Dog and cat cloning update (Proceedings)
Assisted reproductive technologies in small animals are not a 21st century invention; the first artificial insemination in dogs was performed in 1780 and the first description of an oocyte at the microscopic level was that of a canine oocyte, in 1827. However, economic realities have prevented rapid development of reproductive technologies in small animals compared to domestic large animal species. What are the advanced reproductive technologies currently used or proposed in small animals and how likely are they to be commercially available in the near future?
Reproductive physiology and early embryogenesis
The ovaries contain thousands of follicles, each of which contains an egg or ovum. As each estrous cycle begins, a cohort of follicles is selected to begin development. Development is promoted by release of hormones from the hypothalamus (gonadotropin releasing hormone [GnRH]) and pituitary (follicle stimulating hormone [FSH] and luteinizing hormone [LH]). As the follicle develops, it secretes estrogen, which causes the physical and behavioral signs of early heat, or proestrus. In the bitch, estrogen concentrations fall about 9 days after the onset of proestrus; at this time, the bitch will stand to be bred (standing heat or estrus) and a surge of LH is released, causing ovulation. Immature eggs are released from the follicles into the uterine tube, where they undergo two more cell divisions before fertilization can occur. In the queen, estrous behavior occurs when circulating estrogen concentrations are high and copulation stimulates release of GnRH and subsequent ovulation of mature oocytes into the uterine tube.The egg released into the oviduct is surrounded by the zona pellucida and by a layer of granulosa cells from the follicle, the cumulus oophorus. Spermatozoa introduced into the reproductive tract of the bitch undergo capacitation, a calcium-dependent process involving the acrosome reaction on the head of the spermatozoon and achievement of hypermotility. Capacitated spermatozoa digest the layer of cells surrounding the egg and invade the zona pellucida. As soon as one spermatozoon binds to the inner layer of the zona pellucida, entry of other spermatozoa is blocked by an electrochemical reaction so only one spermatozoon fertilizes each egg. Cell division begins immediately.
Repeated doubling of cells occurs (2 cells – 4 cells – 8 cells – 16 cells) with concomitant changes in cell size and placement. The 16-cell stage is called a morula. The 16 to 64 cell stage is called a blastocyst. The blastocyst is a hollow sphere lined with blastomeres (embryonic cells) and filled with fluid. The blastocyst is divided into the inner cell mass, a group of blastomeres at one pole of the blastocyst will go on to form the embryo itself and two of the fetal membranes (yolk sac and allantois), and the trophoblasts, cells lining the outer surface of the blastocyst that go on to form the other two fetal membranes (chorion and amnion).