What are the aims of blastocyst implantation?
Anchor the placenta
Establish basic unit of exchange between the foetus and the mother (chorionic villus)
Establish maternal/fetal blood flow within the placenta
Describe the structure of the umbilical cord
The umbilical cord is a helical structure compose of 2 umbilical arteries carrying deoxygenated blood, and 1 umbilical vein carrying oxygenated blood.
The umbilical cord also contains Wharton's jelly which is a gelatinous substance made up of polysaccharides, fibroblasts and macrophages. It insulates and protects the umbilical vessels.
Describe the process of implantation of the conceptus into the endometrium
At implantation the blastocyst consists of an outer layer of flattened cells (trophoblast) which is destined to bceome the placenta and outer foetal membranes (chorion). The inner cell mass develops into the embryo.
The blastocyst becomes in the endometrium in the fundus of the uterus. The trophoblast differentiates into an outer syncytiotrophoblast and an inner cytotrophoblast.
The syncytiotrophoblast is phagocytic, and invades the endometrium (digests decidual cells, endothelial cells, endometrial glands, subepithelial connective tissue)
At day 12-14 the blastocyst is embeded within the endometriu, and the epithelium grows back over it. For the first month the conceptus is nourised by phagocytosis of the endometrium.
The decidual reaction transforms the functional layer of the endometrium into decidua (part of the placenta). Decidual cells secrete proteins which limit the invasiveness of the blastocyst during implantation.
What are the major substances transported across the placenta?
Describe the factors that influence the passive diffusion of substances across the placenta
Transport of substances across the placenta is determined by molecular weight, solubility and charge.
Substances are transported by:
simple diffusion (Gases, water electrolytes, urea)
facilitated diffusion (glucose)
active transport (amino acids, water soluble vitamins, iron)
pinocytosis (high MW compounds e.g. globulins, Igs, phospholipids)
Describe the structure of the placenta
The placenta is composed of chorionic villi that sprout from the chorion to provide a large contact area between foetal and maternal circulations.
The decidua of the endometrium lies above the myometrium, and contains endometrial vessels which transport blood to the placenta.
The umbilical cord is covered by a chorionic membrane and branches into chorionic villi.
The maternal and foetal vessels are separated by intervillous spaces where gaseous and nutrient exchange takes place.
State three functions of the placenta
Endocrine: Production of steroid and peptide hormones
Transfer: nutriton, waste, gas exchnage
Immunity: physical barrier between foetal and maternal circulation, transport of maternal IgG
State the function of the peptide hormones produced by the placenta
hCG: glycoproetin with high homology to LH. Binds to LH receptors on the corpus luteum to maintain hormone production. Levels peak at 8-10 weeks.
hPL: homologous to prolactin. Alters maternal metabolism to increase blood glucose levels by antagonising the effects of insulin. This increases availability of glucose for transport across the placenta (diabetogenic). Also increases proteolysis, and lipolysis to suppy a.acids to the foetus and FAs to the mother. Also promotes IGFs for foetal growth.
Name four hormones produced by the placenta
Describe the development of chorionic villi
The primary villus is formed by the syncytiotrophoblast which is penetrated by cords of the cytotrophoblast.
The secondary villus is penetrated by mesenchymal cells
The secondary villus is then penetrated by fetal vessels to form the tertiary villus.
Once the chorionic villi have formed, there is thinning of the placental barrier and margination of the foetal vessels.
The villi branch out, increasing the surface area to meet the expanding need of the developing foetus.
Describe the development of the placenta
The placenta orginates from the trophoblast of the conceptus and the uterine endometrium.
Once the embryo attaches to the endometrium, this triggers the decidual reaction at the endometrium. There is an ingrowth of capillaries, increase in capillary permeability. Stromal cells enlarge and become decidual cells.
The proliferating trophoblast develops layer of extraembryonic mesoderm on its inner surface that becomes the chorion. Chorionic villi project into the maternal lacunae surrounding the decidua. Blood enters the lacunae via spiral arteries at low pressure.
Angiogenesis occurs in the mesoderm of the chorionic villi and the newly formed vessels grow into the embryo as the two umbilical arteries and umbilical vein.
Describe the changes that occur in the placental barrier during pregnancy
Initially the embryonic capillaries are separated from the maternal circulation by a 40µm syncytiotrophoblast layer,. This reduces in thickness throughout pregnancy as fetal size and metabolic demands increase, and is 5µm in the later stage.
Describe how maternal-foetal circulation enables exchange across the placenta
The umbilical cord contains vessels which transport blood between the mother and foetus by entering the placenta.
The placenta is made up of chorionic villi and branch from the chorionic membrane. This forms an extensive capillary system which brings foetal and maternal blood into close contact. Under normal circumstances there is no mixing of maternal and foetal blood.
Maternal blood enters the intervillous space through spiral arteries from the endometrium and circulates around the villi, which allows exchange of gas and nutrients.
Deocygenated blood flows back from the intervillous space to the decidua and enters endometrial veins.
A condition where the placenta implants in the lower uterus or cervix. Often causes serious bleeding in the third trimester.
Complete placenta praevia covers the internal os of the cerix and requires delivery via C-section to prevent placental rupture and maternal haemorrhage during delivery.
Partial or complete absence of the decidua. The placental villous tissue adheres directly to the myometrium and the placenta fails to separate.
Significant cause of postpartum bleeding.
When there is partial or complete separation of the placenta from the uterus. Normally occurs in teh 3rd trimester
Can occur due to bleeding between the wall of the placenta (concealed abruption).
Separation of the placenta may cause vaginal bleeding (revealed)
Occurs when a non-viable fertilised egg implants into the uterus.
Trophoblastic cells behave abnormally resulting in a mass of cells that have swollen chorionic villi that form cysts.
Cluster of grapes appearance. Also known as gestational trophoblstic disease.
Blood tests show very high levels of hCG. Sometimes symptoms of hyperthyroidism due to effects of hCG
Name four substances that can be transported across the placenta which cause harm to the developing foetus
Maternal Rh+ antibodies
Drugs e.g. cocaine
Infectious agents e.g. rubella, L. monocytogenes
By the 3rd trimester the barrier between maternal and foetal blood is much thinner. How is this acheived?
1) Thinning of the syncytium
2) The presence of few cytotrophoblasts in each transverse section
3) Margination of the fetal capillaries
4) Reduction in villous diameter overall