At the time of this writing, these fresh samples of new pearl sacs are in the mail. That is to say, what I know is what you are seeing: little glinting dots of nacre pegged to orange mussel flesh (M. edulis). What I wish to see are pearls being born—truly 'dust'-sized beginnings of pearls, and the cell 'sacs' that are assumed to be making natural pearls possible. I am somewhat surprised that the beginning of pearls and of pearl sacs have not been seen yet.
I have received a few of these last year, but the samples had spent too much time in shipping… no hope to get much detail from the subtly disintegrating cells surrounding the new pearls. It was possible to check that new pearls were still just beginning to form around these readily visible ones. Such flesh is rife with novel mineralization!
This year, it seems possible to get usable, fresh samples. That is to say, there will be some good chances to see pearls and pearl sacs just beginning to form.
What gets everything started—that is not up for grabs. Another question might receive some light: how fine pearls are possible; it has never been clear whether any of the dozen, hundreds of 'dust pearls' may grow large at all, or, if the pearls that do grow to a fine size must start forming elsewhere. Samples like the one above could tell if the pearl sacs are connected in any way to the shell-making skin of the mussels, and how new pearls either continue to grow or are expelled out of the sacs.
In my 2016 samples, most pearl sacs are burst open, so some of the latest nacre is exposed in the patches at the top of the pearls, appearing as rounded mounds covered by a thin skin—the mussel's skin, not the pearl sacs—in the shot below.
It is not obvious how the pearl sacs may have opened as they did, with neatly rounded patches and frayed edges missing from the very top. Sample preparation could be a factor, however, it is known, anecdotally, that natural pearls are often expelled from their sacs: some become embedded in shell growth, some are found loose between body and shell, including packets of 'dust pearls'. The exposed nacre areas are closely comparable with shell nacre, down to fine detail (Checa et al. 2013, 2014).
Sure enough, it is easier to get the earliest growth of pearls from inside pearls—all beautifully preserved. Such were most of my last year's samples. However, there is no telling from the beginning of pearls, what the beginning of pearl sacs might be like.
While the snail mail delivers and snail-paced labworks proceed, there will be some old science to review—good detective work, roughly a century's worth thereof! The downside: much of the best sources have only once been translated from the original German, into Japanese.
What I can read is mainly second-hand smoke—reviews in later, English and French publications, which do follow-up with original work. It will have to do… The trail of bibliography stops before 1930, and there has never been a lot. A couple of titles are still on my To Read list.
What I need to understand from these is what the 'theory' was based on—what had been seen in samples. It is crystal clear that the old theory—the idea that pearl sacs (specialized cells) were needed for pearls to grow—was unusually right. It is not clear what observations were available. Molluscs must have made things then much as they do today, so chances are that some of my material will match the works from the days of yore.
Nearby, mineral granules—not of nacre—were found, and true to the old 'sac theory', the surrounding tissue looks different (mineral fiber in bluish-white to the left, organic material in tones of brown):
Likely, even at the end of 2017 I will not have seen as much material as was published in the old sources, let alone what is seen in the field then, or now. Natural pearls seem to form in unexpected, imprecisely documented anatomical locations, some sacs yielding beauty; most producing samples, for better or worse.
I expect this work to last much of the year. To be blogged, one step at a time: the tedious, the exciting, the predictable, the new…
Checa, Antonio G., et al. "Crystallographic control on the substructure of nacre tablets." Journal of Structural Biology 183.3 (2013): 368–376.
Checa, Antonio G., et al. "Crystalline organization of the fibrous prismatic calcitic layer of the Mediterranean mussel Mytilus galloprovincialis." European Journal of Mineralogy 26.4 (2014): 495–505.