Wall of SI is composed of 4 TUNICS: 1) tunica mucosa, 2) tunica submucosa,
3) muscularis externa, 4) tunica serosa

1. Tunica mucosa is composed of 3 layers: a) SIMPLE COLUMNAR epithelium,

b) lamina propria (loose irregular connective tissue), c) muscularis mucosae (smooth muscle tissue).
Surface of tunica mucosa is characterized by 2 types of invaginations and projections:
a) intestinal VILLI , b) intestinal CRYPTS of Lieberkuhn
Villus is protrusion of lamina propria into lumen, covered by epithelium. Lamina propria of villus contains: 1) 2 vessels: a) fenestrated hemocapillary, b) lymphatic capillary (lacteal).
2) smooth muscle cells, providing villus contraction, 3) lymphoid nodules.
Crypt is tubular invagination of epithelium into lamina propria.
Cell types in SI EPITHELIUM:
1) Surface absorptive cells. It are tall columnar cells and have apically located MICROVILLI. Funct.:is terminal digestion and monomers absorption. All microvilli collectivly is brush border.
2). Goblet cells. It are tall columnar cells and have apically located secretory granules. Funct.:is secretion of mucinogen (main component of mucus).
3). Paneth cells. It are pyramid-shaped cells and have apically located EOSINOPHILIC secretory granules. Funct.: is secretion of lysozyme and dipeptidases.
4). Regenerative cells =columnar cells without micrivilli. Locate in crypt bottom. Funct.: is origin for regeneration.
5). Enteroendocrine cells (or DNES cells): EC-, ECL-, G-cells (See topic ‘Stomach”). Excepting enumerated cells, epithelium has S-cells and I-cells. These cells locate ONLY in small intestine.
Lamina propria additionally contains abundance of lymphoid nodules  (it are B-zones).

2. Tunica submucosa is dense irregular connective tissue. It in DUODENUM ONLY contains additionally DUODENAL submucosal GLANDS (or Brunners glands). It are tubuloalveolar mucous glands.
3. Muscularis externa is composed of smooth muscle tissue, that forms 2 layers: inner is circular, outer is longitudinal. Longitudinal layer is continuous.
4. Tunica serosa is composed of: a) loose irregular connective tissue and b) MESOTHELIUM.

Wall of LI is composed of 4 TUNICS: 1) tunica mucosa, 2) tunica submucosa,
3) muscularis externa (smooth muscle tiss.), 4) tunica serosa (see SI)
Tunica mucosa is composed of 3 layers: a) SIMPLE COLUMNAR epithelium (colon, sigmoid).
From pectinate line to external anal orifice – stratif. squam. NONkeratinized epith.
At cutaneous anus - stratif. squam. keratinized epith.
b) lamina propria (loose irregular connective tissue), c) muscularis mucosae (smooth muscle tissue).
Surface of tunica mucosa is characterized by ONE type ONLY of epithelial invaginations:
a) intestinal CRYPTS of Lieberkuhn ONLY. (Villi are absent).
Cell types in LI EPITHELIUM:
1). Surface absorptive cells. It are tall columnar cells but WITHOUT microvilli (compare with SI).
Brush border is absent. Funct.: is water and ions absorption ONLY.
2). Goblet cells. (See above SI). Most numerous in rectum.
3). Regenerative cells - locate strictly in crypt bottom. Funct.: is origin for regeneration.
4). Enteroendocrine  cells (or DNES cells): EC-, ECL-,G-cells (See topic ‘Stomach”).
PANETH cells in large intestine are absent .
Lamina propria and submucosa of appendix infiltrated by LYMPHOID NODULES (it are B-zones).
Appendix is PERIPHERAL hemopoietic organ- it provides anigen dependent prolif. and different. of B-lymphocytes into plasma cells.
Outer longitudinal layer of muscularis externa in LI is NOT continuous and gathered into 3 ribbons, named TAENIAE coli.