RENAL ECTOPIA AND FUSION Embryologic Basis GERALD PIETER

W. FRIEDLAND, DE VRIES,

M.D.

M.D.

From the Department of Radiology, Stanford University School of Medicine, Stanford, and Department of Surgery, Santa Clara Valley Medical Center, San Jose, California

ABSTRACT - To determine when horseshoe kidneys and crossedfused ectopic kidneys might develop, the authors studied the Carnegie collection of human embryos. They found that renalfusion must occur before the kidneus ascend between the umbilical arteries.

In 1914, Kelly and Burnam’ published an illustration in their textbook showing renal ascent (Fig. 1). According to these authors, the kidneys appear at five to six weeks below the umbilical arteries; at seven weeks, they ascend between the umbilical arteries (this is the stage when they believed that the kidneys may fuse, giving rise to horseshoe kidney or crossed fused ectopia); by eight to nine weeks they have ascended above the umbilical arteries. Either the Kelly and Burnam illustration or the Gutierrez’ modification thereof have appeared in several reference works on embryology,3’4 urology, 5 and uroradiology6 to explain renal ascent and fusion. Not everyone agrees with this viewpoint. In 1937 Hinman’ stated that the kidneys appear at the second lumbar level and remain there thereafter (Fig. 2). DuBois* currently supports this concept. Because of this controversy, we have asked two questions: (1) Do the developing kidneys ascend? (2) When and where are the developing kidneys closest to each other? The answers to these questions might explain the development of renal ectopia and fusion. Presented in part at the Western Section meeting, American Urological Association, San Francisco, March 31, 1974, and at a scientific exhibit presented at the International Radiology Congress in Johannesburg, South Africa, August 29, 1974, and the Sixtieth Scientific Assembly and Annual Meeting of the Radiological Society of North America Inc., Chicago, Illinois, December 1, 1974.

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Material

and Methods

We studied the Carnegie collection of human embryos which are divided into 23 age groups according to their stage of development (Fig. 3).‘-11 Each age group shades into preceding and subsequent age groups, since all division into stages of development is of necessity somewhat arbitrary. In two previous studies”,13 we examined age groups VI to XXIII and found that the distance between the tail fold and the umbilical artery at its take-off from the aorta, remained constant.r2 For this report we studied all embryos from age group X to early fetal life, but concentrated on the 231 older embryos of age groups XIII to XXIII. All measurements whenever possible were done on embryos graded excellent. Thin sections (10 microns or less) were chosen to define the evolving structures more accurately. We made our measurements in two planes, sagittal and transverse. We used the sagittal plane for most of our vertical measurements. To determine the width of the aperture between the umbilical arteries through which the kidney must pass, we made several measurements on transverse sections. For our measurements in the sagittal plane we used the tail fold as an anatomic reference point because it is a well-defined point on the longitudinal axis of the embryo. To determine whether certain anatomic structures changed position

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5-6 FIGURE

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week The Kelly and Burnam concept of renal ascent. (Courtesy of Appleton-Century-Croffs,

New York. ‘)

At birth

FIGURE

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Hinman’s view of early renal development

relative to the tail fold at different stages of development, we measured the distance between the tail fold and the following structures: umbilical artery, lower pole of kidney, pelvic peritoneum, and ureteropelvic junction. We also determined the relationship between the umbilical artery and the developing kidney and ureter. To relate the developing kidneys and other structures to future vertebral levels, it was neces-

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NUMBERS

(Courtesy of W. B. Saunders Co., Philadelphia.7)

sary to examine spinal development in detail. We noted which somites became which myotomes and sclerotomes, and which sclerotomes, in turn, became which vertebrae. To explain the change of kidney position relative to the spine, we measured the length of the divisions of the axial skeleton and the angles of curvature of the spine, and defined which spinal segments actually angulated.

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Results Certain general observations had a considerable bearing on the relative position of the urinary organs: the shape of the embryo, for example, and the development of the vertebral column. We will, therefore, dwell on these observations before presenting specific results. Shape

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of embryo

During age group X and XI, embryos have a dorsally concave curve. Subsequently, they straighten, and then become curved convex dorsally during age groups XII, XIII, and XIV - the period when the mesonephric ducts join the hindgut and the ureteric buds and kidney blastemas first appear. By late age group XIV and age group XV the embryos assume an almost semicircular shape. The curve varies not only in degree, but also in shape and position through the various age groups. Through age groups XIII to XV the cervical, thoracic, lumbar, and sacral segments have a similar degree of curvature. Through age groups XVI and XVII, the previous lower thoracic and upper lumbar curve straightens out, while the apex of the curve descends progressively from above the lower limb bud to well below the lower limb bud. The straight upper lumbar and curved lower lumbar segments together impart a fishhook shape to the lower end of the embryo. By age group XVIII the apex of the curve lies at the third and fourth lumbar level; its angle measures 180 degrees. The apex of the curve shifts fi-om the fourth lumbar to the second coccygeal segment during age groups XIX and XX, while its angle is reduced to 45 degrees. The lumbar and sacral area is then straight. As the volume of the embryo increases, particularly ventrally, the shape of the embryo changes from a truncated cone, flattened from side to side, to a somewhat quadrilateral barrel shape. This increase in tissue forces the

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We studied spinal development in detail to determine the relationship of the ureteric bud and developing kidney to future vertebral levels. During the early developmental stages of the kidneys and ureteric buds, somites were present,

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spine to straighten. The embryo thus undergoes marked straightening after the ureteric bud and nephric blastema appear. This straightening is associated with ureteric lengthening and renal ascent.

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FIGURE 3. Diagrammatic illustration of embryonic age and range of embryonic length during Streeter’s age groups X through XXIII. We have indicated some highlights of urinary tract development during these age groups.

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FIGURE 6. Angle between ureter&c bud and mesonephric duct age group XIV-XVI (U = ureter% bud, D = mesonephric duct, M = mesonephros, R = renal blastema, T = comet-like tail of the upper pole of the renal blastema). Early group XIV: ureteric bud projects o?orsallyinto lower pole of renal blastema. Late group XIV: angle about 90 degrees exists between ureteric bud and mesonephric duct. Early group XV: ureteric bud stillforms angle of about 90 degrees wtth mesonephric duct. Mid-age group XV: as renal blastema ascends, angle between ureteric bud and mesonephric duct becomes we acute. Late group XV: course of ureter has become more vertical. Group XVI: ureter nowfnms an angle of about 45 degrees with mesonephric duct; it has lengthened considerably. Upper division of bud has grown into comet-like tail of upper pole; latter is well seen in early and mid-age group XV.

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Renal ectopia and fusion. Embryologic Basis.

To determine when horseshoe kidneys and crossed fused ectopic kidneys might develop, the authors studied the Carnegie collection of human embryos. The...
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