Human fetal ductal plate revisited: II. MUC1, MUC5AC, and MUC6 are expressed in human fetal ductal plate and MUC1 is expressed also in remodeling ductal plate, remodeled ductal plate and mature bile ducts of human fetal livers

Abstract

Mucins are high-molecular-weight glycoproteins, which are heavily decorated with a large number of O-linked oligosaccharides and a few N-glycan chains, linked to a protein backbone. The protein backbone is called mucin core protein or MUC apomucins. MUC expression is down-regulated or up-regulated in malignant neoplasms. These alterations of MUC apomucins, which are regulated by MUC genes, are associated with carcinogenesis and malignant potentials of cancers. MUC expression during human fetal intrahepatic bile duct (IBD) development has been studied only once, and there has been only one histochemical study of mucins in human fetal IBD development. The author herein immunohistochemically investigated the expression of MUC1, MUC2, MUC5AC, and MUC6, and histochemically investigated carbohydrate component of mucins in human fetal cholangiocytes with the use of 32 human fetal livers of various gestational ages. MUC1 is a transmembranous apomucin, while MUC2, MUC5AC and MUC6 are secretory apomucins. Under normal conditions, MUC1 (polymorphic epithelial mucin) is present mainly in the pancreatic epithelium. MUC2 (goblet cell mucin) is mainly located in goblet cells. MUC5AC (gastric foveolar mucin) and MUC6 (pyloric gland-type mucin) are located in the stomach. In the present study, the processes of the human IBD development could be categorized into four stages; ductal plate (DP), remodeling DP, remodeled DP, and mature IBDs. The author identified that MUC1 was present in ductal plate (DP), remodeling DP, remodeled DP, and mature IBD in human fetal livers. MUC5AC and MUC6 were present only in the DP. MUC5AC and MUC6 were absent in remodeling DP, remodeled DP, and mature IBD in human fetal livers. No expression of MUC2 was seen throughout the fetal IBD development. Histochemically, no carbohydrate component of mucins were seen in the remodeling DP and remodeled DP, while neutral and acidic mucins (carboxylated and sulfated mucins) were seen in mature IBD in human fetal livers. The DP showed frequently neutral mucins and less frequently acidic mucins (carboxylated and sulfated mucins residues). These findings suggest that the DP cells have MUC1, MUC5AC and MUC6, and that remodeling DP, remodeled DP, and mature IBDs have MUC1, but not MUC5AC and MUC6. The presence of neutral and acidic carbohydrates in DP suggests that these carbohydrates of mucin are attached to the MUC5AC and MUC6 mucin core proteins. Although the implications are unclear, the expression of these MUC apomucins and their carbohydrate residues are associated with normal development of IBDs in human fetal livers.

Keywords: Ductal plate; MUC apomucins; histochemistry; human fetal liver; immunohistochemistry; intrahepatic bile duct development; mucins.

Figure 1

The HE histologies of ductal plate and intrahepatic biliary cells in human fetal livers. The ductal plate (A, short arrows) is composed of single or double-layered cylindrical structures located in the interface between hepatoblast and mesenchyme of the portal tract. A tubular structure (long arrow) of future bile duct is seen in the ductal plate. Immunostaining of cytokeratin 18 highlights the ductal plate (B) and a tubular formation of future bile duct (B, arrow) in the ductal plate. The ductal plate is composed of single or double layered cuboidal cells (arrows) of beaded appearances. The remodeling ductal plate shows disappearance of some cells of the ductal plate (D, short arrows) and development of tubular structures of future bile ducts, which is moving into the portal mesenchyme (D, long arrow). The remodeled ductal plate is characterized by almost disappearance of ductal plate (E, short arrows) and the tubular structure (E, long arrow) moves from the ductal plate (E, short arrows) into the portal mesenchyme (E, long arrow). The mature intrahepatic bile ducts are mature ducts similar to adult intrahepatic bile ducts (F, arrows). A, C, D, and E: HE stain. A: x40. C, D, and E: x200. F: x300.

Figure 2

Mucin immunohistochemistry in biliary cells in human fetal livers. The cells of ductal plate (A, B, arrows) are negative for mucins in these figures. The tubular formations of future bile ducts are indicated by two arrows. The cells of remodeling ductal plate indicated by arrows (C, D) are negative for mucins. The cells of remodeled ductal plate indicated by arrows (E, F) are negative for mucins. The ductal plate cells show magenta products in d-PAS and combined d-PAS/AB at pH2.5 (G, H, arrows). The mature intrahepatic bile ducts (arrows) are positive for neutral and acidic mucins (I, J). The cells of ductal plate are occasionally positive for acidic mucinous (K, arrows) in AB techniques (K, arrows). A, B: AB at pH 2.5 stain; A: x40; B: x100. C: mucicarmine stain, x200. D: combined d-PAS/AB at pH2.5 stain, x200. E: d-PAS technique, x200. F: mucicarmine technique, x200. G: d-PAS technique, x100. H: d-PAS technique, x400. I: d-PAS technique, x100. J: AB at pH2.5 technique, x100. K: AB at pH2.5 technique, x400.

Figure 3

Immunohistochemistry for cytokeratins in human fetal livers. The cells of ductal plate (A, B, C, and D), remodeling ductal plate, remodeled ductal plate, and mature intrahepatic bile ducts are strongly positive for cytokeratin 8 (A) and 18 (B), while they were weakly positive for cytokeratin 7 (C) and cytokeratin 19 (D). A, B, C, and D: x100. Arrows in B, C and D indicate tubular formations of future bile duct in the ductal plate of human fetal livers.

Figure 4

Immunohistochemistry of MUC1 apomucin in the biliary element of human fetal livers. MUC1 is always expressed in cells of ductal plate (A, short arrows). The tubular formation of future bile duct is indicated by long arrows in A. MUC1 is positive in cells of ductal plate remodeling stages (A). Ductal plate cells (short arrows) and migrating future duct element (long arrow) (B). MUC 1 is positive in cells of remodeled stage of the ductal plate (C, arrow). The mature bile duct (arrow) is strongly positive for MUC1.

Figure 5

Immunohistochemistry of MUC5AC apomucin in the biliary element of human fetal livers. MUC5AC is expressed in cells of the ductal plate (A, arrows), but MUC5AC is negative for cells of ductal plate remodeling stages (B, arrow), is negative for cells of remodeled stage of ductal plate (C, arrow), and is negative for mature bile duct (D, arrow) in human fetal livers. A: x350. B: x 200. C: x 150. D: x80.

Figure 6

Immunohistochemistry of MUC6 apomucin in the biliary element of human fetal livers. MUC6 is expressed in cells of the ductal plate (A, arrows), but MUC6 is negative for cells of ductal plate remodeling stages (B, arrow), is negative for cells of remodeled stage of ductal plate (C, arrow), and is negative for mature bile duct (D, arrow) in human fetal livers. A: x350. B: x200. C: x150. D: x80.