טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentGrinstein Mor
SubjectRegulation of Nephron Patterning in the Chick Mesonephros
DepartmentDepartment of Medicine
Supervisor Professor Thomas Schultheiss
Full Thesis textFull thesis text - English Version


Abstract

The kidney is a vital organ whose main functions include excretion of metabolic waste products and maintenance of water balance. The basic functional unit of the kidney is the nephron. Each nephron consists of a filtering component, the glomerulus, and a tubule extending from the glomerulus. This tubule is connected to the nephric duct, which transmits the contents of the tubule to the outside drainage system. For proper kidney functioning, the differentiation and coordination between its cells are essential. Even small changes in the cells’ arrangement and interactions can have severe pathological implications. 

During amniote development three kidneys are sequentially formed--the pronephros, mesonephros, and metanephros. Most studies of kidney formation have been conducted using the mammalian metanephros.  However, the mesonephros has a simpler structure than the metanephros, and thus serves as a better model for studying certain aspects of kidney development.  In the chick embryo, the early, linear development of the mesonephric kidney simplifies the experimental process, allowing observation with a higher resolution on the cellular level and enables manipulations.

Using the chick mesonephric system, I found that mesonephric gene expression is remarkably similar to the metanephros. I investigated the proximal-distal patterning of the mesonephric nephron during nephrogenesis.  I focused on how the two major nephron segments are specified: the glomerulus, and its main cells, the podocytes that act as a filtration barrier between the internal milieu and the outside world; and the tubules that transfer the filtrate into the collecting duct. I found several lines of evidence that pre-podocyte and pre-tubule cells are separate in early developmental stages. Well before the first morphological or molecular signs of nephron formation, mesonephric mesenchyme can be separated into dorsal and ventral components based on morphology and expression of the transcription factor Pod1, which later is expressed in the podocytes. In explant culture, these dorsal and ventral populations can independently differentiate along tubule and podocyte pathways, respectively. Second, in embryos in which the tubule-inducing activity of the nephric duct is blocked, tubule formation is completely inhibited, but some podocyte differentiation still occurs. I also investigated the molecular regulation of nephron segmentation, and found that canonical Wnt signals (known inducers of nephron formation), which are found in the nephric duct adjacent to the dorsal mesonephric mesenchyme, strongly inhibit podocyte but not tubule differentiation, suggesting that Wnt signaling plays an important role in the segmentation of the mesonephric mesenchyme into tubular and glomerular segments.