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The fourth cleavage event of the developing embryo results in a nonpolarized inner cell mass (ICM, pluriblast, embryoblast), enveloped by the outer, polarized trophoblast layer of cells. The trophoblast cells form an inner cavity (blastocoele), whose formation indicates the bastocyst stage. While the trophoblast will ultimately form the outer chorionic sac and the fetal component of the placenta, the inner cell mass, will give rise to all embryonic tissues and to some of the extraembryonic membranes.
The ICM segregates into a bilaminar embryonic disc (bilaminar blastoderm) which consists of two epithelial layers, each of a distinct lineage: the external (dorsal) epiblast and the internal (ventral) hypoblast. The next developmental stage is gastrulation, in which waves of migrating cells convert the ICM into a trilaminar embryonic disc, which is comprised of three germ layers (ectoderm, mesoderm and endoderm) that contribute to the formation of many organs, often with contributions of two or all three of the germ layers.
The ectoderm forms the central and peripheral nervous systems and epidermis, and contributes to the adipose and heart as well as to numerous other organs. The ectoderm forms many of the sensory organs (eye, ear, nose), and is also the source of Rathke's pouch, an invaginating diverticulum of the stomodeal roof which ultimately detaches from the stomodeum and becomes the adenohypophysis of the pituitary gland.
The mesoderm produces and contributes to the blood, endothelium, heart, kidney, reproductive system, bones, skeletal, smooth muscle and connective tissues. The mesoderm also contributes to tendons, ligaments, dermis and cartilage.