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Embryo Development¹

University of Pennsylvania, Philadelphia 19103^,3

Abstract

The embryo begins its existence as a relatively large cell with a low ratio of nuclear to cytoplasmic volume. Within a few days, cleavage has increased the ratio to near normal and blastocyst formation begins. The formation of the blastocyst is an important event and is the first morphological sign of differentiation. It is also at this time that a variety of metabolic indices markedly increase, including oxygen consumption, incorporation of precursors into nucleic acids and protein synthesis.

The energy metabolism of the early embryo appears to be unusual. Pyruvate is probably an essential substrate for all early cleavage stages of mammalian embryos. Oxidation of pyruvate is much greater than oxidation of glucose during early cleavage, although glucose oxidation increases as early development progresses. However, glucose is an important substrate and is incorporated to a greater extent than any of the amino acids. The unusual early energy substrate requirements may be associated with a defective malate shuttle system.

Protein metabolism appears to be largely controlled by maternal cytoplasmic factors until near blastocyst formation when synthetic rate increases considerably. Despite the suggested early maternal control, it is clear that the embryonic genome does function early in cleavage since proteins coded for by paternal genes can be demonstrated before blastocyst formation.

Although much of our information has been derived from laboratory animals, there is evidence for a similarity among many Eutherian mammals during the first week of development.

¹ Presented at the Symposium on Prenatal and Perinatal Development of Swine sponsored by the American Society of Animal Science and Shell Development Company, July 29, 1973, University of Nebraska, Lincoln.

² I thank Ms. Barbara Hansford for assistance in preparation of this manuscript. Financial support for the author's research is from Grant No. 08452 from the National Institute of Child Health and Human Development and the Population Council.

³ Laboratory of Reproductive Physiology, School of Veterinary Medicine.