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In placental mammals, the umbilical cord (also called the birth cord or funiculus umbilicalis) is the connecting cord from the developing embryo or fetus to the placenta. Developed from the same zygote as the fetus, the umbilical cord normally contains two arteries (the umbilical arteries) and one vein (the umbilical vein), buried within Wharton's jelly. The umbilical vein supplies the fetus with oxygenated, nutrient-rich blood from the placenta. Conversely, the umbilical arteries return the deoxygenated, nutrient-depleted blood.
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Physiology in humans
Development and composition
The umbilical cord develops from and contains remnants of the yolk sac and allantois (and is therefore derived from the same zygote as the fetus). It forms by the fifth week of fetal development, replacing the yolk sac as the source of nutrients for the fetus.[1] The umbilical cord in a full term neonate is usually about 50 centimetres (20 in) long and about 2 centimetres (0.75 in) diameter. This diameter decreases rapidly within the placenta.
The umbilical cord is composed of Wharton's jelly, and not of ordinary skin and connective tissue. The cord contains one vein, which carries oxygenated, nutrient-rich blood (from the mother to the fetus) and two arteries that carry deoxygenated, nutrient depleted blood (from the fetus back to the mother). Occasionally, only two vessels (one vein and one artery) are present in the umbilical cord. This is sometimes related to fetal abnormalities, but it may also occur without accompanying problems.
It is unusual for a vein to carry oxygenated blood, and for arteries to carry deoxygenated blood (the only other examples being the pulmonary veins and arteries, connecting the lungs to the heart). However, this naming convention reflects the fact that the umbilical vein carries blood towards the fetus's heart, whilst the umbilical arteries carry blood away.
Connection to fetal circulatory system
The umbilical vein continues towards the transverse fissure of the liver, where it splits into two. One of these branches joins with the hepatic portal vein (connecting to its left branch), which carries blood into the liver. The second branch (known as the ductus venosus) allows the majority of the incoming blood (approximately 80%) to bypass the liver and flow via the left hepatic vein into the inferior vena cava, which carries blood towards the heart.
The two umbilical arteries branch from the internal iliac arteries, and pass on either side of the urinary bladder before joining the umbilical cord.
Postnatal detachment
Shortly after birth, upon exposure to temperature change, the gelatinous Wharton's Jelly substance undergoes a physiological change that collapses previous structure boundaries and in effect creates a natural clamp on the umbilical cord which halts placental blood return to the neonate, causing the cord to cease pulsation. This process will take as little as five minutes if left to proceed naturally.
Problems and abnormalities
A number of abnormalities can affect the umbilical cord, which can cause problems that affect both mother and child:
- Nuchal cord
- Single umbilical artery
- Umbilical cord prolapse
- Umbilical cord knot
- Umbilical cord entanglement
- Vasa previa
- Velamentous cord insertion
Medical protocols and procedures
Clamping and cutting
General obstetric practice introduces artificial clamping as early as 1 minute after birth of the child. This is followed by cutting of the cord, which is painless due to the lack of any nerves. The cord is extremely tough, like thick sinew, and so cutting it requires a suitably sharp instrument. Provided that umbilical severance occurs after the cord has stopped pulsing (5-20 minutes after birth), there is ordinarily no significant loss of either venous or arterial blood while cutting the cord.
There are umbilical cord clamps which combine the cord clamps with the knife. These clamps are safer and faster, allowing one to first apply the cord clamp and then cut the umbilical cord. After the cord is clamped and cut, the newborn wears a plastic clip on the navel area until the compressed region of the cord has dried and sealed sufficiently. The remaining umbilical stub remains for up to 2–3 weeks as it dries and then falls off.
Nonseverance procedures
Health effects of non-clamping of the cord and delayed umbilical severance are receiving attention in medical journals.[2][3][4] Where cutting the cord is completely omitted (a practice called "lotus birth" by its advocates), the umbilical cord is wrapped up to within an inch of the newborn's belly, and the entire intact cord is allowed to dry like a sinew, which then falls off.[5] A recent Cochrane Review studied effects of the timing of umbilical cord clamping.[6] Infants whose cord clamping occurred later than 60 seconds after birth had a statistically higher risk of neonatal jaundice which required phototherapy. Infants with delayed clamping did have a higher hemoglobin level at 2 months, but this effect did not persist beyond 6 months of age.
Harvesting of cord blood
Recently, it has been discovered that the blood within the umbilical cord, known as cord blood, is a rich and readily available source of primitive, undifferentiated stem cells (i.e. CD34-positive and CD38-negative). These cord blood cells can be used for bone marrow transplant.
Some parents have chosen to have this blood diverted from the baby's umbilical blood transfer through early cord clamping and cutting, to freeze for long-term (and costly) storage at a cord blood bank should the child ever require the cord blood stem cells (for example, to replace bone marrow destroyed when treating leukemia). This practice is somewhat controversial,citation needed with critics asserting that early cord blood withdrawal actually increases the likelihood of childhood disease. The Royal College of Obstetricians and Gynaecologists 2006 opinion states, "There is still insufficient evidence to recommend directed commercial cord blood collection and stem-cell storage in low-risk families."
In the future, cord blood-derived embryonic-like stem cells (CBEs) may also be banked and matched with other patients, much like blood and transplanted tissues. The use of CBEs could potentially eliminate the ethical difficulties associated with embryonic stem cells (ESCs).[7]
The umbilical cord in other mammals
In other mammals, the mother animal generally will gnaw the cord off separating the placenta from the baby. It is often consumed by the mother, which nourishes her, and disposes of tissues that would attract scavengers or predators. In chimpanzees, the mother focuses no attention on umbilical severance, instead staying still and nursing and holding her baby (with cord, placenta, and all) until the cord dries and separates within a day of birth, at which time she leaves the cord and placenta on the forest floor where it is recycled by scavengers. This was first documented by zoologists in the wild in 1974.[8]
Other uses for the term "umbilical cord"
The term "umbilical cord" or just "umbilical" has also come to be used for other cords with similar functions, such as the hose connecting a surface-supplied diver to his surface supply of air and/or heating, or a space-suited astronaut to his spacecraft.
The phrase "cutting the umbilical cord" is used metaphorically to describe a child's breaking away from the parental home.
Additional images
 Diagram illustrating a later stage in the development of the umbilical cord. |
 Fetus of about eight weeks, enclosed in the amnion. Magnified a little over two diameters. |
 Sectional plan of the gravid uterus in the third and fourth month. |
 Fetus in utero, between fifth and sixth months. |
 Scheme of placental circulation. |
 Human embryo with heart and anterior body-wall removed to show the sinus venosus and its tributaries. |
 Newborn child, seconds after birth. The umbilical cord has not yet been cut. |
 Newborn child, soon after birth by Caesarian section. |
 Newborn and mother minutes after birth, umbilical cord still intact. |
References
- ^ The Umbilical Cord
- ^ Hohmann M. (1985). "Early or late cord clamping? A question of optimal time" (Article in German). Wiener Klinische Wochenschrift, 97(11):497-500. PMID 4013344.
- ^ Mercer J.S., B.R. Vohr, M.M. McGrath, J.F. Padbury, M. Wallach, W. Oh (2006). "Delayed cord clamping in very preterm infants reduces the incidence of intraventricular hemorrhage and late-onset sepsis: a randomized, controlled trial." Pediatrics, 117(4):1235-42. PMID 16585320.
- ^ Hutton E.K., E.S. Hassan (2007). "Late vs early clamping of the umbilical cord in full-term neonates: systematic review and meta-analysis of controlled trials." Journal of the American Medical Association, 297(11):1257-58. PMID 17374818.
- ^ Crowther S (2006). "Lotus birth: leaving the cord alone." The Practising Midwife, 9(6):12-14. PMID 16830839.
- ^ "Effect of timing of umbilical cord clamping of term infants on maternal and neonatal outcomes." Cochrane Database Syst Rev. 2008; (2):CD004074
- ^ "Cord blood yields 'ethical' embryonic stem cells.", Coghlin A. New Scientist, August 18, 2005. Accessed June 25, 2007.
- ^ see In the Shadow of Man, by Jane Goodall, who was the first person to conduct any long term studies of chimpanzees in the wild, and reported that they did not sever their offspring's cords.
External links
- Full Policy Statement of the American Academy of Pediatrics on Cord Blood: [1]
- Three frequently asked questions of your Newborn Umbilical Cord: [2]
- Research & web resources on cord clamping options [3]
Wikipedia content modification information:
- This page was last modified on 15 August 2008, at 11:10.
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