Education & Terminology • 01
Reproduction
Glossary.
An A–Z reference for the terms, abbreviations, and concepts used throughout animal reproduction — and throughout Repro with Rhianna.
A
Ampulla
The wider, upper portion of the oviduct where fertilization normally takes place.
Anestrus
A period when a female is not showing estrous cycles. In cattle it’s most often seen after calving or when nutrition and body condition are limiting.
Artificial Insemination (AI)
Placing collected semen into the female reproductive tract by hand rather than through natural mating. AI makes it possible to use genetics from sires that are far away, expensive, or no longer living.
B
Blastocyst
An embryo stage with a fluid-filled cavity (the blastocoel), an inner cell mass that will become the fetus, and an outer layer of cells (the trophoblast) that will help form the placenta.
Breeding Soundness Examination (BSE)
A standardized evaluation of a bull’s ability to breed, typically including a physical exam, scrotal circumference, and semen evaluation.
C
Cervix
The firm, muscular passage between the vagina and the uterus. In cattle it contains a series of ring-like folds that help seal and protect the uterus.
Cleavage
The series of early cell divisions that follow fertilization. The embryo divides into more and smaller cells without growing in overall size.
Conception Rate
The percentage of females bred that become pregnant from that breeding. It measures the success of the breeding itself, not how many animals were bred.
Corpus Luteum (CL)
A temporary structure that forms on the ovary from the follicle after ovulation. It produces progesterone, the hormone that maintains pregnancy.
Cryopreservation
Freezing and storing semen, oocytes, or embryos at very low temperatures, usually in liquid nitrogen, so they can be used later.
D
Dominant Follicle
The follicle that continues to grow while others in the same wave stop growing and regress. Only a dominant follicle present at the right time in the cycle can ovulate.
Donor
In embryo transfer, the genetically valuable female whose embryos or oocytes are collected.
E
Embryo Transfer (ET)
Collecting embryos from a donor female and placing them into recipient females, which carry the pregnancies. ET lets one superior female produce many more offspring than she could carry herself.
Estrogen (Estradiol)
A hormone produced mainly by growing follicles. It drives the behavioral signs of heat and helps trigger the hormone surge that leads to ovulation.
Estrous Cycle
The recurring reproductive cycle of a female, from one heat to the next. In cattle it averages about 21 days.
Estrus (Heat)
The period when a female is sexually receptive and will stand to be mounted. Ovulation follows estrus.
F
Fertilization
The union of a sperm cell and an oocyte, combining genetic material from both parents to form a zygote.
Fertilization Rate
The percentage of recovered oocytes or ova that were fertilized. It helps separate fertilization problems from problems with embryo development.
Flush
The common name for a nonsurgical embryo collection, in which fluid is passed through the donor’s uterus to wash out embryos.
Follicle
A fluid-filled structure on the ovary that holds and nourishes a developing oocyte. Follicles also produce estrogen.
Follicle-Stimulating Hormone (FSH)
A hormone released by the pituitary gland that stimulates a group of follicles to begin growing.
Follicular Wave
A pattern of follicle growth in which a group of follicles begins growing together, one becomes dominant, and the rest regress. Cattle usually have two or three waves per cycle.
G
Gonadotropin-Releasing Hormone (GnRH)
A hormone released by the hypothalamus in the brain that signals the pituitary gland to release FSH and LH. It sits at the top of the reproductive hormone chain.
Grading (Embryo Quality Grade)
A visual assessment of an embryo’s quality under a microscope. The widely used IETS system grades embryos from 1 (excellent or good) to 4 (dead or degenerating).
H
Hatched Blastocyst
A blastocyst that has broken out of the zona pellucida, its protective outer shell. Hatching is a normal step in early development.
I
In Vitro Fertilization (IVF)
Fertilizing oocytes with sperm in a laboratory and culturing the resulting embryos before transfer or freezing.
Infundibulum
The funnel-shaped opening of the oviduct that sits close to the ovary and captures the oocyte at ovulation.
L
Luteinizing Hormone (LH)
A hormone released by the pituitary gland. A sharp rise in LH (the LH surge) triggers ovulation of the dominant follicle.
Luteolysis
The breakdown of the corpus luteum at the end of a cycle when the female is not pregnant. It is triggered by PGF₂α released from the uterus, and it allows a new cycle to begin.
M
Morula
An early embryo stage that looks like a compact ball of cells, before a fluid-filled cavity forms. The name comes from the Latin word for mulberry.
O
Oocyte
The female reproductive cell (egg) that develops inside a follicle and is released at ovulation.
Oocyte Pick-Up (OPU)
Collecting oocytes directly from ovarian follicles using an ultrasound-guided needle, usually so they can be fertilized through IVF.
Ovary
The female reproductive gland. It produces oocytes and reproductive hormones, and it is where follicles and the corpus luteum develop.
Oviduct
The small tube connecting the ovary area to the uterine horn. It carries the oocyte, it is where fertilization happens, and it moves the early embryo toward the uterus. Also called the fallopian tube.
Ovulation
The release of an oocyte from a mature follicle. In cattle, ovulation occurs after the end of standing heat.
P
Pregnancy Rate
The percentage of females that become pregnant. Its definition varies by context. In embryo transfer it usually means pregnant recipients out of those that received an embryo. In herd management it often accounts for both heat detection and conception over a set period. Always ask how a pregnancy rate was calculated.
Progesterone
The hormone produced by the corpus luteum. It prepares the uterus for pregnancy, maintains pregnancy, and prevents the female from returning to heat while it is high.
Prostaglandin F₂α (PGF₂α)
A hormone released by the uterus that causes the corpus luteum to regress (luteolysis), ending the cycle’s progesterone phase.
R
Recipient
In embryo transfer, the female that receives an embryo and carries the pregnancy. Her cycle must be closely matched to the embryo’s stage of development.
Recovery Rate
How successfully ova and embryos were collected from a donor, often compared with the number of ovulations. Like other reproductive figures, it is calculated in different ways.
S
Sexed Semen
Semen that has been sorted to contain mostly X-bearing sperm (to produce females) or Y-bearing sperm (to produce males).
Superovulation
Stimulating a donor female’s ovaries so that multiple follicles mature and ovulate in the same cycle instead of one, producing several embryos for collection.
Synchronization
Using the biology of the estrous cycle to bring a group of females into heat or to ovulation at about the same time. It makes AI and embryo transfer more practical to schedule.
T
Transferable Embryo
An embryo of suitable stage and quality to be transferred into a recipient or frozen. Under the IETS system this generally means grades 1 through 3.
U
Uterine Horns
The two curved branches of the uterus in cattle. Each connects to an oviduct, and the pregnancy usually develops in the horn on the same side as the ovary that ovulated.
Uterus
The muscular organ where the embryo develops into a fetus. In cattle it is made up of a short uterine body and two uterine horns.
Z
Zona Pellucida
The protective outer layer surrounding the oocyte and early embryo. The embryo eventually hatches out of it.
Zygote
The single cell formed when a sperm fertilizes an oocyte, before the first cell division.
Repro with Rhianna provides educational information about animal reproduction and management. Rhianna is not a veterinarian, and content on this site should not be considered veterinary advice.
