Fish to Tetrapod - Tissues and Skeletal Structures

Australia: The Land Where Time Began

Fish to Tetrapod - Tissues and Skeletal Structures

Clack¹ points out that to understand the relationships, physical and developmental, among the various skeletal tissue types, as well as the features formed by them, an explanation of the embryonic origins of these tissue types is necessary. She suggests it is best to observe the characteristics demonstrating the phylogenetic relationships between animals in their embryos, something of their commonality of descent being shown by the developmental history of some skeletal elements. She suggests more detail can be found in standard vertebrate biology textbooks, such as the book by Kardong (2009), or textbooks on developmental biology and embryology (e.g., Raff, 1996; S.B. Carroll et a., 2004; Wolpert et al., 2004).

At a certain size the ball of dividing cells that results from a fertilised ovum reach a point at which the surface to volume ration come into effect, and it becomes necessary to change the shape of the ball in order to keep all component cells close enough to the surface of the ball to allow the continued exchange of nutrients and wastes with the surrounding medium, which it does by invaginating. This is achieved by a small hole appearing in the cell ball and the cells surrounding the hole are swallowed into it, which results in a dimple, then the developing hole, the blastopore, gets progressively deeper. The cell ball eventually becomes hollow as the cells surrounding the hole continue to move into it. Most of the cells have had their eventual fates decided by this stage, with the ball already having a defined top and bottom, front and back. The upper region of the hole’s margin, the dorsal lip of the blastopore, is an important landmark. As the cells move to the inside over this lip they are marked for particular functions in the developing embryo, this entire process in the development of the animal is called gastrulation. Clack¹ suggests it is probably the most crucial moment in the life of the animal; it is at this point that much of the animal’s developmental fate is decided. Some form of this process is common to all multicellular animals.

The embryos of some animals, such as sea anemones, get no further in the process of gastrulation than the 2 layer stage. Most animals progress to the next stage, the triploblastic stage, in which a third layer develops in the space between the other 2 layers. The different parts of a developing embryo are formed by the 3 cell “layers”, though Clack¹ points out that “the third layer is more than just a layer”: in vertebrates the ectoderm is the outer layer, which forms the epidermis, the thin outer layer of skin, and a number of other features. The endoderm is the inner layer that forms the gut lining and other outgrowths from it, such as the liver. The mesoderm is the middle layer of tissue, which forms most structures of the body, including most of the bones, the muscles, and most internal organs, such as the kidneys, blood vessels and glands.

Neurulation is the next stage in embryonic development, the stage at which some of the most basic features of all vertebrates develop. At this stage 2 ridges of tissue gather up to form into parallel crests, the blastopore at one end, which becomes the anus in vertebrates and a small number of other related animal groups, such as echinoderms. Ectodermal layer cells stream towards the ridges, gathering up like waves, both waves eventually meet and merge along the length of the animal, forming a loop at one end and petering out at the other. As the 2 waves meet they enclose a tube beneath, the nerve cord, the loop becoming the head, the swollen end of which becoming the incipient brain. These ridges form within the ectodermal layer, making the brain and nerve cord part of the outer surface, or ectoderm, of the embryo that has become enclosed.

The mesoderm forms a midline structure of its own beneath the ectodermal nerve cord as the embryo grows. The notochord, that is rodlike, forms here, is circular in cross section and extends for the length of the embryo except for the anterior part of the incipient head end. The middle layer of mesoderm begins to divide into segments, beginning at the head end and working towards the rear end. The segments, the somites, become the muscle blocks lying beside the vertebral column and control its movements. Cartilage formed in the mesoderm is the precursor of the bones of the internal limb skeleton, the vertebral column (usually), parts of the skull and braincase, the ribs, and some parts of the limb girdles, the cartilage eventually being replaced by bones as the animal grows. The gill pouches appear just behind the mouth of the growing embryo around the time that the limb buds form.

A population of cells, neural crest cells, that are especially active in the head region, begin to migrate around the body, behaviour that is rather unexpected, though most cells making up an embryo essentially remain in their original location, following an orderly pattern, responding. These cells are formed just at the point where the embryonic ridges make contact with each other, so are of ectodermal origin but they migrate into regions of mesodermal or endodermal structures to reach their destinations. When in their final destination they initiate the development of a wide range of structures that have been found to be unique to vertebrates. Clack¹ suggests it was the development of this tissue, that is highly active, that probably accounts for the development of vertebrates as a group of animals that are diverse and successful. Structures initiated by neural crest cells include jaws, gill arches (including the hyomandibula), the trabecular cartilage of the braincase, sensory capsules housing sense organs, the sensory placodes forming the nasal, optical, ear-related sense organs, and lateral lines, pigment cells, and in concert with mesoderm and normal ectoderm, the teeth, scales, and dermal bones, as well as important structures around the heart.

According to Clack¹ the difference in origin between dermal and endochondral bone is important, as is the bone formed by neural crest and bone formed by mesoderm (all dermal bones originate in neural crest tissue, though not all bone derived from the neural crest is dermal).

Sources & Further reading