Pregnancy, Development, and Lactation Flashcards
Why is the timing of copulation so important? How is the precise timing accomplished?
Timing is important because the spermatozoa must arrive at the oviducts before the ovum to have time to undergo capacitation, a process that enhances their fertility. Nature has an exquisite method for arranging the proper timing. Breeding is only allowed by the female during estrus, or the “heat” period. So, spermatozoa enter the femal reproductive tract when the oocyte in the follicle is fully developed but has not yet been released. Release of the ovum (ovulation) is delayed until near the end of the estrus period in most species. This nifty bit of traffic control helps ensure that the spermatozoa arrive at the oviducts first and have time to undergo capacitation before the ovum shows up ready to be fertilized.
Describe what happens to a zygote between fertilization and implantation.
The fertilized ovum is called a zygote. Immediately after fertilization, the nucleus of the spermatozoon is called the male pronucleus, and the nucleus of the ovum is called the female pronucleus. Each carries the haploid chromosome number. The male and female pronuclei quickly join together to restore the diploid chromosome number and determine the unique genetic makeup of the offspring. As soon as the two pronuclei join to form a single nucleus, the zygote begins to divide rapidly by the normal process of mitosis. This rapid division is called cleavage. The single cell divides into two cells, which quickly divide into four, then eight, then sixteen, and so on.Cleavage occurs so rapidly that the cells of the zygote do not have time to grow between divisions. The number of cells making up the zygote is increasing dramatically, but its overall size is still about the same as the original ovum, even after several days. While cleavage is taking place, the zygote slowly moves down the oviduct toward the uterus. Delicate, muscular contractions and the movements of cilia are gently propelling it along. After a few dayst the zygote is a solid mass of cells that looks like a tiny raspberry; this is known as the morula stage. The cells of the morula continue to divide and gradually form a hollow cavity in the center. By the time it reeaches the uterus a few days later, it is formed into a hollow ball of cells with a bump on one side that eventually forms into the embryo. It is now called a blastocyst, and it is ready to implant itself in the lining of the uterus.
Why is the placenta so important to a successful pregnancy?
The placenta is a life-support system for the developing fetus. The fetus receives all the nutrients and other substances it needs to grow and develop from its mother. Through the placenta, it also depends on her to dispose of the waste products it produces. Fortunately, the placenta grows right along with the fetus and meets its needs.
Describe the relationship between the fetus and the amniotoic and allantoic sacs of the placenta.
The placenta consists of layers of soft membranes that form two fluid-filled sacs around the developing fetus. The layer immediately around the fetus is called the amnion. It forms a sac around the fetus called the amniotic sac. The fetus floats in amniotic fluid inside this sac. Surrounding the amniotic sac is another layer called the allantois, which forms the fluid-filled allantoic sac. The outside of the allantoic sac is covered by the chorion, which attaches to the uterine lining. The chorion is linked to the fetus by the umbilical cord.
Describe the main structures that make up the umbilical cord and the function of each.
The umbilical cord is a tubelike structure that contains blood vessels (the umbilical arteries and vein) and a drainage tube from the fetus’ urinary bladder (the urachus). The two umbilical arteries carry unoxygenated, waste-filled blood from the fetus to the placenta. The single umbilical vein carries nutrient and oxygen rich blood back from the placenta to the fetus. The urachus is a tube that runs from the cranial tip of the fetus’ urinary bladder through the umbilical cord to the allantoic sac. The kidneys of the developing fetus are not fully functional through most of the pregnancy, therefore they do not produce urine as we know it. They do, however, produce a watery fluid that must be removed from the urinary bladder. The urachus drains this fluid out into the allantoic sac.
Which type of placental attachment ot the uterus is the simplest and detaches most easily after parturition? Which is most complicated and often results in retention of the placenta?
Diffuse attachment is the simplest and detaches most easily. Cotyledonary attachment is the most complicated type and often results in retention of the placenta.
What are the basic events of the three trimesters of pregnancy?
During the first trimester, the newly implanted zygote is getting itself organized and developing its life-supporting placenta. The developing offspring is often referred to as an embryo. During the second trimester, the developing offspring is usually called a fetus. This is the fetal development period, when all of the various parts of the fetus (tissues, organs, and systems) are taking shape and differentiating from each other. The third trimester is the period of fetal growth, when all parts of the fetus grow dramatically as it prepares to become a free-living being after birth.
Describe the three stages of labor.
The first stage of labor consists of uterine contractions. The muscle of the uterus (the myometrium) contracts, pressing the membrane-covered fetus down against the cervix and causing the cervix to gradually dilate. Externally, the dam appears restless and uncomfortable. She may repeatedly lie down and get up and may urinate frequently. Some species, such as the dog and pig, may attempt to build a next into which they will deliver their young.
The second stage of labor consists of the delivery of the newborn. This is accomplished by a combination of strong uterine and abdominal muscle contractions. The dam typically lies down and strains in a rhythmic pattern of contractions that gradually become stronger and closer together. The amniotic and allantoic sacs of the placenta (afterbirth) usually rupture before the actual delivery of the newborn.
The third stage of labor is the delivery of the placenta. The placenta separates from the wall of the uterus and is expelled by the weaker uterine contractions. The dam often eats the placenta or placentas. In multiparous species, such as the dog, cat , and pig, the second and thrid stages of parturition intermix with one another. Typically newborns and placentas are delivered alternately; that is, after a newborn is delivered, its placenta is usually expelled before the next newborn is delivered.
Why is it important that uterine contractions continue after the fetus and placenta are delivered?
Uterine contractions continue so that involution can occur. The contractions cause the sloughed endometrium to be expelled so that the sites where the placenta was attached can heal. Pressure from the continued uterine contractions usually stops bleeding relatively quickly.
Why does mastiits in one quarter of a dairy cow’s udder not necessarily spread to the other three quarters?
Each quarter is a completely separate unit from the other three, with its own milk-secreting systems and ducts leading down to their own teats. Infection does not directly spread from one quarter to another. It has to spread down through the teat and duct system of one quarter and up another, or, on rare occasions, it can spread systemically (through the bloodstream).
Describe the suspensory apparatus of the udder.
This suspensory apparatus of the udder consists of a slinglike arrangement of ligaments that run down the center and around the sides of the udder. The medial suspensory ligament contains many elastic fibers that allow it to stretch. It passes down the center between the left and right halves of the udder. The lateral suspensory ligaments are composed largely of strong, but relatively inelastic, collagen fibers. They pass down and around the lateral sides of each half of the udder. The strong lateral ligaments provide firm support for the udder, and the elastic medial ligament acts as a shock absorber for the udder as the animal moves around.
Why don’t mammary glands of male animals usually develop and secrete milk?
Males’ mammary glands don’t devlop and secrete milk because they do not have the balance of hormones required.
Describe the importance of colostrum to the health of a newborn animal.
Before producing actual milk, the mammary gland produces colostrum, which contains larger amounts of proteins, lipids, and amino acids than milk and also contains high levels of various essential vitamins. Colostrum supplies important nutrients to the newborn and has a laxative effect that helps clear the dark, sticky meconium from the newborn’s intestinal tract. In addition, colostrum provides antibodies that provide passive immunity for the newborn. These antibodies are specific for disease-causing organisms that the dam has been exposed to or vaccinated against. If the newborn drinks sufficient colostrum during the first few hours after birth, the large antibody molecules will be absorbed intact into its bloodstream and defend the animal against specific diseases. If colostrum is not consumed within the first few hours, the lining of the newborn’s intestine can no longer absorb the large antibody molecules intact. They will be broken down by the digestive process, and passive immunity will not be transmitted. These animals may die of early infections, or they are weaker and do not grow as rapidly as animals that consumed colostrum at the appropriate time.
Describe how nursing or milking causes milk letdown and also helps sustain lactation.
When milk is produced, it accumulates up high in the mammary gland in the alveoli and small ducts. It does not move down into the larger ducts and sinuses where it is accessible for nursing or milking until milk letdown occurs. Continued physical stimulation of the teat or nipple and regular removal of milk from the gland send sensory nerve impulses to the brain. From there nerve pathways lead to the hypothalamus, which stimulates the anterior pituitary gland to continue its production of the hormones that keep lactation going. It also causes the hypothalamus to release the hormone oxytocin from the posterior pituitary gland. Oxytocin travels to the mammary gland and causes muscle-like myoepithelial cells around the alveoli and small ducts to contract. This squeezes milk down into the large ducts and sinuses, where it can be removed by nursing or milking. When nursing or milking stops, the flow of essential hormones stops also. The lack of hormonal stimulation combined with increased pressure in the gland (because it is no longer being emptied) causes lactation to gradually cease and the mammary gland to dry up. This is called involution of the mammary gland.
