by Peter Kaufmann
1) General Zoological Data
The guinea pig is the best known representative of caviomorph rodents. This is probably due to the fact that it became the only truly domesticated caviomorph. This small South American rodent is considered to be a delectable meal for South American natives and, therefore, it was kept and bred as domestic animal long before becoming largely extinct in the wild. Moreover, in the 20th century it started its triumph around the world as pet. Cavia porcellus is the domestic species and was derived from its wild ancestor Cavia aperea. The sporadic use of the designation Cavia aperea porcellus may point to this origin, but also to existing hybrids (Cavia aperea x Cavia porcellus). The name "guinea" is mistaken; it should have been Guyana, its origin; "cavia" is a Tupi word for rat, and "porcellus" refers to little pig (Gotch, 1979).
With the exception of the human placenta, the guinea pig placenta is today the best studied placenta. The reasons are twofold. First, the guinea pig became one of the favorite experimental animals for pharmacologists and toxicologists, so that basic data concerning its reproduction were essential and asked for. Second, already the first examinations of its placenta by Enders (1965), Vollrath (1965), Müller & Fischer (1968), Kaufmann (1969), and Davidoff & Schiebler (1970) made it clear that (a) due to a series of similarities with the human placenta and (b) because of several advantages over other experimental models, such as the widely-used sheep, the guinea pig might become a perfect animal model for placentologists. The guinea pig, different from other laboratory rodents, has a rather long gestation period (up to 70 days); it is handy, patient and easily bred; it has an endocrine pregnancy control similar to that of the human, and it has a discoidal, hemomonochorial placenta with a fetal/maternal transport barrier which is very similar to that of the human placenta. Consequently, for a period of three decades, the guinea pig started to replace the sheep and became the favorite model in placentology. The success of this animal model has ceased only recently because of the explosively developing human in vitro models, such as primary cell cultures isolated from the human placenta, immortal placental cell lines, placental tissue explants and human placental lobules perfused in vitro. Longevity of domestic guinea pigs is up to six years.
There is a very large variety of guinea pig strains, coat color etc. They are easily accessible through various web sites. These also provide information on diseases, home care, length of expected fertility (4-5 litters), possible problems in assigning sex, and many more topics. One useful site is: http://www.oginet.com/pgurney/. The picture shown comes from another such sites (http://www.meerschweinchen.de by K. Stuber, Germany).
General Gestational Data
Length of gestation: 63 - 70 days
Litter size: 1 - 9, mean 4
Body weight (non pregnant): 750 g, at full term: 1,000 - 1,400 g
Fetal weight at full term: 60 - 100 g
Fetal crown-rump length at full term: mean 100 mm
Weight of placenta and membranes at full term: 5 - 9 g
Organ weight data may be found in the contribution by Webster & Liljegren (1949).
With respect to litter size and gestational length, the experimentator should be aware of some important facts: In wild guinea pigs and in domestic breeds of the sixties, the litter size was rather low (2.3) and gestational length rather short (59 to 63 days) (Weir, 1974; Kaufmann, 1969); in contrast, conventional domestic breeds and inbred strains developed later in the same century tended to produce larger litter sizes and showed an increase in pregnancy length together with an increase in fetal and placental weights; in the meantime, some inbred strains (e.g. Pirbright-White) produce litter sizes up to 12 (mean >6) and have gestational lengths of >72 days. There is also evidence that litter size, gestational length, fetal and placental weight at term increase with increasing age, parity and body weight of the sow (Kaufmann & Davidoff, 1977). For experimental purposes, it is therefore highly advisable to work with colonies that are as homogeneous as possible regarding strain, maternal body weight, parity and age of the female animals.
The general placental type is discoidal, labyrinthine, hemo-monochorial that represent the chorioallantoic main placenta, and with separate subplacenta and yolk sac placenta (Kaufmann & Davidoff, 1977). The allantois is only used for the formation and vascularization of the chorioallantoic placenta (main placenta and subplacenta).
As far as the gross anatomy of the fetal membranes is concerned, the guinea pig has two uterine horns, each horn providing space for one to five implantation sites. The elongated uterine horns unite to form a short, common corpus uteri. Each fetus is surrounded by (Fig. 1) an inner fetal membrane, the amnion (Figures 3a, b), and an outer fetal membrane, which is represented over 95% of its surface (antimesometrially and laterally) by the visceral yolk sac (Figures 3a,b); and only at about 5% of its surface (mesometrially) it is made up by the chorioallantoic main placenta, a small disc-like organ (Figures 3a,b); at its basis the latter has a stem-like connection to the uterine wall, the placental stalk, which contains the subplacenta, representing a special segment of the chorioallantoic placenta (Figures 2, 3a,b).
The gross anatomy of the chorioallantoic main placenta is represented by the main fetal/maternal exchange organ, the disc. As in all caviomorph rodents, it consists of a lens-like disc of syncytiotrophoblast (Figure 2), which is traversed throughout by a system of web-like channels containing maternal blood, the maternal blood lacunae. Additionally it is traversed only in its more central sections by fetal capillaries.
Miglino et al. (2004) have presented a remarkable comparative study of the placentas of agouti, capybara, guninea pig, paca and rock cavy that gives superb details of the vascular organization of these organs.
a consequence of this complete maternal, but only partial fetal vascularization
of the disc, the main placenta is composed of different zones:
The parts perfused by both, maternal and fetal blood channels, make up the so-called labyrinth. It is composed of 60 to 100 cylindrically shaped structures, the labyrinthine lobes which vertically pass the placental disc (Below). They make up about 80% of the main placental volume at term. The lobes measure between 1 and 2 mm in diameter, and 5 to 8 mm in length. All lobes are connected to each other close to their fetal ends. The labyrinth provides the vast majority of fetal/maternal exchange tissue of the guinea pig placenta.
The labyrinthine lobes are embedded into syncytiotrophoblast which is only maternally perfused, the so-called interlobium (about 15% of the main placental volume at term). The interlobium separates the labyrinthine lobes from each other ('interlobar syncytium') and forms the outer mantle of the main placental disc ('marginal syncytium'). The maternal blood lacunae of the interlobium provide the venous blood channels that drain the labyrinth of maternal blood. The syncytiotrophoblast lining the maternal blood lacunae is the secretory source of the progesterone-binding protein (PBP) (Perrot Applanat & David Ferreira, 1982).
In its center, the main placenta is passed by an axis of fetally vascularized mesenchyme, the central excavation (about 5% of the main placental volume at term). This is the continuation of the umbilical cord, across the main placenta, down to the subplacenta. At its basal end, the central excavation widens like an umbrella ('roof of the central excavation'). Those parts of the chorioallantoic placenta, basal to the roof of the central excavation, by definition are designated subplacenta (see section 11 below).
Details of Fetal/Maternal Barrier
The labyrinthine lobes represent the dominating part of the fetal/maternal exchange zone. In caviomorph rodents this is an ideal labyrinthine, hemomonochorial placenta with counter-current arrangement of blood vessels.
Labyrinthine in this context means: a lens-like mass of syncytiotrophoblast (the placental disc) (shown below) is passed, like in Swiss cheese, by a web-like system of channels, half of which contain maternal blood lacunae, which have no own endothelial vessel walls; and the remaining ones contain fetal capillaries, lined by fetal endothelium which is separated from the syncytiotrophoblast by a basal lamina.
in this context means: maternal blood and fetal blood streams are separated
by the following tissue layers (Figures 5, 6):
- an uninterrupted layer of syncytiotrophoblast, beneath which, but only in the first half of gestation, accidental trophoblast cells (cytotrophoblast) can be found;
- a basal lamina, jointly secreted by syncytiotrophoblast and endothelium;
- few connective tissue cells, mostly macrophages, and very few accompanying collagen fibers;
- fetal endothelium which only in the center of the lobes may be accompanied by accidental pericytes.
The counter-current arrangement of vessels (Figure 7) in this context means: Both,
- the maternal blood lacunae (blood flow direction from the center of the cylindrically shaped lobes towards their periphery),
- and the fetal capillaries (blood flow direction from the surface of the lobes towards their centers), are arranged in such a way (Figures 7, 8) that maternal and fetal blood circulates in parallel but opposite directions. According to Faber & Hart (1966) this is the most effective exchange system for diffusional transfer.
its high degree of maternal and fetal vascularization, the guinea pig placenta
is particularly susceptible to postmortem and fixation artifacts. This becomes
evident when comparing the series of semithin sections depicted in Figure
9: All three pictures of the labyrinth come from the same stage of pregnancy
(2 days prior to parturition), are from the same part of the labyrinth (periphery),
are shown at identical magnification, were fixed with the same fixative
(2.2% phosphate-buffered glutaraldehyde, 340 mosmol), but were prepared
with different modes of applying the fixative:
Figure 9a was prepared following immersion fixation(i.e. fresh, excised tissue samples measuring 2x1x1mm were immersed in the fixative for two hours), which caused severe collapse of fetal capillaries and maternal blood lacunas.
Figure 9b was prepared following supra-vital perfusion fixation of the maternal vascular system (i.e. the fixative was instilled into the uterine and ovarian arteries whereas mother animal and fetuses were in barbiturate anesthesia). Note that both, fetal (containing red blood cells) and maternal (devoid of blood) vessel lumina are wider than following immersion fixation, however, the maternal vessels smaller as compared to the fetal vessels.
Figure 9c was prepared following supra-vital perfusion fixation of the fetal vascular system (i.e. instillation of the fixative into the umbilical arteries whereas mother animal and fetuses were in barbiturate anesthesia). Note that both, fetal capillaries (devoid of blood) and maternal blood lacunae (containing erythrocytes) are much wider than following immersion fixation. However, different from the results of maternal perfusion fixation (Figure 9b), the width of the unperfused maternal blood lacunae considerably exceeds that of the perfused fetal capillaries.
comparison of these data with the volume relations measured with physiologic
methods (dilution of radioactive microspheres following administration via
fetal and/or maternal circulation) suggests that perfusion fixation is clearly
superior to immersion fixation. However, which type of perfusion, maternal
or fetal, reflects the true in vivo conditions? Possibly neither of both
do; rather due to the strong and immediate protein-coagulation by the fixative,
the vascular system coming into contact with the fixative first, will shrink
to the benefit of the later-fixed, unperfused vascular bed. Consequently,
even though nearer to truth than immersion fixation, any type of perfusion
fixation will result in distortion of the true volume relations. According
to our experience, we are coming quite close to the in vivo situation when
calculating the means of values obtained after separate fetal and separate
maternal perfusion fixation.
Quantitative structural data:
When calculating the means following fetal and maternal perfusion fixation, quantitative analysis of the guinea pig placenta gives the following data:
Volume composition of the term guinea pig main placenta (interlobium and labyrinth):
- syncytiotrophoblast: 31.3%
- endothelium, other vascular wall cells, connective tissue: 13.7%
- fetal vascular lumina: 17.5%
- maternal lacunar lumina: 37,5%
In conclusion, 55% of the placental volume are vascular lumina filled with blood. This explains the sensitivity of this organ to postpartum vascular collapse, post-mortem changes, fixation artifacts, etc.
Mean materno-fetal diffusion distance at term: 3.2 µm
Surface data at term:
- maternal vascular luminal surface: 0.12 m2/cm3
- fetal vascular luminal surface: 0.11 m2/cm3
Most of these data (volume of syncytiotrophoblast, fetal vessel lumina, maternal blood volume, diffusion distance, ratio of fetal to maternal vascular surfaces) are surprisingly similar to those of the term human placenta. The only remarkable difference is the share of fetal connective tissue which, in the guinea pig, is close to zero whereas in the human placenta it amounts to > 20%.
Trophoblast turnover: The biology of syncytiotrophoblast in the human placenta, that is the mechanisms of its formation and regeneration as well as the extrusion of aged syncytiotrophoblast ('trophoblast turnover') have become well understood in the mean time (for review see Benirschke & Kaufmann, 2000): Villous cytotrophoblast provides a pool of proliferating stem cells which upon leaving the cell cycle start differentiation for about 2 days. Thereafter, syncytial fusion takes place. Upon syncytial fusion, the syncytial nuclei loose their generative potential: DNA replication is completely stopped; transcription of RNA is downregulated to unmeasurable values (Huppertz et al., 1999). Therefore, not only for its growth but also for its own survival and to fulfill the numerous transport and synthetic functions, syncytiotrophoblast depends on continuous input of freshly transcribed mRNA and freshly translated proteins. Without syncytial inclusion of new cytotrophoblast, syncytiotrophoblast dies within two to three days. The fusion rate required for survival by factor 6 exceeds the needs for syncytial growth. The resulting excess quantities of 'aged' nuclei are extruded by apoptotic mechanisms into the maternal blood. The average survival time of a nucleus in the syncytiotrophoblast was calculated to be around 20 to 30 days (Benirschke & Kaufmann, 2000).
Guinea pig syncytiotrophoblast looks like human villous syncytiotrophoblast structurally and the mode of syncytial fusion (Firth et al., 1980) seems to be the same as described for the human (for review see Benirschke & Kaufmann, 2000). Therefore, it is surprising to note, that in the second half of the guinea pig pregnancy cytotrophoblast is largely missing in the placental labyrinth. Syncytial fusion has only been observed throughout the first weeks of guinea pig gestation. This raises the question as to the nature of syncytiotrophoblast in the guinea pig, whether, different from the human, it does not depend on continuous syncytial fusion for growth and its own regeneration; whether syncytiotrophoblast nuclei in the guinea pig perhaps has maintained a certain degree of nucleic acid metabolism; whether syncytiotrophoblast in the guinea pig is really a true syncytium that is exclusively derived by fusion of former cellular trophoblast, or whether it has perhaps a plasmodial character (formation of multinuclear structures by nuclear division without subsequent cellular division); whether syncytiotrophoblast in the human and in the guinea pig are really as analogous structures, as is generally assumed. Unfortunately, up to now respective studies of trophoblast turnover in the guinea pig are completely missing.
6) Umbilical Cord and Larger Fetal Vessels
Two umbilical arteries enter the main placenta via the umbilical cord (Figure 10). On reaching the main placenta, they branch into 4 to 6 big 'chorionic' arteries lying superficially under the amnionic cover. A further branch vertically traverses the main placenta and supplies the subplacenta via the central excavation. The 'chorionic' arteries repeatedly branch dichotomously, always one of the arteriolar daughter branches penetrating the interlobium and following the surface of the labyrinthine lobules. From these arterioles, the labyrinthine capillaries originate. The capillaries fuse in the lobular centers to form one central lobular venule which, upon fusion with the other lobular venules and the subplacental vein, finally form the umbilical vein. Starck (1957) described the cord of the guinea pig as being 2.5 cm long.
Umbilical vessels in the guinea pig are not easy to handle experimentally since they tend to contract irreversibly and completely following even the slightest mechanical irritation. Prior to cannulation of the umbilical arteries, careful paravascular infiltration of 4% formaldehyde solution into the umbilical mesenchyme proved to be useful since it reduced the tactile sensitivity of the vessels.
The maternal blood supply of the guinea pig uterus and placenta are summarized in figures 10 to 12.
Maternal arterial blood to the placenta is jointly supplied by the ovarian and the uterine arteries, which on either side form a long anastomosis, the arcade artery which is located at the base of the mesometrium (Fig. 11). This arcade artery gives rise to numerous mesometrial (radial) arteries, some of which end as myometrial arteries, supplying non-pregnant uterine segments, others transforming into uteroplacental arteries and supplying the implantation sites.
As a specialty of caviomorphs, pregnancy-induced dilatation of uteroplacental arteries extends far beyond the uterine wall deeply into the mesometrium (see. Figs 13 and 14). As will be described in detail in section 9, this process is induced by trophoblast invasion. And when the response to prostaglandin and other mediators of different arterial beds was studied, it was found that the vessels less infiltrated by trophoblast were more responsive (Clausen et al., 2003).
Upon passing the uterine wall, 3 to 4 uteroplacental arteries give off smaller branches to the subplacenta and then enter the main placenta. For the last centimeter before reaching the main placenta already, considerable parts of the arterial wall are infiltrated and partly replaced by invasive trophoblast. Upon reaching the interlobium of the main placenta, the remaining maternal vascular tissue elements are replaced by trophoblast and the arteries are transformed into rigid mere trophoblastic tubes. These pass the interlobium and enter the labyrinthine lobes, where they finally branch into numerous arteriolar blood lacunae that are located in the centers of the labyrinthine lobes. Here, the latter give rise to capillary lacunae which centrifugally pass the labyrinth and supply it with maternal blood (see section 5).
Drainage of venous blood starts at the surface of the labyrinthine lobes. Here, the maternal capillary blood lacunae of the interlobium collect the venous blood in a web-like channel system which embeds all labyrinthine lobes. The venous blood of the interlobium flowing back maternally is collected in a large venous ring that surrounds the transition of the main placenta to the subplacenta (Fig. 12 b, c). This venous ring is partially embedded into the interlobium (where it has a trophoblastic wall) and partially surrounded by endometrial tissues, where its wall is composed of endothelium and medial smooth muscle cells.
This basal venous ring is drained by 3 to 4 uteroplacental veins (Fig. 12 a). These accompany the respective uteroplacental arteries. Additional drainage comes via mesometrial veins and an arcade vein that are finally connected to the ovarian and uterine veins.
The endometrium shows moderate decidualization where it is invaded by trophoblast. In a detailed electronmicroscopic study of decidua during the entire course of pregnancy, Wynn (1965) found remarkable metabolic activity and large numbers of microvilli.
Guinea pigs have 64 chromosomes (Hsu & Benirschke, 1968). Short arm deletions have been reported in the past. Hybrids of domestic guinea pigs with C. aperea, C. cutleri and C. fulgida are reportedly fertile although they are not commonly produced (Gray1972).
Consideration of the extensive studies on immune phenomena in guinea pigs is beyond the scope of this chapter.
16) Physiological Data
Maternal blood flow rates (Bjellin et al., 1975).
(1966) studied the details of transplacental exchange of hexoses and polyols
in this species. She found transfer of fructose as being slower than that
of other hexoses while polyols are exchanged at nearly same rates. Garris
(1984) showed convincingly that fetal growth corresponded to utero-placental
blood flow. DNA synthesis of uteroplacental arteries is diminished at high
altitude according to work by Rockwell et al. (2000), findings that may
relate to growth restriction of fetuses.
17) Other Resources
When one attempts to time the mating of guinea pigs, this results in rather low pregnancy rates. Pregnancy yield can be much improved by keeping the guinea pigs in small, permanently polygamous breeding groups (1 male per 4 to 6 females). Under these conditions, 75% of females conceive within 24 hours following their last delivery, i.e. within the so-called post-partum estrus. The term of conception is then, cum grano salis, identical to the first day after delivery.
When one breeds in this fashion, it becomes important to know whether a sow became pregnant, and how the new pregnancy develops since abortions and periods of halted or retarded fetal development are quite common, as in deer.
A simple method of palpation allows identification of pregnancy from day 10 to 15 onwards, and allows accuracy of timing within +/- 3 days. For the examination of the left uterine horn, the female is immobilized by the right hand as illustrated in Figure 17a. The thumb of the left hand is pressed again the mother's spine, the four other finger tips slightly press the abdominal wall against the thumb. When drawing the hand laterally, the uterus should be felt between thumb and finger tips sooner or later. In the case of pregnancy, swellings of the uterine horn can be detected. The same is repeated with the right hand for the right uterine horn. Depending on size and quality of the palpational findings, at least six pregnancy stages can be identified (Figure 17 b):
Stage 1 (days 10 to 15). Firm, pea-sized swellings from about 5 to 6 mm diameter.
Stage 2 (days 15 to 25). Firm, hazel nut-sized swelling, about 10 to 15 mm in diameter.
Stage 3 (days 25 to 35). Elastic, slightly oval bodies, 15 to 30 mm in diameter.
Stage 4 (days 35 to 45): Cylinder-shaped, heterogeneous bodies, 3.5 to 5 cm long, containing one to two solid parts (head and pelvis).
Stage 5 (days 45 to 55): Length of the cylindrical fetus 5 to 7 cm. Head, thorax with few ribs and pelvis can be identified.
Stage 6 (days 55 to term): Length of the fetus 7 to 10 cm. The hard, knobby head is about 2.5 cm long, orbits and mandible can be identified by palpation.
This method allows a sufficiently safe estimation of pregnancy age of +/- 5 days and, with some experience, accuracy of +/-2 to 3 days can be reached.
Time of approaching term can be identified by palpating the symphysis. The pubic bones start to separate 3 to 4 days before term and stand apart 1 to 2 cm at term.
Other Remarks - What additional information is needed?
The guinea pig placenta has been studied and the data have been interpreted mainly in the 1960s and 1970s. In that period, not too much was known about the biology of the human trophoblast (such as turnover of villous trophoblast, trophoblast apoptosis, and trophoblast invasion) as well as angiogenesis. Re-evaluation of certain aspects of the biology of the guinea pig placenta in the light of these new insights into human placental biology may therefore very likely further or modify our understanding of this organ.
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