Introduction 
The expansion of urban space around the world has become a challenging issue in recent decades. To develop effective strategies to curb urban growth, understanding the forces and driving mechanisms of urban sprawl has particular importance. The phenomenon of urban-regional sprawl as one of the spatial changes is related to the pattern of sprawl of cities in metropolitan areas. Therefore, understanding the dynamics of urban-regional sprawl is vital, since, the spatial growth and expansion of cities and metropolitan areas have not only caused the increasing transformation of agricultural land into cities but also increased the demand for new natural resources that are very vital for the global environment. According to the studies conducted regarding the spatial growth of the metropolitan area, we are witnessing unusual growth of the metropolitan area and its discrete spatial development. The growth that has gone through various stages in the last 50 years and its consequences have been spatially manifested in the form of the formation of the urban area, the fragmentation of the suburbs, and the decline of the main metropolitan. Based on this, in this research, the organizational-institutional structures and mechanisms driving growth and spatial expansion of the Tehran metropolitan region have been measured and evaluated.
Methods and Materials
Based on the background study as well as the nature of the subject, an attempt was made to use mixed methods (a combination of quantitative and qualitative data) to understand the effectiveness of the mechanisms and to understand the causes of the formation of the spatial structures of the region’s development. Qualitative coding of the data obtained from the interview with 18 university professors, city managers, and experts of related organizations and institutions made it possible to identify the main key categories and the components that form the organizational-institutional core categories that affect the growth of the metropolitan area of Tehran. Then, based on the results of these codings, a questionnaire consisting of 52 key questions was prepared with measuring the effectiveness of the mechanism of organizational-institutional structures (separated by 3 departments of government and official organizations and institutions, local institutions, and administrations and semi-official institutions) with calculating the weighted average of the core categories and components that form the mechanisms and ranking these categories and components. Measuring the influence of each of these factors was done in the form of a Likert scale. By examining the results of the questionnaires and calculating the weighted average value, the impact mechanism of each of the central categories and their forming concepts was determined, and in total, the weighted average rating of the assigned points determined which central categories and their forming components won the most privilege and the highest share in the increasing external growth of the metropolitan area of Tehran.
Findings
Local institutions and administrations with an average score of 4.25 among organizational-institutional structures have the greatest impact on the exponential external growth and expansion of the region, and quasi-governmental and semi-official organizations and institutions and governmental and official ones are ranked second and third with an average score of 4.17 and 3.96 respectively. Among the effective mechanisms, the monitoring gap mechanism of local institutions and administrations has the largest effect with a weighted mean of 4.46. and the components of the monitoring gap resulting from the weakness of political divisions among the mechanisms of local institutions and administrations and the lack of effective monitoring tools and the absence of a single supervisory authority in dealing with informal processes among the mechanisms of government and official organizations and institutions have the greatest impact on land acquisition in the region with a weighted mean of 4.50 and 4.42 respectively. 
Conclusion
According to the obtained results, local institutions and administrations as a whole have the greatest influence on land acquisition in the region through the mechanisms of weak management performance, monitoring gaps, and informal processes. While quasi-governmental and semi-official organizations and institutions and governmental and official organizations and institutions of the region are in the second and third ranks with an average score of 4.17 and 3.96, respectively. In fact, by moving towards the periphery and around the metropolis of Tehran and other cities and villages of the province due to fundamental structures and reasons such as the lack of a regionalism approach in structures that lack comprehensiveness and the dominant nature of metropolitan policymaking area and legal structures such as insufficient laws in the sanctuary and outside from privacy, the interactions in these areas are not taken into consideration as they should be. Therefore, the monitoring gap formed due to the power holes in these areas has provided a suitable platform for infiltrating and intensifying the common administrative corruption among some employees and officials of local institutions and offices in these areas. Obviously, this monitoring gap will increase by moving away from the central areas and moving towards the smaller levels of political divisions in the region.