83. The Foregut: The Pancreas and Spleen

  1. The pancreas makes its appearance in week 5 as 2 buds originating from the entodermal epithelium of the duodenum, namely, a dorsal bud (opposite and slightly above the hepatic diverticulum) and a ventral bud (in the angle below the hepatic rudiment)
    1. THE LARGER DORSAL BUD appears first and rapidly grows into the dorsal mesentery
      1. The dorsal bud forms the major portion of the pancreas, namely, the upper half of the head, the isthmus, the body, and the tail
    2. THE VENTRAL BUD develops near the entry of the common bile duct into the duodenum
      1. When the duodenum grows and rotates to the right (clockwise) and becomes C-shaped, the ventral pancreatic bud is carried dorsally along with the common bile duct and comes to lie in the mesoduodenum immediately below and behind the dorsal bud
        1. The parenchyma and the duct systems of both buds then fuse
      2. The ventral bud forms the uncinate process and the inferior part of the head of the pancreas
    3. AS THE PANCREATIC BUDS FUSE, their ducts anastomose
      1. The main pancreatic duct (of Wirsung) forms from the duct of the ventral bud and the distal part of the duct of the dorsal bud
      2. The proximal portion of the duct of the dorsal part often persists as the accessory pancreatic duct (of Santorini), that opens just above the main duct
      3. The common bile duct and the duct of Wirsung open into the ampulla of Vater in the second part of the duodenum, either together or separately, with the bile duct above the pancreatic duct
    4. PANCREATIC PARENCHYMA is of entodermal origin and forms a tubular primitive duct network
      1. Acini, early in the fetal period, develop from cell clusters around the ends of the tubules
      2. The islets of Langerhans develop in month 3 from the parenchymatous pancreatic tissue that separates from the tubules and lies between the acini
      3. Insulin secretion begins at about month 5
        1. Since fetal insulin levels are independent from maternal insulin levels, it is unlikely that insulin crosses the placenta
      4. The connective tissue covering and the pancreatic septa form from the surrounding splanchnic mesenchyme
  2. The spleen is derived from mesenchymal cells found between the layers of the dorsal mesogastrium
    1. THE SPLEEN obtains its characteristic shape early in the fetal period
    2. AS A RESULT OF ROTATION OF THE STOMACH, the left surface of the mesogastrium fuses with the peritoneum over the left kidney, accounting for the dorsal attachment of the lienorenal ligament (connecting the spleen to the left kidney) and the passage of the splenic artery behind the peritoneum on its way to the spleen
    3. MESENCHYMAL CELLS DIFFERENTIATE to form the splenic capsule, the connective tissue framework of the spleen, and its parenchyma
    4. THE SPLEEN serves as a hematopoietic organ until late in fetal life; however, lymphocyte and monocyte production continues through life

the foregut: the pancreas and spleen: image #1