Lecture 37 Second & Third Weeks of Development PDF

37 The Second& Third Weeks of Development ILOs By the end of this lecture, students will be able to 1. Analyze the changes occurring during the 2nd & 3rd weeks of development. 2. Recognize different types of stem cells in relation to stages of fetal development & the significance of...

37 The Second& Third Weeks of Development ILOs By the end of this lecture, students will be able to 1. Analyze the changes occurring during the 2nd & 3rd weeks of development. 2. Recognize different types of stem cells in relation to stages of fetal development & the significance of their differentiation potential 3. Interpret the process of gastrulation. 4. Correlate the process of gastrulation with teratogenesis, and other clinical disorders. Changes occurring during the second week of development: At the beginning of the second week, the blastocyst is partially embedded in the endometrial stroma. 1- The trophoblast differentiates into two layers: (a) Inner, actively proliferating layer, the cytotrophoblast, (b) Outer, the syncytiotrophoblast, which erodes maternal tissues. By day 9, lacunae (cavities) develop in the syncytiotrophoblast. Subsequently, maternalsinusoids are eroded by the syncytiotrophoblast; maternal blood enters the lacunar network, and by the end of the second week, a primitive uteroplacentalcirculation begins. The cytotrophoblast, meanwhile, forms cellular columns penetrating into and surrounded by the syncytium. These columns are primary villi. By the end of the second week, the blastocyst is completely embedded, and the surface defect in the mucosa has healed. 2- The inner cell mass or embryoblast, meanwhile, differentiates into: (a)the epiblastand (b) the hypoblast, together forming a bilaminar disc. Epiblast cells give rise to amnioblaststhat line the amniotic cavity superior to the epiblast layer. Endoderm cells are continuous with the exocoelomic membrane (that lines the inner surface of the cytotrophoblast), and together they surround the primitive yolk sac. 3- By the end of the second week, extraembryonic mesoderm fills the space between the trophoblast externally, and the amnion and exocoelomic membrane internally. When vacuoles develop in this tissue, the extraembryonic coelom or chorionic cavity forms. Page 1 of 6  The extraembryonic mesoderm lining the cytotrophoblast and amnion is called the extraembryonicsomatopleuric mesoderm; the lining covering the yolk sac is known as the extraembryonicsplanchnopleuric mesoderm. Summary:The second week of development is the week of twos, because of the following: Trophoblast differentiates into 2 layers, (Nominate theses 2 layers) Inner cell mass differentiates into 2 layers, (Nominate theses 2 layers) Primary mesoderm splits into somatopleuric primary mesoderm &splanchnopleuric primary mesoderm. Starting of formation of the amniotic and yolk sac cavities. Stages of development of the embryo during the 2nd week. A. Late blastocyst. B, Beginning of implantation at 6 days. The hypoblast has formed and is beginning to spread beneath the cytotrophoblast as the endoderm. C. Implanted blastocyst at 7 days. D. Implanted blastocyst at 8 days. E. Embryo at 9 days. F. Late second week. Page 2 of 6 A 9-day human blastocyst. The syncytiotrophoblast shows a large numberof lacunae. Flat cells form the exocoelomic membrane. The bilaminar disc consists of a layer of columnar epiblast cells and a layer of cuboidal hypoblast cells. The original surface defect is closed by a fibrin coagulum. Differentiation of Extraembryonic mesoderm What is the final product of the first week of development?(slideplayer.com) Page 3 of 6 Embryonic stem cells Stem cells are undifferentiated (unspecialized) cells that have the following properties: 1. Self-renewal: it is the ability of the cell to go through numerous cycles of cell division while maintaining the undifferentiated state. 2. Potency: it is the capacity to differentiate into different cell types. Embryonic stem cells: these are totipotent stem cells that can differentiate into any other types of cells. Clinical Hint: Stem cells have an important role in treatment of many diseases because of their ability to differentiate into different cell types in cell cultures. These cells can be transplanted into the patient to regenerate the damaged cells. Sources of stem cells: a. Embryonic cells that are of limited use due to ethical restriction. b. Adult cells; can be obtained from several sources such as the bone marrow. They are (multipotent) less potent than embryonic stem cells as they can differentiated into limited number of cell types. Figure.Embryonic stem cells Changes occurring during the third week of development: (TRILAMINAR GERM DISC) GASTRULATION: (Formation of Embryonic Germ layers) It is the process by which the bilaminar embryonic disc is converted into a trilaminar embryonic disc. Gastrulation begins with formation of a midline groove on the surface of the epiblast of the embryonic disc with elevated edges, it is called primitive streak. Page 4 of 6  The cephalic end of this steak is called primitive node. A groove appears at the center of the node called primitive pit. Cells of the ectoderm (epiblast) migrate in the direction of the primitive streak; they slip under the epiblastic layer through the primitive streak to replace the hypoblast andform the endodermal layer of embryonic disc. Then, some of invaginatedepiblastic cells form the intraembryonic mesoderm, thus, the ectoderm or the epiblast is the source of the three germ layers of the embryo). The intraembryonic mesoderm migrates in all directions forming a complete layer of cells separating the ectoderm dorsally from the endoderm ventrally except at the area of the oral membrane and the cloacal membrane. Formation of the notochord: Some mesenchymal cells migrate cranially from the primitive node and pit (deep to epiblast), forming a median cellular cord; the notochord.. 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 showing invagination of epiblast cells. The first cells to move inward displace the hypoblast to createthe definitive endoderm. Once definitive endoderm is established, inwardly moving epiblastforms mesoderm. Functions of the notochord: 1. It forms the axis around which the axial skeleton develops. 2. It stimulate the overlying ectoderm to form the central nervous system. 3. The notochord degenerates and disappears as the bodies of the vertebrae form. Its remnant is the nucleus pulposus of the intervertebral discs. (What is the intervertebral discs?) Page 5 of 6 CLINICAL APPLICATION: Gastrulation may be disrupted by genetic abnormalities and toxic insults. In caudal dysgenesis (sirenomelia), insufficient mesoderm is formed in the caudal-most region of the embryo. Because this mesoderm contributes to formation of the lower limbs, urogenital system, and lumbosacral vertebrae, abnormalities in these structures occur. Affected individuals exhibit a variable range of defects, including hypoplasia and fusion of the lower limbs, vertebral abnormalities, renal agenesis, imperforate anus ,and anomalies of the genital organs. In humans, the condition is associated with maternal diabetes and other causes. At the end of the fourth week, the primitive streak shows regressive changes, rapidly shrinks and soon disappears. Sometimes, remnants of the primitive streak persist in sacrococcygeal region. These pluripotent cells proliferate and form tumors known as sacrococcygealteratomas. Caudal dysgenesis (sirenomelia) Sacrococcygealteratoma Page 6 of 6