/. Embryol. exp. Morph. Vol. 32, 2, pp. 355-363, 1974
Printed in Great Britain
355
Comparative remarks on the development of the
tail cord among higher vertebrates
By A. F. H U G H E S 1 AND R. B. F R E E M A N 1
From the Department of Anatomy,
Case-Western Reserve University
SUMMARY
The development of the caudal region of the neural tube is compared in tailed mammals
with that of the chick and human. In rat, mouse, opossum and pig, the lumen of the cord
extends caudally in an even manner, whereas in the chick and in man the addition of small
cavities to the lumen results in a phase of irregular growth. In mammals with unreduced
tails, the site of closure of the posterior neuropore is at the tip of the tail, whereas in pig, man
and in the chick closure occurs before the formation of the tail-bud. The teratological
implications of these findings are discussed.
INTRODUCTION
Formation of the neural tube by the meeting of neural folds ceases on
closure of the posterior neuropore. The cord is still then relatively short.
Further elongation tailwards may be due to endogenous growth or to the addition of new cells from a caudal blastema. In the chick embryo, the second of
these has been described by several authors, the most recent of which is Criley
(1969). Separate small cavities arise beyond the central canal; these are bounded
by cells which are incorporated into the neural tube. Its growth by this means
is subject to irregularities, which are normally later resolved with the production
of a tube of even calibre.
In the human embryo, Lemire (1973) has recently described an essentially
similar process, as a result of which the caudal region of the central canal was
found to be uneven in form in seven out of eight specimens between the ages of
30 and 50 days. Though as yet we lack as close a study of the human sacral and
tail cord as Criley and others have provided for the chick, the question may yet
be raised whether conditions in these two species in this respect are typical of the
higher vertebrates in general. The purpose of this paper is to suggest that this
is not so, and to propose a reason why this facet of bird and human embryology
is exceptional.
1
Authors' address: Department of Anatomy, School of Medicine, Case-Western Reserve
University, Cleveland, Ohio 44106, U.S.A.
356
A . F . H U G H E S AND R. B. FREEMAN
MATERIAL AND METHODS
The observations here described are based on the study of serial sections of
embryos of a selection of mammalian species. We are grateful to our colleague
Dr Jerry Silver for allowing us to examine his rat and mouse material, and to
Mr Jack Cash of the Anatomy School, University of Cambridge, England, for
permitting us to publish his photomicrographs of sections through sheep embryos
from a collection of the late Prof. J. D. Boyd. Sectioned pig and human embryos
were available from departmental collections at Cleveland, as were opossum
series which have been recently studied for other purposes (Hughes, 1973).
All of these section series had been stained with haematoxylin and eosin. The
pig and human material, though well fixed in the embryos we selected for study,
had long since faded, but here the phase-contrast microscope was of great help.
In the tail region of six embryos between 28 and 40 days, photomicrographs
were taken of each section in turn, and from among these the outline drawings of
Fig. 5 were prepared.
RESULTS
Our observations on the development of the posterior neural tube may be
described under three main headings :
(1) Before the formation of the tail-bud
In this category fall the two sheep embryos of 14 (Fig. 1) and 23 somites
respectively, of which we have examined serial photomicrographs. In both, an
open neural groove is succeeded caudally by a flat neural plate, closely applied
to a mesenchymatous blastema beneath which notochord and somites are
forming locally, though whether at these stages this undifferentiated material
makes any contribution to the nervous system, we are unable to decide.
FIGURES 1-4
Fig. 1. Transverse sections through 14 somite sheep embryo. (A) At level of hinder
somites with open neural groove. (B) Further caudally with neural plate dorsal to
undifferentiated blastema. Marker bar equivalent to 20 ji-m.
Fig. 2. Transverse sections near tip of tail of 10-day-old mouse embryo. (A) With
open neuropore; (B) and (C) a few sections further forward, with thin laminae of
neural epithelium and skin ectoderm roofing over the central canal. Notice the
rapid enlargement of the lumen between sections (B) and (C). Marker bar equivalent to 25 ftm.
Fig. 3. Transverse sections near tip of tail of opossum embryo at stage 28, showing
massive neural folds closing over wide central canal. (A) is nearer the tail tip than
is (B). Same magnification as in Fig. 2.
Fig. 4. Median sagittal section through tail tip of 7-5 mm pig embryo, showing
regular form of central canal and neural tube terminating adjacent to tail-bud
mesoderm. Phase contrast. Marker bar equivalent to 40 fim.
Development of tail cord among vertebrates
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358
A. F. HUGHES AND R. B. FREEMAN
(2) Closure of the posterior neuropore during the growth of the tail
In a rat embryo of 9 | days an open neural groove runs dorsally down most of
the tail. At the final stage of closure an open neuropore is found at the tip. This
condition is seen in a 10-day-old mouse embryo of strain C57/B1/6J, shown in
Fig. 2 A, with a shallow neuropore closed in an adjacent section by a thin
cellular lamina (Fig. 2B). A few sections further forward, this is itself covered by
a thin layer of epithelium (Fig. 2C).
In rodent embryos at these stages, there is no sharp distinction between the
cells of the cord and those of the surrounding mesoderm. The notochord has
not yet differentiated at these levels, but a prominent caudal gut runs to the tip
of the tail.
In an opossum embryo (Fig. 3 A, B) of stage 28 of McCrady (1938), 3 days before birth, there is a short tail, with a prominent neural component. At sacral
levels, massive neural folds are meeting over a relatively spacious central canal.
As far as the tail tip, the neural plate and groove are distinct from the surrounding mesenchyme, and the notochord is already present.
In both opossum and mouse, pycnotic cells are common in the basal plate of
the tail cord, as Lemire (1973) has described for the human embryo at comparable stages.
(3) Closure of the posterior neuropore before the stage of the tail-bud
(a) The pig
Figures in Patten (1948) show that in the pig embryo the posterior neuropore
is still open at 17 somites (16 days approximately). At the 5 mm stage (17-18
days) there is a distinct tail-bud; serial sections show that it contains a neural
tube with an even and wide lumen, wholly closed and ending two sections before
the tip of the tail. At 7-5 mm (18-19 days) the tail is drawn out to a fine tip.
A longitudinal section (Fig. 4) shows the neural tube beyond the central canal,
ending among a mass of mesodermal cells.
Thus, in so far as these observations take us, we see that in embryos of tailed
mammals the neural tube terminates at each stage in a wholly even and regular
manner.
(b) Human embryos
In his monograph on the development and reduction of the human tail,
Kunimoto (1918) described how, within the tail-bud of a 4 mm embryo (approx.
28 days), the neural tube and notochord merge caudally into a solid mass of
mesodermal cells. In seven out of eight specimens between the ages of 30 and
48 days (6-25 mm) Lemire (1973) found evidence of 'canalization' within this
tissue, 'with accessory lumens coalescing from the caudal neural cell mass', a
process essentially the same as Criley (1969) had described for the chick.
Development of tail cord among vertebrates
Fig. 5. Outline drawings of transverse sections (10 [im thick) through caudal end
of neural bud and central canal for four human embryos. (A) Five consecutive
sections at 28 days. (B) Six consecutive sections at 30 days. (C) Eighteen consecutive
sections at 32 days. (D) Ten sections at 40 days, consecutive except where numerals
indicate numbers of omitted sections. Arrow: opening to exterior surface. Marker
bar equivalent to 100 /tm.
359
360
A. F. HUGHES AND R. B. FREEMAN
Our own observations of human material are based on embryos between
the ages of 28 and 42 days. In the two earliest of these (of 25 and 28 somites
respectively) a central canal of regular form ends blindly within a dense and
even neural mass of cells (Fig. 5A, B), at this stage differing in no way from that
of the pig at a corresponding stage. In a 32-day-old human embryo, however,
cavitation has begun (Fig. 5C). The whole neural mass is irregular in shape. For
the most part, the outlines of the cavities within are in contact with one another.
In a 40-day-old specimen (13-2 mm, Fig. 5D) they have widened into a system
of branched diverticula, which at one point opens to the outside. In another
slightly older specimen there is also a bifid termination to a wide central canal.
According to Kunimoto the three last somites (36-38) are losing their distinct
outlines at this time, and involution of the hindmost, unsegmented region of the
tail has begun.
Though it seems that the course of these changes within the human cord and
tail is subject to variation, it is clear that at a period before the loss of the
external tail, canalization within the caudal zone of the neural tube does result
in the production of a temporarily branched and irregular lumen. This condition often persists within the post-fetal human equinalcord (Lendon& Emery,
1970).
DISCUSSION
The formation of the tail cord proceeds in three stages. First, there appears
an undifferentiated medullary blastema beyond the sinus rhomboidalis. From
this material a solid rod of neural tissue condenses, in which the central canal is
prolonged caudally. However, not only are there differences in the relationships
between these events among the examples we have cited, but also in the manner
of the extension of the lumen. These differences are represented in Table 1 :
Table 1
Closure of posterior neuropore
Growth of caudal neural tube
By cavitation
By intrinsic growth
Before tail-bud
After tail-bud
Chick, human
Pig
—
Opossum,
rodents
Further study is, however, needed to settle the question whether cavitation
occurs in other species. Bentliff & Gordon (1965), who, from the study of rat
embryos, drew attention to the distinction between primary neurulation by
neural folding and the later stages of formation of the tail cord, here confine
their remarks to the statement that 'secondary neurulation originates from
undifferentiated tissue which would be considered end-bud or tail-bud'.
The delay in its formation until after the closure of the posterior neuropore
Development
of tail cord among vertebrates
361
in chick, man and pig is associated with the reduction of the appendage in the
adult. In the pig, where this loss is still only partial, the embryonic central canal
still extends in the regular manner seen in fully tailed species.
The fact that birds and man share the common feature of an irregular phase
of this process seems to have significance in comparative teratology. As both
lost their tails during the course of evolution, those factors which elsewhere
regulate the even growth of the hinder regions of the embryonic cord and axial
skeleton operated no longer. In consequence, these species now share a special
liability to the various degrees of malformation of the hinder regions of the
spinal cord and of the sacral region. In man, though the incidence of spina
bifida in its various forms varies widely with race (Stevenson, Johnston,
Stewart & Golding, 1966), frequencies as high as 3-6 per 1000 births have been
recorded (Elwood, 1972). On the other hand, in tailed mammals, abnormalities
of the sacral cord seem to be extremely rare, as is indicated in Kalter's work on
malformations of the central nervous system (1968). There we learn that only
one instance of rachischisis has been recorded in the pig (p. 292), whereas
various abnormalities of the brain have been described by many authors. The
same is true for spontaneous malformations of the cord in the sheep (Dennis,
1965) and in the rat.
In the fowl, though figures for the spontaneous occurrence of spina bifida
do not seem to have been recorded as such, treatment of early embryos with
teratogens frequently results in defects of the spinal cord and sacrum. It is a
striking fact that the Veratrum alkaloid cyclopamine (Keeler, 1969), which in
the Sheep causes only cyclopia (Keeler & Binns, 1968), induces neural tube
and tail abnormalities as well in the chick embryo (Freeman & Hughes,
1973). Such abnormalities can be provoked by many other substances which
do not affect eye development. Romanoff (1972) in his tables 11 and 12 has
listed the effects on the chick embryo of a wide and miscellaneous range of
teratogens. Of 63 which were applied between days 0 and 2, nearly half (44 %)
caused abnormalities of the sacrum and spinal cord.
Both in man and in the fowl, defects of the hinder regions of the axial skeleton
appear to have a similar origin in that suppression of sacral and coccygeal
somites are involved. These include both the sacral agenesis associated with
maternal diabetes (Blumel, Butler, Evans & Eggers, 1962), and the various
forms of rumplessness in fowls, either genetical in origin (Dunn & Landauer,
1934) or induced by agents such as insulin (Landauer, 1945; Moseley, 1947).
Besides these close similarities between these two species in tail and cord
development, there is one important point of difference. In man, involution of the
segmented and unsegmented portions of the tail immediately follows the cavitation phase in cord development, whereas in the fowl this period begins at 46 h
(18 somite stage, Criley, 1969), while further somites continue to be formed until
near the end of the fourth day (Romanoff, 1960, p. 931). Reduction of the tail
then proceeds during the next 4 days.
362
A. F. HUGHES AND R. B. FREEMAN
In the chick we are able to follow the manner in which malformations can
arise by the persistence of separate neural cavities unresolved into the main
spinal cord. This condition may result in the multiple lumen defect recently
mentioned by Freeman & Hughes (1973). Another form of abnormality in the
'overlap zone' of Criley (1969) arises when the alar and basal plates remain
distinct, with the former unclosed, the dorsal spinal roots entering near its
lateral borders, and the ventral roots arising from a tubular, basal plate below.
In man there is still much to be learnt about the precise manner in which
abnormalities arise in cord and axial skeleton. It is likely that myeloschisis
can originate in various ways, and during several stages of development. For
instance, Lemire, Shepard & Alvord (1965) have described an example of
posterior neural defect as early as 32 days (5-5 mm C-R length) which must
presumably have arisen by primary failure of closure of the neural tube, and is
thus unrelated to cavitation. On the other hand, it appears that an opening to
the exterior can arise during the phase of dilation of the first formed cavities
(Fig. 5D).
The mouse, in view of the now abundant information available concerning
the inheritance of abnormalities affecting many organ systems, provides a
special case in which there is clear evidence that caudal defect and spina bifida
are phenotypically associated. Among the factors which cause both abnormalities are the recessive gene curly-tail (Grüneberg, 1954) and the dominants
splotch (Russell, 1947; Auerbach, 1954) and tail-short (Morgan, 1950). These
genes cause the tail to be bent or shortened. Others cause a more severe degree
of caudal defect, among which is the semi-dominant gene brachyury (T).
Chesley (1935) found that in the development of these mice the primary defect
was in the notochord at lumbo-sacral and caudal levels, with marked secondary
effects on the growth of the neural tube. These presumably represent degrees
of irregularity greater than that which normally occurs in the development of
tailless species.
These matters are pertinent to the question of what animals should be selected
by the experimental teratologist for research on defects of the spinal cord
aimed at elucidating the factors inducing these conditions in man. There must
always be doubt concerning how far results obtained on experimental animals
can be considered relevant to human development. Generally speaking,
mammals most closely related to man are less subject to these uncertainties
than are others. Yet most infra-human primates have tails, and it is likely that
in them the hinder region of the spinal cord develops in the regular manner
characteristic of other tailed species.
Besides those whom we have mentioned who have aided us in lending their material and
in allowing their photomicrographs to be published, we are grateful to Mrs Judith Freeman
for her skill and patience in the preparation of serial sections. This research was supported
by a grant from the National Foundation (March of Dimes).
Development of tail cord among vertebrates
363
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