Umbilical cord

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. During prenatal development, the umbilical cord is physiologically and genetically part of the fetus and (in humans) 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 fetal heart pumps deoxygenated, nutrient-depleted blood through the umbilical arteries back to the placenta.

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. The cord is not directly connected to the mother's circulatory system, but instead joins the placenta, which transfers materials to and from the mother's blood without allowing direct mixing. The length of the umbilical cord is approximately equal to the crown-rump length of the fetus throughout pregnancy. The umbilical cord in a full term neonate is usually about 50 centimeters (20 in) long and about 2 centimeters (0.75 in) in diameter. This diameter decreases rapidly within the placenta. The fully patent umbilical artery has two main layers: an outer layer consisting of circularly arranged smooth muscle cells and an inner layer which shows rather irregularly and loosely arranged cells embedded in abundant ground substance staining metachromatic. The smooth muscle cells of the layer are rather poorly differentiated, contain only a few tiny myofilaments and are thereby unlikely to contribute actively to the process of postnatal closure.

The umbilical cord is composed of Wharton's jelly, a gelatinous substance made largely from mucopolysaccharides. It contains one vein, which carries oxygenated, nutrient-rich blood to the fetus, and two arteries that carry deoxygenated, nutrient-depleted blood away. 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, while the umbilical arteries carry blood away.

The blood flow through the umbilical cord is approximately 35 ml / min at 20 weeks, and 240 ml / min at 40 weeks of gestation. Adapted to the weight of the fetus, this corresponds to 115 ml / min / kg at 20 weeks and 64 ml / min / kg at 40 weeks.

Connection to fetal circulatory system
The umbilical cord enters the fetus via the abdomen, at the point which (after separation) will become the umbilicus (or navel). Within the fetus, 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 befoin term ones. In contrast to the contribution of Wharton's jelly, cooling causes only temporary vasoconstriction.

Within the child, the umbilical vein and ductus venosus close up, and degenerate into fibrous remnants known as the round ligament of the liver and the ligamentum venosum respectively. Part of each umbilical artery closes up (degenerating into what are known as the medial umbilical ligaments), while the remaining sections are retained as part of the circulatory system.

Problems and abnormalities
A number of abnormalities can affect the umbilical cord, which can cause problems that affect both mother and child:
 * Nuchal cord, when the umbilical cord becomes wrapped around the fetal neck
 * Velamentous cord insertion
 * Single umbilical artery
 * Umbilical cord prolapse
 * Vasa praevia

Clamping and cutting




General hospital-based obstetric practice introduces artificial clamping as early as 1 minute after the birth of the child. In birthing centers, this may be delayed by 5 minutes or more, or omitted entirely. Clamping 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 7–10 days as it dries and then falls off.

Early versus delayed clamping
The health implications of early versus delayed cord clamping are receiving attention in medical journals.

Delayed clamping may be supported by various health benefits: A recent analysis of attended home births over a 6-year period reported that none of the infants experienced adverse outcomes as a result of delayed cord clamping. A meta-analysis showed that delaying clamping of the umbilical cord in full-term neonates for a minimum of 2 minutes following birth is beneficial to the newborn in giving improved hematocrit, iron status as measured by ferritin concentration and stored iron, as well as a reduction in the risk of anemia (relative risk, 0.53; 95% CI, 0.40-0.70). A decrease was also found in a study from 2008. However, a Cochrane Review from 2008 showed that, although there is higher hemoglobin level at 2 months, this effect did not persist beyond 6 months of age.

Negative effects of delayed cord clamping include an increased risk of polycythemia. Still, this condition appeared to be benign in studies. The 2008 Cochrane review found that infants whose cord clamping occurred later than 60 seconds after birth had a statistically higher risk of neonatal jaundice requiring phototherapy. Conversely, a recent randomized, controlled trial noted in the 2008 Examination of the Newborn & Neonatal Health compared the timing of cord clamping on the newborn venous hematocrit and reported an increase in anemia in the infants whose cords were clamped immediately.

Delayed clamping is not recommended for health care providers as a solution to cases where the newborn is not breathing well and needs resuscitation. Rather, the recommendation is instead to immediately clamp and cut the cord and perform cardiopulmonary resuscitation. The umbilical cord pulsating is not a guarantee that the baby is receiving enough oxygen.

Umbilical nonseverance
Some parents choose to omit cord severance entirely, a practice called "lotus birth" or umbilical nonseverance. The entire intact umbilical cord is allowed to dry like a sinew, which then separates naturally (typically on the 3rd day after birth), falling off and leaving a healed umbilicus.

Umbilical cord catheterization
As the umbilical vein is directly connected to the central circulation, it can be used as a route for placement of a venous catheter for infusion and medication. The umbilical vein catheter is a reliable alternative to percutaneous peripheral or central venous catheters or intraosseous canulas and may be employed in resuscitation or intensive care of the newborn.

Storage 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 (of type 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 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 controversial, with critics asserting that early cord blood withdrawal at the time of birth actually increases the likelihood of childhood disease, due to the high volume of blood taken (an average of 108ml) in relation to the baby's total supply (typically 300ml). The Royal College of Obstetricians and Gynaecologists stated in 2006 that "there is still insufficient evidence to recommend directed commercial cord blood collection and stem-cell storage in low-risk families".

The American Academy of Pediatrics has stated that cord blood banking for self-use should be discouraged (as most conditions requiring the use of stem cells will already exist in the cord blood), while banking for general use should be encouraged. In the future, cord blood-derived embryonic-like stem cells (CBEs) may 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).

While the American Academy of Pediatrics discourages private banking except in the case of existing medical need, it also says that information about the potential benefits and limitations of cord blood banking and transplantation should be provided so that parents can make an informed decision.

Cord blood education is also supported by legislators at the federal and state levels. In 2005, the National Academy of Sciences published an Institute of Medicine (IoM) report which recommended that expectant parents be given a balanced perspective on their options for cord blood banking. In response to their constituents, state legislators across the country are introducing legislation intended to help inform physicians and expectant parents on the options for donating, discarding or banking lifesaving newborn stem cells. Currently 17 states, covering two-thirds of U.S. births, have enacted legislation recommended by the IoM guidelines.

Research in this area that has the potential to revolutionize medicine is advancing rapidly and it is difficult for professional medical societies, and other resources that expectant parents turn to for information, to keep pace.

Physicians and researchers are making significant progress evaluating the safety and efficacy of umbilical cord blood stem cells for therapeutic uses far beyond cancers and blood disorders. The use of cord blood stem cells in treating conditions such as brain injury and Type 1 Diabetes is already being studied in humans, and earlier stage research is being conducted for treatments of stroke, and hearing loss.

The fundamental differences between private and public cord blood banking should be noted. Cord blood stored with private banks is reserved for use of the donor child only. In contrast, cord blood stored in public banks is accessible by anyone with a closely matching tissue type. The terms public and private do not necessarily indicate the funding source, but rather the availability of use.

The utilization of cord blood from public banks is rising rapidly. Currently it is used in place of a bone marrow transplant in the treatment of blood disorders such as leukemia, with donations released for transplant through one registry, Netcord, passing 9000. This is usually when the patient cannot find a matching bone marrow donor. It is this "extension" of the potential donor pool which has driven the expansion of public banks.

Private banks which collect for specific individuals store on the premise of future technologies and uses of cord blood. While this is a valid reason for private donation, it must be remembered that for many diseases such as leukemia, it is actually preferable to not use your own cord blood. This is because the disease may be in latent form in your own cord blood, as well as a graft-versus-tumor effect.

Anatomy
The umbilical cord in some mammals contains two distinct umbilical veins, rather than just one (as is the case for humans). Examples include cows and sheep.

Cord disposal
In some animals, the mother will gnaw through the cord, thus separating the placenta from the offspring. It (along with the placenta) is often eaten by the mother, to provide nourishment and to dispose of tissues that would otherwise attract scavengers or predators. In chimpanzees, the mother focuses no attention on umbilical severance, instead nursing her baby with cord, placenta, and all, until the cord dries and separates within a day of birth, at which time the cord is discarded. (This was first documented by zoologists in the wild in 1974. )

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. Engineers sometimes use the term to describe a complex or critical cable connecting a component, especially when composed of bundles of conductors of different colors, thickness and types, terminating in a single multi-contact disconnect.