vulva during estrus or heat. During pregnancy, the mucus is much thicker than at estrus. It plugs the cervix to protect the developing embryo from foreign debris in the vagina. The cervix may also act as a sperm sieve, trapping some abnormal sperm cells and allowing normal sperm cells to travel into the reproductive tract. “The uterus is attached to the cervix and is Y-shaped
Timing and several
hormonal processes go into beef reproduction.
the follicles on the ovaries. This hormone causes her to display estrus. Within 24 hours after the
initial stages of estrus, ovu- lation occurs when 1 of the
with a right and left horn,” said Carpenter. “The horns share a connecting region known as the body. Walls of the uterus are lined with special glands that secrete uterine milk, the substance that nourishes an early embryo. “By approximately 16 to 18 days of gestation, the
placental membranes are well developed and extend into both horns of the uterus. About 38 days into gestation, these membranes begin attaching to the uterine wall at special, raised areas known as caruncles. Located throughout the uterus, these are the exchange points for nutrients coming from the dam.” These attachment points on the placental side are
called cotyledons. The cotyledon with the caruncle forms a combination known as a placentome or button. In mid and late gestation, these buttons become fi rm and are easy to detect when you palpate the uterine surface. The end of each uterine horn is attached to an
oviduct, also known as a fallopian tube. Oviducts are small, tube-like structures with fertilization sites in their upper one-third. At the end of each oviduct is a thin, cup-like membrane called the infundibulum. The ovaries are located close to the infundibulum. “The entire reproductive tract is attached to a thin
suspensory membrane known as the broad ligament,” said Carpenter. “This elastic-like ligament will stretch and move within the pelvic and body cavities to al- low the reproductive tract to move. This movement is necessary because of the weight of the fetus and the crowding of the tract by other internal organs. The broad ligament acts as a cradle for the tract and is at- tached to the upper pelvic and body cavities. It also contains arteries and veins that supply the tract with blood to nourish the tissues.”
Reproductive process The sexual cycle of a normal cow is characterized by
estrus periods that occur at 21-day intervals. At estrus, the cow is infl uenced by estrogen being produced by
tscra.org
follicles ruptures and releases a single ovum, or egg. The infundibulum catches the egg as it is expelled from the follicle and transports it into the oviduct for eventual fertilization. The cavity on the ovary left by the ruptured fol-
licle develops into a new structure known as a corpus luteum. The corpus luteum produces progesterone, the hormone responsible for maintaining pregnancy. If conception does not occur with this ovulation, the uterus releases a hormone called prostaglandin. It re- gresses or destroys the corpus luteum. “Regression is complete by approximately 16 to 17
days into the cycle while follicles continue to grow on the ovary,” said Carpenter. “Since the corpus luteum has regressed and is no longer producing progesterone, a new ovulatory follicle is recruited. Within 4 to 5 days, the cow returns to estrus. The process of development and regression of a normal corpus luteum causes a cow to have her characteristic 21-day sexual cycle. If the cow is mated during estrus, the oviducts transport sperm cells to the fertilization site within minutes. After a 6- to 8-hour maturation period (capacitation), the sperm cells can fertilize an egg.” During natural mating, the vagina serves as a re-
ceptacle for semen. With artifi cial insemination, se- men is deposited in the uterine body. Regardless of the type of breeding, the semen is transported to the fertilization site by the oviducts. When the egg arrives at the fertilization site, the sperm cells are already there and probably have undergone capacitation. The chances of fertilization and pregnancy average about 50 to 70 percent. When fertilization occurs, the cow’s physiologi-
cal cycle begins a dramatic change. This leads to the development of a full-term fetus. About 8 days after fertilization, the embryo is transported to the uterus by the oviducts. At 16 to 17 days after fertilization, the embryo and
its placental membranes begin to release a hormonal signal that prevents the usual release of prostaglandin from the uterus. As a result, the corpus luteum does not regress and continues to release progesterone to maintain the pregnancy. By mid-gestation the placenta takes over as the primary producer of progesterone.
MORE June 2013 The Cattleman 65
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