![]() Development of the bilaminar embryonic disc directly precedes gastrulation, the stage in week 3 of development that involves the transformation of the human blastocyst into a multilayered gastrula with endoderm, mesoderm, and ectoderm. ![]() On the hypoblast is a raised area of columnar cells known as the prechordal plate this is the earliest delineation of cranial from caudal. The cells of the epiblast stretch to form a semi-sphere known as the amniotic cavity, while the cells of the hypoblast extend to surround the yolk sac. The anatomical location of the bilaminar disc is found between the amniotic cavity and the primitive yolk sac. The formation of the bilaminar embryonic disc sets the dorsal/ventral axis as the epiblast cell layer is positioned dorsal to the hypoblast. During the second week of human development, the cells of the ICM spread into a flattened tissue layer and differentiate into a two-layered tissue containing epiblast (columnar epithelial cells) and hypoblast (cuboidal epithelial cells), which are together known as the bilaminar embryonic disc. The inner trophoblast layer, known as the cytotrophoblast, is a single sheet of cells surrounding the extraembryonic mesoderm. This layer also releases human chorionic gonadotropin (hCG, necessary in regulating progesterone secretion), the protein used in many pregnancy tests. The outer trophoblast, known as the syncytiotrophoblast, releases proteolytic enzymes to assist with endometrial implantation. During blastocyst formation, the zona pellucida begins to disappear, allowing the ball of cells to proliferate, differentiate, change shape, and eventually implant into the uterine wall.ĭuring implantation, the trophoblastic layer surrounding the blastocyst further differentiates into two functionally distinct layers. įrom zygote to blastocyst formation, the organism is surrounded by the zona pellucida, a layer of the extracellular matrix that plays a role in the protection and prevention of implantation into the uterine tubes. Together the trophoblastic layer, blastocoel, and inner cell mass are the characteristic features of the human blastocyst. Rather than being arranged as a solid sphere of cells, the ICM is pushed off to one side of the sphere formed by the trophoblast. The cells remaining after blastocoel formation are pluripotent ICM progenitor cells, which give rise to the distinctive formation of the fetus. Trophoblast cells utilize the active transport of sodium ions and osmosis of water to form a fluid-filled cavity known as a blastocoel. ![]() Additionally, the trophoblast cells are essential in the cavitation of the solid morula into a hollowed ball of cells with an internal cavity. The outer trophoblast will develop into structures that provide nutrients, help the growing embryo implant in the uterine lining, and contribute to the formation of the placenta. Cells within the outer shell bind together via gap junctions and desmosomes to undergo compaction, ultimately forming a water-tight shell called the trophoblast. Within the blastocyst, two tissue layers differentiate: an outer shell, known as the trophoblast, and an inner collection of cells termed the inner cell mass (ICM). The morula begins as a solid mass of totipotent blastomeres that undergoes compaction and cavitation to transform into the blastula (non-mammalian term) or blastocyst (human development). Gastrulation also promotes the retention of global left and right symmetry and the loss of bilateral symmetry in specific organs such as the heart.Īfter fertilization, the single-celled zygote undergoes multiple mitotic cleavages of the blastomeres to change from a two-celled to a 16-celled ball or morula. These axes are the dorsal/ventral axis, also termed the anterior/posterior or rostral/posterior axis, and the cranial/caudal or superior/inferior axis. In addition to setting the embryo up for organ formation, gastrulation provides a mechanism to develop a multileveled body plan that demarcates anatomical axis formation. Each germ layer corresponds to the development of specific primitive systems during organogenesis. In triploblastic organisms such as reptiles, avians, and mammals, gastrulation attains a three tissue-layered organism composed of endoderm, mesoderm, and ectoderm. Gastrulation is an early developmental process in which an embryo transforms from a one-dimensional layer of epithelial cells, a blastula, and reorganizes into a multilayered and multidimensional structure called the gastrula. Gastrulation is a critical process during week 3 of human development.
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