Search PubMed⌕ Search

Biomedical subjects

Y Iwasa

Publications and source records attributed to Y Iwasa.

At least 91 records · Page 5Linked to original sources
PubMed1996

Optimal Level of Chemical Defense Decreasing with Leaf Age

A recent study on a biennial plant demonstrated that pyrrolizidine alkaloids decrease with the age of leaves due to reallocation from old leaves to new leaves. Here we study the optimal age-specific pattern of defense chemical concentration in leaves that achieves the maximum growth rate of the plant. We consider a plant growing exponentially in a constant environment. Assumptions are the following: (1) The loss of leaves due to herbivory decreases with defense chemical concentration (2) The daily net photosynthesis of a leaf decreases with its age. (3) Using photosynthetic products, the plant produces new leaves that may contain defense chemicals. (4) Although there is a cost to producing new defense chemicals, such chemicals can be reallocated without cost. In the optimal schedule calculated using Pontryagin's maximum principle, the chemical defense level decreases with leaf age. The optimal level of chemical defense increases with the cost of leaf production and herbivory intensity but decreases with the cost of defense chemical production, effectiveness of the defense chemical, net productivity, and growth rate of the plant. If both generalist and specialist herbivores attack the same plant, the optimal defense level is dependent only on the generalists' abundance and sensitivity, but is independent of the specialists that are unaffected by the defense chemical.

Journal Article↗
PubMed1996

Establishment probability in fluctuating environments: a branching process model.

We study the establishment probability of invaders in stochastically fluctuating environments and the related issue of extinction probability of small populations in such environments, by means of an inhomogeneous branching process model. In the model it is assumed that individuals reproduce asexually during discrete reproduction periods. Within each period, individuals have (independent) Poisson distributed numbers of offspring. The expected numbers of offspring per individual are independently identically distributed over the periods. It is shown that the establishment probability of an invader varies over the reproduction periods according to a stable distribution. We give a method for simulating the establishment probabilities and approximations for the expected establishment probability. Furthermore, we show that, due to the stochasticity of the establishment success over different periods, the expected success of sequential invasions is larger then that of simultaneous invasions and we study the effects of environmental fluctuations on the extinction probability of small populations and metapopulations. The results can easily be generalized to other offspring distributions than the Poisson.

Demography↗
PubMed1996

The evolution of genomic imprinting.

In some mammalian genes, the paternally and maternally derived alleles are expressed differently: this phenomenon is called genomic imprinting. Here we study the evolution of imprinting using multivariate quantitative genetic models to examine the feasibility of the genetic conflict hypothesis. This hypothesis explains the observed imprinting patterns as an evolutionary outcome of the conflict between the paternal and maternal alleles. We consider the expression of a zygotic gene, which codes for an embryonic growth factor affecting the amount of maternal resources obtained through the placenta. We assume that the gene produces the growth factor in two different amounts depending on its parental origin. We show that genomic imprinting evolves easily if females have some probability of multiple partners. This is in conflict with the observation that not all genes controlling placental development are imprinted and that imprinting in some genes is not conserved between mice and humans. We show however that deleterious mutations in the coding region of the gene create selection against imprinting.

Animals↗
PubMed1996

Sclerosing peritonitis associated with luteinized thecoma of the ovary.

A unique case of bilateral luteinized thecomas of the ovary associated with sclerosing peritonitis is reported and the clinical and pathological features of this and previously reported cases are reviewed. The patient, 52 years of age, presented with abdominal distension and diarrhea. Pelvic imaging studies revealed bilateral ovarian tumors with ascites. Total abdominal hysterectomy and bilateral salpingo-oophorectomy with adhesiotomy of the small bowel were performed. Histologically, the ovarian tumor was composed of closely packed spindle to round-shaped cells, and within the spindle cell population, lutein-like cells were scattered singly or in clusters. Mitotic counts of spindle cells revealed 12 mitotic figures (MF) per 10 high-power fields (HPF) in one part of the left ovarian tumor, but other areas of the tumor showed less than 3 MF/10 HPF on average. The lesion from the resected small bowel showed prominent fibrosis, confined to the serosa with no evidence of metastasis from the ovarian tumor. The patient has undergone adhesiotomy with partial resection of the small bowel seven times since the first laparotomy because of the recurrent small bowel obstruction. The patient has survived with complications due to short bowel syndrome for 7 years after the initial surgery and so far no recurrence or metastasis of the ovarian tumor has been identified. The case reported here also supports the idea that luteinized thecoma of the ovary associated with sclerosing peritonitis may be a distinct clinicopathologic entity, in terms of the unique association and of the unique features of thecoma; that is, bilateral, hormonally inactive and apparently benign in spite of its highly mitotic activity. Additional attention should be paid to the patient's quality of life, which is often degraded by peritoneal fibrosis and small bowel obstruction.

Female↗
PubMed1996

Salivary duct carcinoma: a clinicopathologic study of three cases with a review of the literature.

Three cases of salivary duct carcinoma are presented. They occurred in a 60 year old man, a 66 year old man and a 57 year old woman. All of the lesions were located in the parotid gland. The tumor size ranged from 3 to 5 cm across the largest diameter. Facial paralysis was observed in two cases. Histologically, intraductal and invasive adenocarcinoma showing papillary, cribriform, and solid patterns with comedolike necrosis was observed. Immunohistochemically, the tumor cells were positive for keratin and epithelial membrane antigen. No myoepithelial cells were demonstrated within the tumor by staining for S-100 protein, alpha-smooth muscle actin or muscle specific actin. Ultrastructurally, intracytoplasmic lumina with microvilli, a moderate number of mitochrondria, lysosomes, and tight junctions were found. Regional lymph node metastasis was observed in one case, and distant metastasis developed in two cases. All of the patients were treated with adjuvant postoperative irradiation. One patient died of disease at 11 months after the initial diagnosis, another was alive with disease at 8 months, and the third patient was alive without disease at 2 years and 3 months. Salivary duct carcinoma should be differentiated from low-grade salivary gland carcinomas using morphologic and clinical criteria because of its poor prognosis even with aggressive therapy.

Adenocarcinoma↗
PubMed1995

Continual change in mate preferences.

Secondary sexual characters are highly variable both within and between species. Closely related species often differ markedly in sexual morphology but hardly at all in non-sexual traits. Here we show that Fisher's runaway process of sexual selection is intrinsically unstable and naturally leads to continual change in sexual traits. Runaway leads to semi-stable exaggeration of female preference for a male sexual character, followed by a slow decay of both traits until runaway is triggered again in a different direction. The process then repeats itself resulting in continual change in male sexual traits through time. Allopatric populations are thus expected to diverge without drift or substantial changes in selective pressures. If there is significant mutation bias acting on the male trait, continual change stops and a stable equilibrium appears. Such an outcome is more likely when exaggeration of the male sexual trait signals good genes.

Biological Evolution↗
PubMed1995

Papillary serous carcinoma of the peritoneum in women. A clinicopathologic and immunohistochemical study.

BACKGROUND: Papillary serous carcinoma of the peritoneum (PSCP) is a primary peritoneal tumor in women that histologically resembles papillary serous carcinoma of the ovary (PSCO). Recognition of PSCP as an entity is controversial, as is the histogenesis, histopathologic differential diagnosis, and treatment. METHODS: Ten cases of PSCP retrieved from the pathology files of 4 hospitals in Kyoto and Nara, Japan, were studied clinicopathologically and immunohistochemically. RESULTS: Patient ages at presentation ranged from 40 to 74 years (median, 56 years). All patients were Asian (Japanese). None of the patients had a history of asbestos exposure. Most of the patients had abdominal swelling, ascites with positive cytology, and elevated serum CA125. At surgery, omental tumors with dissemination to the abdominal and pelvic peritoneum were found in all patients. The histology was similar to that of Grade 2 to 3 PSCO. Immunohistochemical studies using a panel of monoclonal antibodies against carbohydrates showed that Lewis Y is a good marker, in addition to S-100, placental alkaline phosphatase, CA125, and CD15 for separating PSCP from malignant mesothelioma (MM). With cytoreductive surgery and cisplatin-based combination chemotherapy and in some cases adoptive immunotherapy and radiation, a median survival of 27 months and a 5-year survival rate of 27% were attained. One patient with Grade 3 tumor has survived for more than 6 years after surgery. CONCLUSIONS: (1) Papillary serous carcinoma of the peritoneum is a definite clinicopathologic entity; (2) immunohistochemistry is a useful tool for distinguishing PSCP from MM; (3) cytoreductive surgery and cisplatin-based combination chemotherapy with other adjunct therapies such as immunotherapy and radiation may improve patient survival in PSCP.

Adult↗
PubMed1995

Population persistence and spatially limited social interaction.

We investigate the persistence of a population composed of socially interacting individuals living in a lattice structured habitat and the effect of spatially limited social interaction, reproduction, and migration. Both cooperative interaction (enhancing the survivorship of neighboring individuals) and competitive interaction (reducing it) are examined. Mathematical analysis based on pair approximation (or doublet decoupling approximation) and computer simulation are used. Results are: If migration rate is not very large, the population tends to form clusters of individuals (clumped distribution) due to spatially limited reproduction. Although cooperative interaction is more effective in a spatially structured population, the population is more difficult to persist than in the corresponding population without spatial structure, because the shortage of nearest neighbor vacant sites reduces reproduction. Migration of individuals reduces the clumping of the spatial pattern. Pair approximation predicts the equilibrium density fairly accurately when the predicted density is sufficiently high (i.e., more than 40% sites are occupied). If the predicted density is low however, the pair approximation overestimates the equilibrium population level. To overcome this disagreement, we examined improved pair approximation.

Bias↗
PubMed1994

[Operative indications and treatment in elderly patients with esophageal cancer].

A recent study indicates that infection of human papillomavirus (HPV) may cause esophageal cancer and that it takes several decades from infection of HPV to developing cancer. Even in early stage esophageal cancer, lymph node metastasis is usually observed in the mediastinum, abdomen and neck regions. As patients with esophageal cancer are potentially in a malnutritional state because of disturbance of intake before treatment, active nutritional support should be performed before and after surgery. An objective nutritional assessment is necessary for nutritional support. The nutritional assessment index (NAI) can be applied to elderly patients because there is no difference in nutritional parameters between elderly and young patients. From the point of view that surgical treatment has limitations concerning complete curability, we employed a remote after loading system (RALS) into a multidisciplinary treatment called the Kochi system, which consists of a combination of surgery, radiation therapy and chemotherapy. This multidisciplinary method of treatment greatly contributes to survival and quality of life of esophageal cancer patients especially in elderly cases.

Aged↗
PubMed1993

Overdispersed molecular evolution in constant environments.

According to recent data analysis of DNA sequences, the dispersion index, defined as the variance-to-mean ratio of the number of base substitutions in a lineage, is often much larger than unity, which is in conflict with simple Poisson processes assumed in the molecular clock hypothesis. In this paper, it will be shown that the dispersion index can be much larger than unity in a model in which the fitness of DNA sequences, mutation rate, and population size are all constant with time, and that moderately deleterious sequences are more abundant than the best fit sequences. Since the fixation probability of novel mutations depends on their fitness relative to the current sequence, the neutral mutation rate is enhanced once a deleterious mutation is fixed. In a simple case with two fitness classes, a large dispersion index can be produced by moderately deleterious mutations (3 < 4Ns < 6), but neither by nearly neutral (4Ns < 2) nor by strongly deleterious (4Ns > 7) mutations. Analysis of the case with 100 fitness classes shows that the dispersion index is insensitive to the population size, but greatly changes with the fitness distribution of DNA sequences. The model can explain why non-synonymous substitutions often have a larger dispersion index than synonymous substitutions.

Animals↗