pearl sac tissue

[pearlescence]

I've been wondering this for a time...
Pearl sacs are generated from a tissue graft taken from the cells of a donor mollusc. The tissue is usually from the leading edge of cells making shell. So donors with nice shell colour and lustre are selected..(yes?)
Therefore there is no continuity between generations (apart from a general communal one in that spat will be from a localised genetic pool)and no genetic relationship between donor and recipient/host.
What would happen if a pearl sac was used as the donor tissue - so that a particularly good colour line was not lost.
Also are grafts always accepted, never rejected?
And, why are the insides of shells so coloured anyway? Why have the colours developed? I asked an evolutionary biologist and he had no idea (and he has two doctorates!)

 

[LVLanghanke]

This is all conjecture, because I'm not that knowledgeable (yet!) but I would assume there are times when the grafts are rejected. It is foreign tissue, after all, and if they can reject bead nuclei, certainly they can reject tissue nuclei.

I think maybe the color of the inside of the shells is an epiphenomenon, as I can't see an evolutionary advantage to it. But an epiphenomenon to what, I'm not sure. I don't know enough about mollusk biology to even begin to take a guess.

 

[Lagoon Island Pearls]

The tissue is usually from the leading edge of cells making shell. So donors with nice shell colour and lustre are selected..(yes?)

Generally, yes. Technically, not quite. The leading edge of the mantle is a fiberous, muscular tissue. It serves three purposes. First to keep the mantle from perforating, second to extrude the periostracum and the third, to provide tactile sensitivity. This margin has three folds, with outer, middle and inner surfaces. The periostracum is a water tight lining which acts as a barrier between the mantle and the shell. It's extruded between the inner layer of the outer fold and the outer layer of the middle fold. This tissue is trimmed away and the next level of tissue is sectioned for grafting.

While different epithelial cells perform different functions, they can be divided into two categories. Mineralization and non-mineralization.

The colorful layers of the shell lining are laid up by the outer layer of the outer fold of the mantle and continues along the extrapallial cavity toward the heart. The inner layer of the mantle does not mineralize. Instead having a greater density of mucous cells.

Calcite and aragonite are clear crystals. The color of a shell (or pearl) comes from the protein component. The periostracum is largely protein, but as the mantle recedes along the extrapallial space, the layup gradually becomes more mineralized (hence less protein present). As e-cells mature, they eventually generate the bare minimum protein content and aragonitic layup becomes less ordered, even calcitic. For homogeneic grafting purposes, juvenile tissue is preferred. Color is not necessarily a factor to vital regrowth, but a priority in donor selection criteria for aethstetic purposes.

What would happen if a pearl sac was used as the donor tissue - so that a particularly good colour line was not lost.

One of several reasons why re-grafting is performed (not technically a graft, insomuch as a transplant). Although grafting with sacs can be done, there are many reasons why it isn't. First of all, a pearl sac is a super-thin membrane, scarcely measuring a few microns in thickness. Once introduced to air, dries out rapidly or shrinks. It may roll up, invert or become necrotic in the host during convalescence. It may be too mature to regenerate at high rates or yield poor grade pearls. Sectioned juvenile mantle tissue remains viable for a much longer period, hence giving the technician a longer working time and a greater chance for compatibility.

Likewise, grafting would have to occur simultaneous to harvesting. This would require twice the crew. It would also involve seasonal compromises. Pearls are (generally) grafted in spring and harvested in winter. Hence about one year, nine months growth (on average) depending on farm and location. Harvesting tends to be a dirty operation, whereas grafting is preferred in a more sterile environment.

This is why I loathe the word "irritant" as it applies to pearl onset. In human or plant grafts, viability is dependent upon reducing irritation and inflammation otherwise giving rise to infection and septicemia. Epithelial cells do not appear spontaneously in the presence of an irritant. They divide and multiply in the adjacent space only. Natural pearls form largely as a result of "perforation" of epithelial structures. Sometimes, xenogeneic pearls occur when shell bearing parasites become lodged in the mantle. Even though the animal itself dies, it's mantle tissue lives and grows within the host's vascular stream. This is a plausible cause for my octopus pearl. Perhaps a tiny piece of mantle tissue from a clam (it's lunch) became lodged in a healing wound (amputation) near it's beak.

Also are grafts always accepted, never rejected?

It depends on the farm environment, season, species and technician. Warmer water gives rise to a greater number of detrimental pathogens. It's important to remember that mantles do not always build shells. All living things need calcium. During periods of low salinity or reduced food supply, e-cells may "revert", by producing acids which dissolve the shell, that the soft tissues may uptake and re-metabolize the calcium. Grafting during these periods (namely winter) would invariably reduce success rates.

And, why are the insides of shells so coloured anyway? Why have the colours developed? I asked an evolutionary biologist and he had no idea (and he has two doctorates!)

E-cells are skin cells. Like many species of animals, there are slight variations in the components and how they present to the eye. It's almost always a genetic factor, rarely (if ever) environmental.

 

[MSC]

Unless said evolutionary biologist's speciality were in bivalves, they wouldn't be able to answer specific questions such as "how did coloration on the inside of shells evolve and for what purpose," because they wouldn't necessarily know about the mechanisms behind shell formation. That question would be better asked of an aquatic invertebrate biologist that specializes in bivalves. This is particular pet peeve of mine, general public assuming biologists know everything about everything that exists in the world; that's a plainly ridiculous assumption.

 

[pearlescence]

Not assuming that at all MSC. The two I asked were a reproductive biologist who specialised in genetics and evolution and had a sideline in advising SF authors on plausible exo-biology, for which he is well known and the other is a very highly qualified mathematician who specialises in pattern and who has collaborated with Terry Pratchett. They are sufficiently broadly read and out of the box to give an opinion, as well as secure enough to say they didn't know.
In any case a bivalve specialist might not have a handle on why the colourations either. An evolutionary biologist looks at how and why anything evolves, not its biology.
These two exposed the goldilocks exo theories as rubbish years ago (although they still persist, disappointingly)

 

[pearlescence]

You covered it mostly Dave, for which huge thanks.
Couple of follow up questions...I meant reject as in host body recognising transplant as alien (as happens with humans etc) not as in host ejects grafted material (nuke or tissue)
2 I am right then...promising colour selection is limited to looking at the shell. No point whatsoever at looking at the pearl?
(apart from second graft etc into same sac obviously) Do all the tissue graft materials from one donor produce pearls of the same colour?
I recently had a white SS pearl which had a little blue pimple on it, for example)

 

[Lagoon Island Pearls]

Couple of follow up questions...I meant reject as in host body recognising transplant as alien (as happens with humans etc) not as in host ejects grafted material (nuke or tissue)

Immunology (T-cell behavior namely) may vary across animals of the same species. For example, rheumatoid arthritis. While most humans can tolerate a streptococcus infection, a small percentage of the population will develop arthritis or other "autoimmune" diseases, even though affected areas show no signs of the pathogen itself.

2 I am right then...promising colour selection is limited to looking at the shell. No point whatsoever at looking at the pearl? (apart from second graft etc into same sac obviously) Do all the tissue graft materials from one donor produce pearls of the same colour? I recently had a white SS pearl which had a little blue pimple on it, for example)

Yes, for the first part. As to the second, I couldn't say for certain, but I'd speculate there would be near similarities. That would be a good study, by collecting photographic evidence of the donor shells, then segregating the graft recipients for comparison later.

I'm glad you mentioned the blue dot. I've touched on this in other threads in the past. While still highly speculative, I'm inclined to postulate a theory that the classic purple we see in pearls and shells, is a result of an extra thin epithelium. In your case... caused by a prolapsed pearl sac.

So thin is the mantle, the crystalline structure is limited in it's ordering, hence only ultraviolet light is reflected. this is not exclusive to pearls, but to the lining of many mollusks, especially where two shells meet. As mentioned earlier, mantles mineralize on one side only, but in tight spaces mantles may mineralize from both sides, thus producing purple hues.

I'll further advance this theory a little further by discussing SoC pearls, which predominate in purple hues. Perhaps our friend and father-of-pearl Douglas would weigh in, but if I were a betting man... would suggest the pearl sacs of Pteria pearls are markedly thinner than their Pinctada cousins. Likewise Quahogs (Mercenaria mercenaria) being the thinnest of all.

 

[Lagoon Island Pearls]

On other thing worthy of mention is a relationship between color and growth rate.

Mammals are not the only creatures that have monthly lunar periods. in fact, it's highly evident in mollusks. Looking closely at the latticed patterns of Nuttall's Cockle (Clinocardium nuttallii), it's observed to be about six years of age by counting the wide depressions. Between these depressions are nine consecutive layers, ranging broadly from light to dark. each of these layers is one growth cycle. Wider in summer (warm water, more food), narrower in winter (cooler water, less food).

In theory, twelve layers should be present, but as mentioned earlier, mantles do not always build shell, but for periods of quiescence and reversion result in nine distinguishable growth cycles. Each of which presenting with markedly different colors.

 

[pearlescence]

Thanks for all that info Dave. I need to go and absorb it now. Absolutely fascinating. Wow

 

[Pearl Dreams]

This is the most interesting thread I've read in a long time!

 

[pearlescence]

Okay
new questions
still not on same page re rejection of tissue - I mean like a heart transplant. With mammals there are tissue types, blood groups etc and compatibiltiy issues. Can any mollusc be a donor and any a recipient. within a species, within a genus? Hybridisation. Humans are very fussy. Are oysters bovvered?
And second question - still not addressed - why colour when it's dark down there and who's looking anyway? It's not like one little oyster is hoping to be especially pretty for another one.

 

[GemGeek]

Go Dave!

 

[Pearl Dreams]

I had to look up "bovvered". For those of us who are not from the UK, it comes from a comedy sketch with Catherine Tate and means "bothered" (Not bothered = doesn't care.)

 

[Lagoon Island Pearls]

Okay new questions still not on same page re rejection of tissue - I mean like a heart transplant. With mammals there are tissue types, blood groups etc and compatibiltiy issues. Can any mollusc be a donor and any a recipient. within a species, within a genus? Hybridisation. Humans are very fussy. Are oysters bovvered?

There are two types of rejection. Physical and systemic. Physical rejection can happen in several ways. For example, a misplaced or oversized incision. A mollusk's foot is a highly dexterous appendage. When extended or contorted may re-position or expel the graft or bead. Systemic rejection is something yet again. It's difficult to diagnose, even with sophisticated lab work.

Let's speculate for a moment to assume there is an autoimmune response causing rejection. It's nearly impossible to detect unless we monitor the animal 24/7 in a controlled environment. In pathology, systemic issues are difficult determine post mortem, especially in the absence of tumors or other chromosome abnormalities (aberration).

I touched previously upon homogeneic and xenogeneic tissue compatibility. Especially in the case of natural pearls, both are observed having little or no affect upon the overall health of the host. That's not to say it doesn't occur naturally to some degree. Unless a shell is riddled with holes, deformed or discolored, it's infinitely difficult to determine a cause of death unless tissues are sampled perimortem (at or near time of death).

And second question - still not addressed - why colour when it's dark down there and who's looking anyway? It's not like one little oyster is hoping to be especially pretty for another one.

Color is largely coincidental to the structure of the shell or pearl. Most mollusks are epifaunal or benthic in nature. In other words, laying on the bottom or buried in sediments. Silt, mud or other marine organisms will invariably mask outward appearances. There are exceptions though. All mollusks swim in the first stages of life. Swimming scallops and nautilus are quasi-pelagic for their entire life span. Meaning they swim to some degree within the water column. At those times, they're vulnerable to predation. Even in the intertidal zone, where no such masking occurs, abberant coloration (albino for example) would undoubtedly increase the likelihood of mortality by birds or mammals.

 

[BWeaves]

Wow, I agree with Pearl Dreams. This is the most interesting thread I've read here. Lots of amazing information. Thank you, Dave, for answering Pearlescene's great questions. I know we were all wondering the same things, but never asked.

 

[pearlescence]

more brilliant info, more thanks Dave
But of course more questions
What I meant was not death of the host itself but death of the tiny fragment of implant tissue due to the host's immune system spotting it doesn't belong. Obviously a huge bead can be physically rejected (as in ptoui <spits>) but what keeps the tissue graft alive?
and while I take the point about the outside of the shell, which may indeed resemble a muddy mulm or sandy sediment the inside can be colourful - gold, white, pink, blue etc etc etc
Loving this conversation!

 

[Katbran]

Really interesting! Dave is amazing . I'm going to have to read this all a few times ..this is a bit complex for my morning cup f tea in bed !

 

[Lagoon Island Pearls]

what keeps the tissue graft alive?

The vascular supply of the host.

inside can be colourful - gold, white, pink, blue etc etc etc

Yes, internally it's presumed as coincidental to the structure of the shell lining. But maybe not.

A few years ago, I stumbled on something interesting that warrants further investigation. I'll often do autopsies on things washed ashore. Prior to sending tissues samples to the lab for detailed analysis, I'll perform cursory examinations. Sometimes, a physical examination is suitable where damage to tissues are readily observed. Occasionally, no physical damage or anomalies are evident, in which case I will inject stain into the heart. Sometimes I'll observe a rise in blood pressure, meaning the heart and vascular system is good. Cold shock (for example) may be the cause of of death. If it infiltrates the adjacent tissues, there's fibrosis hence the specimen may have died of old age or other tissue atrophy. I use e-Osin Y (yellow) or e-Osin B (blue). These will flouresce under special lighting.

We had a mass stranding of giant squid on our local beaches a few years ago. One of the scientists I worked with provided a radio isotope that was injected in specimens. Out of curiosity, while working with Pododesmus macrochisma (my clear green pearls) in the lab, I used the residual material (it has a short life). Interestingly enough, electrical activity was detected by an oscilloscope when the mantle tissues were stimulated while probing under the microscope.

Why is this important? I suspect cells within the epithelium of mollusks may behave like crude chromatophores. These are pigmented and light reflecting cells, common in many animals such as fish, reptiles and cephalopods. The latter... octopus have visible changes in color by chromatophores. All cephalopods are mollusks. If what I'm speculating is true, these cells might use bioluminescence to order the layup of aragonite during the production of nacreous structures. I do know for certain, nacre is not a random structure, but a result of complex cellular behavior. My ongoing work with Ana, a brilliant young scientist actively pursues "modified behavior" of epithelial cells as applied to mineral structure.

As such color may have some bearing, but how or why is yet to be understood. We're still trying to sort out which factors are implicated in the natural changes between prismatic, nacreous and calcitic structures of the same animal.

 

[Lagoon Island Pearls]

Loving this conversation!

Me too. I'm not a fashionista insomuch as curious about natural processes.

I'm going to have to read this all a few times ..this is a bit complex for my morning cup f tea in bed !

Yes. I try to be concise, but that can be over one's head, so I endeavor to support phraseology with lay terms and analogies that they may be easier to understand.

I can talk about pearls all day long. For anyone lurking, don't be shy to ask questions. While I'll never have all the answers, I strive to find ecological pathways while studying biology and habitat. Climate change for one, as ocean acidification is evident, especially where biology and archaeology meet, but that's a topic for another thread.

 

[pattye]

Great discussion, Dave and Wendy and All! I'll be rereading this more than once! Recommend we all rate this thread up at the top of the page, so it will stay at the top of the Tahitian Pearls forum section where we can come back to it easily. Thanks!!