Embryology & Physiology: Development of the Placenta PDF

Last edited: 9/2/2021 7. DEVELOPMENT OF THE PLACENTA Embryology | Development of the Placenta OUTLINE I) PRE-IMPLANTATION II) IMPLANTATION III) POST-IMPLANTATION IV) MACROANATOMY V) FUNCTIONS VI) APPENDIX VII) REVIEW QUESTIONS VIII) REFRENCES I) PRE-IMPLANTATION (1) In the ampulla of the fallopian t...

Last edited: 9/2/2021 7. DEVELOPMENT OF THE PLACENTA Embryology | Development of the Placenta OUTLINE I) PRE-IMPLANTATION II) IMPLANTATION III) POST-IMPLANTATION IV) MACROANATOMY V) FUNCTIONS VI) APPENDIX VII) REVIEW QUESTIONS VIII) REFRENCES I) PRE-IMPLANTATION (1) In the ampulla of the fallopian tube Medical Editor: Ilia-Presiyan Georgiev (3) The morula turns into a blastocyst Lots of water starts flowing into the morula o Forms a water-filled cavity - blastocoel The blastocyst consists of o Blastocoel o Outer cell mass  Will form the trophoblast Will form the placenta o Inner cell mass  Will form the embryoblast Will form the endoderm, mesoderm and ectoderm that will form the entire structure of the fetus there are sperm and secondary oocytes in metaphase 2 Their fusion forms the zygote o Has a lining – zona pelucida (2) The zygote divides (cleaves) into 2 cells o The two-cell stage o The cells are surrounded by the zona pelucida It cleaves again into 4 cells o The four-cell stage o The cells are surrounded by the zona pelucida It cleaves again into 8 cells o The eight-cell stage o The cells are surrounded by the zona pelucida It cleaves again into 16 cells o The sixteen-cell stage o Forms a hollow ball of cells – the morula o The cells are surrounded by the zona pelucida Figure 2 A blastocyst. II) IMPLANTATION (1) The uterus lining consists of o Endometrium o Myometrium o Perimetrium (2) The endometrium is divided into two: Basal part o Called decidua basalis o Made up of stratum basalis cells undergone decidualization  Bigger cells  Filled with glycogen  Filled with lipids  More sustainable for fertilization Apical part o Called decidua functionalis o Made of the replication and proliferation of decidua basalis (3) The blastocyst Figure 1 Cleavage of the zygote. DEVELOPMENT OF THE PLACENTA normally attaches to and invades the decidua functionalis Invading other layers of the uterine lining leads to complications o Usually require the uterus to be taken out after giving birth  Invading the decidua basalis - placenta acreta  Invading the myometrium - placenta increta  Invading the perimetrium - placenta percreta EMBRYOLOGY: Note #1. 1 of 6 Forms two types of attachment with the endometrium (i) Loose attachment:  Microvilli On the trophoblastic cells  Pinopodes Protrude from the decidua functionalis Prevents the corpus luteum from becoming corpus albicans and stimulates it to continue to produce progesterone The uterine blood vessels are dependent on progesterone If the concentration of progesterone decreases → the vessels spasm → ischemia of the surrounding tissue → rupturing and sloughing off of the endometrium lining (2) The embryoblast Figure 3 Loose attachment. differentiates into: o Bilaminar disc  Epiblast  Hypoblast o Amniotic cavity  Above the epiblast o Primary yolk sac  Bellow the hypoblast (ii) Tight attachment  Integrines Molecules expressed by the trophoblastic cells  Selectins Contained in carbohydrates on the decidua functionalis Sometimes covered with collagen  Chemokines Released by the trophoblast Stabilize the strong connection Figure 5 Week 1 development. (B) DAY 9 (1) The syncytiotrophoblast continues to invade trough the stromal tissue o Forms spaces of stromal tissue between its projections - lacunae Figure 4 Tight attachment. These attachments allow the blastocyst to start invading the stroma of the endometrium and complete the implantation Releases proteolytic enzymes o Break down the blood vessels lining o Allow the blood to seep in the lacunae In result the lacunar spaces are filled with blood and become intervillous spaces III) OST-IMPLANTATION (A) WEEK 1 (1) The trophoblast differentiates into o Cytotrophoblast  The cytotrophoblastic cells proliferate rapidly  Their membranes break down  The cytoplasm and the nuclei fuse with those of other cells  Form a big pool of protoplasm and numerous nuclei without cell borders - the syncytiotrophoblast o Syncytiotrophoblast  Releases hydrolytic enzymes Allow it to break the stromal tissue of the endometrium and invade it  Secretes Human Chorionic Gonadotropin hormone (hCG) 2 of 6 EMBRYOLOGY: Note #7. Figure 6 Day 9 Development. DEVELOPMENT OF THE PLACENTA (C) DAY 12 (1) The intervillous spaces increase in number and fill with more blood (2) The hypoblast (and the primary yolk sac, according to some sources) will start producing connective tissue - extraembryonic mesoderm Under the cytotrophoblast and above the amniotic cavity/primary yolk sac Later will form the chorion of the placenta o The fetal part of the placenta (E) WEEK 3 (1) The somatopleuric extraembryonic mesoderm invades the primary villi made by cytotrophoblastic cells o Forms a core inside them o The villi are still surrounded by the syncytiotrophoblast These villi are called secondary chorionic villi o Can be anchored and floating Figure 9 Week 3 development. (F) WEEK 4 Figure 7 Day 12 development. (D) DAY 14 (1) The extraembryonic mesoderm starts breaking down forming: o The extraembryonic coelom – an empty space  Between two layers of extraembryonic mesoderm  It will become the chorionic cavity o The somatopleuric extraembryonic mesoderm  The layer towards the trophoblast o The splanchnopleuric extra embryonic mesoderm  The layer towards the amniotic cavity/the primary yolk sac o The connecting stalk – connects the two layers (2) The cytotrophoblastic cells proliferate and form primary chorionic villi that: o Penetrate the syncytiotrophoblast o Move out and surround the intervillous spaces o Form the outer cytotrophoblastic shell there is already an embryonic folding The mesodermal cells start differentiating o Form capillaries The chorionic plate (a thick part of the chorion) comes off the chorionic villi o Forms the chorionic frondosum The connecting stalk o Forms the umbilical cord o Connects the chorionic plate with the embryonic folding (1) In the umbilical cord the mesodermal cells form three blood vessels o One umbilical vein o Two umbilical arteries  Move out into the chorionic plate and connect to the chorionic arteries  The chorionic arteries move up the chorionic villi and eventually become cotyledonal arteries  The villi will become tertiary chorionic villas by week 4 Figure 10 Week 4 development. (2) There is an exchange Figure 8 Day 14 development. between the tertiary chorionic villas and the maternal blood in the intervillous spaces o From the fetus to the mother  CO2 and breakdown products o From the mother to the fetus  Oxygen, nutrients, even pathogens This remains the exchange system until week 20 DEVELOPMENT OF THE PLACENTA EMBRYOLOGY: Note #1. 3 of 6 (G) WEEK 4-8 IV) MACROANATOMY (1) The tertiary villi Branch out o Still completely covered by the cytotrophoblast and the syncytiotrophoblast Purpose - to increase the surface area for exchange (1) The fetus is located inside the uterus It is connected to the uterine lining with the umbilical cord o Has extraembryonic mesoderm (2) The amniotic membrane is the layer closest to the fetus (3) The chorion is the next layer o Chorionic fondosum – formed from the continuation of the extraembryonic mesoderm in the umbilical cord  The collective name of all chorionic villi o Chorionic leave - a thin, flat membrane  The part of the chorion facing the uterine cavity  Has no extensive villi system (4) The decidua Figure 11 Week 4-8 development. (H) WEEK 16-20 (1) Tissue from the decidual membrane starts to branch in o The branches are called placental septa o They separate the tertiary villas into septations cotyledons  Around 15-20 cotyledons  Each consist of 2-3 tertiary villi The decidual tissue in them swells up and fills up with glycogen, lipids, etc. is the last layer o Decidua basalis  Interacts with the chorionic frondosum  Both of them collectively form the placenta o Decidua capsularis  The part of the decidua facing the uterine cavity o Decidua parietalis  The decidua where there is no fetal involvement Eventually, as the fetus grows, the decidua capsularis will merge with the decidua parietalis and obliterate the entire uterine cavity Figure 14 Macroanatomy. Figure 12 Week 16-20 development. (I) WEEK 20 V) FUNCTIONS (1) The cytotrophoblast regresses o Makes the exchange of substances more quick and more efficient (A) METABOLIC FUNCTIONS (1) Gas exchange is the primary metabolic function o From the mother to the fetus  Oxygen o From the fetus to the mother  CO2 (2) Nutrient delivery Figure 13 Week 20 development. Ways of delivery o Simple diffusion o Facilitated diffusion o Active transport  Different types Types of nutrients o Glucose o Amino acids o Fatty acids o Water soluble vitamins  B vitamins 4 of 6 EMBRYOLOGY: Note #7. DEVELOPMENT OF THE PLACENTA (3) Natural passive immunity is conferred by IgG antibodies The mother has already made antibodies against different foreign antigens o Transfers them to the fetus through the placenta (4) Hemolytic disease of the fetus and newborn A condition also known as Rhesus disease A Rh negative mother that have had previously a Rh positive baby (or a spontaneous abortion of a Rh positive baby) has created IgG antibodies against the Rh (Duffy) antigen o The antibodies then destroy the blood cells that are Rh positive There is a medication called Rhogam o An anti IgG antibody erected against the Rh antigens o Given to the mother so the immunological reaction is blocked  The immunological reaction takes place when the placenta breaks away during the third phase of delivery Some of the blood that is Rh positive can go into the mother’s circulation If the mother is already alloimmunized o The baby’s middle cerebral artery blood flow is checked  Done through a Doppler  Velocity is calculated based upon fetal anemia (5) Waste removal of a lot of different types of waste products o Urea o Uric acid, etc. (B) PATHOGENS (1) Different pathogens can be transported across the placenta Most important is the TORCH series o Toxoplasmosmosis  Caused by toxoplasma gondii  Causes significant fetal defects and damage  Treatment Spiramycin if the fetus has not been affected yet Combination of Pyrimethamine and Sulfadiazine if the fetus has already been affected o Others  HIV  Syphilis  Hepatitis (especially hepatitis B) o Rubella  Can cause a lot of cardiac and congenital defects as well as hearing loss. o Cytomegalovirus (CMV) o Herpes simplex virus type 2 Another danger that people should be aware nowadays is the Zika virus as it can be transmitted vertically as well Bacteria like Listeria monocytogenes o Comes from pasteurized products and lunch meats Viruses are the biggest threat because of their ability to easily cross the placental membrane. (C) HORMONAL FUNCTIONS (1) Estrogen and progesterone Generally produced around 10th -12th week Take over for the corpus luteum secretion during the rest pregnancy Make the endometrial lining thicken up o Increasing the vasculature o Provide a nutritive environment Increase secretion o Plugs up the cervix with a mucus plug o Prevent external environment factors from harming the fetus Play a crucial role in the development of the fetus (2) Thyroid hormone Promotes the development of the central nervous system Lack of thyroid hormone can lead to cretinism o Incomplete development of the CNS o Can lead to mental retardation Figure 15 Metabolic functions of the placenta. (3) Human placental lactogen Decreases the insulin release from the pancreas Acts on the mother’s cells o Promotes lipolysis  Provides fatty acids for the baby o Promotes gluconeogenesis  Provides glucose for the baby o Increases the cells insulin resistance  So it can’t shuttle the glucose into the mom’s cells (4) Relaxin Relaxes (increases the laxity of) specific ligaments of the pubic symphysis o The pelvic inlet and outlet are widened out o Ensure easier passage of the baby DEVELOPMENT OF THE PLACENTA EMBRYOLOGY: Note #1. 5 of 6 (5) Corticotrophin-releasing hormone In addition to the mother’s pituitary gland Stimulates adrenocorticotropic hormone (ACTH) production o Stimulates cortisol production Cortisol plays an important role in lung development and surfactant production If the baby is born prematurely (before 34 week) there is not enough cortisol → not enough surfactant → the alveoli of the baby will be collapsed → it is almost impossible for the baby to get enough strength through the diaphragm and the intercostal muscles to pull air in and pop the alveoli open This leads to infant respiratory distress syndrome VI) REVIEW QUESTIONS What is the name of the ball of cells formed in the 16-cell stage of cleaving? a. Zygote b. Morula c. Blastocyst d. Blastocoel Which is a part of the blastocyst? a. Blastocoel b. Trophoblast c. Embryoblast d. Primary yolk sac Which does not take part in forming a tight attachment? a. Integrines b. Selectins c. Chemokines d. Microvilli Which hormone does the syncytiotrophoblast secrete? a. Progesterone b. Adrenocorticotropic hormone c. Human Chorionic Gonadotropin hormone d. Estrogen Figure 16 Hormonal functions of the placenta. Which week do primary chorionic villi develop? a. 1 b. 2 c. 3 d. 4 When do cotyledons form? a. Week 4 b. Week 4-8 c. Week 16-20 d. Week 20 Which layer of the placenta regresses during week 20? a. Chorion b. Syncytiotrophoblast c. Amniotic membrane d. Cytotrophoblast Which part of the decidua has no fetal involvement? a. Decidua basalis b. Decidua functionalis c. Decidua capsularis d. Decidua parietalis Which medication is not given for toxoplasmosis? a. Rhogam b. Spiramycin c. Pyrimethamine d. Sulfadiazine Which hormone is not produced by the placenta a. Thyroid hormone b. Human placental lactogen c. Human Chorionic Gonadotropin hormone d. Corticotrophin-releasing hormone CHECK YOUR ANSWERS 6 of 6 EMBRYOLOGY: Note #7. DEVELOPMENT OF THE PLACENTA