Tissue Development Page 1 During embryogenesis, the three germ layers (endoderm, mesoderm, and ectoderm) are formed from which all body tissues will later develop. Key Points o The endoderm, the inner germ layer, forms the stomach, the colon, the liver, the pancreas, the urinary bladder, the lining of the urethra, the epithelial parts of trachea, the lungs, the pharynx, the thyroid, the parathyroid, and the intestines. o The mesdoderm, the middle germ layer, forms skeletal muscle, the skeleton, the dermis of skin, connective tissue, the urogenital system, the heart, blood (lymph cells), the kidney, and the spleen. o The ectoderm, the outer germ layer, forms the central nervous system, the lens of the eye, cranial and sensory nerves, the ganglia and nerves, pigment cells, head connective tissues, the epidermis, hair, and mammary glands. Terms • Endoderm One of the three tissue layers in the embryo of a metazoan animal that produces tissue within the lungs, thyroid, pancreas, and the digestive system of the adult. • Mesoderm One of the three tissue layers in the embryo of a metazoan animal. Through embryonic development, it will produce many internal organs of the adult, e.g. muscles, spine, and circulatory system. • Ectoderm Outermost of the three tissue layers in the embryo of a metazoan animal. Through development, it will produce the epidermis (skin) and nervous system of the adult. In humans, fertilization of an egg leads to the formation of a zygote. During the next stage, termed cleavage, mitotic cell divisions transform the zygote into a tiny ball of cells, called a blastula. This early embryonic form undergoes gastrulation, forming a gastrula with either two or three layers (the germ layers). In all vertebrates, these are the forerunners of all adult tissues and organs. After about three days, the zygote forms a solid mass of cells by mitotic division, called a morula. This then becomes a blastocyst, consisting of an outer layer called a trophoblast, and an inner cell mass called the embryoblast. Filled with uterine fluid, the blastocyst breaks out of the egg coating and undergoes implantation. The inner cell mass initially has two layers: the hypoblast and epiblast. At the end of the second week, a primitive streak appears, this is the site of the beginning of bilateral symmetry. The epiblast in this region moves towards the primitive Tissue Development Page 2 streak, dives down into it, and forms a new layer, called the endoderm, pushing the hypoblast out of the way (this goes on to form the amnion). The epiblast keeps moving and f orms a second layer, the mesoderm. The top layer is now called the ectoderm. The endoderm produces tissue within the lungs, thyroid, and pancreas. The endoderm is one of the germ layers formed during animal embryogenesis. Cells migrating inward along the p rimitive gut form the inner layer of the gastrula, which develops into the endoderm. The endoderm consists at first of flattened cells, which subsequently become columnar . It forms th e epithelial lining of the whole of the digestive tube, except part of the mouth and pharynx, and the terminal part of the rectum (which are lined by involutions of the ectoderm). It also forms the lining cells of all the glands which open into the digestive tube, including those of the liver and pancreas; the epithelium of the auditory tube and ear cavity; the trachea, bronchi , and air cells of the lungs; the urinary bladder and part of the urethra; and the follicle lining of the thyroid gland and thymus. The endoderm forms: the stomach, the colon, the liver, the pancreas, the urinary bladder, the lining of the urethra, the epithelial parts of trachea, the lungs, the pharynx, the thyroid, the parathyroid, and the intestines. The mesoderm aids in the production of cardiac muscle, skeletal muscle, smooth muscle , tissues within the kidneys, and red blood cells . The mesoderm germ layer forms in the embryos of triploblastic animals. During gastrulation, some of the cells migrating inward contribute to the mesoderm, an additional layer bet ween the endoderm and the ectoderm. The formation of a mesoderm leads to the development of a coelom , a fluid filled cavity. Organs formed inside a coelom can freely move, grow, and dev elop independently of the body wall while fluid cushions and protects them from shocks . The mesoderm forms skeletal muscle, the skeleton, the dermis of skin, connective tissue, the uroge nital system, the heart, blood (lymph cells), the kidney, and the spleen. Mesoderm Image illustrates the types of cells produced by the mesoderm (midd le germ layer) of the developing embryo. Tissue Development Page 3 The ectoderm produces tissues within the epidermis, aids in the formation of neurons within the brain, and constructs melanocytes (the cells that produce melanin) . The ectoderm is the start of a tissue that covers the body surfaces. It emerges first and forms from the outermost of the germ layers. The ectoderm forms the c entral nervous system, the lens of the eye, cranial and sensory nerves, the ganglia and nerves, pigment cells, head connective tissues, the epidermis, hair, and mammary glands. Ectoderm This image illustrates the types of cells produced by the ectoderm (external germ layer) of the developing embryo. Endoderm Image illustrates the types of cells produced by the endoderm (inner germ layer) of the developing embryo. Source: Boundless. “Tissue Development.” Boundless Anatomy and Physiology . Boundless, 21 Jul. 2015. 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