The umbilical cord is a cord that connects the placenta to the fetus during pregnancy, allowing transfer of nutrients and other material between the mother and fetus. Its full len
gth is usually 50-60 cm in length and the diameter generally increases with gestational age [1,6]. It is usually twisted in a spiral shape in either a clockwise or counterclockwise direction, however counterclockwise has been found to be more common[1]. The 
cord contains three large blood vessels: two arteries that carry deoxygenated blood from the fetus to the placenta, and one vein that carries oxygenated blood to the fetus in the opposite direction. These are embedded in a mucous connective tissue known as Wharton’s Jelly. The umbilical cord does not contain lymphatic vessels or nerve fibers[1,5].* Although the general agreement is that there are no nerve
fibers in the cord, there have been some studies that have found different results[1].
Development of the Umbilical Cord
The umbilical cord begins to develop with the formation of an extraembryonic coelom that almost surrounds the early embryo and also remains attached to the chorion by a connecting stalk of mesenchyme, which later becomes ventrally located during development. The amniotic sac expands as the embryo grows, filling the extraembryonic coelom and compressing the remnants of both the yolk sac and a duct called the vitello-intestinal duct, against the connecting stalk. Eventually these structures all fuse to form the umbilical cord, which is surrounded by the amnion and amniotic cavity at this point. By the middle of the 5th month of gestation, many of these structures disappear, and all that remains are two umbilical arteries and a single umbilical vein embedded in the Wharton’s Jelly, which consists mainly of ground substance[3].
Figure 2: Development of the umbilical cord (click here to enlarge figure)Histology
Epithelium
The umbilical cord is covered in amniotic epithelium that varies along the length of
the cord. The area near the umbilicus (navel), is covered in an unkeratinized, stratified squamous epithelium which provides the transition from the abdominal wall to the cord surface. As the distance increases away from the naval, the epithelium changes into 2 to 8 layers of stratified columnar epithelium, and finally into a simple columnar epithelium. Unlike the 
amniotic surface of the placenta, the amnion of the cord grows firmly into the central connective tissue core and cannot be moved[1].Wharton's jelly
The bulk of the umbilical cord is comprised of mucous connective tissue known as Wharton’s Jelly, shown in the figures below. It contains ground substance rich in GAG’s, primarily hyaluronic acid, as well as fibroblasts and a delicate network of collagenous fibers[1,4]. The extracellular matrix is hydrophilic giving the jelly-like consistency. The fibroblasts are spindle shaped and evenly distributed with long extensions, and there are numerous mast cells present that surround the vessels and are also found beneath the cord surface. Immunohistochemically, the interstitial collagens types I, III, and VI, as well as the basal lamina molecules collagen type IV, laminin and heparin sulphate have all been found[1].
According to certain studies, stromal cells in Wharton’s Jelly show different degrees of differentiation from mesenchymal cells to myofibroblasts depending on their location. The most immature cells are close to the amniotic surface and still proliferating, however, with increasing distance of the amniotic surface, the cells acquire cytoskeletal features of contractile cells. The stromal cells close to the umbilical vessels were found to be highly differentiated myofibroblasts[1].
Pathology
Umbilical cord problems account for 9% of fetal deaths, and are related to a large variety of abnormalities and other negative consequences. There are numerous types of problems that may occur with the umbilical cord, however not all of their causes and consequences are fully understood. The following are brief descriptions about some of the problems that may occur.
Short Cord Length
A cord is considered short when is has a total length of 40 cm or less, although they are relatively uncommon. Those less than 15 cm are often associated with abdominal wall defects, as well as spinal and limb deformities. Short cords are correlated with depressed IQ, and have been found in offspring who suffer from Fetal Alcohol syndrome as well as Down’s syndrome.
Long Cord Length
Long cords are typically those that have a total length of over 70 cm and have been found to be associated with a significant increase in risk of brain abnormalities and/or abnormal neurological function. Knots and fatal strictures are nearly always found with cords of excessive length. Also, long cords may prolapsed or become entangled around itself or the fetus. When a cord loops around a fetus’s neck, it is referred to as a nuchal cord loop. This occurs in 20-33% of normal term pregnancies, is slightly more common in males, and includes 2 types:
Type A: The cord encircles the neck in an unlocked pattern
Type B: The cord encircles the neck in a locked pattern
The latter type is usually associated with more severe consequences, however for the most part, nuchal cords do not affect the outcome of the pregnancy or the fetal weight at birth.
Knots
Fetuses with knots in their umbilical cords are at a 4-fold risk of intrauterine death compare to those with normal cords. Knots may cause compression of Wharton’s jelly, lead to significant prepartum hypoxia with lasting damage, or cause death. There are 2 types of knots:
True knots – These are associated with thrombosis of placental surface veins and an increase in
still births (figure 7).
False Knots – These may be large, but are named poorly, as they are actually local redundancies
of umbilical vessels rather than knots (figure 8).
Figure 7: True Knot [1]
Abnormal cord insertion
Furcate cord Insertion – Occasionally a rare abnormality occurs in which the umbilical vessels separate from the cord substance prior to reaching the surface of the placenta, therefore losing the protection of the Wharton’s jelly. This greatly increases occurrences of thrombosis and injury.
Velementous cord insertion – This occurs when the insertion of the umbilical cord is in the membrane of the placenta rather than within the placenta, and may be close to the placenta itself or farther away at the apex of the membrane. It is common in multiple births, and those where IUD’s are found in placental membranes. Haemorrhages are the more frequent complication of these membranous vessels.
Cysts and Edema
Cysts in Whartons jelly are rare but occur, and are most obvious in edementous umbilical cords that have been associated with some cases of respiratory distress syndrome of newborns. Usually, cysts are not accompanied by fetal disease.
Edema is apparent in the markedly swollen, glistening umbilical cord.
Strictures
Strictures are significant reductions in the size of the umbilical cord and are not uncommon. They are found in long, heavy spiralled cords, and although fetal grasping or knots may cause them, the primary cause is a deficiency of Wharton’s jelly.
Hematomas
Umbilical cord hematomas are associated with 50% of fetal mortality. Some studies have showed that short cords, trauma, and entangling play a role in the formation of a hematoma, but the primary cause remains generally unknown[1].
Interesting Facts
* Wharton’s Jelly liquefies when touched[1]
* Leonardo da Vinci suggested that the length of the umbilical cord is about the same as the length of the baby, which has been supported by many current studies[1].
* A 2006 study found that infants who had their umbilical cord clamped immediately after birth had much lower iron levels later in life than those who had a two minute delay in the clamping of their umbilical cord. The delay increased iron stores by about 27-47mg, suggesting that delaying cord clamping could help prevent iron deficiency from developing early in life[2].
References
Baergen, Rebecca; Kurt Benirschke, Peter Kaufmann. (2006). Pathology of the human placenta, 5th edition. Springer New York. pp. 380-451 [1]
Chapero, C; L. Neufeld, G. Tena Alavez, R. Eguia-Liz Cedillo, K. Dewey. (2006). Effect of timing of umbilical cord clamping on iron status in mexican infants: a randomised controlled trial. Lancet. (9527) pp 1997-2004 [2]
Heath, John W.; James S. Lowe, Alan Stevens, Barbara Young. (2006). Wheater's functional histology: A text and colour atlas, 5th edition. Elsevier. p 385 [3]
Junqueira, Luiz Carlos; Jose Carneiro. (2005). Basic Histology: Text and atlas, 11th edition. McGraw-Hill companies. [4]
Meyer, David B; (1985). Laboratory guide for human histology, revised edition. Wayne State University Press. p 76 [5]
Zhang, Shu-Xin. (1999). An atlas of histology. Springer, New York, Berlin. pp 334-336[6]
