The Umbilical Cord
Development of the Placenta
The umbilical cord extends from the fetal umbilicus to the placenta and conveys fetal blood to and from the placenta. This is formed by an outer covering of amniotic ectoderm, containing the vitello-intestinal duct and the yolk sac or umbilical vesicle. The umbilical cord incorporates within itself the body stalk and its contained umbilical vessels (two umbilical arteries and one umbilical vein) and the allantois. It is spirally twisted and it increases in length so that at the end of pregnancy it is about 50 cm long. On cross-section, the cord shows the covering of amniotic epithelium, generally two arteries and one vein and the remains of the allantois with the whartonian jelly which binds everything together.
After the embedding of the fertilised ovum, a mass of syncytiotrophoblast (the trophoblastic shell) - develops around the periphery. At first these cells simply digest the endometrial and stromal cells. But the ovum is in urgent need of oxygen, and soon the maternal vessels and thereafter the larger spiral arterioles supply blood to the ovum, thus bathing it in a lake of maternal blood. This ensures a supply of oxygen and food. The chorionic villi attach the fetus to the uterus. Quite soon, small buds of syncytiotrophoblast can be seen on the trophoblastic shell. These buds grow at a truly remarkable pace, surrounding the whole ovum with a mass of delicate villous structures in a short span. At first these are composed of the syncytiotrophoblast, then there appears a core of discrete, pale cells called the cytotrophoblast or Langhan's layer. About the same time the mesoderm is growing into the villus and the afferent and efferent vessels are being developed in situ. The villus is now mature. Most of the chorionic villi which branch and rebranch in a complicated manner float freely in the maternal blood lake and expose a very large area to the blood circulating there; these are called nutritive villi. The vessels in the villi join with each other until they are collected in the body stalk to form the umbilical vessels. Other villi are attached directly to the decidua, particularly in the region of the decidua basalis. Their main function is to ensure anatomical fixation of the growing ovum. The trophoblast from these anchoring villi spread over the surface of the decidua basalis, so that the whole of the maternal blood space is lined with trophoblastic cells. The villar surface of the syncytiotrophoblast is bathed directly by maternal blood, but fetal blood is contained within fetal capillaries in the villi of the placenta. This type of placenta is called hemochorio-endothelial placent. Fetal blood is separated from the syncytiotrophoblast by the wall of the fetal capillaries, the mesenchyme in the villous space and the cytotrophoblasts. Thus fetal and maternal blood do not come into direct contact.
THE STRUCTURE OF A VILLUS
Villi can be seen in the human placenta as early as 12 days after fertilisation. The trophoblast, undergoes rapid proliferation and forms numerous processes which are known as the primary chorionic villi on the surface of the chorion. These increase in size and ramify, and the chorionic mesenchyme invades the solid trophoblastic column forming the secondary villi. Branches of the umbilical vessels grow into these villi and in this way they are converted into tertiary chorionic villi.
The young villus is a delicate structure covered by a layer of syncytiotrophoblast which is sometimes condensed into deeply staining knots at its free tip. Beneath this is the cytotrophoblast composed of discrete cubical cells . At first this is two layers thick, then a single layer thick, and in the mature villus it disappears entirely. The mesodermal core of the young villus is comparatively thick but is very thin in the mature organ. The capillaries of the villus are thin walled structures with no special features. Hence the metabolites must pass through these several layers on their way to and from the fetal bloodstream. In the mature or ageing villus, certain stigmata of age are apparent. The syncytium may be represented by an extremely thin film, and at a few places may be replaced by little clots of fibrin the fibrin nodes. The Langhan's layer has disappeared and the mesodermal core is greatly attenuated.
The Placenta at Term
This connects the fetus to the uterine wall and is the organ by means of which the nutritive, respiratory and excretory functions of the fetus are carried on.
At term the human placenta is a discoid organ measuring 15-20 cm in diameter, 2-3 cm in thickness and weighing about 500 g. Usually it is implanted on the posterior or anterior surface of the uterine cavity, well towards the fundus. On examination of the maternal surface of the expelled placenta, one can see that it is divided by the depressed areas of varying depths into irregularly shaped lobes known as cotyledons. In the placenta, there are two circulations - fetal and maternal. The cotyledons are filled with loose villi attached only to the major villous stalks. The number of cotyledons varies from 10 to 38. The depressed areas are the bases of the placental septa which are mostly made up of fibrous tissue. The septa, which are maternal in origin, extend only to about three quarters of the way to the chorionic plate; very few functional vessels are seen in it at term.
The average increase in weight of the fetal placenta is proportional to the term from 12 to 30 weeks of pregnancy. Thereafter, to term, the weight increase is small and there is no further rise in the postmature period. At term, about 150 ml of blood may be transferred from the placenta to the infant if there is a delay of ten minutes before the cord is tied.
The Functions of the Placenta
THE TRANSFER OF IMMUNITY The transfer of immunity to certain diseases furnishes an excellent example of the species differences in the placental barrier between the maternal blood and fetal blood. In humans, immune bodies are transferred from mother to fetus through the chorionic villi.
TRANSFER OF ESSENTIAL SUBSTANCESSubstances required for the maintaining fetal life and eliminating its waste products are handled largely by diffusion across the placental barrier. Oxygen, carbon dioxide, water, electrolytes and urea are included in this group.
OXYGEN ABSORPTION FROM PLACENTAL SOURCESAt first, the blood which flows through the placental site is only slightly altered by its contact with the villi, but later, when the growth of the fetus is accelerated, marked changes take place. Due to the continuous passage of oxygen from the maternal blood in the intervillous space to the fetus, the oxygen saturation of this blood resembles that in the maternal capillaries and is less than that of the mother's arterial blood. The average oxygen saturation of the intervillous space blood is estimated to be 65-75 per cent with a partial pressure of oxygen of about 30-40 mm Hg. The oxygen saturation of the umbilical vein blood is approximately 60 per cent with a partial pressure of oxygen of about 20 mm Hg. These data should be regarded only as approximations. Despite the relatively low partial pressure of oxygen, the fetus normally does not suffer from lack of oxygen. The cardiac output in a human fetus in late pregnancy is considerably greater (per unit of weight) than that in an adult. This, along with the increased oxygen carrying capacity of the fetal blood due to a higher hemoglobin concentration, compensates effectively for the low oxygen tension.
ENZYMATIC PROCESSES Enzymatic processes supply many fetal nutritional needs. Glucose, aminoacids, calcium, phosphorus, iron and probably some of the vitamins cross the placental barrier by this means.
CARBOHYDRATE METABOLISM In the early months of pregnancy, glycogen is stored in the placenta until the liver and pancreas are sufficiently developed to take over the function of storing and utilising this carbohydrate. As it contains glycolytic enzymes, the placenta too is able to convert its store of glycogen into absorbable glucose. Most of the fetal carbohydrate is absorbed from the mother's blood in the form of glucose which passes readily through the placenta.
VITAMINS The water-soluble vitamins B, and C pass through the placenta readily and the latter is retained in excess in the fetal blood. Riboflavin is transmitted by the absorption and degradation of its dinucleotide precursor. The fat soluble vitamins A, E and K are also probably transmitted through the placenta but definite evidence concerning the permeability of the placenta to vitamin D is lacking.
HORMONES The placenta produces many steroid and protein hormones. It produces large amounts of estrogens and progesterone. It also produces large amounts of human chorionic gonadotrophin (hCG), human placental lactogen (hPL), chorionic adrenocorticotrophin (ACTH) as well as other products of propiomelanocortin, chorionic thyrotrophin, growth hormone variant, parathyroid related protein (PTHrP), calcitonin and relaxin; hypothalamic-like releasing and inhibiting hormones, including thyrotropin releasing hormone (TRH), gonadotrophin releasing hormone (GnRH), corticotropin releasing hormone (CRH), somatostatin and growth hormone releasing hormone (GHRH). The human placenta also produces inhibins, activins and atrial natriuretic peptide.