[Diagnostic image (23). Congenital lobar emphysema]

A 6-months-old-boy failed to thrive. Congenital emphysema of the left upper lobe of the lung was diagnosed, and lobectomy was performed, with good results.

Normal development of the male anterior urethra

A histological study was performed on serially sectioned human and mouse embryos to study the influences of programmed cell death (PCD) during morphogenesis for clarifying the existing controversies on the morphology and basic processes involved in the embryonic development of the male anterior urethra. The following new insights into the development of the anterior urethra could be established. The formation of the urethra starts with the early adhesion of the arms of the genital tubercle. In this way an epithelial plate is formed, located in the ventral midline, that is in continuity with the cloacal membrane. Male sex differentiation takes place following rupture of this cloacal membrane through programmed cell death. Fusion of the urogenital swellings with primary luminization gives rise to the penile urethra, whereas the glandular part of the urethra is formed through secondary luminization of the epithelial cord that is formed during fusion of the arms of the genital tubercle, i.e., the glans. In both fusion processes, apoptosis plays a key role. The consequence of fusion of the urogenital swellings is that their mesodermal cores unite on the ventral aspect of the penile urethra, where they differentiate into the integumental structures. The prepuce starts to develop as a fold of ectoderm with a mesodermal core after complete fusion of the entire urethra. Finally, the scrotum was found to develop through merging of the labioscrotal swellings and not by fusion.

Normal and abnormal embryonic development of the anorectum in human embryos

In the literature, some controversy still exists about the normal and abnormal development of the human anorectum. Therefore, a three-dimensional and histological study was performed on human embryos. In early anorectal development (< or = 49 days postfertilization), the cloaca plays a crucial role, separated from the amniotic cavity by its cloacal membrane. In the cloaca, the yolk sac/primitive hindgut and allantois/primitive urogenital sinus enter. During the embryonic caudal folding process, incorporation of these structures occurs, including their surrounding extraembryonic mesoderm, which fuses to form the urorectal septum. Consequently, this septum does not grow in the direction of the cloacal membrane, and fusion of these structures is likewise never observed. The cloaca remains as such until the cloacal membrane ruptures by apoptotic cell death. The dorsal part of the cloaca then becomes part of the amniotic cavity, and is by no means involved in the development of the anorectum. The tip of the urorectal septum will become the perineal area. Soon after rupture of the cloacal membrane, during late anorectal development (> or = 49 days postfertilization), a secondary occlusion of the anorectal canal occurs, first due to adhesion, followed by formation of an epithelial "plug" at the level of the anal orifice. Recanalization, by apoptotic cell death, of this secondary occluded anal orifice occurs later during development. Based on these embryological observations, congenital anorectal malformations with an abnormal communication to the exterior are best explained as early embryonic defects. The abnormal communications, usually called fistulae, should be regarded as ectopic anal orifices. Anorectal malformations with the anus in normal position are best explained as late embryonic defects.

MR imaging of anorectal malformations and associated anomalies

Congenital anorectal malformations are found in many forms, and are frequently associated with other anomalies, especially of the spinal cord, spine, and urogenital system. Decisions concerning initial management of children with anorectal malformations can be made only after accurate determination of (a) the level and type of malformation, (b) the type of fistula, (c) the developmental state of the sphincter muscle complex, and (d) the presence of associated anomalies. Magnetic resonance imaging has proven to be the only modality to answer all these crucial questions, and has contributed to a better insight in the morphology and pathogenesis of such complex congenital malformations.

Normal and abnormal embryonic development of the anorectum in human embryos

In the literature, some controversy still exists about the normal and abnormal development of the human anorectum. Therefore, a three-dimensional and histological study was performed on human embryos. In early anorectal development (< or = 49 days postfertilization), the cloaca plays a crucial role, separated from the amniotic cavity by its cloacal membrane. In the cloaca, the yolk sac/primitive hindgut and allantois/primitive urogenital sinus enter. During the embryonic caudal folding process, incorporation of these structures occurs, including their surrounding extraembryonic mesoderm, which fuses to form the urorectal septum. Consequently, this septum does not grow in the direction of the cloacal membrane, and fusion of these structures is likewise never observed. The cloaca remains as such until the cloacal membrane ruptures by apoptotic cell death. The dorsal part of the cloaca then becomes part of the amniotic cavity, and is by no means involved in the development of the anorectum. The tip of the urorectal septum will become the perineal area. Soon after rupture of the cloacal membrane, during late anorectal development (> or = 49 days postfertilization), a secondary occlusion of the anorectal canal occurs, first due to adhesion, followed by formation of an epithelial "plug" at the level of the anal orifice. Recanalization, by apoptotic cell death, of this secondary occluded anal orifice occurs later during development. Based on these embryological observations, congenital anorectal malformations with an abnormal communication to the exterior are best explained as early embryonic defects. The abnormal communications, usually called fistulae, should be regarded as ectopic anal orifices. Anorectal malformations with the anus in normal position are best explained as late embryonic defects.

Abnormal anatomy of the lumbosacral region imaged by magnetic resonance in children with anorectal malformations

OBJECTIVE

To investigate the frequency of lumbosacral anomalies, the association with urogenital abnormalities, and the correlation with defaecation pattern by magnetic resonance imaging (MRI).

METHODS

A prospective analysis was performed of routine MRI in patients with anorectal malformations. Between 1990 and 1994, MRI was performed in 43 such patients: 31 boys and 12 girls. Twenty four had a high anorectal malformation, 16 had a low anorectal malformation, and three had Currarino's triad. MRI was performed before reconstruction in 26, and postoperatively in 17. Urogenital anomalies were found in 21.

RESULTS

Abnormalities of the spinal cord and spine were found with MRI in 20 patients (46.5%); caudal regression syndrome in 10, tethered cord in two, a combination of both in three, and other spinal anomalies in five. These anomalies were found in 30% of the patients with low anorectal malformations, and in 50% with high anorectal malformations. In patients with urogenital malformations, MRI more often showed spinal anomalies (13/21, 62%) than in patients without (7/22, 32%). In high anorectal malformations, defaecation was more often a problem in patients with spinal anomalies (12/15, 80%) than in patients without (2/8, 25%).

CONCLUSIONS

Spinal anomalies in the lumbosacral region were found with MRI in 46.5% of patients with anorectal malformations. Since presence of these anomalies seems to be related to clinical outcome, MRI should be performed routinely in all such patients.

MR of the caudal regression syndrome: embryologic implications

PURPOSE

To evaluate the spectrum of developmental anomalies observed in patients with the caudal regression syndrome and relate them to the pathogenesis of this syndrome.

METHODS

Nineteen children with caudal regression were investigated with MR.

RESULTS

The level of vertebral agenesis varied from T-11 to S-5. In 9 of the 19 children the characteristic high-ending wedge-shaped cord terminus was observed. A separation of the anterior and posterior spinal roots of the cauda equina was observed in 9 patients. Four patients had a tethered spinal cord, in 1 in combination with a wedge-shaped cord terminus.

CONCLUSIONS

The pathogenesis of the caudal regression syndrome can be divided into two kinds: there is usually a disturbance of the primary neurulation process; in other cases there is a derailment of the process of degeneration and differentiation of an initially normally developed primary and secondary neural tube. MR aids understanding of the morphology and pathogenesis of congenital malformations involved (including the associated anomalies of the genitourinary and gastrointestinal systems), but other studies are still necessary to determine the exact mechanism of this syndrome.

Embryonic development of the mammalian caudal neural tube

In the literature, some controversy still exists about the embryonic developmental processes involved in the formation of the caudal neural tube. Therefore, a three-dimensional and histological study concerning the normal development of the caudal neural tube was performed on both mouse and human embryos. Three developmental processes can be distinguished in caudal neural tube development: caudal neuropore closure, secondary neurulation, and degeneration and differentiation of the secondary neural tube. Caudal neuropore closure occurs at the level of somite 32-34 in both species. Therefore, primary neurulation leads to the formation of all spinal cord segments and ganglia. Secondary neurulation involves cell deposition from a cluster of neurectodermal cells at the caudal end of the closed neural tube, directly around a lumen, the lumen always in contact with the lumen of the primary neural tube. This process leads only to the formation of the primordia of the filum terminale and ventriculus terminalis and, possibly, part of the conus medullaris. Secondary neurulation is followed by a period characterized by degeneration and differentiation of the secondary neural tube. Its lumen and neural tissue will disappear, whereas part of the secondary neurectodermal cells differentiate to a fibrous layer comparable and continuous with the marginal layer of the primary neural tube. This fibrous layer represents the future filum terminale. The embryological processes indicated above can be helpful in the interpretation of congenital anomalies affecting the caudal spinal cord and spine.