Information on Harvesting Oocytes and ICSI (Intra-Cytoplasmic Sperm Injection)

How are oocytes harvested from a mare and what is ICSI?

Oocytes are eggs that come from the follicles on the mare’s ovaries. The oocyte is released from the follicle on the ovary during ovulation and travels into the fallopian tube where it meets a sperm cell from the male and fertilization occurs. The resulting embryo then travels to the uterus for implantation and development.

Harvesting oocytes is the process by which the oocytes are removed from the follicles and then in the lab are cultured and matured (maturation is roughly 12-30 hours, this is dependent upon the stage of the oocyte at the recovery). These oocytes are subsequently injected with individual sperm from the stallion, a procedure we refer to as ICSI. The resulting embryo is then matured in the laboratory for about 7-10 days and then is available for transfer into a recipient mare or frozen for later use.

You may ask what is the advantage of harvesting oocyte’s from a mare and doing ICSI vs an embryo transfer?

  • Some mare’s may be unable to become pregnant themselves (chronic uterine infections, complicating cervical abnormalities, blocked oviducts or other oviductal pathology, abnormal ovulations (anovulatory follicles), or any other conditions that prevent the mare from conceiving or providing a healthy environment for the embryo)
  • Stallions with limited sperm availability. Some stallions may simply have low fertility. Other stallions may be deceased and only a certain amount of semen is available. ICSI should be considered when only 1 or 2 straws of frozen semen remain from a stallion that is no longer alive or additional semen cannot be collected. Sperm injection could theoretically be used to produce multiple foals from a single straw of semen. For example, a small piece of the frozen straw could be snipped off under liquid nitrogen and thawed, while the rest of the straw remained frozen (i.e. cut-straw approach). Alternatively, the entire straw could be thawed and the contents diluted in additional semen extender and refrozen in many smaller straws (i.e. refrozen semen approach). In either situation, the overall principle is conservation and maximum use of a limited number of sperm from a valuable stallion.

Success Rates

Oocytes can be recovered by transvaginal aspiration once every two weeks. Multiple oocytes are typically recovered through each attempt. Of those immature follicles aspirated, we will recover oocytes from 50-60% of them. Mature follicles are a little higher at 75%. Typically one third of the resulting embryo’s created will make it into recipients. Therefore, each aspiration session yields on average 1.2 embryo’s. ICSI transferred embryo’s result in a slightly higher early embryonic loss than our naturally produced embryo’s. The mare’s age and her oocyte quality significantly affects these numbers. Older mares, with poor oocyte quality can take several attempts to result in a pregnancy. Some mares may have decreased numbers of follicles available at the time of aspiration which also affects overall success. Viable pregnancies are highly mare-dependent. Some mares may produce multiple embryos and pregnancies in the first cycle, others may take several more attempts, especially if they are older, with the average being around 3 cycles to result in one pregnancy.

Post-mortem Oocyte Recovery

Unfortunately, sudden death or terminal disease or injury may affect your mare. In this case, the ovaries can be harvested from the mare in order to utilize for ICSI immediately following death or euthanasia. Please contact our office for more information on this procedure.


– Click here for Routine Reproductive Pricing

– Click here for Equine Fertility Institute Pricing

– Click here for New Client Form

– Click here for General Check In

– Click here for Foaling Mare Check In

– Click here for the link to Oocyte/ICSI Contract

– Click here for the link to Normal Foaling Events

– Click here for the Mare Foaling Date Calculator, simply put in the ovulation date of your mare and find out her due date along with scheduled vaccinations and checkups!

– Click here for the link to Foal Vaccination Date Calculator, simply put in the date your foal was born and find out when vaccinations are due!


Feel free to contact us if you have any questions concerning our Oocyte/ICSI Services at PEVS.

We are always available for a consultation.

If you have any questions and would like to speak with a Veterinarian, please call us at 803-641-0644