A study of the three-dimensional organization of the human diaphragmatic lymphatic lacunae and lymphatic drainage units

PTX3 is expressed in terminal lymphatics and shapes their organization and function

Introduction

The lymphatic system is a multifaceted regulator of tissue homeostasis and an integral part of immune responses. Previous studies had shown that subsets of lymphatic endothelial cells (LEC) express PTX3, an essential component of humoral innate immunity and tissue homeostasis.

Methods

In the present study using whole-mount imaging and image-based morphometric quantifications, Ptx3-targeted mice and in vivo functional analysis, we investigated the involvement of PTX3 in shaping and function of the lymphatic vasculature.

Results

We found that PTX3 is localized in the extracellular matrix (ECM) surrounding human and murine lymphatic vessels (LV). In murine tissues, PTX3 was localized in the ECM close to LV terminals and sprouting. Ptx3-deficient mice showed LV abnormalities in the colon submucosa and diaphragm, including a disorganized pattern and hyperplasia of initial LV capillaries associated with altered distribution of tight junction-associated molecules. Mice with LEC-restricted PTX3 gene inactivation showed morphological and organization abnormalities similar to those observed in Ptx3-deficient animals. Ptx3-deficient mice showed defective fluid drainage from footpads and defective dendritic cell (DC) trafficking.

Discussion

Thus, PTX3 is strategically localized in the ECM of specialized LV, playing an essential role in their structural organization and immunological function.

Embryological, anatomical and clinical considerations on pleuroperitoneal communication

The pleural and peritoneal cavity share many related features due to their common celomic origin. Normally these two spaces are completely separated with the development of the diaphragm. Defects in diaphragm morphogenesis may result in congenital diaphragmatic hernias, which is the most known form of communication between the pleural and peritoneal cavity. However, in several cases, findings of pleuroperitoneal communication (PPC) have been described in adults through an apparently intact diaphragm. In this comprehensive review we systematically evaluate clinical scenarios of this form of "unexpected" PPC as reported in the literature and focus on the possible mechanisms involved.

Immunohistochemical and Scanning Electron Microscopic Confirmation of the Lymphatic Lacunae in the Uterine Tube Mucosal Folds. What Are the Clinical Implications?

Uterine tubes (UTs) are essential during physiological reproduction. The most intriguing part of its wall is the mucosa. Apart from the epithelial cells vital for its normal function, the connective tissue lamina propria contains wide spaces whose function, morphology and structure are yet to be elucidated. The present study used bioptic samples from 25 premenopausal (mean age 48.3 years, σ=3.56) and 25 postmenopausal women (mean age 57.8 years, σ=7.79). In both study groups, samples were obtained from two anatomically distinct parts of the UT – ampulla and infundibulum with fimbriae. The specimens were processed for scanning electron microscopy (SEM) and immunohistochemical detection of podoplanin (clone D2-40) and VEGFR-3 – two markers of lymphatic endothelial cells. The results showed that specimens from premenopausal and postmenopausal women contain wide lymphatic spaces, also known as lymphatic lacunae. The most probable function of the lacunae in the fimbriae is oocyte pick-up upon ovulation thanks to their ability to get engorged with lymph, thus serving as an erectile-like tissue. The ampullary lacunae are probably responsible for tubal fluid maintenance and recirculation. These results indicate that they are vital for normal reproduction because tubal fluid dynamics are as important as fluid composition. Further research on this topic is highly warranted because more detailed insights into UT function have a great potential to refine the methods of reproductive medicine, e.g. in vitro fertilization (IVF), which are still far from optimal regarding fertility outcomes.

Peritoneal carcinomatosis secondary to metastatic lung cancer complicated with acute suppurative appendicitis: A case report and literature review

RATIONALE

Lung cancer (LC) is a malignant tumor with the highest morbidity and mortality in the world. The most common metastatic sites of LC are the brain (47%), bone (36%), liver (22%), adrenal glands (15%), thoracic cavity (11%) and distant lymph nodes (10%). Peritoneal carcinomatosis (PC) is a rare clinical event in LC patients. Considering the rarity and nonspecific clinical symptoms of peritoneal metastasis among LC patients, a case of peritoneal metastasis secondary to LC incidentally observed by laparoscopic appendectomy is unusual.

PATIENT CONCERNS

Here, we present a 53-year-old never-smoker woman who presented to the emergency department with a 2-day history of pain in the right abdominal quadrant. Later, laparoscopy revealed acute suppurative appendicitis accompanied by a peritoneal metastatic mass.

DIAGNOSIS

The patient was diagnosed with PC secondary to metastatic LC complicated with acute suppurative appendicitis by immunohistochemistry. Positron emission tomography computed tomography (PET CT) findings further strengthen the evidence of PC from LC.

OUTCOMES

Based on the results of genomic analysis, the patient received targeted therapy with osimertinib 80 mg/d.

LESSONS

Due to the discovery of new targets, the use of molecular therapies improved progression-free survival (PFS) and overall survival (OS), which increases the chance of identifying peritoneal metastasis of LC. For LC patients with abdominal symptoms, clinicians should be aware of the possibility of peritoneal metastasis from LC, especially for patients diagnosed with lung adenocarcinoma or with pleural effusion.

Mechanisms of Peritoneal Mesothelial Cells in Peritoneal Adhesion

A peritoneal adhesion (PA) is a fibrotic tissue connecting the abdominal or visceral organs to the peritoneum. The formation of PAs can induce a variety of clinical diseases. However, there is currently no effective strategy for the prevention and treatment of PAs. Damage to peritoneal mesothelial cells (PMCs) is believed to cause PAs by promoting inflammation, fibrin deposition, and fibrosis formation. In the early stages of PA formation, PMCs undergo mesothelial–mesenchymal transition and have the ability to produce an extracellular matrix. The PMCs may transdifferentiate into myofibroblasts and accelerate the formation of PAs. Therefore, the aim of this review was to understand the mechanism of action of PMCs in PAs, and to offer a theoretical foundation for the treatment and prevention of PAs.

Immune Niche Within the Peritoneal Cavity

There are numbers of leukocytes present in peritoneal cavity, not only protecting body cavity from infection but also contributing to peripheral immunity including natural antibody production in circulation. The peritoneal leukocytes compose unique immune compartment, the functions of which cannot be replaced by other lymphoid organs. Atypical lymphoid clusters, called "milky spots", that are located in visceral adipose tissue omentum have the privilege of immune niche in terms of differentiation, recruitment, and activation of peritoneal immunity, yet mechanisms underlying the regulation are underexplored. In this review, I discuss the emerging views of peritoneal immune system in the contexts of its development, organization, and functions.

Patterns of diaphragm involvement in stage 3B/3C ovarian-tubal-peritoneal epithelial cancer patients and survival outcomes

PURPOSE

Demonstrate survival outcomes of stage 3B/3C ovarian-tubal-peritoneal epithelial cancer patients who had metastases on diaphragm.

METHODS

141 patients who had undergone diaphragm surgery as a part of primary cytoreductive surgery procedures performed for stage 3B and 3C ovarian-tubal-peritoneal epithelial cancers were reviewed retrospectively. Patients who were administered neoadjuvant chemotherapy and patients who were suboptimally cytoreduced were not included to the study.

RESULTS

Median follow-up time was 42 months. Median overall survival of the patients who underwent diaphragm full-thickness resection (n = 18) because of tumors infiltrating diaphragm muscle was 40 months. Median overall survival of the patients who did not have a transdiaphragmatic thoracotomy (n = 113) was 77 months. Patients who underwent a willful full-thickness diaphragm resection because of tumors invading diaphragm muscle had significantly shorter survival compared with patients who did not have a transdiaphragmatic thoracotomy (p = 0.033). Seven (38.9%) of the 18 patients who underwent diaphragm full-thickness resection developed a recurrence in thorax. Twelve (9.8%) of the remaining 123 patients developed a recurrence in thorax. Patients who underwent full-thickness diaphragm resection because of tumors infiltrating diaphragm muscle developed recurrence in the thorax more frequently (p = 0.001).

CONCLUSIONS

Diaphragm muscle involvement is a predictor of thorax recurrences and worse survival outcomes in stage 3B and 3C ovarian-tubal-peritoneal epithelial cancers. Thus, this should be considered when selecting appropriate adjuvant treatment and route of administration (intravenous/intraperitoneal) in patients who had diaphragm implants infiltrating diaphragm muscle.

Role of asbestos clearance in explaining long-term risk of pleural and peritoneal cancer: a pooled analysis of cohort studies

OBJECTIVES

Models based on the multistage theory of cancer predict that rates of malignant mesothelioma continuously increase with time since first exposure (TSFE) to asbestos, even after the end of external exposure. However, recent epidemiological studies suggest that mesothelioma rates level off many years after first exposure to asbestos. A gradual clearance of asbestos from the lungs has been suggested as a possible explanation for this phenomenon. We analysed long-term trends of pleural and peritoneal cancer mortality in subjects exposed to asbestos to evaluate whether such trends were consistent with the clearance hypothesis.

METHODS

We used data from a pool of 43 Italian asbestos cohorts (51 801 subjects). The role of asbestos clearance was explored using the traditional mesothelioma multistage model, generalised to include a term representing elimination of fibres over time.

RESULTS

Rates of pleural cancer increased until 40 years of TSFE, but remained stable thereafter. On the other hand, we observed a monotonic increase of peritoneal cancer with TSFE. The model taking into account asbestos clearance fitted the data better than the traditional one for pleural (p=0.004) but not for peritoneal (p=0.09) cancer.

CONCLUSIONS

Rates of pleural cancer do not increase indefinitely after the exposure to asbestos, but eventually reach a plateau. This trend is well described by a model accounting for a gradual elimination of the asbestos fibres. These results are relevant for the prediction of future rates of mesothelioma and in asbestos litigations.

Therapeutic delivery to the peritoneal lymphatics: Current understanding, potential treatment benefits and future prospects

The interest in approaches to deliver therapeutics to the lymphatic system has increased in recent years as the lymphatics have been discovered to play an important role in a range of disease states such as cancer metastases, inflammatory and metabolic disease, and acute and critical illness. Therapeutic delivery to lymph has the potential to enhance treatment of these conditions. Currently much of the existing data explores therapeutic delivery to the lymphatic vessels and nodes that drain peripheral tissues and the intestine. Relatively little focus has been given to understanding the anatomy, function and therapeutic delivery to the peritoneal lymphatics. Gaining a better understanding of peritoneal lymphatic structure and function would contribute to the understanding of disease processes involving these lymphatics and facilitate the development of delivery systems to target therapeutics to the peritoneal lymphatics. This review explores the basic anatomy and ultrastructure of the peritoneal lymphatics system, the lymphatic drainage pathways from the peritoneum, and therapeutic and delivery system characteristics (size, lipophilicity and surface properties) that favour lymph uptake and retention after intraperitoneal delivery. Finally, techniques that can be used to quantify uptake into peritoneal lymph are outlined, providing a platform for future studies.

The anatomy of fetal peripheral lymphatic vessels in the head-and-neck region: an immunohistochemical study

Using D2-40 immunohistochemistry, we assessed the distribution of peripheral lymphatic vessels (LVs) in the head-and-neck region of four midterm fetuses without nuchal edema, two of 10 weeks and two of 15 weeks' gestation. We observed abundant LVs in the subcutaneous layer, especially in and along the facial muscles. In the occipital region, only a few LVs were identified perforating the back muscles. The parotid and thyroid glands were surrounded by LVs, but the sublingual and submandibular glands were not. The numbers of submucosal LVs increased from 10 to 15 weeks' gestation in all of the nasal, oral, pharyngeal, and laryngeal cavities, but not in the palate. The laryngeal submucosa had an extremely high density of LVs. In contrast, we found few LVs along bone and cartilage except for those of the mandible as well as along the pharyngotympanic tube, middle ear, tooth germ, and the cranial nerves and ganglia. Some of these results suggested that cerebrospinal fluid outflow to the head LVs commences after 15 weeks' gestation. The subcutaneous LVs of the head appear to grow from the neck side, whereas initial submucosal LVs likely develop in situ because no communication was evident with other sites during early developmental stages. In addition, CD68-positive macrophages did not accompany the developing LVs.

A pleuro-peritoneal communication through the diaphragm affected with lymphangioleiomyomatosis

A 30-year-old Japanese woman with lymphangioleiomyomatosis (LAM) developed a left chylothorax and chylous ascites. A pleuro-peritoneal communication was confirmed by a scintigram with (99)mTc-labeled macroaggregated-albumin injected into the peritoneal cavity. Video-assisted thoracic surgery revealed a protruding papillary lesion on the left diaphragm. Chyle was oozing into the pleural cavity through this lesion. Histopathological analyses demonstrated that the protrusion was a diaphragmatic LAM lesion and that LAM-associated lymphangiogenesis enabled communication between the pleural and peritoneal cavities through lymphatic vessels. This case demonstrated a new mechanism for chylous pleural effusion in LAM and illustrates the significance of LAM-associated lymphangiogenesis.

Lymphangiogenesis-Mediated Shedding of LAM Cell Clusters as a Mechanism for Dissemination in Lymphangioleiomyomatosis

Lymphangioleiomyomatosis (LAM) affects exclusively women of reproductive age, involves the lungs and axial lymphatic system, and is frequently complicated with renal angiomyolipomas. LAM lesions are generated by the proliferation of LAM cells with mutations of one of the tuberous sclerosis complex (TSC) genes. Recent studies indicate that LAM cells can migrate or metastasize to form new lesions in multiple organs, although they show a morphologically benign appearance. In the previous study, we reported LAM-associated lymphangiogenesis and implicated its role in the progression of LAM. In this study, we further focused on the lymphatic abnormalities in LAM: LAM-associated chylous fluid (5 pleural effusion and 2 ascites), surgically resected diaphragm (1 patient), and axial lymphatic system including the thoracic duct, lymph nodes at various regions, and diaphragmatic lymphatic system (5 autopsy cases). We demonstrated that LAM cell clusters enveloped by lymphatic endothelial cells (LCC) in all chylous fluid examined. We identified LAM lesion in the diaphragm (2 of 5 autopy cases and one surgical specimen), thoracic duct (5 of 5), and lymph nodes (retroperitoneal (5 of 5), mediastinal (4 of 5), left venous angle (5 of 5) with total positive rate of 68% to 88% at each region of the lymph node, but less frequent or none at remote lymph nodes located away from the axial lymph trunk (cervical [1 of 5] and axillary [0 of 5]). LCCs were identified in intra-LAM lesional lymphatic channels where LAM cells proliferate along lymphatic system. In in vitro culture system, LCC can fragment into each proliferating LAM cell. These findings suggest that LAM-associated lymphangiogenesis demarcates LAM lesion into bundle- or fascicle-like structure and eventually shed LCC into the lymphatic circulation and that LCCs play a central role in the dissemination of LAM lesion.

HIV-1-derived lentiviral vectors and fetal route of administration on transgene biodistribution and expression in rhesus monkeys

The gene transfer efficiency of lentiviral vectors pseudotyped with vesicular stomatitis virus-glycoprotein (VSV-G) driven by the MND or CMV promoters and expressing the enhanced green fluorescent protein (EGFP) was investigated in fetal rhesus monkeys (Macaca mulatta) (N=21). Fetuses (50+/-10 days gestation; term 165+/-10 days) were injected under ultrasound guidance using an intraperitoneal (i.p.) or intrahepatic (i.h.) approach with a range of 1 x 10(7)-2 x 10(8) infectious particles/fetus. Analysis of transgene biodistribution and expression was performed in multiple tissues at 3-7 months postgene delivery using quantitative techniques. Overall, results indicated the following: (1) i.p. gene transfer at 40 days gestation resulted in a more diffuse distribution of the vector compared to administration at 60 days gestation; (2) vector biodistribution was similar after administration by the i.p. or i.h. routes; and (3) gene expression analysis in transduced tissues showed the presence of mRNA transcripts that correlated with the level of gene transfer. These studies suggest that fetal gene transfer using the i.p. and i.h. routes results in prolonged transduction and expression of the transgene in multiple tissues.

Transendothelial transport and migration in vessels of the apparatus lymphaticus periphericus absorbens (ALPA)

The vessel of the apparatus lymphaticus periphericus absorbens (ALPA) represents the sector with high absorption capacity of the canalization of the lymphatic vascular system. It plays a basic role in preserving tissue homeostasis and in directing interstitial capillary filtrate back to the bloodstream. ALPA lymphatic endothelium differs from the endothelia of conduction and flowing vessels (precollectors, prelymph nodal and postlymph nodal collectors, main trunks), since it presents a discontinuous basement membrane, which is often absent, and lacks pores and fenestrations. The mesenchymal origin of the ALPA lymphatic vessel, morphological and ultrastructural aspects, intrinsic contractile properties, the presence of valves, innervation, and specific lymphatic markers that reliably distinguish it from blood capillaries are studied. Furthermore, its role in lymph formation through different mechanisms (hydrostatic pressure and colloidal osmotic-reticular mechanisms, vesicular pathway, and intraendothelial channel) is investigated. We have studied morphological and biomolecular mechanisms that control the transendothelial migration, from the extracellular interstitial matrix into the lumen of the lymphatic vessel, of cells involved in immune response and resistance (lymphocyte recirculation, etc.) and in the tumoral metastatic process via the lymphatic system. Finally, future research prospects, clinical implications, and therapeutic strategies are considered.

Development of the peritoneal lymphatic stomata and lymphatic vessels of the diaphragm in mice

The generation and development of the peritoneal lymphatic stomata (PLS) and lymphatic vessels of the diaphragm were studied in mice at gestational ages from the embryonic to the postnatal period with TEM, SEM and enzyme histochemistry and the PLS data were quantitatively analyzed with computer-assisted image processing technology (Elescope image analysis software). The results showed that the diaphragmatic mesothelium was covered only by flattened mesothelial cells (FMC) at the 13th embryonic day (ED 13). At ED 15, some cuboidal mesothelial cells (CMC) and immature lymphatic stomata (NLS) were found scattered on the diaphragmatic mesothelium. The sub-peritoneal lymphatic capillaries did not appear until ED 18. However, no absorptive function was observed in NLS when trypan blue granules were injected into the peritoneal cavity. At postnatal day 1 (PND 1), the endothelial cytoplasm processes of the diaphragm lymphatic capillaries span the connective tissue fibers and the basal membrane of CMC to form the subperitoneal channels. These channels were connected with NLS and serve as the absorptive route between the peritoneal cavity and the sub-peritoneal lymphatic vessels. The trypan blue absorption test demonstrated that postnatal PLS possessed an absorptive function and had transformed to mature lymphatic stomata (MLS) by PND 1. Thus, NLS were renamed of MLS. At PND 5, the cuboidal mesothelial cell ridge (CMCR) appeared with increased CMC areas. At PND 10, CMCR were fused to form the band-like CMC area with much more MLS distributed in the muscular portion of the diaphragm. With distribution area and density of PLS increasing and growth of lymphatic vessels, an increased absorptive function from the peritoneal cavity was observed in the experiment.

Ultrastructure and three-dimensional study of the lymphatic stomata in the costal pleura of the rabbit

The aim of this report was to investigate the ultrastructure and three-dimensional organization of the pleural lymphatic stomata in the adult rabbit costal pleura by electron microscopy. A computer image processing system attached to the scanning electron microscopy was used to get statistical evaluation of the dimensions of pleural lymphatic stomata. Mesothelial cells were digested by 2 mol/L NaOH solution in order to expose the submesothelial connective tissue with macula cribriformis. Two kinds of mesothelial cells were observed on the costal pleura: the flattened cells and the cuboidal ones. Both had microvilli on their surface. Pleural lymphatic stomata were located only in the regions of cuboidal mesothelial cells. The average area of a stoma was 7.20 +/- 3.69 microm2 and their average density 121 +/- 72/mm2. Pleural cavity is connected with the lympo-vascular system by lymphatic stomata and the interstitial layer with a dense network of lamina cribriformis. The macula cribriformis (7-60 microm in diameter) were found in subpleural connective tissue below the cuboidal mesothelial cells. Consequently on cross-section, the pathway represents a channel consisting of the stoma, the connective tissue space, and the gap between endothelial cells of the lymphatics. Closed lymphatic stomata and the milky spots composed of macrophages could be observed on the costal pleura. Our results suggest that the pleural cavity is connected with the lymphatic capillaries through the lymphatic stomata and the subpleural channel. This is the only "highway" from the pleural cavity to the vessels. This pathway may be importantly involved in the material exchange and the immunity of the pleura cavity.

The distribution and morphology of lymphatic vessels on the peritoneal surface of the adult human diaphragm, as revealed by an ink-absorption method

Application of india ink to the peritoneal and pleural surfaces of the adult human diaphragm allowed visualization of the distribution and morphology of the lymphatic vessels by light microscopy and scanning electron microscopy. The diaphragms examined had been fixed and stored in 10% formalin. Numerous lymphatic vessels were stained black with india ink, presenting reticular, radial-meshwork, ladder-like and lacy patterns. They were distributed throughout the entire sternocostal part. Analysis by light and scanning electron microscopy of the areas indicated by india ink revealed the presence of primary lymphatic vessels that formed lymphatic lacunae and stomatal openings to the peritoneal cavity. A layer of secondary collecting lymphatic vessels was located cranially with respect to the layer of primary lymphatic vessels. Thus, the peritoneum had at least two layers of lymphatic vessels. These lymphatic vessels were not tubular vessels but resembled flat cisternae, as has been suggested in the case of the mouse diaphragm. The pleura lacked lymphatic stomata and had no such double-layered lymphatic organization. This is the first report that showed distribution and morphology of the lymphatic vessels in the diaphragmatic peritoneum of the formalin-fixed, adult human diaphragm. The method and results in the present study may contribute to morphological analysis of the lymphatic system in the wall of the human body cavity.

Study on the mechanism of regulation on the peritoneal lymphatic stomata with Chinese herbal medicine

AIM

To study the mechanism of Chinese herbal medicine (CHM), the prescription consists of Radix Salviae Miltiorrhizae, Radix Codonopsitis Pilosulae, Rhizoma Atractylodis Alba and Rhizoma Alismatis, Leonurus Heterophyllus Sweet,etc on the regulation of the peritoneal lymphatic stomata and the ascites drainage.

METHODS

The mouse model of live fibrosis was established with the application of intragastric installations of carbon tetrachloride once every three days; scanning electron microscope and computer image processing were used to detect the area and the distributive density of the peritoneal lymphatic stomata; and the concentrations and NO in the serum were measured and analyzed in the experiment.

RESULTS

Two different doses of CHM could significantly increase the area of the peritoneal lymphatic stomata, promote its distributive density and enhance the drainage of urinary ion such as sodium, potassium and chlorine. Meanwhile, the NO concentration of two different doses of CHM groups was 133.52+/-23.57 micromol/L and 137.2+/-26.79 micromol/L respectively. In comparison with the control group and model groups (48.36+/-6.83 micromol/L and 35.22+/-8.94 micromol/L, P<0.01),there existed significantly marked difference, this made it clear that Chinese herbal medicine could induce high endogenous NO concentration. The effect of Chinese herbal medicine on the peritoneal lymphatic stomata and the drainage of urinary ion was altered by adding NO donor(sodium nitropurruside,SNP) or NO synthase (NOS) inhibitor (N(G)-monomethyl-L-arginine, L-NMMA) to the peritoneal cavity.

CONCLUSION

There existed correlations between high NO concentration and enlargement of the peritoneal lymphatic stomata, which result in enhanced drainage of ascites. These data supported the hypothesis that Chinese herbal medicine could regulate the peritoneal lymphatic stomata by accelerating the synthesis and release of endogenous NO.

Surgical embryology and anatomy of the diaphragm with surgical applications

This article reviews the development, surgical anatomy, and teratology of the diaphragm, and discusses the diagnostic procedures, surgical therapy, and prognosis of congenital disturbances. Special attention is paid to the traumatic rupture of the diaphragm, concerning incidence, cause, diagnosis, prognosis, and surgical repair.

The lymphatic vessels and the so-called "lymphatic stomata" of the diaphragm: a morphologic ultrastructural and three-dimensional study

We studied the absorbing peripheral lymphatic vessel with the light microscope, the transmission electron microscope, the scanning electron microscope, and three-dimensional models of the diaphragm of several rodents and insectivores under normal and experimental conditions (lymphatic stasis and dehydration). To clarify the delicate and complex mechanism that permits drainage of the abdominal cavity contents into the lymphatic circulatory system, we introduced Polystyrene latex spherules, China ink, and Trypan blue into the abdominal cavities. After anatomical comparisons of the superficial and deep networks of absorbing peripheral lymphatic vessels at the tendinous and muscular portions of the diaphragm and after classification of lymphatic vessels into absorbing and conducting functions, we examined the stomata, which, owing to morphologic and topographic findings, we defined as stable structures. Furthermore, we observed that the stomata and submesothelial connective channel are fundamental elements that facilitate the flow of the corpuscular and liquid contents of the peritoneal cavity to the submesothelial absorbing lymphatic vessel wall. Also, we underlined that the genesis of the connective channel depends on the secondary cytoplasm extensions of two distinct adjacent endothelial cells, which, to facilitate the flow of the absorbed abdominal contents, completely coat this channel. Additionally, our observations illustrate that the secondary cytoplasm extensions do not engage in continuous relationships with the basal lamina of the mesothelium and with the margins of the stoma, and, hence, the hypothesis of "lymphatic stomata" as an expression of the anchoring of the borders of the open interendothelial junctions to the orifice margins of the stoma cannot be confirmed. Moreover, we describe the presence and formation of intraendothelial channels in the lymphatic endothelial wall. We affirm that this morphological entity is a dynamic unit, because its numerical density varies according to different physiological and experimental conditions to degrees of hydrostatic and colloidal osmotic pressure and, perhaps, also to the particular characteristics of the substances that the connective channel liberates into the endothelial wall of the lymphatic vessel. In conclusion, we affirm that the absorbing peripheral lymphatic vessels of the diaphragm, by way of intraendothelial channel formations, membrane diffusion, and the vesicular path of the endothelial cells, constitute the fundamental draining elements for the corpuscular and liquid contents of the abdominal cavity.

Lymphatic system of the mouse diaphragm: morphology and function of the lymphatic sieve

BACKGROUND

The diaphragm has a unique system that collects peritoneal fluid and carries it into the lymphatic system. However, our understanding of the morphology and function of this system is still incomplete.

METHODS

Twelve C57BL/6 mice of 13 to 25 weeks of age were used without regard to sex. In one series of experiments, the diaphragm was isolated and fixed 10-15 minutes after injection of india ink into the peritoneal cavity and then the peritoneal mesothelium was peeled off from the submesothelial connective tissue. The lymphatic vessels attached to the mesothelial strip were examined by scanning electron microscopy. The diaphragm was also observed in plastic-embedded semithin and ultrathin sections. In another series of experiments, the diaphragm was stained by 5'-nucleotidase histochemistry (Wachstein and Meizel, 1957a. Am. J. Clin. Pathol., 27:13-23), and several microdrops of india ink were placed on the peritoneal or pleural surface to reveal the profile of the lymphatic vessels.

RESULTS

The lymphatic vessels on the peritoneal side of the diaphragm were flattened. They usually ranged from several to 100 microns in width and from close to zero to a few micrometers in thickness. In other words, they formed extremely flat lumina, differing from the more usual tubular lymphatic vessels. Several lymphatic vessels extended radially and parallel to one another from the central tendon to the thoracic wall, with numerous connecting branches, forming an area of lymphatic vessels. The india ink that had been injected intraperitoneally and the staining with 5'-nucleotidase revealed that there were seven to nine such lymphatic areas in one hemisphere of the diaphragm. The lymphatic areas spread in parallel with the peritoneal surface of the diaphragm and all the areas together appeared to occupy more than half the surface area of the sternocostal part of the diaphragm. Each area was a relatively distinct functional unit with respect to the draining of india ink. Microdrops of india ink placed on the pleural surface did not enter the lymphatic vessels, while those placed on the peritoneal surface immediately entered the peritoneal lymphatic vessels and migrated to the pleural lymphatic vessels via the transmuscular lymphatic branches.

CONCLUSIONS

The peritoneal lymphatic vessels of the diaphragm have extremely flat lumina that spread in parallel with the peritoneal surface of the diaphragm and form a lymphatic sieve that covers approximately half or more of the surface area of the sternocostal region for drainage of fluid and particulate matter from the peritoneal cavity. The lymphatic system has been characterized by the presence of openings (= stomata) to the peritoneal cavity and the amplitude of the lumina (= lacunae). However, the fundamental characteristic of the system is the extremely flat lumen (= vadum), which facilitates the formation of the lymphatic sieve.