Thymic aplasia

In thymic hypoplasia, thymic refers to the thymus which is an immune organ that sits between the lungs, hypo- refers to under, and -plasia refers to development.

So, thymic hypoplasia is a condition where the thymus is underdeveloped and has a reduced number of cells.

By week 4 of development, the embryo takes on a more recognizably “human” form—but to be honest, it still looks more like a shrimp than a baby.

At the head end of this little shrimp-like creature, a set of structures called the pharyngeal apparatus begins to develop, consisting of pharyngeal arches, clefts, and pouches.

The components of the pharyngeal apparatus develop into various head and neck structures, and sometimes multiple arches join together to give rise to a single structure.

Now, the epithelial tissue of the embryo’s third and fourth pouch turns into the inferior parathyroid glands and superior parathyroid glands, while the epithelial tissue that lines the ventral region of the third pouch forms the thymus.

Both glands then go on to break off from the pharyngeal wall and eventually attach to the posterior side of the thyroid.

The thymus now free, migrates down the middle of the pharynx, until it ends up in its final position in the front of the thorax where it fuses with its counterpart from the opposite side.

During childhood, the thymus occupies considerable room behind the sternum, in a part of the chest known as the mediastinum, a space in the chest between the lungs that also contains the heart.

But when people become older, it atrophies and is replaced by fatty tissue.

It's here in the thymus, where certain immune cells from the bone marrow mature into T lymphocytes or T cells, where the T stands for Thymus.

Once mature, these T cells help defend the body against infections by activating other cells of the immune system and checking the body’s cells for viral infection or abnormalities like cancer.

In thymic hypoplasia, a small portion of chromosome 22 is deleted in a condition known as DiGeorge syndrome or 22q11.2 deletion syndrome.

On chromosome 22, there’s a gene called TBX1 that controls the development of the 3rd and 4th pharyngeal pouch during the prenatal period.

So, when there’s a 22q11.2 deletion, there’s no TBX1 gene, and the thymus and parathyroid glands ends up underdeveloped.

In thymic hypoplasia, the immature T cells from the bone marrow don’t have a place to go to mature, and so these people often have a deficiency in mature T cells.

The parathyroid glands also remains underdeveloped.

Normally it secretes parathyroid hormone which promotes the absorption of calcium in the intestines as well as kidneys while breaking down bones to release more calcium into the blood.

But when the parathyroid glands are underdeveloped, there’s less parathyroid hormone which results in hypocalcemia.

Thymic hypoplasia also happens in people with ataxia-telangiectasia syndrome, and there It's caused by a defect in the ATM gene, or Ataxia-Telangiectasia Mutated gene, located on chromosome 11.

The ATM gene encodes a protein whose job is to recognize DNA damage and activate proteins to fix the damage.