MadSci Network: Cell Biology
Query:

Re: Why is it more efficient for a holocrine cell to die than use exocytosis?

Date: Tue Nov 4 10:52:11 2003
Posted By: Brian Foley, Molecular Genetics Staff Scientist
Area of science: Cell Biology
ID: 1065190328.Cb
Message:

Let me first say that I am not an expert at all on holocrine cells, so my answer might not be what you are looking for.

The short answer to your question is another question: Are you sure that cell death is in fact more efficient than exocytosis?

Holocrine cells are cells in glands that release hormones or other secretions when they die and break open. As far as I know there is no evidence that this process is more efficient than exporting hormones or other secretions via exocytosis. I assume that you got the idea that it was more efficient because you have the idea that life always evolves to the "most fit" and/or "most efficient" state possible. This is not the case, there are seemingly infinite numbers of examples of very unfit and/or ineficient systems in biology. Darwin's sound byte should have been "elimination of the weakest" rather than "survival of the fittest", or as Richard Dawkins has stated: "survival of the survivors".

Systems that work tend to survive, whether or not they work "optimally" or "most efficienctly". All living organisms are complex combinations of many different systems, and only through genetic recombinations (primarily reassortment of chromosomes during sexual reproduction, but also including gene transduction by phages and viruses, and other mechanisms of DNA exchange) can one system be exchanged for another. For example wolves don't breed with bears, so there is no chance of wolves inheriting all of the systems involved in hibernation.

At the cellular level, similar principles apply. A gland that produces a secretions by cell death can't somehow observe that another gland produces its secretions by exocytosis and convert to using exocytosis itself. Glands that produce excretions via cell death probably did not evolve from the type of cell that has an exocytosis pathway "turned on". All cells in an organism carry the full genetic code, and thus the potential for turning on any such pathway seems to be present. However, the developmental process that differentiates different cell types seems to be quite irreversible in most cases.

Very early in the development of a fertilized egg, some type of genetic "switching mechanism" begins in sequential order. An early switch forms 3 main types of cells: endoderm, mesoderm, ectoderm. Later each of these main types of cell will be switched into sub-specialties. For example, some nerve cells will on the track to become brain tissue, while other never cells will be on the track to become nerves that control muscle contraction. "Glands" are defined as any tissue that secretes something. Examples are sweat glands, mammary glands, pituitary glands, salivary glands, etc...

I am not sure, but I suspect that some glands are formed from cells that began on the nerve tissue pathway, while others are fromed from cells that began on the "gut" or muscle pathways. It is possible that the "switches" that turn on or off the exocytosis pathway are already thrown before the final development to a particular gland tissue occurs. If so, this could be an explanation of why some glands are able to use exocytosis, and others are not. I don't have a developmental biology textbook handy to be able to look this up. A GOOGLE search for "developmental biology glands" finds some interesting pages:

Nature Developmental Biology
"...mammary glands, sweat glands and hair follicles all develop from the same discrete accumulation of stem cells resting in the primitive epidermis, the outermost cell layer of an embryo..."

The end result, is that the things we group together under the label "gland" are not at all similar really. They all secrete something, but some secrete milk, others secrete sweat, and others secrete hormones. Their methods of making secretions are as diverse as the products they secrete.


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