Developmental Anatomy 12

Transcription

1 Developmental Anatomy 12 Last day: development of the urogenital sinus. A. Development of the urinary tract. MS urogenital sinus. Ectoderm covers the anterior part of the UGS and forms the PHALLUS. The perineal body develops posterior to the UGS. The hindgut develops dorsally. The intermediate mesoderm from the stage of the primitive streak also contributes to the formation of the urogenital system. TS intermediate mesoderm. The mesoderm of the dermatome forms the body wall. Interior to that is the gut tube. The intermediate mesoderm is present between the VC and the lateral mesoderm of the body wall. It lies between the median structures and the lateral mesoderm and exists in two parts. The reason it is in two parts is that it is the UROGENITAL MESODERM, giving rise to two separate structures, the gonad and the kidney. The NEPHROS/NEPHROAE give rise to the kidney. The MESONEPHRIC RIDGE is the lateral part of the urogenital mesoderm; the GONADAL RIDGE is the medial part.

2 Anterior view mesonephros The nephros gives rise to the nephrons of the primitive kidney. It contains many vesicles that give rise to the Bowman s capsules. The aorta give off lateral branches to the nephros which form the efferent arterioles of the nephron by invaginating the Bowman s capsule. This happens in large numbers to the nephrons (not adult type, but primordial). The afferent arterioles from the capsules join together forming the MESONEPHROS. There is a primitive kidney in the neck called the PRONEPHROS, but this is obliterated in utero. The mesonephros is a long paravertebral structure that runs along the length of the body wall. The duct grows down just lateral to the vertebral column and sweeps forwards around the sides of the hindgut and opposes itself to the wall of the UGS. The ridge cells of the leading edge will drill their way through the mesenchyme and will find the UGS. This forms a ureter. The urogenital membrane breaks down around 6wks and the foetus urinates into the amniotic fluid. B. Lateral view mesonephros and UGS. The UGS has changed its appearance dramatically to a nearly tear-shaped structure with an inferior anterior projection (the precursors of the bladder and female/prostatic urethra; the remainder of the ureter in the male forms). The upper caudal end of the UGS is the VESICOURETHRAL CANAL, and the upper part is the ALLANTOIS, becoming the URACHUS, the fibrotic remnant of the primordial bladder. The caudal end of the mesonephros forms the third kidney, the METANEPHROS. The caudal end is still not differentiated

3 Lateral view definitive bladder and urachus. The VID loses lumen and closes, same as the urachus. There can be a persistent urachus forming a cyst or, if entirely patent, a URINARY FISTULA. The DEFINITIVE UROGENITAL SINUS is the most inferior part of the primitive UGS. It becomes the vestibule of the vagina in the female and must be reconstructed in the male to form the proximal penis and urethra. The formation of the penile urethra occurs in four parts: 1. The UGS grows forward into the phallus, forming the URETHRAL PLATE. 2. The urogenital membrane breaks down. TS penis, the development of the urethral plate. The urethral plate grows along on the deep surface of the ectoderm surrounded by mesenchyme. The logical thing would be for the urethral plate to undergo cavitation to produce the urethra. This doesn t happen. 3. The URETHRAL GROOVE is formed. At this stage, the opening of the definitive UGS is open and so is the urethral groove. So the whole of the spongy urethra is open. Driven by cellular proliferation, the two ridges of the groove grow together and fuse.

4 4. The mesenchyme grows posteriorly from the urethral plate which closes off the posterior part of the urethra. This process can fail. 1. There can be an entirely open urethra in the male perineum or partly open. This is called HYPOSPADIAS and necessitates reconstruction; this causes impaired urination and sexual function. 2. There can be breakdown of the mesenchyme anterior to the vesicourethral canal which causes an opening on the anterior abdominal wall. It can be called ECTOPIA VESICAE if the bladder opens onto the anterior abdominal wall. 3. Or, it can be EPISPADIAS if the opening is on the dorsal surface of the penis. C. What about the metanephros? The ureter will grow backwards off the posterior surface of the bladder as a typical bud, the URETERIC BUD, with a solid leading edge. This grows up and branches many time in the intermediate mesoderm that is caudal to the mesonephros. These branches are the calyces, minor calyces, collecting ducts, etc. Ureteric bud The end of the ureteric bud (approximately 13 th order branch) will come into contact with a vesicle in the intermediate mesoderm. The vesicle will be invaginated by blood vessels forming the afferent and efferent arterioles. The other end of the bud will become elongated and the Bowman s capsule meets the ureteric bud at the collecting duct. Most of the nephron is produced by the vesicle. If there is failure of junction: 1. The ureteric bud may not appear, leading to AGENESIS, an absent kidney. 2. The branching can take place abnormally early. This leads to a BIFID URETER. 3. If the branching occurs at the very beginning of the ureteric bud, there will be a DOUBLE URETER. This may lead to double kidneys on the side of the double ureter. 4. The breakdown of the membrane between the vesicle and the ureteric bud will lead to collection of fluid in the vesicle, causing POLYCYSTIC KIDNEYS. Sometimes this problem is only manifest in late youth or middle age. There can by huge masses in the abdomen; these cysts can compress normal renal tissue leading to renal failure. These variations are associated with urinary tract infections. Repeated UTIs suggest congenital anomalies.

5 D. Look at the region where the ureter comes off the bladder. The mesonephros gives rise to the ductus deferens but it develops separately from the ureter. The kidney in the foetus develops around sacral levels and must ascend to adult levels during growth. In truth, the body wall grows around it causing it to move up. Strangely, the kidney does not keep its original blood supply but rather grows new vessels as it ascends. These are ACCESSORY RENAL ARTERIES. These accessory vessels are supposed to compress the ureter leading to statis of urine in the renal pelvis which will lead to persistent UTIs in the new-born.