Amniotic fluid is produced by several means which include fluxes of fluid across the amnionic epithelium and unkeratinized fetal skin, production by the lungs, and, in later
pregnancy, by urine. During most of gestation, urine produced by the fetus is expelled into the bladder and through the urachus within the umbilical cord into the allantoic
cavity. Near term, however, it has been shown that the composition of amniotic fluid changes to become more like urine. Fetuses older than 240 days gestation urinate
Ž .
through the urethra into the amniotic cavity Reeves et al., 1972 . The amount of amniotic and allantoic fluid is approximately equal in the 3-month gestation bovine
Ž .
fetus, but by term, the volume of allantoic fluid 10–15 l is greater than the volume of Ž
. Ž .
amniotic fluid 2.5–7.5 l Russe and Sinowatz, 1991 .
4. Histology
As noted, trophoblast cells form a continuous epithelial layer of the chorion, over the entire surface of the chorioallantois. Subpopulations of the trophoblast cells, however,
are present, and have different functions. Two populations of mononuclear trophoblast cells have phagocytic phenotypes.
Ž These are the trophoblast cells lining the arcade area located towards the fetal side of
. Ž .
the placentome between the bases of cotyledonary villi Fig. 5 . These cells line the
‘‘hematophagus organs’’ where maternal erythrocytes accumulate between the surfaces of endometrial and chorioallantois tissues and are phagocytized by the trophoblast cells.
A second area of specialized phagocytic trophoblast cells is found in the interpla- centomal chorioallantois overlying the openings of endometrial glands. Microscopic
accumulations of uterine milk secreted by endometrial glands raise the overlying
Ž .
chorioallantois to form small microscopic dome-shaped spaces referred to as ‘‘areolae’’. Trophoblast cells over these chambers are tall and contain phagocytized uterine milk.
While areolae are not visible grossly, careful examination of the surface of fresh placentomes reveals small uniformly distributed black areas that are the hematophagus
organs.
Approximately one fifth of bovine trophoblast cells are binucleated. The cellular biology of these cells is amazing. Although they may have a similar microscopic
appearance, lectin cytochemistry clearly demonstrates the existence of many subpopula- Ž
. tions Munson et al., 1989; Jones et al., 1994 .
Trophoblast cells produce a number of hormones and growth factors including Ž
. Ž
. Ž progesterone Reimers et al., 1985 , bovine placental lactogen bPL
Duello et al., .
Ž . Ž
1986 , pregnancy associated glycoproteins bPAG-1, bPAG-2, bPAG-3 Roberts et al.,
. Ž
. 1995 , and transforming growth factor beta Munson et al., 1996 . These hormones are
associated with fetal growth and development and the maintenance of pregnancy. Progesterone receptors are present within nuclei of many different tissues of the
Ž placentome, including trophoblast cells, suggesting paracrine function Shuler et al.,
. 1999 .
In addition to the production of a remarkable array of products, trophoblast cells also undergo intriguing migration into the maternal epithelium. Ultrastructural studies using
phosphotungstic acid to stain cytoplasmic granules of binucleate cells clearly demon- strated the translocation of binucleate trophoblast cells from the chorionic layer of the
cotyledonary villi across to penetrate the single epithelial cell layer lining the caruncular Ž
. crypts see Wooding et al., 1980; Wooding, 1982, 1992 . Similar migration occurs in the
interplacentomal areas. As the binucleate cells migrate, they routinely fuse with a single endometrial epithelial cell and discharge their cytoplasmic granules. The fusion of a fetal
cell, the binucleate trophoblast with a maternal cell temporarily forms a hybrid cell with three nuclei. This hybrid represents the union of two separate individuals into a single
cell
Figs. 8 and 9 show the dramatic difference in immunohistochemical staining using Ž
monoclonal antibody SBU-3 SBU-3 is the designation and antigen prepared from sheep .
trophoblast cells; Lee et al., 1986 stained sections of placentomal tissues comparing late
Fig. 8. Photomicrography of an SBU-3 immunohistochemically stained section of bovine placentome from the placenta of a cow in the 8th month of gestation.
Cross-sections of villi are surrounded by crypt walls of the caruncle. The larger cells that appear darker in this photomicrograph are binucleate trophoblast cells that are specifically stained with the SBU-3 antibody. There
is slightly linear staining along the basement membrane of crypt epithelium. Compare this discrete cellular pattern with that in Fig. 9.
Fig. 9. Photomicrography of an SBU-3 immunohistochemically stained section of bovine placentome from the placenta collected during cesarean section.
Note the deposition of stain along the area of the basement membrane of the caruncular crypts. Binucleate trophoblast cells migrate from the surface of the villi to embed within and fuse with an endometrial epithelial
cell followed by dramatic degranulation. They are then lost. Several stained trophoblast cells remain in this photomicrograph. The exact role binucleate cell migration, fusion and degranualtion play in parturition andror
placental release is not known.
Ž gestation with the degranulated binucleate cells found immediately before delivery Fig.
. 9 . The monoclonal antibody, originally ‘‘raised’’ against ovine placental tissues,
specifically labels binucleate trophoblast cells in ruminant placentas. It is probable that the antigen labeled is a bPAG. The bPAGs have been localized to binucleate cells, and
peripheral systemic levels have been shown to rise over the last 10 days of pregnancy Ž
. Ž
Patel et al., 1997 . This is when marked binucleate cell degranulation occurs Figs. 8 .
and 9 . It has been suggested that bPAG-1 is a proteinase and may function as an Ž
. enzyme Roberts et al., 1995 . Its specific role in placental release is not known.
Another important population of cells found in the bovine placenta are the fetal macrophages. These cells are referred to in human medicine as ‘‘Hofbauer Cells’’
Ž .
Benirschke and Kaufmann, 1995 . Fetal placental macrophages may originate either from chorionic mesenchyme early in gestation, or from fetal bone marrow-derived
monocytes. These cells produce pro-inflammatory cytokines and function in antigen presentation suggesting that they can function as sentinel cells, are important in fetal
defense and may contribute to transplacental transmission of microbial agents.
5. Disease states
