The evolution of a viviparous reproduction strategy in teleosts included several adaptations: a shift from external to internal fertilization, the ability to retain embryos in the female reproduction system, the utilization of the ovary as a site of gestation, and modifications of the endocrine reproductive control mechanisms as well as structural and functional modifications of the embryo and the female reproductive system (see Wourms and Lombardi, 1992). Viviparous reproduction has evolved in an estimated 2–3% of the known teleost species (Wourms and Lombardi, 1992). Although numerous cytoplasmic vacuoles and secretory granules were present in both epithelial and endothelial cells, the fate of synthesized protein remains to be determined. Evidence for protein synthesis in the ovarian lining was found in the form of Golgi apparatus and rough endoplasmic reticulum. Heavy investment in keratin filaments suggests that follicles are tissues of high structural integrity. Junctional complexes between cells were predominantly anchoring junctions with the occurrence of occasional occluding junctions, supporting the possibility of paracellular transport from maternal serum to ovarian fluid of small molecular weight compounds. The barrier between ovarian fluid and maternal blood consisted of the endothelial cells of the maternal blood vessels and a layer of epithelial cells lining the ovarian lumen, with an intermittent layer of loose connective fibers. Follicular capillary beds were continuous with those in the ovary wall and were eventually drained by the ovarian and intestinal venous systems. The follicles had a rich capillary network with diffusion distances between maternal blood and ovarian fluid comparable to those found for gill epithelia, suggesting this is the primary site of gas exchange between maternal plasma and ovarian fluid. Casts of the ovarian vasculature showed that blood supply to the ovary is initially directed to the follicular appendages lining the ovarian wall through thick-walled muscular arteries running along the ovary wall and within the follicular appendages. For a complete diagram of body fluid compartments, see body fluid compartments of a 70-kg man and body fluid compartments of a 55-kg woman.The structural basis for exchange between maternal serum and ovarian fluid in the viviparous teleost Zoarces viviparus was investigated. Note that this diagram places focus only on these three major fluid compartments. Plasma is the smallest fluid compartment (~8% of total body water). Interstitial fluid contains ~25% of the total body water. The intracellular fluid compartment contains most of the water in the body (~67% of total). The right diagram shows the three major fluid compartments drawn to scale. The left diagram allows for a better demonstration of the relationship between the intracellular fluid, interstitial fluid, and plasma, however, the relative size of each of the compartment is not drawn to scale. Waste products produced by cells follow the reverse path from the cytoplasmic compartment to plasma. They then must cross the plasma membrane to enter the cytoplasmic compartment of cells. Nutrient molecules traveling in the blood must first cross the capillary endothelium to enter the interstitial fluid. ![]() The capillary endothelium is the physical barrier that separates the interstitial fluid from plasma. The physical barrier separating the intracellular fluid compartment (i.e., cytoplasm) and the interstitial fluid is the cell plasma membrane. Fluid, molecules, and ions flow across physical barriers between the fluid compartments. These are the (1) intracellular fluid compartment, (2) interstitial fluid, and (3) plasma. In the human body plan, there are three major fluid compartments that are functionally interconnected.
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