Morphology of Starch Particles along the Passage through the Gastrointestinal Tract in Laboratory Mice Fed Extruded and Pelleted Diets

Abstract

Diet processing impacts on starch properties, such as the degree of starch gelatinization. This affects digestibility, as shown in laboratory mice fed either a pelleted or an extruded diet. In the present study, the morphology of starch particles throughout the digestive tract of mice was visualized. Thirty-two female C57BL/6J mice were used for a feeding trial. They were fed a commercial maintenance diet for laboratory mice, which was available in pelleted and extruded form, for seven weeks. The mice were sacrificed after the feeding period, and chyme samples were collected from five sites (stomach, anterior and posterior small intestine, caecum, colon). Samples of diets, chyme and faeces were analyzed via stereomicroscopy (stained with Lugol's iodine) and scanning electron microscopy (SEM). The starch granules appeared more compact in the pelleted diet, showing first signs of degradation only in the small intestine. The caecum content of both diets group was intensively stained, particles as well as fluid phase, indicating that it contained mainly starch. The SEM pictures of caecum content showed abundant bacteria near starch particles. This suggests selective retention of prae-caecally undigested starch in the murine caecum, likely the site of microbial fermentation.

Keywords: amylase; caecum fermentation; carbohydrate metabolism; laboratory animal diets; processing.

Figure 1

Centrifuged samples of the diets, showing a narrow blue band in PEL (on the left) and staining throughout the complete sediment in EXT (on the right).

Figure 2

Diets. (A) Intact pellets (PEL) and extruded kibbles (EXT); (B) Intact pellet stained with Lugol´s iodine; (C) Intact extruded kibble stained with Lugol´s iodine.

Figure 3

Caecum content stained with Lugol´s iodine. On the left is a sample of group PEL, stained entirely blue, and on the right a sample of group EXT.

Figure 4

Diet samples stained with Lugol’s iodine. (A) PEL with a high number of small, round stained starch particles; (B) EXT with less compact stained particles, larger fragments and starch in the soluble fraction (stereomicroscopy, 40 ×; ○ marking larger fragments of starch, ↓ marking small round particles).

Figure 5

Stomach content samples stained with Lugol’s iodine. (A) PEL with compact, round stained starch particles; (B) EXT with round and irregular stained particles and a blue haze in the fluid phase (stereomicroscopy, 25 ×; ○ marking larger starch fragments, ↓ marking small round particles, ∆ marking blue haze).

Figure 6

Samples from the anterior small intestine stained with Lugol’s iodine. (A) PEL showing a higher amount of small, round stained particles than (B) EXT (stereomicroscopy, 25 ×; ○ marking larger fragments of starch, ↓ marking small round particles in B).

Figure 7

Caecum content stained with Lugol’s iodine. (A) PEL and (B) EXT, both with stained streaks dominating the picture (stereomicroscopy, 25 ×; ○ marking larger fragments of starch).

Figure 8

Colon content stained with Lugol’s iodine. (A) PEL with smears of stained starch; (B) EXT with hardly any stained particles visible (stereomicroscopy, 25 ×; ○ marking larger fragments of starch, ↓ marking small round particles, ∆ marking blue haze).

Figure 9

Starch granules in the diets (A) PEL as round, compact granules of different size, and (B) EXT as irregularly shaped particle (Scanning electron microscopy; ∆ marking irregularly shaped starch particles in B).

Figure 10

Starch particles in the stomach content of group (A) PEL, and (B) EXT; note the rod-shaped bacteria in the surroundings of the starch particles. (Scanning electron microscopy; ↓ indicating bacteria, ∆ marking irregularly shaped starch particles in B).

Figure 11

Starch granules with small pinholes, indicating the beginning of enzymatic degradation in the small intestine. Example from diet PEL (Scanning electron microscopy; ↓ indicating pinholes).

Figure 12

Starch granules from the posterior small intestine, showing large and partly confluencing holes (indicated by ↓). Example from diet PEL, scanning electron microscopy.

Figure 13

Caecum content sample from diet group EXT showing a large, crystalline shape (Scanning electron microscopy, ↓ indicating bacteria in the background).

Figure 14

Starch granules in the course of degradation, surrounded by bacteria and small fragments. Sample of caecum content from group PEL (Scanning electron microscopy, ↓ indicating bacteria, ∆ marking the starch granules).