Moran Edwin T, Bedford Michael R
Poultry Science Department, Auburn University, AL 36849-5416, USA.
AB Vista, Woodstock Court, Blenheim Road, Marlborough Road, Wiltshire, SN8 4NA, UK.
Anim Nutr. 2023 Jul 12;14:403-410. doi: 10.1016/j.aninu.2023.06.010. eCollection 2023 Sep.
Endogenous protein leaving the ileum largely consists of accrued mucins from the upper gastrointestinal tract (GIT) that had resisted digestion. The amounts released rely on their mucosal generation during enteral feeding which vary with age as well as diet. These digestion resistant proteins of endogenous origin continue to be unavailable in the large intestine, whereas those of dietary origin provide amino acids that largely support the existing microbial population while denying limited amounts for absorption. Other mucins pre-exist within the large intestine as two layers at the lumen surface. A loose layer harboring a diverse microbial population is superimposed on the unstirred water layer (USWL) which simultaneously acts as an obstacle to microbes at the loose layer while performing as a molecular sieve for nutrients. The USWL is formed through interplay between enterocyte and goblet cells; however, the basis for presence of the loose layer is elusive. Large intestinal fermentation predominates within the colon of swine, whereas fowl employ their ceca. Motility within the colon of swine segregates fine materials into haustrae out-pocketings that parallel their placement within the ceca of fowl. Viscous mucins from small intestinal endogenous losses may envelop microbes within the large intestinal lumen to present successive adherents on the USWL that assemble its loose layer. The loose layer continually functions as a microbial reservoir in support of lumen fermentation. Microbial catabolism of mucin within the loose layer is known to be slow, but its proximity to the enterocyte is of advantage to enterocyte absorption with by-product amino acids fostering the USWL.
离开回肠的内源性蛋白质主要由来自上胃肠道（GIT）且未被消化的累积粘蛋白组成。其释放量取决于肠内喂养期间它们在黏膜中的生成情况，而这会随年龄和饮食而变化。这些内源性来源的抗消化蛋白质在大肠中仍然无法利用，而饮食来源的蛋白质则提供氨基酸，这些氨基酸在很大程度上支持现有的微生物群体，同时仅提供少量用于吸收。其他粘蛋白在大肠内以两层的形式预先存在于管腔表面。一层含有多种微生物群体的疏松层叠加在未搅动水层（USWL）上，USWL同时对疏松层的微生物起到阻碍作用，同时又作为营养物质的分子筛。USWL是通过肠上皮细胞和杯状细胞之间的相互作用形成的；然而，疏松层存在的基础尚不清楚。猪的结肠内以大肠发酵为主，而禽类则利用它们的盲肠。猪结肠内的蠕动将精细物质分隔到袋状肠袋中，这些肠袋与它们在禽类盲肠中的位置相对应。小肠内源性损失产生的粘性粘蛋白可能会包裹大肠管腔内的微生物，从而在USWL上形成连续的附着体，进而形成其疏松层。疏松层持续作为微生物库，以支持管腔发酵。已知疏松层内粘蛋白的微生物分解代谢缓慢，但其靠近肠上皮细胞有利于肠上皮细胞吸收，其副产物氨基酸促进了USWL的形成。
