Embryonic Period Lecture PDF

38 The Embryonic period ILOs By the end of this lecture, students will be able to 1. Interpret the importance of embryonic folding during development. 2. Differentiate between the derivatives of the three germ layers. 3. Appraise their dysregulated development in occurr...

38 The Embryonic period ILOs By the end of this lecture, students will be able to 1. Interpret the importance of embryonic folding during development. 2. Differentiate between the derivatives of the three germ layers. 3. Appraise their dysregulated development in occurrence of birth defects 4. Distinguish how the timeline of embryogenesis is the most sensitive period for teratogens and structural defects. The embryonic period or period of organogenesis, occurs from the third to the eighth weeks of development and is the time when each of the three germ layers, ectoderm, mesoderm, and endoderm, gives rise to a number of specific tissues and organs. Also, folding of the embryo occurs at the beginning of this period. FOLDING OF THE EMBRYO Definition: Conversion of the flat trilaminar embryonic disc into a cylindrical embryo. Time: begins by the end of the 3rd week. It is completed by the 4th week. Types: 1. Cephalocaudal folding (head fold & tail fold): It is caused by the rapid longitudinal growth of the central nervous system. A. The head fold: Before folding: the following structures are present in the midline of intraembryonic mesoderm arranged in a craniocaudal direction: a. septum transversum (future central tendon of diaphragm) b. cardiogenic plate (future heart) the pericardial cavity lies dorsal to the cardiogenic plate. c. Oral membrane (future mouth opening). Late in folding: The following structures lie ventral to the embryo and arranged in a craniocaudal order: a. Oral membrane, b. Cardiogenic plate, c. Septum transversum. Due to folding, a constriction appears at the junction of the embryo and yolk sac. Part of the endodermal yolk sac is included in the cranial part of the embryo it is called foregut. B. Tail fold: Page 1 of 5 Before folding, the allantois is caudal to the yolk sac. The yolk sac lies ventral to the endodermal layer of embryonic disc. The cloacal membrane lies in the caudal part of the embryo. After folding: The allantois and the cloacal membrane shifted ventrally to the embryo. Part of the yolk sac incorporated in the caudal part of the embryo forming the hind gut. The terminal part of hind gut dilates to form the cloaca (future urinary bladder and rectum) Sagittal midline sections of embryos at various stages of development to demonstrate cephalocaudal folding and its effect on position of the endoderm-lined cavity. A. Presomite embryo. B. Embryo with 7 somites. C. Embryo with 14 somites. D. End of the first month. Note the angiogenic cell clusters in relation to the buccopharyngeal membrane. 2. Lateral folding (right and left folds) It is due to the rapid growth of the somite (part of intraembryonic mesoderm). Lateral folding lead to formation of midgut, also it leads to constriction of the wide communication between the extraembryonic ceolom and midgut. This constricted communication called vitellointestinal duct or yolk stalk. The amniotic cavity increases in size on expense of the extraembryonic coelom. Page 2 of 5 Transverse sections through embryos at various stages of development to show the effect of lateral folding on the endoderm-lined cavity. A. Folding is initiated. B. Transverse section through the midgut to show the connection between the gut and yolk sac. C. Section just below the midgut to show the closed ventral abdominal wall and gut suspended from the dorsal abdominal wall by its mesentery. Results of folding: 1. It gives the embryo its cylindrical form. 2. Formation of the gut. 3. As a result of formation of the head fold: the buccopharyngeal membrane, heart & septum transversum become ventral in position and arranged in a craniocaudal order. 4. As a result of formation of the tail fold: the cloacal membrane and allantois become ventral in position. 5. The umbilical cord is formed and the connecting stalk is shifted ventrally. 6. The allantois becomes ventral instead of dorsal. Differentiation& Derivatives of the three germ layers: Differentiation of the secondary (intra-embryonic) mesoderm: Secondary mesoderm is divided into three columns on each side of the notochord. 1. The paraxial mesoderm: - It is the medial part that lies on either side of the notochord. - It is divided by transverse grooves into body blocks called somites (4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 8-10 coccygeal). - The somites give the axial skeleton (bones & cartilage), vertebral muscles and covering skin. 2- The intermediate cell mass (or nephrogenic cord). It gives rise to the urinary and genital systems. 3. The lateral plate of mesoderm: Page 3 of 5 - A cavity appears in the lateral plate of mesoderm called the intraembryonic coelom. This ceolom splits the lateral plate of mesoderm into two layers, the somatic and splanchnic layers. A. The somatic layer: gives the muscles of body wall. B. The splanchnic layer: It lies next to the endoderm. It gives the smooth or involuntary muscles (heart, bronchial tree & gut) Differentiation of the intraembryonic mesoderm Endodermal Derivatives: The endodermal germ layer provides the epithelial lining of the gastrointestinal tract, respiratory tract, and urinary bladder. It also forms the parenchyma of the thyroid, parathyroids, liver, and pancreas. Ectodermal Derivatives: The ectodermal germ layer gives rise to the organs and structures that maintain contact with the outside world: (a) central nervous system; (b) peripheral nervous system; (c) sensory epithelium of ear, nose, and eye; (d)skin, including hair and nails; and (e) pituitary, mammary, and sweat glands and enamel of the teeth. Neurulation: The neural plate appears as a thickening of embryonic ectoderm induced by the developing notochord. A longitudinal neural groove develops in the neural plate, which is flanked by neural folds. Fusion of the folds forms the neural tube (the primordium of the central nervous system). As the neural folds fuse to form the neural tube, neuroectodermal cells form a neural crest between the surface ectoderm and the neural tube. Page 4 of 5  Formation of the neural tube; A: Day 17; B: Day 19; C: Day 20; D: Day 21 neuroembryology Flashcards | Quizlet C L I N I C A L C O R R E L A T E S: Birth Defects Most major organs and organ systems are formed during the third to eighth week. This period, which is critical for normal development, is therefore called the period of organogenesis. Neural tube defects can develop during this period due to abnormal neurulation process. Page 5 of 5