Embryology 3 PDF

EMBRYOLOGY3 Dr.Ban A.Ghani Third week trilaminar germ layer: Gastrulation & Neurulation Early development:Morula,blastocyst,and bilaminar disc  Zygote: diploid cell resulting from the fusion of 2 haploid gametes  Blas...

EMBRYOLOGY3 Dr.Ban A.Ghani Third week trilaminar germ layer: Gastrulation & Neurulation Early development:Morula,blastocyst,and bilaminar disc  Zygote: diploid cell resulting from the fusion of 2 haploid gametes  Blastomeres: individual cells at the 2-, 4-, and 8-cell stages  Morula: “ball of cells” starting at the 16-cell stage  Blastocyst:Outer cell mass, Inner cell mass  Zona pellucida: o A layer of extracellular matrix surrounding embryo through the blastocyst stage o Prevents embryo from implanting in the fallopian tubes (where fertilization typically occurs)  Outer cell mass → trophoblast (cytotrophoblast and syncytiotrophoblast)  Inner cell mass → embryoblast → bilaminar disc: o Epiblast: dorsal layer o Hypoblast: ventral layer -Amniotic sac: a cavity of fluid that develops “above” the epiblast (between epiblast and cytotrophoblast) -Primary yolk sac: a cavity that forms “below” the hypoblast (between hypoblast and cytotrophoblast) 1 EMBRYOLOGY3 Dr.Ban A.Ghani Implantation  Occurs around days 7–9 after fertilization Trophoblast cells in contact with uterine wall that have lost their outer membranes  As the cell membranes rupture, hydrolytic enzymes are released, allowing the embryo to “eat its way” into the uterine wall.A layer of endometrium covers the invading blastocyst=implantation. 2 EMBRYOLOGY3 Dr.Ban A.Ghani Both gastrulation and neurulation are critical events that occur during the 3 rd week of embryonic development. Gastrulation is the process by which the bilaminar disk differentiates into a trilaminar disc (an embryo transforms from a one-dimensional layer of epithelial cells, a blastula, and reorganizes into a multilayered and multidimensional structure called the gastrula), made up of the 3 primary germ layers: the ectoderm, mesoderm, and endoderm. During gastrulation , a structure called the notochord is formed in the midline in the mesodermal layer; the notochord is critical in inducing neurulation. Neurulation is the process by which some of the ectoderm in the trilaminar embryo develops into the neural tube and neural crest cells, which will go on to form all of the neural tissue in the body. This process is completed by the end of the 3rd week. 3 EMBRYOLOGY3 Dr.Ban A.Ghani Development of the bilaminar disc directly precedes gastrulation, where the cells of the bilaminar disk (epiblast and hypoblast) undergo gastrulation a highly specialized process , the two cell layers become three germ cell layers, and the bodily axes observed in the mature adult are created. So during gastrulation cellular rearrangement which involves migration, invagination and differentiation of the epiblast occurs ,it is largely controlled and orchestrated by the primitive streak. The primitive streak :is a groove in the midline of the epiblast which appears during the third week. Within the primitive streak lies a primitive node at the cranial end, and within the primitive node lies the primitive pit. PS appears as a thickened linear band in the caudal end of the dorsal aspect of the embryonic disk. It’s appearance enables identification of embryonic axes, cranial and caudal ends, top and bottom surfaces, and sides of the embryo. Cells of the epiblast layer break off and migrate toward the primitive pit., they detach and penetrate through the epiblast layer to form three new germ cell layers. These three cell layers are then responsible for forming the different tissues of the fetus. Endoderm – formed by epiblast cells that migrate through the primitive pit and displace the hypoblast cells. Mesoderm – formed by epiblast cells that migrate through the primitive pit and lie between the epiblast layer and the newly created endoderm. Ectoderm – formed by the epiblast cells that remain in position. 4 EMBRYOLOGY3 Dr.Ban A.Ghani Cell Layer Structures Derived Endoderm  Epithelial lining of digestive and respiratory tracts,  Lining of urethra, bladder and reproductive System  Liver and pancreas Mesoderm  Notochord  Musculoskeletal system  Muscular layer of stomach, intestine etc  Circulatory system Ectoderm  Epidermis of skin  Cornea and lens of eye  Nervous system Gastrulation occurs in the following sequence: a-The embryo becomes asymmetric. b-The primitive streak forms. c-Cells from the epiblast at the primitive streak undergo an epithelial to mesenchymal transition and ingress at the primitive streak to form the germ layers. 5 EMBRYOLOGY3 Dr.Ban A.Ghani  At about (day 16 )cells of the epiblast migrate medially toward the streak, enter the primitive groove, then migrate laterally between the embryonic ectoderm and endoderm to organize into a layer, the intraembryonic mesoderm.  Mesoderm also lies outside the embryo as extra-embryonic mesoderm  The epiblast, through the process of gastrulation, is the source of all of the germ layers, and cells in these layers will give rise to all of the tissues and organs in the embryo.  As more and more cells move between the epiblast and hypoblast layers, they begin to spread laterally and cranially.  Gradually, they migrate beyond the margin of the disc and establish contact with the extraembryonic mesoderm covering the yolk sac and amnion.  Some cells of the primitive streak migrate cranially, pass on each side of the notochordal process, around the prochordal plate(small circular area of columnar endodermal cells), and meet cranially in the cardiogenic area where the heart will be formed. 6 EMBRYOLOGY3 Dr.Ban A.Ghani By the middle of week 3, intraembryonic mesoderm separates the ectoderm and endoderm everywhere except at : a- oropharyngeal membrane cranially b- cloacal membrane caudally c- in the midline, cranial to the primitive knot where the notochordal process extends. Because embryonic ectoderm and endoderm fuse at these sites and prevent mesenchymal cells from migrating between them. Growth of the embryonic disc  The embryonic disc, initially flat and almost round, gradually becomes elongated, with a broad cephalic and a narrow caudal end. Growth and elongation of the cephalic part of the disc are caused by a continuous migration of cells from the primitive streak region in a cephalic direction.  Invagination of surface cells in the primitive streak and their subsequent migration forward and laterally continues until the end of the 4th week.  At that stage, the primitive streak shows regressive changes, rapidly shrinks, and soon disappears. 7 EMBRYOLOGY3 Dr.Ban A.Ghani  Gastrulation, or formation of the germ layers, continues in caudal segments while cranial structures are differentiating, causing the embryo to develop cephalocaudally. Neurulation  Neurulation refers to the folding process in vertebrate embryos, and the formation of the neural tube from the ectoderm of the embryo.  It follows gastrulation in all vertebrates which includes the transformation of the neural plate into the hollow neural tube that will eventually differentiate into the brain and the spinal cord of the central nervous system.. The embryo at this stage is termed the neurula.  The notochord (a flexible, rod-shaped body that runs along the back of the embryo) is formed from the mesoderm.  During the 3rd week of gestation the notochord sends signals to the overlying ectoderm, inducing it to become neuroectoderm.  This results in a strip of neuronal stem cells that runs along the back of the fetus. This strip is called the neural plate. 8 EMBRYOLOGY3 Dr.Ban A.Ghani  As the neural plate lengthens, its lateral edges elevate to form neural folds, and the depressed mid region forms the neural groove.  Gradually, the neural folds approach each other in the midline, where they fuse forming the neural tube (the neural tube will later differentiate into the spinal cord and the brain), eventually forming the central nervous system.  Until fusion is complete, the cephalic and caudal ends of the neural tube communicate with the amniotic cavity by way of the anterior (cranial) and posterior (caudal) neuropores respectively.  By the end of the 4th week of gestation closure of the cranial neuropores occurs at approximately day 25, whereas the posterior neuropore closes at day 28.  Neurulation is then complete, and the central nervous system is represented by a closed tubular structure with a narrow caudal portion, - the spinal cord, and a much broader cephalic portion. 9