Trilminar Layer Derivatives PDF

WE MAKE DOCTORS WEEK 3 TO 8: TRILAMINAR LAYER DERIVATES DR. GUADALUPE RODRIGUEZ EMBRYOLOGY OBJECTIVES: Describe general features of the structures derived from the ectoderm, mesoderm and endoderm. Describe the process of neurulation (neural tube development) and its clinical correlation. Describe...

WE MAKE DOCTORS WEEK 3 TO 8: TRILAMINAR LAYER DERIVATES DR. GUADALUPE RODRIGUEZ EMBRYOLOGY OBJECTIVES: Describe general features of the structures derived from the ectoderm, mesoderm and endoderm. Describe the process of neurulation (neural tube development) and its clinical correlation. Describe the paraxial mesoderm, intermedium mesoderm, lateral mesodermal. ECTODERMAL GERM LAYER  At the beginning of the third week of development, the ectodermal germ layer has the shape of a disc that is broader in the cephalic than in the caudal region. Appearance of the notochord and prechordal mesoderm induces the overlying ectoderm to thicken and form the neural plate. Neurulation Process where the neural plate forms the neural tube One of the key events in this process is lengthening of the neural plate and body axis by extension, whereby there is a lateral to medial movement of cells in the plane of the ectoderm and mesoderm. As the neural plate lengthens, its lateral edges elevate to form neural folds, and the depressed midregion forms the neural groove.  The neural folds approach each other in the midline, where they fuse. Fusion begins in the cervical region (fifth somite) and proceeds cranially and caudally. As a result, the neural tube is formed.  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. Closure of the cranial neuropore occurs at approximately day 25, whereas the posterior neuropore closes at day 28. Neural Crest Cells As the neural folds elevate and fuse, cells at the lateral border or crest of the neuroectoderm begin to dissociate from their neighbors. The neural crest cells undergo an epithelial-to-mesenchymal transition as it leaves the neuroectoderm by active migration and displacement to enter the underlying mesoderm. Crest cells leave the neuroectoderm after closure of the neural tube and migrate along one of two pathways: 1. A dorsal pathway through the dermis, where they will enter the ectoderm to form melanocytes in the skin and hair follicles. 2. A ventral pathway through the anterior half of each somite to become sensory ganglia, sympathetic and enteric neurons, Schwann cells, and cells of the adrenal medulla. NCC also form and migrate from cranial NEURAL FOLDS, leaving the neural tube before closure in this region. These cells contribute to the craniofacial skeleton as well as neurons for cranial ganglia, glial cells, melanocytes, and other cell types. https://www.youtube.com/watch?v=3vkRS1AqTpE Neural Crest Derivates Neural Tube Defects Mesordermal Germ Layer Initially, cells of the mesodermal germ layer form a thin sheet of loosely woven tissue on each side of the midline. By approximately the 17th day, however, cells close to the midline proliferate and form a thickened plate of tissue known as PARAXIAL MESODERM. More laterally, the mesoderm layer remains thin and is known as the lateral plate. With the appearance and coalescence of intercellular cavities in the lateral plate, this tissue is divided into two layers: Somatic or parietal mesoderm layer: a layer continuous with mesoderm covering the amnion Splanchnic or visceral mesoderm layer: a layer continuous with mesoderm covering the yolk sac. Paraxial Mesoderm By the beginning of the third week, paraxial mesoderm begins to be organized into segments, known as SOMITOMERES. These segments first appear in the cephalic region and downwards to the caudal region. In the cephalic region this somitomeres along with the segmentation of the neural plaque forms the NEUROMERES that contributes to the cephalic mesenchyme. After the occipital region, the somitomeres are organized caudally in somites. The first pair of somites appear in the 20th day. From this moment new somites are formed in a cephalo-caudal direction. Around 3 pairs per day are formed, so that at the end of the 5th week there are from 42 - 44 pairs. 4 occipitals 8 cervical 12 thoracic 5 lumbar 5 sacral 8-10 coccygeal At the beginning of the 4th week, the cells that form the ventral and medial wall of the somite, loose their organization and start to surround the notochord and neural tube. This cells are now named SCLEROTOME, thar forms a lax tissue named mesenchyma. Some of this cells originates tendons, the others that surrounds the spinal cord and notochord will form the vertebral column. After migration from the muscular cells and the sclerotome cells, and the cells from the dorsomedial of the somite proliferate and migrate to the ventral side forming a new layer: MYOTOME The remain dorsal epithelium forms the DERMATOME Intermedium Mesoderm Intermediate mesoderm, which temporarily connects paraxial mesoderm with the lateral plate, differentiates into urogenital structures. Cervical and upper thoracic regions form segmental cell clusters which will be future nephrotomes Caudal region will form an unsegmented mass of tissue called the nephrogenic cord Excretory units of the urinary system and the gonads develop from this partly segmented, partly unsegmented intermediate mesoderm Lateral Mesoderm Lateral plate mesoderm splits into parietal (somatic) and visceral (splanchnic) layers, which line the intraembryonic cavity and surround the organs. The somatic mesoderm along with the ectoderm forms the lateral body walls. The splanchnic mesoderm and endoderm will form the intestine wall.  Blood Vessels Blood vessels form in two ways: Vasculogenesis: vessels arise from blood islands Angiogenesis: sprouting from existing vessels The first blood islands appear in mesoderm surrounding the wall of the yolk sac at 3 weeks of development and slightly later in lateral plate mesoderm and other regions. These islands arise from mesoderm cells that are induced to form hemangioblasts, a common precursor for vessel and blood cell formation. The definitive hematopoietic stem cells are derived from mesoderm surrounding the aorta in a site near the developing mesonephric kidney called the aorta-gonad-mesonephros region (AGM). These cells colonize the liver, which becomes the major hematopoietic organ of the embryo and fetus from approximately the second to seventh months of development. Stem cells from the liver colonize the bone marrow, the definitive blood-forming tissue, in the seventh month of gestation; thereafter, the liver loses its blood-forming function. vascular endothelial growth factor Focal capillary hemangioma Diffuse capillary hemangioma involving the oral cavity Endodermal Germ Layer  The gastrointestinal tract is the principal organic system that derivates from the endodermic germinative layer. This layer covers the superficial ventral layer of the embryo and is the top of the yolk sack.  Two lateral body wall folds form and also move ventrally to close the ventral body wall. As the head and tail and two lateral folds move ventrally, they pull the amnion down with them, such that the embryo lies within the amniotic cavity. The ventral body wall closes completely except for the umbilical region where the connecting stalk and yolk sac duct remain attached. Failure of the lateral body folds to close the body wall results in ventral body wall defects. As a result of cephalocaudal growth and closure of the lateral body wall folds, a continuously larger portion of the endodermal germ layer is incorporated into the body of the embryo to form the gut tube. The tube is divided into 3 regions: Foregut Midgut Hindgut References: Chapter 6. Third to Eight Weeks: The Embryonic Period Bevis, R. (1992). Langman’s Medical Embryology, 6th edition T W Sadler Langman’s Medical Embryology, 6th Edition Williams & Wilkins 412pp £20.50 0-683- 07473-8. Nursing Children and Young People, 4(3), 14. https://doi.org/10.7748/paed.4.3.14.s14 Thank You!