236 Development of the Urinary System

Development of the Urinary System

The urinary system develops from the intermediate mesoderm during prenatal development.

Learning Objectives

Outline the development of the urinary system

Key Takeaways

Key Points

  • The embryonic structures preceding the adult organs of the urinary system include the pronephros, the mesonephros and the metanephros of the kidney, and the Wolffian and Müllerian ducts of the sex organs.
  • The pronephros and the mesonephros degenerate and disappear early during development.
  • The Wolffian ducts are retained in the male reproductive system.
  • The Müllerian ducts are retained in the female reproductive system.
  • Urine formation begins towards the end of the first trimester of fetal development and is excreted into the amniotic fluid.
  • The urinary bladder develops from the upper end of the urogenital sinus.

Key Terms

  • chromaffin cells: Neuroendocrine cells are found in the medulla of the adrenal gland (suprarenal gland, located above the kidneys) and in other ganglia of the sympathetic nervous system.
  • Wolffian ducts: Mesonephric ducts connect the primitive kidney Wolffian body (or mesonephros) to the cloaca and serve as the anlage for certain male reproductive organs.

The urogenital system arises during the fourth week of development from urogenital ridges in the intermediate mesoderm on each side of the primitive aorta. The nephrogenic ridge is the part of the urogenital ridge that forms the urinary system.

Three sets of kidneys develop sequentially in the embryo. The pronephros is rudimentary and nonfunctional, and regresses completely. The mesonephros is functional for only a short period of time and remains as the mesonephric (Wolffian) duct. The metanephros remains as the permanent adult kidney. It develops from the uteric bud, an outgrowth of the mesonephric duct, and the metanephric mesoderm, derived from the caudal part of the nephrogenic ridge.

Urine excreted into the amniotic cavity by the fetus forms a major component of the amniotic fluid. Urine formation begins towards the end of the first trimester (weeks 11 to 12) and continues throughout fetal life. The kidneys develop in the pelvis and ascend during development to their adult anatomical location at T12-L3. This normally happens by the ninth week. The adrenal medulla forms from neural crest cells that migrate into the fetal cortex and differentiate into chromaffin cells. The urinary bladder develops from the upper end of the urogenital sinus, which is continuous with the allantois and lined with endoderm. The lower ends of the metanephric ducts are incorporated into the wall of the urogenital sinus and form the trigone of the bladder. The connective tissue and smooth muscle surrounding the bladder are derived from adjacent splanchnic mesoderm. The allantois degenerates and remains in the adult as a fibrous cord called the urachus (median umbilical ligament).

This diagram of the intermediate mesoderm indicates the ectoderm, endoderm, neural tube, neural canal, notochord, aorta, somite, Wolffian duct, celom, splanic mesoderm, somatic mesoderm.

Intermediate mesoderm: Intermediate mesenchyme or intermediate mesoderm is a type of embryological tissue called “mesoderm” that is located between the paraxial mesoderm and the lateral plate. It develops into the part of the urogenital system (kidneys and gonads), as well as the reproductive system.

Aging and the Urinary System

As kidneys age, the number of filtering units and nephrons decreases, slowing down function.

Learning Objectives

Summarize the changes that may occur in the urinary system with age

Key Takeaways

Key Points

  • Aging kidneys manifest changes at the level of filtering units, nephrons, and blood supply. These changes lead to a less functional system.
  • In aging individuals, disorders may arise as a result of a slower kidney function. These could range from infections to kidney failure.
  • Cancer is a disease of aging. Urinary cancers (bladder and prostate ) are more common in individuals older than age 55.

Key Terms

  • cancer: A disease in which the cells of a tissue undergo uncontrolled and often rapid proliferation.
  • kidney failure: Any condition, acute or chronic, in which the kidneys cease to function properly.
  • urinary incontinence: Any involuntary leakage of urine that could affect quality of life.

The kidneys filter the blood and help remove waste and extra fluid from the body. They also play an important role in controlling the body’s chemical balance. As with other organs, kidney function may be reduced with age.


Distribution of blood vessels in cortex of kidney: As kidneys age, blood vessels harden and blood filtering becomes less efficient.

The kidneys are part of the urinary system, which also includes the ureters and the bladder. Bladder control can be affected by muscle changes and changes in the reproductive system. As the kidneys age, a number of events occur. The number of units that filter waste from the blood (nephrons) decreases, as does the overall amount of kidney tissue. The blood vessels supplying the kidney can harden, slowing filtration of blood. Additionally, the bladder wall changes with age. The elastic tissue becomes tough, and the bladder becomes less stretchy. Muscles weaken, and the organ may not empty completely when urinating.

Kidney function usually remains normal in the elderly, albeit sometimes working more slowly than in a younger individual. However, illness, medications, and other conditions can affect a kidney’s ability to function properly. Changes in the kidneys may affect an elderly person’s ability to concentrate urine. Dehydration can occur if fluid intake is reduced in an attempt to reduce bladder control problems. Aging also increases the risk for urinary disorders such as acute and chronic kidney failure, urinary incontinence, leakage, or retention, and bladder and other urinary tract infections.

Urinary system cancers are associated with advanced age and are more common in the elderly, especially prostate cancer (men) and bladder cancer.


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