By Andrea Mess & Peter Kaufmann
Suborder: Hystricognathi (sensu Tullberg 1899), as a member of South American hystricognaths, the Caviomorpha
1) General Zoological Data
Systematics: The degu belongs to the rodent suborder Hystricognathi. This group was originally defined by Tullberg in 1899, but its validity was later doubted because of the disjunct distribution into a South American group or Caviomorpha on the one hand (including the North American tree porcupines), and the African hystricognaths on the other (with a few species in Europe and Asia). From the mid -1980's on, phylogenetic studies respecting characters of fetal membranes and placentation support the monophyletic hypothesis of Hystricognathi (Luckett, 1985) and have been accepted by former critics of the concept (Wood, 1985). Moreover, Hystricognathi is well supported by molecular data studies (e.g. Nedbal et al., 1994, Huchon et al., 2000), but the relationships within the group are not that well resolved, and data are often not congruent. For instance, there is no morphological support for Caviomorpha (e.g. McKenna & Bell, 1997), although this clade is supported by molecular data sets. The degu is a member of Octodontidae, an ecomorphologically diverse, but monophyletic family (see Honeycutt et al., 2003), which is poorly studied in regard to placentation. Thus, we refer to Hystricognathi when comparing the degu with relatives. Special attention is given to the guinea pig as the best investigated species of the group.
Biological background: Octodon degus is one of the smallest South American hystricognath rodents (Figure 1) and lives in rocky habitat and bush habitat in areas of medium high elevation (up to 1,200 m) native to Chile. Octodon is reported as one of the most common mammals of central Chile with densities of about 40 to 80 individuals per hectare (Nowak, 1999). Regarding body shape and climbing behavior in bushes, trees, and rocks, this agile diurnal species may easily be mistaken for a small squirrel. The tail is not bushy, however, but has a small black brush at the tip (Woods & Boraker, 1975; Nowak, 1999). The degu is grayish to brownish in color, and possesses digits with sharp, curved claws and stiff bristles that extend over the claws. The grinding area of the cheek teeth are shaped like a figure eight, reflecting to the scientific name of the genus (Woods & Boraker, 1975; Nowak, 1999). These teeth are high crowned and ever-growing, forming a powerful tool for manipulating fibrous plant material. The diet includes grass, leaves, bark, herbs, seeds and fruits; the amount of animal material intake is doubtful (Woods & Boraker, 1975; Fulk, 1975, 1976; Nowak, 1999; Veloso & Bozinovic, 2000). The degu lives in small, polygamous colonies with individual territories, characterized by communal burrow systems (Woods & Boraker, 1975; Kleiman et al., 1979; Nowak, 1999). Food and nesting material are accumulated in the burrows. In the wild, Octodon seems to breed once per year, whereas in captive colonies litters occur more frequently. Females of one group may rear their young together in one nest (Weir, 1974; Nowak, 1999). The offspring are born in a relatively advanced developmental condition (Figure 1b), although the exact condition at birth (e.g. if the eyes are open or not) differs in laboratory colonies in different continents (cf. Weir, 1974; Rojas et al., 1982; Nowak, 1999). According to our personal observations, the newborn are kept hidden in their nests for the first one or two weeks of life. Thus, the eyes of such young are usually closed, but could be opened if necessary, e.g. when the surrounding of the nest is disturbed. Lactation is reported to last 2 to 4 weeks, and the medium age of first conception is about 6 months (Weir, 1974; Nowak, 1999). In addition to the references cited, there is a broad range of papers available on Octodon, covering the fields of energetic and nutritional biology, circadian rhythm, thermoregulation, and adaptations in skull morphology and sensory systems. Additional information is available on various degu web sites through Google.com.
General Gestational Data
Estrous cycle: Octodon does not possess a regular estrous cycle, but it depends on male presence to induce ovulation. Postpartum ovulation takes place, but not regularly (Weir, 1974).
Length of gestation: 87 - 93 days (Weir, 1974).
Litter size: 1 - 10, mean 5 (Weir, 1974), under laboratory conditions, according to our data: 1 - 7, mean 4.
Body weight (non pregnant): 170 to 230 g according to our data.
Fetal weight at full term: about 10 g according to our data.
Weight of placenta and membranes at full term: about 8 g (Roberts & Perry, 1974).
3) Implantation and Formation of the Fetal Membranes
Primary and completely interstitial implantation at the antimesometrial pole of the uterus takes place around day 6 ½ to 7 post conception (Roberts & Perry, 1974). This type of implantation is regarded as a defining character of Hystricognathi (Luckett, 1985).
The yolk sac is characterized by very early and complete inversion (Roberts & Perry, 1974; personal observation). As a striking characteristic of hystricognaths (Luckett, 1985), the abembryonic trophoblast degenerates soon after implantation is finalized, coupled with the inversion of the germ layers at this time (Roberts & Perry, 1974).
Formation of the amniotic cavity takes place on day 12 ½ p.c. The amnionic cavity is formed by cavitation as is usual for hystricognaths (Roberts & Perry, 1974).
Formation of the allantois takes place on day 30 p.c. (Roberts & Perry, 1974).
Formation of the chorioallantoic placenta takes place around day 30 to 35 p.c. (Roberts & Perry, 1974).
As is typical for hystricognaths (see Luckett, 1985), the chorion seems to be restricted exclusively to the chorioallantoic placenta (personal observation).
General Characterization of the Chorioallantoic Placenta
General placental type: Discoidal, labyrinthine, hemomonochorial, chorioallantoic placenta with main placental disc and a special subplacenta as well as a separate yolk sac placenta (cf. Figures 2 to 7).
Allantois: Only used for formation and vascularization of the chorioallantoic placenta (main placenta and subplacenta).
Organization of the Placenta and the Fetal/Maternal Barrier
According to investigations of histological serial sections and semi-thin slides (Mess, 2001, 2003; personal observation) as well as the ultrastructural descriptions by King (1992) and Kertschanska, Schroeder, & Kaufmann (1997), the structural organization of the degu's main placenta and the ultrastructure of its barrier mainly correspond to that of the guinea pig. However, some differences exist with regard to the macroscopic organization of the placenta.
As is typical for hystricognaths, the chorioallantoic main placenta (Figures 2 and 3) consists of a labyrinthine zone (including fetal vessels) in the middle region of the placental disc and a more spongy component (without fetal vessels), that is mainly situated at the outer border of the placental disc, but also found in between parts of the labyrinth (Figure 2). Based on the nomenclature introduced for the closely related guinea pig placenta (Kaufmann & Davidoff, 1977), this area is here recognized as interlobium. Other names for this region are for instance "spongy zone", "trophospongium" or "ectoplacenta".
In the degu, the chorioallantoic placenta is only very moderately lobulated, even in late ontogenetic stages. That means that the labyrinth is not composed of finger-like lobes, each lobe representing its own circulatory unit, nearly completely separated by interlobium, as in most other hystricognaths such as the guinea pig. Instead, the separation of the labyrinth by the interlobium is incomplete in the degu. Folds of the interlobium push out medially from a ring-like outer region of the placental disc, but do reach the center of the placenta and do not fully separate parts of the labyrinth (Figure 2). Thus, the labyrinthine parts of the placenta are largely joined together (Figure 3). Additionally, in the area of the central excavation, a layer of interlobium is present that appears to divide the labyrinth into two halves (Figure 2).
Besides the moderate lobulation of the degu placenta, the fine structure and the vascular architecture of the fetomaternal exchange area, the labyrinth, do not differ from other hystricognaths:
• the maternal blood flows from a central arterial lacuna (Figures 2 and 3) centrifugally via a web-like system of trophoblast-lined blood lacunae (Figures 4 and 5) to the periphery of the labyrinth, i.e. the spongy zone or interlobium, where the blood is drained by a web of venous lacunae (Figure 3);
• the fetal blood enters the same region via several fetal arterioles (Figure 3), located at the lobular surface, i.e. at the border between labyrinth and interlobium; there they branch into radially oriented capillaries which lead the fetal blood centripetally; the blood is collected by fetal venules in the center of the labyrinth (see Figure 3) and then returned back to the embryo by the umbilical vein.
As a result of the very moderate lobulation of the degu placenta, the blood flow arrangement does not represent a perfect counter-current exchanger as in the guinea pig. In contrast, the system in the degu is characterized by the fact that the exchange vessels (fetal capillaries and maternal capillary lacunae) do not span similar distances all over the placenta, but differ in length depending on their place in the placenta. Usually, the capillary length is shorter in regions where the interlobium is protruding medially, whereas it is longer in the outer parts of the labyrinthine lobes (see Figure 2). The phylogenetic interpretation suggests that a purely lobulated condition of the placenta represents the ancient condition of hystricognaths (plesiomorphic character state), from which highly lobulated placentas evolved (Mess, 2003).
The fetal/maternal barrier of the degu is hemomonochorial (Figure 6), as in all hystricognaths. The maternal and fetal blood systems are separated by a layer of syncytiotrophoblast which is situated in between the maternal capillary lacunae and the fetal capillaries (Figure 5). The latter possess fetal endothelium as well as a basal lamina towards the syncytiotrophoblastic side (Figure 6). The fetal capillaries are not fenestrated (Figure 6). Intervening connective tissue in the trophoblastic region is rare, and the syncytiotrophoblast is characteristically thin (Figures 5 and 6), as in other hystricognaths (see Kaufmann & Davidoff, 1977; Mess, 2003).
Cytotrophoblast can be found throughout the placenta only in the first weeks of pregnancy. In later stages, cytotrophoblast occurs in the outer region of the interlobium or spongy zone of the main placenta. In addition, the so-called subplacenta (see below) consists mainly of cytotrophoblast, which also continues into late ontogenetic stages.
Fernandez, R.: Karyotype of Octodon degus (Rodentia ochotonidae)(Molina 1782). Arch. Biol. Med. Exp. (Santiago) 5:33-37, 1968. (In Spanish).
Fulk, G.W.: Population ecology of rodents
in the semiarid shrublands of Chile. Occas Papers Mus. Texas Tech. Univ.
T.: Ueber das System der Nagethiere. Eine phylogenetische Studie. Akademische
Buchdruckerei. Upsala, 1899.
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