Development of Kidneys

Embryonic Origins

• The mammalian kidney develops from the intermediate mesoderm.
• This mesoderm lies between the paraxial and lateral plate mesoderm on the posterior abdominal wall of the embryo.
• Three distinct pairs of embryonic kidneys develop sequentially during gestation.
• The pronephros is the first to form but is non-functional and undergoes apoptosis in mammals.
• The mesonephros forms next and functions temporarily during embryogenesis.
• The mesonephros largely degenerates, but remnants persist in the male reproductive system.
• The metanephros develops last and forms the mature, functional mammalian kidney.

Metanephric Kidney Formation

• Human metanephric kidney development begins in the fifth week of gestation.
• Kidney formation relies on reciprocal inductive interactions between two primary intermediate mesoderm derivatives.
• The ureteric bud emerges as an epithelial evagination from the caudal portion of the nephric (Wolffian) duct.
• The metanephric mesenchyme condenses around the ureteric bud ampulla.
• Glial cell line-derived neurotrophic factor (GDNF) is secreted by the metanephric mesenchyme.
• GDNF binds to the RET receptor tyrosine kinase on the ureteric bud to drive its outgrowth and branching.
• The ureteric bud secretes WNT9b, which induces the adjacent metanephric mesenchyme to condense into cap mesenchyme.

Nephrogenesis

• Nephron formation occurs progressively through several defined morphological stages.
• The cap mesenchyme represents multipotent nephron progenitor cells.
• These progenitor cells self-renew under the influence of the SIX2 transcription factor.
• Secretion of WNT4 from the cap mesenchyme drives the cells to undergo a mesenchymal-to-epithelial transition.
• The transition forms the first epithelial structure, the renal vesicle.
• The renal vesicle elongates and differentiates into a comma-shaped body.
• Further elongation and folding produce an S-shaped body.
• The lower limb of the S-shaped body differentiates into specialized glomerular podocytes.
• Vascular endothelial growth factor (VEGF) recruits endothelial cell precursors into the vascular cleft of the S-shaped body to form glomerular capillaries.
• The middle and upper limbs of the S-shaped body elongate extensively.
• These limbs form the proximal tubule, the loop of Henle, and the distal convoluted tubule.
• Nephrogenesis is normally complete by 34 to 36 weeks of gestation in humans.
• The final nephron endowment ranges widely from 200,000 to 1.8 million nephrons per kidney.
• No new nephrons can be formed postnatally.

Collecting System Development

• The ureteric bud undergoes repeated branching morphogenesis to form the entire collecting system.
• Approximately 15 generations of bifurcations occur during development.
• The earliest branches remodel and coalesce to form the renal pelvis and major and minor calyces.
• Subsequent branching forms the collecting ducts and papillary ducts.
• Ureteric branching morphogenesis is largely completed by the 20th to 22nd week of human gestation.

Renal Ascent and Functional Maturation

• The developing kidneys initially form in the pelvic cavity.
• Between 6 and 9 weeks of gestation, the kidneys ascend to their final lumbar position.
• The kidneys ultimately rest just below the adrenal glands.
• Glomerular filtration begins at 5 to 9 weeks of gestation, initiating fetal urine formation.
• Beyond 16 weeks of gestation, fetal urine production becomes the principal source of amniotic fluid.

Development of Gonads and Genital Ducts

Undifferentiated Stage

• The primitive genital tract in early embryos includes both the Wolffian (mesonephric) and Mullerian (paramesonephric) ducts.
• The gonads are initially bipotential and structurally identical in both sexes.
• Phenotypic sexual differentiation is dictated by genetic, gonadal, and hormonal factors.

Male Gonadal and Genital Development (46,XY)

• The SRY gene on the Y chromosome functions as the primary testis-determining factor.
• SRY triggers the primitive gonad to differentiate into a testis starting at 6 to 7 weeks of gestation.
• Testicular Sertoli cells differentiate and produce Anti-Mullerian Hormone (AMH).
• Testicular Leydig cells differentiate and commence testosterone production around 8 weeks of gestation.
• Testosterone acts locally to stimulate the differentiation of the Wolffian ducts.
• The Wolffian ducts mature into the epididymis, vas deferens, and seminal vesicle.
• AMH causes the active regression of the Mullerian ducts in the male fetus.
• Peripheral conversion of testosterone to dihydrotestosterone is necessary for the development of external male genitalia.

Female Gonadal and Genital Development (46,XX)

• Female sexual differentiation begins approximately two weeks later than male differentiation.
• Development proceeds in the absence of the SRY gene and resulting testicular hormones.
• The lack of local testosterone production results in the passive regression of the Wolffian ducts.
• The absence of AMH allows the Mullerian ducts to persist and grow.
• The Mullerian ducts differentiate into the fallopian tubes, the uterus, and the upper one-third of the vagina.
• At 10 weeks of gestation, the caudal portions of the Mullerian ducts fuse in the midline.
• This fusion creates a Y-shaped structure, where the fused lower section forms the uterovaginal canal.
• The unfused upper portions of the Mullerian ducts remain separate to form the fallopian tubes.
• The fused medial walls initially form a central septum within the uterine cavity.
• This uterine septum undergoes apoptosis and is normally fully resorbed by 20 weeks of gestation.

Summary Tables

Embryonic Origins of Genitourinary Structures

Key Genetic Regulators of Genitourinary Development