The thyroid cells are bounded by a plasma membrane that appears as a single dark line about 80 Å thick. In those areas where two cells lie close together these lines are separated by a space of fairly constant width, about 145 Å. That part of the plasma membrane which bounds the cell from the colloid forms microvilli, which have a maximum length of about 0.44 μ and a maximum width of about 0.14 μ.
The endothelial lining of the capillaries exhibits cytoplasmic discontinuities, the cytoplasm being replaced by a thin (50 Å) membranous structure. The width of the discontinuities is about 400 Å.
The thyroid mitochondria are mostly rod-shaped. They are bordered by a triple-layered membrane consisting of two opaque layers with a less opaque layer interposed. In the interior of the mitochondria there is a large number of similarly triple-layered membranes. The mean thickness of the outer membranes is 165 Å and of the inner membranes 180 Å.
The Golgi zone contains pairs of membranes, vacuolar spaces and small vesicles.
In the cytoplasm there is a well developed system of membranes (α-cytomembranes). These consist of basic membranes, 55 Å thick, to one side of which dense particles with a diameter of about 145 Å are attached. The membranes bound more or less wide spaces which contain a homogeneous material of low opacity.
Two types of big granules are observed delimited by a single membrane, about 50 Å thick. One type has a homogeneous, rather dense content and a regular rounded shape. The granules of the other kind are filled with an inhomogeneous matter and their outline is somewhat wavy.
The nuclear membrane appears as a double membrane consisting of two opaque layers separated by a less opaque interspace. The outer opaque layer seems to belong to the α-cytomembrane system.
