesicles by fibroblasts was not seen during cultivation. In contrast with the two-dimensional cell cultures grown on artificial supports, the vesicle cells show organization similar to that of natural epithelia. The polarization and cytoarchitecture of normal gastrointestinal epithelial cells cultured as 3-D vesicles are comparable to those known for the native tissue. This study was undertaken to provide a morphological baseline for subsequent infection experiments.

Transimmortalized mouse intestinal cells (m-ICc12) that maintain a crypt phenotype

This study describes the properties of a clone of immortalized cells (m-ICc12 cells) derived from the bases of small intestinal villi from 20-day-old fetuses of L-type pyruvate kinase (L-PK)/ TAg1 transgenic mice. The mice harbor the simian virus 40 large T antigen under the control of the 5' regulatory sequence from the L-PK gene. m-ICc12 cells expressed nuclear large T antigen, had a prolonged life span, and were nontumorigenic when injected into nude mice. They formed confluent monolayers of cuboid cells separated by tight junctions, developed dense, short apical microvilli, and formed domes. They also possessed cytokeratins, villin, aminopeptidase N, dipeptidyl-peptidase IV, and glucoamylase and retained crypt cell features, including intracellular sucrase isomaltase and alpha-L-fucose glycoconjugates accumulation and expression of the polymeric immunoglobulin receptor and the cystic fibrosis transmembrane conductance regulator gene. Thus the m-ICc12 cell line obtained by targeted oncogenesis in transgenic mice maintained in culture several important properties and differentiated functions of intestinal crypt cells.

Conditionally immortalized intestinal epithelial cells: novel approach for study of differentiated enterocytes

Clonal cell lines have been established from primary fetal rat intestinal epithelial cells by stable transfection with plasmids containing either the simian virus 40 (SV40) large T-antigen gene under the control of the heavy metal inducible metallothionein promoter (pMTWt) or the thermolabile SV40 T-antigen gene under the control of the SV40 early promoter (pZipSVtsa58). pMTWt-transfected cells produced sufficient T-antigen to allow them to proliferate both when the metallothionein promoter was induced and uninduced. No differences were observed in the pattern of intestinal epithelial markers expressed when the cells were cultured in the presence or absence of inducing agent (zinc). In contrast, fetal rat intestinal epithelial cells transfected with pZipSVtsa58 were immortalized conditionally; cells proliferated at 32 degrees C but ceased to proliferate between 48 and 72 h of culture at 39 degrees C. Four of these cell lines were characterized in detail; they showed microvilli and tight junctions as well as dome formation and expressed functional and biochemical markers of intestinal epithelial cells, including keratins 8, 19, and 21, aminopeptidase N, and dipeptidyl peptidase IV. One cell line, 2/4/A1, expressed in addition a low level of lactase and sucrase-isomaltase. The amount and/or activity of some of these markers changed during the switch from the proliferative to the nonproliferative state (switch from culture at 32 to 39 degrees C), resulting in a more differentiated phenotype and mimicking similar changes taking place during intestinal epithelial cell differentiation in vivo.

Transplantation of fetal mouse colon under the kidney capsule of an adult mouse: a model for the study of colonic development

Fourteen-day fetal mouse colon was transplanted under the kidney capsule of an adult mouse to determine whether this system could be used as a model of embryonic colonic development. The 14-day fetal colon was transplanted and left for a period of 7 days. Comparisons of the normal one day postnatal colon and the transplanted colon were made morphologically and morphometrically. It was found that the transplanted fetal colon resembled its postnatal counterpart with respect to morphology; the cell types seen in the transplanted colon were similar to those observed in the in situ colon of the same age. However, morphometric analysis showed that the transplanted colon was significantly smaller than its postnatal counterpart, suggesting that conditions in the host were not optimal to support the full growth of the colon. In spite of this, it appears that the fetal colon can differentiate normally under the kidney capsule and this model can be used to study both epithelial-mesenchymal interactions and the role of hormones in fetal colonic development.

Factors affecting in vitro growth of harvested enterocytes

Selective enterocyte transplantation may be an alternative to whole organ transplantation for increasing absorptive capacity. Our aim was to determine the effect of initial cell number and viability, proportion of intact crypts, and basement membrane components (BMC) on the in vitro growth of rabbit enterocytes. Enterocytes were harvested using warm trypsinization from ileal segments in 40 rabbits. Initial cell viability was 92 +/- 4% (mean +/- SD), cell yield was 7.7 +/- 3.6 x 10(6) cells/cm, and there were 0.51 +/- 0.33 crypts/100 cells. Initial cell viability correlated with cell yield (r = -0.508, p < 0.001) and % crypts (r = 0.313, p < 0.05). Cell yield also correlated with % crypts (r = -0.645, p < 0.001). Enterocytes (5 x 10(6)) were incubated in growth media in plain or BMC coated growth culture vessels for 14 days. There was a correlation between both number of cells seeded (r = 0.824, p < 0.001) and cell viability (r = -0.696, p < 0.01) and % growth colonies containing epithelial cells at 14 days. Both total growth colonies (r = -0.565, p < 0.05) and colonies with epithelial cells (r = -0.589, p < 0.05) had a negative correlation with % crypts. Incubating cells in BMC coated vessels (n = 6) resulted in significantly more dispase liberated cells after 14 days than in plain vessels (n = 6) (6.6 +/- 1.1 vs. 3.8 +/- 1.0 x 10(6), p < 0.05) but viability was similar (97 +/- 2% vs. 96 +/- 2%).(ABSTRACT TRUNCATED AT 250 WORDS)

Intestinal tissue and cell cultures

The culture of animal cells and tissues is a widely used technique in the field of cellular and molecular biology; one of the most interesting aspect being linked to the study of the mechanisms of cell differentiation. In the specific case of intestinal epithelial cells, various tissue culture technologies have proved to be important tools for the study of precise facets related to intestinal function, pathology and differentiation. Concerning this latter aspect, organ culture experiments have brought about interesting data on the hormonal or nutritional control of intestinal maturation. Nevertheless, the study of the precise mechanisms underlying epithelial proliferation and/or differentiation at the cellular level needs more adequate cell culture model systems. One of them has been described for two cell lines derived from human colonic adenocarcinomas, in which the cells can be induced to achieve enterocytic-like differentiation. Up to date, none of the continuous cell lines starting from normal undifferentiated cells have allowed generation of morphological or functional enterocytic polarity. In contrast, primary cell cultures which allow maintenance of a more physiological environment for the epithelial cells like contacts with their in vivo counterparts, mesenchymal cells or extracellular matrix molecules, have proved to be promising approaches.

Importance of a fibroblastic support for in vitro differentiation of intestinal endodermal cells and for their response to glucocorticoids

Microexplants of 14- or 15-day-old fetal rat intestinal endoderm, separated from mesenchyme by collagenase, were placed on culture dishes coated with different extracellular matrix components or on confluent monolayers of intestinal mesenchymal cells or of fetal skin fibroblasts. Only small variations in the attachment or spreading of the endodermal cells could be observed when they were cultured on the different acellular substrata, and their survival never exceeded one week. When cocultured with intestinal or skin fibroblasts, endodermal cells proliferated and the survival time was prolonged to 2 or 3 weeks. Furthermore, differentiation, as assessed by the polarization of the cells, occurred and was characterized by the maturation of apical brush borders and by the synthesis of microvillar digestive enzymes visualized immunocytochemically with monoclonal antibodies. Glucocorticoids accelerated structural differentiation and stimulated or induced brush border enzymes only in the coculture conditions. These experiments emphasize the role of a fibroblastic support without tissue specificity on the cytodifferentiation of intestinal endodermal cells. They also suggest a mesenchymal dependence on the hormonal response.

Cellular senescence revisited: a review

The field of cellular senescence (cytogerontology) is reviewed. The historical precedence for investigation in this field is summarized, and placed in the context of more recent studies of the regulation of cellular proliferation and differentiation. The now-classical embryonic lung fibroblast model is compared to models utilizing other cell types as well as cells from donors of different ages and phenotypes. Modulation of cellular senescence by growth factors, hormones, and genetic manipulation is contrasted, but newer studies in oncogene involvement are omitted. A current consensus would include the view that the life span of normal diploid cells in culture is limited, is under genetic control, and is capable of being modified. Finally, embryonic cells aging in vitro share certain characteristics with early passage cells derived from donors of increasing age.

Morphogenesis in vitro of dissociated fetal rat small intestinal cells upon an open surface and subsequent to collagen gel overlay

The morphogenesis of cells dissociated from fetal rat intestine at 18 days of gestation was compared in vitro on two different substrates, tissue culture plastic and collagen gel. During the first 2 days in culture growth on both substrates was similar, resulting in the formation of layers of epithelial cells and of small lumina within them. On plastic, the cell layers contracted over time, resulting in the formation of small mounds of cells bearing on their surface small protrusions covered with epithelial cells which had densely packed microvilli on their apical surfaces. When the cultures on collagen gels were overlaid with more collagen gel, vesicles lined with epithelial cells developed. These cells were joined by junctional complexes and displayed an apical brush border which was periodic acid-Schiff and alkaline phosphatase positive. After 1 week in culture, when the vesicles reached their maximum extent, numerous epithelial cells were actively incorporating labeled thymidine and cells were being extruded into the lumen. These results demonstrate that the dissociated intestinal cells have the capacity to form intestine-like organoids in vitro and that surrounding the cells with the collagen gel allows expression of this potential. The collagen gel cultures thus undergo morphogenetic processes which can be modified by the surrounding environment and should provide a useful in vitro model system for the study of intestinal development.

Formation of organoid structures and extracellular matrix production in an intestinal epithelial cell line during long-term in vitro culture

During the long-term in vitro maintenance of an epithelial cell line established from rat duodenum (IEC-17) we have observed progressive morphological changes which, after approximately 4-5 months in culture, led to a loss of substrate adherence and to the formation of organoid structures characterized by organized layers of cells separated by continuous extracellular-like material and delimiting close lumina. The cells exhibited a defined polarity with deposition of extracellular matrix components on one side and development of microvilli on the opposite surface. The morphological changes observed did not appear to be the expression of spontaneous transformation since the cells retained a normal diploid rat karyotype and did not grow in soft agar. In this report we present the optical and electron microscopical characterization of the progressive organotypic differentiation of the cell line. Further studies are currently in progress to characterize the extracellular matrix during the process of differentiation.

Establishment and characterisation of six human colorectal adenocarcinoma cell lines

The establishment and characterisation (morphology, ultrastructure, tumourigenicity) of six cell lines from primary human colorectal adenocarcinomas is described. These lines were established from surgical specimens, from 49 unselected patients, without the use of 'feeder' cells, 'conditioned' medium or passage of cells in nude mice. The six cell lines exhibit considerable variation in morphology, CEA secretion and tumourigenicity in nude mice. At least two of the lines retain some of the differentiated characteristics of colorectal epithelium.

Fetal gut mesenchyme induces differentiation of cultured intestinal endodermal and crypt cells

An experimental model was designed to analyze the effect of fetal gut mesenchyme on the cytodifferentiation of crypt cells and of embryonic progenitor cells. The cells used were the rat intestinal crypt cell line, IEC-17, and primary cell cultures prepared form isolated 14-day-old fetal intestinal endoderm (EC). Both cultures prepared from isolated 14-day-old fetal rat intestinal endoderm (EC). Both types of cells were associated with 14-day-old fetal rat gut mesenchyme (Rm) and grafted under the kidney capsule of adult rats. Seventy percent of the Rm/EC and ten percent of the Rm/IEC recombinants, recovered after 9 days, exhibited well-vascularized structures in which the mesenchyme had induced morphogenesis of the cells into a villus epithelium. The four main intestinal epithelial cell types, absorptive, goblet, endocrine, and Paneth cells, were identified using electron microscopy. Biochemical determinations of enzyme activities associated with brush border membranes revealed that alkaline phosphatase, lactase, sucrase, and maltase were expressed in both types of associations. These results were confirmed by immunofluorescence staining using monoclonal antibodies to brush border enzymes. Both enzyme assays and immunocytochemistry showed that the amount of enzymes present in the brush border membrane of Rm/IEC grafts was in general lower than that of the Rm/EC recombinants. The results indicate that fetal rat gut mesenchyme enables morphogenesis and cytodifferentiation of both crypt and embryonic progenitor cells.

Organ specificity of epithelial cells grown in tissue culture from explants obtained from various levels of the rat gut

Monolayer cultures of epithelial cells grown from explants of fetal rat small intestine can differentiate into columnar as well as goblet cells in tissue culture (Kondo et al., Exp cell res 153 (1984) 12) [9]. In this study we have cultured tissue from various levels of the fetal rat gastrointestinal tract in order to characterize those cell types that can be cultured from these tissues and to determine the growth potential of these cells using this culture system. Explants of esophagus and forestomach tissue yielded monolayer outgrowths of squamous epithelial cells which grew as closely apposed polygonal cells capable of developing cornified envelopes. Explants of the glandular portion of the stomach yielded outgrowths of densely packed cells which were more pleiomorphic and which did not desquamate or form cornified envelopes. Explants of small intestine and colon yielded outgrowths of epithelial cells, some of which differentiated into goblet cells, while others developed a 'columnar' cell morphology. The following differences between squamous- and non-squamous cultures were observed. Squamous epithelium showed self-sustained growth, while growth of non-squamous epithelial cells became self-limiting. Cytochalasin B (CB) (5 micrograms/ml for 1 h) induced contraction of the whole cell sheet of non-squamous cells, but of only individual squamous cells. Squamous cell outgrowth was observed from tissues derived from fetuses of all ages (13-day fetuses through to 3-week-old rats); whereas non-squamous epithelial outgrowth was poor when rats older than the 21st gestational day were studied. These results indicate that cultures established in this manner developed the general characteristics of the cells of the organ from which they were derived, even when undifferentiated fetal tissue was used. The growth conditions needed for columnar epithelium are more stringent than those needed for squamous tissues. This technique opens the way for further characterization of growth requirements of gastrointestinal columnar epithelium.