Developmental Brain Research, 47 (1989) 263-274 263 Elsevier

BRD 50919

Transplanted mesencephalic quail cells colonize selectively all primary visual nuclei of chick diencephalon: a study using

heterotopic transplants

Salvador Martinez and Rosa-Magda Alvarado-Mallart 1.N.S.E.R.M. Unit~ 106, H6pital de la Salp~tri~re, Paris (France)

(Accepted 17 January 1989)

Key words: Chick/quail chimera; Mesencephalic transplant; Cell migration; Visual system; Neural development; Cytodifferentiation

A portion of the quail mesencephalic alar plate (10-12 somites embryos, second day of incubation) was heterotopically transplanted to replace a portion of the diencephalie alar plate of a similar stage chick embryo. Analysis of the chimeric embryos on day 18 of incubation was performed both by Feulgen and Rossenbeck histochemical staining to recognize the transplanted cells, and by cytoarchitectonic methods. The heterotopicaUy transplanted neuroepithelia were integrated in the host pretectal area, although their precise location, substituting some missing host pretectal nuclei, varied slightly from case to case. The cytoarchitecture of the graft and its extension allowed to distinguish two types of transplants: in 50% of the cases the graft developed a laminated, tectal-like structure appearing as a supernumerary optic tectum, whereas in the other 50% of the cases it gave rise to a smaller, not well-defined, non-laminated structure, which could not be recognized as tectal. Independent of the extension and cytoarchitecture of the grafts, in all cases numerous transplanted quail cells were observed beyond the limits of the graft spreading along the optic tract, into all the retino-recipient diencephalic nuclei and into the mesencephalic tectal gray. Conversely, the host optic teetum and the non-primary visual nuclei, even those in close apposition to the transplant, were always devoid of transplanted cells. Analysis of 5- to 10-day-old chimeric embryos has shown that the ectopically located mesencephalic quail cells start migrating from the transplant on day 7 of incubation and follow a tangential pathway at the surface of the diencephalon, throughout the optic tract and between the optic tract and the incipient primary visual nuclei. On day 10, many of these cells have already invaded most of the host retino-recipient nuclei. These observations are discussed with respect to both the phenotypic expression of the transplanted primordium and the tangential migration of tectal cells previously observed in homotopically transplanted chimeric embryos. The possible significance of these results is also discussed.

INTRODUCTION

Chick/quail chimeras, in t roduced by Le

Doua r in 14'15 as a powerful technique to analyze the

migra t ion and different ia t ion of neural crest cells 16,

have also shown to be useful for the study of the

deve lopmen t of central neural s tructures 1'2'27. In

chimeric embryos , resulting from the substi tut ion of

the diencephal ic alar plate of the chick by a por t ion of the dorsal mesencephal ic vesicle of the quail 1"26,

it was observed that some of the mesencephal ic

t ransp lan ted cells invade the host brain 26. The

present pape r descr ibes the final emplacement of

these heterotopica l ly loca ted cells, their migra tory

pa thway and the possible significance of this abnor-

mal migrat ion.

MATERIALS AND METHODS

L Microsurgical procedure

White Leghorn chick and Japanese quail eggs

were incubated in a forced-draf t incubator at

38+1 °C and p repa red for heterospecif ic t ransplan-

tat ion at the stage of 10-14 somites. This corre-

Correspondence: R.M. Alvarado-Mallart, INSERM U. 106, H6pital de la Salp~tri~re, 47, Bld. de l'H6pital, 75651 Paris C6dex 13, France.

0165-3806/89/$03.50 t~ 1989 Elsevier Science Publishers B.V. (Biomedical Division)

264

D

.°

0

@ ¥

Fig. 1. Schematic representation of quail donor neural tube (stippled) and of host neural tube (in white) to show the portion of the mesencephalic alar plate which was trans- planted, and the location in which this primordium was positioned in the host embryo.

sponds to stage 12 of Hamburger and Hamiltott j2

(about 42 h of incubation) for the chick, and to stage 8-10 of Zacchei 2') (about 36 h of incubation) for the

quail. Quails were always donors and chicks were

always hosts.

The microsurgical procedure has been described

in detail elsewhere ~. Briefly, and as illustrated in

Fig. l, the right alar plate of the diencephalic vesicle

of the chick was removed together with the neural

crest cells and the ectodermal cells covering it. Then,

the graft was prepared by excising a portion of the

right alar plate of the mesencephalic vesicle of the

quail embryo, transferred to the host with a thin

glass pipette, and positioned in the previously

prepared neuroepithelial hole, keeping the original

anterior/posterior and inside/outside orientation. At

the moment of surgery chick and quail embryos were

of similar size but, since the mesencephalic vesicle is

larger than the diencephalic one, the graft cannot be larger than one-third to one-half of the mesencepha-

Fig. 2. Microphotographs of two sections from two different El8 chimeric embryos stained for cytoarchitectony, to illustrate the two types of transplants (limited by the stippled lines) obtained in this study.a: the transplant (TST) in this case (K364) has developed a structure which presents the characteristic tectal lamination; ×20. b: in this case (K546) the transplant (niT) has developed a non-laminated structure; ×32.

265

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co

62 ,38 )]

Fig. 3. a,b: camera lucida drawings of spaced serial sections, going from caudal to rostral, of the same chimeric embryos illustrated respectively in Fig. 2a,b (the plane of section is represented in d). The numbers in the upper right corner of each drawing represent the number of sections which separate them. The grafts are represented by the stippled areas; the quail transplanted cells which have left the transplanted mass of cells and have invaded the host are represented by dots. Note that these cells have invaded the optic tract and all retino-recipient nuclei (see also c). c: ideal lateral view of the diencephalon of a E17-18 normal chick embryo in which the optic tract and the various retino-recipient nuclei are represented in relation to the segmental pattern of the diencephalon: synencephalon (S), posterior parencephalon (PP) and anterior parencephaion (PA) (from Puelles and Zabala23).

260

Ion in order to fit correctly into the operated host

diencephalon. After grafting, the eggs were sealed

with tape and kept in the incubator until the moment

of fixation.

Histological methods Eight of the chimeric embryos used in this study

(fixed at stage 43-44 HH, E17-E18) belong to the

collection previously used to analyze retinotectal projections in the presence of an ectopic supernu-

merary tectum (see ref. 1), and to correlate these

projections to retinal ganglion cell survival (see ref.

26). These embryos received an injection of 5/,tCi

tritiated proline (spec. act. 20 Ci/mmol) in the eye

contralateral to the graft. Sixteen to twelve hours

later they were intracardially perfused with 4% paraformaldehyde in 0.12 M, pH 7.3, phosphate

buffer and postfixed for several days in the same

fixative. The brains were dissected, dehydrated in a

graded series of ethanol and embedded in paraffin. Serial sections 7.5 ktm thick, were cut in a transverse

plane, and alternate series of 7 sections were

mounted on parallel slides. One of the series was

treated for autoradiography; the other, after being

postfixed overnight with Zenker's fixative, were

stained with the Feulgen and Rossenbeck histoche-

mical nuclear method "~, in order to better recognize the transplanted quail cells ~. For autoradiography

the slides were dipped in K-5 Ilford emulsion,

exposed for 3-5 weeks at 4 °C, developed in DI9,

and counterstained with Cresyl violet-thionine.

In order to follow the migration of transplanted cells from the transplants (see Results), 8 additional

chimeric embryos were fixed at younger stages, from HH28 (E5.5) to HH36 (El0), with no previous intraocular injection. In these cases the brains were

fixed by immersion in Zenker's fixative, either

before or after dissection in physiological Tyrode's solution. They were embedded in paraffin and alternate series of 7.5 ~m thick sections were mounted on parallel slides; one of these series was processed for cytoarchitectonics and the other one

with the Feulgen and Rossenbeck histochemical method.

The nomenclature used for mesencephalic and diencephalic grisea follows that of Ehrlich and Mark s,9.

RESU13"S

Chimeric embryos fixed at EI7-E18 Macroscopically, the brains of these 8 embryos

did not look all the same. Four of them presented on

the right side a globular structure, of variable size,

prominent between telencephaton and tectum (see

Fig. 22, in ref. 1). On the contrary, the other 4

Fig. 4. Microphotographs of two serial sections stained with Cresyl violet-thionine (a) and by the Feulgen and Rossenbeck nuclear method (b). The area illustrated in both sections corresponds to the host EM. The asterisks show the same blood vessel in both sections. The arrows indicate transplanted quail cells found in this nucleus, a, × 170; b, x800.

267

Fig. 5. Low magnification microphotograph of a section from an El8 chimeric embryo, stained for cytoarchitecture (b) and high magnification microphotographs of a consecutive section, stained with the Feulgen method (a and c). The frames in b correspond to the details illustrated respectively in the Feulgen stained section of a and c. It is conspicuous that quail transplanted cells are present in the GLV (c, arrows) whereas they are absent from the ROT (a), although this latter nucleus is in close apposition to the transplant, which has been delimited by a discontinuous line in both a and b. a, x500; b, x50; c, x800.

268

embryos looked quite normal dorsally, but on the

right, operated side, the ventral pretectal region was

somewhat larger than on the left side and, in two

cases, it even presented a small protrusion.

The microscopic analysis showed that the cytoar-

chitecture of the grafts is totally different in these

two groups of embryos. In the first group of

embryos, as described previously (see ref. 1), the

transplanted neuroepithelium has given rise to a

structure of variable size, formed by quail cells, in

which a large cortical area displays the 15 alternate cellular and plexiform layers that characterize the

normal avian optic tectum (Fig. 2a). The resulting

transplant can be assimilated to a supernumerary

optic tectum (TST), although its size is always

smaller than the normally positioned host tecta. In

the second group of embryos, the grafted neuroepi-

thelium has developed a mass of quail cells, also of

variable size, whose cytoarchitecture is more diffi-

cult to characterize and which, in any case, cannot be

recognized as tectal since it presents no lamination

(Fig. 2b; see also ref. 26). These grafts will be

referred to as non-laminated transplants (niT). In

spite of this important difference in the development

of the transplants, all 8 embryos exhibit a common

and conspicuous feature: numerous transplanted

quail cells, recognizable by their condensed hetero-

chromatine have left the graft and have entered several host nuclei, where they are found mixed with

the chick cells (Figs. 4b, 5c, 6). Although a detailed analysis of the cytology of the

two groups of transplants will be the subject of a

further publication, a brief description of their

integration within the host diencephali is necessary

Fig. 6. Detail of GLV nucleus from a section stained with Cresyl violet-thionine and previously treated for autoradiography (El8 chimeric embryo). The small arrows point to several transplanted quail cells which have invaded this nucleus. Most of these cells present a cytoplasm rich in basophilic tigroid material characteristic of neurons (detailed in inset). A few quail cells, with scarce lightly stained cytoplasm are also observed both within the optic tract and this nucleus (large arrows). Notice that both optic tract and GLV are densely labelled by the autoradiographic silver grains, x380.

269

for the understanding of our results. The TST are always larger than the niT, as can be appreciated in Fig. 3a,b which represents a reconstruction of a representative case of each group (K364 for TST and K546 for niT). The TST are integrated in a region that corresponds to the locaton of some host pre- tectal nuclei (not always exactly the same) which are missing in these cases. In case K364 (Fig. 3a) the surface of the TST appears integrated between the mesencephalic tectal gray (griseum tectalis, GT), caudally, the nucleus geniculatus lateralis pars inter° calaris (GLi) and the nucleus rotundus (ROT) rostrolaterally, and the nucleus precommissuralis principalis (PPC) rostromedially (see Fig. 3a, sec- tions 45-70). Rostrodorsally, the transplanted tec- turn is contiguous with the host habenula (Fig. 3a, section 95), whose cytoarchitectony is somewhat disturbed. Other TST are similarly integrated al- though their limits are not exactly the same. The

missing pretectal host structures are in this case (K364): nucleus superficialis synencephali (SS), nu- cleus pretectalis principalis (P), nucleus subpretec- talis (SP) and also the lateral and medial spiriformis nuclei (SL, SM).

The niT are also integrated in a region corre- sponding to some missing pretectal nuclei of the host, from either precommissural or commissural regions. As in the chimeric embryos with TSTs, the missing nuclei are not exactly the same in each case. In the illustrated case K546 (Fig. 3a) the missing nuclei are precommissural ones (PPC, SS and the spiriformis complex). It is clear that the relative position of the remaining host nuclei is perturbed in both types of transplants and that, in the operated side, the diencephalon appears somewhat distended.

In both groups of transplants, quail cells have left the transplant and have invaded the same nuclei of the host. Quail cells are found spreading throughout

Fig. 7. Low magnification microphotograph stained for Cresyl violet-thionine, a: the transplant (TST) is conspicuous. The framed area corresponds to the region enlarged in b, taken from a consecutive section, which was treated by the Feulgen and Rossenbeck nuclear staining. A few quail transplanted cells, invading the host superficial diencephalon are indicated by arrows, a, x40; b, x600.

27O

the optic tract, and reach caudodorsally into the GT, the ectomammillary nucleus (EM, Fig. 4), the GLi, the geniculatus lateralis ventralis (GLV, Fig. 5c), the

nucleus ventrolateralis (VLT), the nucleus dorsola- teralis pars lateralis (DLL) and its pars anterior laterorostralis (DLAlr) and the nucleus lateralis

anterior (LA). In the cases where the nucleus superficialis synencephali (SS) and the pretectal optic area (AOP) are present, transplanted quail

cells are also found in these two nuclei. It is worthwhile to emphasize that all these nuclei are retino-recipient structures and are located in close apposition to the optic tract (Fig. 3c). Moreover, as demonstrated by the autoradiographic labelling, all

Fig. 8. Camera lucida drawing (a) and microphotographs (b,c) of a section stained by the Feulgen method, a: the stippled area indicates the transplant. The dots schematize transplanted quail cells observed in the corresponding areas, b: photocom- position of the framed area in a to illustrate the large number of quail cells which have already invaded host GLi. They are better recognized in c which is an enlargement of the framed area of this picture, b, ×220; c, ×500.

these nuclei receive a qualitatively norlnal optic innervation in the chimeric embryos (Fig. 6, see also ref. 1).

It is interesting to note that the sole primary visual structure of the chimeric embryos without quail cells is the host optic tectum: the transplanted cells never cross the border between host optic tectum and GT, the only mesencephalic host nucleus invaded by the transplanted cells.

It should also be noted that non-primary visual host nuclei are always devoid of transplanted cells, including those situated very close to the host/graft interface. Fig. 5a illustrates the ROT, the main target of the tectum, which in most cases is contig- uous with the transplanted mass (see Fig. 5a,b), but always free of quail cells.

In most cases, the transplanted cells can also be recognized in Cresyl violet-stained preparations (Fig. 6). This staining allows the conclusion that a great proportion of the 'ectopically' located quail cells are neurons, since they are provided with a large cytoplasm, rich in basophilic tigroid material corresponding to Nissl bodies. Nevertheless, a num- ber of the 'ectopically' located quail cells, particu- larly, among those found within the optic tract (Fig. 6, large arrows), have scarce, lightly stained cyto- plasm and are interpreted as gila.

Chimeric embryos fixed at younger stages The observations made at short-term survival

after transplantation indicate that quail cells do not leave the transplant before E7. In the embryos fixed at early stages, quail cells are found only within the radial limits of the grafted germinative neuroepithe- lium, Conversely, in the embryos fixed at E7.5-E8, a migratory stream of quail cells is present at the superficial host diencephalon, in the region corre- sponding to the optic tract, and between the optic tract and the primordia of several primary visual nuclei (Fig. 71).

In the embryos fixed at El0, many tangentially migrating quail cells have already reached all host visual nuclei (Fig. 8), Moreover, although no quan- titative analysis has been performed, the proportion of quail cells which have entered host visual nuclei seems to be similar to that observed in the cases analyzed at later stages (see Fig. 9).

271

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Fig. 9. Camera lucida drawings (at the same magnification) of two sections (Feulgen and Rossenbeck staining) from two different chimeric embryos. All cells present in GLV have been drawn in. The level of the section is illustrated. The proport ion of transplanted quail cells (schematized by dots) is about 12% in both sections.

DISCUSSION

Chick/quail chimeras have been extensively used to study the migration and differentiation of neural crest cells, particularly those at the origin of the peripheral nervous system (see ref. 16). Although still less broadly used, xenoplastic transplantation of portions of neural tube between these two species also provides excellent material for the study of neuronal proliferation, migration and differentiation of central neurons and establishment of projection maps of the C N S 1"13'27.

Two main observations have been made in this study after heterotopic transplantation of a large medial portion of the quail mesencephalic alar plate:

(1) this primordium when replacing the caudal portion of the chick diencephalon develops either as a laminated, tectum-like, structure (TST), or as a less distinctive, non-laminated cell mass (niT); (2) the transplanted neuroepithelium always gives rise to cells that leave the graft and invade selectively all primary visual nuclei of host diencephalon as well as the host mesencephalic GT.

The fact that the mesencephalic alar plate forms a TST in 50% of chimeric embryos indicates that this neuroepithelium already contains all the necessary information for the development of the optic tectum. This result is in agreement with previously obtained data in chimeric embryos 1'27. Moreover, we have shown that the mesencephalic alar plate also con-

272

tributes to the formation of other mesencephalic nuclei 27, as well as to isthmic grisea and to the rostralmost cerebellum 2. Therefore, one may won- der if, when the graft gives rise to a niT, the transplanted primordium was solely formed by non- 'tectal' neuroepithelium. However, this does not seem to be the case since the transplants were always taken from the same mesencephalic region. It is thus conceivable that unknown factors might interfere, in some cases, with the normal development of the transplanted neuroepithelium and might prevent the formation of a supernumerary tectum. New experi- ments are in progress, aimed to answer this impor- tant question.

Concerning the tangential migration of trans- planted quail cells, it is interesting to recall that, in previous experiments using homotopic transplanta- tion of the mesencephalon, tangential migration of tectal neurons has also been disclosed 27. Such a type of migration only occurs in the outer retino-recipient layers of the tectum. These tangentially migrating cells might correspond to the superficial horizontal type I1 neuroblasts described by Puelles and Bendala 2°. All these observations lead us to suppose that some mesencephalic cells might have a prefer- ential adhesiveness to molecules present within the optic tract territory and that they might follow a neurophilic migration 25. It is worth noting in this respect that the monoclonal antibody JONES, de- scribed by Constantine-Paton et al. 5, recognizes in the rat an epitope related with cell migration and neurite elongation ts.

However, it is clear that the migratory behavior of the tangentially migrating cells differs in homotopic and heterotopic grafts. In the former they never leave the chimeric optic tectum, being at the border between the optic tectum and the GT. Conversely, in the heterotopic grafts - - as reported in the present study - - these cells invade all host primary visual centers but never penetrate the host tectum being, also, arrested at the GT/tectal boundary. The GT/ tectal boundary seems to be of a capital importance in the avian visual system on other grounds too, since it corresponds to the locus of inversion of the naso-temporal axis of visual projections 9"H'21'22

The morphology of quail cells observed in the host diencephalon indicates that they belong both to neural and glial categories. The presence of mesen- cephalic neurons within the diencephalon opens the possibility that their phenotypic expression could be influenced by either their migratory pathway and/or by environmental factors of their new location. It is by now well known that the determination of neural crest cells is influenced by the territories through which they migrate and in which they differentiate (see ref. 16). Moreover, the contact of these cells with different molecules of the extracellular matrix influences their phenotypic expression 3"19. Along the

same line of thought, it is worthwhile to recall the experiments of Prochiantz's group on the influence of astroglia on the differentiation of embryonic mesencephalic and striatal neurons 6'7. Further stud- ies are needed to clarify whether the tangentially migrating quail cells of our chimeric embryos de- velop according to their mesencephalic origin, or if their phenotype is influenced by their atypic dien- cephalic environment.

Glial cells are known to migrate long distances in the CNS 4A7. It has been demonstrated that type 2 astrocyte precursors 24 migrate, in the rat, from the brain into the optic nerve 28. Therefore, it is tempting to assume that the observed migration of quail glial cells is an expression of a normal migration of this type of cells throughout the optic tract.

In conclusion, although the implications of the observed neurophilic tangential migration of trans- planted cells is not yet fully understood, the present results demonstrate that these cells are able to migrate over long distances in the host diencephalon and selectively invade all primary visual nuclei.

ACKNOWLEDGEMENTS

We would like to thank Drs. L. Puelles and C. Sotelo for stimulating discussions, M.P. Morel, J.P. Rio and R. Wherl6 for technical assistance and D. Le Cren for photographic work. Part of this work has been carried out while Dr. S. Martinez was a recipient of a fellowship from INSERM.

ABBREVIATIONS

AOP CP D DLAIr DLL dsv EM GLi GLV GT HM iot IPS LA NE ot niT Ov PA

pretectal area posterior commissure diencephalon nucleus dorsolateralis anterior, rostrolateral part nucleus dorsolateralis lateralis ventral supraoptic decussation ectomammillary nucleus lateral geniculatus nucleus, intercalated part lateral geniculatus nucleus griseum tectalis habenular medial nucleus isthmo optic tract nucleus interstitio pretecto-subpretectalis lateral anterior nucleus external nucleus optic tract non-laminated transplant nucleus ovoidalis parencephalon anterior

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273

PM pretectal medial nucleus PP parencephalon posterior PPC precommissural principal nucleus pROT perirotundic nucleus PT pretectal nucleus PV posteroventral thalamic nucleus ROT nucleus rotundus S synencephalon SAC stratum album centrale SGC stratum griseum centrale SGFS stratum griseum et fibrosum superficiale SGPv stratum griseum periventriculare SP subpretectal nucleus sROT nucleus subrotundus SS superficial synencephalic nucleus T nucleus triangularis TeO optic tectum TS torus semicircularis TST transplanted supernumarary tectum v ventricle VLT ventrolateral thalamus

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