Table 1.

Summary of similarities and differences in periconceptional physiology and ART procedures between the woman and the mare. LUF: Luteinized Unovulatory Follicle; OPU: Ovum Pick-up; PCOS: Polycystic ovary syndrome.

		Woman	Mare
Similarities	Physiology	Formation of primordial follicles during fetal life
		Two or three follicular waves per ovarian cycle
		Mono-ovulatory cycles, LUF syndrome
		Timing of early developmental stages until the blastocyst stage
		Easy and efficient in vitro embryo culture up to the blastocyst stage in humans, a bit less efficient in horses
	ART procedure	ICSI is widely used in both species
		Blastocoel collapsing is used for freezing embryos
		Development of several tools for embryo selection in ART procedures, including embryo biopsy and time-lapse
		Shared interest in non-invasive preimplantation genetic testing
	Impact of environment	Reduction of fertility with age
		Frequency of embryonic aneuploidies
		Menopause for woman; ovarian senescence for mares
		Potential impact of excess sport on spontaneous reproduction
		Effects of obesity on cycles, inflammation, follicular fatty acids and triglycerides, lower oocyte and embryo quality
Differences	Physiology	Preovulatory follicles: 2 cm diameter	Large follicles (×2.1 human)
		ZP surrounding the embryo	Presence of a capsule in addition to the ZP
		Uterine prostaglandin secretion not needed for luteolysis	Uterine prostaglandin secretion needed for luteolysis
		Human blastocyst only reaches a maximum size of 200 μm and a few hundred cells before implantation	Horse blastocyst is larger on Day 7 (200–800 μm)
		Hatching and implantation on Day 7–10	No hatching, implantation on Days 35–40
		PCOS	No PCOS
	ART procedure	Oocyte collection limited to OPU OPU mostly performed after ovarian stimulation	Oocytes can be collected from live (by OPU) or dead mares OPU only performed in non-stimulated mares Oocyte embedded in the follicular wall making it necessary to scrape-off the follicular wall
		Possibility of ovarian stimulation, IVM, IVF, ICSI	Possibility of only IVM and ICSI; IVF not efficient
		Effective embryo freezing at all stages of development	Effective embryo freezing on embryos < 300 µm in size and in vitro produced embryos; methods need to be improved
		Oocyte vitrification is mastered	The technique of oocyte freezing needs to be improved and is not used commercially