M iscellaneous elementary lesions that can be encountered in presence of a diffuse lung disease are (among others) the following.
Hyaline membranes. Characteristic of the exudative phase of a diffuse alveolar damage (DAD), they consist of proteinaceous exudate which adheres to the inner surface of the alveoli. During the subsequent proliferative phase, the membranes are incorporated within the alveolar septa, which become thick.
Diffuse alveolar hemorrhage (DAH). Besides red cells, hemorrhage is characterized by fibrin and hemosiderin-laden macrophages, which can be appreciated both on cytology in BAL fluid and at histology. Capillaritis is often part of the picture; nevertheless, it represents a transient phenomenon and therefore it's inconsistently seen.
Pulmonary Alveolar Proteinosis (PAP). Pulmonary alveolar proteinosis consists of amorphous, lipid-rich, eosinophilic material with small clumps and cholesterol clefts, within the alveolar spaces with a rim of retraction at the edge.
Diffuse lung damage (DAD), pulmonary alveolar proteinosis (PAP) and diffuse alveolar hemorrhage (DAH).
Top left: exudative phase of DAD with hyaline membranes. The membrane is the pink amorphous material (blue star) that adheres to the inner surface of the alveolus (white).
Top right: proliferative phase of DAD. The hyaline membranes are now incorporated within the alveolar septa that are thickened (blue star).
Bottom left: pulmonary alveolar proteinosis (PAP). An amorphous, lipid-rich, eosinophilic material completely fills the alveolar spaces leaving a white thread-like rim of retraction at the edge.
Bottom right: diffuse alveolar hemorrhage (DAH). Red cells, fibrin and hemosiderin-laden macrophages speckle the alveolar spaces.
Amyloid. Whether nodular or interstitial, isolated finding or associated to chronic inflammation or low grade lymphomas, it consists of the deposition of an homogeneous, acellular, pink material which can calcifify or ossify.
Microlitiasis. Tiny, calcified micronodules fill the alveolar spaces.
Lymphangioleiomyomatosis (LAM). It is characterized by the presence of thin-walled cysts containing smooth muscle bundles. LAM cells are spindle shaped and plump, with vacuolated cytoplasm.
Necrosis. Necrosis refers to the premature death of cells in living tissues; this is a detrimental event as opposed to apoptosis, which is actually the programmed cell death. It mostly appears as amorphous material with or without inflammatory cells.
Histologically, tissue cells and lung architecture (including vessels) are no longer appreciable in any kind of necrosis; so, the necrotic portion of a process results avascular in a computed tomography performed after administration of contrast medium. Cavitation of the necrotic areas is also possible and frequent.
Coagulative necrosis is characteristic of hypoxic conditions, such as infarction. It consists of homogeneous, eosinophilic material almost devoid of inflammatory cells.
Suppurative (colliquative) necrosis, on the contrary, is characteristic of infectious diseases (bacterial, viral or fungal) but it can be also observed in other clinical contexts (toxic, immunological, aspiration, etc.) and it is also characteristic of vasculitides, such as Wegener granulomatosis (WG) and Churg Strauss syndrome (CSS). Suppurative necrosis is characterized by numerous neutrophils associated with fibrin and abscess formation, filling and often destroying the alveolar structures.
Caseous necrosis refers to the gross appearance of this necrosis of mycobacterial, namely, tubercular origin. Its amount, architecture and distribution depend on the immunological settings of the patient.
Tumoral necrosis. This necrosis presents intermediate features between coagulative and colliquative and may beobserved in several types of malignancy, mostly epithelial (e.g., large-cell neuroendocrine carcinoma-LCNEC) and lymphoid (e.g., large cell lymphoma).
Amyloid, microlitiasis, lymph-angioleiomyomatosis, necrosis.
Top left: homogeneous, acellular, pink material typical of amyloidosis.
Top right: tiny, calcified micronodules fill the alveolar spaces, typical of microlitiasis.
Bottom left: spindle shaped cells with vacuolated cytoplasm, typical of lymphangioleiomyomatosis.
Bottom right: large cavitated caseous necrotic tubercular lesion.