Dental embryology: Third Week of Development, Slides of Dental Anatomy

Download Dental embryology: Third Week of Development and more Slides Dental Anatomy in PDF only on Docsity! Third Week of Development: Trilaminar Germ Disc Maya Kiladze, PhD General Embryology - Detailed Animation On Gastrulation.mp4 Gastrulation (the third week of development), begins with the appearance of the primitive streak, which has at its cephalic end the primitive node. Through the node and streak, epiblast cells move inward (invaginate) to form new cell layers, endoderm and mesoderm. Cells that remain in the epiblast form ectoderm. Hence, epiblast forms all three germ layers in the embryo, ectoderm, mesoderm, and endoderm, and these layers form all of the tissues and organs in a cephalocaudal direction as gastrulation continues. A. Dorsal side of the germ disc from a 16-day embryo indicating the movement of surface epiblast cells (solid black lines) through the primitive streak and node and the subsequent migration of cells between the hypoblast and epiblast (broken lines). B. Cross section through the cranial region of the streak at 15 days with invagination of epiblast cells. The first cells to move inward displace the hypoblast to create the definitive endoderm. Once definitive endoderm is established, inwardly moving epiblast forms mesoderm. Development of a villus. * Primary villi: cytotrophoblastic core covered by syncytial layer * Secondary villi: mesodermal cells penetrate the core of primary villi * Tertiary villi;wwhen blood vessels appear Third to Eighth Weeks: The Embryonic Period During embryonic period, ectoderm, mesoderm, and endoderm give rise to tissues and organ systems. The ectoderm gives rise to the organs and structures that maintain contact with the outside world: • central and peripheral nervous system; • sensory epithelium of ear, nose, and eye; • skin, including hair and nails; and • pituitary, mammary, and sweat glands and enamel of the teeth. The mesoderm give rise to mesenchyme of the head and organize into somites in occipital and caudal segments. Somites give rise to the supporting tissues of the body: • myotome (muscle tissue), • sclerotome (cartilage and bone), and • dermatome (dermis of the skin). Mesoderm also gives rise to • vascular system, • urogenital system: kidneys, gonads, and their ducts (but not the bladder) • spleen and cortex of the suprarenal glands. The endoderm provides the epithelial lining of • gastrointestinal tract, • respiratory tract, • urinary bladder, • tympanic cavity, and • auditory tube It also forms the parenchyma of the thyroid, parathyroids, liver, and pancreas. General Embryology - Detailed Animation On Neurulation.mp4 At the end of the third week, the neural tube is elevating and closing dorsally, while the gut tube is rolling and closing ventrally. Mesoderm holds the tubes together and the lateral plate mesoderm splits to form a visceral (splanchnic) layer associated with the gut and a parietal (somatic) layer that, together with overlying ectoderm, forms the lateral body wall folds. The space between the visceral and parietal layers of lateral plate mesoderm is the primitive body cavity. Transverse sections through embryos at various stages of closure of the gut tube and ventral body wall. A. At 19 days. B. At 20 days, the lateral plate is divided into somatic and visceral mesoderm layers that line the primitive body cavity. C. By 21 days, the primitive body cavity is still in open communication with the extraembryonic cavity. D. By 24 days, the lateral body wall folds are approaching each other in the midline. E. At the end of the fourth week, visceral mesoderm layers are continuous with parietal layers as a double- layered dorsal mesentery. Parietal mesoderm will form the parietal layer of serous membranes lining the peritoneal, pleural, and pericardial cavities. The visceral mesoderm will form the visceral layer of serous membranes covering the lungs, heart, and abdominal organs. These layers are continuous at the root of each organ. Visceral and parietal layers are continuous with each other as the dorsal mesentery (extends from foregut to hindgut). Ventral mesentery exist only in the region of esophagus, stomach and the upper portion of duodenum. SEROUS MEMBRANES Mesenteries provide a pathway for vessels, nerves, and lymphatics to the organs. The septum transversum is a thick plate of mesoderm occupying the space between the primitive thoracic and abdominal cavities. DIAPHRAGM AND THORACIC CAVITY B. Portion of an embryo at 5 weeks with parts of the body wall and septum transversum removed to show the pericardioperitoneal canals. Growth of the lung buds Into the pericardioperitoneal canals. Note the pleuropericardial folds. Third Month to Birth: The Fetus and Placenta • The period from the beginning of the ninth week to birth is fetal period. It is characterized by maturation of tissues and organs and rapid growth of the body. The length of the fetus is indicated as the crown-rump length (CRL) (sitting height) or as the crown-heel length (CHL), the measurement from the vertex of the skull to the heel (standing height) in centimeters. • Growth in length is striking during the third, fourth, and fifth months ( 5 cm per month), while an increase in weight is most striking during the last 2 months of gestation ( 700 gr per month). • In general, the length of pregnancy is considered to be 280 days, or 40 weeks after the onset of the last normal menstrual period (LNMP) or, more accurately, 266 days or 38 weeks after fertilization. At the beginning of the third month, the head constitutes approximately half of the CRL. By the beginning of the fifth month, the size of the head is about one third of the CHL, and at birth, it is approximately one quarter of the CHL. Hence, over time, growth of the body accelerates but that of the head slows down. A 9-week fetus. Note the large head size compared with that of the rest of the body. The yolk sac and long vitelline duct are visible in the chorionic cavity. Note the umbilical cord and herniation of intestinal loops. One side of the chorion has many villi (chorion frondosum), while the other side is almost smooth (chorion laeve). PLACENTA Functions of the placenta are: (1) exchange of gases; (2) exchange of nutrients and electrolytes; (3) transmission of maternal antibodies, providing the fetus with passive immunity; (4) production of hormones, such as progesterone, estrogen, and hCG (5) detoxification of some drugs. The fetal component of placenta is derived from trophoblast and extraembryonic mesoderm (the chorionic plate); the maternal component is derived from uterine endometrium (decidua). Maternal blood is delivered to the placenta by spiral arteries in the uterus. Erosion of these maternal vessels to release blood into intervillous spaces is accomplished by endovascular invasion by cytotrophoblast cells. These cells, released from the ends of villi, invade the terminal ends of spiral arteries, where they replace maternal endothelial cells in the vessels’ walls, creating hybrid vessels containing both fetal and maternal cells. Human embryo at the beginning of the second month of development PLACENTA A 6-week embryo. The amniotic sac and chorionic cavity are opened to expose the embryo, showing the bushy appearance of the trophoblast at the embryonic pole (chorion frondosum) in contrast to small villi at the abembryonic pole (chorion laeve). Note the connecting stalk and yolk sac with its extremely long vitelline duct. End of the second month End of the third month By the beginning of the fourth month, the placenta has two components: (1) a fetal portion, formed by the chorion frondosum and (2) a maternal portion, formed by the decidua basalis – functional layer of endometrium. The space between the chorionic and decidual plates is filled with intervillous lakes of maternal blood. Villous trees (fetal tissue) grow into the maternal blood lakes and are bathed in them. The fetal circulation is always separated from the maternal circulation by (1) a syncytial membrane (a chorion derivative) and (2) endothelial cells from fetal capillaries. Hence, the human placenta is of the hemochorial type. PLACENTAL BARRIER Initially 4 layer » Endothelial linining of fetal vessels >» Connective tissue in villous core » Cytotrophoblastic layer » Synecytium * From4 months Barrier formed by 1, Syneytum > Endothelium Sees teone 4. Endothelium > Syneytium AMNIOTIC FLUID The amnion is a large sac with amniotic fluid in which the fetus is suspended by its umbilical cord. The fluid: (1) absorbs jolts, (2) allows for fetal movements, and (3) prevents adherence of the embryo to surrounding tissues. The fetus swallows amniotic fluid, which is absorbed through its gut and cleared by the placenta. The fetus adds urine to the amniotic fluid, but this is mostly water. PARTURITION (BIRTH) Signals initiating parturition (birth) are not clear, but preparation for labor usually begins between 34 and 38 weeks. Labor itself consists of three stages: (1) effacement (thinning and shortening) and dilatation of the cervix, (2) delivery of the fetus, and (3) delivery of the placenta and fetal membranes. To study: Langman‘s Medical Embryology Chapters 5, 6, 7, 8