1 Embryology of the Kidney Rizaldy Paz Scott | Yoshiro Maezawa | Jordan Kreidberg | Susan E. Quaggin CHAPTER OUTLINE MAMMALIAN KIDNEY DEVELOPMENT, 2 MOLECULAR GENETICS OF MODEL SYSTEMS TO STUDY KIDNEY NEPHROGENESIS, 22 DEVELOPMENT, 8 GENETIC ANALYSIS OF MAMMALIAN KIDNEY DEVELOPMENT, 15 KEY POINTS • The development of the kidney relies on reciprocal signaling and inductive interactions between neighboring cells. • Epithelial cells that comprise the tubular structures of the kidney are derived from two distinct cell lineages: the ureteric epithelia lineage that branches and gives rise to collecting ducts and the nephrogenic mesenchyme lineage that undergoes mesenchyme to epithelial transition to form connecting tubules, distal tubules, the loop of Henle, proximal tubules, parietal epithelial cells, and podocytes. • Nephrogenesis and nephron endowment requires an epigenetically regulated balance between nephron progenitor self-renewal and epithelial differentiation. • The timing of incorporation of nephron progenitor cells into nascent nephrons predicts their positional identity within the highly patterned mature nephron. • Stromal cells and their derivatives coregulate ureteric branching morphogenesis, nephrogenesis, and vascular development. • Endothelial cells track the development of the ureteric epithelia and establish the renal vasculature through a combination of vasculogenic and angiogenic processes. • Collecting duct epithelia have an inherent plasticity enabling them to switch between principal and intercalated cell identities. MAMMALIAN KIDNEY DEVELOPMENT The filtration function of the kidneys is accomplished by basic units called nephrons (Fig. 1.1). Humans on average have 1 million nephrons per adult kidney but the range of ANATOMIC OVERVIEW OF THE 4 MAMMALIAN KIDNEY total nephrons is highly variable across human populations. Each mouse kidney may contain up to 12,000–16,000 nephrons The kidney is a sophisticated, highly vascularized organ that depending on the strain.5 This wide range in nephron number plays a central role in overall body homeostasis. In humans, is influenced by genetic background, fetal nutrition and the kidneys filter as much as 180 liters of blood per day, environment, and maturity at birth.6,7 Nephron endowment receiving as much as ~20% of the total cardiac output. Renal can be clinically important as markedly reduced nephron filtration of blood removes metabolic waste products (e.g., numbers raises the susceptibility risk to hypertension and urea, ammonia, and by-products of bile from the liver) as chronic kidney disease.1–3,8,9 At the core of the nephron is the urine while concomitantly adjusting the levels of water, renal corpuscle or glomerulus (see Fig. 1.1). The glomerulus electrolytes, and pH of tissue fluids. Additionally, the kidneys consists of a porous and highly convoluted capillary bed regulate blood pressure via the renin-angiotensin-aldosterone composed of highly fenestrated glomerular endothelial cells. system, secrete erythropoietin that stimulates erythrocyte These glomerular capillaries are circumscribed by morpho- production, and contribute to the activation of vitamin D to logically elaborate and interdigitating cells called podocytes. control calcium and phosphate balance. These capillaries are further structurally supported by pericytes 2 CHAPTER 1 — EmbRyology of the KidNEy 3 Fig. 1.1 Anatomic organization of the kidney. (A) Spatial distribution of nephron within the metanephric kidney. Glomeruli, the filtration compartments of the nephrons, are found in the cortex. (B) Segmental structure of nephrons. The vascularized glomerulus is found at the proximal end and is connected through a series of renal tubules where urinary filtrate composition is refined through resorption and secretion. (C) Cellular organization of the glomeruli. AA, Afferent arteriole; BS, Bowman space; CD, Collecting duct; DT, distal tubule; EA, efferent arteriole; GEC, glomerular endothelial cell; LOH, loop of Henle; MC, mesangial cell; PEC, parietal epithelial cell; Pod, podocyte; PT, proximal tubule. Reproduced with permission from Scott RP, Quaggin SE. The cell biology of renal filtation.J Cell Biol. 2015;209:100–210. called mesangial cells. Blood filtration occurs through this capillary tuft, generating primary urine that collects within the Bowman capsule, an enclosure formed by parietal epithelial cells. From the Bowman capsule, urine drains through a series of tubules starting with the proximal tubules, the loop of Henle, the distal tubules, and the collecting ducts. These tubules are responsible for dynamic resorption and secretion processes that help recycle filtered small molecules; they also adjust water, electrolyte, and acid–base balance by fine-tuning the composition of the final urine output before it exits the ureter and is excreted via the bladder. Supporting the main functions of the nephrons are interstitial fibroblasts and a heterogenous network of extraglomerular vasculature. DEVELOPMENT OF THE UROGENITAL SYSTEM The vertebrate kidney derives from the intermediate meso- derm of the urogenital ridge, a structure found along the posterior wall of the abdomen in the developing fetus.10,11 Mammalian kidneys develop in three successive stages, generating three distinct excretory structures known as the Fig. 1.2 Three stages of mammalian kidney development. The pronephros, the mesonephros, and the metanephros (Fig. pronephros and mesonephros develop in a rostral-to-caudal direction 1.2). The pronephros and mesonephros are vestigial structures and the tubules are aligned adjacent to the wolffian or nephric duct in mammals and degenerate before birth; the metanephros (WD). The metanephros develops from an outgrowth of the distal end is the definitive mammalian kidney. The early stages of kidney of the wolffian duct known as the ureteric bud epithelium(UB) and a development are required for the development of the adrenal cluster of cells known as the metanephric mesenchyme (MM). The glands and gonads that also form within the urogenital ridge. pronephros and mesonephros are vestigial structures in mice and Furthermore, many of the signaling pathways and genes that humans and are regressed by the time the metanephros is well play important roles in the metanephric kidney appear to developed. play parallel roles during the development of the pronephros 4 SECTion I — NoRmAl STRuCTure and FuNCTioN and mesonephros. The pronephros consists of pronephric the epithelial ureteric bud (UB) at the caudal end of the tubules and the pronephric duct (also known as the precursor urogenital ridge. The UB is first visible as an outgrowth at to the wolffian duct) and develops from the rostral-most the distal end of the wolffian duct approximately between region of the urogenital ridge at 22 days of gestation in the fourth and fifth week of gestation in humans or E10.5 humans and 8 days post coitum (embryonic stage E8) in in mice. The MM becomes histologically distinct from the mice (Table 1.1). Throughout the rest of this chapter, most surrounding mesenchyme and is found adjacent to the UB. timelines of kidney development are with reference to the Upon invasion of the MM by the UB, signals from the MM mouse. The pronephros serves as the principal excretory cause the UB to branch into a T-tubule (at around E11.5 in organ of the larval stages of fishes and amphibians. The mice) and then to undergo iterative dichotomous branching, mesonephros develops caudal to the pronephric tubules in giving rise to the urinary collecting duct system (Fig. 1.3). the midsection of the urogenital ridge. The mesonephros Simultaneously, the UB sends reciprocal signals to the MM, becomes the functional excretory apparatus in lower verte- which is induced to condense along the surface of the bud. brates (adult fish and amphibians) and may perform a filtering Following condensation, a subset of MM cells aggregates function during embryonic life in mammals. Prior to its adjacent and inferior to the tips of the branching UB. These degeneration, endothelial, peritubular myoid, and steroido- collections of cells, known as pretubular aggregates, undergo genic cells from the mesonephros migrate into the adjacent mesenchymal-to-epithelial conversion to become the renal adrenogonadal primordia, which ultimately form the adrenal vesicle (Fig. 1.4). gland and gonads.12 Abnormal mesonephric migration leads to gonadal dysgenesis, a fact that underscores the intricate URETERIC BRANCHING MORPHOGENESIS association between these organ systems during development The collecting duct system is composed of hundreds of tubules and explains the common association of gonadal and renal through which the filtrate produced by the nephrons is defects in congenital syndromes.13,14 conducted out of the kidney, to the ureter, and then to the bladder. Water and salt resorption and excretion, ammonia DEVELOPMENT OF THE METANEPHROS transport, and H+ ion secretion required for acid–base The metanephros is the third and final stage, representing homeostasis also occur in the collecting ducts, under different the definitive adult kidney of higher vertebrates. It results regulatory mechanisms, and using different transporters and from a series of reciprocal inductive interactions that channels than are active along tubular portions of the occur between the metanephric mesenchyme (MM) and nephron. The collecting ducts are all derived from the original Fig. 1.3 Ureteric branching morphogenesis. Rapid reiterative branching of the UB within a 5-day period in mice