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Development of urinary system

Dr. Archana

Introduction

• The urogenital system can be functionally divided into two entirely different components, the urinary system and the genital system , but the embryonic development of the two are intimately interwoven.
• Both are derived from the nonsegmented intermediate mesoderm , which is also referred to as the nephrogenic plate, and the neighbouring mesodermal coelomic epithelium.

Please focus the differentiation of mesoderm

to paraxial, intermediate and lateral mesoderm

Pronephros

• During the early developmental period, when somites are present, cells of the intermediate mesoderm in the cervical region separate into an outer parietal layer and an inner visceral layer, forming a cavity, the nephrocoele, between the two layers.

Contd…

• The pronephric duct grows towards the cloaca and becomes canalised.
• As more caudal pronephric tubules develop, they open into the primordial pronephric duct.
• The lumen of each pronephric tubule becomes continuous with the nephrocoele which opens into the coelomic cavity through an aperture termed as nephrostome.

Contd…

• Branches from the dorsal aorta form tufts of capillaries called glomeruli, which may invaginate either into the coelomic epithelium, or alternatively into the wall of each pronephric tubule.
• Glomeruli which invaginate into the coelomic epithelium are referred to as external glomeruli ; those which invaginate into the tubular wall are termed internal glomeruli.
• Formation of external glomeruli, a feature of lower vertebrates, results in a filtration arrangement which is less efficient than internal glomerular filtration.

Mesonephros

• Towards the end of the post-somite stage of development, a column of tissue referred to as the urogenital ridge develops from proliferating intermediate mesoderm in the thoraco-lumbar region and projects into the coelomic (peritoneal) cavity.
• Later, this structure divides into a medial genital ridge and a lateral urinary ridge.

Focus the medial genital ridge and lateral urinary ridge

• Invagination of the medial end of each mesonephric tubule by a glomerular tuft induces the formation of Bowman’s capsule by the mesonephric tubule epithelium.
• The combination of Bowman’s capsule and the glomerular tuft forms a filtration unit known as a renal corpuscle.

Contd…

• The lateral end of each mesonephric tubule enters separately into the pre-existing pronephric duct, which at this stage is referred to as the mesonephric duct.
• With the development of the mesonephric system, the pronephric tubules and the cranial portion of the pronephric duct atrophy.

• The ureteric bud extends cranially towards the metanephric blastema and dilates at its cranial end where it becomes almost completely surrounded by metanephric tissue.

• The dilated portion of the ureteric bud gives rise to ureter, the pelvis and the collecting ducts of the definitive kidney.
• The formation of the collecting ducts induces the metanephric tissue to give rise to metanephric tubules.

• Accordingly, the anatomical form of mammalian kidneys ranges from unilobar to distinct multilobar structures, with intermediate gradations of partial and complete lobar fusion.

Points to remember....