Читать книгу Genetic Disorders and the Fetus - Группа авторов - Страница 95

Decades ago, the mean protein concentration in AF at term was observed to be less than one‐tenth that in maternal serum.⁶³ Subsequently, the ratios of albumin, transferrin, γ‐globulin, ceruloplasmin, α₁‐antitrypsin, and Gc⁵¹ have been established, and provide direct evidence as to their maternal or fetal origin.⁵¹ Many maternal serum proteins gain access to the AF, thereby complicating the use of this fluid for prenatal diagnosis. Proteins of fetal origin probably derive from skin, amnion, chorion, umbilical cord, urine, and bronchial, buccal, and gastrointestinal secretions, and may be cellular, free organelles, or in solution.^{51, 66} Protein constituents of basement membrane have been detected,⁶⁷ as has Tamm–Horsfall glycoprotein.⁶⁸

The glycosaminoglycan composition of human AF reveals the major constituent to be hyaluronic acid at 12–21 weeks; the rest is mostly chondroitins and small amounts of heparan sulfate.^{69, 70} Gestational age is an important variable affecting glycosaminoglycan composition in both normal and pathologic pregnancies.⁶⁹ The determination of glycosaminoglycan composition for prenatal diagnosis of the mucopolysaccharidoses is not recommended. The prenatal detection of hemophilia B by assay of factor IX and prothrombin in AF was unsuccessful,⁷¹ but direct analysis of DNA is successful (see Chapter 14).

There is striking variability in the total protein concentration during pregnancy, increasing from a mean of about 3.5 mg/mL at 12 weeks to a maximum of about 8 mg/mL at 25 weeks.^{72, 73} The concentration gradually falls to about 3 mg/mL between 25 and 35 weeks, with little change occurring thereafter. The highest concentrations of albumin, α₁‐antitrypsin, Gc, and transferrin have been noted between 20 and 30 weeks of gestation.

Although most proteins in AF may be of maternal serum origin, nonserum proteins derived from the epithelial cells of the amnion^{74, 75} or from the maternal uterine decidua,⁵¹ as well as other α₂‐proteins and α₁‐fetoprotein, have been described. Chitayat et al.⁷⁶ used a colon epithelial‐specific monoclonal antibody (Mc‐Ab) to determine the contribution of fetal colonic mucosal cells to the amniocyte population, and concluded that cell‐specific Mc‐Ab can be used to detect colon cells and that colonocytes are an important component of the AF cell population. The S100B protein, a product of nervous system glia⁷⁷ and the amnion,⁷⁸ can be elevated in preeclampsia, intrauterine restriction,⁷⁸ and fetal death.⁷⁹