EL TIEMPO DE DEDICACIÓN DE LAS MUJERES AL TRABAJO

The placenta also produces cytokines which are growth factors that regulate movement within and between cells. They have been suggested to play a role in the development of the embryo and fetus (Robinson et al., 1995).

Chapter 1 INTRODUCTION

1.4 PLACENTAL GROV\n*H RATE

The normal placenta grows and gains in weight throughout gestation, though the cellular events occurring during this growth and the rate of growth are not uniform throughout. Over the course of pregnancy in the rat (term 21 days) its placenta shows three phases of growth, hyperplasia, hyperplasia and hypertrophy, and hypertrophy alone. Placental growth in the human (term about 40 weeks - 280 days) has been shown to be similar. Between 25 and 36 weeks gestation it has been demonstrated that hyperplasia is occurring, then for the last month of pregnancy, after about 36 weeks, placental growth is by hypertrophy alone (Winick et a i, 1967). The most rapid period of placental growth is during the first half of pregnancy (Fig. 1.10).

Placental growth is not easy to measure, particularly in animals such as the sheep, which has a cotyledonary placenta. Usually placental size is determined either at delivery, in humans, or at post-mortem, in experimental animals, by weighing. Placental growth is assessed by killing animals at different gestational ages. However, measurements of placental volume have been made during mid-pregnancy in the human using ultrasound (Howe et a i, 1994). In the sheep measurements of placentome diameter have been made between 45 d and 141 d G A using ultrasound (Kelly et a i, 1987). But, the technique can only give mean placentome diameter and it does need further validation.

Chapter 1 INTRODUCTION

4000

3000

|) 2000

Î

1000

0.5

1

0

Figure 1.10. Ontogeny o f fetal (o) and placental ( • ) growth. Adapted from Page (1993) - human, and Owens (1995) - sheep.

1.5 FETAL GROWTH RATE

Conversely to the placenta, the period of maximal fetal growth is during the second half of pregnancy, particularly late gestation (Owens et a/., 1995). (Fig. 1.10)

Technological limitations mean that it is difficult to measure fetal growth in early gestation, due to the small size of the fetus. In experimental animals fetal growth has been measured by killing animals at different gestational ages. The limitation of this method is that longitudinal measurements of growth within individual animals are obviously not possible. Mellor and Matheson (1979) developed a technique for measuring fetal growth during late gestation using what they called a CRL measuring device. This consisted of a nylon monofilament threaded through a sleeve of transparent polyethylene tubing which was sealed at one end. The open end of the device was attached at the rump and the monofilament was tunnelled sub-cutaneously along the spine of the fetus to the crown where it was secured, ensuring that the nylon thread

Chapter 1 INTRODUCTION

was fully inserted into the tubing, which was then exteriorised through the ewe's flank. Growth of the fetus caused the nylon to be drawn out of the polyethylene tube. Measurement of the distance between the end of the sealed tube and the nylon added to CRL measured at surgery gave the CRL on any given day. Using this device curves of growth in the fetal sheep between 100 and 140 days gestation were obtained. This technique has also been implemented more recently to look at the effect of various factors (e.g. cortisol) on fetal growth during late gestation (Fowden, 1995). Longitudinal measurements of fetal growth are also available for the human fetus, growth being assessed by serial ultrasound measurements (Robson & Chang, 1995). It is possible to measure CRL by ultrasound in the human from 6 weeks gestation. Recently Anjari et al. (1996) have successfully measured growth of alpaca fetuses from as early as 33 d G A (term=345 d) using ultrasound.

1.6 DETERMINANTS OF FETAL GROWTH

N utrient supply, endocrine status, genetic m ake-up, and various environmental factors all influence fetal growth. It is difficult to quantify the extent to which these different factors determine the growth of the fetus, however it is possible to indicate their relative importances. For example, without oxygen the fetus would die, therefore oxygen is essential for growth. However, adrenalectomised fetuses are able to survive (Fowden et a l, 1990), therefore cortisol is not essential for growth and survival of the fetus. Likewise, thyroidectomised fetuses survive but are growth retarded (Fowden & Silver, 1995), which suggests that the thyroid hormones are necessary for normal growth, but are not essential.

Chapter 1 INTRODUCTION

Requirements for Importance

fetal growth Oxygen TTT Glucose TTT Lactate TTT Fructose T Amino acids TTT Lipids TTT Insulin TTT Thyroid hormones TT Cortisol T Growth hormone T IGFs TTT

Other growth factors TT

Genetic T

Maternal size T

Table 1.2. Relative importance of the various factors that determine fetal growth.

1.6.1 Nutrition

Nutrient supply to the fetus is the major regulator of fetal growth. There are three different sources from which the fetus may receive nutrients: the maternal circulation, substances synthesised by the placenta, endogenous production by the fetal tissues. Under normal circumstances the maternal pool is the primary source of nutrients for the fetus (Fowden, 1995). The main nutrients required for fetal growth are oxygen, glucose, lactate, amino acids and lipids.

1.6.1J Oxygen

Pa0 2 values in the mother are higher (aorta: 95 mmHg) than those of the fetus (umbilical vein: 35 mmHg), which facilitates diffusion of oxygen from

Chapter 1 INTRODUCTION

mother to fetus. The rate of oxygen uptake by the placenta is about 50% greater, per gram of tissue, than that of the fetus, which significantly reduces the amount of oxygen that eventually reaches the fetus (Fowden, 1994 &

1995). Oxygen affinity is higher in the fetal than in the maternal blood and tissue perfusion is relatively high, so adequate supply of oxygen to the fetal tissues is ensured. Oxygen consumption by the fetal carcass (skin, bone, muscle) is highest, about 50% of total oxygen consumption, and the heart, brain and liver together make up 35-40% of total oxygen consumption, the heart consuming almost double that of the other two organs on a weight- specific basis (Fowden, 1994).

1.6.1.Ü Carbohydrates

Glucose and lactate are the major carbohydrates metabolised by the fetus, though some species (ruminants, pig, horse) also utilise fructose (Fowden,

1994).

Glucose

Glucose is the main substrate for oxidation in utero. Maternal and fetal blood glucose concentrations and the ratio between the two vary from species to species. Typical values measured in the sheep are 2.25-3 mmol/1 in the mother and 0.75-1.25 mmol/1 in the fetus. The maternal to fetal concentration gradient means that facilitated diffusion of glucose across the placenta occurs. As with oxygen, a large amount o f the glucose leaving the maternal circulation is consumed by the placenta, 60-75% of the total uterine glucose uptake in late gestation (Fowden, 1995). The glucose taken up by the placenta may either be oxidised or may be used for lipogenesis, glycogenesis and conversion to lactate, fructose and amino acids, some of which may be released into the umbilical circulation. The fetus can also produce glucose endogenously by glucogenesis in the liver and kidneys, though this usually only occurs in adverse conditions. The carcass utilises most of the glucose supplied to the fetus, though the brain also consumes fairly large amounts (15%). (Fowden, 1994 & 1995).

Lactate

The second most important carbohydrate fuel in the fetus is lactate. It is produced in large quantities by the placenta as a result of the breakdown of glucose (both maternal and fetal in origin) and fructose (fetal), and is released

Chapter 1 INTRODUCTION

into both maternal and fetal circulations. Its production is affected by fetal, but not placental, metabolism. There is also endogenous production of lactate by the fetal tissues at almost double the rate of that produced by the placenta. Most endogenous lactate is derived from glucose, though some may be formed from fructose and amino acids. 70% of the lactate consumed by the fetus is oxidised to CO2 and the rest contributes to the fetal carbon pool. A large proportion (30%) of the lactate taken up by the fetal organs is utilised by the heart and liver. Although some lactate is produced anaerobically, generally it does not imply anaerobic metabolism in the fetus. In stressful conditions, lactate may be a substrate for glucogenesis by the liver (Fowden, 1994 &

1995).

Fructose

Fructose is not an important carbohydrate for metabolism in the fetus. It is produced at a very low rate from glucose in the placenta of the sheep. Only very small amounts are oxidised and it may be converted to lactate. (Fowden,

1994).

1.6.1.in Amino acids

In the fetus amino acids are essential for oxidation, protein accretion, and as a source of carbon and nitrogen. Concentrations are higher in the fetal than in the maternal circulation and higher in the placenta than in either, therefore transport is an active process. The essential amino acids must come from the mother, whilst non-essential amino acids may be synthesised either by the placenta or endogenously by the fetus. Therefore, the supply of some amino acids to the fetus is dependent on placental metabolism as well as tranplacental transport (Battaglia, 1992; Fowden, 1994 & 1995).

I.S .I.iv Lipids

Lipids are essential for fetal growth and they may play a minor role in oxidative metabolism. The source of free fatty acids may be maternal, de novo

synthesis by the feto-placental tissues, or the breakdown of triglycerides and phospholipids. It is thought that the triglycerides are produced endogenously. (Fowden, 1994 & 1995).

Chapter 1 INTRODUCTION

1.6.2 Endocrine and paracrine control of fetal growth

Fetal growth involves not only supply of substrates to the fetus, but is a complex interaction between nutrition and various endocrine and paracrine regulators.

Growth during childhood is known to be dependent on a wide variety of hormones, in particular GH and the IGFs, thyroid hormone, insulin, glucocorticoids, and the sex steroids.. However, the endocrine regulation of growth in the fetus is much less clearly defined. Maternal hormones that are involved in the regulation of growth, e.g. GH, thyroid hormones, insulin, and steroid hormones do not cross the placenta in concentrations that are enough to influence fetal growth. Therefore, the hormones regulating fetal growth seem to be contained within the fetus itself. (Fowden, 1995; Han & Hill, 1994; Han & Fowden, 1994).

1.6.2J Insulin

Insulin is essential for fetal growth, and is first detected at 11 weeks GA in the human fetus, 42 d GA in the sheep, and 18 d GA in the rat (Parkes, 1988). The action of insulin in allowing the cellular uptake of glucose and amino acids in part explains the role of insulin in fetal growth. However, it is also thought to modulate growth by its actions on the IGFs (the IGFs and fetal growth are discussed in section 1.6.2.iii). It enhances IGF-I secretion and at high concentrations can bind to IG F-IR , as well has having an inhibitory effect on IGFBP-1 production. Thus, it is likely that insulin does not have a direct effect on fetal growth, but regulates other factors that do. (Han & Fowden, 1994; Han & Hill, 1994; Bassett, 1995; M ilner & Gluckman, 1996).

1.6.2.Ü Thyroid hormones

The hypothalamic-pituitary-thyroid axis is active from 10 weeks GA in the human fetus (Milner & Gluckman, 1996), 50 d G A in the sheep and 19 d GA in the rat (Parkes, 1988). Triiodothyronine (T3), the active form of thyroid hormone, promotes fetal growth by increasing protein synthesis, and by its synergistic action with GHRH in the secretion of GH and the IGFs. It has been suggested that increased thyroxine (T4) levels may cause an increase in IGF-1 and asymmetrical growth retardation is observed in hypothyroid sheep (Han & Fowden, 1994). Fetal thyroid function is significantly affected by

Chapter 1 INTRODUCTION

nutrient availability. Reduced substrate supply, e.g. due to maternal starvation, uterine artery occlusion, etc., leads to a decrease in plasma T4 concentrations and lUGR (Symonds, 1995).

Cortisol

Cortisol concentrations are low (about 10 ng ml ') for most of gestation in fetal sheep, then start to rise gradually 10-15 days before birth (up to about 20 ng ml '), with a final surge 3-5 days before birth (up to about 70 ng ml ') (Fowden, 1995). Cortisol is maternal and placental in origin, as well as from the fetal adrenal. Cortisol is thought to be a modulator of fetal growth because there is a decline in fetal growth concomitant with the rise in cortisol towards the end of gestation, and adrenalectomised fetuses are heavier than intact fetuses. However, the mechanisms by which cortisol may affect growth are not known. Postnatally, cortisol has an antagonistic effect on insulin and down-regulates IGF gene expression.

Cortisol is known to be involved in the maturation of various organs perinatally. In the lung it is involved in the manufacture of pulmonary surfactant (Han & Fowden, 1995; Milner & Gluckman, 1996). In the gut it is important for villi proliferation and the induction of digestive enzymes (Fowden, 1995). In the liver it is involved in the maturation of glycogen synthetase which is needed for the accumulation of hepatic glycogen during the last trimester, and it also induces p receptors and hepatic gluconeogenic activities, which are important for the supply of glucose to the neonate after birth (Fowden et a l , 1990; Fowden et al., 1992; Fowden et at., 1995; Fowden, 1995). These effects on tissue maturation may in part be due to the effects of cortisol on IGF production. In the ovine fetus, liver and adrenal gene expression of IGF-II is depressed, expression of IGF-I is enhanced in the liver, and hepatic GH receptor mRNA is increased (Fowden 1995).