Pax-3,

Pax-7, Pax-3,

BMP-4 Parodis c-med

Wnt

Shh

Pax-1,

Pax-9

BMP-4

Fig. 29.4  Ectodermal BMP-4 induces the lateral mesoderm to also produce BMP-4. The sonic hedgehog produced in the neural tuve and notochord stimulates expression of Pax-1 and Pax-9 inside the somite, and as a consequence there is a loss of cell adhesión molecules (N-cadherina), favoring cell emigration typical of monostrati ed pleural mesothelium

Fig. 29.5  Relationship between parietal pleura (PP) and visceral pleura (PV) stained with hematoxylin-eosin. Below the parietal pleura, adipose tissue is observed (A) and below the visceral pleura, the lung parenchyma (P)

Pleural Histology

Both the parietal and visceral pleura are characterized by their thinness and transparency. It isrmly attached by elastic bers to the underlying endothoracic fascia elastic layer (costal pleura) or to the underlying alveolar wall [7] (visceral pleural), respectively.

The visceral pleura structure is made up ofve thin layers distinguishable by light microscopy (Figs. 29.5, 29.6, 29.7) which are from super cial to deep:

•\ Mesothelial layer, which is the outermost layer, formed by mesothelial cells.

•\ Submesothelial connective tissue layer.

•\ Super cial elastic layer, whose thickness varies during the respiratory cycle.

•\ Subpleural connective tissue layer, which constitutes the pulmonary decortication cleavage plane. It contains the blood and lymphatic vessels.

•\ Deep broelastic layer is attached to the alveolar elastic membrane.

Fig. 29.6  Hematoxylin-eosin stained parietal pleura showing the mesothelial cell layer (M), submesothelialbrous tissue (F) and subpleural adipose tissue (A)

Fig. 29.7  Visceral Pleura. Vong Gieson stain for elastin, that stains black the external elástic layer (EE), and brown the collagen bers (C)