91. The Urinary or Excretory System: Intermediate Plate, Nephrogenic Cord, and Pronephros

  1. Introduction: embryologically and anatomically, the urinary system (excretes waste products and excess water via an intricate tubular system in the kidneys) and the genital system (assures continuity of the race by production of germ cells) are closely associated, especially in early stages of development. Both develop from a common mesodermal ridge along the posterior abdominal wall, and the excretory ducts of both systems initially enter a common cavity, the cloaca. In the male, the urethra conveys both urine and semen; although separate in the female, the urethra and vagina both open into a common vestibule. The genital system is discussed elsewhere
  2. The intermediate plate: the urinary system is derived from the intermediate mesoderm or plate (lying between the paraxial (somite-forming) mesoderm and the lateral plate) and the cloaca
    1. THE URINARY SYSTEM develops in a craniocaudal direction in successive chronologic steps
      1. The definitive kidney or metanephros is preceded by 2 transitory structures
        1. The pronephros may be thought of as a "rough draft" and is rapidly replaced
        2. The mesonephros reaches complete development but later predominantly regresses. Remnants of this system are incorporated into the urogenital system
  3. The nephrogenic cord: the intermediate mesoderm migrates ventrally and loses its connections with the somites. This longitudinal mass of nephrogenic mesoderm on each side of the body becomes the nephrogenic cord
    1. THE NEPHROGENIC CORD is at first continuous with the paraxial mesoderm (internally) and the lateral plate (externally), but it later separates yet remains close to the intraembryonic coelom
    2. LIKE SOMITE MESODERM, the nephrogenic cord undergoes metameric segmentation into nephrotomes
      1. Metamerization is clear at the cranial end of the embryo, rudimentary in its middle portion, and almost nonexistent at its caudal end, where the nephrogenic mesoderm remains undivided
      2. The nephrogenic cords give rise to the renal tubules of the kidney
    3. THE NEPHROGENIC CORDS produce bilateral longitudinal bulges, the urogenital ridges, on the dorsal wall of the coelomic cavity. The ridges give rise to both nephric and genital structures
  4. Development of the nephrogenic cords: arising from the long ribbons of nephrogenic cords, each transitory kidney develops from 3 primordia which succeed each other, not only in time but in space, and differentiate progressively from the cervical to the caudal region of the embryo. The primordia are referred to as the pronephros, mesonephros, and metanephros, from cervical to caudal region, respectively
    1. THE PRONEPHROS (forekidney) differentiates at the end of week 3 in the cervical region and is nonfunctional. It disappears at the end of week 4. Cranialcaudal development is especially true of this transitory kidney
      1. First, the nephrogenic cord cleaves into nephrotomes
      2. Second, each nephrotome hollows out into a nephrotomal vesicle, which becomes oval in shape
      3. Third, the union of vesicles forms the beginning of the pronephric duct, which progresses toward the cloaca
      4. Fourth, while above is taking place, the pronephros degenerates and disappears
      5. Currently, it is thought that the pronephros disappears completely, leaving no vestiges, thus, it is the mesonephros that forms its own collecting duct
        1. The classical conception was that the pronephric duct persisted and was used by the mesonephros

the urinary or excretory system: intermediate plate, nephrogenic cord, and pronephros: image #1