Histochem Cell Biol (2006) 126:149–158 DOI 10.1007/s00418-006-0214-1 REVIEW Cell death in development: shaping the embryo Carlos Penaloza · Lin Lin · Richard A. Lockshin · Zahra Zakeri Accepted: 8 June 2006 / Published online: 1 July 2006 © Springer-Verlag 2006 Abstract Cell death in animals is normally classiWed ter, as in other embryos, cell death plays a major role in as type I (apoptotic), type II (autophagic) or necrotic. shaping and sculpting the embryo. In those situations Of the biologically controlled types of death, in most that have been carefully studied, cell death is under embryos apoptosis is the most common, although in tight genetic control (including regulation of gene metamorphosis and in cells with massive cytoplasm products whose function in cell death is not yet known, type II is often seen, and intermediate forms are seen. such as cdk5), with activation of apoptosis sometimes For vertebrate embryos other than mammals, apopto- regulated by local environmental variables. sis is not seen prior to gastrulation but thereafter is used to sculpt the organs of the embryo, while overpro- Keywords Cell death · Apoptosis · Development · duction of cells with subsequent death of excess cells is cdk5 · Caspases a common means of generating high speciWcity with low information cost. In zebraWsh at least, the inability It is now well understood that cell death is a funda- of embryos prior to the maternal-zygotic transition to mental aspect of development of almost all organ- undergo apoptosis appears to derive from the inability isms. Historically cell death was mostly noticed in of the cells to resist lysis once apoptosis begins, rather developing systems and for many years considered to than any inhibition of apoptosis. In mammalian be important only in development. In the last 15 years embryos, apoptosis is seen during cavitation. Thereaf- this image has changed, and the importance of cell death has been realized in many other systems and situations that govern our lives. However, the study of cell death in development still has much to teach us about the embryo and about the mechanisms of cell Histochemistry and Cell Biology Lecture presented at the 48th death. Embryonic development is dynamic and well Symposium of the Society for Histochemistry in Stresa, Lake orchestrated. Cells in very close proximity proliferate, Maggiore, Italy, 7–10 September 2006 diVerentiate, and die, and their decision making X C. Penaloza · L. Lin · Z. Zakeri (&) appears to be in uenced by their microenvironment. Department of Biology, Queens College and Graduate How one cell dies whereas the neighboring cells Center of CUNY, Flushing, NY 11367, USA divide is presumptively regulated by the internal and e-mail: [email protected] external signals that they receive. To understand how R. A. Lockshin these signals are presented and recognized during Department of Biological Sciences, St. John’s University, development we as well as others have used many Jamaica, NY 11439, USA methodologies and systems. Several laboratories had to develop speciWc markers and in doing so learned Present Address: L. Lin much about the steps taken by a dying cell and how Preclinical Department, Medarex Inc., Bloomsbury, developmental cell death resembles and is diVerent NJ 08804, USA from other types of cell death. 123 150 Histochem Cell Biol (2006) 126:149–158 Cell death, its types and its characterization (MZT). However, when zebraWsh embryos are exposed to cycloheximide, both 256-cell embryos Cell death has been classiWed into three classical types: (prior to the MZT) and 1,024-cell embryos (after the apoptosis, autophagy (also referred as lysosomal cell MZT) activate caspase-3 after approximately 5 h. Pre- death), and necrosis (for review, see Zakeri and Lock- MZT embryos lyse almost immediately thereafter and shin 2002), and all three are seen during development. thus show no morphological signs of apoptosis, Both apoptosis and autophagy are considered to be whereas the older embryos persist for two more hours under biological control and thus programmed (pro- and develop all the signs of classical apoptosis, includ- grammed cell death, PCD). PCD was originally deWned ing phosphatidylserine exposure, DNA fragmentation, by the demonstration of a clear physiological and bio- and margination of the chromatin (Negron and Lock- chemical sequence leading to death in a developmental shin 2004; J.F. Negrón and R.A. Lockshin, in press). situation (metamorphosis), but now the term is under- stood to refer to any biologically controlled and man- Lysosomal, autophagic PCD (Type II) aged cell death. The diVerent types of cell death have speciWc features that are used both to identify them and The appearance of large autophagic vacuoles, lysosomal to characterize them. Although many features are spe- derivatives that consume the bulk of the cytoplasm, is ciWc the boundaries among the diVerent types are not characteristic of this type of PCD. This type of cell death nearly as well deWned as many assume, and there are unlike apoptosis is not characterized by the prompt overlapping features among the diVerent types. It is also destruction of DNA. In most of the cells undergoing noteworthy that there is much crosstalk among the Type II PCD, the cytoplasm is bulky and cannot be easily diVerent organelles; this crosstalk allows autophagy to shed to be phagocytosed. The function of the phagocyte is convert to apoptosis or vice versa, and it allows an apop- conducted within the cell. When approximately 80% of tosis typically triggered by ligation of receptor molecules the cytoplasm has been destroyed, the condensation of to follow the pathway of metabolically activated apopto- the cytoplasm and coalescence and migration of the chro- sis. Thus if one pathway is blocked, a cell may still die by matin become apparent, and only then it is possible to a second, highly organized and biological pathway. This detect by electrophoresis a DNA ladder, indicating DNA result is quite common and has led to considerable con- cleavage by an apoptosis-type DNase. It is not certain fusion among researchers not attentive to the possibility. that the autophagy is truly a type of cell death, as opposed to the response of a cell under stress and attempting to Apoptotic PCD (Type I) survive by consuming its resources. Thus, as in other instances in which cells undergo autophagy, there is Apoptosis or type I cell death is characterized by a potential recovery, and the autophagy is a recycling cell’s loss of adherence to neighboring cells as well as mechanism, used under stressful conditions. In the docu- to extracellular matrix. Morphological changes include mented deaths of cells by autophagy, as in insect meta- cell rounding and condensation of nuclear material. morphosis, organelles are eliminated in waves. Glycogen, The chromatin coalesces into one or a few masses ribosomes, and mitochondria are eliminated in sequence. along the nuclear membrane. PCD type I is also char- In other instances of autophagic death, such as death of acterized by fragmentation of DNA to form a ladder prostatic epithelium following castration and possibly when electrophoresed. There is release of cytochrome post-lactational involution of mammary epithelium, the c and other materials from mitochondria and activation actual commitment to die occurs relatively late, and cells of caspases, speciWcally caspase 3. Cells ultimately frag- can revert and recover if given the appropriate hormonal ment into blebs that are taken up by phagocytes. At support. PC12 cells undergoing starvation from nerve the Wnal stages of apoptosis, phosphatidylserine (PS) is growth factor (NGF) withdrawal can recover if the NGF actively extruded from the internal face of the cell is provided. They are capable of recovering until almost membrane of the dying cell; the exteriorized PS serves immediately before they lyse; this point of no return is as one of the markers that identify the cell as a target coincident with the Wnal autophagic destruction of mito- for phagocytosis. This type of cell death can be greatly chondria, presumptively terminating the cells’ ability to aVected by levels of ATP: if levels of energy are not maintain ionic pumps (Xue et al. 1999, 2001). suYcient, cells can undergo partial apoptosis, leading Both type I and type II cell death have been referred to diVerent appearance and possibly misinterpreta- to as programmed cell death. Like proliferation and tions. For instance, it is commonly noted that the cells diVerentiation, cell death, especially PCD, also plays a of early non-mammalian embryos cannot undergo prominent role in normal development. It is essential apoptosis prior to the maternal-zygotic transition for the removal of unwanted cells and is critical both 123 Histochem Cell Biol (2006) 126:149–158 151 for restricting cell numbers and for tissue patterning in early embryos is a solution to the problem of how during development (Chanoine and Hardy 2003; Cou- much genetic information would be needed to con- couvanis et al. 1995; Glücksmann 1951). struct an animal. To take an easily understood image, the quantity of information to connect 1,000 neurons to Necrosis 1,000 speciWc targets will vary according to several assumptions, but even in the simplest version, the Necrosis characterized as uncontrolled death, occurs as information required would be immense. It is surpris- a result of high levels of stress, such as injury. Rapid and ing but mathematically demonstrable that much less large changes in critical ions, osmolarity, and pH lead to genetic information (DNA sequences) is needed to the lysis of the cell. In the absence of adequate mito- produce an excess number of presynaptic neurons that chondrial function and eVective ion pumps, the high can generally and with little requirement for speciWcity concentration of impermeable protein as the primary Wnd their way to the target area, and to allow only intracellular anion tends to draw water in, and under those that make successful synapses to survive.