BIOLOGY OF EGGS AND IMPLANTATION 



861 



other sites have swung the pendulum back 

 to the embryo and the role that it may 

 l)lay in nidation (Asshcton, 1894; von Spec, 

 1901; Schoenfeld, 1903; Mossman, 1937; 

 Fawcett, Wislocki and Waldo, 1947; Run- 

 ner, 1947; Blandau, 1949a; and Boving, 

 1954, 1961 j. 



The extensive i)rolil'eralion and differen- 

 tiation in the endometrium of certain ani- 

 mals after ovulation undoubtedly arc im- 

 l)ortant in the nourishment and maintenance 

 of the ovum in utero and in providing a suit- 

 able implantation site. The considerable 

 growth and differentiation which the blasto- 

 cysts of many animals undergo before they 

 make contact with the uterine mucosa would 

 indicate that more nutrients are required 

 than are stored in the ooplasm of most mam- 

 malian eggs. The widespread occurrence of 

 glucose, glycogen, lipids, phosphatases, iron, 

 calcium, and many other substances, includ- 

 ing vitamins and enzymes, in the endome- 

 trium may provide the necessary nourish- 

 ment during the very early stages of implan- 

 tation ( Wislocki and Dempsey, 1945) . Bloch 

 (1939) described the secretion of an osmo- 

 l)hilic substance by the uterine epithelium 

 which is thought to be absorbed by the free 

 mouse blastocyst. The work of Daron 

 (1936), Markee (1940), Phelps (1946), 

 Parry (1950), and Boving (1952a, 1961) 

 has demonstrated that there is an increased 

 blood supply immediately below the uterine 

 epithelium at about the time of blastocyst 

 attachment. The increased vascularity may 

 not only provide nutrition to the uterine epi- 

 thelium, but more importantly it provides 

 blood vessels for specific physicochemical re- 

 actions between the trophoblast and endo- 

 metrium (Boving, 1959a) . A similar increase 

 in the blood supply in the antimesometrial 

 area has been observed in the guinea pig 

 (Bacsich and Wyburn, 1940). This is the 

 area in which implantation invariably oc- 

 curs in this species, and the localized hyper- 

 emia is considered to be a factor in tlie anti- 

 mesometrial implantation. 



It is well established that the presence of 

 an actively secreting corpus luteum is essen- 

 tial if implantation is to be complete and 

 successfully maintained. In rabbits proges- 

 terone is necessary, not only for the nutri- 

 tion of the free blastocyst in utero, but also 

 for implantation (Fraenkel, 1903; Corner, 



1928b; Corner and Allen, 1929; Hafez and 

 Pincus, 1956a, b) . Histochemical and quan- 

 titative tests have indicated that lipids are 

 present in the endometrium in greater 

 amounts during the luteal phase of the re- 

 productive cycle than at any other time 

 (Krehl)iel, 1937; van Dyke and Chen, 1940; 

 Alden, 1947). 



It is clear that the presence of an embryo 

 in the cornu exerts a significant effect on the 

 secretion of luteotrophic hormone and on the 

 functional life of the corpus luteum. How 

 these effects are producecl remains a chal- 

 lenging problem. We need to determine 

 whether direct invasion of the endometrium 

 is essential or whether mere expansion of the 

 embryo can act as a trigger mechanism. 

 Nalbandov and St. Clair (1958) have shown 

 that if plastic beads of more than 2 mm. in 

 diameter are inserted into the cornua on the 

 8th day of the estrous cycle in sheep, the 

 cycle is significantly lengthened. Denerva- 

 tion of the cornu containing the beads pre- 

 vented this change in length. 



It has been found repeatedly that endo- 

 metrial sensitivity to the formation of de- 

 ciduomata is limited normally to the period 

 of implantation and placentation (Loeb, 

 1908; Allen, 1931; Selye and McKeown, 

 1935; Krehbiel, 1937; Greenwald, 1958b). 

 The traumatizing substances were physical, 

 chemical, and electrical stimuli. From these 

 studies, three facts were revealed: (1) The 

 formation of the "maternal placenta" can 

 be induced in the complete absence of the 

 blastocyst (Krehbiel, 1937; Mossman, 1937; 

 Dawson and Kosters, 1944). (2) Even 

 though tissue destruction in the endome- 

 trium can be brought about by specific and 

 nonspecific stimuli and even though the end- 

 result may appear similar, the mechanisms 

 producing the changes do not necessarily 

 stem from the same basic stimulus. (3) All 

 of the stimuli used are presumed to have as 

 the basis of their action some kind of tissue 

 injury.^ Notwithstanding, the histologic 

 transformations of the deciduomas corres- 

 pond exactly to those occurring normally in 



^ The passage of an electric current of sufficient 

 magnitude through the endometrium to induce the 

 decidual response gives no evidence of tissue dam- 

 age that can be detected microscopically. This of 

 course does not eliminate the possibility^ that cel- 

 lular injury has not occurred. 



