Septum Transversum

At the end of the third week, intercellular clefts appear in the mesoderm on each side of the midline. When these spaces fuse, the intraembryonic cavity (body cavity), bordered by a somatic mesoderm and a splanchnic mesoderm layer, is formed (Figs. 10.1 and 10.2). With cephalocaudal and lateral folding of the embryo, the intraembryonic cavity extends from the thoracic to the pelvic region. Somatic mesoderm will form the parietal layer of the serous membranes lining the outside of the peritoneal, pleural, and pericardial cavities. The splanchnic layer will form the visceral layer of the serous membranes covering the lungs, heart, and abdominal organs. These layers are continuous at the root of these organs in their cavities (as if a finger were stuck into a balloon, with the layer surrounding the finger being the splanchnic or visceral layer and the rest of the balloon, the somatic or parietal layer surrounding the body cavity). The serous membranes in the abdomen are called peritoneum.

The diaphragm divides the body cavity into the thoracic and peritoneal cavities. It develops from four components: (a) septum transversum (central tendon); (b) pleuroperitoneal membranes; (c) dorsal mesentery of the esophagus; and (d) muscular components of the body wall (Fig. 10.6). Congenital

Pleuropericardial MembraneCases Aeromonas

Figure 10.7 Congenital diaphragmatic hernia. A. Abdominal surface of the diaphragm showing a large defect of the pleuroperitoneal membrane. B. Hernia of the intestinal loops and part of the stomach into the left pleural cavity. The heart and mediastinum are frequently pushed to the right and the left lung compressed. C. Radiograph of a newborn with a large defect in the left side of the diaphragm. Abdominal viscera have entered the thorax through the defect.

Figure 10.7 Congenital diaphragmatic hernia. A. Abdominal surface of the diaphragm showing a large defect of the pleuroperitoneal membrane. B. Hernia of the intestinal loops and part of the stomach into the left pleural cavity. The heart and mediastinum are frequently pushed to the right and the left lung compressed. C. Radiograph of a newborn with a large defect in the left side of the diaphragm. Abdominal viscera have entered the thorax through the defect.

diaphragmatic hernias involving a defect of the pleuroperitoneal membrane on the left side occur frequently.

The thoracic cavity is divided into the pericardial cavity and two pleural cavities for the lungs by the pleuropericardial membranes (Fig. 10.5).

Double layers of peritoneum form mesenteries that suspend the gut tube and provide a pathway for vessels, nerves, and lymphatics to the organs. Initially, the gut tube from the caudal end of the foregut to the end of the hindgut is suspended from the dorsal body wall by dorsal mesentery (Fig. 10.2, C and E). Ventral mesentery derived from the septum transversum exists only in the region of the terminal part of the esophagus, the stomach, and upper portion of the duodenum (see Chapter 13).

Problems to Solve

1. A newborn infant cannot breathe and soon dies. An autopsy reveals a large diaphragmatic defect on the left side, with the stomach and intestines occupying the left side of the thorax. Both lungs are severely hypoplastic. What is the embryological basis for this defect?

2. A child is born with a large defect lateral to the umbilicus. Most of the large and the small bowel protrude through the defect and are not covered by amnion. What is the embryological basis for this abnormality, and should you be concerned that other malformations may be present?

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