Expression of nestin, mesothelin and epithelial membrane antigen (EMA) in developing and adult human meninges and meningiomas

The spatial and temporal pattern of appearance of nestin, epithelial membrane antigen (EMA) and mesothelin proteins was immunohistochemically determined in the cells of normal developing and adult human meninges and meningiomas. Human meninges developed as two mesenchymal condensations in the head region. The simple squamous epithelium on the surface of leptomeninges developed during mesenchymal to epithelial transformation. Nestin appeared for the first time in week 7, EMA in week 8, while mesothelin appeared in week 22 of development. In the late fetal period and after birth, nestin expression decreased, whereas expression of EMA and mesothelin increased. EMA appeared in all surface epithelial cells and nodules, while mesothelin was found only in some of them. In adult meninges, all three proteins were predominantly localized in the surface epithelium and meningeal nodules. In meningothelial meningiomas (WHO grade I), EMA was detected in all tumor cells except in the endothelial cells, mesothelin characterized nests of tumor cells, while nestin was found predominantly in the walls of blood vessels. The distribution pattern of those proteins in normal meningeal and tumor cells indicates that nestin might characterize immature cells, while EMA and mesothelin appeared in maturing epithelial cells. Neoplastic transformation of these specific cell lineages contributes to the cell population in meningiomas.     

Role of mesothelial and submesothelial stromal cells in matrix remodeling following pleural injury.

The pleural response to injury is a complex and poorly understood multifactorial process that can result in the development of fibrosis or obliteration of the pleural space. Pleural fibroblasts are considered the main source of extracellular matrix but cell culture studies have demonstrated synthesis of matrix components by mesothelial cells. We assessed the mesothelial cell contribution to extracellular matrix during pleural healing using immunohistochemical technique. Paraffin-embedded tissue of 3 normal adult lungs and 7 adults with active pleuritis were studied using monoclonal antibodies to cytokeratin, type IV collagen, vimentin, and type I procollagen (PCI). Normal pleural had a single layer of cytokeratin-positive and PCI-negative mesothelium over a thin, continuous type IV collagen-positive basement membrane and PCI-negative submesothelial stroma. Areas of active pleuritis showed loss of the continuous linear staining with anti-type IV collagen antibody. Coexpression of cytokeratin, vimentin and PCI was identified in spindle and/or cuboidal cells located in the fibrin layer, submesothelial connective tissue layer, or on the pleural surface. These findings suggest that reactive mesothelial cells play an active role in the production of extracellular matrix during pleural injury, and that disruption of the submesothelial basement membrane is a key event in determining subsequent fibrous organization of pleural exudate.     

An Electron-microscope Study of the Response of Mesothelial Cells to the Intrapleural Injection of Asbestos Dust

Studies of the pleural mesothelium in rats, mice, and guinea-pigs following the intrapleural injection of asbestos dust, showed that for 6 months at least this dust did not induce mesothelial hyperplasia. During the first few days after injection some areas of mesothelial cells became rounded and less clearly attached to one another, and a few were found to contain small numbers of asbestos fibres. During this period there was evidence of the penetration of asbestos fibres between the mesothelial cells, into the submesothelial connective tissues. Later in the studies the mesothelium covering most of the pleural cavity returned to normal, but where mesothelium covered asbestos granulomata, the cells were found to be extremely flattened, and without surface microvilli. Usually the mesothelial covering was complete, but in some areas pores were found penetrating the mesothelial cell cytoplasm and leaving areas of connective tissue in direct contact with the pleural cavity. In a few cases mesothelial cells were found lining clefts within the connective tissue of asbestos granulomata.     

The use of immunohistochemistry in distinguishing reactive from neoplastic mesothelium. A novel use for desmin and comparative evaluation with epithelial membrane antigen,p53, platelet-derived growth factor-receptor,P-glycoprotein and Bcl-2

AIMS: To evaluate the expression of the intermediate filament desmin in reactive mesothelium and malignant mesothelioma and to compare its utility with five other previously reported immunomarkers claimed to be of use in distinguishing reactive from neoplastic mesothelium. METHODS AND RESULTS: Sixty cases of malignant pleural mesothelioma and 40 cases of reactive mesothelial hyperplasia formed the study group. Cases were immunohistochemically stained with desmin, epithelial membrane antigen (EMA), p53, Bcl-2, P-glycoprotein and platelet-derived growth factor receptor (PDGF-R) beta-chain by the avidin-biotin complex method. The cohort of malignant pleural mesotheliomas were immunoreactive to desmin, EMA and p53 in 6/60 (10%), 48/60 (80%) and 27/60 (45%), respectively. In comparison, the cohort of reactive mesothelial hyperplasias were immunoreactive to desmin, EMA and p53 in 34/40 (85%), 8/40 (20%) and 0/40 (0%), respectively. In a smaller cohort (n = 15) of malignant pleural mesotheliomas, Bcl-2, P-glycoprotein and PDGF-R beta-chain were expressed in 0/15 (0%), 2/15 (13%) and 15/15 (100%), respectively. In a small cohort (n = 15) of reactive mesothelial hyperplasias, Bcl-2, P-glycoprotein and PDGF-R beta-chain were immunoreactive in 0/15 (0%), 0/15 (0%) and 6/15 (40%), respectively. CONCLUSIONS: Desmin and EMA appear to be the most useful markers in distinguishing benign from malignant mesothelial proliferations. Desmin appears to be preferentially expressed in reactive mesothelium and EMA appears to be preferentially expressed in neoplastic mesothelium. The complementary use of both markers is advocated in ascertaining the nature of mesothelial proliferations. Immunohistochemical detection of mutated p53 oncoprotein appeared to be of less utility in this study on account of the low marker sensitivity for malignant mesothelioma. However, p53 antibody may be of use as a second-line marker of neoplastic mesothelium within a standard immunohistochemical panel of antibodies. In this study, Bcl-2, P-glycoprotein and PDGF-R beta-chain appear to be of no use in distinguishing reactive from neoplastic mesothelium, although more formal evaluation of these markers is required.     

Cytological differential diagnosis among adenocarcinoma,epithelial mesothelioma,and reactive mesothelial cells in serous effusions by immunocytochemistry

The objective of the study is to estimate the expression of some antibodies in the metastatic adenocarcinomas, malignant epithelial mesotheliomas, and reactive mesothelial cells in serous effusions and to choose effective panel to the differential diagnosis. Totally 113 effusion cytology samples (80 pleural fluid, 30 ascitic, and 3 pericardial fluid) from 60 cases of metastatic adenocarcinoma (ACA), 18 cases of malignant epithelial mesothelioma (MM), and 35 cases of reactive mesothelium (RM) were included in this study. The cytological diagnoses of these cases were confirmed by histopathology or clinical datas. Smears and cell blocks were prepared for each case. Immunocytochemical study was performed on the cell block sections. The sensitivity of E‐cadherin, CEA, MOC‐31, and Ber‐EP4 for adenocarcinoma was 86.7%, 80%, 70%, and 76.4%, respectively. The specificity was 98.1%, 96.2%, 92.5%, and 86.8%, respectively. The sensitivity of calretinin, HBME‐1, and thrombomodulin for RM/MM was 83%, 79.2%, and 47.2% respectively. The specificity was 88.3%, 21.7%, and 70%, respectively. The expression of E‐cadherin, CEA, MOC‐31, Ber‐EP4, calretinin, and thrombomodulin showed significant difference between ACA and RM/MM (P < 0.01). The reactivity of EMA and Des showed significant difference between RM and MM (P < 0.01). In our opinion, the antibody panel that consists of E‐cadherin, CEA, calretinin, and thrombomodulin should be the best for differential diagnosis between metastatic adenocarcinomas and RM/MM in serous effusions. EMA and Des should be used to differentiate malignant epithelial mesothelioma and reactive mesothelial cells. EMA positive and Des negative favor MM, while Des positive and EMA negative favor RM. Diagn. Cytopathol. 2011.     

大鼠膈腹膜间皮孔的电镜观察

用扫描电镜和透射电镜观察了正常大鼠膈腹膜间皮，并观察了腹膜腔内注射中国墨汁和兔血液后大鼠膈腹膜间皮的变化以及腹膜腔和间皮下毛细淋巴管的关系。     

Immunohistochemical distribution patterns of epidermal growth factor receptor in malignant mesothelioma and non-neoplastic mesothelium

Summary An immunohistochemical study of the epidermal growth factor (EGF) receptor in non-neoplastic pleural mesothelium (35 cases) and in human malignant mesothelioma (36 cases) was made, using a murine monoclonal antibody OM-11-951. All malignant mesotheliomas and non-neoplastic pleural biopsies exhibited a strong cytoplasmic immunoreactivity in mesothelial cells. Nuclear immunoreactivity was detected in mesothelial cells of all specimens of both malignant and non-neoplastic pleura. No statistically significant differences were found between malignant mesothelioma and non-neoplastic pleural mesothelium. There were differences, between the three subtypes of mesothelioma, in the number of cells that exhibited nuclear staining. Statistically significant differences were noted between the epithelial subtype and the mesenchymal subtype (P< 0.005), epithelial subtype versus the mixed cell type (P< 0.005) and between the mesenchymal component of the mixed cell type and the mesenchymal type (P<0.0005). We conclude that there is strong expression of EGF receptor in both malignant mesothelioma and in non-neoplastic pleural mesothelium. Different staining patterns are seen when comparing the different subtypes of mesotheliomas with each other. EGF receptor expression cannot be used to distinguish between malignant and benign mesothelium.     

p53 immunostaining in the distinction between benign and malignant mesothelial proliferations using formalin-fixed paraffin sections.

The distinction between reactive mesothelium and mesothelioma in pleural biopsy specimens is notoriously difficult, and conventional immunohistochemical markers have provided no relief. The object of this study was to examine the frequency of immunohistochemically detectable p53 overexpression in routinely processed, paraffin-embedded tissue from pleural mesotheliomas and from pleura showing reactive mesothelial hyperplasia, using a polyclonal antibody to formalin-resistant p53 epitopes, and to consider the diagnostic utility of this antibody in the distinction between mesothelioma and reactive mesothelium in pleural biopsy specimens. Immunostaining was enhanced by pepsin predigestion prior to the application of the primary antibody. Positivity occurred in 10/16 epithelial mesotheliomas, 9/19 biphasic mesotheliomas, 2/12 sarcomatous mesotheliomas but in none of 20 reactive pleura. Immunostaining was particularly intense in some of the biopsy specimens, which may be due to the rapidity with which these small pieces of tissue were fixed. In conclusion, this study suggests that p53 immunostaining can help to distinguish epithelial or biphasic mesothelioma from reactive mesothelial hyperplasia in formalin-fixed, paraffin-embedded pleural biopsy specimens.     

Light and electron microscope observations of the lymphatic drainage units of the peritoneal cavity of rodents

Fluid, particles, and cells are taken up from the peritoneal cavity by lymphatic drainage units, which, in the mouse and rat, are located along the peritoneal surface of the muscular portion of the diaphragm. The drainage units are composed of three specifically differentiated components: a lymphatic lacuna, a covering of lacunar mesothelium, and intervening submesothelial connective tissue. The units are drained by connecting lymphatic vessels that cross the diaphragm to empty into collecting lymphatic vessels running along the pleural surface of the diaphragm. The collecting lymphatics empty into parasternal lymphatic trunks. In this report, we briefly review critical features of the drainage apparatus and describe new observations, summarized below, about their structure. Around the rim of stomata, the mesothelial openings that lead into the lymphatic lacunae, plasma membranes of lacunar mesothelial cells and of lacunar enidothelial cells abut but are not linked to one another by recognizable junctional specializations. Lacunarendothelial cells often extend valve-like processes that bridge the distal end of the channel beneath the stoma. The configuration of the endothelial processes may be complex. Occasionally, processes from fibroblasts in the submesothelial connective tissue adjacent to stomata make contact with the interstitial surface of lacunar endothelial cells. A discontinuous elastic layer in the submesothelial connective tissue spans the roof of each lacuna. Connecting and collecting lymphatics, which drain lymphatic lacunae, possess endothelial valves. Possible functions for each of these newly described structural features are discussed.     

Mesothelial regeneration is not dependent on subserosal cells

It has been proposed that after mesothelial injury, resident cells within the subserosal connective tissue proliferate, differentiate, and migrate to the serosal surface. The aim of this study was to examine the temporal and spatial changes of proliferating cells in a murine model of testicular mesothelial healing and assess the potential of submesothelial cells to reconstitute the damaged mesothelium. Histology and autoradiography were employed to determine the number of cells within the submesothelial connective tissue, as well as the proportion of cells undergoing DNA synthesis on and beneath the injured serosa. Mesothelial cells surrounding the wound demonstrated maximal DNA synthesis 48 h after injury (27. 82+/-5.64% SEM, compared with 0.17+/-0.16% (3)H-TdR labelled cells for resting mesothelium), whereas a significant increase in proliferating submesothelial cells was not seen until day 4 post-injury (7.79+/-3.31% compared with 0.85+/-0.64% (3)H-TdR labelled cells at day 2). Furthermore, this small number of dividing submesothelial cells must include cells other than the proposed mesothelial precursors, indicating a very low proportion of precursor cells in the submesothelial cell population. As large numbers of mesothelial cells were seen at the wound centre by 3-4 days after injury, it is unlikely that submesothelial cells contributed significantly to the repopulation of the injured mesothelium. It is hypothesized that regenerating mesothelium is more likely to originate from the surrounding uninjured mesothelial cell population.     

Pleural mesothelioma of connective tissue type, localized fibrous tumour of the pleura, and reactive submesothelial hyperplasia. An immunohistochemical comparison

Ten diffuse pleural mesotheliomas of connective tissue type have been compared with 14 examples of pleural granulation tissue and 7 localized fibrous tumours of the pleura, using immunohistochemistry to identify cytokeratins of low and high molecular weight and vimentin. Low molecular weight cytokeratin and vimentin were both detected in 8 of the 10 mesotheliomas and in 12 of the 14 reactive lesions. High molecular weight cytokeratin was rarely detected in either lesion. The seven localized fibrous tumours of the pleura were all positive for vimentin and negative for both cytokeratins. These findings support an origin of connective tissue type mesotheliomas from multipotential submesothelial spindle cells and of localized fibrous tumours of the pleura from either conventional fibroblasts or resting submesothelial spindle cells. Antibodies to cytokeratin help distinguish these two neoplasms but provide no assistance in the more difficult diagnostic problem of distinguishing mesotheliomas of connective tissue type from pleural reactions characterized by abundant granulation tissue.     

Keratin and epithelial membrane antigen immunoreactivity in nonneoplastic fibrous pleural lesions: implications for the diagnosis of desmoplastic mesothelioma

The histologic distinction between desmoplastic mesothelioma and reactive fibrosis and between biphasic desmoplastic mesothelioma and pleural metastases with desmoplasia can be difficult. The presence of such epithelial markers as keratin and epithelial membrane antigen (EMA) within the fibrous areas of a pleural lesion might be construed as evidence in support of the diagnosis of desmoplastic mesothelioma. To test this hypothesis ten desmoplastic mesotheliomas were studied with monoclonal antisera to keratin and EMA and compared with ten hyaline pleural plaques, 11 pleural scars, five benign localized fibrous mesotheliomas, and eight desmoplastic pleural metastases. Although the hyalinized fibrous areas within the desmoplastic mesotheliomas stained with keratin (ten of ten) and EMA (five of ten), the following reactive fibrous pleural lesions were also positive: plaques (four of ten keratin-positive, five of ten EMA-positive); scars (six of 11 keratin-positive, four of 11 EMA-positive); and fibrosis induced by metastases (six of eight keratin-positive, one of eight EMA-positive). Benign localized fibrous mesotheliomas were keratin- and EMA-negative. This study suggests that both benign and malignant fibrous pleural lesions may be of mesothelial origin and demonstrates the need to determine by histologic criteria whether the spindle cell component is benign or malignant before assessing the significance of its keratin or EMA immunoreactivity. The lack of keratin and EMA staining in benign localized fibrous mesotheliomas might be of use in distinguishing benign localized cellular fibrous mesotheliomas from sarcomatous mesotheliomas.     

Mesothelial proliferation due to asbestos and man-made fibres. Experimental studies on rat omentum

The intraperitoneal test in rats has proven to be an appropriate method controlling fibrogenicity and carcinogenicity of asbestos fibres and other fibrous dusts. We analyzed the reaction patterns of mesothelial cover layer to different natural mineral fibres (crocidolite, chrysotile, actinolite, erionite, wollastonite) and man-made mineral and synthetic fibres (glass fibres 104/475, polypropylene, aramide fibres). The injection of doses between 0.01 and 100 mg dust suspended in saline solution led to a continued repairing proliferation of submesothelial connective tissue cells and focal submesothelial fibrosis. These changes were never observed after application of granular dusts as mine dust and quartz. After 15 to 28 months we often found an association of fibrosis and local reactive hyperplasia of partly atypical proliferation of rat omentum mesothelium. These changes were also demonstrated in cases without macroscopically visible tumors. In later stages the underlying fibrosis was often infiltrated and dissolved by mesotheliomas.     

Immunoperoxidase staining of malignant cells in pleural effusions

The indirect immunoperoxidase method was used to detect the presence of carcinoembryonic antigen (CEA), epithelial membrane antigen (EMA), keratin and alpha-1-antichymotrypsin (alpha 1ACT) in cells of pleural fluid sediments from 30 patients with pleural malignancies (18 mesotheliomas and 12 metastatic carcinomas). CEA was negative in all mesotheliomas and positive in all carcinomas but one (an ovarian serous cystadenocarcinoma); alpha 1ACT was positive in mesotheliomas and negative in carcinomas; EMA and keratin were positive in both types of tumor. These data suggest that the use of immunostaining against CEA and alpha 1ACT seems to improve the cytologic differential diagnosis between malignant mesothelioma and metastatic carcinoma in pleural effusions.     

Serosal Tissue: Reactive Tissue as a Model for Understanding Mesotheliomas

Serosal tissues consist of a surface mesothelial layer and subsurface spindled connective tissue cells. Surface cells are decorated with antibodies to both low and high molecular weight cytokeratin whereas subserosal cells only express the intermediate filament vimentin. Serosal injury results in the proliferation of multipotential subserosal cells (MSC) which have the ultrastructural morphology of myofibroblasts and yet co-express low molecular weight cytokeratin and vimentin. These cells appear responsible for the re-establishment of surface mesothelium during which they acquire high molecular weight cytokeratin and loose vimentin. There are many parallels between reactive and neoplastic serosal tissues. Desmoplastic/sarcomatoid mesotheliomas resemble the MSC and co-express low molecular weight cytokeratin and vimentin and epithelial mesotheliomas resemble surface mesothelium and express both low and high molecular weight cytokeratin. The ability of normal serosal tissue to modulate its cell shape and intermediate filament expression helps understand the diversity of serosal tumors.     

Serosal tissue: reactive tissue as a model for understanding mesotheliomas

Serosal tissues consist of a surface mesothelial layer and subsurface spindled connective tissue cells. Surface cells are decorated with antibodies to both low and high molecular weight cytokeratin whereas subserosal cells only express the intermediate filament vimentin. Serosal injury results in the proliferation of multipotential subserosal cells (MSC) which have the ultrastructural morphology of myofibroblasts and yet co-express low molecular weight cytokeratin and vimentin. These cells appear responsible for the re-establishment of surface mesothelium during which they acquire high molecular weight cytokeratin and loose vimentin. There are many parallels between reactive and neoplastic serosal tissues. Desmoplastic/sarcomatoid mesotheliomas resemble the MSC and co-express low molecular weight cytokeratin and vimentin and epithelial mesotheliomas resemble surface mesothelium and express both low and high molecular weight cytokeratin. The ability of normal serosal tissue to modulate its cell shape and intermediate filament expression helps understand the diversity of serosal tumors.     

Use of antisera to epithelial membrane antigen for the cytodiagnosis of malignancy in serous effusions.

A new human antigen, designated epithelial membrane antigen (EMA), has recently been described on surface membranes of a wide variety of normal epithelium but not on connective tissue cells. The antigen is only weakly expressed on normal or reactive mesothelium. Increased expression of the antigen has been observed in most neoplasms of epithelial origin and in malignant mesothelioma. We have investigated the possibility of using this difference in the expression of the antigen to distinguish between mesothelial cells and malignant cells in cytological smears of serous effusions. This distinction cannot always be made on morphological grounds alone and problems of differential diagnosis are encountered in about 15% of all specimens of serous effusions sent for cytological examination. Using antisera to EMA we have applied an indirect immunoalkaline phosphatase technique to alcohol-fixed smears prepared from serous effusions and have found that intense staining of the antigen is confined to effusions from patients in whom there is either clinical or cytological evidence of malignancy. The technique proved to be especially useful in cytologically equivocal cases, where there were problems of differential diagnosis.     

The expression pattern of beta-catenin in mesothelial proliferative lesions and its diagnostic utilities

beta-Catenin is a component of the E-cadherin-catenin cell adhesion complex. It plays an important role in the Wnt/wg pathway, which conveys critical signals for cell proliferation and transformation. The beta-catenin mutation is an important event in the progression of a number of malignancies. In this study, we evaluated the immunohistochemical (IHC) pattern of beta-catenin in a spectrum of mesothelial lesions. Sixty-five formalin-fixed, paraffin-embedded blocks from 54 serous effusions and 11 pleural biopsies were examined. These cases consisted of 33 invasive mesotheliomas, 9 early mesotheliomas (with negative radiologic finding), so-called mesotheliomas in situ, and 23 reactive mesothelial proliferations. A distinct membranous and/or submembranous staining pattern was seen in 23 cases with normal and reactive mesothelium. In contrast, reduced membranous and/or submembranous antibody staining and markedly increased ectopic cytoplasmic and nuclear staining was seen in 26 cases of 33 mesotheliomas. Seven of 9 early mesotheliomas showed increased ectopic cytoplasmic and/or nuclear stain. On the basis of our findings, identification of beta-catenin staining pattern offers a useful marker in the diagnosis of mesothelial lesions and may help identify neoplastic change.     

Electron microscopic alterations of the rat's pleura after experimental haemothorax.

The initial signs of pleural reactivity and the subsequent mechanisms of pleural healing still remain unsolved. The visceral and parietal (costal and diaphragmatic) pleura were investigated following an experimental haemothorax (EH) by transmission electron microscopy. Young-adult Wistar rats were divided in five groups and survived 6 hours, 1, 3, 8 and 15 days respectively after EH. Six hours after EH the mesothelial cells had a more prominent lysosomal system and electron-dense material in the vesicles, as in the dilatated intercellular spaces. On the 1st day of the EH the mesothelial cytoplasm formed a thin interrupted band. The extravasal cells built multiple layers over the basal lamina, leading to a thicker submesothelial layer, occupying the superficial position toward the pleural cavity. The activated mesothelial cells covered both pleural sheets on the 3rd day after EH. Eight days after EH different membrane bodies, large apical evaginations, elastic-like formations, an extensive vesicular and cytofilamentous systems characterized the mesothelium. The wider elastic membrane showed thickenings, protrusions, bifurcations and double course. Fifteen days after EH larger zones in both pleural sheets displayed thinner basal lamina, remnants of elastic membrane and a thicker submesothelial layer. In conclusion, different newly formed structures (reversible and stable) retain the tendency of enlargement of the pleural surface in all investigated periods. Simultaneous intercellular and transcellular transport, as an increase of the lysosomal system characterize the passing of the electron-dense material through the mesothelium. The early period (until 3rd day after EH) is characterized by more prominent mesothelial changes, involving activated cells. The initiation of the late period (on the 8th day after EH) begins with the appearance of lamellar bodies and newly formed elastic membrane. The following late changes (on the 15th day after EH) concern predominantly the components of the connective tissue layer, such as collagen accumulations and blood capillaries. The present data suggest that the alterations over the entire pleura are irregular and asynchronous, showing significant morphological differences in both pleura sheets, some of them are diffuse in character, the final ones appear to be stable and ensure incomplete pleural restoration.     

Postinflammatory changes of the diaphragmatic stomata

Numerous investigations concerning the fine morphology of diaphragmatic stomata have been performed, but its ultrastructural changes in experimental conditions remain unclear. The present study demonstrates the peritoneal side of the diaphragm in adult Wistar rats by transmission electron microscopy. Ten experimental animals were observed 5 and 8 days after Pseudomonas aeuriginosa instillation (PI) into the peritoneal cavity. A control group of 6 rats showed flat mesothelial covering on basal lamina (BL) and connective tissue layer, as well as cubic mesothelial cells, single stomata over underlying lymphatic lacunae (LL). Five days after PI the mesothelial cells had more numerous microvilli, microvesicles, vacuoles, lysosomes and a lesser number of specialized contacts. The multiplication of the extravasal cells and larger intercellular spaces lead to thickenings of the connective tissue around LL. LL were larger and located in close proximity of the mesothelium. Intercellular spaces in the mesothelial layer and different types of contacts between mesothelial cells and endothelial protrusions of LL (with common BL or without BL) were encountered. Eight days after PI the mesothelium, endothelium of LL, their BL and surrounding connective tissue were interrupted and structurally modified to form typical new channels--stomata. The larger portion of the channels were formed of mesothelial cells, while the endothelial cells participated in the submesothelial part. LL were more numerous than in the previous period, and were arranged in groups. LL increased their vertical (50.59 microm) and horizontal (155.57 microm) diameter, as compared with control animals (respectively 12.37 microm and 74.08 microm). Neighbouring LL were separated by thin or thick septae. Peristomatal mesothelial cells or more rarely endothelium formed valve- or bridge-like structures. Valves on the opposite side of LL were observed. Groups of electron-dense bodies characterized some tall endothelial cells of LL. Cubic mesothelium, endothelium of the LL, both BL, the cell connections that formed new stomata, LL and surrounding connective tissue underwent rapid and parallel changes after PI. Among these elements of the lymphatic regions mentioned above, the mesothelium and endothelium of LL had a main role in experimental conditions.