University:
Kentucky State University
Course:
BIO 111 | Principles of Biology
Academic year:

2024
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368

Pages:

17
Author:

elongeaborblid

Chapter 18 Intermediate mesoderm: The kidney Intermediate mesoderm: the kidney • The mammalian kidney development progresses through three major stages. • The first and the second stages are transient, while the third stage persists as a functional kidney. • Mesonephros: sometimes called the mesonephric kidney, a set of kidney tubules that is induced in the adjacent mesenchyme by the middle portion of the nephric (Wolffian) duct. Intermediate mesoderm: the kidney Functions of mesonephros during human development: • It is one of the main sources of the hematopoietic stem cells necessary for blood cell development. • In male mammals, some of mesonephric tubules persist to become the tubules that transport the sperm from the testes to the urethra (epididymis and vas deferens). Intermediate mesoderm: the kidney To produce the metanephros, the permanent kidney of amniotes, complex set of interactions (reciprocal interactions) occurs between the epithelial and mesenchymal components of the intermediate mesoderm. Reciprocal interactions of developing kidney tissues • The kidney forms two different progenitor cell populations derived from the intermediate mesenchyme: the ureteric bud and the metanephric mesenchyme. • The mesenchyme derived tubules form the mature nephrons of the functioning kidney. • The branched ureteric bud gives rise to the collecting ducts and to the ureters which drain urine from the kidney into the urinary bladder. Mechanisms of reciprocal induction Step 1: Formation of metanephric mesenchyme and the ureteric bud. • The metanephric mesenchyme and the ureteric bud are similar because both come from the intermediate mesoderm, and both are generated through the action of Wnt and FGF signaling pathways. Mechanisms of reciprocal induction Step 1: Formation of metanephric mesenchyme and the ureteric bud. • The ability of Wnt and FGF signals to generate the ureteric bud epithelium or metanephric mesenchyme has been shown using human induced pluripotential cells (human iPS). Mechanisms of reciprocal induction Step 2: the metanephric mesenchyme secrete GDNF to induce and direct the ureteric bud But how the glial-derived neurotrophic factor (GDNF) causes the outgrowth of the ureteric bud? • GDNF may be locally suppress activin, which suppresses proliferation in nephric duct regions, and therefore GDNF induces the formation of ureteric bud. Wild type mouse kidney Retinoic acid Ret receptors Out growth of ureteric bud Gdnf +/- mouse Gdnf -/- mouse Mechanisms of reciprocal induction Step 3: the ureteric bud secretes Fgf2 and BMP7 to prevent mesenchymal apoptosis. • The factors secreted from the ureteric bud to the metanephric mesenchyme include Fgf2, Fgf9 and BMP7. • FGFs and BMP7 have three modes of actions in that they: 1. Inhibit apoptosis. 2. Promote the condensation of mesenchyme cells. 3. Maintain the synthesis of WT1, a transcription factor necessary for bud outgrowth. Mechanisms of reciprocal induction Step 4: Signals from the mesenchyme induce the branching of the ureteric bud. • Several paracrine factors including GDNF, Wnts, FGFs and BMPs have been implicated in the branching of the ureteric bud. • These factors probably work as “pushes” and “pulls”. • GDNF not only induce the formation of the initial ureteric bud from the nephric duct, but can also induce secondary buds from the ureteric bud once the bud enters the mesenchyme. Mechanisms of reciprocal induction Step 4: Signals from the mesenchyme induce the branching of the ureteric bud. • GDNF also appears to induce Wnt11 synthesis in the responsive cells at the tip of the bud. • The cooperation between the GDNF/ Ret pathway and the Wnt pathway appears to coordinate the balance between branching and metanephric mesenchyme proliferation such that continued kidney development is insured. Mechanisms of reciprocal induction Step 5: Wnt signals convert the aggregated mesenchyme cells into a nephron • Wnt9b induces the metanephric mesenchyme to condense. • Both Wnt9b and Wnt6, which have receptors on the mesenchyme cells, are important for transforming the mesenchyme cells into tubular epithelium. • Wnt4 is induced in the mesenchyme cells by the Wnt9b and Wnt6. • Wnt4 acts in an autocrine fashion to complete the transformation of the mesenchymal cells into tubular epithelium. Mechanisms of reciprocal induction Step 5: Wnt signals convert the aggregated mesenchyme cells into a nephron • The tubular epithelium hollows to form the neural vesicle and becomes polarized in a proximal to distal direction. Mechanisms of reciprocal induction Step 6: Inserting the ureter into the bladder • The branching ureteric epithelium becomes the collecting duct. • The original stalk of the ureteric bud, situated above the first branch point, becomes the ureter which is the tube that carries urine into the bladder. • Hydronephrosis: is a birth defect leading to abnormalities of renal filtration, occurs when the junction between the ureter and bladder is not properly placed and urine cannot enter the bladder. Mechanisms of reciprocal induction Step 6: Inserting the ureter into the bladder • The ureter is made into a watertight connecting duct by the condensation of mesenchyme cells around it. • The mesenchymal cells around the ureter secrete BMP4, which upregulates the expression of the genes that encode for uroplakin. • Uroplakin is a cell membrane protein that causes differentiation of the ureteric bud region that gives rise to the ureter. • The urinary bladder develops from a portion of the cloaca. • Cloaca: is an endodermally lined chamber at the caudal end of the embryo that will become the waste receptacle for both the intestine and the kidney. • In mammals, the cloaca becomes divided by a septum into the urogenital sinus and the rectum. • The space between the rectal opening and the urinary opening is the perineum. • After the ureteric bud outgrowth (on day 11), both the ureter and nephric duct empty in the bladder. • In females, the entire nephric duct degenerates, while in males the nephric duct forms the sperm outflow track (epididymis and vas deferens).

Chapter 18 Intermediate Mesoderm The Kidney

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