Chapter 20 With the exception of the epithelial lining of the bladder and urethra, which are of endodermal origin, the urinary system of vertebrates develops from intermediate mesoderm. The urinary system has a number of important functions which include elimination of metabolic waste products by filtration and excretion, regulation of electrolyte levels in the body and reabsorption of water and low molecular weight molecules, all of which are essential aspects of homeostasis. In addition, the kidney, through the production of the enzyme renin, has a role in the regulation of blood pressure. An important endocrine function of the kidney is the production of erythropoietin in the renal cortex, which has a regulatory role in erythrocyte production by cells of the bone marrow. The primordial kidney consists of tubular units, nephrons, which function by selective filtration, reabsorption and finally excretion of waste products. As mammalian evolution progressed, functional kidney units developed from primitive structures to highly complex, efficient filtration units. Developing vertebrate nephric tubules exhibit increased complexity, as those which form in the cervical region are sequentially replaced in the thoraco‐lumbar and sacral regions by more functionally competent structures. These structures are referred to as the pronephros, mesonephros and metanephros respectively. As the more caudal structures develop and become functional, the pronephric and mesonephric tubules atrophy and the metanephros persists as the definitive functioning kidney. While these three structures are no longer considered as distinct successive functional kidneys but rather as three successive morphological manifestations of a single excretory organ, the holonephros, the terms pronephros, mesonephros and metanephros have been retained solely for descriptive purposes. The evolutionary development of the kidney is illustrated by the increasing refinement of renal structure and function evident in vertebrate animals. Lower vertebrates have relatively primitive kidneys in comparison with higher vertebrates. In fish and amphibians, the mesonephros, which replaces the pronephros, becomes the functional kidney. In reptiles, birds and mammals, the definitive kidney is formed by the metanephros, an additional structure which succeeds the pronephros and mesonephros, both of which atrophy. 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. At the level of each somite, cords of cells referred to as nephrotomes, which grow out from the dorsal (parietal) wall of the intermediate mesoderm, later form pronephric tubules (Fig 20.1). The distal end of each tubule proliferates and extends initially in a lateral direction and then caudally, before fusing with the corresponding proliferating cells of the tubule developing immediately caudal to it. The primordium of the excretory pronephric duct arises from fusion of the distal ends of each tubule. The pronephric duct grows towards the cloaca and becomes canalised. As more caudal pronephric tubules develop, they open into the primordial pronephric duct. Figure 20.1 Cross‐section, at the level indicated, through an early embryo, A, and an embryo at a later stage of development, B, showing formation of a pronephric duct and an internal and external glomerulus. The lumen of each pronephric tubule becomes continuous with the nephrocoele which opens into the coelomic cavity through an aperture termed a nephrostome. Branches from the dorsal aorta form tufts of capillaries, 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 (Fig 20.1). The term ‘Bowman’s capsule’ is used to describe the invaginated epithelium surrounding each glomerulus. Formation of external glomeruli, a feature of lower vertebrates, results in a filtration arrangement which is less efficient than internal glomerular filtration, as the filtrate has to be propelled from the coelomic cavity to the pronephric tubule by the ciliary action of cells surrounding the nephrostome. With the formation of internal glomeruli, a feature of higher vertebrates, the connection between the pronephric tubules and the coelomic cavity is lost. Water and some electrolytes are reabsorbed from the pronephric tubules and waste products are conveyed to the cloaca. In placental mammals, these waste products are transported from the foetus to the placenta for excretion by the dam. 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. Lateral to the urinary ridge, the pronephric ducts, which extend caudally towards the cloaca, induce the mesonephric tissue to form S‐shaped tubules within the urinary ridge (Fig 20.2). 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. 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 (Fig 20.3). With the development of the mesonephric system, the pronephric tubules and the cranial portion of the pronephric duct atrophy (Figs 20.4 and 20.5). Figure 20.2 Cross‐sections through an embryo showing successive stages in the formation of a mesonephric tubule and paramesonephric duct. Figure 20.3 Cross‐sections through embryos at the levels indicated, showing the formation of a mesonephric tubule and duct (A to C). Figure 20.4 Stages in the formation of the pronephros, mesonephros and metanephros and their relationships to other developing structures (A to E). Figure 20.5 Dorsal views of the developing pronephros, mesonephros and metanephros (A and B). The development of a peritubular capillary network around the mesonephric tubules assists in the reabsorption of water and electrolytes. In contrast to the structure of the pronephros, where only one tubule develops at the level of each somite, in the mesonephros multiple tubules may develop at the level of each somite. The developing left mesonephros and right mesonephros project into the abdominal cavity as distinct anatomical structures in the developing embryo and are especially prominent in porcine embryos up to 35 days of gestation (Fig 20.4
Urinary system
Kidney
Pronephros
Mesonephros
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
