Human Developmental Biology. Danton Inc. O'Day
term “eggs” refers to all stages of oogenesis not just mature eggs. In short it reflects the reproductive status of the ovary—the number of potential eggs that a female could produce at each stage of her life. As can be seen from the graph these changes in number are massive.
Figure 3.5. The changes in germ cell number in the female ovary.
Here’s what the graph reveals:
•The number of eggs increases to around 7 million by six months in utero.
•Then the number of eggs decreases due to apoptosis (controlled cell death).
•At birth approximately two million eggs remain.
•By the time of puberty onset only around half a million eggs are present.
•A continual decline occurs until menopause due to ovulation and egg death.
Apoptosis: The Major Events
The death of cells is an important part of human cell biology and embryonic development. It occurs during oogenesis, brain development and the formation of our toes and fingers, to name a few events. Apoptosis is the formal name for the controlled regulation of cell death. It involves the activation of specific genes and the activation of signal transduction pathways that underlie the cell death program. Unlike tissue necrosis caused by external damage, apoptosis is a controlled process in which cells show a precise breakdown in a series of well-defined steps that are under active study. Apoptosis allows the body to remove specific cells without damaging surrounding cells and tissues.
In biomedical terms, understanding how to control apoptosis would open up potential avenues for the targeted destruction of tumors and other diseased tissues while leaving normal tissues intact. Brain cell death is a major problem in Alzheimer’s and other neurodegenerative diseases, so understanding what triggers this death and finding ways to prevent it would be a major advance in human medicine.
Apoptosis is characterized by a series of well-defined stages as summarized in the following figure (Figure 3.6) and detailed below.
Figure 3.6. The sequence of events in apoptosis.
The major cellular events in apoptosis:
1.Cell shrinkage: cells become smaller and lose normal cell-to-cell contacts.
2.Chromatin condensation: chromatin initially condenses to the periphery of the nucleus and ultimately nuclear fragmentation occurs. During these events DNA is digested in specific ways leading to what is called “laddering” in DNA gels.
3.Cell membrane blebbing: small and numerous bulges form on the cell surface.
4.Cell fragmentation: cells break up producing what are called “apoptotic bodies”
5.Phagocytosis of “apoptotic bodies”: cell fragments are ingested by macrophages.
A Cascade of Caspases Cleaves Proteins
Apoptosis is regulated by a diversity of signaling pathways all of which involve caspases (Figures 3.7, 3.8). There is a large family of caspases in humans that exist in an inactive form (pro-caspase) that becomes activated by limited proteolysis: pro-caspase → active caspase.
Figure 3.7. Caspase activation mediates apoptosis.
Caspases are a family of cysteine proteases, protein-digesting enzymes that that cleave proteins after aspartic acid residues. The caspases work in cascades (a number of caspases working in sequence) digesting a diversity of proteins that underlie specific apoptotic events. Typically initiator caspases (e.g., caspase-2, 8, 9, 10) activate effector caspases (e.g., caspase-3, 6, 7) that digest specific proteins or activate other specific caspases (e.g., caspase-1, 4, 5, 11, 12, 13, 14) that have roles in inflammation.
Caspase Cascade:
Initiator Caspases → Effector/Executioner Caspases
Effector caspases are sometimes called executioner caspases.
Apoptosis is Induced by the Intrinsic or Extrinsic Pathway
There are many variations on how apoptosis is regulated. As summarized in Figure 3.8, the events of apoptosis leading to cell death can be induced via one of two major pathways. Since the way this occurs during oogenesis is still a mystery, we will detail each of these. Continued research will reveal the importance of each pathway in egg cell death.
The intrinsic pathway, which occurs from within the cell, involves the release of cytochrome C from mitochondria which then activates the caspase cascade. In contrast the extrinsic pathway begins with the activation of death receptors on the cell surface, which in turn initiate signal transduction events leading to caspase activation and resultant cell death.
Figure 3.8. The intrinsic pathway and extrinsic pathway of apoptosis.
The Intrinsic Pathway: Cell Death Starts Inside
As discussed above, apoptosis can be initiated by internal events (i.e., "intrinsic pathway") involving the disruption of mitochondria and the specific enzyme cytochrome C, in turn leading to the downstream activation of caspases. Before we continue, let’s quickly review mitochondrial structure and function. Mitochondria are called the “powerhouses of the cell” because they are the site where most of the ATP is generated via aerobic metabolism. The following figure (3.9) shows that an outer membrane surrounds each mitochondrion. The inner membrane is folded into the matrix as cristae. F1 particles, which are ATPase enzymes, are localized on the cristae and oriented into the matrix.
Figure 3.9. The structure of a mitochondrion.
This design is very effective because it organizes the metabolic functions of the mitochondria as shown in the following figure (Figure 3.10). Thus glycolysis (anaerobic metabolism) generates ATP in the cytoplasm as well as providing the substrate molecules for aerobic metabolism, leading to ATP production within the mitochondria. This occurs via tightly linked events: the Krebs cycle in the matrix
and the electron transport chain in the cristae, coupled with ATP synthesis via F1 ATPases bound to the cristae.
Figure 3.10. Compartmentalization within a mitochondrion.
Most readers will have learned the details behind mitochondrial structure and function in high school and introductory biology books. But what is important here is the presence of cytochrome C in the electron transport chain and its role in apoptosis (Figure 3.11).
Figure 3.11. Apoptosis initiated by mitochondrial damage.
Typically this molecule is tightly bound within the cristae. But, as shown in Figure 3.11, when mitochondria are disrupted, the cytochrome C located on the inner mitochondrial membrane gets released allowing it to bind to and activate caspase 9. Similarly Apaf-1 (apoptotic activating factor-1) is bound to mitochondria via Bcl-2 (beta-cell lymphoma-2). Bcl-2 is found in a number of cancers where it suppressed apoptosis. Mitochondrial disruption leads to the release of Apaf-1 which also