Ontophylogenetic nature of malformations
Many congenital anomalies of the human urogenital system belong to the group of ontophylogenetic malformations. Their development is directly linked to the phenomenon of recapitulation, in which the embryo during maturation repeats anatomical features characteristic of distant evolutionary ancestors. If normal embryogenesis fails, these temporary ancestral states may persist permanently in the fetus.
Furthermore, these malformations are often caused by disruptions in the sequential replacement of kidney types or failures in their intrauterine migration. For instance, delayed regression and persistence of elements of the pronephros or mesonephros lead to cyst formation and various atypical supernumerary structures.
Renal heterotopia (dystopia)
During normal embryogenesis, the kidney must migrate (so-called "ascent") from the pelvic region, where it initially forms, to its final bed in the lumbar region. Disruption of this process or an initial shift in the site of organogenesis leads to heterotopia (renal ectopia).
Depending on the stage at which the kidney stopped or where it aberrantly shifted, several types of heterotopia are distinguished by localization:
- Pelvic: the kidney remains deep in the pelvis (dystopia renis). This position is normal for lower vertebrates (fish and amphibians) and reflects the fixation of an early embryonic stage.
- Sacroiliac: the organ is fixed at the level of the iliac bones and sacrum.
- Lumbar (low): the kidney fails to reach its normal level, remaining below the physiological norm.
- Thoracic: an abnormally high position of the organ, where the kidney is located in the thoracic cavity.
Structural anomalies of the kidneys and urinary tract
In addition to anatomical positioning defects, anomalies in the shape and number of urinary system structures can occur:
- Horseshoe kidney: formed as a result of pathological fusion of the lower poles of the two kidneys.
- Duplication of kidneys and ureters (ureter duplex): this defect arises due to premature branching or splitting of the ureteric bud in early developmental stages.
- Persistence of the cloaca: an extremely severe anomaly. In this case, the rectum, bladder, and reproductive tracts do not separate, opening into a single common cavity. This anatomical structure is normally characteristic of reptiles, amphibians, and monotreme mammals.
Bladder exstrophy
Bladder exstrophy is a severe congenital malformation in which the organ is turned inside out and lies directly on the body surface.
Pathomorphologically, this condition is characterized by the complete absence of the anterior bladder wall, as well as a pronounced defect in the corresponding adjacent area of the anterior abdominal wall.
Upon objective clinical examination of such a patient, the following anatomical landmarks are identified:
- The mucosa of the posterior bladder wall is directly visualized.
- The ureteric orifices (openings) are clearly visible.
- The exposed urethral plate is revealed.
- Associated anomalies of the external genitalia are present, most commonly manifesting as their clefting.
Associated anomalies of the reproductive system
Since the urinary and reproductive systems are closely linked during embryogenesis, their anomalies are often considered together. Developmental defects of the female genital tracts are primarily caused by the failure of normal fusion of the paramesonephric (Mullerian) ducts. Evolutionary recapitulation is also evident here:
- Uterine and vaginal duplication (uterus didelphys): complete non-fusion of the ducts occurs. This anatomy is normally found in lower mammals.
- Bicornuate uterus (uterus bicornis): occurs with partial fusion of the ducts, which is characteristic of carnivores and insectivores.
- Arcuate uterus: the mildest form of non-fusion, manifested only by a slight depression in the uterine fundus.
Briefly: answers to topical questions
Which types (stages) of kidneys sequentially replace each other during normal human embryogenesis?
During human embryogenesis, three types of kidneys are sequentially replaced: the pronephros, a transitory precursor functioning in early stages; the mesonephros (primary kidney), which replaces the pronephros at 4–8 weeks of gestation; and the metanephros (secondary kidney), which becomes the definitive excretory organ.
- Pronephros (pronephros) — a transitory, temporary precursor.
- Mesonephros (mesonephros) — the primary kidney replacing the pronephros.
- Metanephros (metanephros) — the secondary kidney developing into the definitive organ.
What anomalies of kidney number are distinguished in urology?
In urology, anomalies of kidney number include agenesis (complete absence of the organ), aplasia, and the presence of a supernumerary kidney.
- Renal agenesis (Renal agenesis) — complete absence of one kidney.
- Supernumerary kidney — the presence of an additional organ.
Which embryonic structure normally divides the cloaca into the urogenital sinus and the rectum?
Normally, the cloaca is divided into the ventral urogenital sinus and the dorsal anorectal part by the urorectal septum. This process occurs in a craniocaudal direction during weeks 3–7 of gestation. The caudal part of this septum forms the perineal body.
- Urorectal septum (urorectal septum) — the structure dividing the cloaca.
From which embryonic duct does the ureteric bud originate?
The ureteric bud originates from the mesonephric (Wolffian) duct near its opening into the cloaca.
- Ureteric bud (ureteric bud) — the structure branching off from the mesonephric duct.
What types of renal dystopia are distinguished in relation to the body's midline?
Sources describe crossed renal ectopia: the dystopic kidney is located on the side opposite to the insertion of its ureter into the bladder; such a kidney crosses the body's midline.
To remember the types of renal heterotopia from bottom to top (along the path of normal ascent), use the mnemonic.
Class and exam questions
What is renal dystopia and why does it occur?
Dystopia (heterotopia) is the abnormal anatomical location of a kidney. It occurs due to the arrest of organ migration from the pelvis to the lumbar region during embryogenesis or an initial alteration in the site of the primary anlage.
What explains the development of a horseshoe kidney?
This anomaly is the result of pathological fusion of the lower poles of the right and left kidneys during early stages of intrauterine development.
How does bladder exstrophy manifest on physical examination?
In exstrophy, the anterior abdominal wall and anterior bladder wall are absent. The physician observes the everted mucosa of the posterior wall, ureteral orifices, urethral plate, and clefting of the external genitalia.
What is the cause of uterus didelphys and vaginal duplication?
This ontophylogenetic malformation arises due to complete non-fusion of the paramesonephric (Mullerian) ducts, mirroring the anatomical structure of lower mammals.
What else to study on this topic
- Embryogenesis of the paramesonephric (Mullerian) ducts
- Formation and development of the ureteric bud
- Mechanisms of recapitulation in the development of the excretory system
- Structural differences between the pronephros, mesonephros, and metanephros
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