Umbilical cord


In placental mammals, the umbilical cord is a conduit between the developing embryo or fetus and the placenta. During prenatal development, the umbilical cord is physiologically and genetically part of the fetus and normally contains two arteries and one 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 low-oxygen, nutrient-depleted blood through the umbilical arteries back to the placenta.

Structure and development

The umbilical cord develops from and contains remnants of the yolk sac and allantois. It forms by the fifth week of development, replacing the yolk sac as the source of nutrients for the embryo. The cord is not directly connected to the mother's circulatory system, but instead joins the placenta, which transfers materials to and from the maternal 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 newborn is usually about long and about 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 post-natal closure.
The umbilical cord can be detected by ultrasound by six weeks of gestation and well-visualized by eight to nine weeks of gestation.
The umbilical cord lining is a good source of mesenchymal and epithelial stem cells. Umbilical cord mesenchymal stem cells have been used clinically to treat osteoarthritis, autoimmune diseases, and multiple other conditions. Their advantages include a better harvesting, and multiplication, and immunosuppressive properties that define their potential for use in transplantations. Their use would also overcome the ethical objections raised by the use of embryonic stem cells.
The umbilical cord contains Wharton's jelly, a gelatinous substance made largely from mucopolysaccharides that protects the blood vessels inside. 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 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. However, this naming convention reflects the fact that the umbilical vein carries blood towards the fetus' 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.
The proximal part of an umbilical cord refers to the segment closest to the embryo or fetus, and to the placenta, and opposite the distal part.

Function

Connection to fetal circulatory system

The umbilical cord enters the fetus via the abdomen, at the point which will become the 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, which carries blood into the liver. The second branch bypasses the liver and flows 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 bladder into the umbilical cord, completing the circuit back to the placenta.

Gas exchange

The umbilical cord lacks vasa vasorum, which means that the cells within Wharton's Jelly are dependent on the diffusion of gases and nutrients for their metabolic needs. It has been hypothesized that this diffusion might explain the persistence of the dual umbilical artery, while the second umbilical vein atrophies early in gestation. The dual umbilical artery increases the surface area of the arterial walls with roughly 45% compared to a single umbilical artery, while the same placental flow is maintained. This might provide a threefold protective mechanism against the effect of uterine contractions during childbirth. These contractions disrupt the maternal blood flow towards the placenta, and therefore might result in potentially harmful fetal acidosis. During a uterine contraction, flow through the umbilical cord, and thus diffusion with Wharton's Jelly, is maintained. First of all, Wharton's jelly acts as an oxygen reserve, which delays the onset of anerobic metabolism, ultimately delaying the onset of fetal acidosis. Secondly, the diffusion of carbon dioxide from the fetal blood into Wharton's jelly dampens the increase of acidity in the fetal blood, delaying the onset of acidosis as well. Finally, the diffusion of carbon dioxide back into the arterial blood after the uterine contraction accelerates fetal recovery, as it raises the carbon dioxide content of the fetal blood entering the placenta. In the placenta, this increase in carbon dioxide increases the uptake of oxygen from the maternal blood as a result of the Bohr effect. In turn, this increased amount of oxygen increases the rate with which carbon dioxide is removed from the fetal tissues, as a result of the Haldane effect. Since all three effects are proportional to the diffusion rate, the protective properties are reduced for fetuses with a single umbilical artery, which provides an explanation for the increased amount of complications seen for these fetuses during childbirth.

Changes after birth

After birth, the umbilical cord stump will dry up and drop away by the time the baby is three weeks old. If the stump still has not separated after three weeks, it might be a sign of an underlying problem, such as an infection or immune system disorder.
In absence of external interventions, the umbilical cord occludes physiologically shortly after birth, explained both by a swelling and collapse of Wharton's jelly in response to a reduction in temperature and by vasoconstriction of the blood vessels by smooth muscle contraction. In effect, a natural clamp is created, halting the flow of blood. In air at 18 °C, this physiological clamping will take three minutes or less. In water birth, where the water temperature is close to body temperature, normal pulsation can be five minutes and longer.
Closure of the umbilical artery by vasoconstriction consists of multiple constrictions which increase in number and degree with time. There are segments of dilations with trapped uncoagulated blood between the constrictions before complete occlusion. Both the partial constrictions and the ultimate closure are mainly produced by muscle cells of the outer circular layer. In contrast, the inner layer seems to serve mainly as a plastic tissue which can easily be shifted in an axial direction and then folded into the narrowing lumen to complete the closure. The vasoconstrictive occlusion appears to be mainly mediated by serotonin and thromboxane A2. The arteries in cords of preterm infants contract more to angiotensin II and arachidonic acid and are more sensitive to oxytocin than in 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, while the remaining sections are retained as part of the circulatory system.

Clinical significance

Problems and abnormalities

A number of abnormalities can affect the umbilical cord, which can cause problems that affect both mother and child:
The cord can be clamped at different times; however, delaying the clamping of the umbilical cord until at least one minute after birth improves outcomes as long as there is the ability to treat the small risk of jaundice if it occurs. Clamping is followed by cutting of the cord, which is painless due to the absence of nerves. The cord is extremely tough, like thick sinew, and so cutting it requires a suitably sharp instrument. While umbilical severance may be delayed until after the cord has stopped pulsing, there is ordinarily no significant loss of either venous or arterial blood while cutting the cord. Current evidence neither supports, nor refutes, delayed cutting of the cord, according to the American Congress of Obstetricians and Gynecologists guidelines.
There are umbilical cord clamps which incorporate a 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 length of umbilical left attached to the newborn varies by practice; in most hospital settings the length of cord left attached after clamping and cutting is minimal. In the United States, however, where the birth occurred outside of the hospital and an emergency medical technician clamps and cuts the cord, a longer segment up to in length is left attached to the newborn.
The remaining umbilical stub remains for up to ten days as it dries and then falls off.