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Development of the Urinary System

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									Development of the Urinary System
• Urinary and genital systems are closely
  associated
• Both develop from intermediate mesoderm
  – 7th- 28th somite level(3-d week)
  – Nephrogenic mass (cord)
     • Dorsal side of coelom each cord produces a bulge
       into the coelom called the urogenital ridge
• Urinogenital Ridge
  – Form the urinary and genital structures
  – Nephrogenic tissue from 7-14th somite breaks
    up into segments called nephrotomes
Fig. 1 -   Transverse section and dorsal view of an embryo (trilaminar)
     Transverse section of the three-layered embryo
(ca. 21 days)


     towards the end of the 3rd week of development.
                                                                          1.Paraxial mesoderm
                                                                          2.Intermediate
                                                                          mesoderm
                                                                          3.Lateral mesoderm
                                                                          4.Notochord
                                                                          5.Amnion
                                                                          6.Intraembryonic coelom
                                                                          7.Endoderm
                                                                          8.Ectoderm
                                                                          9.Somatopleural
                                                                          (mesoderm and
                                                                          ectoderm)
                                                                          10Splanchnopleural
                                                                          (mesoderm and
                                                                          endoderm)
                                                                          11.Neural groove
                                                                          12.Neural ridge
Mesonephros enlargement point A
                           1.Neural tube
                           2.Notochord
                           3.Aorta dorsalis
                           4.Dorsal mesentery
                           5.Intestinal tube
                           6.Ectoderm
                           7.Somite
                           8.Inferior cardinal
                           vein
                           9.Mesonephric duct
                           (Wolffian duct)
                           10.Mesonephric
                           tubule
                           11.Urogenital ridge
The urinary tract develops from the 3rd week of the embryonic
period from the intermediate mesoderm as well as from the
urogenital sinus. The kidneys develop from the 4th week in
three steps: As a first one, a cranial anlage, the pronephros,
forms that then later atrophies in the 8th week and is never
active functionally. It is followed by a further anlage from the
intermediate mesoderm, the mesonephros, that is formed
between the 6th and 10th weeks, but is only transitory, and the
anlage of the definitive kidneys, the metanephros. They
develop from a metanephric anlage (mesodermal origin) and
the ureter anlage (that has its origin in the caudal part of the
wolffian duct).
The urine-excreting part of the kidneys, the nephron, mainly
arises from the metanephric anlage (glomerulus, proximal,
intermediate and distal tubules), while the rest of the upper
urinary tract (collecting ducts, calices, renal pelvis and ureter)
develop from the ureter anlage.
The lower urinary tract differentiates from the cloaca between
the 5th and 8th weeks in that it becomes subdivided by the
urorectal septum. The ventral part of the cloaca forms the
primary urogenital sinus, out of which the urethra forms in the
lower part and the bladder in the upper part. The ureter anlage
discharges into the upper posterior wall of the urogenital sinus.
In males, the wolffian duct remains present and forms a
connection to the genital tract in the lower part of the
urogenital sinus.
The numerous induction mechanisms between ureter anlage
and metanephric mesenchyma during the development of the
renal system, as well as the ascent of the kidneys,
originating at the level of the sacrum and moving up to the
diaphragm at the end of the development, make it possible for
a large number of abnormalities to arise. Many remain
asymptomatic whereas others are not compatible with survival.
      Intermediate Mesoderm
• Cervical region
  – Loses contact with the somite
     • Forms nephrotomes which acquire a lumen and
       open medially into the intra-embryonic coelom
     • Caudal growth unite and form longitudinal duct
     • Branches off dorsal aorta form glomeruli (ext & int)
• Thoracic, lumbar, sacral regions
  – Loses contract with coelomic cavity
     • Ext glomeruli fail to develop
     • Segmentation disappears (nephrogenic cord)
        – 2 or more excretory tubules per prior segment
             Urinary Tubules
• Associated with a vascular tuft =
  glomerulus
  – Open tubules = external glomerulus
     • One end opens into the coelom
     • Other end opens into the collecting duct
  – Closed tubules = internal glomerulus
     • Open only into the collecting ducts
  Formation of 3 kidney systems
• Pronephros (simplest & most primitive)
  – 7-10 solid or tubular arranged cell groups in the
    cervical region (head kidney)
  – Gone by the end of the 4th week
• Mesonephros (intermediate-more advanced)
  – Appear during regression of pronephros
  – 10-26th somite level
• Metanephros (permanent kidney)
  – Begins to develop early in 5th week, functions by
    the 11th week
Pronephros (forekidney): transitory structure
                         1.Nephrogenic cord
                         2.Mesonephric duct
                         (Wolff)
                         1+2.Mesonephros
                         3.Intestinal tube
                         4.Cloaca
                         5.Atrophying
                         nephrotomes
                         6.Yolk sac (umbilical
                         vesicle)
                         7.Allantois
                         8.Outflow of the
                         mesonephric duct into
                         the cloaca
              Mesonephros
• Tubules develop from nephrogenic cord (NC)
  – Opens into the excretory/mesonephric duct
  – Gone by week 10 in females, in males some
    tubules persist & become vas deferens
• Approximately 38 pairs of closed tubules
  – S shaped bend
  – Surrounds internal glomerulus
• Mesonephric duct develops laterally from NC
  & extends from 8th somite to urinogenital
  sinus
Mesonephros: transitory kidney
                      1.Nephrogenic cord
                      2.Mesonephric duct
                      1+2.Mesonephros
                      3.Intestine
                      4.Cloaca
                      5.Atrophied
                      nephrotome
                      6.Yolk sac (umbilical
                      vesicle)
                      7.Allantois
                      8.Outflow of the
                      mesonephric duct
                      into the cloaca
                      9.Ureter bud (anlage)
Mesonephros enlargement point A
                           1.Neural tube
                           2.Notochord
                           3.Aorta dorsalis
                           4.Dorsal mesentery
                           5.Intestinal tube
                           6.Ectoderm
                           7.Somite
                           8.Inferior cardinal
                           vein
                           9.Mesonephric duct
                           (Wolffian duct)
                           10.Mesonephric
                           tubule
                           11.Urogenital ridge
The metanephros: definitive kidney 5th week

                           1a-Pronephros
                           (atrophying)
                           1b-Mesonephros
                           (atrophying)
                           2-Mesonephric duct
                           (Wolffian duct)
                           3-Nephrogenic cord
                           4-Ureter anlage
                           5-Metanephric
                           blastema
                           6-Liver anlage
                           7-Cloaca
Metanephric outflow
               1.Cloaca
               2.Ureter anlage
               3.Metanephric
               blastema
               2+3.Metanephros
               4.Mesonephric
               duct (Wolffian
               duct)
               5.Nephrogenic
               cord
               4+5.Mesonephros
                Metanephros
• Nephrons/tubules develop from nephrogenic
  mass (26th-28th somite level)
  – Located lateral to mesonephric duct
  – Internal dense layer which forms tubules/nephrons
  – Outer loose layer forms connective tissue capsule
• Duct system derived from ureteric bud
  – Ureter, renal pelvis, calyces, collecting ducts
  – Ureteric bud elongates and makes contact with
    nephrogenic mass which surrounds bud like a cap
• Tubules are closed (internal glomerulus)
• Migrate from pelvis to abdomen as fetus grows
  – Blood supply from aorta changes as ascent occurs
• Becomes functional in second ½ of pregnancy
Kidney at the end of its development
                         1.Ureter
                         2.Renal pelvis
                         3.Vena renalis
                         4.Arteria renalis
                         5.Major calix
                         6.Minor calix
                         7.Cortex
                         8.Capsula renis
                         9.Medullary rays
                         10.Papilla renalis
                         11.Sinus renalis
                         12.Columna renalis
                         13.Medullary
                         pyramid
Schematic cut through the kidney (sagittal section)
                                    1.Ureter
                                    2.Major calix
                                    3.Minor calix
                                    4.Renal pelvis
                                    5.Collecting
                                    duct
                                    6.Metanephric
                                    vesicle
                                    7.Kidney lobe
Enlargement of the inset Nephron



                       8.Distal tubule
                       9.Proximal tubule
                       10.Glomerulus
                       11.Connecting
                       tubule
                       13.Intermediate
                       tubule
                  Cloaca
• Caudal end of the hindgut (dilated)
• In 3 week old embryo the hindgut ends
  blindly at the cloacal membrane
• Blind end = cloaca
• Allantois and mesonephric ducts open into
  cloaca
• Cloaca is latin for sewer, a system of pipes
  used to transport human waste
               Urinary Bladder
• During 4th to 7th week cloaca subdivided
   – Posterior portion = anorectal canal
   – Anterior portion = primitive urogenital sinus
• Bladder is formed from primitive urogenital sinus
   – Bladder is upper and largest part of urogenital sinus
• Initially bladder is continuous with the allantois
   – Allantois lumen obilterated & urachus formed
     connecting apex of bladder with umbilicus
   – In adult urachus = median umbilical ligament
• Ureter is outgrowth of mesonephric duct
   – Terminal ends of mesonephric ducts become part of
     bladder wall
   – Ureter obtains separate entrance into bladder with time
   Production of urine by fetus
• Fetal urine mixes with amniotic fluid
• Amniotic fluid enters fetal intestinal tract
  where it is absorbed into bloodstream
• From the bloodstream to the placenta
  which transfers metabolic waste to the
  mother
• Fetal kidneys are not necessary for
  exchange of waste products
             Ascent of Kidneys
During the fifth and sixth weeks of development,
the mature kidneys lie in the pelvis with their hila
pointed anteriorly. As the pelvis and abdomen
grow, the kidneys slowly move upward. By the
seventh week, the hilum points medially and the
kidneys are located in the abdomen. As the
embryo continues to grow in a caudal direction,
the kidneys are left behind and eventually come to
lie in a retroperitoneal position at the level of L1
by the ninth week of development. In the
meantime, the kidneys have completed rotation
and the hila now face anteromedially.
   Development of the reproductive
             system
• Makes its appearance during 5th & 6th week
  – Indifferent stage-sex cannot be determined
• Gonads (testes & ovaries) develop from
  – Coelomic epithelium
  – Inner mesenchyme tissue
  – Primordial germ cells
• Thickening of ventromedial surface of
  urogenital ridge forming genital ridge
              Genital ridge
• Covered by coelomic epithelium
  – Primary sex cords
    • Grow into underlying mesenchyme
• Inner mass is composed of mesenchyme
• Outer layer called cortex
• Inner layer called medulla
  – Males- medulla differentiates, cortex regresses
  – Females-cortex develops, medulla regresses
  Primordial Germ Cells (PGC)
• Differentiate in the neck of the yolk sac
  – Early in the 4th week
• Migrate to genital ridge
  – Amoeboid movement
  – By end of 6th week the PGC become
    incorporated into the primary sex cords
        – migration of primordial germ cells
 Development of Genital Ducts
• Indifferent stage
  – Both male and female genital ducts present
     • Male develop from mesonephric/wolffian ducts
     • Female develop from paramesonephric/mullerian
       duct
        – Undifferentiated gonad

• Males:Mesonephric ducts form epididymis,
  ductus deferens, ejaculatory duct
  – Cranial mesonephric tubules Þ efferent ducts
     • Open into epididymis
  – Process begins about the 3rd month
 Development of Genital Ducts
• Females: Paramesonephric duct/Mullerian
  duct develops on each side of the body
  – Longitudinal invagination of coelomic
    epithelium on the lateral surface of
    mesonephros
  – Ducts open into coelom
  – Runs along side of mesonephric duct
  – Fuse at caudal end
    • Y shaped uterovaginal complex Þ uterus & vagina
       – uterovaginal complex
        Development of testes
• Primary sex cords of testes containing the
  primordial germ cells = testes cords
  – Well defined cords within the medulla
  – Contain two types of cells
     • Epithelial cells Þ Sertoli cells
     • Primordial germ cells Þ spermatoblasts
        – development of testes
• Testes cords remain solid until puberty
  – Canalize to form seminiferous tubules (ST),
    tubuli recti, rete testis
     • ST seperated from each other by mesenchyme that
       gives rise to interstitial cells (Cells of Leydig)
   Development of the Ovaries
• Primary sex cords are not well defined
  – Extend into the medulla but later dissappear
• PGC migrate near the cortex (surface
  epithelium
  – Forms cortical cords
  – At about 16th week cortical cords break up into
    isolated clusters called primordial follicles
       – development of ovary

								
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