![]() The dorsal part of the yolk sac, composed of endoderm, is compressed by the lateral folding of the embryo and is incorporated as a rim during the fourth week. 2 At the same time, the human embryo elongates craniocaudally and folds laterally. Mesoderm proliferation and segmentation, which takes place between the endoderm and ectoderm, induces numerous transformations in the endoderm. This process ends with the formation of 33 to 35 somites by day 31 of embryo development. By day 28, the paraxial mesoderm fragments progressively from cranial to caudal into cubes of tissue called somites. By day 21, the mesoderm is thickened and forms longitudinal masses called the paraxial mesoderm. On day 15, the third embryonic layer, the mesoderm, appears and provides the substrate for the connective tissue, angioblasts, smooth muscle, and serosal layers of the gut. The neural crest cells, placed between the dorsal neural tube and the overlying epidermis, migrate out to form the peripheral nervous system by week 4. The neural tube is the precursor for the spinal cord and brain. Through the neurulation process, the neural plates evolve to neural tube and neural crest cells. The ectoderm gives rise to epidermis and neural plates. The endoderm is the scaffold for the future digestive tract. From days 0 to 14, the human embryo develops into a bilaminar disk of ectoderm and endoderm, with the endoderm forming the lining of the yolk sac. The fetal period lasts from the end of the week 9 to birth. The embryonic period extends from fertilization to week 9. The first stages of life are divided into the embryonic and fetal periods. Esophageal peristalsis appears in the first trimester, and gastroesophageal reflux can be documented in the second trimester.The development of various elements of esophageal wall requires coordination of a variety of genes and mediators.The upper esophagus is derived from branchial arches 4, 5, and 6, but the derivation of the lower esophagus is not known.At either end of the esophagus the ciliated epithelium gives rise to esophageal glands. Beginning at 4 months, the ciliated epithelium starts to be replaced by squamous epithelium.Esophagus develops from foregut and by week 10 is lined by ciliated epithelial cells.The function of the myenteric plexus in the striated esophagus is not well understood.The myenteric plexus is well developed in the smooth muscle, but is also present in the striated muscle part of the esophagus.The esophagus wall is composed of striated muscle in the upper part, smooth muscle in the lower part, and a mixture of the two in the middle.The adult human esophagus is an 18- to 25-cm long muscular tube that has cervical, thoracic, and abdominal parts.
0 Comments
Leave a Reply. |