There are several samples of the banding periods and growing end ribosomes, along with a tangential portion showing a grid arrangement of ribosomes on what I presume is an RER profile that contains an abundance of surfactant protein A.  The relative sizes marked in black dotted lines are the intervals seen in each separate period of the bands (visible either as 3, 5, 7, or sometimes 9 separate bands) and a bar marker in red (which is the approximate equivalent of the total period (100 nm) and a sample from the internet of a shadow cast molecule of surfactant protein A — in a tiny circle (red circle, white background) which was estimated by the authors to be about 20 nm,  Relative to the width of the micrographs which I have used in this image, the single surfactant molecule is sized accordingly. Because the surfactant A protein was shadow cast it appears more defined that would be seen in reality.

I used an image with a tangential section on the growing end of the protein found in some alveolar type II cells, where clearly the ribosomes were aligned (not in spirals) in a grid.  Using the pretty standard measurement of 100 nm for the distance between major dense bands in the periodicity found in these protein aggregations within the RER, it seems that in a complete period, dense to dense band there are four ribosomes (4 ribosomes per 100 nm) and if one calculates the continuous stacking of these bands (rather than having each period bound individually by RER membranes) then it becomes 3 ribosomes.  The number of ribosomes found in routine sections along the leading edge of protein production is approximately 1.7/period (when counting hundreds of sites, and two species of mammal). The count just in this particular image along the leading edge is @1.6 ribosomes, while measured from the tangential grid of ribosomes, it is 4 per 100 nm.

Images from ferret alveolar type II cells give a little more information about the ribosomal organization, as they are more regular and banding is more easily seen, profiles are more orderly, whereas images from guinea pig intracisternal bodies are more often curved, with the banding crossing, or being circular and disorderly, and also there are larger composites of banding patterns. In mongrel dogs… there is typically a single period with 7 bands…. (in some sense the dog intracisternal bodies resemble a little bit… the birbeck granules in Langerhans cells).  Here is the image used for these numbers.

(red lines are @ 100 nm, centered over contiguous ribosomes, the 100 nm length was calculated from the mean of the proximate banding patterns, shown in dotted lines)

I found another set of very organized (grid style) ribosomes at the growing end of what is likely a profile of RER which contains the layered protein I think might be surfactant protein A (in over-production).  More data are being gathered. Here is yesterdays video with the second tangential set of a dozen or more ribosomes added.

Since the size of surfactant protein A is about half that of the proposed width of a period (the 3, 5, and 7 sometimes prominent layers that general run parallel to the long axis of the RER cisterna (but not always) it would be really convenient to think that just on either side of the dense band there is a ribosome that is producing the protein.  That would be too tidy for words.

Here is a tangential view of the end of an intracisternal body in a type II cell of a ferret which shows, opportunely, an area of about 45 ribosomes (which would be producing the protein within the RER I am calling an intracisternal body, or a granule.  This is an educated guess, since what is behind or in front of the tangential is not really visible, but can be assumed. I am quite certain the original block for transmission electron microscopy has been cut past this field.  The grid overlay in this animation shows the spacing of about 3 ribosomes @ 100 nm.  This is not a complete match for the number of ribosomes I had counted on the growing end of an intracisternal body, which was inbetween 2 –  4, 2 for an incidentally cut period, and 4 for the most opportune.

What is amazing is the regularity of their assembly here, and their regularity in size.

Decades (literally) ago when I began trying to figure out what this organized protein was in type Ii cells, I received some peripheral canine lung tissue from from a researcher whose name I actually don’t remember.  Several blocks were processed as for routine transmission electron microscopy and occasionally a type II cell would show cisternae of RER which had a protein that was organized in layers similarly to the intracisternal bodies in ferret and guinea pig.  Picture is below.  Intracisternal space with paracrystalline protein organized parallel to the long axis of the RER membrane, cyan; funny organization of something (too small to be ribosomes) at the end on the left, green; lamellar bodies, gold; ribosomes, purple. No nuclear or mitochondrial  profiles are in this cropped image of a type II alveolar cell.