Embryology and Anatomy of the Urogenital System

The urinary system originates from the dorsal body wall of the developing embryo as a longitudinal elevation of intermediate mesoderm referred to as the urogenital ridge. The urinary bladder is formed as dilated proximal portion of the allantois and is separated from the hindgut by acraniocaudal growth of urorectal septum. This septum divides the rectum from the urogenital sinus; the latter structure eventually develops into part of the urinary bladder and all of the urethra. The newborn foal's kidneys are located retroperitoneally along the dorsal abdominal wall and weigh approximately 175g in the average newborn foal, as compared to 500–600g in adult horse. Urine departs the renal pelvis via the proximal ureter, and in the horse, the structures are lined with mucous glands and goblet cells that secret a thick viscous mucus. The kidneys receive neural input from the sympathetic nervous system that originates from the thoracolumbar spinal cord via aorticorenal and celiacomesenteric ganglia.