/. Embryol. exp. Morph. Vol. 31, 1, pp. 247-261, 1974
Printed in Great Britain
247
Genetic and environmental factors
determining the morphological maturation of the
mouse and its relationship with weight growth
By G. GARRARD, 1 G. A. HARRISON AND J. S. WEINER
From the Anthropology Laboratory, Department of Human Anatomy, Oxford,
and the School of Hygiene and Tropical Medicine, London
SUMMARY
Data on the morphological development of inbred and Fx hybrid mice reared at 22-8 °C dry
bulb, 17-8 °C wet bulb, and 32-2 °C dry bulb, 26-7 °C wet bulb are presented. Animals were
weighed at birth, 1 and 2 weeks of age and at each developmental stage. All animals were
examined for the freeing of the ear pinnae, eruption of the lower incisor teeth and perforation
of the eye membranes and in addition females were examined for the appearance of the first
pair of nipples and for perforation of the outer part of the vagina.
Within each environment, but especially at 23 °C, there are genotypically characteristic
differences in the ages at maturation of each variable. Though the ages of maturation of the
different variables are highly correlated within genotypes, early maturation of one variable is
not necessarily associated with the early maturation of others. Reciprocal Fx hybrid differences
indicate a maternal effect upon the age of maturation which is more marked at 23 than at 32 °C.
Comparisons of the ages at maturation in the two environments showed that the only
character to mature earlier at 23 than at 32 °C was pinna freeing. The most marked influence
of the 32 °C environment in advancing development was on the appearance of nipples
and on vaginal opening. In almost all cases, animals reared at 32 °C were lighter at the time
of maturation than those at 23 °C. Hence when allowance was made for environmental
differences in body weight the association between an environment at 32 °C and early
development was further enhanced. Those animals that were relatively heavy when weighed
at the weekly interval prior to maturation matured earlier than the lighter ones, especially at
23 °C, indicating that morphological maturation was closely related to overall growth.
Both the within, and between, litter variation in the ages and body weights at maturation
of a strain tended to be greater in animals exposed to the higher temperature. In bilaterally
distributed characters, asymmetrical maturation was more common at 32 than at 23 °C.
INTRODUCTION
The effects of environmental and genetic variation on mammalian growth and
size have been investigated (Mills, 1945; Cawley, McKeown & Record, 1954;
Acheson, 1960; Hammond, 1961; Harrison, 1963; Chevillard, Portet & Cadot,
1963; Porter & Festing, 1969) but little attention has been paid to their effects
on postnatal morphological development. These are of interest for, in principle,
they afford a means of analysing the extent to which the correlations between
1
Author's address: Division of Human Physiology, National Institute for Medical Research
(Hampstead Laboratories), Holly Hill, London NW3 6RB, U.K.
16-3
248
G. GARRARD AND OTHERS
maturation patterns and growth can be broken and the extent to which the
processes can be shown to be independent.
Where morphological maturation has been investigated in relation to the
body weights of rats or mice, results varied according to the conditions in which
the animals were maintained and to the particular morphological characteristics
observed (Engle & Rosasco, 1927; Parkes, 1929; Outhouse & Mendel, 1933;
Kennedy, 1957; Biggers, Ashoub, McLaren & Michie, 1958; Widdowson &
McCance, 1960; Knudsen, 1962; Barnett & Burn, 1967; Gall & Kyle, 1968).
Among mice the issue is complicated by the existence of differences in the ages
at morphological maturation among inbred strains (van Ebbenhorst-Tengbergen,
1942; Yoon, 1955) and between reciprocal F± hybrids (Deringer, Heston &
Andervont, 1945). These genetic observations have not been related to differences in the growth patterns of the various genotypes.
This paper is concerned with examining, among inbred strains of mice and
the jFi hybrids between them, the effects of a difference in environmental temperature upon the time of appearance of a number of external morphological
characters, and relating these measures of development to variation in the overall
growth of the animals, as assessed from body weight.
MATERIALS AND METHODS
The general design of the study and the environmental specifications have been
detailed elsewhere (Garrard, Harrison & Weiner, 1974a). In summary, animals
were bred and reared either at 22-8 ± 1-6 °C dry bulb, 17-8 ± 1-0 °C wet bulb or
at 32-2 ±0-5 °C dry bulb and 26-7 ±0-5 °C wet bulb. At 23 °C the following
inbred strains were observed: C57BL, BALB/c and CBA/H. The latter strain
included, mainly because of its availability, an isogenic substrain with a T6
translocation, CBA/H.T6T6 (referred to as CBA/T6). The six reciprocal Fx
hybrids derived from these inbred strains (but using only CBA/T6 to represent
the CBA genotype) were also reared and are identified as IFX = C57 x CBA/T6,
I l f i = C57 x BALB and III.Fi = CBA/T6 x BALB. At 32 °C observations were
limited to C57, BALB and reciprocal crosses of the hybrid between them, i.e.
IIi<i. Data on C57 are limited because of the difficulty experienced in breeding
and rearing this strain at high temperatures (Garrard et al. 1974a).
The developmental features observed were the chronological ages at which
freeing of the ear pinnae, eruption of the lower incisor teeth, and the first perforation of the eye membranes occurred. These were recorded for males and
females but two additional characters were scored for females: the age of
appearance of the first pair of nipples (usually one of the inguinal pairs) and the
age at perforation of the vagina. The latter is not the same stage as that identified
by other authors, which more closely coincides with the first oestrus cycle, but
is the time when the external part of the vagina is perforated. This, like the other
morphological features examined, matures in the preweaning period.
Morphological maturation of the mouse
249
Animals were examined each morning to determine any changes in their
developmental status which had occurred over the preceding 24 h. Since there was
not always symmetry in the development of two of the bilateral characters,
pinna freeing and eye opening, separate observations were made for the right
and left sides. When development was asymmetrical the mean age at maturation
for each animal was used in the statistical analysis.
At each morphological developmental stage the body weight of an individual
was recorded to 0-5 g, a mean value again being calculated when development
was asymmetrical. Where the character was observed in both sexes the data for
males and females have been combined as analysis showed no statistically
significant sex effect. Animals were also weighed at birth, and at 1 and 2 weeks
of age.
RESULTS
Morphological maturation at 23 °C
Data on the ages at morphological maturation of male and female litter mates
have been combined to give means since no sex differences were detected. These
means have been used to derive the mean ages at which the ear pinnae became
free, the lower incisor teeth erupted and the eye membranes became perforated
in offspring of each of the mating types (see Table 1). Mean strain values for the
ages of appearance of nipples and the opening of the external part of the vagina,
in females, are also included in the table. Genotypic comparisons of the mean
ages at maturation have been made and statistically significant differences are
set out in Table 2. The morphological character whose developmental age was
most influenced by genotype was the eruption of the lower incisor teeth, while
the age at pinna freeing was least responsive to genotype.
BALB is particularly advanced, as compared with all other genotypes in the
eruption of the lower incisors. With the exception of opening of the eyes, the
development of BALB litters generally tends to be advanced with respect to
that of the other inbred strains, especially CBA and CBA/T6. Some consistent
hybrid genotypic differences which emerged were the relatively early development of IFX (C57 x CBA/T6) litters and the late development of H1F1 (CBA/T6
xBALB) litters. Among litters with a CBA/T6 mother there was a marked
tendency for inbred offspring to mature earlier than hybrid offspring.
Inter-litter variation in the ages at maturation of five of the characters tends
to be low but is greater at the time of vaginal opening than at development of
the remaining four characters. Certain strain differences in inter-litter variation
in ages at maturation were found, and can be summarized as follows. Both the
inbred and hybrid litters of BALB mothers usually mature at a more uniform
age than litters of C57 mothers. BALB litters are also less variable than CBA/T6
litters, but in this case a reciprocal hybrid CBA/T6 x BALB difference did not
emerge.
S.E.
Weight
S.E.
No.
Age
S.E.
Weight
S.E.
No.
Age
S.E.
Weight
S.E.
No.
Age
S.E.
Weight
S.E.
No.
Age
S.E.
Weight
S.E.
Age
No. of
litters ...
81
3-4
008
2-2
004
48
9-4
017
4-6
010
68
13-5
010
60
Oil
92
6-8
008
3-6
006
58
10-9
018
5-2
010
C57
28
3-2
015
20
005
16
9-3
0-24
4-4
0-22
19
12-5
014
5-5
0-25
22
71
0-21
3-3
009
15
10-8
0-37
50
0-20
CBA
46
3-5
010
1-9
003
26
100
0-22
4-2
014
32
12-8
014
4-9
015
43
7-4
019
3-2
005
29
10-4
0-37
4-6
013
CBA/T6
93
3-4
005
2-5
003
56
8-9
Oil
50
Oil
56
13-5
010
6-9
016
76
6-6
008
40
007
69
9-8
014
5-5
013
BALB
47
31
012
2-2
005
30
9-9
015
4-8
017
37
130
013
5-8
0-20
42
6-6
013
3-7
008
33
10-3
016
50
016
C57?
28
30
009
2-4
003
22
9-6
012
5-3
015
22
12-8
014
61
019
24
6-3
016
3-8
008
23
10-2
010
5-4
016
CBA/T6 ?
40
3-5
012
2-3
006
21
10-2
0-21
4-5
0-20
24
140
0-21
5-8
0-21
27
70
0-22
3-7
013
25
10-9
0-27
4-6
016
C57?
I
44
3-4
008
2-7
005
28
9-4
014
5-2
015
39
13-5
Oil
6-7
0-20
39
6-5
009
41
010
34
9-7
Oil
5-2
012
BALB?
30
3-7
012
2-2
003
32
10-2
Oil
4-3
013
34
13-8
015
5-1
014
31
6-3
019
30
008
33
12-2
0-35
4-7
009
CBA/T6 ?
III
27
3-6
010
2-3
004
29
10-2
010
51
015
30
13-8
017
60
0-21
25
5-5
Oil
2-9
005
26
111
0-30
5-3
016
BALB?
I = Pinna freeing, II = lower incisor eruption, III = eye opening, IV = nipple appearance, V = vagina opening. The means are derived from averaging
mean values for litters within each mating type.
V
IV
III
II
I
Strain
]
Table 1. Mean ages (days) and body weights (g) at morphological maturation of mice reared at 23 °C
on
w
X
/^
H
5/
H
>
O
0
%
IF,**
C57*
—
CBA/T6***
—
—
C57*
C57**
—
—
BALB**
BALB***
IF,*
—
IF,***
—
—
BALB*
IF,***
IF,**
—
—
IF,**
IIF,***
—
IIF, (BALB $)*
—
—
IIF,(BALB?)**
—
—
—
—
IF,*
IF,**
—
CBA/T6***
—
—
CBA/T6***
Eyes
C57*
CBA*
BALB**
BALB***
Lower incisors
—
—
—
—
Pinna
—
—
IF,***
IIIF,***
—
BALB***
—
—
IIIF,***
—
IIF, (BALB $)*
IIIF, (BALB ?)***
C57***
—
—
BALB***
Nipples
—
—
CBA/T6***
—
IIIF,(BALB?)***
IF,*
IF,*
IF,***
IIF,***
—
IIF, (BALB £)***
IIIF, (BALB 5)*
—
—
BALB***
—
Vagina
The genotypes in the body of the table are those which matured earlier. *, **, and *** indicate differences in t values that are significant at the 50,
10 and 0 1 % probability levels, respectively.
Inbred lines
C57/CBA/T6
CBA/CBA/T6
C57/BALB
CBA/T6/BALB
Reciprocal F, hybrids
IF, (C57/CBA/T6)
IIF,(C57/BALB)
IIIF, (CBA/T6/BALB)
Different hybrid genotypes
but same maternal genotype
C57 mother IF,/IIF,
CBA/T6 mother IF,/IIIF,
BALB mother IIFJIIIF,
Hybrid with inbred offspring
with same maternal genotype
C57/IF, (C57 ?)
C57/IIF, (C57 ?)
CBA/T6/IF, (CBA/T6 ?)
CBA/T6/IIIF, (CBA/T6 ?)
BALB/IIF, (BALB $)
BALB/niF, (BALB $)
Offspring compared
Table 2. Significant differences in the mean ages at maturation of offspring of different mating types reared at 23 °C
to
S
s
5
***
•**
**-
JC57?
ICBA/T6?
UF
ri
(C579
1BALB9
/CBA/T6 9
IIIF
MBALB9
TF
iri
C57
CBA
CBA/T6
BALB
128
46
61
160
153
87
102
118
177
144
No.
+ 0-79***
+ 0-74***
+ 0-71***
+ 0-53***
+ 0-77***
+ 0-33**
+ 0-48***
+ 0-69***
+ 0-68***
+ 0-60***
+ 0-64***
+ 0-21
+ 0-70***
+ 0-46***
+ 0-61***
+ 013
-003
+ 0-65***
+ 0-58***
+ 0-52***
+ 0-68***
+ 009
+ 0-72***
+ 0-38***
+ 0-68***
-010
+ 0-70***
-010
+ 0-55***
+ 0-42***
Incisors with
eyes
63
21
29
75
78
47
47
65
104
67
No.
Incisors with
vagina
+ 016
+ 0-53*
+ 0-58***
+ 007
+ 0-55***
+ 010
+ 0-65***
+ 0-69***
+ 0-31**
+ 0-48***
Pinna with
nipples
+ 0-73***
+ 0-67***
+ 0-69***
+ 0-42***
+ 0-34**
+ 004
+ 0-21
+ 0-54***
+ 0-42***
+ 0-29*
Females
***, Coefficients significant at the 50 %, 10 % and 0-1 % probability levels, respectively.
Pinna with
eyes
Pinna with
incisors
Males and females
+ 0-53***
+ 0-37
+ 0-56**
+ 0-21
+ 0-48***
+ 012
+ 0-64***
+ 0-36**
+ 0-33***
+ 0-28*
Nipples with
vagina
Table 3. Correlations between the ages at development of five parameters of maturation, for individuals reared at 23°C
O
H
X
u
Z
d
GAR RARD A
ERS
Morphological maturation of the mouse
253
Inter-character correlations between developmental ages at 23 °C
The relationship between the ages of individual animals, within a mating type,
at maturation of pairs of morphological characters have been estimated by
calculating correlation coefficients. The coefficients are presented in Table 3.
Data for males and females are combined. Between most of these pairs of
characters there is a highly significant correlation but especially between the
times of pinna freeing and eye opening. For some characters the offspring of
certain mating types may show high correlations, while in others the correlations
are not statistically significant. Some of the non-significant correlations may be
attributable to the very small variance in the time of maturation of some of the
characters.
The effects of body weight on maturation at 23 °C
One factor which might have produced the observed correlations between the
ages at maturation of the various morphological characters is the integrated
nature of general growth. The relationship between body weight and age at
maturation was examined by linear regression analysis using the mean weights
of litters at that weekly age which immediately preceded the development of the
character and the mean ages of litters at the time of maturation. For pinna
freeing and eye opening there is a statistically significant, negative regression
coefficient relating age of maturation and body weights at fixed ages in all
mating types. The majority of the remaining negative regressions are also
significant for the other three characters. On the basis of this analysis it is
evident that, within genotypes, light animals matured later than heavy ones.
To some extent the relationship between weight growth and maturation
explains the observed differences in the mean ages at maturation of the different
mating types. Few of the regression coefficients vary significantly according to
mating type, but the regression constants do vary with mating type. For
instance, among reciprocal cross hybrids, none of the regression coefficients
differ significantly from one another but there are significant maternal effects
upon the intercepts for each parameter.
Ages at morphological maturation have also been related to the body weights
of animals at the time of maturation. The mean weights at the times of maturation are included in Table 1. Although the relationship between weight and age
at maturation is less than between weight at a fixed age and age at maturation,
some of the coefficients for the former are nevertheless significant for some
characters in some mating types. This is particularly evident at eye opening
when, with only one exception, there is a highly significant negative regression
between the age of development of the character, and body weight at that age.
Pinna freeing shows the same phenomenon, though to a smaller extent. In
contrast, appearance of nipples, eruption of lower incisors and, to some extent,
opening of the vagina are more affected by weight growth, there being very few
254
G. GARRARD AND OTHERS
Table 4. Mean ages (days) and body weights (g) at morphological maturation of
mice reared at 32 °C
I.
Strain
I
No. of
litters
Age
S.E.
Weight
S.E.
II
No.
Age
S.E.
Weight
S.E.
III
No.
Age
S.E.
Weight
i—i
S.E.
No.
Age
S.E.
Weight
S.E.
V
No.
Age
S.E.
Weight
S.E.
...
C57
BALB
6
41
015
2-2
006
6
9-9
0-29
4-3
0-41
5
12-9
0-34
5-8
0-38
6
5-5
0-34
2-4
0-20
4
90
056
4-6
014
36
4-2
014
2-3
005
34
8-7
0-29
3-9
0-20
28
130
014
5-7
019
27
5-5
0-28
2-8
010
25
90
0-23
4-3
0-22
r
BALB$
40
3-8
013
2-3
005
39
93
014
4-3
013
32
12-6
014
5-6
017
32
5-5
010
2-7
010
35
8-9
017
41
013
24
41
015
2-5
009
25
9-4
012
4-6
019
24
13-1
018
5-9
0-27
21
5-2
013
30
012
21
8-8
018
4-4
016
I = Pinna freeing, II = lower incisor eruption, III = eye opening, IV = nipple appearance, V = vagina opening.
Means are genotypic means of litter means.
significant regression coefficients between ages at development of these characters
and body weights at the time of maturation.
Maturation at 32 °C
The mean ages of litters reared at 32 °C at the time of maturation of five
morphological characteristics are shown in Table 4. There was much less variation between the offspring of the different mating types at 32 than at 23 °C.
Ages at pinna freeing, nipple appearance and vaginal opening did not vary with
mating type, although ages at lower incisor eruption and eye opening did so.
Incisor eruption occurred later in C57 than in BALB (P = < 1-0 %) and in the
reciprocal cross hybrids maturation occurred at the same age, which was intermediate between that of the two parental strains. In contrast, at eye opening
BALB
/C57$
\BALB?
82
101
96
No.
+ 0-37***
+ 0-88***
+ 0-46***
Pinna with
incisors
+ 0-49***
+ 0-86***
+ 0-51***
Pinna with
eyes
Males and females
+ 0-31**
+ 0-73***
+ 0-53***
Incisors with
eyes
45
53
41
No.
Incisors with
vagina
+ 0-27*
+ 0-29*
+ 0-46**
Pinna with
nipples
+ 0-39***
-0-30*
-0-36*
Females
+ 0-31**
+ 009
0
Nipples with
vagina
Table 5. Correlations between the ages at development of five parameters of maturation, for individuals reared at 32°C
to
I
s-
3
I
256
G. GARRARD AND OTHERS
there was a reciprocal cross difference (P = < 5-0 %). For this character both
the inbred and F1 hybrid litters of C57 mothers matured before the inbred and
hybrid litters of BALB mothers.
There are few systematic differences in within, and between, litter variances
that can be related to mating type, although between litter variances in the times
of nipple appearance and lower incisor eruption were significantly higher in
BALB than in the reciprocal cross hybrids. BALB also had high levels of within
litter variation in the age of development of the same two characters, as compared with the hybrid offspring of BALB mothers.
Inter-character correlations between developmental ages at 32 °C
The correlation coefficients relating the ages at maturation of pairs of
characters are presented in Table 5. With the exception of C57, for which the
data are too limited, in those characteristics which are common to both males
and females the coefficients are all positive and almost all are highly significant.
The relationship between ages at nipple appearance and vaginal opening is not
significant in hybrids. Contrary to the results obtained at 23 °C and to other
results from animals reared at 32 °C, there is a significant (P = < 5-0 %) inverse relationship between ages at pinna freeing and nipple appearance among
each of the reciprocal cross hybrids.
At 32 °C many of the coefficients for the linear regressions of age at maturation on body weight at the immediately preceding weekly interval do not differ
significantly from zero. Where coefficients are significant they are negative,
indicating that lighter animals matured later than heavy ones. With the exception of the difference between the inbred and hybrid litters of BALB mothers
at vaginal opening, the relationship between the weights of animals at a fixed
age and their ages at maturation of a particular morphological character is not
influenced by genotype.
Variation in age at maturation, with respect to body weight at that age, was
also examined by linear regression analysis. There was no evidence at 32 °C for
the negative weight effect that was found at 23 °C. Indeed the only significant
regression coefficient occurred in BALB, at opening of the vagina, and the
relationship was positive.
Comparison of maturation at 23 and 32 °C
Among the offspring of the four mating types which were reared at both
temperatures, the mean ages at freeing of the ear pinnae were always significantly less at 23 °C than in animals reared at 32 °C. However, the appearance of
nipples and opening of the vagina occurred earlier at 32 °C (P = < 1-0%, in
each comparison). A similar retardation in development at 23 °C was apparent
at eye opening, though only in BALB and hybrid litters of C57 mothers was the
difference statistically significant (P = < 5-0% and < 0-1 % respectively). The
environmental effect upon the age of eruption of the lower incisors was less
Morphological maturation of the mouse
257
marked, the only significant difference occurring in C57 mothers' hybrid
offspring, with animals at 32 °C maturing the earlier.
In almost all cases animals reared at 23 °C were heavier, at the time of maturation, than those reared at 32 °C. Mean body weights at the times of nipple
appearance and vaginal opening particularly evidenced the environmental effect
for animals not only matured at an earlier age at 32 °C but they also matured
at a lighter body weight. Regression analysis showed that lower incisor eruption
occurred earlier in animals at 32 °C than in animals of the same weight and
genotype reared at 23 °C. Further, the effect of the higher temperature in
advancing eye opening becomes even more marked when environmentally
associated differences in body weight are taken into account. Consideration of
body weight does not modify the conclusion that pinna freeing occurred earlier
at 23 than at 32 °C.
Some genotypic differences between the ages at maturation of particular
characters were altered by the environmental temperature. For example, for all
the morphological characters except eye opening BALB animals reared at 23 °C
tended to mature earlier than the other inbred lines but this was not so at 32 °C,
though the data for C57 are scant. Other genotypic relationships in ages at
maturation were unaffected by the temperature of the environment. Whilst
maternal and genotypic offspring effects were detected among hybrids, in both
environments there were no consistent differences in the ages at maturation of
inbred and hybrid lines.
Age at maturation was also regressed on litter size at birth and, as was to be
expected from the relationship between body weight and litter size at birth
(Garrard, Harrison & Weiner 1974ft), there was a marked effect of litter size on
developmental age. Although these regressions did not distinguish hybrids from
inbreds any more clearly than did the weight regressions, there was evidence
that at 23 °C, though not at 32 °C animals from inbred lines mature later than
Fx hybrids from litters of the same size.
Comparisons of the between, and within, litter variances of both the ages and
the body weights at which maturation occurred showed that the environment
affected the magnitude of the variance. Where significant differences occurred
it was usually the variance of animals reared at 32 °C which was the greater.
This was true of between litter variances at pinna freeing, lower incisor eruption
and nipple appearance among the offspring of BALB mothers. The hybrid
offspring of C57 mothers were exceptional, their between litter variation in age
at nipple appearance being significantly greater at 23 than at 32 °C. Estimates of
within litter variation in age at maturation were greater at 32 °C at pinna freeing
and eye opening for BALB and reciprocal 11F1 (C57/BALB) hybrids but the
environmental effect was reversed at nipple appearance and vagina opening.
258
G. GARRARD AND OTHERS
The environmental effect upon the symmetry of maturation
Two of the characters that were considered in the previous analyses, pinna
freeing and eye opening, have a bilateral disposition which leant itself to scoring
for symmetry or asymmetry in development. When the development was
asymmetrical the interval between development on the two sides rarely exceeded
one day. A contingency analysis was used to assess the effects of genetic and
environmental variation on the numbers of symmetrically and asymmetrically
developing animals in the two environments. Within neither environment did
the frequency of asymmetrical maturation vary according to mating type but
there were significant differences according to the temperature at which the
animals had been reared. The freeing of the pinnae and eye opening were less
symmetrical in their development among BALB animals reared at 32 °C than
when reared at 23 °C (P = < 5-0 % and 0-5 %, respectively). Among the reciprocal IIjFi (C57/BALB) hybrids, those with a C57 mother showed a greater
asymmetry at pinna freeing (P = < 0-5 %) and those with a BALB mother at
eye opening (P = < 0-5 %) at 32 than at 23 °C. There is thus evidence that
asymmetrical maturation is more common in the hotter conditions.
DISCUSSION
This study has demonstrated that the age of maturation of certain external
morphological features is influenced by genetic and environmental factors.
These factors may interact with each other so that the relative ages at maturation
of mice of different strains varies according to the environmental temperature.
Other investigations have shown that environmental conditions, especially in
association with nutritional variation (Parkes, 1929; Yoon, 1955; Kennedy,
1957; Widdowson & McCance, 1960), affect the age of maturation of some of
the characters that were also examined in this study. Age at maturation was
shown to depend upon growth in body weight. In the present study too the ages
at eruption of the lower incisor teeth, the appearance of the nipples and, to a
lesser extent, the opening of the vagina are closely related to general growth in
animals reared at 23 °C. In contrast, ages at pinna freeing and eye opening of
the same animals indicate that the development of these characters tends to be
at a fixed age, the age being strain-specific. Nevertheless, for all the morphological characters examined there is evidence that slowly growing animals, those
with low body weights at the fixed weekly intervals, mature morphologically
more slowly than rapidly growing animals.
From studies in which morphological development was examined in relation to
environmental temperature variation earlier eye opening (Biggers et ah 1958) and
vaginal opening (Biggers et ah 1958; Knudsen, 1962) in hot than in temperate
conditions were associated with comparatively high body weights. However,
the temperate conditions which these workers employed were less stressful
Morphological maturation of the mouse
259
than those in the present study, and promoted weight growth. In the work
reported here high temperatures caused early maturation in these characters and
also in nipple appearance and lower incisor eruption, even when early postnatal growth in weight was reduced. Certainly at the time of maturation, animals
reared at 32 °C weighed less than those reared at 23 °C. The time at maturation
of four of the characters which developed earlier at 32 than at 23 °C coincided
with that period when the growth rate was greater in the animals reared at 32 °C
(Garrard et al. 19746). The one character that matured earlier at 23 °C - pinna
freeing - did so in the first week when the 32 °C environment had a retarding
effect on growth, as assessed from body weight. Despite this and the high correlations between weights and ages at maturation within each environment,
the pattern of growth in body weight cannot fully account for the observed
environmental and genotypic differences in maturation. Under the stress of an
environmental temperature of 32 °C there is evidence for a breakdown in the
relationship between the pattern of growth in body weight and the maturation
of particular morphological characters that was found at 23 °C. Just what is responsible for the dissociation of weight growth and morphological maturation is
obscure. There is some evidence that specific growth factors may be involved in
the maturation of some of the tissues. Cohen (1962), for instance, has been able
to^isolate from the submaxillary gland of the adult male mouse a factor which,
when injected into neonates, causes premature incisor eruption and eye
opening. It is possible that environmental temperature could affect the production of, or receptivity of tissue to, such factors. Since a cold environment
retards development independently of its effect on body weight (Biggers et al.
1958; Barnett & Coleman, 1959) there is evidence for a specific temperature
effect, with the heat promoting and the cold retarding maturation.
Even more marked than the environmental temperature effects are the differences in the developmental ages of offspring of the different mating types,
especially at 23 °C. A few differences appear to be related to maternal environmental factors acting mainly, though not invariably, on weight growth. In
addition, there is evidence for offspring genotypic effects, not only among the
different inbred strains, but also between Fx hybrids of different genotypes. The
latter are most manifest at pinna freeing and eye opening, when a C57 contribution to a hybrid, irrespective of whether it is derived from a maternal or a
paternal source, advances maturation; whilst a BALB contribution retards
development. These patterns are not predictable from the relative developmental
ages of the respective inbred parental strains although these strains are themselves variable. Thus BALB, which is characterized by the early maturation of
most parameters, is markedly retarded in eye opening. When allowance is made
for the relationship between body weight growth and maturation age, the
development of BALB is not as precocious as would at first appear. There are,
however, problems in assessing the effects of body weight, for the analysis has
shown that although weight growth and maturation within any one strain
260
G. GARRARD AND OTHERS
are closely related, the form of the relationship is a characteristic of the
genotype.
There is very little evidence from either environment for systematic differences
in the rates of maturation of hybrids as a whole as compared with all the inbred
strains. The type of variation occurring within and between the inbreds and
hybrids reared at 23 °C is much more consistent with specific heterozygotic/
homozygotic contrasts but before this can be firmly concluded a detailed genetic
analysis is required. Specific genetic effects might be expected to operate either
through variable receptivity of tissues to local morphogenetic factors or through
quantitative variation in the production of such factors. At a developmental
level the genetic variation could therefore be causally produced in much the
same way as the environmental effect. However, the data from animals reared
at 32 °C, though much more limited, indicate that genetic variety is less expressed
in age at maturation in this environment than at 23 °C. So far as can be judged
the maturation promoting effect of the hotter environment occurs more or less
independently of genotype.
The results indicate that, while growth and the features of morphological
development that have been examined are closely interrelated, at least some of
the underlying processes are different and can be independently affected by
genetic and environmental factors. Although the growth pattern has been
described in terms of weights at particular ages or when particular events
occurred, the data indicate that even fine monitoring of differences in growth
rates and changes in these rates would be unlikely to account for some of the
observed independence between growth and maturation.
We are grateful to Mr M. Potter for his technical assistance and to the Science Research
Council for a grant to G. Garrard.
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{Received 17 September 1973)