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Organization of Placenta

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  1. Trophoblast ultrastructure
  2. Chorionic villi
  3. Placental cotyledons
  4. Breaks in the placental 'barrier'
  5. Placental size and weight
  6. The placenta at term
  7. Placental aging
  8. Conclusion
  9. Bibliography

From the electron microscopic studies of Wislocki and Dempsey (1955), data were provided that permitted a functional interpretation of the fine structure of the placenta. There are prominent microvilli on the syncytial surface, corresponding to the "brush border" described by light microscopy. Associated pinocytotic vacuoles and vesicles are related to the absorptive and secretory placental functions. The inner layer of the villi?the cytotrophoblasts?persists to term, although often compressed against the trophoblastic basal lamina, and retains its ultrastructural simplicity.

[...] As the placenta matures, the short, thick, early stem villi branch repeatedly, forming progressively finer subdivisions and greater numbers of increasingly small villi. Each of the main stem (truncal) villi and their ramifications (rami) constitute a placental cotyledon (lobe). Each cotyledon is supplied with a branch (truncal) of the chorionic artery; and for each cotyledon, there is a vein, constituting a 1:1:1 ratio of artery to vein to cotyledon. BREAKS IN THE PLACENTAL "BARRIER" The placenta does not maintain absolute integrity of the fetal and maternal circulations. [...]

[...] The stroma of the villi also exhibits changes associated with aging. In placentas of early pregnancy, the branching connective tissue cells are separated by an abundant loose intercellular matrix. Later, the stroma becomes denser and the cells more spindly and more closely packed. Another change in the stroma involves the Hofbauer cells, which likely are fetal macrophages. These cells are nearly round with vesicular, often eccentric nuclei and very granular or vacuolated cytoplasm. These cells are characterized histochemically by intracytoplasmic lipid and are readily distinguished from plasma cells. [...]

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