Equis ISSN 2398-2977

Reproduction: oocyte collection

Synonym(s): Oocyte aspiration, Follicular puncture

Contributor(s): Madeleine Campbell, Camilla Scott


  • Oocyte collection from mares can involve either oocyte aspiration, ie sucking/flushing the oocyte from the ovary, in live mares; or opening the follicle and scraping free the granulosa layer in excised ovaries, from ovariectomised or dead mares. Both of these techniques are described below.
  • Oocyte aspiration is most commonly performed in the mare using transvaginal ultrasound guided oocyte collection, though it can be performed via flank puncture. Both of these methods are described below. Flank puncture is typically used when aspirating a single pre-ovulatory follicle, or less commonly in cases where the mare has an infectious vaginal discharge (which could otherwise be carried into the peritoneal cavity during transvaginal aspiration), or abnormal perineal conformation, which makes transvaginal aspiration difficult.


  • Oocyte aspiration is most commonly performed to obtain oocytes for in vitro maturation, fertilization with intra-cytoplasmic sperm injection (ICSI) and culture of blastocysts prior to either embryo cryopreservation or immediate transfer in a recipient mare. Oocyte aspiration followed by ICSI and embryo transfer may be used to overcome fertility issues in both the mare and stallion.
  • Oocyte aspiration for transfer into the oviduct of a recipient mare is useful in cases of mare subfertility involving the uterus; cervix; oviduct or ovary, where the quality of the mare’s oocyte is acceptable, but some other factor in those parts of the reproductive system is limiting fertility and the mare’s ability to either donate an embryo, or to carry a foal to term herself. When oocyte aspiration is being undertaken in such cases, a single oocyte recovered from a stimulated preovulatory follicle of the donor mare provides the most successful outcome.
  • Oocyte collection and subsequent fertilization by ICSI can also overcome some cases of stallion infertility, where standard insemination techniques are not sufficient to obtain good pregnancy rates or in cases where there is very limited availability of semen, eg dead stallions.
  • Oocyte aspiration is also used to acquire oocytes in order to use them as host cytoplasts for the transfer of somatic cell nuclei during somatic cell nuclear transfer (‘cloning’). When oocyte aspiration is being undertaken for that reason, immature or mature oocytes may be collected, but it is common to collect immature oocytes as more are available.
  • Oocytes may be collected from ovaries excised from mares in which ovariectomy has been performed, or who have died, in order to preserve valuable genetics that would otherwise be lost.


  • Overcomes some types of subfertility in the mare, and thus facilitates breeding from valuable mares from whom pregnancies cannot otherwise be obtained.
  • Facilitates breeding from mares that have recently been ovariectomised or died/euthanized.
  • Certain stallions are only available for breeding via ICSI either due to limited availability of semen or stallion fertility issues, in which case oocyte collection is required.
  • Oocyte recovery from pre-ovulatory follicles has a high recovery rate (70-80%) as the oocyte cumulus complex has already loosened from the wall of the follicle so is easier to retrieve.
  • Oocytes collected from dominant follicles have already reached developmental competence following gonadotropin stimulation and only need to be cultured in vitro until the predicted time of ovulation.
  • Aspiration of immature follicles has an overall higher pregnancy rate per cycle than that of pre-ovulatory follicles as the increased oocyte number recovered per aspiration session overcomes any developmental and quality issues associated with immature oocytes.


  • Oocyte collection is a highly specialized assisted reproductive technique, which requires technical expertise and specialized equipment.
  • The equipment needed for transvaginal ultrasound guided oocyte aspiration is expensive.
  • Success rates at collecting oocytes vary between around 25% and 70%, depending on the stage of maturity of the oocyte, and technical expertise.
  • The procedure is semi-invasive for the donor mare and although rare, complications such as peritonitis and rectal tears can occur during the procedure.
  • Oocyte recovery from pre-ovulatory follicles is limited to one or two follicles per cycle; superovulation does not help, as aspiration of subsequent follicles can be difficult.
  • Follicular growth needs to be closely monitored when aspirating pre-ovulatory follicles, whereas immature oocytes can be collected at any stage of the year and estrous cycle.
  • Aspiration of immature follicles is a non-selective procedure with juvenile, growing and atretic follicles being recovered. As such only 50-60% of all recovered oocytes are expected to mature.
  • Immature follicles need to be cultured in vitro with gonadotropins and have lower developmental rates than those obtained from stimulated dominant follicles.


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Further Reading


Refereed papers

  • Recent reference from PubMed and VetMedResource.
  • Velez I C, Arnold C, Jacobson C C et al (2012) Effects of repeated transvaginal aspiration of immature follicles on mare health and ovarian status. Equine Vet J Suppl 43, 78–83 PubMed.
  • Carnevale E M & Maclellan L J (2006) Collection, evaluation and use of oocytes in equine assisted reproduction. Vet Clin North Am Equine Pract 22 (3), 843-856 PubMed.
  • Carnevale E M, Coutinho da Silva M A, Preis K A et al (2004) Establishment of pregnancies from oocytes collected from the ovaries of euthanized mares. Proc Am Assoc Equine Pract 50, 531-533 VetMedResource.
  • Bogh I B, Brink P, Jensen H E et al (2003) Ovarian function and morphology in the mare after multiple follicular punctures. Equine Vet J 35 (6), 575–579 PubMed.
  • Vanderwall D K & Woods G L (2002) Severe internal hemorrhage resulting from transvaginal ultrasound-guided follicle aspiration in a mare. J Equine Vet Sci 22, 84–86 VetMedResource.
  • Cook N L, Squires E L, Ray B S et al (1993) Transvaginal ultrasound guided follicular aspiration of equine oocytes. J Equine Vet Sci 15, 71-74 WileyOnlineLibrary.

Other sources of information

  • Ortis H & Foss R (2014) How to Collect Equine Oocytes by Transvaginal Ultrasound-Guided Follicular Aspiration. In: Proc Am Assoc Equine Pract 59, 519-524 VetMedResource.
  • Hindrichs K (2012) Assisted reproduction techniques in the horse. Reprod Fertil Dev 25 (1), 80-93 PubMed.
  • Hinrichs K (2011) Immature Oocyte Collection and Maturation. In: Equine Reproduction. Eds: McKinnon A O, Squires E L, Vaala W E & Varner D D. Blackwell Publishing Ltd, UK. pp 2931-2935.
  • Carnevale E M (2011) Mature Oocyte Collection. In: Equine Reproduction. Eds: McKinnon A O, Squires E L, Vaala W E & Varner D D. Blackwell Publishing Ltd, UK. pp 2936-2940.