Ventral Mesentery

Figure 13.2 Diagrams of the mid- and hindgut regions. The morphogen sonic hedgehog (SHH) is secreted by gut endoderm and induces a nested expression of HOXgenes in surrounding mesoderm. HOX expression then initiates a cascade of genes that "instruct" gut endoderm to differentiate into its regional identities. Signaling between the two tissues is an example of an epithelial-mesenchymal interaction.

Mesenteries

Portions of the gut tube and its derivatives are suspended from the dorsal and ventral body wall by mesenteries, double layers of peritoneum that enclose an organ and connect it to the body wall. Such organs are called intraperitoneal, whereas organs that lie against the posterior body wall and are covered by peritoneum on their anterior surface only (e.g., the kidneys) are considered retroperitoneal. Peritoneal ligaments are double layers of peritoneum (mesenteries) that pass from one organ to another or from an organ to the body wall. Mesenteries and ligaments provide pathways for vessels, nerves, and lymphatics to and from abdominal viscera (Figs. 13.3 and 13.4).

Initially the foregut, midgut, and hindgut are in broad contact with the mesenchyme of the posterior abdominal wall (Fig. 13.3). By the fifth week,

Splanchnic Mesoderm

Figure 13.3 Transverse sections through embryos at various stages of development. A. The intraembryonic cavity, bordered by splanchnic and somatic layers of lateral plate mesoderm, is in open communication with the extraembryonic cavity. B. The intraembryonic cavity is losing its wide connection with the extraembryonic cavity. C. At the end of the fourth week splanchnic mesoderm layers are fused in the midline and form a double-layered membrane (dorsal mesentery) between right and left halves of the body cavity. Ventral mesentery exists only in the region of the septum transversum (not shown). D. Scanning electron micrograph of a mouse embryo at approximately the same stage as in B. The mesoderm (arrowheads) surrounds the gut tube (G) and suspends it from the posterior body wall into the body cavity (C). E. Scanning electron micrograph of a mouse embryo at approximately the same stage as in C. Mesoderm suspends the gut tube from the posterior body wall into the body cavity (C) and is thinning to form the dorsal mesentery (arrow). NT, neural tube; A, dorsal aorta.

Figure 13.3 Transverse sections through embryos at various stages of development. A. The intraembryonic cavity, bordered by splanchnic and somatic layers of lateral plate mesoderm, is in open communication with the extraembryonic cavity. B. The intraembryonic cavity is losing its wide connection with the extraembryonic cavity. C. At the end of the fourth week splanchnic mesoderm layers are fused in the midline and form a double-layered membrane (dorsal mesentery) between right and left halves of the body cavity. Ventral mesentery exists only in the region of the septum transversum (not shown). D. Scanning electron micrograph of a mouse embryo at approximately the same stage as in B. The mesoderm (arrowheads) surrounds the gut tube (G) and suspends it from the posterior body wall into the body cavity (C). E. Scanning electron micrograph of a mouse embryo at approximately the same stage as in C. Mesoderm suspends the gut tube from the posterior body wall into the body cavity (C) and is thinning to form the dorsal mesentery (arrow). NT, neural tube; A, dorsal aorta.

Lesser

Lesser

Ventral And Dorsal Mesentery

Figure 13.4 Primitive dorsal and ventral mesenteries. The liver is connected to the ventral abdominal wall and to the stomach by the falciform ligament and lesser omentum, respectively. The superior mesenteric artery runs through the mesentery proper and continues toward the yolk sac as the vitelline artery.

Figure 13.4 Primitive dorsal and ventral mesenteries. The liver is connected to the ventral abdominal wall and to the stomach by the falciform ligament and lesser omentum, respectively. The superior mesenteric artery runs through the mesentery proper and continues toward the yolk sac as the vitelline artery.

however, the connecting tissue bridge has narrowed, and the caudal part of the foregut, the midgut, and a major part of the hindgut are suspended from the abdominal wall by the dorsal mesentery (Figs. 13.3C and 13.4), which extends from the lower end of the esophagus to the cloacal region of the hindgut. In the region of the stomach it forms the dorsal mesogastrium or greater omentum; in the region of the duodenum it forms the dorsal mesoduodenum; and in the region of the colon it forms the dorsal mesocolon. Dorsal mesentery of the jejunal and ileal loops forms the mesentery proper.

Ventral mesentery, which exists only in the region of the terminal part of the esophagus, the stomach, and the upper part of the duodenum (Fig. 13.4), is derived from the septum transversum. Growth of the liver into the mesenchyme of the septum transversum divides the ventral mesentery into (a) the lesser omentum, extending from the lower portion of the esophagus, the stomach, and the upper portion of the duodenum to the liver, and (b) the falciform ligament, extending from the liver to the ventral body wall (Fig. 13.4).

Foregut

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Responses

  • Leah
    Where is the ventral cavity?
    8 years ago

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