Measurement of intra-embryonic pH during the early stages of development in the chick embryo

Summary Measurements have been made of the pH in the extracellular space, adjacent to the neural tube, in 73 isolated chick embryos in vitro at stages from 4–22 somites. A pH of 7.8–8.4 was observed in the segmented region, while caudally, in the segmental plate, the pH was consistently lower falling by as much as 0.5 pH units at the regressing primitive streak. Variations were noted in the pH of embryos of the same age but the regional variation in pH was a consistent finding in all of the embryos examined. The buffering capacity of the extracellular space was found to be 12.9 mequiv/pH unit/1 in the segmented region and 13.9 mequiv/pH unit/1 in the segmental plate. Thus it is unlikely that the regional variations in pH result from local variations in the buffering power of the extracellular space. Varying the K⁺, Cl^-, Mg²⁺ or HCO ₃ ^- ion concentrations in the bathing medium caused little change in the intra-embryonic pH, while reducing the concentrations of Na⁺ or Ca²⁺ caused a small acidification. This suggests that the ectoderm and endoderm form an effective barrier between the embryo and the external environment. Exposure of the embryo to KCN reduced the intra-embryonic pH suggesting that the alkaline environment is maintained by active processes.     

Intralobular lymphatic vessels and their relationship to blood vessels in the mouse thymus

Summary The spatial distribution and fine structure of the lymphatic vessels within the thymic lobules of normal and hydrocortisone-injected mice were studied by light- and electron microscopy. The lymphatic vessels of the cortex and medulla of normal thymus are irregularly shaped spaces closely associated with branches of the intralobular artery and vein. The overall distribution of these vessels in the greatly involuted thymus of hydrocortisone-treated mice is essentially the same as in the normal thymus. The wall of the lymphatic vessels consists of only a layer of endothelial cells supported by underlying reticular cells. The luminal surface of the endothelial cell is smooth, but trabecular processes are often seen. There are three morphological types of intercellular contacts between contiguous cells, namely, end-to-end, overlapping and interdigitating. The lymphatic vessel has anchoring filaments and collagen fibrils, but a basal lamina is either absent, or if present, is discontinuous. This is in contrast to the continuous basal lamina of the venule. The perivascular space surrounding the postcapillary venule opens into a terminal lymphatic vessel at the cortico-medullary junction and in the medulla. Lymphocytes are seen penetrating the lymphatic endothelium, particularly in acutely involuted thymuses. These findings suggest that the intralobular lymphatic vessels may originate from the vacuities that surround the postcapillary venules, and the lymphatic system may function as a pathway for the migration of lymphocytes into or out of the lymphatic circulation.     

The formation of intercellular spaces in the cotyledons of developing and germinating pea seeds

Summary Electron microscopic observations have shown that the intercellular spaces in the storage parenchyma of the cotyledons of pea (Pisum sativum L.) seeds arise schizogenously. The wall segments adjoining future intercellular spaces display a triple zonation and contain regions with electron-dense material. Space formation starts at the central point of contact between several cells and spreads then further up to the electron-dense, intra-wall structures. As a result of this the electron-dense material is found again in the corners of intercellular spaces. It is proposed that the intra-wall structures may have an important function in limiting the schizogenous process. The localization of the intercellular spaces is thus predetermined. The amount of electron-dense material in their corners increases considerably during further development of the embryo. During germination wall segments between two intercellular spaces diverge resulting in a fusion of several spaces.     

Internal segmentation of rhizomes of Phragmites australis: protection of the internal aeration system against being flooded

Segmentation of the aeration system and its anatomy was studied in rhizomes of common reed ( Phragmites australis ). Segmentation is achieved by nodal diaphragms which allow the passage of pressurised gas-flow but which also form effective barriers against flooding of the internal space in case of local injury. The pressures required to force water through the diaphragms were measured and compared with the anatomical structure of the diaphragm. The fine hydrophilic stellate parenchyma of the diaphragms was shown to act as a matrix supporting menisci of air–water interfaces and therefore preventing water movement until the 'limit' pressure (measured values ranged from 18 to >40 kPa) was overcome. Although the surfaces of the large stellate parenchyma and sclerenchyma strands covering the diaphragm are hydrophobic these components do little to prevent the ingress of water.     

Distribution of O2 within infected cells of soybean root nodules: a new simulation

Summary A simulation model is presented for the distribution and consumption of O₂ in infected cells of soybean root nodule central tissue. It differs from earlier models in closer adherence to observed structure and embodies new morphometric data about the distribution of > 12,000 mitochondria per cell and about the geometry of the gas-filled intercellular spaces near which the mitochondria are located. The model cell is a rhombic dodecahedron and O₂ enters only through interfaces (totalling 26% of the cell surface) with 24 gas-filled intercellular spaces. These spaces are located at the edges of each rhombic face of the cell, forming an interconnected network over the cell suface. Next, O₂ is distributed through the cytoplasm by a leghaemoglobin-facilitated diffusive process, initially between the mitochondria and amyloplasts in the outer layers of the cell and then between > 6,000 symbiosomes (each containing 6 bacteroids) towards the central nucleus. The symbiosomes and mitochondria consume O₂, but impede its diffusion; all O₂ entering symbiosomes is considered to be consumed there. For the calculations, the cell is considered to consist of 24 structural units, each beneath one of the intercellular spaces, and each is divided into 126 layers, 0.2 m thick, in and through which O₂ is consumed and diffused. Rates of consumption of O₂ and of N₂ fixation in each diffusion layer were calculated from previously-established kinetics of respiration by mitochondria and bacteroids isolated from soybean nodules and from established relationships between bacteroid respiration and N₂ fixation. The effects of varying the O₂-supply concentration and the concentration and type of energy-yielding substrates were included in the simulations. When the model cell was supplied with 0.5 mM malate, mitochondria accounted for a minimum of 50% of the respiration of the model cell and this percentage increased with increased concentration of the O₂ supply. Gradients of concentrations of free O₂ dissolved in the cytoplasm were steepest near the cell surface and in this location respiration by mitochondria appeared to exert a marked protective effect for nitrogen fixation in layers deeper within the cell. Estimates of N₂ fixation per nodule, calculated from the model cell, were similar to those calculated from field measurements.Abbreviations Lb leghaemoglobin - LbO₂ oxyleghaemoglobin - [O₂] concentration of free, dissolved O₂ - e.m. electron micrograph Dedicated to the memory of Professor John G. Torrey     

Dimensions and distribution of intercellular spaces in cryo-planed soybean nodules

The ability of legume nodules to regulate their permeability to gas diffusion has been attributed to physiological control over the size and distribution of gas-filed intercellular spaces within the nodule cortex. To examine the size and distribution of intercellular spaces and to determine whether they were filled with gas (high diffusion permeability) or liquid (low diffusion permeability), whole nodules were frozen in liquid nitrogen slush (-210°C), and then either cryo-fractured or cryo-planed before being examined by cold-stage scanning electron microscopy (SEM). The cryo-planed tissue was found to have many advantages over cryo-fractured nodules in providing images which were easier to interpret and quantify. Intercellular spaces throughout the nodule were examined in both tangential and medial planed faces. Since no differences were observed between views in either the size or shape of the open intercellular spaces, it was concluded that the intercellular spaces of nodules were not radially oriented as assumed in many mathematical models of gas diffusion. The inner cortex region in the nodules had the smallest intercellular spaces compared to other zones, and less than 10% of the intercellular spaces were occluded with any type of material in the central zone regions. Vacuum infiltration of nodules with salt solutions and subsequent cryo-planing for SEM examination showed that open and water-filled intercellular spaces could be differentiated. The potential is discussed for using this method to study the mechanism of diffusion barrier regulation in legume nodules.     

The fate of apoplastic sucrose in sink and source leaves of Urtica dioica

Experiments were performed with developing and mature leaves of Urtica dioica L. to trace differences which could be interpreted in terms of cell wall-bound acid invertase (EC 3.2.1.26) participating in phloem unloading in a sink leaf. The pH of apoplastic fluid that was collected by gentle centrifugation of entire leaves was identical (7.1) in the two types of leaves; also, fluorometric determination with esculetin showed a neutral apoplastic pH between 7.0 in the source and 7.2 in the sink leaf. To detect whether differences in apoplastic pH occur within limited leaf areas, such as of the tissue surrounding the veins, the metabolic fate of [¹⁴C]–(fructosyl)-sucrose that was administered via the xylem was investigated. In source leaves, there was a large transitory decrease in [¹⁴C]-sucrose followed by a substantial resynthesis of this compound. In sink leaves, resynthesis was less significant and carbon was incorporated mainly in starch, charged soluble compounds and cell walls. However, after correction for resynthesis, the two types of leaves showed an identical capacity for sucrose cleavage. Finally, activation of the apoplastic invertase by administering labelled sucrose in buffered solution of pH 5.0 did not result in an enhanced degradation. By contrast, apoplastic fluid collected from leaves which had been infiltrated with buffer solutions of pH 5.5 and 8.0, respectively, showed a rapid adjustment of the pH close to the natural neutral value by the mesophyll tissue. The results are incompatible with the idea of an active invertase in the sink (and the source) leaves apoplast, and hence do not lend support to the theory of apoplastic cleavage of sucrose being required for phloem unloading in this kind of a utilization sink.     

基于DTTD-MCR-PLUS模型的三生空间格局优化——以长沙市为例

优化三生空间格局有助于实现区域国土空间可持续高质量发展。研究提出利用动态交通时间数据优化最小阻力成本模型指标体系，对生态累计最小阻力与生活最小累计阻力差值进行聚类分析，利用突变点对各情景下的三生空间数量进行优化，从生活扩张、生态优先及基于资源与环境承载力视角下的粮食安全3个方面，提出了一种基于DTTD-MCR-PLUS的三生空间格局优化方法。研究发现：1) 基于动态数据优化的长沙市生态功能优化分区结果显示生态空间保护核心区面积为4111.41 km2，生态空间保护边缘区面积为2285.29 km2，生产空间开发重点区面积为2144.79 km2，生产空间开发边缘区面积为1928.59 km2，生活空间扩张集中区面积为1235.55 km2； 2) 耦合DTTD-MCR-PLUS模型模拟的多情景结果表明：生活优先情景下，生活空间面积增幅高达43.57%，主要分布在望城区南部，长沙县西部和雨花区东部；生态优先情景下，生态空间转出速度最低，与生活优先情景相比下降了3.11%；粮食安全情景下，生产空间侵占生态空间速度加快，增幅高达58.79%；3) 协调基本农田、生态保护红线、以及自然保护区下的2030 年长沙市三生空间格局优化布局方案结果表明：生产空间、生活空间和生态空间比例分别为37.63%、7.67%和54.70%。     

Vegetation cover change in growing urban agglomerations in Chile

Cities in Latin America expose high rates of urbanization and poorly controlled processes of creation of new urban peripheries. In this study we evaluated the changes in vegetation cover as a proxy of the success of urban planning in the creation or conservation of elements able to provide ecosystem services to citizens and therefore strengthening urban sustainability. Three urban agglomerations in Chile located in different climates were analysed. Four indicators were processed to understand the changes and correlations between vegetation and urban dynamics: normalized difference vegetation index (NDVI), vegetation cover, normalized difference built-up index (NDBI) and built-up area. The indicators were calculated for a period over 20 years covering two parts of the city as an urban development: the urban core and the new peripheries. An overall loss of vegetation was observed in all cities has a consequence of urban expansion despite their geographical location. Moreover, the greatest losses were in new peripheries. Santiago broke this pattern of change. First its urban core showed a small increase in indicators for vegetation cover despite the increase in indicators for urban dynamics. Secondly, despite their peripheries experiencing a decrease in vegetation cover, a more detailed analysis found differences on the northern and eastern peripheries where increases of vegetation cover were observed, and other new peripheries where vegetation loss was massive. Urban planning needs to play a role not only to facilitate the creation of green spaces or other public spaces able to host vegetation, but also to form an urban structure supported by regulations that facilitate the planting and maintenance of vegetation in private spaces.