SECTION 3: THE FORMATION AND FUNCTIONS OF FETAL APPENDAGES

1. The placental

1.1 Placental composition

The Placenta is limited internally（fetal aspect）by the amniotic membrane and chorionic plate; externally （maternal aspect）by the basal plate and in between these two lies the intervillous space containing the stem villi with their branches, the space being filled with maternal blood.

Maternal and blood streams flow side by side, but in opposite direction. This counter flow facilitates material exchange between the mother and fetus. Fetal and maternal blood are separate and there is no admixture in this hemochorial placenta.

There are three layers existing between fetal blood and maternal blood—villous capillaries wall, mesoderm, cytotro-phoblast and syncytiotrophoblast, the arterial blood enters the space under pressure. Lateral dispersion occurs, after it reaches the chorionic plate. Villi help in mixing and slowing of the biood flow. Mild stirring effect by the villi pulsations help migration of the blood towards the basal plate and thence to the uterine veins. The maternal blood enters intervillous space through decidual spiral artery of the uterus, then flow towards the maternal body through spiral vein.

1.2 Gross anatomy of the placenta at term

The placenta, at term, is almost a circular disc with a diameter of 15-20cm and thickness of about 2.5cm at its centre. It is spongy and weighs about 450gm to 650gm.It presents two surfaces, fetal and maternal, and a peripheral margin. The maternal surface is divided into 20 somewhat convex polygonal areas known as lobes or cotyledons.

1.3 The functions of placenta

1.3.1 Exchange of Oxygen and Carbon Dioxide

It has long been recognized that the placenta serves as the fetal lung. In general, exchange of fetal Oxygen and Carbon Dioxide is accomplished by diffusion.

1.3.2 Nutrition supply

Glucose which is the principle source of energy is trans-ferred to the fetus by facilitated diffusion. Amino acids are transferred by active transport （energy requiring transport）. Triglycerides and fatty acids are directly transported from the mother to the fetus. Electrolytes and vitamins cross through the fatal membrane by active transport. Numerous enzymes are elaborated in the placenta.

1.3.3 Fetal metabolites such as urea, creatinine can be excreted by the substance exchange in the placenta, then dis-charged by maternal system.

1.3.4 Fetal Immunocompetence

Placenta can prevent some harmful substances in maternal blood reaching the fetal blood. Immune antibodies in maternal blood can through the placenta reach the fetal circulation. Thus they provide a short period of passive immunity for fetal survival. But different kinds of viruses, Drugs with small molecular weights which are harmful to the embryo and fetus can reach the fetus through the placenta. This may result in fetal anomalies and even death. Some microbes such as bacteria, toxoplasma, chlamydia, can't enter though the placenta directly. However a focus of infection firstly forms in the placenta. and breaks down the structure of chorionic villus in the placenta, and thus invades into the fetus.

1.3.5 Synthetic function

The human placenta can synthesizes an enormous amount of protein and peptide hormones.

Human Chorionic Gonadotropin （HCG）: HCG is produced almost solely in syncytiotrophoblasts, which is a glycoprotein with a high carbohydrate content. The hormone can be detected in maternal plasma or urine as early as 6 days following fertilizing, serum hCG level increases from the day of implantation and reaches peak levels at 8 to 10 weeks. There-after, the concentration declines slowly reaching a low level of 10-20IU/ml between 100-130 days, the levels remain constant throughout pregnancy, it can't be detected within two weeks after delivery if the placenta is discharged completely.

Clinical application: The detection of HCG by immunoradiometric assays in blood or urine after ten days of fertilization is almost always indicative of pregnancy. This is an extremely sensitive test of pregnancy.

Biological Functions of HCG: HCG prevents involution of the corpus luteum. The best-known biological function of hCG is the so-called rescue and maintenance of function of the corpus luteum--that is, continued progesterone production It is also known that hCG stimulates fetal testicular testosterone secretion. In the fetus, it acts as an LH surrogate to stimulate replication of Leydig cells and testosterone synthesis to promote male sexual differentiation.

Human Placental Lactogen （HPL）is produced only by the syncytiotrophoblast of the placenta. The hormone is chemically and immunologically similar to pituitary growth hormone and prolactin. Its secretion goes up gradually with the process of pregnancy, and reaches the peak at 34 to 36 weeks, It drops rapidly after delivery and can't be detected up to 7 hours.

Biological Functions of HPL: Prolonged maternal star-vation in the first half of pregnancy leads to an increase in the plasma concentration of hPL. Short term changes in plasma glucose or insulin, however, have relatively little effect on plasma hPL levels. In vitro studies of syncytiotrophoblast suggest that hPL synthesis is stimulated by insulin and insulin-like growth factor-1 and inhibited by PGE2 and PGF2α.

Estrogen: The corpus luteum secretes estrogen in the early pregnancy. Following the development of trophoblast from 10 weeks, estrogen is synthesized and secreted in increasing amount from the placenta, the concentration of estriol is thousand times higher in the third trimester pregnancy than non-pregnancy.

Progesterone: Before 6 weeks of pregnancy, the corpus luteum secretes progesterone. Following the development of trophoblast from 8 to 10 weeks, progesterone is synthesized and secreted in increasing amount from the placenta

Oxytocinase enzyme: It is produced by syncytiotrophoblast which inactivate oxytocin to maintain pregnancy process.

2. Fetal membrane

The composition of fetal membrane: It consists of chorionic villi and amnion. The outermost layer of fetal membrane is Chorionic villi, which degenerates and atrophies to be the chorion laeve. The innermost layer of fetal membrane is amnion, it lacks smooth muscle cells, nerves, lymphatics, and importantly, blood vessels.

The functions of fetal membrane: To keep the balance of amnionic fluid, and play a role in the steroid hormone metabolism and also contribute to the initiation of parturition.

3. Umbilical Cord

The cord at term normally has two arteries and one vein.

Its diameter is 0.8 to2.0 cm, with an average length of 55 cm and a range of 30 to 70 cm.

Umbilical cord function: Umbilical cord forms the only connecting link between the fetus and the maternal system. This plays an important role in gas exchange, nutrient supply and metabolites discharge. This blood flow is obstructed when umbilical cord is compressed. This could lead to fetal distress in uterus, and even fetus death.

4. Amnionic Fluid

4.1 Origin of amniotic fluid: As a transudate from the maternal serum across the fetal membranes or from maternal circulation in the placenta early in trimester. In mid-preg-nancy, the fetal urine is the main source of amniotic fluid. In late trimester of pregnancy, pulmonary fluid contributes a small proportion of the amnionic volume, and fluid filtering through the placenta accounts for the rest.

4.2 Volume and features of amniotic fluid volume

Amniotic fluid volume: Amniotic volume measures about 5-8ml at 8 weeks, 30ml at 10 weeks, 400ml at 20 weeks, and reaches its peak of 1000ml at 38 weeks. Therefore the amount diminishes, till at term it measures about 800ml.As the preg-nancy continues post term, further reduction occurs to the extent of 300ml

Physical features: the fluid is faintly alkaline with low specific gravity of 1.007 to 1.025. In the early pregnancy, it is colourless but near term, it becomes pale straw coloured due to the presence of exfoliated lanugo and epidermal cells from the fetal skin.

4.3 Functions of amniotic fluid

4.3.1 To protect the fetus. It acts as a shock absorber, protecting the fetus from possible extraneous injury. Maintains an even temperature. The fluid distends the amniotic sac and thereby allows for growth and free movement of the fetus and prevents adhesion between the fetal parts and amniotic sac. What's more, amniotic fluid also plays a role in supporting the fluid balance of fetus.

4.3.2 To protect the maternal: reducing the discomfort feltwhen the fetus moves, Amniotic fluid sac can expand the cervix and vagina in labor. Lubricates and washes the vagina after the rupture of membranes.

（冯祥 潘苗苗）