Cells in the first weeks of embryo growth are in a hpyoxic enviroment and they are glyolytic. Glycolysis does not consume oxygen. As oxygen levels in the placenta slowly increase over the weeks, the cells rely more on mitochondrial oxidative metabolism. This is called oxidative phophorylation or OxPhos because the process consumes oxygen and the end result is the conversion of adenosine diphosphate (ADP) to adenosine triphosphate (ATP). Oxygen is burned and phosphate groups are transferred onto ADP.
ROS are Reactive Oxygen Species. This is when electrons “bounce out of” enzyme systems supporting oxidative metabolism and they become vacuumed up by oxygen to form superoxide. They are produced by mitochondria at high levels. They can oxidize lipids causing oxidative damage. This is how the system is designed.
Let’s review.
Glycolysis
- Does not use Oxygen
- Ancient . ~4 billion years old
- Happens in the cytoplasm
- All cells can do it
- Cells take in glucose or fructose
- The sugars are converted to pyruvate
- Pyruvate can be exported as lactate
- Cells make a small amount of ATP
- Cytoplasmic NAD+ is reduced to NADH as a byproduct
- The carbon backbone of the glucose can be retained as pyruvate to use for building fats and many amino acids
- Pyruvate CAN BE burned in oxidative phosphorylation in oxidative cells
- ROS is minimal
- Stem cells, cancer, immune cells, adipocytes of obese humans
- Reasonably clever
Mitochondrial Oxidative Phosphorylation
- Requires oxygen as the terminal electron acceptor
- Less ancient: ~2 billion years ago
- Happens in mitochondria
- Carbon backbone of fuels is released as CO2
- Produces a large amount of ATP
- Mitochondria produce ROS. This is BY DESIGN.
- Only Eukaryotic cells can do it: plants, animals, fungi, yeast. NOT bacteria or archaea
- Can burn carbohydrates (via glycolysis), fats, amino acids (ie protein), alcohol
- Fire In A Bottle
- Higher ROS levels
- Differentiated cells
- More clever
So we can add to our word association.
Hypoxic: Anabolic. Growthy. Stemness. Fetal. Glycolytic.
Normoxic: Catabolic. Stable. Adult. Differentiated. Oxidative. ROS.
Sponges are Neat
Sponges are the most ancient things that we call animals. They’re perpetually in the just figuring out stage. It has been suggested that fibrous material found in 890 million year old reef fossils could be from sponges. (Turner, 2021) Sponges are primordial.
Sponges are neat to think about because they only have about 5 cell types. The archaeocytes are within the tissues of the sponge where oxygen penetration is limited. The are “totipotent”, meaning they are stem cells which can form any of the other types of cells in the sponge.
Hypoxic. Glycolytic. Stemness.
This system is primordial. It has been this way since animals.
600 Million year old sponge
| Cell type | Main roles | Where in the body | Totipotent? | Typical O₂ micro-environment* |
|---|---|---|---|---|
| Pinacocytes | Form the external “skin” (pinacoderm); some contract to control size & shape | Thin surface sheet over the whole sponge | No | Normoxic – cortex and sub-cortical spaces remain well-oxygenated while the animal is pumping |
| Porocytes | Tubular valves that make each ostium; regulate inflow | Span the pinacoderm from exterior to sub-dermal canal | No | Normoxic – lumen is flushed with ambient seawater; cytoplasm sits in the same oxygen-rich cortex as pinacocytes |
| Choanocytes (collar cells) | Drive water current with flagella; capture bacteria; can transform into sperm | Line the internal flagellated chambers (choanoderm) | No (pluripotent at most) | Variable – normally oxygenated when pumping, but chambers turn hypoxic → anoxic within minutes after pumping stops or during osculum closure |
| Archaeocytes / amoebocytes | Digest and distribute food; give rise to gametes and every other somatic cell | Motile throughout the mesohyl (gelatinous matrix between pinacoderm & choanoderm) | Yes – principal totipotent stem cells (BioMed Central) | Sub-oxic to anoxic – reside in the sponge interior that becomes O₂-depleted 0.5–6 mm below the surface when pumping slows or ceases |
| Sclerocytes / spongocytes / collencytes | Secrete skeletal elements: mineral spicules (sclerocytes) or spongin/collagen fibres | Embedded in the mesohyl near growing spicules/fibres | No (lineage-restricted derivatives of archaeocytes) | Sub-oxic to anoxic – same mesohyl environment as archaeocytes |
Hoffmann, Friederike. “Microbial sulfate reduction in the tissue of the cold-water sponge Geodia barretti (Tetractinellida, Demospongiea).” (2003).
Kumala L, Larsen M, Glud RN, Canfield DE. Spatial and temporal anoxia in single-osculum Halichondria panicea demosponge explants studied with planar optodes. Mar Biol. 2021;168(12):173. doi:10.1007/s00227-021-03980-2
Turner EC. Possible poriferan body fossils in early Neoproterozoic microbial reefs. Nature. 2021;596(7870):87-91. doi:10.1038/s41586-021-03773-z
Yin Z, Zhu M, Davidson EH, Bottjer DJ, Zhao F, Tafforeau P. Sponge grade body fossil with cellular resolution dating 60 Myr before the Cambrian. Proc Natl Acad Sci USA. 2015;112(12). doi:10.1073/pnas.1414577112