84. Development of The Midgut: General Introduction

  1. The midgut: in the 5 mm embryo, it is seen beginning just caudal to the entrance of the bile duct into the duodenum and terminating at the beginning of the last third of the transverse colon (from anterior to posterior intestinal portals). It is suspended from the dorsal abdominal wall by a short mesentery and communicates with the yolk sac via the vitelline duct. Its derivatives consist of the small intestines (except the first part of the duodenum to the common bile duct entrance); the cecum, the appendix, the ascending colon; and the right one-half to two-thirds or proximal part of the transverse colon. It is supplied by the superior mesenteric artery and vagus nerve
  2. Midgut development is characterized by a rapid elongation of the gut and its mesentery
    1. ROTATION AND FIXATION initially, the midgut communicates with the yolk sac, but this connection narrows to the yolk stalk or vitelline duct. Elongation of the gut occurs faster than elongation of the embryo's body, thus, a series of intestinal changes takes place, usually in 3 stages
      1. Physiologic herniation of the midgut
        1. As it elongates, the midgut forms a ventral U-shaped umbilical loop of gut, the primary intestinal loop, which projects into the umbilical cord
        2. This "herniation" takes place at weeks 6 to 10 and is a normal migration of the midgut into the extraembryonic coelom. It occurs because there is not enough room in the abdomen to accommodate the fast-growing midgut due to the space occupied by the massive liver and the kidneys
          1. Thus at this stage, the intraembryonic and extraembryonic coeloms communicate at the umbilicus, and the midgut develops entirely outside the abdominal cavity
        3. The midgut has 2 limbs: a proximal (cranial) and a distal (caudal) lim The yolk stalk is attached to the apex of the loop at their junction. If the duct persists, it is called Meckel's diverticulum
          1. The proximal limb grows rapidly to form the small intestinal coils (distal duodenum, jejunum, and ileum)
          2. The caudal limb changes little except for developing the lower ileum, the cecal diverticulum, the appendix, the ascending colon, and proximal two-thirds of the transverse colon
        4. In the umbilical cord, the midgut loop rotates 90? counterclockwise (seen from in front) around the axis of the superior mesenteric artery, bringing the proximal limb of the loop to the right and the distal limb to the left
          1. From the artery arise the colic branches for the caudal limb and jejunoileal branches for the proximal limb
      2. Return of the midgut (reduction of the midgut hernia)
        1. During week 10, the intestines return to the abdomen
        2. The proximal limb (jejunum of the small intestines) returns first and passes behind the superior mesenteric artery to the left sid The later returning loops settle more to the right
          1. The cecal swelling is seen in the l2 mm embryo as a small conical swelling on the caudal limb of the primitive intestinal loop and is the last part to reenter the cavity
        3. As they return, the gut undergoes another 180? counterclockwise rotation, placing the cecum and appendix near the right lobe of the liver, from where they descend into the right iliac fossa at a later date
          1. Total rotation is thus: 90? + 180? = 270?
        4. Return to the abdomen is related to a decrease in relative size of the liver and mesonephric kidneys, as well as abdominal enlargement and expansion
      3. Fixation of the midgut: see Section 85 next

development of the midgut:  general introduction: image #1