After fertilization, the single-celled zygote will undergo multiple mitotic cleavages of the blastomeres to change from a two-celled to a 16-celled ball or morula. The morula begins as a solid mass of totipotent blastomeres but then undergoes compaction and cavitation to transform into the blastula (non-mammalian term) or blastocyst (human development). 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). Cells within the outer ring/shell bind together via gap junctions and desmosomes to undergo compaction, which ultimately forms a water-tight ring/shell called the trophoblast. The outer trophoblast will develop into structures that provided nutrients, helps the growing embryo implant in the uterine lining, and will become part of the placenta. Additionally, the trophoblast cells are essential in the cavitation of the solid morula into a hollowed-ball of cells with an internal cavity. Trophoblast cells utilize the active transport of sodium ions and osmosis of water to form a fluid-filled cavity known as a blastocoel. The cells remaining after cavitation/blastocoel formation are pluripotent ICM progenitor cells, which will give rise to the distinctive formation of the fetus. Rather than being an arrangement of a solid sphere of cells, the inner cell mass is pushed off to one side of the sphere formed by the trophoblast. Together the trophoblastic layer, blastocoel, and ICM define the human blastocyst. From zygote to blastocyst formation, the organism has been surrounded by the zona pellucida, which is a layer of the extracellular matrix that plays a role in the protection and prevention of implantation into the uterine tubes. During blastocyst formation, the zona pellucida begins to disappear from the blastocyst, allowing the ball of cells to proliferate, differentiate, change shape, and eventually implant into the uterine wall. During implantation, the trophoblastic layer, which surrounds the blastocyst, further differentiates into two functionally distinct layers. The outer trophoblast, known as the syncytiotrophoblast, releases digestive enzymes to assist with implantation to the endometrium, this layer also releases human chorionic gonadotropin (hCG, necessary in regulating progesterone secretion) the protein used in many pregnancy tests. The inner trophoblast layer, known as the cytotrophoblast, is a single sheet of cells surrounding the extraembryonic mesoderm. Within the cytotrophoblast is the ball of ICM, and during the second week of human development, the ICM cells spread into a flattened tissue layer and differentiate into a two-layered tissue containing epiblast(columnar epithelial cells) and the hypoblast (cuboidal epithelial cells), which are together known as the bilaminar disc. The formation of the bilaminar disc sets the dorsal or ventral axis as the epiblast cell layer is positioned dorsal to hypoblast. Anatomical location of the bilaminar disc is found between the amniotic cavity and the primitive yolk sac. 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 sack. On the hypoblast is a raised area of columnar cells known as the prechordal plate, this is the earliest delineation of cranial from caudal. Development of the bilaminar disc directly precedes gastrulation, where the end goal during week 3 of development is to transform the human blastocyst into a multi-layered gastrula with endoderm, mesoderm, and ectoderm. Submit Your Manuscript at https://www.imedpub.com/submissions/anatomical-science-research.html or mail us to anatomicalsci@scholarlymed.com Regards Jacqueline Managing editor Journal of Anatomical Sciences and Research