Embryonic Development (3rd Week) PDF

(004) 3RD WEEK OF DEVELOPMENT: TRILAMINAR GERM DISC & 3RD TO 8TH WEEKS: THE EMBRYONIC PERIOD DR. PEREDO | 10/22/2020 OUTLINE 3rd WEEK OF DEVELOPMENT: TRILAMINAR DISC...

(004) 3RD WEEK OF DEVELOPMENT: TRILAMINAR GERM DISC & 3RD TO 8TH WEEKS: THE EMBRYONIC PERIOD DR. PEREDO | 10/22/2020 OUTLINE 3rd WEEK OF DEVELOPMENT: TRILAMINAR DISC I. GASTRULATION II. FORMATION OF NOTOCHORD III. ESTABLISHMENT OF BODY AXES 3RD TO 8TH WEEKS: THE EMBRYONIC PERIOD I. PERIOD OF ORGANOGENESIS I. GASTRULATION C. Dorsal view of an embryo showing the primitive node and streak and a cross section through the streak.  most characteristic event occurring the 3rd week  Begins with the formation of primitive streak (PS) on the surface  Cells migrate inwards towards the PS this is called Invagination, of the epiblast: cephalic end, primitive node, surrounding controlled by Fibroblast Growth Factor 8 (FGF8) primitive pit FGF8 then Controls cell specification into the mesoderm by regulating BRACHYURY (T) expression.  Cells invaginated and displace hypoblast creat endoderm  Between epiblast and endoderm (that was produced previously) form mesoderm  Cells remaining in epiblast form ectoderm. The epiblast cells continous to move laterally, cranially and caudally. Those epiblast cells that moves cranially are formed into two:  Precordial plate which will be the induction of forebrain  Oropharyngeal membrane, the future opening of oral cavity A.Implantation site at the end of the second week. A. Dorsal side of the germ disc from a 16-day embryo indicating the movement of surface epiblast cells [solid black lines] B. Representative view of the germ disc at the end of the second through the primitive streak and node and the subsequent week of development. The amniotic cavity has been opened to migration of cells between the hypoblast and epiblast [broken permit a view of the dorsal side of the epiblast. The hypoblast lines]. and epiblast are in contact with each other, and the primitive streak forms a shallow groove in the caudal region of the embryo. PREPARED BY GROUP 2  (004) 3RD WEEK OF DEVELOPMENT: TRILAMINAR GERM DISC & 3RD TO 8TH WEEKS: THE EMBRYONIC PERIOD DR. PEREDO | 10/22/2020 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 create the definitive A.Drawing of a sagittal section through a 17-day embryo. The endoderm. Once definitive endoderm is established, inwardly most cranial portion of the definitive notochord has formed, moving epiblast forms mesoderm. whereas prenotochordal cells caudal to this region are intercalated into the endoderm as the notochordal plate. Note that some cells migrate ahead of the notochord. These  Epiblast gives rise to all 3 germ layers in the embryo, ectoderm, mesoderm cells form the prechordal plate that will assist in mesoderm, and endoderm (deepest). All these layers form all of forebrain induction. the tissues and organs during organogenesis  The Primary villi, small capillaries, villous capillaries and the connecting stalk (chorionic plate) supply the embryo with nutrients and oxygen Remember that the tropoblast will be the future placenta and this placenta will produce oxygen and nutrients to the embryo. In the cephalic direction, they pass on each side of the prechordal plate. Later, the prechordal píate will be important for induction of the forebrain B.Schematic cross section through the region of the notochordal plate. Soon, the notochordal plate will detach from II. FORMATION OF NOTOCHORD the endoderm to form the definitive notochord.  Prenotochordal cells invaginate in primitive node and the epiblast cells are invaginating moving lateral wise, they are at the cranial region, are your prechordal plate. As they continue to move lateral wise upon reaching the middle, they will form two layers. It will be the notochondral plate. As the two layers reach the central region, it will now becomes a single layer. It will form the notochord because the endoderm will now be detached. So it now only form a single layer and that will become the definitive chord, future neural tube and future axial skeleton - The notochord and prenotochordal cells extend cranially to C.Schematic view showing the definitive notochord. the prechordal plate (an area just caudal to the oropharyngeal membrane) and caudally to the primitive pit. III. ESTABLISHMENT OF BODY AXES At the point where the pit forms an indentation in the epiblast, the neuromeric canal temporarily connects the  Anterior- posterior AP cranio caudal, dorso-ventral DV, left-right amniotic and yolk sac cavities. LR occurs early in embryogenesis, initiated during morula stage - The cloacal membrane is formed at the caudal end of the  AP axis forms the anterior visceral endoderm (AVE) at the embryonic disc. When the cloacal membrane appears, the cranial end of the endoderm which will become the head region posterior wall of the yolk sac forms a small diverticulum that extends in the connecting stalk. This diverticulum, the  Mesoderm will ventralize to contribute to kidneys in the allantoenteric diverticulum, or allantois, appears around presence of FGF8, bone morphogenetic protein 4 (BMP4), the 16th day of development. transforming growth factor (TGF B)  Node is the organizer  Nodal in involved in intiating and maintaining the primitive streak  Laterally (L-R sidedness) – primitive streak appears, FGF8 secreted, nodal expression restricted to L side. Abnormal expression includes laterality defects: situs inversus, dextro cardia Lateral defects – if the nodal expressions are abnormally expressed Dextro cardia – if the organ that should be on the left but is located on the right, eg. heart PREPARED BY GROUP 2  (004) 3RD WEEK OF DEVELOPMENT: TRILAMINAR GERM DISC & 3RD TO 8TH WEEKS: THE EMBRYONIC PERIOD DR. PEREDO | 10/22/2020 Situs inversus - if the organ is positioned in a reverse manner - MOLECULAR REGULATION OF NEURAL INDUCTION such as in a mirror image arrangement. For example, the organ - Neurulation is the process whereby the neural píate forms that is supposedly on the right but is placed on the left and one the neural tube. that is placed on the left is placed on the right. - NEURAL CREST. As the neural folds elevate and fuse, cells at the lateral border or crest of the neuroectoderm FATE ESTABLISHED DURING GASTRULATION begin to dissociate from their neighbors. - Paraxial Mesoderm are those migrating at the lateral edges of the node and from the cranial end of the streak - Intermediate Mesoderm are cells migrating through the mid streak region - Lateral Plate Mesoderm forms those migrating through the more caudal part of the streak. - Cells migrating through the caudal most part of the streak contribute to extraembryonic mesoderm GROWTH OF 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. At that stage, the primitive streak shows regressive changes, rap- idly shrinks, and soon disappears. A. Dorsal view of a 16-day presomite embryo. The primitive - That the primitive streak at the caudal end of the disc streak and primitive node are visible. continues to supply new cells until the end of the fourth week has an important bearing on development of the embryo. - Thus, gastrulation, or formation of the germ layers, continues in caudal segments while cranial structures are differentiating, causing the embryo to develop cephalocaudally. I.PERIOD OF ORGANOGENESIS  formation of tissues and organs  3 germs layers, ectoderm, mesoderm, and endoderm, gives rise to a number of specific tissues and organs  Ectoderm germ layer (everything that is in contact with the outside world) e.g. CNS, PNS, sensory epithelium of era, nose, eye, skin, hair, nails, pituitary, mammary, and sweat glands B. Dorsal view of an 18-day presomite embryo. The embryo is  Mesoderm germ layer (tissues that support the body) e.g pear-shaped, with its cephalic region somewhat broader than its somitomeres, myotome (muscle tissue), sclerotome (cartilage, caudal end. bone), dermatome (skin)  Endoderm germ layer (organs withing the body) e.g. GIT, RT, UB, thyroid, parathyroid, liver, pancreas, tympanic cavity, auditory tube  Induction of the neural plate caused by the upregulation of Fibroblast Growth factor (FGF), inhibition of the activities of Bone Morphogenetic Protein 4 (BMP4), Transforming growth factor B (TGF B)  Formation of neural tube from neural plate (NEURULATION) occurs by lengthening of neural plate (cranial to caudal) and body axis, lateral edges elevate to form NEURAL FOLDS and the depressed mid region form NEURAL GROOVE  Neural fold approach each other in midline where they fuse and form the cervical region that proceeds cranially and caudally. Neural tube is formed  Anterior cranial neuropore closes at day 25 C. Dorsal view of an 18-day human embryo. Note the primitive  Posterior caudal neuropore closes at day 28 node and, extending forward from it, the notochord. The yolk  Neurulation complete with its narrow caudal portion/ spinal cord sac has a somewhat mottled appearance. The length of the and broader cephalic portion/ brain vesicles embryo is 1.25 mm, and the greatest width is 0.68 mm.  DERIVATIVES OF THE ECTODERMAL GERM LAYER PREPARED BY GROUP 2  (004) 3RD WEEK OF DEVELOPMENT: TRILAMINAR GERM DISC & 3RD TO 8TH WEEKS: THE EMBRYONIC PERIOD DR. PEREDO | 10/22/2020 A. Dorsal view of a late presomite embryo [approximately 19 days]. The amnion has been removed, and the neural plate is clearly visible. B. Dorsal view of a human embryo at 19 days A. Dorsal view of an embryo at approximately day 22. Seven distinct somites are visible on each side of the neural tube. B. Dorsal view of a human embryo at 21 days. C. Dorsal view of an embryo at approximately 20 days showing somites and formation of the neural groove and neural folds. C. Dorsal view of an embryo at approximately day 23. Note the D. Dorsal view of a human embryo at 20 days. pericardial bulge on each side of the midline in the cephalic part of the embryo. D. Dorsal view of a human embryo at 23 days. PREPARED BY GROUP 2  (004) 3RD WEEK OF DEVELOPMENT: TRILAMINAR GERM DISC & 3RD TO 8TH WEEKS: THE EMBRYONIC PERIOD DR. PEREDO | 10/22/2020 A.Lateral view of a 14-somite embryo (approximately 25 days]. Note the bulging pericardial area and the first and second pharyngeal arches B. The left side of a 25-somite embryo approximately 28 days old. The first three pharyngeal arches and lens and otic placodes are visible The ectodermal germ layer gives rise to organs and structures that maintain contact with the outside world: - The central nervous system - The peripheral nervous system - The sensory epithelium of the ear, nose, and eye - The epidermis, including the hair and nails - The subcutaneous glands - The mammary glands - The pituitary gland - Enamel of the teeth REFERENCE Sadler, T. W. & Langman, J. (2012). Langman's Medical Embryology (13th Ed). Wolter's Kluwr Health / Lippincott Williams and Wilkins. PREPARED BY GROUP 2