Diaphragm

The fourth to sixth weeks of development mark the division of the coelom into the definitive pericardial, pleural, and peritoneal spaces. The pleuropericardial membranes, mesenchyme derived from the septum transversum, the dorsal mesentery of the esophagus, and myoblasts from the abdominal wall all contribute to the formation of the diaphragm (Fig. 2-3).

Failure of the diaphragmatic components to properly unite may leave an opening, especially on the left, for abdominal contents to pass into the thorax (foramen of

Intraembryonic Coelom

Fig. 2—1. Cross-sectional schematic through the midportion of the embryo early in the fourth week shows infolding of the ectoderm and mesoderm (somatopleure) as it begins to encase the intraembryonic coelom. This will eventually encompass the body cavity. The splanchnopleure's (endoderm and mesoderm) contribution to the formation of the midgut is evident as well.

(From Javors BR, Sloves JH.4)

Fig. 2—1. Cross-sectional schematic through the midportion of the embryo early in the fourth week shows infolding of the ectoderm and mesoderm (somatopleure) as it begins to encase the intraembryonic coelom. This will eventually encompass the body cavity. The splanchnopleure's (endoderm and mesoderm) contribution to the formation of the midgut is evident as well.

(From Javors BR, Sloves JH.4)

Bochdalek hernia) ' (Fig. 2-4). A retrosternal weakness (more frequently on the right) at the defect through which the superior epigastric vessels pass may allow herniation of omentum and, less likely, colon through the resultant foramen of Morgagni.4

both its ventral and dorsal mesenteries. The ventral mesentery that lies between the distal foregut (stomach and duodenum) and the liver forms the gastrohepatic and, more distally, the hepatoduodenal ligaments.5 That portion of the ventral mesentery that lies between the developing liver and the anterior abdominal wall persists as the falciform ligament, with the obliterated umbilical vein (ligamentum teres) lying in its free edge. However, during the second month of development, the distal foregut undergoes asymmetric growth with the dorsal aspect growing much more rapidly. Along with this dorsal bulge, the distal foregut rotates clockwise (as seen from the front) about its anteroposterior axis and as clockwise (as seen from below) about its longitudinal axis. Therefore, the dorsal bulge presents to the left and is convex inferolateral, forming the greater curvature of the stomach. The original ventral concavity of the developing stomach is carried to the right, forming the definitive lesser curvature (Fig. 2-5).

The change from the foregut to the midgut is marked by a change in arterial supply, from the celiac to the superior mesenteric arteries, respectively. This occurs at the level of the duodenal papilla. As the stomach rotates to the left, the duodenum, which had buckled ventrally (completing an S-shaped configuration with the dorsal gastric bulge), is carried to the right into its definitive position. Its dorsal mesentery is eventually resorbed, resulting in its "retroperitoneal" location, although its anterior surface is still covered by peritoneum. Failure of the mesentery to be completely absorbed results in an elongated and redundant appearance of the proximal duodenum (Fig. 2-6).

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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