Stomach

The stomach appears as a fusiform dilation of the foregut in the fourth week of development (Fig. 13.8). During the following weeks, its appearance and position change greatly as a result of the different rates of growth in various regions of its wall and the changes in position of surrounding organs. Positional changes of the stomach are most easily explained by assuming that it rotates around a longitudinal and an anteroposterior axis (Fig. 13.8).

The stomach rotates 90° clockwise around its longitudinal axis, causing its left side to face anteriorly and its right side to face posteriorly (Fig. 13.8, A-C). Hence the left vagus nerve, initially innervating the left side of the stomach, now innervates the anterior wall; similarly, the right vagus nerve innervates the

Longitudinal Rotation Stomach
Figure 13.8 A, B, and C. Rotation of the stomach along its longitudinal axis as seen anteriorly. D and E. Rotation of the stomach around the anteroposterior axis. Note the change in position of the pylorus and cardia.

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Intraembryonic Mesogastrium
Figure 13.9 A. Transverse section through a 4-week embryo showing intercellular clefts appearing in the dorsal mesogastrium. B and C. The clefts have fused, and the omental bursa is formed as an extension of the right side of the intraembryonic cavity behind the stomach.

posterior wall. During this rotation the original posterior wall of the stomach grows faster than the anterior portion, forming the greater and lesser curvatures (Fig. 13.8C).

The cephalic and caudal ends of the stomach originally lie in the midline, but during further growth the stomach rotates around an anteroposterior axis, such that the caudal or pyloric part moves to the right and upward and the cephalic or cardiac portion moves to the left and slightly downward (Fig. 13.8, D and E). The stomach thus assumes its final position, its axis running from above left to below right.

Since the stomach is attached to the dorsal body wall by the dorsal meso-gastrium and to the ventral body wall by the ventral mesogastrium (Figs. 13.4 and 13.9A), its rotation and disproportionate growth alter the position of these mesenteries. Rotation about the longitudinal axis pulls the dorsal meso-gastrium to the left, creating a space behind the stomach called the omental bursa (lesser peritoneal sac) (Figs. 13.9 and 13.10). This rotation also pulls the ventral mesogastrium to the right. As this process continues in the fifth week of development, the spleen primordium appears as a mesodermal proliferation between the two leaves of the dorsal mesogastrium (Figs. 13.10 and 13.11). With continued rotation of the stomach, the dorsal mesogastrium lengthens, and the portion between the spleen and dorsal midline swings to the left and fuses with the peritoneum of the posterior abdominal wall (Figs. 13.10 and 13.11). The posterior leaf of the dorsal mesogastrium and the peritoneum along this line of fusion degenerate. The spleen, which remains intraperitoneal, is then connected to the body wall in the region of the left kidney by the lienorenal ligament and to the stomach by the gastrolienal ligament (Figs. 13.10 and

Lesser omentum Liver

Stomach

Lesser omentum Liver

Stomach

Intraembryonic Mesogastrium

Umbilical

Falciform ligament

Spleen

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