Morphological characteristics of dying cells in axial structures of developing human embryos

Programmed cell death (PCD) is a widespread phenomenon in the development of vertebrates. In most cases, dying cells during development exhibit generalized morphological features typical of apoptosis. We analyzed the morphological features of dying cells in the developing axial structures of 5 human embryos between 5 and 8 weeks of postovulatory age. Cell death in the axial structures, i.e. spinal cord, notochord and surrounding mesenchyme and somites, was analyzed using light and electron microscopy. Tissue samples were taken from the cervicothoracic region of normal human conceptuses. Two morphological types of cell death were found: apoptosis which was characterized by round or semilunar nuclear chromatin condensations, condensation and shrinkage of the cytoplasm and formation of apoptotic bodies, and cell death without the morphological features of apoptosis which was characterized by pyknotic nuclear chromatin condensations, vacuolated cytoplasm and the formation of numerous intercellular spaces. Apoptotic death occurred during the 5th week of normal development in all the axial structures. Later, apoptotic death appeared in all the axial structures, with the exception of the notochord, where some dying cells displayed features of secondary necrosis. According to our findings, apoptosis seems to be the most frequently observed type of PCD, but it is not the exclusive type of morphological cell death during the development of axial structures in human embryos.