The subcutaneous fat, like the dermis, is derived embryologically from mesenchyme. Primitive mesenchymal cells give rise to fibrocytes and to adipocytes. An adipocyte synthesizes so much fat in its cytoplasm that the lipid displaces and flattens the nucleus against the periphery of the cell, thereby causing an adipocyte to appear as a large clear cell whose nucleus either is a dark speck at its border or is not visualizable at all in a random section.
The basic unit of the subcutaneous fat is a collection of adipocytes that seem to be cohesive, i.e., a primary microlobule. An aggregation of primary microlobules, termed a secondary lobule, is surrounded by an easily discernible rim of fibrous tissue known as a septum or trabecula (Fig. 1.86).
Struts, termed septa or trabeculae, composed largely of bundles of collagen, divide conglomerations of subcutaneous adipocytes into lobules. The fibrous septa also house muscle-containing vessels, lymphatics, and nerves. (x25)
Arteries and veins in the subcutaneous fat are positioned within the major fibrous septa (Figs. 1.87 and 1.88). Smaller vessels that emanate from large blood vessels in septa branch to supply portions of a secondary lobule. Every adipocyte in the subcutaneous fat is encircled by a capillary.
Architecture of subcutaneous fat, including its vasculature.
Vascular supply to microlobules in the subcutaneous fat.
Knowledge of the architecture of the subcutaneous fat is crucial to application effectively of the algorithmic method based on pattern analysis for diagnosis with specificity of panniculitides. The judgment by a histopathologist to be made first at scanning magnification is whether a particular panniculitis is septal mostly or lobular mostly. The stereotypical example of septal panniculitis is erythema nodosum, whereas the quintessential example of lobular panniculitis is pancreatic panniculitis. Sometimes, a panniculitis is both septal and lobular, which is the situation in nodular vasculitis, the arteritis fundamental to that process being present in a septum and the necrosis of adipocytes and infiltrates of inflammatory cells being distributed throughout lobules.
In sum, mastery of the material set forth in this chapter is essential in preparation for utilizing an algorithmic method based on pattern analysis for diagnosis of inflammatory diseases of the skin and subcutaneous fat. Parenthetically, the information presented here is requisite also for diagnosis, utilizing principles integral to the very same method, for diagnosis of all kinds of pathologic processes, especially of neoplasms, both benign and malignant, with adnexal differentiation. Those neoplasms can be thought of as faulty attempts at formation of structures that come into being during embryogenesis and, that being the case, thorough knowledge of embryology and histology are necessary to diagnosis accurately and comprehension insightfully of those disorders.