DEFINED MEDIA AND DEVELOPMENT OF MAMMALIAN EGGS IN VITRO
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DEFINED MEDIA AND DEVELOPMENT OF MAMMALIAN EGGS IN VITRO '
R. B. L. Gwatkin King Ruirch Laboratory of Reprodirctive Physiology, Department of Animal Biology,
School of Veterinary Medicine, University o/ Pennsylvania, Philadelphia, Pa.
Compared with tissue cells in culture, the nutritional requirements of mam- malian ova have scarcely been investigated. Yet the potential significance of doing so is enormous, for defined media permitting the routine development of ova through the free-living stage before implantation would allow many experi- ments now difficult, or impossible, in vivo.
Omitting the special problem of in vitro fertilization, this paper will outline recent progress made in developing defined media for fertile mammalian eggs. Use of these media to study the effect of viruses on early development will then be considered briefly. Finally, some previously unpublished experiments on the culture of mouse eggs beyond the blastocyst stage will be presented.
The Culture of Mammalian Ova Although the eggs of many species have been seen to cleave once or twice
in vitro, only those of the rabbit and the mouse undergo appreciable develop- ment. Rabbit eggs will cleave readily from one-cell to morula in a medium containing large proportions of serum. However, blastocyst expansion fails to take place (Austin, 1961). Mouse ova, on the other hand, can be cultured to fully expanded blastocysts, but one must start from the two-cell stage. The first division is refractory. Cleavage from the one-cell stage through to blasto- cyst will take place within organ cultures of the oviduct (Biggers, Gwatkin & Brinster, 1962). Under these conditions much of the cellular integrity of the duct is preserved (Gwatkin & Biggers, 1963) and, presumably, one or more essential factors are made available by the oviduct for this first cleavage division.
In 1949, Hammond cultured eight-cell mouse ova to blastocysts in balanced salt solution with glucose and egg white. Whitten in 1956 made the medium more defined by using crystalline bovine serum albumin instead of egg white, in Krebs-Ringer bicarbonate with glucose (TABLE 1 ) . At least some of the blasto- cysts produced in this medium were normal, since they developed into mice on transfer to the uteri of foster mothers. In these experiments, carried out by McLaren and Biggers in 1958, a genetic marker was used to distinguish between the embryos originating from the transplanted and native blastocysts.
A basic clue to the successful culture of mouse eggs from the two-cell stage was discovered by Whitten in 1957, when he showed that the addition of one mg. of lactic acid per ml. of his medium (Krebs-Ringer bicarbonate with glucose and albumin) allowed the development of some eggs from the two-cell stage. Whitten speculated that lactic acid might be made available to the egg in vivo by sperm glycolysis, but it is more likely that lactate is produced by the oviduct itself. Data are not available for the mouse, but the secretions of rabbit (Bishop, 1961) and human oviducts (Mastroianni, Winternitz & Lowi, 1958) both con- tain large supplies of lactate. Of special significance is the recent finding of Mastroianni and Wallach (1961) that the lactate concentration in the oviduct fluid of the rabbit increases during the first three days of pregnancy.
Brinster (1963, 1965a, b, c ) has much extended Whitten's original observa- tion that the early cleavage stages of the mouse egg require a 3-carbon compound
80 Annals New York Academy of Sciences TABLE 1
PARTIALLY DEFINED MEDIA FOR THE CULTURE OF MOUSE O V A TO THE ELASTOCYST STAGE
Component From 2-cell stage From S-cell stage whitten* Whittent Brinsterz Brinsters
mM mM mM mM
NaCl 119.32 119.32 109.23 119.32 KCl 4.78 4.78 4.78 4.78 CaCl, 2.54 - 1.71 1.71 KH,PO, 1.19 1.19 1.19 1.19 MgSO, * 7H20 1.19 1.19 1.19 1.19 NaHCOS 25.07 25.07 25.07 25.07 Glucose 5.5 5.5 Calcium lactate - 2.54 Sodium lactate - - 10.15 - Sodium pyruvate - - - 0.316 Crystalline bovine serum albumin 1 gm/L 1 gm/L 1 gm/L l g m / L Penicillin - - 100 U/ml 100 U/ml Streptomycin - - 50 pg/ml 50 pg/ml * Whitten, W. K. 1956. Nature 177: 96. t Whitten, W. K. 1957. Nature 179: 1081. $ Brinster, R. L. 1963. Exp. Cell Res. 32: 205. Brinster, R. L. 1965. J. Exp. Zool. 158: 59.
- - - -
for development. He has established that this requirement may be met by pyru- vate, oxaloacetate, and phosphoenolpyruvate, but not by a number of other compounds of the Krebs cycle, Embden-Meyerhof, or pentosephosphate path- ways (Brinster, 1965b). The reason for this specificity was not established. It may be that the enzymes dealing with some of these inactive compounds are not yet developed at the two-cell stage. However, other possibilities may also be involved. The egg membrane may be impermeable to some of the compounds. A third possibility, which does not appear to have been mentioned previously, is that such compounds as pyruvate may be readily lost from the intracellular pool to the surrounding medium, a situation that has been shown to limit the growth of isolated tissue cells in cloning experiments (Harris, 1964).
Mintz (1964a) has shown that mouse ova, as early as the two-cell stage, incorporate H3-leucine, and one might therefore expect that there is a require- ment for amino acids which is being met by the albumin. Brinster ( 1 9 6 5 ~ ) has in fact shown that the albumin in his media can be replaced by polyvinyl- pyrrolidone (PVP), a synthetic polymer providing some of the physical proper- ties of protein, and a mixture of the 21 individual amino acids of albumin. Omitting either the polymer or all of the amino acids blocked development. However, no one amino acid was proved to be essential, since when the amino acids were omitted singly, development still occurred.
Osmolarity and hydrogen ion concentration are not particularly critical for the development of two-cell ova to blastocysts. In Krebs-Ringer bicarbonate with lactate and albumin, Brinster (1965a) found the optimum osmolarity to be 0.2760 osmols with a range of 0.2718 to 0.2801 osmols. The optimum pH was about 6.82 with a range of 5.87 to 7.78
Effect of Viruses on Eggs In Vitro Brinsters media, chemically defined except for their albumin component,
have been applied in studies to determine the effect of viruses on early develop-
Gwatkin : Development of Mammalian Eggs 81
ment. The mammalian egg, perhaps due partially to its regulative capacity, is relatively resistant to radiations and chemical carcinogens (Wilson, 1964). However, no one knows whether the egg can become infected with viruses, which may remain latent to cause damage in later development, or even in adult life. Recently, I have shown that infection of the mouse ovum is possible, at least with one virulent virus (Gwatkin, 1964; Gwatkin and Auerbach, 1966).
Two-cell mouse ova were cultured in drops of Brinsters lactate medium under mineral oil and in the presence of Mengo virus. This RNA-virus penetrated the zona pellucida, the thick acellular membrane which surrounds the egg until the time of implantation, and blocked further development of the egg in vitro (Gwatkin, 1963). In subsequent experiments the egg was found to support the synthesis of new Mengo virus particles even before fertilization (Gwatkin, 1964). When a one-step growth curve of this virus in the egg was determined, a relatively long latent period and low virus yield per unit volume were encountered. These characteristics set the egg apart, as a host, from other mouse cells which have been studied. However, the significance of this phenomenon is not understood. There is some evidence that the ribosomes are forming during cleavage in the rat (Szollosi, 1965) and only reach a high concentration in the trophoblast cells. Since virus-protein synthesis presumably occurs on the ribosomes, their relatively low numbers early in cleavage could account for the peculiar behavior of the egg as a host. I hope soon to test this hypothesis by infecting late blastocysts.
In contrast to the cytopathology exhibited by Mengo virus, there was no obvious reaction to high concentrations of Polyoma or Rubella viruses (Gwatkin, unpublished). Whether latent infection may have occurred which will affect later development is being studied by transfer of virus-treated eggs to foster mothers. Since this technique is tedious, and since usually only a fraction of the eggs develop, I have recently attempted to continue in vitro culture beyond the blastocyst stage. The remainder of this paper will report some of these experiments.
Cellular Outgrowth f r o m Mouse Blastocysts In Vitro
Cole, Edwards, and Paul (1965) have succeeded in growing cell strains from rabbit blastocysts using reconstituted collagen and Waymouths medium 7521 1
FIGURE 1. Free-floating blastocysts which have shed their zonae pellucidae. Shown after culture for seven days in pyruvate-lactate medium (Brinster, 1965d) with 0.1 per cent glucose. x 60.
82 Annals New York Academy of Sciences
with calf and human serum. Mintz (1964b) recently noted that mouse blasto- cysts grow out into cell cultures in a medium consisting of equal parts of fetal calf serum and Earles balanced salt solution with lmg. lactic acid per ml. This process was not described in detail and no attempt was made to determine the conditions necessary for it. In Brinsters media attachment and outgrowth does not take place. In his most recent formulation, combining the optimal interact- ing concentrations of both lactate and pyruvate (Brinster, 1965d), and which I supplement with 0.1 per cent glucose, the two-cell eggs grow into blastocysts in three days. The zona pellucida is shed by 3?h days, and the blastocysts then remain free-floating (FIG