**7. Diabetic pregnancy**

Diabetes in pregnancy is divided in two groups, pre-gestational diabetes mellitus (PGDM) and gestational diabetes mellitus (GDM). In PGDM, the pregnant woman suffers from diabetes acquired prior to onset of pregnancy. PGDM is subdivided in type 1 (insulin deficiency) or type 2 (insulin resistance). Type 1 diabetes (DM1) is caused by an autoimmune reaction against the insulin producing pancreatic β cells [52]. DM1 is often diagnosed in early childhood, and DM1 patients will require exogenous insulin. Type 2 diabetes (DM2) is the most common, less severe type of diabetes. In DM2, the skeletal muscle and adipose tissue are insensitive to insulin, and the β cells fail to compensate.

In normal pregnancy, maternal tissues become progressively insensitive to insulin. This effect is likely caused by hormones from the placenta. In order to maintain a euglycemic state, the woman must increase her insulin secretion by 200–250%. About 3–10% of the pregnant population is unable to produce an adequate insulin response to compensate this insulin resistance and they develop GDM. The choice of animal model should depend on the type of diabetic pregnancy that the research aim to study. Diabetes can be induced pre-gestationally or gestationally as either an insulin-resistant or insulin-deficient model and the following methods can be used to induce PGDM or GDM in animals: (1) surgical induced (partial pancreatectomy), (2) chemical induced (streptozotocin or alloxan), (3) diet induced and (4) genetic models.

#### **7.1. Surgical-induced diabetic pregnancy by partial pancreatectomy**

Partial pancreatectomy is provided by removal of up to 95% of pancreas prior to mating, leading to onset of PGDM with concomitant insulin deficiency [53]. This model was introduced in female rats in 1970 [54], but the model is hampered by several factors: surgical complexity, high post-surgical mortality, a long time between surgery and development of diabetes (2–3 months) and sequelae like digestive problems from the missing exocrine pancreas. However, partial pancreatectomy in sheep fetuses in late gestation has been used to study fetal insulin and glucose metabolism *in utero* [55].

#### **7.2. Chemical-induced diabetic pregnancy**

A widely used method for induction of experimental diabetes is chemical destruction of pancreatic β cells, resulting in insulin deficiency. This approach resembles a DM1 model, but it has been used to mimic GDM. Streptozotocin and alloxan are the most used drugs, especially in rats and mice. The amount of time required to induce diabetes and the phenotype (mild to server diabetes) depend on factors such as animal species, strain, dose and mode of administration (sc, iv, ip or im) [56]. In rats, streptozotocin has shown to cause ovarian dysfunction [57], and untreated diabetes generally results in subfertility. For these reasons, streptozotocin is often administered on the day of mating in order not to interfere with a successful mating and where the risks of direct toxic effects on the embryo are little [56].

#### **7.3. Diet-induced diabetic pregnancy**

There are several other ways to induce preeclampsia, including adriamycin-induced, chatechol-O-methyltransferase-deficient and BPH/5 mice strain. However, these methods have

Diabetes in pregnancy is divided in two groups, pre-gestational diabetes mellitus (PGDM) and gestational diabetes mellitus (GDM). In PGDM, the pregnant woman suffers from diabetes acquired prior to onset of pregnancy. PGDM is subdivided in type 1 (insulin deficiency) or type 2 (insulin resistance). Type 1 diabetes (DM1) is caused by an autoimmune reaction against the insulin producing pancreatic β cells [52]. DM1 is often diagnosed in early childhood, and DM1 patients will require exogenous insulin. Type 2 diabetes (DM2) is the most common, less severe type of diabetes. In DM2, the skeletal muscle and adipose tissue are

In normal pregnancy, maternal tissues become progressively insensitive to insulin. This effect is likely caused by hormones from the placenta. In order to maintain a euglycemic state, the woman must increase her insulin secretion by 200–250%. About 3–10% of the pregnant population is unable to produce an adequate insulin response to compensate this insulin resistance and they develop GDM. The choice of animal model should depend on the type of diabetic pregnancy that the research aim to study. Diabetes can be induced pre-gestationally or gestationally as either an insulin-resistant or insulin-deficient model and the following methods can be used to induce PGDM or GDM in animals: (1) surgical induced (partial pancreatectomy), (2) chemical induced (streptozotocin or alloxan), (3) diet induced and (4) genetic models.

Partial pancreatectomy is provided by removal of up to 95% of pancreas prior to mating, leading to onset of PGDM with concomitant insulin deficiency [53]. This model was introduced in female rats in 1970 [54], but the model is hampered by several factors: surgical complexity, high post-surgical mortality, a long time between surgery and development of diabetes (2–3 months) and sequelae like digestive problems from the missing exocrine pancreas. However, partial pancreatectomy in sheep fetuses in late gestation has been used to

A widely used method for induction of experimental diabetes is chemical destruction of pancreatic β cells, resulting in insulin deficiency. This approach resembles a DM1 model, but it has been used to mimic GDM. Streptozotocin and alloxan are the most used drugs, especially in rats and mice. The amount of time required to induce diabetes and the phenotype (mild to server diabetes) depend on factors such as animal species, strain, dose and mode of administration (sc, iv, ip or im) [56]. In rats, streptozotocin has shown to cause ovarian dysfunction

only been used in mice and will not be discussed further [51].

356 Experimental Animal Models of Human Diseases - An Effective Therapeutic Strategy

insensitive to insulin, and the β cells fail to compensate.

**7.1. Surgical-induced diabetic pregnancy by partial pancreatectomy**

study fetal insulin and glucose metabolism *in utero* [55].

**7.2. Chemical-induced diabetic pregnancy**

**7. Diabetic pregnancy**

Obesity is a well-known risk factor for DM2 and GDM [58]. Feeding with high-fat diets and/ or high concentrations of sucrose and fructose induces insulin resistance, and this approach is used to create animal models of DM2 and GDM in rats, mice and sheep (**Table 5**) [56] [59]. This method is cheap and accessible, but relatively more time-consuming than chemical induction. Holemans et al. fed female rats with a diabetogenic diet 4 weeks prior to mating and during gestation [59]. They found that diabetes was not present prior to mating, but was confirmed at gestation day 20, resembling a GDM model. Liang et al. used a similar protocol in mice, but diabetes was developed pre-gestational in this study [60]. In sheep, a 60 days of diabetogenic diet before mating resulted in insulin resistance and increased fetal adipose tissue and β cell mass in mid-gestation (gestation day 75) [61]. Another way to study hyperglycemia and hyperinsulinemia and the impact on the fetus is by continuous iv glucose infusion during gestation [62]. However, this method is considered too simple and lacks the complexity of a diabetic pregnancy.

#### **7.4. Genetic models of diabetic pregnancy**

Several genetic mice models of diabetic pregnancy exist. Genetic engineering and inbreeding are unfortunately impossible in several species [56]. The "non-obese diabetic" mice and "bio breeding" rats are inbreed strains spontaneously developing DM1. They are used to study fertility and fetal complications in DM1 diabetic pregnancy [53]. The "db/db" mouse is a classic DM2 model with a mutation in the leptin receptor gene (ObR) resulting in excessive appetite and hence obesity [63] [56]. These mice are infertile, but the heterozygote "db/+" mouse are fertile and develops insulin resistance during gestation, and they are therefore providing a model of GDM [64]. Newborns of "db/+" mice show complications related to GDM like


**Table 5.** Diabetic pregnancy models (number refers to reference list).

macrosomia regardless of fetal genotype. An important factor is that the diabetic phenotype of the "db/+" mouse is not present prior to gestation, making this model more transferable to GDM than many other models [56].

With genetic models of diabetic pregnancy, it is important to remember the genetic predisposition to diabetes in the fetus. Embryo transfer can be used to study the influence of maternal diabetes separately from the fetal genotype [65]. Many genes affecting β cell function in pregnancy can be mutated in mice to induce a diabetic phenotype [56].
