The renal parenchyma is enclosed by a thin but distinct glistening membrane known as the ﬁbrous (true) capsule of the kidney, which extends into the renal sinus. Immediately surrounding the ﬁbrous capsule is a variable amount of perinephric fat (perirenal fat capsule), which forms a matrix around the structures within the renal sinus. The perinephric fat also surrounds the ipsilateral suprarenal gland.
The kidneys, suprarenal glands, and perinephric fat are all contained within a condensed, membranous layer of renal fascia. The renal fascia consists of a stronger posterior and more delicate anterior layer, previously described as two separate structures (posterior fascia of Zuckerkandl and anterior fascia of Gerota) that fused laterally to form the lateral conal fascia. At present, however, the renal fascia is regarded as a single structure.
The posterior layer originates from the lateral aspect of the psoas fascia, fusing variably with the anterior layer of thoracolumbar fascia (quadratus lumborum fascia) and transversalis fascia as it passes posterior and lateral to the kidney. It then wraps around the anterior aspect of the kidneys as the anterior layer. The medial continuation of the anterior layer ensheaths the renal vessels and fuses with the sheaths of the abdominal aorta and inferior vena cava. In some individuals, these fusions are very delicate and may rupture under pressure, permitting midline crossing of accumulated ﬂuid. Another delicate fascial prolongation extends inferomedially along each ureter as periureteric fascia.
There is substantial disagreement over the craniocaudal boundaries of the renal fascia, reﬂecting its tenuous and elusive structure. In their cranial aspect, the anterior and posterior layers are generally thought to fuse superior to the suprarenal glands. In several studies this fused fascia appears to deﬁne a closed space on each side, which is then continuous with the diaphragmatic fascia in the region of the coronary ligament on the right and the gastrophrenic ligament on the left. Other studies, however, have challenged the notion that these spaces are closed, ﬁnding the perinephric space to be continuous with the bare area between liver and diaphragm on the right and the subphrenic extraperitoneal space on the left.
Caudally, fusion of the anterior and posterior layers is incomplete, which allows perinephric ﬂuid to seep into the iliac fossa of the greater pelvis. Likewise, air injected into the presacral space is known to reach the perinephric space through this same opening; this technique was formerly used to visualize the kidneys in a procedure known as retroperitoneal pneumography.
External to the renal fascia lies the retroperitoneal paranephric fat (pararenal fat body), a continuation of the extraperitoneal fat. The perinephric and paranephric fat are both traversed by variably developed strands of collagenous connective tissue that extend from the renal fascia, which may cause them to appear multilaminate in sectional studies.