Growth and Development Symposium: Intestinal Development and Growth

The gastrointestinal tract (GIT) in chickens, turkeys and ducks is a dynamic organ system. During early posthatch growth, there is a tremendous energetic allocation to GIT growth. While most research has focused on ontogenic changes to nutrient transporter and/or enzyme production, the structural aspects of intestinal maturation often is not studied in tandem, which may be more limiting to growth. Similarly, researchers often use simple morphometric measures of intestinal structure (villus height, crypt depth, goblet cell numbers). However, these crude measures may not be as reflective of enterocyte proliferation, migration, apoptosis, and necrosis. Remarkably, the turnover of the bird′s intestine ranges from less than 2 to over 5 d, but our knowledge of what factors affect cell cycle and functionality are largely lacking in poultry. Recent work has noted that appearance of indigestibility by the young bird is due in part to changes in the pH of the proventriculus and endogenous secretion differences. For example, 60 to 80% of the apparent digestibility differences in amino acid utilization during the first week and later in life are due to differences in basal endogenous amino acid losses. Recent research has also focused on the maintenance needs of the intestine, particularly as it relates to preserving its barrier functionality with or without sub-therapeutic antibiotics. The GIT barrier is far from being static and responds to many challenges through changes to: a) peristalsis, b) enterocyte turnover, c) mucin production, adaptation of commensal microflora, d) innate immune responsiveness (including inflammation and acute phase response), and/or e) alterations to secretions. The extent and duration of each of these responses encompasses the nutrient and growth cost for maintenance of this barrier function. While most antibiotic replacement strategies are not capable of similar physiological and microbiological responses, some probiotic bacteria and plant extracts are able to modify mucin production, preserve intestinal tight junctions and enterocyte cell cycle, as well as alter inflammatory responses within the intestine.