Diaphragmatic structures

Introduction

The diaphragm is a muscular fibrous structure between the chest and the abdomen. It separates two areas with different physiological functions: an upper area geared to the circulation and the exchange of gases and a lower area intended for metabolic assimilation and elimination. Its role is not just respiratory but, as we will see, it takes part in abdominal movement and the mobilization of fascial transmission, and through the intermediary of these it interacts with the cranial, cervicothoracic, and perineal regions.

Embryology

In week 4 we see the appearance in the embryo of three sections, which will divide the body cavity into the pleural, pericardial, and peritoneal cavities. The first to form is a horizontal layer: the transverse septum of our future diaphragm. It divides the body cavity into a primitive pericardial cavity, the future chest, and in the peritoneal cavity, the future abdomen (see Fig. 1.10.1).

Fig. 1.10.1 • Embryologic development of the diaphragm.

The differential growth of the head, the neck, and the trunk will gradually displace the transverse septum from the neck region in a caudal direction until the definitive placement of the future diaphragm. Its anterior margin finally attaches to the anterior wall of the trunk at the level of the 7th thoracic vertebra, while posteriorly it adheres to the esophageal mesenchyme at the level of the 12th thoracic vertebra.

The frontal pleuropericardial folds appear on the lateral wall of the primitive pericardial cavity and grow in a medial direction to unite with each other, as well as with the anterior facet of the mesoblast of the anterior intestine to form the definitive pericardial cavity and the pleural cavities. The muscle fibers participate in the formation of the lower part of the esophagus (Botros et al. 1983). Note that these two cavities initially communicate with the peritoneal cavity through the pleuroperitoneal canals passing behind the transverse septum/ampullary crest.

At this stage there is fascial continuity between the chest and the abdomen. Thereafter, a pair of transverse pleuroperitoneal membranes develops in an anterior direction to join with the transverse septum/ampullary crest and form with this the definitive diaphragm which closes the pleuroperitoneal canals in the 7th week (Greer et al. 2000). The left pleuropericardial canal is wider than the right one and closes later. The mesenchyme associated with the anterior intestine contracts to form the right and left crura. This closure does not constitute an interruption between the thoracic and abdominal cavities; the diaphragm merely represents an intermediary point between the pleuropericardial and abdominal structures. This continuity is an important element in the transmission of forces and pressures between the regions above and below the diaphragm, particularly during movement of the diaphragm.

At the same time, the myoblasts become differentiated in the transverse septum/ampullary crest and will constitute the origin of the muscular part of the diaphragm. They are innervated by segments C3, C4, and C5 and on joining constitute the phrenic nerves which follow the migration of the transverse septum/ampullary crest, provide numerous collaterals (as we will see), and innervate the diaphragm, although the peripheral portion coming from the para-axial mesoblast is innervated by the spinal nerves T7 and T12. The definitive diaphragm is formed from four embryonic structures: the transverse septum/ampullary crest, which will provide the central tendon; the pleuroperitoneal membranes; the para-axial mesoblast of the trunk wall; and the esophageal mesenchyme.

During embryonic development, the nonclosure of the pleuroperitoneal canals is the cause of congenital diaphragmatic hernias, which have a frequency of 1/2000 to 1/4000 births. This nonclosure is more often partial than total and the left side is more often affected than the right, in a proportion of four to eight times, probably because the right cavity is wider, closes later, and contains the liver, the capsule of which is dependent on the diaphragm. Genetic factors also play a part in diaphragmatic hernias (Holder et al. 2007).

Organization

Three parts can be distinguished at the level of the diaphragm (see Fig. 1.10.2).

Fig. 1.10.2 • Diaphragmatic interactions.

Central part

The phrenic center: This is a fibrous layer shaped like a trefoil, stemming from the transverse septum and comprising three leaflets: the anterior leaflet is the most extensive and very rich in lymphatic vessels. Then there is also one leaflet on the right and another one on the left.

It is formed from two types of fibers arranged in three groups (Menck et al. 1990): fundamental fibers directed sagittally in the anterior leaflet, oblique fibers in the lateral leaflets, as well as the connecting inferior and superior semicircular bands. As they cross, they circumscribe a nonstretchable fibrous opening for the passage of the inferior vena cava. This multidirectional, multilayered conformation reinforces the diaphragm’s strength and distributes the forces over the whole surrounding area to reduce the exertion of the vertical components on the abdominal mass. The fibers of the central tendon are made up of three groups.