The basis for the project was a comparison of the prenatal growth of
the largest and smallest pigs within litters. The differential growth rate
of these two groups provides a natural experiment for determining
factors important in limiting growth. The investigation concentrated on
muscle growth because of its agricultural importance, but gross
measurements of the fetus and its membranes were also made in an
attempt to ascertain the cause of the size difference.
The material consisted of 12 gravid uteri of Large White x Landrace
gilts sacrificed at approximately 10 days intervals from 38 days gestation
until term, together with 2 new born litters. The project can be divided
into 3 sections according to the techniques employed.
1. Fetal and Placental growth
Gross measurements of fetal weight, length and position within the
uterine horn were taken from each animal together with x-rays of the
forelimbs of fetuses from two litters. The results showed that the small
littermates, as well as being lighter, were shorter, had a lower ponderal
index, had a less well ossified skeleton and tended to be found in the
more crowded of the two uterine horns. All but the last characteristic
is descriptive of small-for-dates babies in comparison to normal. Mean
fetal weight was found to decline with increasing number in a uterine
horn but the spacing between fetuses was always even. Examination of
farm records (247 pigs) showed a small (80gm.) but significant difference
in the birth weights of male and female pigs.
The fresh weight, in situ length, circumference and area of all the fetal
membranes were measured. All the above parameters were found to
increase significantly during gestation except for placental length. This
last parameter was found to show a strong position effect with the
shortest placentae generally occurring in the middle region of the uterine
horn. Macroscopic placental area was found to be an adequate measure
of the exchange area as no difference could be demonstrated between
placentae in terms of the surface enlargement due to their microscopic
ridges. In general, fetal membrane weight and area correlated well with
fetal weight, in contrast to placental length and circumference. The
endometrium at the site of each fetus was thrown into macroscopic folds
of up to 2 cm in height. These folds were found to be significantly
greater in more crowded horns so that these placentae partly
compensated for their shorter length.
2. Muscle histology and histochemistry
Frozen sections of the semitendinosus muscle stained with haemotoxylin
and eosin were used to calculate fibre number and the mean sizes of
primary and secondary fibres. Fibre number increased from 38 days
until 85-90 days gestation when total fibre number was about 350,000.
The time when fibre hyperplasia ceased was estimated from the time
when total fibre number and the secondary/primary fibre ratio became
constant. From 60 days onwards the larger littermate had more fibres
in the m. semitendinosus than its smaller sibling and the difference at
birth was 17%.
Primary fibres were observed at 38 days and continued to form until 60
days. Equal numbers of primaries (about 18,000) formed in both large
and small littermates. Primary fibre size increased from 38 to 70 days
gestation after which these fibres declined in diameter. The initial
increase in size was largely caused by the development of a region of
myofibril free cytoplasm in the centre of these fibres giving them a
tubular shape which was lost in the second half of gestation due to their
collapse. Primaries in both large and small were of equal size at 38
days but a significantly greater maximum size was attained by primaries
in large littermates (23 ^ m) compared to the small (17 ^ m). This size
difference was due to differences in the extent of the myofibril free
region, described above, which could account for 60% of the fibre
diameter in large as opposed to only 30% in the small.
Secondary fibres were formed between 54 and 85 days gestation and
there appeared to be no difference in the duration of hyperplasia
between the large and small animal. This generation of fibres was found
to be formed only on the surface of primary fibres. Secondary fibres
showed little change in size until 100 days when hypertrophy started.
There was a small but significant difference in their diameter between
large and small (7 f* m and 6 j* m respectively for most of gestation).
Comparison of the secondary/primary fibre ratio during gestation showed
that the difference in fibre number mentioned above was due to fewer
secondaries forming on each primary in the small as compared with the
A hypothesis was put forward that a difference in the available surface
area of primary fibres, caused by their different diameters, had resulted
in fewer secondaries forming on the surface of primary fibres in small
littermates. Sections were taken of the m. semitendinosus of all 13
members of a litter of 70 days gestation. A significant correlation
between fetal weight and primary fibre diameter was found, together
with a significant correlation between primary fibre diameter and the
secondary/primary fibre ratio.
Frozen sections of the m. semitendinosus were histochemically stained
for Adenozine triphosphatase after acid preincubation. From ^6 days
until 102 days only primary fibres in the deep region of the muscle were
stained, indicating that there were presumptive slow twitch fibres.
After 102 days secondary fibres immediately adjacent to the above
primary fibres also started to be stained so that by birth, characteristic
bundles of slow twitch fibres were seen in the deep region.
Assays for DNA, RNA and two fractions of protein (sacroplasmic and
fibrillar) were carried out on the semitendinosus muscles of large and
small littermates. All the above parameters increased during gestation
but significantly lower total amounts were found in small littermates.
No significant difference could however be found between large and
small in the concentrations of these constituents. DNA concentration
increased until 100 days after which it declined. The concentration of
nuclei (measured from sections) described a similar pattern but showed a
significantly higher concentration of nuclei in small littermates. RNA
concentration declined with age while protein concentration, after an
initial decline, increased until term. Some of these results were
discussed in terms of the histology reported in the previous section.
Towards term smaller fetuses had significantly higher
sacroplasmic/fibrillar protein ratios which is believed to be a sign of
malnutrition. RNA/DNA ratios showed an initial decline but were
constant for the rest of gestation. Protein/DNA ratios also showed a
similar pattern with an initial decline followed by a constant level until
it increased after 100 days. No significant difference could be
demonstrated between large and small in the results for either ratio.
The conclusion of this project was that the smallest littermates were
growth retarded due to prenatal malnutrition. The most important
effect of this was the failure of small littermates to form as many
secondary fibres as their large siblings. A theory linking this to their
low primary fibre size was presented. Other factors affecting fibre
number were discussed together with the effect this has on postnatal
growth and performance.