The Placenta

Question 1

Describe what happens to the endometrium within a few days of fertilization

Answer 1

• the trophoblasts begin to produce (hCG) ensuring that th endometrium will be receptive to the implanting embryo
• the endometrium increases in vascularity and undergoes a series of changes known as decidualization in preparation for implantation, hence it is known as the decidua during pregnancy

Question 2

Describe the different regions of the decidua

Answer 2

• decidua basalis
  - -> lies between the developing embryo and the stratum basalis of the uterus at the implantation site
• decidua capsularis
  - -> covers the developing embryo separating it from the uterine cavity
• decidua vera
  - -> lines the remainder of the uterine cavity

Question 3

Describe the immune adaptation that occurs to prevent rejection of the fetus

Answer 3

• the decidua is invaded by macrophages which become immunosuppressive
• adapted t-regulator cells become less effective as part of the specific hormonal response to antigens and the effect of natural killer cells is reduced so their cytotoxicity becomes impaired and they are less likely to destroy foreign cells
• microchimerism is the term for the presence of a small number of cells in one individual that originated in a different individual
• some fetal cells actively move into the mothers circulation, tissues and organs in the 1st trimester without triggering an immune response

Question 4

Describe the pre lacunar stage of implantation of the placenta

Answer 4

• seven days post conception the blastocyst makes contact with the decidua (apposition) and the process of placentation takes place
• the process of implantation is extremely aggressive
  - -> chemical mediators, prostaglandins and proteolytic enzymes are released by both the decidua and the trophoblasts and maternal connective tissue is invaded
  - -> nearby maternal blood vessels ensure there is optimum blood flow to the placenta
  - -> at this stage cytotrophoblasts form a double layer and further differentiate into various types of syncytiotrophoblasts
  - -> the supply of syncytiotrophoblasts is a result of continued mitotic proliferation of the cytotrophoblastic layer below

Question 5

Describe the lacunar stage of implantation of the placenta

Answer 5

• increasing numbers of synctiotrophoblasts surround the blastocyst and small lakes form within these cells known as lacunae
  
  • -> these lacunae will become the intervillous spaces between the villi and will be bathed in blood as maternal spiral arteries are eroded 10-12 weeks following conception
• the trophoblasts have a potent invasive capacity which if left unchecked would spread throughout the uterus
  
  • -> this potential is moderated by the decidua which secretes cytokines and protease inhibitors that modulate trophoblastic invasion
• the layer of Nitabusch is a collaginous layer between the endometrium and myometrium which assists in preventing invasion further than the decidua

Question 6

Describe the chorionic villous tree

Answer 6

• chorionic villi are finger-like projections of chorion surrounded by cytotrophoblastic and synctiotrophoblastic layers
• initially new blood vessels develop from pregentor cells within the chorionic villi (vasculogenesis), a small amount of oxygen promotes this
• further growth of these vessels (angiogenesis) produces a vascular network that connects with the blood vessels developed independently in the embryo via the umbilical arteries and vein through the connecting stalk
• the villi proliferate and branch out approx 3 weeks after fertilization
• over time the villi can differentiate and specialize, resulting in different functions
• villi are most profuse in the decidua basalis as the blood supply is the richest
• the part of the trophoblastic layer known as the chorion frondosum eventually becomes the placenta
• the villi under the decidua capsularis gradually degenerate due to lack of nutrition forming the chorion laeve which is the origin of the chorionic membrane
• the syncytiotrophoblasts erode the walls of maternal vessels as they penetrate the lower myometrium opening them up into a funnel shape forming a lake of blood in which the villi float
• the maternal blood circulates enabling the villi to absorb nutrients and oxygen and to excrete waste (nutritive villi)
• a few villi are more deeply attached to the decidua and are called anchoring villi

Question 7

What happens to the decidua capsularis as the fetus enlarges?

Answer 7

• it is pushed towards the decidua Vera on the opposite wall of the uterus until at around 27 weeks gestation it disappears

Question 8

Describe the structure of chorionic villus

Answer 8

• each chorionic villus is a branching structure like a tree arising from one stem
• it’s centre consists of mesoderm and fetal blood vessels as well as branches of the umbilical artery and vein
• these are covered by a single layer of cytotrophoblast cells and the external layer of the villus is synctiotrophoblast
• this means that 4 layers of tissue separate the maternal blood from the fetal blood making it impossible for the two circulations to mix unless any villi are damaged

Question 9

Describe the placenta at term

Answer 9

• at term the placenta is discoid in shape about 20cm in diameter, 2.5cm thick at its centre
• the maternal surface (basal plate) is dark red in colour due to maternal blood and partial separation of the basal decidua
  
  • -> the surface is arranged in up to 40 lobes which are separated by furrows into which the decidua dips down to form walls
  • -> the lobes are made up of lobules each of which contains a single villus with its branches
• the fetal surface (chorionic plate) has a shiny appearance due to the amnion covering it
  - -> branches of the umbilical vein and arteries are visible, spreading out from the insertion of the umbilical cord which is normally in the centre
  - -> the amnion can be peeled off the surface of the chorion as far back as the umbilical cord whereas the chorion being derived from the same trophoblastic layer as the placenta cannot be separated from it

Question 10

Describe the storage function of the placenta

Answer 10

• the placenta metabolizes glucose and stores it in the form of glycogen and reconverts it to glucose as required
• it can also store iron and fat-soluble vitamins

Question 11

Describe the endocrine function of the placenta (steroid hormones)

Answer 11

• oestrogen
  
  • -> both maternal and fetal adrenal production provide precursors for oestrogen production by the placenta
  • -> pregnalone sulphate is converted to oestriol by the veto-placental unit from 6-12 weeks onwards, rising steadily until term
  • -> oestrogens influence uterine blood flow, enchanted RNA and protein synthesis, and aid growth of uterine muscle
  • -> they also increase the size and motility of the nipple and cause alveolar and duct development of the breast tissue
• progesterone
  
  • -> production is maintained by the corpus luteum for approx 8 weeks until the placenta takes over this function and is dependent on maternal cholesterol stores
  • -> it is produced by the syncytial layer of the placenta in increasing quantities until immediately before the onset of labour when it’s level falls
  • -> it maintains the myometrium in a quiescent state during pregnancy
  • -> it is involved in preparing breast tissue during pregnancy and when levels reduce after birth of the placenta, prolactin stimulates lactation

Question 12

Describe the endocrine function of the placenta (protein hormones)

Answer 12

• human chorionic gonadotrophin (hCG)
  - -> is produced under the influence of placental gonadotrophic releasing hormone (GnRH) by the trophoblasts
  - -> initially it is present in very large quantities with peak levels being achieved by the 7th and 10th week but these reduce as the pregnancy advances
  - -> the function of hCG is to stimulate the corpus luteum to produce mainly progesterone
  - -> it also increases fetal leydig cells to affect male sexual development prior to fetal luteinizing hormone production
• human placental lactogen (hPL)
  - -> stimulates somatic growth but also stimulates proliferation of breast tissue in preparation for lactation
  - -> in early pregnancy hPL stimulates food intake, weight gain mobilizing free fatty acids and functions with prolactin to increase circulating insulin levels
  - -> can be used as a screening tool in pregnancy to asses placental function
• human placental growth hormone (hPGH)
  - -> levels rise throughout pregnancy
  - -> it is involved with hPL as a determinant of insulin resistance in late pregnancy
  - -> it mobilizes maternal glucose for transfer to the fetus and contributes to lipolysis, lactogenesis and fetal growth
• there are also other factors, insulin growth factor (IGF) and vascular endothelial growth factor (VEGF) playing a variety of roles in metabolism, growth, vasculogenesis and regulation of utero-placental blood flow

Question 13

Describe the respiration function of the placenta

Answer 13

• gaseous exchange to and from the fetus as a result of diffusion
• transfer of gases is assisted by a slight maternal respiratory alkalosis in pregnancy
• the fetal haemoglobin level is high in utero to facilitate transport of gases
• the fetal haemoglobin also has a high affinity for oxygen

Question 14

Describe the protection function of the placenta

Answer 14

• the placenta provides a limited barrier to infection
  - -> few bacteria can penetrate with the exception of treponema of syphils and the tubercle bacillus
  - -> however many viruses can penetrate the placental barrier such as HIV, hepatitis strains, parvo virus, CMV and rubella
  - -> some parasitic and protozoal diseases such as malaria and toxoplasmosis will cross the placenta
• the placenta filters substances of a high molecular weight and therefore some drugs and medicines may transfer to the fetus
  - -> many will be harmless, some positively beneficial and other substances such as alcohol and chemicals associated with smoking cigarettes and recreational drug use are not filtered out
  - -> these substances can cross the placental barrier freely and may cause congenital abnormalities and subsequent problems for the baby
• immunoglobulins will be passed from mother to fetus transplacentally in late pregnancy
  - -> this provides about 6-12 weeks naturally acquired passive immunity to the baby

Question 15

Describe the excretion function of the placenta

Answer 15

• the main substance excreted from the fetus is carbon dioxide
• bilirubin will also be excreted as red blood cells are replaced relatively frequently
• there is very little tissue breakdown apart from this and the amounts of urea and uric acid excreted are very small

Question 16

Describe the nutrition function of the placenta

Answer 16

• the fetus requires nutrients for its ongoing development such as amino acids and glucose (required for growth and energy) calcium and phosphorous (bones and teeth) and iron and other minerals for blood formation
• these nutrients are actively transferred from the maternal to the fetal blood through the walls of the villi
• the placenta is able to select those substances required by the fetus even depleting the mothers own supply in some cases
• water, vitamins and minerals also pass to the fetus
• fats and fat soluble vitamins (A, D and E) cross the placenta only with difficulty and mainly in the later stages of pregnancy

Question 17

What are the different transport mechanisms used to transfer substances to and from the fetus?

Answer 17

• diffusion (gases and lipid soluble substances)
• osmosis (water-soluble substances)
• facilitated diffusion (glucose using carrier proteins)
• active transport (against concentration gradients of ions, calcium and phosphorus)
• endocytosis (pinocytosis of macromolecules)

Question 18

Describe placental circulation

Answer 18

• invading trophoblasts modify maternal spiral arteries to accommodate an increase in blood flow where there is open circulation around the chorionic villi
• maternal blood (oxygenated) is discharged in a pulsatile fashion into the intervillous space by 80-100 spiral arteries in the decidua basalis after 10-12 weeks gestation
• the blood flows slowly around the villi eventually returning to the endometrial veins and the maternal circulation
• there are about 150ml of maternal blood in the intervillous spaces which is exchanged 3/4 times per minute
• fetal blood (deoxygenated- low in oxygen) is pumped by the fetal heart towards the placenta along the umbilical arteries and transported along their branches to the capillaries of the chorionic villi where exchange of nutrients takes place between mother and fetus
• having yielded carbon dioxide and waste products and absorbed oxygen and nutrients, the blood is returned to the fetus via the umbilical vein

Question 19

Describe the placental membranes

Answer 19

• the basal and chorionic plates come together and meet at the edges to form the chorioamnion membrane where the amniotic fluid is contained, it is composed of two membranes
• amnion
  - -> Inner membrane derived from the inner cell mass and consists of a single layer of epithelium with a connective tissue base
  - -> it is a tough, smooth and translucent membrane, continuous with the outer surface of the umbilical cord which moves over the chorion aided by mucous
  - -> it contains amniotic fluid which it produces in small quantities as well as prostaglandin E2 (PGE2) which plays a role in the initiation of labour
• chorion
  - -> outer membrane that is continues with the edge of the placenta, it is composed of mesenchyme, cytotrophoblasts and vessels from the extended spinal arteries of the decidua basalis
  - -> it is a rough, thick, fibrous, opaque membrane which lines the decidua vera during pregnancy, although loosely attached
  - -> it produces enzymes the can reduce progesterone levels and also produces prostaglandins, oxytocin and platelet-activating factor which stimulate uterine activity

Question 20

What is amniotic fluid?

Answer 20

• it is a clear alkaline and sightly yellowish Iiquid contained within the amniotic sac
• it is derived from the maternal circulation across the placental membranes and exuded from the fetal surface
• the fetus contributes to the amniotic fluid through metabolism in small quantities of urine and fluid from its lungs
• the fluid is returned to the fetus by intramembranous flow across the amnion into the fetal vessels and through the mechanism of the fetus swallowing

Question 21

Describe the function of the amniotic fluid

Answer 21

• it distends the amniotic sac allowing for growth and free movement of the fetus and permitting symmetrical musculoskeletal development
• it equalises pressure and protects the fetus from jarring and injury
• it maintains a constant intrauterine temperature protecting the fetus from heat loss and providing it with small quantities of nutrients
• in labour when membranes are intact it protects the placenta and umbilical cord from the pressure of uterine contraction
• it also aids effacement of the cervix and dilatation of the uterine os, especially when pp is poorly applied

Question 22

What are the constituents of amniotic fluid?

Answer 22

• 99% water an 1% dissolved solid matter including food substances and waste products
• the fetus also sheds skin cells (vernix caseosa) and lanugo into the fluid
• volume increase from 20ml at 10 weeks to approx 500ml at term

Question 23

What is the umbilical cord?

Answer 23

• it extends from the fetal surface of the placenta to the umbilical area of the fetus
• it is formed by the 5th week of pregnancy and originates from the duct that forms between the amniotic sac and the yolk sac which transmits the umbilical blood vessels

Question 24

What is the function of the umbilical cord?

Answer 24

• to transport oxygen and nutrients to the developing fetus and remove waste products

Question 25

Describe the structure of the umbilical cord

Answer 25

• it contains two arteries and one vein which are continuous with the blood vessels in the chorionic villi of the placenta
• the blood vessels are enclosed and protected by wharton’s jelly
• the whole cord is covered in a layer of amnion that is continuous with that covering the placent

Question 26

Describe the anatomical variations of the placenta and cord

Answer 26

• succenturiate lobe -> a small extra lobe is present that is separate from the main placenta and is joined to it by blood vessels that run through the membranes to connect it
• circumvallate placenta -> opaque ring is seen on fetal surface of placenta, it is formed by a doubling back of the fetal membrane onto the fetal surface of the placenta and may result in membranes leaving the placenta near the centre instead of at the edge
• bipartite placenta -> two complete and separate lobes where the main cord bifurcates to supply both parts, tripartite has 3 parts
• battledore insertion -> cord is attached to very edge of placenta
• vellamentous insertion -> cord is inserted into the membranes some distance away form the edge of the placenta, the umbilical vessels run through the membranes from the cord to the placenta