BMS 150 Embryology 3 - Week 8 PDF

Embryology The Early Fetal-Maternal Circulation References: The Developing Human – Clinically-Oriented Embryology Editors: Moore, Persaud, Torchia Chapters 4 & 7 BMS 150 Week 8 Early Extraembryonic Structures - Review By the end of the second week the embryo (epiblast, prior to gastrulation) is surr...

Embryology The Early Fetal-Maternal Circulation References: The Developing Human – Clinically-Oriented Embryology Editors: Moore, Persaud, Torchia Chapters 4 & 7 BMS 150 Week 8 Early Extraembryonic Structures - Review By the end of the second week the embryo (epiblast, prior to gastrulation) is surrounded by: ▪ An amniotic cavity at the dorsal surface – the amniotic cavity becomes lined by amnioblasts Extraembryonic somatic mesoderm surrounds the amnioblasts and forms the connecting stalk ▪ The umbilical vesicle at the ventral surface – this will eventually become smaller and smaller as the embryo develops and undergoes folding The amniotic cavity and chorionic membranes get larger and larger ▪ The chorion – extraembryonic somatic mesoderm on the inside and trophoblastic cells on the outside Early Extraembryonic Structures - Review The trophoblast consists of the cytotrophoblast and the syncytiotrophoblast ▪ The syncytiotrophoblast invades the endometrial stroma and helps induce the apoptotic death of decidual cells These decidual cells release energy-rich glycogen and lipids that can freely diffuse to (and feed) the embryo ▪ The syncytiotrophoblasts secretes hCG, which allows the ovary to continue to secrete high levels of progesterone → Prevents the endometrium (and embryo) from being lost during menstruation ▪ The cytotrophoblast begins to form primary chorionic villi that project into the lacuna The Maternal-Fetal Circulation By the 4th week, the embryo and the chorion have grown large enough that diffusion of nutrients between embryo and endometrium are inadequate for metabolic demands ▪ A circulation that exchanges between the maternal and fetal tissues is necessary ▪ The embryonic heart begins to beat at day 21 – by that stage blood vessels and red blood cells are present in the fetus, and gas/nutrient/electrolyte/waste exchange between mother and embryo increases The maternal-fetal circulation depends on: ▪ A fetal cardiovascular system ▪ Structures within the endometrium that allow exchange with the fetal cardiovascular system Chorionic Villi Primary chorionic villi: present in week 2 ▪ only composed of cytotrophoblasts surrounded by a syncytiotrophoblastic shell Secondary chorionic villi: present in week 3 ▪ Extraembryonic mesenchymal core surrounded cytotrophoblastic and syncytiotrophoblastic shell Tertiary chorionic villi: present by the end of week 3 ▪ Blood vessels (capillaries) within the mesenchymal core ▪ Fetal blood can now exchange substances between the maternal lacuna through the membrane formed by the tertiary villus Secondary villus Tertiary villus Maternal-Fetal Circulation – Week 3 Day 16 Day 21 Chorionic villi As the villi develop, the cytotrophoblast proliferates and extends through the syncytiotrophoblast layer ▪ This forms the cytotrophoblastic shell The cytotrophoblastic shell anchors the chorionic sac and attaches it to the endometrium Anchoring villus – a villus that attaches to the endometrium via cytotrophoblastic extensions Diagram of full-term placenta, just prior to birth Development of Fetal Membranes The decidua is the endometrium in a pregnant woman – 3 separate areas: Decidua basalis – the decidua that forms the placenta ▪ Connected to the embryo by the umbilical cord (which develops from the connecting stalk) Decidua capsularis – the decidua that does not include the basalis, but is still associated with/covers the embryo Decidua parietalis – formed by the endometrium that is not part of the embryo ▪ Far from the site of implantation As the embryo develops, the villi within the capsularis deteriorate and the amniotic cavity grows larger and larger, eventually filling the uterus and contacting the decidua parietalis Development of Fetal Membranes Development of the fetal membranes, placenta, and amniotic cavity From weeks 5 22 Pregnant Uterus “Close-up” Approximate Age – End of Week 4 Smooth chorion = the chorionic membrane that is associated with the decidua capsularis ▪ The villi degenerate (don’t contact maternal blood) Maternal blood is found in the intervillous spaces ▪ These spaces are separated by septae Development of the Fetal Cardiovascular System As the extra-embryonic tissues develop vascular associations with the maternal decidua, the fetal cardiovascular system also develops Begins in the umbilical vesicle and neighbouring connecting stalk/chorion ▪ Arises from the extra-embryonic mesoderm, starts early in 3rd week Embryonic vessels develop from mesoderm about 2 days after the extraembryonic vessels develop Divided into vasculogenesis & angiogenesis ▪ Vasculogenesis → development of brand new blood vessels from mesoderm ▪ Angiogenesis → “sprouting” of blood vessels formed by vasculogenesis Connects blood vessels to each other Vasculogenesis & Angiogenesis Angioblasts derived from mesoderm develop in specialized regions known as blood islands ▪ The blood islands develop lumens and become blood vessels ▪ This is vasculogenesis The angioblasts also give rise to red blood cells within and outside the embryo ▪ The blood cells are derived from the inner lining of the new vessels (endothelium) ▪ Extra-embryonically – develop from the endothelium in the allantois and the umbilical vesicle ▪ Intra-embryonically – develop from the endothelium of the dorsal aorta (see later) Vasculogenesis and Angiogenesis Within the embryo, vasculogenesis → formation of angioblastic cords ▪ Occurs in the mesenchyme during gastrulation/neurulation ▪ The cords form a pair of endocardial heart tubes anterior to the prechordal/oropharyngeal membrane ▪ The caudal head fold brings the paired heart tubes caudally, towards the umbilical vesicle ▪ The heart tubes then fuse at the end of the 3rd week to form a primordial heart tube Angiogenesis links the separate cords/blood islands together to form a linked circulatory system that allows blood flow through the embryo and the developing placenta Early embryo – 3rd week Early Embryo – 3rd and 4th week Note that cranial folding brings the heart tube ventrally and caudally ▪ The intra-embryonic coelom near the heart tube develops into the pericardial cavity ▪ The paired heart tubes are connected with the extraembryonic vessels once the heart starts to beat at day 21 ▪ The red blood cells develop first in the extra-embryonic vessels Allantois, umbilical vesicle vessels ▪ By the 5th week RBCs arise from the dorsal aorta Embryonic cardiovascular system – early 4th week By day 21 or 22, the heart begins to beat ▪ Blood slowly flows through the embryonic circulation into tertiary villi Embryonic cardiovascular system – early 4th week Key vessels: Dorsal aorta – blood to the embryo and then connects to: ▪ umbilical artery ▪ cardinal veins via the dorsal intersegmental arteries The flow from dorsal aorta to cardinal veins feeds the intraembryonic tissues Embryonic cardiovascular system – early 4th week Key vessels: Vitelline artery and vein ▪ Blood flow through the umbilical vesicle Embryonic cardiovascular system – early 4th week Key vessels: Umbilical vein ▪ Returns blood from tertiary villi back to the embryonic heart Folding of the embryonic heart By the 4th week, the paired heart tubes have fused, resulting in a single “ventricle” and atrium As the inflow tract – the sinus venosus – bends in a counter-clockwise direction, it brings the atria superiorly and posteriorly ▪ Completed by end of 4th week The septae and valves take longer to develop