BIOLOGY OF EGGS AND IMPLANTATION 



86;: 



l)hases of embryonic attachment to the uter- 

 ine epithelium, the subepithelial mucosa 

 shows no reaction whatever. When joined, 

 these observations remind us that as yet 

 there is no conclusive evidence that the im- 

 planting embryo secretes cytolytic enzymes 

 but may secrete other substances. 



The most imaginative experimental ap- 

 proach to the problems of embryo spacing, 

 attachment, and implantation is the work 

 of Boving (1959a, b and c, 1961) on the 

 rabbit. He has clearly shown that, in this 

 animal, invasion is promoted by a chemical 

 substance elaborated witliin the blastocyst 

 and transferred to the maternal circulation. 

 The invasion-promoting substance has been 

 characterized as being in the form of bi- 

 carbonate which induces a localized high 

 1)H. Circulating progesterone increases the 

 level of endometrial carbonic anhydrase 

 and accelerates the removal of bicarbonate 

 from the embryo by catalyzing the forma- 

 tion of carbonic acid. The carbonic acid is 

 converted to carbon dioxide which is re- 

 moved by the maternal circulation. The 

 local pH rises and the various blastocysts' 

 membranes become very sticky, particu- 

 larly at the site of attachment. The physico- 

 chemical interrelationship of the tropho- 

 blast and endometrial epithelium effects a 

 dissociation of the epithelium, thus opening 

 a path for the trophoblast. 



At this writing, the precise roles of the egg 

 and endometrium during implantaton are 

 unknown and remain a challenging problem. 

 The numerous modifications of the implan- 

 tation processes in the different mammalian 

 families create difficulties of interpretation 

 in what is already an unusually complex 

 problem. As more detailed descriptions of 

 the embryo-endometrial relationships ap- 

 pear, it seems clear that neither the ovum 

 nor the endometrium is primarily responsi- 

 ble for implantation, but that both play mu- 

 tual and overlapping roles. One of the 

 greatest gaps in our knowledge of implanta- 

 tion for any animal is a detailed description 

 of the process itself and the precise timing of 

 the events in this phenomenon. The various 

 experimental approaches to the physiologic 

 and biochemical mechanisms of implanta- 

 tion have quickened our interest and broad- 

 ened our view of the complex metabolic 



processes required if implantation is to be 

 successful, but our efforts to interpret cor- 

 rectly the data from biochemical, physio- 

 logic, and pharmacologic investigations will 

 be limited until more accurate information 

 has been obtained bearing on the morpho- 

 logic features of the process itself. 



V. References 



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 reco\'eiy and transplantation of ov^a in the im- 

 matuie rabbit. In Mammalian Germ Cells, pp. 

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Alden, R. H. 1942a. The periovarial sac in the 

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Alden, R. H. 1942b. The oviduct and egg trans- 

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Alden, R. H. 1942c. Aspects of the egg-ovary- 

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Alden, R. H. 1945. Implantation of the rat egg. 



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Alden, R. H. 1947. Implantation of the rat egg. 



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Alden, R. H. 1948. Implantation of the rat egg. 



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Alfert, M. 1950. A cytochemical study of oogen- 

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Allen, B. M. 1911. The origin of the sex-cells of 

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Allen, E. 1922. The oestrus cycle in the mouse. 

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Allen, E. 1923. Ovogenesis during sexual ma- 

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Allen, E., Pratt, J. P., Newell, Q. U., and Bland, 

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Allen, W. M. 1931. I. Cyclic alterations of the 

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Allen, W. M., and Corner, G. W. 1929. Physiol- 

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