The basement‐membrane‐like matrix of the mouse EHS tumor: I. Ultrastructure

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Abstract
The fine structure of the extracellular matrix was examined in the Engelbreth‐Holm‐Swarm (EHS) tumor of the mouse. The matrix is composed of layers parallel to the surface of the associated cells; the layers are poorly defined close to the cells (proximal region) but quite distinct at a distance from the cells (distal region). In the proximal region of the matrix, the indistinct layers are composed of three types of structures: (1) a network of 3‐to 8‐nm‐thick „cords”︁ makes up the bulk of the tissue. (2) Within the network are scattered few 7‐ to 10‐nm‐wide, hollow rods of indefinite length, referred to as „basotubules”︁; their cross section has a dense, more‐or‐less‐circular or pentagonal wall and a light lumen containing a spherule. In addition, many plae profiles similar to these cross sections are present; they are interpreted as small, independent structures. (3) Minute structures composed of two parallel, 3.5‐nm rodlets are referred to as „double pegs.”︁ In the distal region of the matrix, the distinct layers include the same three types of structures, but basotubules are numerous and prominent; in each layer, they are arranged in picket‐fence fashion along two parallel planes. Cords are packed between them. Double pegs are scattered throughout the clear interlayer spaces. Inasmuch as cord network, basotubules, and double pegs are present in the two regions of the tumor matrix, both regions resemble basement membrane. To explain the contrast between the paucity of basotubules in the proximal region and their abundance in the distal region, it is proposed that, as the production of newer matrix by the cells causes the older matrix to be displaced distally, the independent structures seen as pale profiles in the proximal region gradually assemble into basotubules.