Elsevier

Livestock Science

Volume 133, Issues 1–3, September 2010, Pages 10-19
Livestock Science

Classical and post-genomic methods to study GIT function with emphasis on the pig

https://doi.org/10.1016/j.livsci.2010.06.014Get rights and content

Abstract

The main roles of the gastrointestinal tract (GIT) comprise the transient storage of discontinuously consumed food, the transport of chyme through the alimentary canal, the digestion and absorption of nutrients, and barrier functions to prevent pathogenic microorganisms, toxins and allergenic macromolecules to enter the interior space of the body. As such, intestinal function directly or indirectly affects animal health. During the course of the developmental genetic program and as a response to a number of challenging events during the life cycle the GIT undergoes a maturational program including cellular, morphologic, digestive, absorptive, metabolic and immunological changes. It is important to recognize that enteral feed intake in itself is a major factor influencing the function and health of the GIT. This review provides an overview on classical and more recent methods applied to investigate GIT structure and functions including i) intestinal and mucosal mass and functional morphology, ii) digestive function, nutrient absorption and absorptive capacity, iii) intestinal metabolism, and iv) barrier function. The methods described focus on in vivo tests of GIT function supplemented by examples from the experimental literature. Due to the similarities between human and pigs GIT physiology and maturation, many of these methods have been applied in pigs as model animals for the examination of GIT development, function and disease in humans. Especially the neonatal and infant pig is increasingly used as a model for human neonates and infants.

Introduction

The gastrointestinal tract (GIT) forms the contact surface between the environment and the interior of the body and acts as a dynamic interface allowing exchange of molecules but also serves as an internal barrier. Among the main roles of the GIT are the transient storage of discontinuously consumed food, the transport of chyme through the alimentary canal, and the digestion of food. During digestion food derived macromolecules are disintegrated which coincides with their absorption thereby transferring water and nutrients to the circulation where it is further utilized in adjacent tissues. Starting already in the terminal small intestine (SI) unabsorbed residues and endogenous secretions serve as substrate for the indigenous microbiota entering the intestine with the diet. In addition, the GIT has barrier functions to prevent pathogenic microorganisms, toxins and allergenic macromolecules to enter the interior space of the body. As such, intestinal function directly or indirectly affects animal health.

The timely and correct development of the GIT is a prerequisite for its proper function and adaptability to a number of challenging events during the life cycle. During transition from the fetal to the postnatal period, nutrient supply changes from the parenteral route via the umbilical cord and some swallowing of amniotic fluid to enteral nutrition with colostrum containing compounds new to the individual (e.g. fat, lactose, immunoglobulins) and protective and growth-stimulating factors (e.g. IGF-1, epidermal growth factor) (Pluske et al., 1997, Trahair and Sangild, 1997, Sangild, 2003).

The second transition refers to the change from suckling the dam, i.e. the ingestion of liquid feed, to the uptake of dry feed associated with the abrupt withdrawal of beneficial milk constituents, and a major change in the complexity of dietary carbohydrates. These alterations are accompanied by the exposure to a novel pattern of microbes or the encounter with enterotoxigenic microbes, and the social stress associated with the separation from the sow and the mixing with unfamiliar piglets (Lalles et al., 2007). It is important to recognize that enteral feed intake (i.e. luminal stimulation) in itself is a major factor influencing the function and health of the GIT (e.g. McCracken et al., 1999, Burrin et al., 2000a, Lalles et al., 2007).

During the course of the developmental genetic program and as a response to the above mentioned changes the GIT undergoes a maturational program including cellular, morphologic, digestive, absorptive, metabolic and immunological changes (Trahair and Sangild, 1997, Pluske et al., 1997). The intestinal epithelium is continuously and rapidly renewed from the stem cell population located at the bottom of the crypt to the extrusion of the terminally differentiated cells at the tip of the villus. As a consequence intestinal cells have a short life span of between 2 and 10 d (Radecki et al., 1992, Fan et al., 2001). This enables it to react quickly to changing substrate types and concentrations as well as pathogens, toxins and allergenes.

Related to the above described roles of the GIT this contribution provides an overview on classical and more recent methods applied to investigate GIT structure and functions including i) intestinal and mucosal mass and functional morphology, ii) digestive function, nutrient absorption and absorptive capacity, iii) intestinal metabolism, and iv) barrier function. The description of the principles of the methods is supplemented by examples from the experimental literature. This review is focused on in vivo functional tests of GIT function, because in vitro methods do not always describe functions in vivo completely. Due to the similarities between human and pigs GIT physiology and maturation, many of these methods have been applied in pigs as model animals for the examination of GIT development, function and disease in humans (Reisenauer et al., 1989, Sangild et al., 2002, Burrin et al., 2003, Chang et al., 2006). Especially the neonatal and infant pig is increasingly used as a model for human neonates and infants.

Section snippets

Intestinal and mucosal mass and functional morphology

One of the simplest methods to obtain information on gut development as influenced by age or diet is to measure intestinal, mucosal mass or mucosal proportion. These variables provide an approximate measure of potential digestive and absorptive mass. Also intestinal area over a specific section of the GIT or total intestinal mucosa mass can be calculated when length and circumference are obtained (Sangild et al., 2002, Adeola and King, 2006). Many researchers have shown the importance of

Digestive function, nutrient absorption and absorptive capacity

In vivo measurements of digestion or digestibility belong to the classical repertoire of animal nutrition and are important for feed ingredients evaluation and pig ration formulation (Fuller, 1991, Fuller, 2003). These methods are principally based on the disappearance of nutrients in the GIT which implies the involvement of not only digestive but also absorptive processes. The most important example is amino acid (AA) digestibility which reflects enzymatic hydrolysis, the contribution of

Intestinal metabolism

The gut is a metabolically active organ supplied by nutrients from the luminal as well as from the basolateral side and responsible for 17–25% of whole-body oxygen consumption (Cant et al., 1996). To fulfill its various roles the SI takes up a share of the nutrients provided enterally to generate ATP used for the many synthetic processes (e.g. mucin, digestive, absorptive, and transporter proteins, cell turnover). Stable isotope tracer methodologies provide powerful tools to dissect the various

Barrier function

Propulsion of food, chyme and fluid through the alimentary canal limits opportunities of opportunistic pathogens for colonization intestinal sections and bacterial translocation, decrease the contact time between toxic or cancerogenic compounds in the luminal contents (e.g. products of microbial proteolytic processes) and the epithelial surface, and thus can be considered a component of barrier function. In that way GIT infections caused by ingested pathogens leading to diarrhea can be reduced.

Conflict of Interest

I declare that there is no actual or potential conflict of interest including any financial, personal or other relationships with other people or organizations within 3 years of beginning the submitted work that could inappropriately influence, or be perceived to influence.

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