Mammalian Development: Formation of blastocyst to development of organs
II - Early Development A. Cleavage Cleavage is when the fertilized egg divides several times to create a multicellular embryo. This process occurs as the embryo travels along the oviduct towards the uterus (figure 1). Cleavage ends when a blastocyst is formed. 1. Events in the ampulla The first cleavage starts when "arrested" meisos in the egg resumes. At this time sperm DNA are involved. The first cleavage is usually meriodional, producing 2 cells (figure 2a). Production of four cells is achieved through rotational cleavage where one blastomere divides meriodionally and the other one equatorially (figure 2b). 2. Events in the Isthmus The blastomeres divide further to produce 8 cells. At this point compaction happens. During compaction, blastomeres adhere to each other forming a compact ball of cells. These compacted cells divide further to produce a morula (figure 2d). The 8-cell morula is composed of a small group of internal cel...