IMPLANTATION OF THE RAT EMBRYO: FURTHER HISTOCHEMICAL OBSERVATIONS ON CARBOHYDRATE, RNA, AND LIPID METABOLIC PATHWAYS

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Abstract
The routes of metabolism of carbohydrate, lipid and RNA in relation to the implanting rat embryo are examined histochemically, using the standard dehydrogenase technique and, as substrates, [alpha]-glycero-phosphate, [beta]-hydroxy-butyrate, glucose-6-phos-phate, 6-phospho-gluconate, lactate, isocitrate, succinate, malate (with both NAD and NADP as co-factors), alcohol (ethanol), furfuryl alcohol and glutamate. Lipid (Sudan III) and RNA (chrome-alum-gallocyanin with RNAS controls) were also investigated. The results divide the enzymes into four groups: [alpha]-glycerophosphate dehydrogenase and [beta]-hydroxy-butyrate dehydrogenase, which are found mainly in the uterine epithelium where they accumulate just before its breakdown and then disappear, and which correspond well to the lipid distribution. glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase, whose distribu-tion corresponds well with that of RNA suggesting that ribose production for RNA may be via the pentose shunt in this site. These enzymes are found in the primary decidua initially, and then mainly in the meso-metrial stroma. Lactate, isocitrate, succinate and malate dehydro-genases which accumulate chiefly in the antimesometrial decidua up to 8 to 8 1/2 days thereafter falling off in concentration, and whose dis-tribution suggests that they may represent the route of glycogen break-down in this site for energy production. Alcohol dehydrogenase (func-tion unknown), furfuryldehydrogenase whose distribution suggests that it may be concerned with RNA turnover, and glutamate dehydrogenase, which is found in sites probably engaged in rapid protein turnover. A lesser degree of staining of isocitric dehydrogenase as compared to other Krebs'' cycle enzymes is demonstrated, and it is suggested that this is due to an associated decrease in NADPH diaphorase, which decrease may suggest transhydrogenation and associated stimulation of protein synthesis. The function of the yolk-sac placenta is examined, and a possible route of protein absorption, breakdown to simpler sub-stances, and re-secretion into the yolk sac cavity for direct access to the embryo is suggested. Finally the function of the decidua in relation to the nutrition of the embryo is examined and it is suggested that at least one of these functions may be the synthesis and transport to the embryo of protein for tissue growth.

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