Many types of high-polymeric substances,which might be termed fibrous, occur in the tissues of animals. Some of these substances, however, consist predominantly of one type of macromolecule, and aggregations of such macromolecules form sizeable fibres which serve principally in structural, mechanical and protective roles. Amongst these long chain compounds are polysaccharides such as chitin and tunic in and proteins such as keratin, collagen, elastin and myosin.
Since such compounds consist of chains of regularly repeated groups of atoms, they lend themselves to X-ray diffraction studies. Also, information on their axial periodicities, orientation and surface contours can be gleaned by use of the electron microscope. In recent years the application of these techniques has provided much data on the submicroscopic structure of animal fibres which has greatly supplemented data on their coarser structure obtained by classical histological methods.
The chemistry of such fibres has also been the subject of intensive research. Here, however, investigators of the fibres of animal tissues have been faced with the major problem of obtaining pure samples for analyses. The isolation of fibres free from extraneous materials presents difficulties but after this isolation has been accomplished there remains the question of whether the fibres are chemically homogeneous. Thus Haurowitz (1950 p, 175) says of the Insoluble structural proteins "we can neither reprecipitate nor recrystallize them and hence have no criteria to test conclusively whether they are uniform or consist of mixtures of various similar substances" . An understanding of the mode of operation of animal fibres also requires a knowledge of the chemical constitution and mode of linkage of other macromolecules such as certain polysaccharides and nucleic acids with which they are normally associated. Such a knowledge is not easy to obtain for there are many complicating factors. Chargaff (1955) for example pointed out that there are systematic differences in the nucleotide components of nucleic acids.
In spite of these difficulties a considerable body of data has accumulated on the chemistry of animal fibres. However, it is becoming clear that studies of the biosynthesis, mode of deposition and biological role of each of these fibres is essential for a full evaluation of the results obtained by recent physico-chemical investigations . Also it is necessary that these results should be put in to their proper perspective in modern biology.
Furthermore, the emphasis in recent research on fibrous structures has been placed on those occurring in vertebrates and, although chitinous structures have received considerable attention, other invertebrate fibrous structures have been comparatively neglected. It is of great importance from the viewpoint of phylogenetic relationships with in the animal kingdom and from considerations with respect to possible biochemical evolution of the fibrous structures of animals that these structures be investigated fully in each animal group.
Partly because of the above considerations it was decided to investigate the macroscopic and microscopic structure, chemical constitution, mode of formation and deposition and the biological role of the connective tissue in the test of a local ascidian, Pyura stolonifera (Heller) and also in the Cuvierlan tubules of a local holothurian, Holothuria leucospilota (Brandt). Other factors which prompted the choice of experimental material were:
(1) An adequate supply of fresh specimens was assured.
(2) The test of Pyura stolonifera and the Cuvierian tubules of Holothuria leucospilota contain an abundance of connective tissue which is readily separable from most other tissues.
(3) Because of phylogenetic considerations it was thought that study of connective tissues in the Tunicata and Echinodermata might throw some light on the lineage of vertebrate connective tissue.
It soon became apparent that necessary prerequisites for the understanding of the elaboration of fibres in the ascidian and holothurian studied were investigations of the cytochemistry and roles of blood corpuscles and coelomocytes respectively...