Mine eyes have been opened. I've always liked histology—much of my thesis work involved fixing, slicing, and staining tissues to characterize the cells therein—but I've always viewed it with a vertebrate bias. Most histology classes (the relative few you can find any more, at least…it's a discipline that isn't as well represented in the undergrad curriculum as it used to be) are packed full of pre-meds, and they focus almost entirely on characterizing vertebrate and especially mammalian tissues.

So it was a pleasant surprise to find a paper by Cole and Hall on comparative histology, with an extensive reference list that's leading me to all kinds of stuff I've neglected before. This is one of the joys of studying science—that you can stumble across an interesting question that you hadn't considered before, and surprise, you find the library and the labs are full of interesting data. And even better, much of the data is just beautiful.

The paper is focused on just one tissue, cartilage, and seeks to characterize its distribution across different animal groups. Here's the abstract:

Tissues similar to vertebrate cartilage have been described throughout the Metazoa. Often the designation of tissues as cartilage within non-vertebrate lineages is based upon sparse supporting data. To be considered cartilage, a tissue should meet a number of histological criteria that include composition and organization of the extracellular matrix. To re-evaluate the distribution and structural properties of these tissues, we have re-investigated the histological properties of many of these tissues from fresh material, and review the existing literature on invertebrate cartilages. Chondroid connective tissue is common amongst invertebrates, and differs from invertebrate cartilage in the structure and organization of the cells that comprise it. Groups having extensive chondroid connective tissue include brachiopods, polychaetes, and urochordates. Cartilage is found within cephalopod mollusks, chelicerate arthropods and sabellid polychaetes. Skeletal tissues found within enteropneust hemichordates are unique in that the extracellular matrix shares many properties with vertebrate cartilage, yet these tissues are completely acellular. The possibility that this tissue may represent a new category of cartilage, acellular cartilage, is discussed. Immunoreactivity of some invertebrate cartilages with antibodies that recognize molecules specific to vertebrate bone suggests an intermediate phenotype between vertebrate cartilage and bone. Although cartilage is found within a number of invertebrate lineages, we find that not all tissues previously reported to be cartilage have the appropriate properties to merit their distinction as cartilage.

Another of the joys of science is the delight we take in feeling ignorance spall and flake away as we hammer at our brains with new information. This was one of those papers where I was constantly thinking, "I did not know that—neat!" as I read it.

Cartilage is a connective tissue: that means that it consists of scattered cells imbedded in a matrix of extracellular material. In the case of cartilage, that matrix is a hydrophilic mucopolysaccharide (chondroitin sulfate) interlaced with collagen and/or elastin fibers. A kind of fiber-reinforced, dense snot, in other words. The matrix has characteristic staining properties, and the tissue is easily recognized in sections; the image to the left is a slice through a piece of vertebrate cartilage, and what you see is pinkish-purple stained cells called chondrocytes within the smooth, grayish-blue matrix. You can also see the purplish, collagen-rich connective tissue sheath that envelops the cartilage, the perichondrium. I've often used simple Alcian Blue staining to visualize cartilage, which turns the matrix a strong, bright blue color.

Cole and Hall developed their own pentachromatic staining protocol, which labels multiple common properties of cartilage in one procedure, and allows them to more accurately determine whether a tissue actually is appropriately called a cartilage. The protocol stains the tissues with the following colors:

They applied these stains to a regular zoo of invertebrates, and found cartilage and chondroid tissues (like cartilage, but lacking the chondrocytes) all over the place…and made some very pretty pictures, too. Here, for instance, are the feeding tentacles of a couple of sabellid polychaete worms, which possess a core of cartilage:

(d) Feeding tentacle from the sabellid polychaete Potamilla. The cartilage is composed of two distinct regions: an inner cellular region (cc) and an outer acellular matrix (arrows). (e) Tentacle from the sabellid polychaete Myxicola showing similar organization as in (d). The large vacuolar chondrocytes (*) are more apparent than in Potamilla.

And here's a lovely example of convergent evolution. At the top is a piece of funnel cartilage from a cuttlefish; at the bottom is a piece of cartilage from the skull of a salmon.

(j) Funnel cartilage from a juvenile cuttlefish, Sepia officinalis, showing newly formed ECM with low mucopolysaccharide content (arrows) at the periphery. Chondrocytes within the interior are large and spherical, similar to vertebrate chondrocytes. (k) Cartilage undergoing endochondral ossification from the head of the Atlantic salmon Salmo salar. Zone of calcification (arrows) retains higher mucopolysaccharide staining than the adjacent hyaline cartilage (hc).

The similarities are impressive. Both have rounded chondrocytes nested in the mucopolysaccharide matrix, with a bounding region rich in collagen, the perichondrium.

Perhaps it isn't too surprising that cephalopods have evolved a tissue similar to our cartilage. One of the points of this paper is that the universality of cartilage-like tissues in the metazoa suggests that this is one of those properties that we inherited from a common ancestor—that using these mucopolysaccharides to build a more rigid framework was a useful feature found in early metazoans.

The data on invertebrate cartilaginous endoskeletons presented here offer unique insights into the evolution of vertebrate skeletal tissues. The ability to form cellular connective tissues structurally similar to cartilage without type II collagen is a feature that appeared before the evolution of vertebrates, supporting the notion that cartilage is not simply an embryonic adaptation (as per Romer), but was present in vertebrates before calcification evolved. Vertebrate cartilage and bone may have arisen from the same ancestral chondroid connective tissue that gave rise to the invertebrate cartilages, as evidenced by the fact that invertebrate cartilages share features not only with vertebrate cartilage but also with bone, such as cell-cell connections, use of type I collagen and possibly osteonectin and bone sialoproteins.

Cole AG, Hall BK (2004) The nature and significance of invertebrate cartilages revisited: distribution and histology of cartilage and cartilage-like tissues within the Metazoa. Zoology 107:261-273.