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Calf health and gastrointestinal development

Milena Saqui-Salces Published on 24 November 2014

We constantly hear that a healthy gut is the key for proper animal development, but what does gut health mean? It means that all the cells and layers that form the gastrointestinal (GI) tract maintain their integrity and composition; thus, the function of the gut is optimal.

The GI (gut) has three main functions: motility, nutrient absorption and endocrine secretion. These functions are regulated by luminal stimuli (diet, drugs, commensal and pathogenic biota, etc.) and the animal’s immune and nervous system response to those luminal stimuli and the general environment. Gut health then depends on the development, health and environment of the animal.

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The main player on gut health is the epithelium, the layer of cells that lines the intestine and is in contact with all that enters the lumen of the GI tract. From the stomach to the rectum, the epithelium of the GI is formed of a single layer of cells that looks like standing bricks tightly bound to each other.

Each fragment of the GI tract has a specific organization and cell type composition that defines its function. In general, the stomach is organized in tubular structures of secretory cells called glands.

The stomach epithelium has two different parts: the oxyntic mucosa of the fundus and corpus that is in charge of producing acid, the hormone ghrelin, bicarbonate, intrinsic factor, pepsinogen and mucous, among other products; and gastric antrum that produces the hormone gastrin and mucous.

The small intestine has three parts: duodenum, jejunum and ileum; followed by the cecum and large intestine or colon. The epithelium of the small intestine is arranged in invaginations called crypts and finger-like structures called villi.

The crypts contain the stem cells and Paneth cells. The villi have functional (differentiated) cells: enterocytes, goblet, endocrine, brush and tuft cells. The length and width of the villi change along the small intestine to maximize the area for nutrient absorption.

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Enterocytes absorb nutrients (carbohydrates, proteins and lipids), Paneth cells produce lysozyme and antimicrobial agents, goblet cells secrete mucous, and endocrine cells produce a large number of different hormones, making the gut the largest endocrine system in the body.

The colon is formed of crypts with mucous cells and colonocytes. Most nutrient absorption occurs in the small intestine; the colon absorbs water, salts, some vitamins and minerals.

One of the most important considerations for gut health is that the GI tract is not fully developed at the time of birth. The muscular and connective parts of the GI tract are functional at birth. The epithelium, on the other hand, is immature and requires luminal stimuli (i.e., the presence of food) to complete development.

In most animals, the intestinal villi and most cell types are present at the time of birth, but endocrine and Paneth cells do not reach functional maturity until around 16 days after birth. At birth, the stomach epithelium is not developed. Gastric glands are formed only after birth, and functional cell lineages appear during the first postnatal weeks. The stomach achieves full maturity only around day 21.

The development of the GI has numerous implications on gut health. The obvious one is the incapacity of young animals to digest complex food, a fact well known by animal producers. During the first few weeks of life, the stomach does not have sufficient acid-secretion ability.

This compromises not only digestion but also the immune response by allowing the survival of bacteria that otherwise would be killed by the acidic gastric environment.

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In addition, at this stage the Paneth cells are just initiating the production of antimicrobial products, further limiting the innate immune response of the newborn. Young animals exposed to pathogens or non-commensal bacteria are thus more susceptible to infection and inflammation that may not show clinical signs but may compromise gut integrity and function.

The neonatal stage is critical for the establishment of adequate gut microbiota and the lifelong health of the animal.

The animal environment in utero and during early life determines how the GI develops and sets the programming of the newborn for growth, health, reproduction, fat deposition and metabolic control. The hypothesis behind this is that epigenetic adaptions occur at embryonic stages to warrant the survival and development of the product in the conditions cued by the intrauterine environment.

When there is a significant difference between the offspring and the mother’s environments, the adaptations programmed in utero may be disadvantageous for the product. This may happen when the young are kept in environmental conditions different from the mother’s.

In addition to the maternal nutritional status and an adequate transition of the offspring to adult feed, the microbiota that populates the intestine is defined by the sanitary conditions of the animal in early life.

Studies have shown that immature and juvenile individuals exposed to unsanitary conditions, i.e., contamination with feces, develop impaired intestinal barrier function, endotoxemia and chronic inflammation. Although hygiene is not directly a nutritional approach to health, it may be the difference between healthy and poor-performing animals, independent of the diet fed to them.

Diet composition and some nutrients in particular are associated with changes in the cell populations and function of the intestine. Glutamine is a key amino acid that participates in protein synthesis in the enterocyte and the regulation of tight-junction proteins that help maintain the integrity of the intestinal epithelium.

Some supplements, like sodium butyrate, are important in the developmental stages but not in the adult. Butyrate is produced by bacteria in the intestine once the gut microbiota is established.

In the stomach, butyrate promotes the development of gastric glands, acid-secreting parietal cells and endocrine cells. Once the stomach is mature, butyrate is negligible for adult epithelial differentiation and supplementation after the first two weeks post-weaning has no effect on gain-to-feed ratio.

It is common to find reports of changes in intestinal villus height and crypt depth in response to diet modification. The most common observation is the increase of goblet cells in animals fed high-fiber diets. High-fiber diets are associated with increased fecal discharges, decreased energy density and masking the absorption of other nutrients as well as elevated intestinal mass due to engrossment of the epithelium.

Feeding high-fat diets, on the other hand, seems to affect the differentiation and function of GI endocrine cells and increase the number of enterocytes.

These effects of diets are important because the expansion of one intestinal cell type occurs only at the expense of other cell types: An increase in goblet cells probably means a reduction in enterocytes and thus a reduction in the absorptive capacity and the need to compensate with longer villi and heavier guts.

Any treatment or diet that results in a loss of cell types has the consequent alteration in function associated to the cell types affected.

In summary, the establishment of favorable gut microbiota and the maintenance of the integrity and cell types of the GI tract are the main conditions for gut health. To optimize gut health and performance, take into consideration the animal’s developmental needs, diet and environmental conditions. PD

Milena Saqui-Salcesis an assistant professor in theDepartment of Animal Science with theUniversity of Minnesota. Email Milena Saqui-Salces.

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