Alimentos varios y bebidas i) Panificables y pastas

The postnatal period can be divided into the:  

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1.2.2.1 Alveolar Phase

Alveolar Phase: In some mammals alveolarisation starts before birth and proceed postnatally, whereas in humans, alveolarisation begins at 36 weeks of gestation and lasts 1-3 years postnatally. Although a certain degree of maturation of the lung is associated with the appearance of fully mature alveoli at 36 weeks, new alveoli will continue to form for approximately three years after birth. Alveolar formation is closely linked to the deposition of elastin in the saccular lung (Grant et al., 2001). At birth, a newborn infant would have developed approximately 50 million alveoli and has the potential to expand it in number to about 250 million alveoli with an increase in surface area from approximately 3 to 70m2 (Grant et al., 2001, Langston et al., 1984).

In rats and mice, alveoli form entirely after birth (Hislop, 2005). At postnatal day 4, cells of the alveolar walls show high proliferation rates. The inter-airspace walls consist of double capillary layers and a cellular sheet of connective tissue which is the basis for rapid alveolar formation from postnatal days 4 to 13, resulting in an increased capillary volume as well as a larger alveolar-capillary surface area (Bolle et al., 2008). Angiogenesis is promoted by VEGF, whereas the inhibition of VEGF receptor will result in a decrease in alveolar number (Jakkula et al., 2000).

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1.2.2.2 Microvascular Maturation

Microvascular Maturation (human birth – 2-3years/adulthood), where double capillary layers of the immature alveolar septa is reduced to a single capillary layer (Roth-Kleiner et al, 2003). In rats, the finalization o f septal restructuring is complete at around postnatal day 21 (Bolle et al., 2008). The double capillary network that merges into a single layer is an important event in postnatal lung growth as angiogenesis and alveolarization goes hand in hand (Joshi and Kotecha, 2007).

1.2.2.3 Stage of Late Alveolarization

Stage of Late Alveolarization (3-5 years to young adult age in humans): Alveolar formation appears to continue by peripheral (perhaps repetitive) septation at a slow pace (Burri, 2006).

1.3 WINDOWS OF PLASTICITY DURING LUNG DEVELOPMENT

The respiratory system consists of approximately 40 different cell types with multiple functions. Common air pollutants such as airborne particles, oxidant gases, and environmental tobacco smoke exposure affects the fetus and impacts on target cells, which are likely to affect critical signals or mediators expressed during distinct stages of lung development. Consequently, it impacts the pre- and postnatal periods of life in the offspring (Pinkerton and Joad, 2006). Studies have shown that the anti-oxidant capacity of the developing lung is low during most of

Page | 7 its development, but rapidly increases just before birth (Barker, 1995). There is also a natural neonatal susceptibility to environmental pollutants due to direct or indirect hits on a number of cell types to influence cell differentiation, proliferation and/or maturation (Pinkerton and Joad, 2006). This indicates that exposure to foreign substances, in utero such as nicotine, at the critical periods of lung development, can alter the normal “program” that controls lung growth and maturation. This could result in the offspring becoming more prone to diseases such as cardiovascular disease, and type II diabetes mellitus in their adult life, as well as inadequate lung maintenance (Barker, 1995). An example of the effect of a change in the in utero environment is that inadequate nutrient supply in utero leads to metabolic changes in the developing fetus that enhance their chance of postnatal survival in a supposedly deprived environment. These adaptions are generally beneficial only after birth (Somm et al., 2009). However, if the fetus is programmed during pregnancy to survive in an environment of low nutrient supply, but is born into an environment of abundant nutrient supply, they become more prone to adult obesity (Eriksson and Forsen, 2002). Maternal under- nutrition, vascular diseases, drug abuse and placental pathology are of the major causes resulting in reduced oxygen and/or nutrient supply to the developing fetus and the consequent adjustment of metabolism in the offspring. Studies have shown that the degree and type of nutrient restriction, as well as gestational timing, affects the final structure of the fetal lungs. Experiments done in sheep showed that they had fewer, larger alveoli with thicker septa and blood-gas barriers in their adult life when intra-uterine growth restriction (IUGR) was induced. This was due to an inadequate nutrient supply from the mother during

Page | 8 gestation (Maritz and Harding, 2011). It is therefore, plausible that changes in the in utero environment due to maternal smoking or nicotine replacement therapy (NRT) may interfere with the development of the offspring and increase susceptibility to disease.

1.4 THE ROLE OF LYSYL OXIDASE DURING LUNG

DEVELOPMENT

Lysyl Oxidase (LO), for example, functions by initiating the cross linking of collagen and elastin by oxidizing specific peptidyl lysine residues, stabilizing collagen or elastin fibres in the extracellular matrix, thereby playing a central role in lung morphogenesis and tissue repair. A LO reactio n produces hydrogen peroxide which may possibly regulate gene expression and cell behavior. It is also considered a tumor suppressor gene. The molecular structure of LO is synthesized by the fibrogenic cells as a 46 preproenzyme. LO is a metalloenzyme that requires copper II (Cu II) as a cofactor for enzymatic function. Cu (Raunio et al.) binds at its active site: one Cu per molecule of the enzyme. Loss of Cu results in total inactivation of LO. Studies have shown that other divalent metals such as Cd, Fe, Co, Zn and Ni cannot replace Cu as a cofactor for LO (Chen et al., 2005). Therefore, animals that received a diet with reduced Cu showed in induction of lythyritic manifestation due to the reduction of elastin deposition in the lung which resulted from suppression in LO activity (Harris, 1986). Studies have also shown that the catalytic activity of LO may be suppressed by cigarette smoke condensate (CSC) and cadmium (Cd), which are majo r components of cigarette smoke. It has indeed been shown that smoke induced emphysema in chick lungs

Page | 9 was due to Cu deficiency, causing a disruption in lung structure (Li et al., 2011). This suggests that Cu is essential for the functioning of LO and thus normal lung growth, maintenance and aging and that a lack of adequate intake will compromise lung development.

1.5 FACTORS AFFECTING THE DEVELOPING LUNG