Oncofetal IGF2BP3-mediated control of microRNA structural diversity in the malignancy of early-stage lung adenocarcinoma.

The nature of microRNA (miRNA) dysfunction in carcinogenesis remains controversial because of the complex connection between miRNA structural diversity and biological processes. Here, we found that oncofetal IGF2BP3 regulates the selective production of a subset of 3'-isoforms (3'-isomiRs), including miR-21-5p and Let-7 family, which induces significant changes in their cellular seed occupancy and structural components, establishing a cancer-specific gene expression profile. The D-score, reflecting dominant production of a representative miR-21-5p+C (a 3'-isomiR), discriminated between clinical early-stage lung adenocarcinoma (LUAD) cases with low and high recurrence risks, and was associated with molecular features of cell cycle progression, epithelial-mesenchymal transition pressure, and immune evasion. We found that IGF2BP3 controls the production of miR-21-5p+C by directing the nuclear Drosha complex to select the cleavage site. IGF2BP3 was also involved in the production of 3'-isomiRs of miR-425-5p and miR-454-3p. IGF2BP3-regulated these three miRNAs are suggested to be associated with the regulation of p53, TGF-ß, and TNF pathways in LUAD. Knockdown of IGF2BP3 also induced a selective upregulation of Let-7 3'-isomiRs, leading to increased cellular Let-7 seed occupancy and broad repression of its target genes encoding cell cycle regulators. The D-score is an index that reflects this cellular situation. Our results suggest that the aberrant regulation of miRNA structural diversity is a critical component for controlling cellular networks, thus supporting the establishment of a malignant gene expression profile in early stage LUAD.

APRIL/BAFF upregulation is associated with clonal B-cell expansion in Hunner-type interstitial cystitis.

Hunner-type interstitial cystitis (HIC) is a chronic inflammatory disease of the urinary bladder with an unknown etiology. We conducted comprehensive immunogenomic profiling of bladder specimens obtained by biopsy and cystectomy from 37 patients with HIC. Next-generation RNA sequencing demonstrated abundant plasma cell infiltration with frequent light chain restriction in HIC-affected bladder tissue. Subsequent analysis of the B-cell receptor repertoire revealed spatial and temporal expansion of B-cell clones. The extent of B-cell clonal expansion was significantly correlated with the gene expression levels of TNFSF13 and TNFSF13B, which encode APRIL and BAFF, respectively. These findings indicate that APRIL and BAFF are the key regulators of clonal B-cell expansion in HIC and might serve as therapeutic targets in this debilitating disease. © 2024 The Author(s). The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

Research autopsy programmes in oncology: shared experience from 14 centres across the world.

While there is a great clinical need to understand the biology of metastatic cancer in order to treat it more effectively, research is hampered by limited sample availability. Research autopsy programmes can crucially advance the field through synchronous, extensive, and high-volume sample collection. However, it remains an underused strategy in translational research. Via an extensive questionnaire, we collected information on the study design, enrolment strategy, study conduct, sample and data management, and challenges and opportunities of research autopsy programmes in oncology worldwide. Fourteen programmes participated in this study. Eight programmes operated 24 h/7 days, resulting in a lower median postmortem interval (time between death and start of the autopsy, 4 h) compared with those operating during working hours (9 h). Most programmes (n = 10) succeeded in collecting all samples within a median of 12 h after death. A large number of tumour sites were sampled during each autopsy (median 15.5 per patient). The median number of samples collected per patient was 58, including different processing methods for tumour samples but also non-tumour tissues and liquid biopsies. Unique biological insights derived from these samples included metastatic progression, treatment resistance, disease heterogeneity, tumour dormancy, interactions with the tumour micro-environment, and tumour representation in liquid biopsies. Tumour patient-derived xenograft (PDX) or organoid (PDO) models were additionally established, allowing for drug discovery and treatment sensitivity assays. Apart from the opportunities and achievements, we also present the challenges related with postmortem sample collections and strategies to overcome them, based on the shared experience of these 14 programmes. Through this work, we hope to increase the transparency of postmortem tissue donation, to encourage and aid the creation of new programmes, and to foster collaborations on these unique sample collections. © 2024 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

Prognostic Value of Cancer-Associated Fibroblast Marker Expression in the Intratumoral and Marginal Areas of Soft Tissue Sarcoma.

INTRODUCTION: The tumor microenvironment of sarcomas has not been studied in detail; in particular, little is known about cancer-associated fibroblasts (CAFs). Sarcoma cells are difficult to distinguish from CAFs, either histomorphologically or immunohistochemically. METHODS: We scored the expression of individual CAF markers (fibroblast-activating protein [FAP], CD10, and podoplanin) in the intratumoral and marginal areas of 133 sarcomas. We also examined the association between these markers, as well as the number of CD163-positive macrophages (i.e., tumor-associated macrophages), and clinical outcome. RESULTS: In all cases, the log-rank test revealed that those with high marker scores and macrophage counts (except for marginal CD10+ CAFs) showed significantly worse disease-free survival (DFS). Grade 2/3 cases with high CAF scores (excluding the marginal FAP and CD10 scores) showed significantly worse DFS, whereas those with high intratumoral FAP/CD10 and marginal podoplanin scores showed significantly worse metastasis-free survival (MFS), and those with high intratumoral CD10 score showed significantly worse local recurrence-free survival (LFS). Multivariate analysis identified intratumoral CD10/podoplanin scores and marginal FAP/podoplanin scores as independent prognostic factors for DFS, intratumoral FAP/CD10 and marginal FAP/podoplanin/CD163-positive macrophage scores as independent prognostic factors for MFS, and the intratumoral podoplanin score as an independent prognostic factor for LFS. There was a weak-to-moderate correlation between each score and CD163-positive macrophage counts. CONCLUSION: Patients with high CAF marker expression in the intratumoral and marginal areas have a poorer outcome.

Identification of uromodulin deposition in the stroma of perinephric fibromyxoid nephrogenic adenoma by mass spectrometry.

Nephrogenic adenoma (NA) is an epithelial lesion that usually occurs in the mucosa of the urinary tract. Rare cases of deep infiltrative or perinephric lesions have also been reported. Recently, NA with characteristic fibromyxoid stroma (fibromyxoid NA) has been proposed as a distinct variant. Although shedding of distal renal tubular cells due to urinary tract rupture has been postulated as the cause of NA in general, the mechanism underlying extraurinary presentation of NA and fibromyxoid stromal change in fibromyxoid NA remains unknown. In this study, we performed mass spectrometry (MS) analysis in a case of perinephric fibromyxoid NA of an 82-year-old man who underwent right nephroureterectomy for distal ureteral cancer. The patient had no prior history of urinary tract injury or radiation. Periodic acid-Schiff staining-positive eosinophilic structureless deposits in the stroma of fibromyxoid NA were microdissected and subjected to liquid chromatography/MS. The analysis revealed the presence of a substantial amount of uromodulin (Tamm-Horsfall protein). The presence of urinary content in the stroma of perinephric fibromyxoid NA suggests that urinary tract rupture and engraftment of renal tubular epithelial cells directly cause the lesion.

[Case of Mixed Neuroendocrine-Non-Neuroendocrine Neoplasm of the Papilla of Vater Metastasizing to the Lung after Resection].

A 79-year-old man visited a hospital for right upper abdominal pain and nausea. After conservative treatment for cholangitis and pancreatitis owing to a pancreatic head lesion, he was referred to our hospital for further evaluation and treatment of the lesion. He was diagnosed with pancreatic head cancer or carcinoma of papilla of Vater and underwent subtotal stomach- preserving pancreaticoduodenectomy. Postoperative histopathological examination revealed the coexistence of adenocarcinoma( 60%)and neuroendocrine carcinoma(40%)components, consistent with the diagnosis of mixed neuroendocrine- non-neuroendocrine neoplasm(MiNEN). In addition, regional lymph node metastasis of the adenocarcinoma component was found. Adjuvant chemotherapy was not administered because of a poor performance status. Lung metastasis occurred 13 months after surgery. Chemotherapy with S-1 was administered, and partial response was obtained 17 months after surgery. Herein, we report this rare case of MiNEN of the papilla of Vater with lung metastasis.

A New Chemotactic Mechanism Governs Long-Range Angiogenesis Induced by Patching an Arterial Graft into a Vein.

Chemotaxis, the migration of cells in response to chemical stimulus, is an important concept in the angiogenesis model. In most angiogenesis models, chemotaxis is defined as the migration of a sprout tip in response to the upgradient of the VEGF (vascular endothelial growth factor). However, we found that angiogenesis induced by performing arterial patch grafting on rabbits occurred under the decreasing VEGFA gradient. Data show that the VEGFA concentration peaked at approximately 0.3 to 0.5 cm away from the arterial patch and decreased as the measurement approaches the patch. We also observed that the new blood vessels formed are twisted and congested in some areas, in a distinguishable manner from non-pathological blood vessels. To explain these observations, we developed a mathematical model and compared the results from numerical simulations with the experimental data. We introduced a new chemotactic velocity using the temporal change in the chemoattractant gradient to govern the sprout tip migration. We performed a hybrid simulation to illustrate the growth of new vessels. Results indicated the speed of growth of new vessels oscillated before reaching the periphery of the arterial patch. Crowded and congested blood vessel formation was observed during numerical simulations. Thus, our numerical simulation results agreed with the experimental data.

[Postmortem diagnosis of intravascular large B-cell lymphoma after atraumatic splenic rupture due to splenic infiltration].

Atraumatic splenic rupture (ASR) is a rare but fatal complication of malignant lymphoma. However, only one case of intravascular large B-cell lymphoma (IVLBCL)-related ASR (IVLBCL-ASR) has previously been reported, and the mechanism of IVLBCL-ASR is unknown. We present the case of a 78-year-old man who died unexpectedly and was diagnosed with IVLBCL-ASR pathologically by autopsy. A massive intraperitoneal hemorrhage and four lacerations on the splenic surface were discovered during the autopsy. CD20-positive lymphoma cells that infiltrated into small vessels were highly concentrated in the center of the spleen and were only slightly distributed in the lacerations on the splenic surface. Therefore, increased intrasplenic pressure due to lymphoma cell proliferation was identified as the cause of ASR. The patient had undergone 18F-fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG PET/CT) for tongue cancer evaluation 3 months earlier, and positive uptake was found in the right adrenal gland, where lymphoma cell infiltration was confirmed by the autopsy. Our findings suggest that clinicians should be aware that the advanced stage of IVLBCL can cause fatal ASR via increased intrasplenic pressure. Therefore, early diagnosis and early treatment intervention are desirable to prevent the onset of IVLBCL-ASR, and 18F-FDG PET/CT is useful for the early diagnosis of IVLBCL.

CADM1 promotes malignant features of small-cell lung cancer by recruiting 4.1R to the plasma membrane.

Cell adhesion molecule 1 (CADM1), which mediates intercellular adhesion between epithelial cells, is shown to be highly expressed in small-cell lung cancer (SCLC) and to enhance tumorigenicity of SCLC cells in nude mice. Here, we investigated the molecular mechanism underlying the oncogenic role of CADM1 in SCLC. CADM1 promoted colony formation of SCLC cells in soft agar. Analysis of deletion and point mutants of the conserved protein-binding motifs in CADM1 revealed that the 4.1 protein-binding motif in the cytoplasmic domain is responsible for the promotion of colony formation. Among the actin-binding 4.1 proteins, 4.1R was the only protein whose localization to the plasma membrane is dependent on CADM1 expression in SCLC cells. Knockdown of 4.1R suppressed the colony formation enhanced by CADM1, suggesting that 4.1R is required for the oncogenic role of CADM1 in SCLC. In primary SCLC, CADM1 expression was correlated with membranous localization of 4.1R, as was observed in a SCLC cell line. Moreover, membranous co-localization of CADM1 and 4.1R was associated with more advanced tumor stage. These results suggest that the formation of CADM1-4.1R complex would promote malignant features of SCLC.

Collision of Epstein-Barr virus-positive and -negative gastric cancer, diagnosed by molecular analysis: a case report.

BACKGROUND: Epstein-Barr virus (EBV)-positive gastric carcinoma (GC) is defined by the proliferation of GC cells with EBV infection. The co-existence of EBV-positive and -negative components in a single GC is rare. We report a case of GC with the co-existence of EBV-positive and EBV-negative components, in which we performed-for the first time-various molecular analyses to elucidate their histogenesis. CASE PRESENTATION: An 81-year-old man was diagnosed with GC based on the results of endoscopy and a pathological examination of the biopsy specimen. Systemic chemotherapy was performed, since lymph node and lung metastases were diagnosed based on computed tomography. Total gastrectomy and lymph node dissection were performed after chemotherapy, after confirming that the size of the metastatic lymph nodes had decreased and that the lung metastasis had disappeared. Grossly, a type 3 tumor was located in the middle posterior part of the stomach body. At the cut section, the tumor consisted of a white and solid part on the anal side of the tumor and a flat and elevated part on the oral side. Histologically, the former part consisted of GC with lymphoid stroma and the latter part was composed of poorly differentiated adenocarcinoma without prominent lymphocytic infiltration. The two histopathological components were clearly separated from each other. On EBV-encoded small RNA (EBER)-in situ hybridization (ISH), the part with the lymphoid stroma component was positive, while the other part was negative. Immunohistochemistry revealed that both components showed the overexpression of p53. Sequencing of TP53 using DNA extracted from the two components was conducted, and revealed different patterns. Targeted next generation sequencing revealed MYC amplification in the EBV-positive component of the tumor and HER2 amplification in the EBV-negative part. Immunohistochemistry revealed that the EBV-positive part was C-MYC( +)/HER2(-) and the EBV-negative part was C-MYC(-)/HER2( +). Correspondingly, chromogenic ISH and dual-color ISH showed amplification of C-MYC and no amplification of HER2 in the EBV-positive part, and no amplification of C-MYC and amplification of HER2 in the EBV-negative part. CONCLUSION: We presented a case of collision of two different GCs composed of EBER-ISH ( +)/C-MYC ( +) and EBER-ISH (-)/HER2 ( +) cells.

An extragonadal yolk sac tumor presumed to be of postmeiotic germ cell origin by genetic zygosity analysis via single nucleotide polymorphism array.

An extragonadal yolk sac tumor (YST) is a rare malignant germ cell tumor that usually occurs in childhood. The pathogenesis of extragonadal YST remains largely unknown, especially with regards to its cell of origin. Herein, we report a case of extragonadal YST arising in the uterine round ligament. A 31-year-old Japanese woman, para 2, underwent partial resection of a left-sided, 5-cm, solid inguinal mass. Intraoperative findings showed enlargement of the uterine round ligament in the inguinal canal. Pathological evaluation diagnosed the mass as YST with a mature teratoma (MT) component. The preoperative α-fetoprotein level was markedly elevated, at 24 790 ng/mL. Postoperative magnetic resonance imaging revealed a right ovarian MT and a 3-cm mass remaining in the left lower abdominal wall. The patient underwent total abdominal hysterectomy, bilateral adnexectomy, and left inguinal mass resection. We sampled three frozen tissues (YST, right ovarian MT, and left normal ovary) and performed a single nucleotide polymorphism (SNP) array. Pathological evaluation revealed remnant extragonadal YST in the left inguinal region. The SNP array demonstrated a completely homozygous YST genotype. Copy number variations were gains of 1p, 1q, 2p, 3p, 7p, 8p, 10q, 14q, 18p, 20q, Xp, and Xq and losses of 12q, 20p, and Xq. The right ovarian MT and left normal ovary were partially homozygous and heterozygous, respectively. The evidence suggests that this neoplasm is presumed to be a postmeiotic germ cell origin.

Detection of MEAF6-PHF1 translocation in an endometrial stromal nodule.

Endometrial stromal nodule (ESN) and low-grade endometrial stromal sarcoma (LG-ESS) are rare uterine tumors known as endometrial stromal tumors (ESTs). In addition to their similarity in morphological features, recent studies have shown that these two tumors share common genetic alterations. In particular, JAZF1-SUZ12 fusion is found with high frequency in both ESN and LG-ESS. In LG-ESS, some minor fusions have also been described, which include rearrangements involving PHF1 and its partner genes, such as JAZF1, EPC1, MEAF6, BRD8, EPC2, and MBTD1. Because of the rarity of ESN, genetic alterations other than JAZF1 fusion have not been investigated in detail. In this study, we performed a next-generation sequencing-based analysis in a case of ESN with peripheral metaplastic bone formation and detected MEAF6-PHF1 fusion, which has been reported in a small subset of uterine LG-ESSs and soft tissue ossifying fibromyxoid tumors. The finding that MEAF6-PHF1 fusion is a background genetic abnormality detected both in ESN and LG-ESS, along with JAZF1-SUZ12, provides further support for the similarity and continuum between these two types of ESTs. Furthermore, the association between metaplastic bone formation and MEAF6-PHF1 fusion may not be limited to soft tissue tumors.

Clinicopathological and molecular analyses of linearly expanded epithelial cells with ß-catenin alterations, "ß-catenin signature", in the normal fallopian tube.

AIMS: Recent advances in next-generation sequencing have made it clear that clonal expansion of cells harbouring driver gene mutations occurs in physiologically normal epithelium. Molecular analysis of tubal epithelium has been almost exclusively confined to the TP53 pathway, which is involved in serous carcinogenesis. Other oncogenic events have not been explored in detail. Here, we report the linear expansion of fallopian tubal epithelial cells exhibiting an altered ß-catenin profile (ß-catenin signature). Through molecular analyses, we determined the incidence and clinicopathological significance of ß-catenin signatures. METHODS AND RESULTS: We evaluated 64 specimens of surgically removed bilateral fallopian tubes. Thirty-three ß-catenin signatures were identified in 13 cases (20.3%); these patients were significantly younger than those without ß-catenin signatures (median ages of 44 and 57 years, respectively, P = 0.0317). No correlation between ß-catenin signature and any clinical factor was observed. CTNNB1 mutations were detected in three of eight ß-catenin signatures when tissues were microdissected and subjected to Sanger sequencing in two representative cases. CONCLUSIONS: This is the first report of the CTNNB1 mutation in clusters of morphologically bland tubal epithelial cells. The results of this study indicate that ß-catenin signatures are common, and they may be a part of diverse molecular alterations occurring in normal tubal epithelium.

Molecular Taxonomy of Interstitial Cystitis/Bladder Pain Syndrome Based on Whole Transcriptome Profiling by Next-Generation RNA Sequencing of Bladder Mucosal Biopsies.

PURPOSE: We systematically characterized gene expression, inflammation and neovascularization in patients with interstitial cystitis/bladder pain syndrome to obtain biological evidence supporting diagnosis and classification. MATERIALS AND METHODS: We sequenced RNA obtained from bladder mucosal biopsies of 33 patients with 3 subtypes of interstitial cystitis/bladder pain syndrome, including Hunner lesions in 12, no Hunner lesions in 11 but with glomerulations and neither Hunner lesions nor glomerulations in 10, and 9 controls. Differentially expressed genes of each subtype were searched to identify subtype specific biological pathways and candidate genes important for pathogenesis. Candidate genes were validated by quantitative polymerase chain reaction and immunohistochemistry. Digital immunohistochemical quantification was performed to assess subepithelial lymphoplasmacytic cell and microvessel density. Relationships between candidate gene over expression and symptom severity were explored. RESULTS: Patients with Hunner lesions showed a distinct gene expression profile associated with significant up-regulation of biological processes involving immune responses and infection, and an increase in subepithelial lymphoplasmacytic cell and microvessel density. Over expression of 2 candidate genes, VEGF and BAFF, correlated with symptom severity. Meanwhile, the gene expression profiles of patients with the 2 subtypes without Hunner lesions were similar to those of controls. No difference in biological pathways or subepithelial lymphoplasmacytic cell and microvessel density were detected between these 2 subtypes and controls. CONCLUSIONS: Interstitial cystitis/bladder pain syndrome with Hunner lesions shows distinct genomic and histological features associated with immune responses and infection. In addition, VEGF and BAFF are potential disease biomarkers and therapeutic targets. This subtype should be considered separate from the syndrome.

Adenomatoid tumour of the uterus is frequently associated with iatrogenic immunosuppression.

AIMS: Uterine adenomatoid tumour (AT) is a benign proliferation of cells showing mesothelial differentiation within the myometrium that usually presents as a single nodule. Rare diffuse uterine ATs have been reported, often in patients undergoing immunosuppressive therapy. Herein, we aimed to elucidate the general association between the incidence of uterine AT and iatrogenic immunosuppression by cohort analysis. METHODS AND RESULTS: We analysed 611 consecutive hysterectomy specimens to determine the incidence of AT and its correlation with the immunosuppressive status. Mesothelial lineage, p16 expression, mismatch repair (MMR) protein alterations, and the possible integration of tumorigenic viruses were examined by in situ hybridizasion and immunohistochemistry. ATs were detected in 14 of 611 hysterectomy cases (2.3%). The incidence of AT was significantly higher in the immunosuppressed (IS) group (5/20, 25.0%) than in the non-IS group (9/591, 1.52%), with a relative risk of 16.4. Of the five ATs in the IS group, three were multifocal or diffuse. Latent uterine AT was detected, by in toto sectioning, in one of four immunosuppressed autopsy cases. The tumor cells of ATs commonly expressed calretinin and podoplanin. Characteristic block-type (≥90%) positivity for p16 was observed in most ATs. None of the ATs were positive for human herpes virus type 8, Merkel cell polyomavirus, SV40 large T antigen, Epstein-Barr virus, and human papilloma virus, and the MMR proteins were retained. A TRAF7 mutation was identified from macrodissected tissue in one of 12 ATs by Sanger sequencing. CONCLUSION: Uterine AT is an immunosuppression-associated mesothelial lesion characterised by p16 overexpression.

Digital quantitative analysis of mast cell infiltration in interstitial cystitis.

AIMS: To evaluate the significance of mast cell infiltration in interstitial cystitis (IC) by comparison with equally inflamed controls using a digital quantification technique. METHODS: Bladder biopsy specimens from 31 patients with Hunner type IC and 38 patients with non-Hunner type IC were analyzed. Bladder biopsy specimens from 37 patients without IC, including 19 non-specific chronic cystitis ("non-IC cystitis") specimens and 18 non-inflamed bladder ("normal bladder") specimens, were used as controls. Mast cell tryptase-, CD3-, CD20-, and CD138-immunoreactive cells were quantified using digital image analysis software to evaluate both mast cell and lymphoplasmacytic cell densities. Mast cell and lymphoplasmacytic cell densities were counted independently in the entire lamina propria and detrusor areas and compared among the four groups. RESULTS: In the lamina propria, there were no significant differences in mast cell and lymphoplasmacytic cell densities between Hunner type IC and non-IC cystitis or between non-Hunner type IC and normal bladder specimens. In the detrusor, the mast cell densities were not significantly different among the four groups. Mast cell density was correlated with lymphoplasmacytic cell density, but not with clinical parameters. CONCLUSIONS: Mast cell density is not significantly different between IC specimens and non-IC control specimens with a similar degree of background inflammation. The intensity of mast cell infiltration generally correlated with that of lymphoplasmacytic cells. We conclude that mast cell count is of no value in the differential diagnosis between IC and other etiologies.

Development of targeted therapy and immunotherapy for treatment of small cell lung cancer.

Targeted therapy against druggable genetic aberrations has shown a significantly positive response rate and longer survival in various cancers, including lung cancer. In lung adenocarcinoma (LADC), specific thyroxin kinase inhibitors against EGFR mutations and ALK fusions are used as a standard treatment regimen and show significant positive efficacy. On the other hand, targeted therapy against driver gene aberrations has not been adapted yet in small cell lung cancer (SCLC). This is because driver genes and druggable aberrations are rarely identified by next generation sequencing in SCLC. Recent advances in the understanding of molecular biology have revealed several candidate therapeutic targets. To date, poly [ADP-ribose] polymerase (PARP), enhancer of zeste homologue 2 (EZH2) or delta-like canonical Notch ligand 3 (DLL3) are considered to be druggable targets in SCLC. In addition, another candidate of personalized therapy for SCLC is immune blockade therapy of programmed death-1 (PD-1) and its ligand, PD-L1. PD-1/PD-L1 blockade therapy is not a standard therapy for SCLC, so many clinical trials have been performed to investigate its efficacy. Herein, we review gene aberrations exploring the utility of targeted therapy and discuss blockade of immune checkpoints therapy in SCLC.

Clusters of proliferating endothelial cells and smooth muscle cells in rabbit carotid arteries.

Schwarz and Benditt found clustering of replicating cells in aortic endothelium in 1976 and discussed how homeostasis of the arterial wall is maintained through this nonrandom distribution of replicating cells. However, it is still unclear how cells of vascular walls turnover. In order to address this issue, we evaluated distribution of the cells in mitotic cycle, labeled by Ki67-immunostaining, in serial histological sections of twelve carotid arteries of six adult male Japanese rabbits. As a result, a total of 1713 Ki67-positive endothelial cells (ECs) and 1247 Ki67-positive smooth muscle cells (SMCs) were identified. The Ki67-positivity rate in ECs and SMCs were about 0.048% and 0.0027%, respectively. Many of the Ki67-positive cells clustered in two (EC, 37%; SMC, 33%), three to four (EC, 8%; SMC, 28%), and five to eight cells (EC, 5%; SMC, 10%). Clusters having more than eight cells were not found. Thus, it can be speculated that the cell division of proliferating ECs and SMCs occur four times at most. These novel findings offer great insights for better understanding of the mechanism that underlies cell number regulation of the blood vessel.