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The Development of the Notochord in Chick Embryos

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The Development of the Notochord in Chick Embryos The Development of the Notochord in Chick Embryos by A. JURAND1 From the Institute of Animal Genetics, Edinburgh WITH EIGHT PLATES THE literature on the notochord, a structure characteristic of all vertebrates, is very extensive, due to the phylogenetic importance of this organ, its role in early embryonic development, and its central position in the developing vertebral column. As early as 1834 the notochord tissue was described by Miiller as being similar in appearance to the parenchyma of plants. Surprisingly, however, in the chick embryo, which is so widely used by embryologists, its development has not very often been the subject of descriptive or experimental investigations. From the early days most work on this fundamental organ was done on fish and amphibians, probably because the notochord in lower vertebrates is more suitable for investigations, as it persists longer, carrying out its function as an embryonic and larval skeleton. Papers have been published in the past describing the fate of the notochord in birds, but mostly in connexion with wider problems, such as the development of the vertebral column (Bruni, 1912; Harman, 1922; Piiper, 1928; Kuhlenbeck, 1930; Williams, 1942; and others) or dealing with some details connected with the notochord, as, for instance, its sheath (Klaatsch, 1895; Held, 1921; Baitsell, 1925; Duncan, 1957 a, b; and others). The life-span of the notochord in birds and mammals is comparatively short. Its transient role, which is to provide rigidity of the longitudinal axis of the embryo, and its inductive potencies in cartilage formation become obsolete as soon as the definite skeleton begins to be formed. In chick embryos the whole period of development and existence of the notochord is limited to about 10 days (i.e. from the 2nd until the 1 lth day of embryonic life). During this period the formation of the prechordal material, its moulding into a rod-like organ, and, later, profound intracellular changes take place. The rate of the morpho- genetic process, as well as the process of internal cytodifferentiation, is un- doubtedly much faster in the notochord than in embryonic structures which develop more slowly but persist longer. For the reasons given, investigations on the morphogenesis and cytodifferen- tiation of the chick notochord appear to be particularly interesting. In the 1 Author's address: The Institute of Animal Genetics, West Mains Road, Edinburgh 9, U.K. [J. Embryol. exp. Morpb. Vol. 10, Part 4, pp. 602-21, December 1962] A. JURAND—DEVELOPMENT OF THE NOTOCHORD 603 normal developmental table of the chick prepared by Hamburger & Hamilton (1951), which still remains the best guide for all embryological work on the development of chick embryos, the changes in the notochord are not actually taken into account, as this work was based exclusively on external characteristics. Some general information on the development of the chick notochord can be found in monographs or textbooks such as those of Waddington (1952, 1956), Hamilton (1952), and Romanoff (1960). More detailed data based on light microscopy have been published by Kuhlenbeck (1930) and Williams (1942). It seemed desirable to investigate more closely the process of normal develop- ment of the chick notochord by means of light- and electron-microscope tech- niques in order to try to co-ordinate all that is known so far. MATERIAL AND METHODS Chick embryos at all developmental stages from the 5th to the 31st were obtained from Brown Leghorn eggs incubated at 38-5° C. (The stages are num- bered throughout the paper according to Hamburger & Hamilton (1951).) For histological reasons, after being dissected from the eggs while the entire yolk was immersed in Pannet-Compton's saline solution, the embryos were fixed for 1 hour in Carnoy's fluid (with chloroform), rinsed thoroughly with abso- lute alcohol, and, after clearing in methyl benzoate, were embedded in wax (m.p. 54° C). Serial transverse or sagittal sections were stained with methyl green-pyronine. For electron-microscope examination the embryos were fixed in toto for 15-45 minutes, depending on age, after being dissected in Pannet-Compton's solution. As a fixative, a modified 1 or 2 per cent, osmium tetroxide solution buffered with sodium barbitone and sodium acetate was used (Palade, 1952; Caulfield, 1957). As the original Palade buffer proved unsatisfactory, a number of modifications were tried. As a result of these technical approaches it was established that the best preservation of tissues in general, and of the notochord in particular, can be achieved by using a 2 per cent, osmium tetroxide solution made up according to the following formula: Sodium barbitone B.D.H. 0-117 g. Sodium acetate cryst. 0-078 g. Sucrose 0-18 g. Osmium tetroxide 0-2 g. 0-1 N hydrochloric acid 2 ml. Distilled water up to 10 ml. The pH of the above buffer solution without osmium tetroxide is 8-15. Its osmotic pressure is similar to that of the original Palade's buffer. The use of a higher pH than that recommended by Palade and other authors greatly im- proves the results, and so do the replacement of sodium chloride by sucrose 604 A. JURAND—DEVELOPMENT OF THE NOTOCHORD (Caulfield, 1957) and the increased percentage of osmium tetroxide (Pease, 1960). In preliminary investigations, a methyl-butyl methacrylate mixture proved to be unsatisfactory as an embedding medium, presumably due to its tendency to polymerization damage. All final embeddings were carried out with Araldite epoxy resin made up according to the original formula recommended by Glauert & Glauert (1958). After being fixed at about 4° C. the embryos were dehydrated with 35, 70, and 95 per cent, alcohol (5-10 minutes in each), and then with three changes of absolute alcohol. Embedding was carried out either with prepolymerized methacrylate mixture after two changes of liquid methacrylate or with the final Araldite mixture after replacing the absolute alcohol by two changes of 1:2- epoxy-propane. In the majority of cases ultra-thin sections were cut transversely to the longi- tudinal axis at the following levels: (1) In head-process and head-fold embryos, approximately in the middle of the length of the chorda-mesoblast concentration. (2) In embryos with 1-12 somites, at the level of the first somite or just anterior to it. (3) In older embryos in the cervical region of the notochord. The methacrylate sections were viewed without using electron 'stains', whereas the Araldite sections were contrasted either by floating, after being mounted on grids with collodion-carbon films, on potassium permanganate (1 per cent.) with uranyl acetate (2-5 per cent.) solution (modification of Lawn's method, 1960) for 20 minutes, or mounted straight on uncovered grids (e.g. Athene, No. 483) and subsequently stained by immersing in saturated lead acetate solution in ether-absolute alcohol (1:1) for 20-30 minutes. The latter method proved to be the more satisfactory. Some Araldite sections were treated by floating them on 10 per cent, phosphotungstic acid solution for 15 minutes and then washed for 5 minutes (Watson, 1958). The material used in the present investigations consisted of 185 chick embryos at various stages. RESULTS Light-microscope observations The aim of this part of the investigations was to have a preliminary and more general look at the developmental process taking place in the notochord, as, for example, the changes in the arrangement of the cells in the course of morpho- genesis of the organ. In particular, an attempt was made to re-examine the changes in the relationship of the chorda-mesoblast and, subsequently, of the notochord to the surrounding derivatives of the germ-layers, to determine the time of formation of the rod-like notochord, and to trace the vacuoliza- tion process and the subsequent appearance of the first involutive changes in the cells. A. JURAND—DEVELOPMENT OF THE NOTOCHORD 605 The histological material was also used in observations on the distribution of dividing cells by counting them at early stages (5-9) to get more information about the mechanism of elongation and growth of the notochord. The chorda-mesoblast condensation first appears at stage 5, at a point anterior to Hensen's node, as an extension of the primitive streak along the axis, in the form of an elongated, dorso-ventrally flattened strip of loosely arranged cells (Plate 1, fig. 1). It is distinctly separated from the medullary plate throughout its entire length by a definite boundary, but at the same time it remains very closely attached to it. The contact surface between the chorda-mesoblast and the medullary plate, which appears to be very intimate, is, however, not even; at regular intervals of about 30-50 ^ along the longitudinal axis of that surface distinct indentations can be found in the medullary plate, into which notochordal cells protrude. These indentations, which continue up to stage 7, can be seen particularly well in sagittal sections. It seems that during these stages the whole chorda-mesoblast tissue is being anchored by means of these indentations to the bottom of the medullary plate (Plate 2, fig. 11). Laterally, the central, comparatively loose mass of the presumptive notochord tissue is confluent with the paraxial mesoblast whose cells are, however, much looser than the axially situated prechordal cells. On the ventral side the chorda-mesoblast condensation at this stage, as well as in a few later stages, appears to be intimately fused with the unicellular layer of the endoderm. At the median line, therefore, the endodermal cells are difficult to distinguish from the prospective notochord cells as described by Hamilton (1952) and also by Fraser (1954) in somewhat older embryos in the pharyngeal region. The unification of both tissues at the head-process stage was described by Ussow (1906) as 'entochorda'.
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