Atrophy, aplasia, and hypoplasia

Definitions & Key takeaways

Atrophy, aplasia, and hypoplasia all refer to degeneration or poor growth of cells and tissues. Atrophy refers to the reduction in size of a tissue, or organ, after it had been normally formed and attained its normal growth. With aplasia there is a complete congenital lack of the cells, tissue or organ, whereas in hypoplasia, precursor cells are present, but they do not develop into their intended organs during embryogenesis. All three conditions can be caused by a variety of factors, including disease, injury, or genetic abnormalities

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Introduction0:00–0:47

Growing is an important part of living. In fact, everything from an individual muscle cell, to a baby blue whale - strives to grow, in order to live and perhaps replicate or reproduce.
Sometimes, however, growth fails to occur, or even reverts back, and we call that atrophy, aplasia, or hypoplasia, depending on the situation.
Let’s break down these words. Atrophy, “a” means “no”, and “trophy”, means nourishment.
So, atrophy means “no nourishment”. Aplasia, “a” means “no” and “plasia” means development.
So aplasia means “no development”, and “hypo” means “under” so hypoplasia is “under formation”. In a nutshell, atrophy is the reduction in size of a cell, organ, or tissue, after it has attained its normal, matured growth.

Atrophy0:47–3:43

This happens either through decrease in cell number or decrease in cell size. Decrease in cell number most commonly happens due to apoptosis, which is controlled type of cell death - a bit like cellular suicide.
An example would be weight loss. In the first few weeks to months of eating healthy and losing weight, the fat cells or adipocytes get smaller but are ready to fill up again with fat.
Over months to years of eating healthy, however, the adipocytes undergo apoptosis - and at that point it’s a bit more difficult to gain back the weight.
Decrease in cell size, however, is a bit more complex. Usually, the first step is the loss of nerve or hormonal supply, both of which provide nourishment to cells.
Then there’s something called the ubiquitin proteasome pathway. You see, cells have a cytoskeleton, which is a framework of various filaments that keep the cell propped up.
As cells start getting less nourishment, those filaments get “tagged” for demolition with a protein called ubiquitin. Ubiquitin proteins start to attach to one another - a process known as polyubiquitination.
And then an intracellular protein complex called a proteasome comes in to destroy all polyubiquitinated filaments, causing the cell to decrease in size.
Some organelles can also be tagged with ubiquitin; and when that happens, a bubble of phospholipid bilayer membrane forms around the organelle, creating a vacuole.
Next, lysosomal vesicles which are filled with degradative enzymes, fuse with the vacuole; destroying the unfortunate organelle.
Like being sent off to the firing squad. An example is muscle atrophy.
Anytime there’s long-standing disuse of muscles, like extended bedrest, zero gravity, or during long study sessions, there can be a loss of muscle mass and strength.
The good news is that this type of muscle atrophy is reversible with exercise. There’s also a more severe form of muscle atrophy which happens in cachexia - or whole body wasting syndrome.
That form of muscle atrophy is often so severe that it’s not typically reversible and happens in severe illnesses like AIDS, or in some forms of cancer patients.
But atrophy is not necessarily pathological - in fact, it’s often a part of normal development. For example, it happens in the thymus.
The thymus a small organ that sits over the heart and is responsible for the maturation of T cells which are part of the immune system.
As children grow and their immune system gets established, the thymus begins to atrophy. This process is called thymus involution.
As a result, the thymus is much larger in young children then it is in adults. Now, aplasia and hypoplasia are somewhat different; they’re both ways in which growth can go wrong during embryogenesis - when our organs develop within the fetus.

Aplasia and hypoplasia3:43–5:08

With aplasia, the precursor cells are completely absent, while in hypoplasia, there are some precursor cells present, but not enough of them.
With aplasia, the organ never forms in the first place- so this is the more severe case. Going back to the thymus again.
In DiGeorge syndrome, a small part of the 22nd chromosome is deleted, so the the genes that code for the thymus are gone.
As a result the cells never have the DNA they need to express the proteins that would turn them into thymic precursor cells.
As a result, the thymus never forms, and the person is left with a severely impaired immune system. Hypoplasia, or underdevelopment, is more common and usually results in milder consequences.
Here there are only some precursor cells available, and that results in a smaller, or misshapen organ. One example is a genetic disease called optic nerve hypoplasia, where there are fewer neurons than normal serving the eye.
As a result, the optic nerve is abnormally thin. The result can vary wildly - from near blindness, to near normal eyesight, all depending on how many axons formed in the first place.

Review5:08–5:32

Alright, as a quick recap - atrophy, aplasia, and hypoplasia are terms for lack or absence of growth in an organ or a tissue.
Atrophy means the wasting of a previously normally mature organ or tissue, aplasia means usually a complete congenital lack of an organ or a tissue, while hypoplasia means a relative congenital lack of cells in an organ or a tissue.
Atrophy, aplasia, and hypoplasia: Video and Causes | Osmosis