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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.
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.
Thymic hypoplasia is a condition in which the thymus is underdeveloped or involuted, leading to a reduced number of T cells. There are two main causes of thymic hypoplasia, DiGeorge syndrome, and Ataxia-telangiectasia syndrome, both cause a reduction of thymic cells. In DiGeorge syndrome the parathyroid glands are underdeveloped, this results in less parathyroid hormones which results in hypocalcemia. On the other hand, in Ataxia-telangiectasia syndrome, the parathyroid glands develop normally, this results in normal levels of parathyroid hormones which results in normal levels of calcium in the blood.
Symptoms of thymic hypoplasia include repeated infections because of a weakened immune system due to a T cell deficiency. In DiGeorge syndrome, there are also symptoms caused by hypocalcemia such as osteoporosis and tetany, or involuntary contraction of muscles. Other symptoms associated with this disorder are congenital cardiac defects like tetralogy of Fallot, cleft palate, and learning disabilities. In ataxia telangiectasia, there can also be symptoms like ataxia which is a problem with muscle coordination, telangiectasia, which are dilated blood vessels in the skin and eye, and an increased risk of cancer.
Treatment is focused on managing symptoms and preventing complications, and can include antibiotics and antiviral medications to prevent infections, nutritional support to promote growth, and surgical interventions for cardiac abnormalities or cleft palate.
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