Embryology Course, Second year, Faculty of Medicine, Damascus University

1. Urinary System Development
www.marwanalhalabi.com
Marwan Alhalabi
Professor of Reproductive Medicine and Infertility,
Damascus University
Head of Assisted Reproduction Unit, Orient Hospital
President of Middle East Fertility Society
Past President of Syrian Society of Obstetricians and Gynecologists

2. The Urogenital System

3. Development of Urinary System
The Urinary System
Urinary
Bladder
Ureters
and
Urethra
Kidneys

5. Intermediate Mesoderm
The midline NOTOCHORD secretes factors that organize the dorsal-
ventral and medio-lateral axes of the embryo.

6. Functionally the urogenital system can be divided into two entirely
different components:
1. The urinary system
2. The genital system.
q Embryologically and anatomically they are intimately
interwoven.
q Both develop from a common mesodermal ridge (intermediate
mesoderm) along the posterior wall of the abdominal cavity,
q Initially the excretory ducts of both systems enter a common
cavity, the cloaca.
Development of Urinary System

7. Formation of The Nephrogenic Cord

8. • Three slightly overlapping kidney systems are formed in a cranial
to caudal sequence during intrauterine life in humans:
1. The pronephros, (rudimentary and nonfunctional).
2. The mesonephros, (function for a short time during the early fetal
period).
3. The metanephros, (forms the permanent kidney)
Urinary System (KIDNEY SYSTEMS)

9. Urinary System Development

10. • At the beginning of the fourth week, the pronephros is
represented by 7 to 10 solid cell groups in the cervical
region.
• These groups form vestigial excretory units, nephrotomes,
that regress before more caudal ones are formed.
• By the end of the fourth week, all indications of the
pronephric system have disappeared.
Pronephros

11. Mesonephros

12. Mesonephros

13. • The mesonephros and mesonephric ducts are derived from
intermediate mesoderm from upper thoracic to upper
lumbar (L3) segments.
• Early in the fourth week, the first excretory tubules of the
mesonephros appear.
Mesonephros

14. • They lengthen rapidly, form an S-shaped loop, and acquire a tuft of
capillaries that will form a glomerulus at their medial extremity.
• Around the glomerulus the tubules form Bowman’s capsule, and
together these structures constitute a renal corpuscle.
• Laterally the tubule enters the longitudinal collecting duct known
as the mesonephric or wolffian duct.
Mesonephros

19. • Since the developing gonad is on its medial side, the ridge formed by both
organs is known as the urogenital ridge.
• The caudal tubules are still differentiating,
• The cranial tubules and glomeruli show degenerative changes, and by the
end of the second month the majority have disappeared.
• In the male a few of the caudal tubules and the mesonephric duct
persist and participate in formation of the genital system, but they
disappear in the female.
Mesonephros

21. Mesonephros

22. Mesonephros

23. Mesonephros

24. Mesonephros

25. Mesonephros

26. • The third urinary organ, the metanephros, or permanent kidney,
appears in the fifth week.
• Its excretory units develop from metanephric mesoderm in the same
manner as in the mesonephric system.
• The development of the duct system differs from that of the other
kidney systems.
Metanephros: The Definitive Kidney

27. Metanephros

28. Nephrons

29. Nephrons

30. • Collecting ducts of the permanent kidney develop from the
ureteric bud.
• The bud penetrates the metanephric tissue.
• The bud dilates, forming the primitive renal pelvis, and splits into
cranial and caudal portions (the future major calyces).
Collecting System

31. • Each calyx forms two new buds while penetrating the metanephric
tissue.
• These buds continue to subdivide until 12 or more generations of
tubules have formed.
• Meanwhile, at the periphery more tubules form until the end of
the fifth month.
Collecting System

32. Relation of the
hindgut and
cloaca at the end
of the 5th week.
The ureteric bud penetrates the metanephric mesoderm (blastema).
Metanephros

34. • The tubules of the second order enlarge and absorb those of
the third and fourth generations, forming the minor calyces
of the renal pelvis.
• Collecting tubules of the fifth and successive generations form
the renal pyramid.
• The ureteric bud gives rise to the;
• ureter,
• renal pelvis,
• major and minor calyces,
• 1 million to 3 million collecting tubules.
Collecting System

35. Nephrons

40. Metanephros

41. Metanephros

42. Metanephros

43. • Each newly formed collecting tubule is covered at its distal
end by a metanephric tissue cap.
• Cells of the tissue cap form small vesicles, the renal vesicles,
• Renal vesicles give rise to small S-shaped tubules.
• Capillaries grow into the pocket at one end of the S and
differentiate into glomeruli.
Excretory System

44. • The proximal end of each nephron forms Bowman’s capsule.
• The distal end forms an open connection with one of the collecting
tubules, establishing a passageway from Bowman’s capsule to the
collecting unit. Continuous lengthening of the excretory tubule results
in formation of the proximal convoluted tubule, loop of Henle, and
distal convoluted tubule.
Excretory System

45. • The kidney develops from two sources:
• (a) metanephric mesoderm, which provides excretory units.
• (b) the ureteric bud, which gives rise to the collecting system.
• Nephrons are formed until birth, at which time there are
approximately 1 million in each kidney.
• Urine production begins early in gestation, soon after differentiation of
the glomerular capillaries, which start to form by
the 10th week.
• At birth the kidneys have a lobulated appearance, but the
lobulation disappears during infancy as a result of further growth of
the nephrons, although there is no increase in their number
Excretory System

47. • The kidney, initially in the pelvic region,
• later shifts to a more cranial position in the abdomen.
• This ascent of the kidney is caused by diminution of body curvature
and by growth of the body in the lumbar and sacral regions.
• During its ascent to the abdominal level, it is vascularized by arteries
that originate from the aorta at continuously higher levels.
• The lower vessels usually degenerate, but some may remain.
Position of The Kidney

48. Relocation of The Kidney

49. Definitive Metanephros

50. Relocation of The
Kidney

51. Ascent of the kidneys: Gonad descent

52. Ascent of the kidneys: Gonad descent

53. Changes of developing kidney
• Ascend of the kidney: It ascends from pelvic cavity to its adult
site in the lumbar region on posterior abdominal wall. This is
done by dorso-cranial elongation of the ureter pushing the
kidney.
• Change of blood supply: during ascend.
• Loss of fetal lobulation: surface of the kidney becomes
smooth.
• Change of direction of hilum: from anterior to medial.

54. Partitioning of The Cloaca

55. Partitioning of The Cloaca

56. Partitioning of The Claoca

57. • The definitive kidney formed from the metanephros becomes
functional near the 12th week.
• Urine is passed into the amniotic cavity and mixes with the amniotic
fluid.
• The fluid is swallowed by the fetus and recycles through the kidneys.
• During fetal life, the kidneys are not responsible for excretion of
waste products,
• The placenta serves this function.
Function of The Kidney

58. • During the fourth to seventh weeks of development the cloaca
divides into the urogenital sinus anteriorly and the anal canal
posteriorly.
• The urorectal septum is a layer of mesoderm between the
primitive anal canal and the urogenital sinus.
• The tip of the septum will form the perineal body.
Bladder And Urethra

59. • Three portions of the urogenital sinus can be distinguished:
1. The urinary bladder , (the upper and largest part).
2. The pelvic part of the urogenital sinus, (narrow canal)
3. The phallic part of the urogenital sinus,(flattened from side to
side).
Bladder And Urethra

60. Divisions of the cloaca into the urogenital sinus and anorectal
canal. The mesonephric duct is gradually absorbed into the
wall of the urogenital sinus, and the ureters enter separately.

61. Initially the bladder is continuous with the allantois,
• but when the lumen of the allantois is obliterated, the urachus, remains
and connects the apex of the bladder with the umbilicus.
• In the adult, it is known as the median umbilical ligament.
Bladder And Urethra

62. • The caudal portions of the mesonephric ducts are absorbed into the wall of the
urinary bladder.
• The ureters enter the bladder separately.
• As a result of ascent of the kidneys, the orifices of the ureters move farther
cranially.
During Differentiation of the Cloaca

63. • those of the mesonephric ducts move close together to enter
the prostatic urethra and in the male become the ejaculatory
ducts.
• With time the mesodermal lining of the trigone is replaced by
endodermal epithelium, so that finally the inside of the bladder
is completely lined with endodermal epithelium.
During Differentiation of the Cloaca

64. Separation of The Ureter

65. Separation of The Ureter

66. Urinary Bladder

67. Urinary Bladder & Urethra

68. Male Urethra

69. Female Urethra

70. The prostate gland is formed by buds from the urethra
Prostate Gland

71. ProstaticGland

72. • The epithelium of the urethra in both sexes originates in the
endoderm;
• At the end of the third month, epithelium of the prostatic urethra
begins to proliferate and forms a number of outgrowths that
penetrate the surrounding mesenchyme.
• In the male, these buds form the prostate gland .
• In the female, the cranial part of the urethra gives rise to the
urethral and paraurethral glands.
Urethra

73. Wolffian ducts
Art

74. Netter
Müllerian ducts
Art

75. Acknowledgement

76. 76
Thank you

79. Development of Kidneys and Ureters
Three sets of successive kidneys develop in human embryos.
The first set—pronephroi—
is rudimentary and
nonfunctional
The second set—
mesonephroi—is well
developed and functions
briefly during the early fetal
period.
The third set—
metanephroi forms the
permanent kidneys.

80. Pronephros
These bilateral structures appear early in the fourth week.
They are represented by a few cell clusters and tubular structures
in the neck region
The pronephric ducts run caudally and open into the Cloaca.
The pronephroi soon degenerate; however, most parts of the
pronephric ducts persist and are used by the next set of kidneys.

81. Mesonephros
These large, elongated excretory organs appear late in the fourth week, caudal to the
pronephroi.
The mesonephroi are well developed and function as interim kidneys for approximately 4
weeks, until the permanent kidneys develop and function.
The mesonephric kidneys consist of glomeruli (10–50 per kidney) and tubules.
The mesonephric tubules open into bilateral mesonephric ducts which open into the
cloaca.
The mesonephroi degenerate toward the end of the first trimester.

82. Metanephroi
Metanephroi—primordia of permanent kidneys—begin to develop in
the fifth week and become functional approximately 4 weeks later.
The permanent kidneys develop from two sources:
(1) The ureteric bud (metanephric diverticulum)
(2) The metanephrogenic blastema (metanephric mass of mesenchyme)

83. A uriniferous tubule
consists of two
embryologically
different parts:
A nephron derived
from the
metanephrogenic
blastema
A collecting tubule
derived from the
ureteric bud

84. Initially the primordial permanent kidneys lie close to each other in the pelvis.
As the abdomen and pelvis grow, the kidneys gradually relocate to the abdomen and move farther
apart. They attain their adult position by the ninth week (contact with the suprarenal glands).
This “ascent” results mainly from the growth of the embryo’s body caudal to the kidneys
Initially the hilum of each kidney, where blood vessels, the ureter, and nerves enter and leave, face
ventrally; however, as the kidneys relocate (“ascend”), they rotate medially almost 90 degrees.
By the ninth week, the hila are directed anteromedially. Eventually the kidneys become
retroperitoneal (external to the peritoneum) on the posterior abdominal wall.
Positional Changes of Kidneys

85. Development of Urinary Bladder
• The urogenital sinus is divided into three parts (imaginary):
A vesical part that forms most of the urinary bladder and is continuous
with the allantois.
A pelvic part that becomes the urethra in the neck of the bladder, the
prostatic part of the urethra in males, and the entire urethra in females
A phallic part that grows toward the genital tubercle (primordium of
the penis or clitoris