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The Development of the Chick Embryo Diencephalon and Mesencephalon During the Initial Phases of Neuroblast Differentiation

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The Development of the Chick Embryo Diencephalon and Mesencephalon During the Initial Phases of Neuroblast Differentiation /. Embryol. exp. Morph., Vol. 16, 3, pp. 497-517, December 1966 497 With 2 plates Printed in Great Britain The development of the chick embryo diencephalon and mesencephalon during the initial phases of neuroblast differentiation By KATHERINE M. LYSER1 Harvard Biological Laboratories, Cambridge, Massachusetts, and Hunter College of the City University of New York The development of the nervous system presents many interesting problems as a developing system with numerous parameters of differentiation as well as from the point of view of the establishment of adult structure and function. With our growing understanding of developmental processes in general, and inter- actions at various stages of development in particular, it should be profitable to study more closely events of each period in a developing system, looking for information concerning their immediate control and their relation to events of other periods. In the nervous system, one phase which should be investigated much more thoroughly—especially from the point of view of the control of cellular differentiation—is that of the initial appearance of neuroblast cells and formation of the first nerve processes. Most studies of normal embryos which have included the period of initial differentiation have been primarily concerned with tracing the origins of definitive nuclei and fiber tracts, though possible mechanisms controlling various aspects of their development have of course been discussed. The present study is concerned specifically with the period of initial differen- tiation of cells and fibers in the diencephalon and mesencephalon of the chick embryo. This region has been chosen because it is among the early areas of differentiation, and it contains a number of different centers, which are not continuous with other areas of differentiation at first. This study was begun as part of a thesis (Lyser, 1960) and reconsidered in the light of recent work in related fields. In the chick embryo, neuroblasts with processes appear first in the hind brain and shortly thereafter in the diencephalon and mesencephalon, where the first neuroblasts with processes have been reported at 17- or 18-somite stages 1 Author's address: Department of Biological Sciences, Hunter College, New York, 10021, U.S.A. 498 K. M. LYSER (Tello, 1923; Windle & Austin, 1936). The initial differentiation of neurites thus takes place quite early in the development of the central nervous system. In the spinal cord of the chick embryo (Hamburger, 1948), and presumably in the brain also, initial neuroblast differentiation begins while or before pro- liferation has reached its peak (cf. Hamburger, 1948; Tello, 1923; Windle & Austin, 1936) and so overlaps this phase. It is of course continuous with the later phases of development, including histological differentiation, but it begins well before these become apparent. In the diencephalon and mesencephalon the mantle layer does not become distinct from the inner cell layer nor can the longitudinal columns of cells be distinguished until about the fourth day (Palmgren, 1921; Rendahl, 1924; Kuhlenbeck, 1937). MATERIALS AND METHODS Forty-five 13- through 30-somite chick embryos were studied. All the em- bryos were serially sectioned, either sagitally or transversely, and stained for nerve fibers with silver. Eighteen embryos were from the collection of Professor Leigh Hoadley. These embryos had been fixed in 95 % ethanol and stained with pre-war German Protargol by a modified Bodian (1936) method. The other twenty-seven embryos were prepared for this study. A number of fixatives recommended for the embryonic nervous system and several silver stains were tried in various combinations. Staining by a modified Holmes's (1942) method was most satisfactory. For these young embryos, the following fixatives were found to be useful: 95 % ethanol, Bodian's fixative no. 2 or no. 4 (Bodian, 1937), Mahdissen's fixative as given by Gray (1954, p. 192), Lavdowsky's mixture as given by Guyer (1953, p. 236), or Lavdowsky's mixture modified by substituting formic acid (1-6 ml) for acetic acid (2-0 ml). Embryos remained in ethanol for H h or in one of the other fixatives for approximately 24 h. They were stored in 70 % or 80 % ethanol, dehydrated in ethanol, cleared in cedar-wood oil, and embedded in 60-63 °C Tissuemat (Fisher). Serial sections were cut at 10 or 12 /*. Graphic reconstructions of some of the younger embryos were made by drawing neuroblast cells with processes and other segments of fibers on camera lucida tracings of each section and then tracing these on to an outline of the brain (Text-figs. 1-3). All cells and fibers which could be seen were recorded. In addition, diagrams of the pattern in some of the older embryos were made by sketching representative cells and fibers on an outline of the brain as the sections were studied (Text-figs. 4-6). In these drawings the actual number of cells present is not indicated; only a few are shown, illustrating the locations and orientations of the neuroblasts and fibers observed. Initial neuroblast differentiation 499 OBSERVATIONS There is some variation in the development of the neuroblasts and nerve fibers among embryos of the same stage as determined by somite count. This appears to be due to a difference in the time at which differentiation of axons begins in this area of the brain in relation to the development of the somites. However, development is fairly regular in general location and arrangement of cells and fibers and in the order of their appearance. The embryos can be arranged in sequence by considering the numbers and distribution of cells and fibers together with the number of somites and incubation time. The earliest stage at which fibers were identified in the diencephalon and mesencephalon in the group of embryos studied was the 14-somite stage. For purposes of description, development from the earliest appearance of nerve fibers in this region through the 30-somite stage has been divided arbitrarily into five periods: (A) 14- to 16-somite embryos in which axon differentiation in this region is just beginning, (B) 16-somite embryos in which differentiation is slightly more advanced, (C) 17- to 18-somite embryos, (D) 19- to 22-somite embryos and (E) 23- to 30-somite embryos. At the beginning of the period of development under consideration, the wall of the neural tube is essentially a pseudostratified columnar epithelium, the cells of which may be referred to as neural epithelial cells. Those which are undergoing division move toward the neurocoel; the nuclei of interphase cells are at various levels (Sauer & Walker, 1959; Sidman, Miale & Feder, 1959; Fujita, 1963). At the outer edge is a nucleus-free zone, consisting of the outer ends of the epithelial cells, where the marginal layer will subsequently form. This will be referred to here as the 'peripheral zone'; the area between the peripheral zone and the neurocoel will be called the 'nuclear zone'. Neuroblasts and fibers that are parallel to the surface of the neural tube and oriented in a dorso-ventral direction or obliquely will be referred to as 'circumferential', those which are parallel to the longitudinal axis of the neural tube as 'longi- tudinal \ and those which are perpendicular to the margin of the neural tube as 'radial'. In these preparations, neuroblasts with processes stand out because the cytoplasm is more darkly stained than the cytoplasm of adjacent epithelial cells. Their processes and other segments of fibers are black. It usually is not possible to trace a fiber that extends through several sections from one section to the next, even in the younger embryos where there are only a few fibers. As with other methods for identifying nerve cells and fibers, the question of whether all neuroblasts are stained can be raised. It seems likely that with this method most neuroblasts are recognizable, but it cannot be definitely determined that all are stained. Silver methods may demonstrate only nerve cells with neuro- fibrillae (Guillery, 1965; Gray & Guillery, 1966), but there is no evidence at present that there are nerve processes in the early embryo which do not contain 500 K. M. LYSER neuro-fibrillae or neuro-tubules. In electron micrographs of motor neuroblasts of chick embryos, for example, all the axons which could be definitely identified contained fibrillar structures (Lyser, 1964). Two other problems encountered in studying silver-stained sections should be remembered in regard to the intent and the basis of interpretation of observa- tions. As indicated above, in well-stained sections nerve fibers and the cell bodies of neuroblasts with processes are usually distinct. Sometimes, however, the edges of epithelial cells are dark and difficult to distinguish from axons, or fibers do not show up well and neuroblasts from which processes arise cannot be clearly distinguished. In descriptions of individual embryos the lower number of fibers recorded includes only those which can be identified with certainty; the higher number also includes those cellular structures which are thought to be neuroblasts or fibers but which are not clearly identifiable. Both have been included in the figures. The neuroblasts which can be seen in any one embryo represent less than the total number present, since the plane of section must be nearly parallel to the long axis of a neuroblast in order to see the origin of the process from the cell body. It is difficult to see cross-sections of individual fibers if they are scattered singly, even though groups of transversely sectioned fibers show up well. To obtain an adequate picture of the pattern of nerve cell bodies, which are oriented in various directions, both transversely and sagittally sectioned embryos must of course be studied. Also, deviation of the plane of section from a true sagittal or transverse plane must be taken into account.
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