Onset of sexual maturity in female Göttingen minipigs
B. Peter1, E.P.C.T. De Rijk1, H. Lorentsen2, A. Zeltner2, H.H. Emmen1
1
WIL Research Europe B.V., ‘s-Hertogenbosch, The Netherlands
2
Ellegaard Göttingen Minipigs A/S, Dalmose, Denmark
l
Introduction
l
Results, continued
l
Results, continued
In drug development, minipigs are used as a non-rodent alternative to dogs or non-human
primates, though the exact age range when female minipigs reach sexual maturity is unknown.
This, however, is pivotal for the design of general toxicity studies when a reliable interpretation
of female reproductive functioning is required. Based on breeding experience, female Göttingen
minipigs are considered to reach sexual maturity at 4–5 months of age. To investigate this further,
a study with young sows at 3–4 months of age was initiated to determine the age range when
sexual maturity is reached.
IMMATURE
AF
CA
l
Method
A total of 14 female Göttingen minipigs (3-4 months old) were used that were housed in the
same room as adult boars. Twice weekly, they were observed for signs of heat (vulva redness/
swelling, mounting) and blood samples were taken for measurement of the reproductive
hormone progesterone. In addition, body weights were recorded every 2 weeks. Females were
sacrificed either after completion of 1-2 cycles of progesterone release or at an age of 7 months,
if no progesterone was detected. The reproductive organs (ovaries, uterus, pituitary gland) were
weighted, and histopathological examination of reproductive-related organs (ovaries, uterus and
cervix, vagina, mammary gland and pituitary) was performed to confirm sexual maturity.
l
Results
Age and body weight at first ovulation
18
ght (kg)
Body weight
Bod
(k
AF
Figure 2: Schematic presentation of the estrous cycle in Göttingen minipigs based on
progesterone measurements. Day 0 of the cycle depicts ovulation, i.e. the time point when
the ovarian follicle ruptures and releases the egg from the ovary. Without the ovum, the
follicle folds inward on itself, transforming into the corpus luteum, a steroidogenic cluster of
cells that produces progesterone. Levels of progesterone increase until the Mid Luteal phase,
whereupon they decrease again if no mating occurs. The estimated cycle length was 17–22
days. F/P = Follicular phase/Proliferation phase; EL/ES = Early Luteal phase/Early Secretory phase;
ML/MS = Mid Luteal phase/Mid Secretory phase; LL/LS = Late Luteal phase/Late Secretory phase.
16
SEXUAL MATURITY
80
70
60
O
V
U
L
A
T
I
O
N
O
V
U
L
A
T
I
O
N
50
40
30
20
O
V
U
L
A
T
I
O
N
10
14
0
12
6
2
4
6
8
Age (months)
Figure 1: For the 13 females that reached sexual maturity, the first cycle of progesterone release
indicative for ovulation was noted in 7 sows at 3.7-4.2 months (8.4-12 kg) and for another 6 sows
at 6.1-6.5 months (12.1-16 kg) of age. There was no correlation between body weight and the age
at onset of sexual maturity.
F
AF
AF
Figure 4: Photomicrographs of an immature (no ovulation has yet occurred) and a mature ovaria
in the Mid Luteal phase. On the left: The ovarium of an immature minipig with small to medium
sized anthral follicles (AF). No corpora lutea are present. On the right: In the ovarium of a mature
minipig, also small to medium sized anthral follicles (F; growing into a Grafiaan follicle in the
following cycle just before ovulation), a fresh corpora luteum (CL) from the present cycle, and
a corpus albicans (CA) from a previous cycle can be noted. (PAS/Hematoxylin-staining; original
objective lens magnification 2x)
l
Discussion/Conclusion
The age range when female Göttingen minipigs reach sexual maturity (3.7–6.5 months) is much
longer than generally thought. Hence, care should be taken when designing toxicity studies with
young sows and interpreting the data hereof. For minipigs, a detailed description of the normal
morphological changes of the reproductive organs during the various phases of the estrous cycle is
lacking. In the present study, progesterone analysis proved to be a reliable method to detect sexual
maturity during in-life and to determine the stage of the estrous cycle. This provides an accurate
tool to investigate in the minipig the normal morphology of the reproductive organs during the
various phases of the estrous cycle, which will ultimately allow the evaluation of female minipig
reproductive function as a relevant endpoint within toxicity studies.
l
Acknowledgments
5.5
6
6 .5
Age (months)
7
7 .5
10
8
F
CL
AF
Pro esteron (ng/mL)
Progester
(ng/mL)
Before reaching sexual maturity, very low concentrations of progesterone (< 1 ng/mL) and/or
signs of heat were sporadically detected. The first cycle of progesterone release, indicative of a
functional corpus luteum, was noted in 7 sows at 3.7–4.2 months (8.4–12 kg) and for another
6 sows at 6.1–6.5 months (12.1–16 kg) of age. Neither body weight nor weight of the reproductive
organs appeared to be a useful predictor for the onset of sexual maturity. Based on progesterone
data, the estimated cycle length was 17–22 days. Signs for heat were not always noted around
the expected ovulation. Histopathological examination of the reproductive organs confirmed
that all 13 sows with progesterone peaks were sexually mature. One other female did not show
any progesterone release until necropsy at 7 months of age and histological examination of her
reproductive organs confirmed that she had not reached sexual maturity.
MATURE
Figure 3: A representative example of estrous cycles in the Göttingen minipig. For this female,
first ovulation was seen at the age of approximately 6.3 months. The graph depicts 2.5 cycles of
progesterone release. Before reaching sexual maturity, very low concentrations of progesterone
(< 0.234 ng/mL) were measured on one occasion at approximately 4.5 months of age (not shown)
and signs of heat (vulva redness/swelling to a slight or severe degree; see arrows) were noted
sporadically. With onset of sexual maturity, signs of heat are often, but not always, seen around
the time of ovulation. Photomicrographs of representative ovaria for the different stages of sexual
maturity are added. For further details on the structure of the ovaria, see Figure 4.
The authors would like to thank the dedicated staff of Ellegaard Göttingen Minipigs A/S and WIL
Research Europe B.V. for their expertise and assistance during the conduct of this joint project.
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