Essay: Embryological Development: The Formation and Anatomy of the Kidney



Embryological Development

The development of the kidney begins in the 4th  week of gestation by inductive interaction between the ureteric bud and the metanephric blastema (Babu et al, 2015).

Ureteric bud arising from the mesonephric duct gives rise to the collecting tubules and the pelvicalyceal system and the metanephros develop into excretory part formed by nephrons (Babu et al, 2015).

Initially the developing kidney lies in the sacral region and acquires blood supply from the neighboring vessels (Babu et al, 2015).

During 6th  to 9th  weeks of development, the lobulated kidney undergoes a complex process of ascent, probably due to differential growth of abdominal and pelvic regions, to reach its adult position in the upper part of posterior abdominal wall (Babu et al, 2015).

During its ascent, the kidney successively receives arterial supply from median sacral, internal iliac, common iliac and dorsal aorta. These arteries represent lateral splanchnic branches and as the migrating kidney acquires new arteries cranially, the caudally placed arteries undergo degeneration (Babu et al, 2015).

When the kidney reaches the renal fossa it undergoes 90° axial rotation such that the hilum which was initially anteriorly placed becomes medial  (Babu et al, 2015).

Renal gross anatomy

The kidneys are paired retroperitoneal structures that are normally located between the transverse processes of D12­L3 vertebrae, with the left kidney typically somewhat more superior in position than the right. The upper poles are normally oriented more medially and posteriorly than the lower poles (Cheuck et al., 2013).

The kidneys serve important functions, including filtration and excretion of metabolic waste products (urea and ammonium); regulation of necessary electrolytes, fluid, and acid­base balance; and stimulation of red blood cell production (Cheuck et al., 2013).

They also serve to regulate blood pressure via the renin­angiotensin­aldosterone system, controlling reabsorption of water and maintaining intravascular volume. The kidneys also reabsorb glucose and amino acids and have hormonal functions via erythropoietin, calcitriol, and vitamin D activation  (Cheuck et al., 2013).

The renal gross and CT anatomy is shown in the figures below (1.1 & 1.2 & 1.3 & 1.4)

    Figure 1.1   Surface anatomy of kidneys (Quoted from Cheuck et al., 2013)

     Figure 1.2  gross renal anatomy (Quoted from Kung, 2015)

Figure 1.3 : gross renal anatomy (Quoted from Kung, 2015)

Figure 1.4 : Normal axial CT anatomy of kidneys (L=liver & GB = gallbladder & D= duodenum & P= pancreas & A= aorta &RK = right kidney & LK = left kidney & V= vertebral body & SB = small bowel & S= spleen) (quoted from Quaia et al., 2011).

Grossly, the kidneys are bean shaped structures and weigh about 150 g in the male and about 135 g in the female. They are typically 10­12 cm in length, 5­7 cm in width, and 2­3 cm in thickness (Bajpai et al., 2014).

The relationship of neighboring organs to the kidneys is important, as described below:

Superiorly, the suprarenal (adrenal) glands sit adjacent to the upper pole of each kidney

On the right side, the second part of the duodenum (descending portion) abuts the medial aspect of the kidney

On the left side, the greater curvature of the stomach can drape over the superomedial aspect of the kidney, and the tail of the pancreas may extend to overlie the renal hilum

The spleen is located anterior to the upper pole and is connected by the splenorenal  (lienorenal)  ligament

Inferiorly to these organs, the colon typically rests anteriorly to the kidneys on both sides

Posteriorly, the diaphragm covers the upper third of each kidney, with the 12th  rib most commonly crossing the upper pole

The kidneys sit over the psoas (medially) and the quadratus lumborum muscles (laterally) ( Cheuck et al., 2013).

Renal lymphatics

The lymphatic drainage parallels the venous drainage system. After leaving the renal hilum, the left primary lymphatic drainage is into the left lateral aortic lymph nodes, including nodes anterior and posterior to the aorta between the inferior mesenteric artery and the diaphragm. On the right, it drains into the right lateral caval lymph nodes (Cheuck et al., 2013).

Collecting system (figure 1.5)

Once the filtrate gets to the collecting ducts in the medulla of the kidney, they converge to a renal papilla, which represents the tip or apex of the renal pyramid. Urine then collects in typically 9­12 minor calyces, which then converge into 3­4 major calyces (significant variation is possible) ( Cheuck et al., 2013).

The major calyces then empty into the renal pelvis, which passes urine through the UPJ and into the ureter, which then propels urine distally to the bladder through peristalsis ( Cheuck et al., 2013).

   Figure 1.5 : Anatomy of collecting system (Quoted from Kung, 2015)

The ureter may course posterior to the renal artery (or a lower pole branch) at its superior point, cross over the psoas muscle, and then pass posteriorly to the gonadal vessels. As it proceeds further distally, it passes over the iliac vessels and into the pelvis, finally traversing an intramural tunnel into the bladder and ending at the ureteral orifice on the trigone of the bladder ( Cheuck et al., 2013).

Renal nerve anatomy/autonomic innervation

The kidney receives autonomic supply via both the sympathetic and parasympathetic portions of the nervous system. The pre-ganglionic sympathetic nervous innervation to the kidneys arises from the spinal cord at the level of D8­L1. They synapse onto the celiac and aortic-renal ganglia and follow the plexus of nerves that run with the arteries. Activation of the sympathetic system causes vasoconstriction of the renal vessels ( Cheuck et al., 2013).

Parasympathetic innervation arises from the 10th cranial nerve (X), the vagus nerve, and causes vasodilatation when stimulated  ( Cheuck et al., 2013).

CT anatomy and classification of normal renal variants:

1 ) Horseshoe kidney: consists of two distinct renal masses lying

vertically on either side of the midline and fused in their lower

poles by an isthmus parenchymal or fibrous tissue that crosses the

median plane of the body (Gutiérrez et al., 2013). (figure 1.6& 1.7 & 1.8)

3 ) Crossed renal ectopia: one or both kidneys cross the midline,

reaching opposite the corresponding VUJ. It may be unilateral or bilateral and can also occur with or without fusion contralateral kidney ( figure 1.9) (Gutiérrez et al., 2013).

2 ) Simple renal ectopia  or ectopic kidney  ipsilateral: a kidney that is on the same side of its corresponding VUJ, but in an abnormal position (outside flank, L1- L3) . It may be unilateral or bilateral (figure 1.10) (Gutiérrez et al., 2013).

Embryology and pathophysiology of renal variants

Congenital renal anomalies in the position and in the renal fusion are the result of impaired cephalic migration from the pelvis to the flank of the ureteric bud and metanephric blastema, a process that

begins in the 5th week of gestation and ends in the 9th  week (Gutiérrez et al., 2013).

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   Figure 1.6  : Horseshoe kidney (Quoted from Cheuck et al., 2013)

Figure 1.7: Axial MDCT images show horseshoe kidneys. A, aorta (Quoted from Türkvatan et al., – C- , 2009)

Some of these variations may be associated with pathological conditions, such as hydronephrosis and UPJ obstruction, they can also remain completely asymptomatic and undiscovered until a diagnosis is made by radiographic study ( Cheuck et al., 2013).

Figure 1.8 a, b.Coronal VR (a) and MIP (b) MDCT images show the arterial supply of the horseshoe kidney consisting of two main renal arteries arising from the aorta with the addition of one accessory LRA arising from the right iliac artery. The RRA has prehilar branching. The isthmus receives a branch from  the RRA. A stone in the left kidney is also seen (b) (Quoted from Türkvatan et al., – C- ,  2009)

 

Figure 1. 9  Right-left crossed fused ectopia   (A) Oblique coronal MPR, excretory phase. Note the fusion of the upper pole of the ectopic kidney with orthotopic kidney lower pole (white arrows). (B) Oblique volumetric CT Reconstruction shows the full right collecting system and left collecting system, (Note the independence of the two collecting systems) (Quoted from Gutiérrez et al., 2013).

Importantly, in an ectopic kidney (figure 1. 10) , the adrenals should still be in the superior portion of the posterior peritoneum, since their embryologic origin is different from that of the kidneys ( Cheuck et al., 2013).

Figure 1. 10. Bilateral ectopic kidney. a woman of 46 years. (A) and (B) coronal CT volumetric reconstruction, excretory phase. Empty right renal fossa, ectopic right kidney iliac or lumbar and abdominal ectopic left kidney. Note the right renal axis vertical orientation and shorter right ureter and altering the rotation of both kidneys ( Quoted from Gutiérrez et al., 2013).

Variants may also exist in the collecting system drainage. Duplication anomalies may develop, wherein more than a single collecting system may form and drain separately into the bladder (complete duplication) or join at some point proximally before draining into a single orifice into the urinary bladder (partial duplication) (Doery et al., 2015).

In a complete duplicated system, the upper pole moiety drains inferomedially into the bladder, and the lower pole moiety drains superolaterally, as described by the Weigert­Meyer rule (Doery et al., 2015).