Volumetric Investigations on Overgrowth
(Hypermorphosis) in Chick Embryo Brains
by HARRY BERGQUIST1
From the Tomblad Institute, University of Lund, and the Department of Anatomy,
University of Gothenburg
T H E present paper is intended as a continuation of preceding publications
(Bergquist, 1959 a, b) on possible causes of the phenomenon of 'overgrowth'
(Patten, 1952) in the brain of the chick embryo. Reference should be made to
these publications for the relevant literature. In these earlier investigations, it
was observed that the mitotic rate in the altered portions of the neural tube
seemed higher than in the normal regions, and that the hemispheres and tectum
opticum showed marked folding.
The first aim of the present investigation was to compare the volumes of the {gf^«r
hemispheres in normal chick embryos
and in corresponding stages of embryos
with overgrowth, the study being confined to the distinctly evaginated portions of both hemispheres. The embryos
measured consisted of those used in
the previous work. The surface areas
of sections through the walls of both
hemispheres were determined plani-
metrically. In small embryos all sections
were measured, and in bigger ones every
fifth or tenth. The volume of the wall
was then computed from section thick3SO
ness.
Both normal embryos (designated as TEXT-FIG. 1. Hemisphere volume (logarithmic scale) in normal chick embryos (x) and
x in Text-fig. 1) and embryos with over- in
embryos showing overgrowth (o). The
growth (designated as o in Text-fig. 1) volumes of the embryos with overgrowth lie
scattered around the normal ones.
were examined. In Text-fig. 1 the logarithm of the volume of the hemispheres
forms the ordinate and the age of the embryos the abscissa. It will be seen that
Author's address: Zoological Institute, Postgatan 35, Gothenburg, Sweden.
[J. Embryol. exp. Morph. Vol. 8, Part 2, pp. 69-72, June 1960]
5584.8
p
70
H. BERGQUIST—OVERGROWTH IN CHICK EMBRYO BRAINS
there was no significant difference in hemisphere volume between normal
embryos and those with overgrowth throughout the period studied.
The second aim of the present investigation was to study the mitotic number
in normal and overgrown chick brains. In order to obtain a well-delimited region
the optic tectum was chosen for this study.
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TEXT-FIG. 2. Total mitotic number in the tectum opticum in
normal chick embryos (x) and in embryos showing overgrowth (o).
The mitotic number of the overgrown hemispheres are obviously
higher than the corresponding normal values.
The number of mitoses in the optic tectum was determined in 9 normal and
in 6 overgrown embryos and was counted under high magnification in every
tenth section. The total number of mitoses in the different embryos is shown in
Text-fig. 2 as a function of embryo age in hours. The number is obviously higher
in chick embryos with overgrowth than in corresponding stages of normal
embryos.
It has previously been demonstrated that an increased number of mitoses next
to the ventricular lumen is correlated with evagination of the wall of the neural
tube to form bulges during the neuromere phase (Bergquist, 1932; Kallen, 1952)
or more or less deep furrows during the migration area phase (Bergquist &
Kallen, 1954). Moreover, by treating the embryos with colchicine, Kallen (1953)
temporarily arrested the mitotic process so that cells in mitosis aggregated along
the inside of the neural tube. This caused the ventricular wall to bulge out in a
similar manner to that described here.
The excessive formation of folds in the ventricular wall of embryos with over-
H. BERGQUIST—OVERGROWTH IN CHICK EMBRYO BRAINS
71
growth might consequently be due, to some extent, to the increased number of
mitoses. Another possibility might be that the overproduction of neural tissue
within the fixed space available inside the skull had made folding a necessity.
But the volumetric studies presented here have shown that overgrowth is not
accompanied by a significantly increased volume of the nervous substance. At
first sight the higher number of mitoses might seem to be in conflict with this
finding. However, it is well known that volumetric growth—particularly in the
later stages of development—is mainly due to increase in the volume of individual nerve-cells and to growth of fibres.
SUMMARY
1. There is no increase or decrease of the volume of the hemispheres in brains
with 'overgrowth'.
2. The optic tectum of chick embryos showing the overgrowth phenomenon
have a markedly higher number of mitoses than do those of normal embryos of
corresponding stages.
3. It is suggested that the formation of the overgrowth is fundamentally similar to the development of proliferation furrows and of neuromeric bulges.
RESUME
Recherches volumetriques sur Vexces de croissance (hypermorphose) dans le
cerveau de I'embryon de poulet
1. II n'y a pas d'augmentation ni de diminution de volume des hemispheres
cerebraux dans Texces de croissance'.
2. Le toit optique des embryons de poulet manifestant le phenomene 'd'exces
de croissance' montre une augmentation marquee du nombre des mitoses par
rapport aux embryons normaux des stades correspondants.
3. II est suggere que Texces de croissance' est dans l'ensemble similaire a la
proliferation des sillons et des dilatations neuromeriques.
ACKNOWLEDGEMENT
The cost of this investigation was defrayed by grants from the Swedish Medical
Research Council.
REFERENCES
BERGQUIST, H. (1932). Zur Morphologie des Zwischenhirns bei niederen Wirbeltieren. Ada zool.
Stockh. 13, 58-303.
(1959fl). Experiments on the 'overgrowth' phenomenon in the brain of chick embryos.
/. Embryol. ex p. Morph. 7, 122-7.
(1959/)). 'Overgrowth im Kiickengehirn, experimentell dargestellt. Anat. Anz. 106, 429-41.
•
& KALLEN, B. (1954). Notes on the early histogenesis and morphogenesis of the central
nervous system in vertebrates. J. comp. Neurol. 100, 627-60.
72
H. BERGQUIST—OVERGROWTH IN CHICK EMBRYO BRAINS
HAMBURGER, V., & HAMILTON, H. L. (1951). A series of normal stages in the development of the
chick embryo. /. Morph. 88, 49-92.
KALLEN, B. (1952). Notes on the proliferation processes in the neuromeres in vertebrate embryos.
Ada Soc. Med. Uppsal. 57, 111-18.
(1953). On the significance of the neuromeres and similar structures in vertebrate embryos.
J. Embryol. exp. Morph. 1, 387-92.
PATTEN, B. M. (1952). Overgrowth of the neural tube in young human embryos. Anat. Rec. 113,
(Manuscript received 28:v:59)