Fourth pharyngeal pouch

The fourth pharyngeal pouch is an embryological structure that plays a crucial role in the development of the human endocrine system. It is one of the four pairs of pharyngeal pouches that form during the early stages of embryogenesis.

Development

The fourth pharyngeal pouch arises from the endoderm of the developing embryo. During the fourth week of gestation, the pharyngeal pouches begin to form as outpocketings of the lateral walls of the pharynx. The fourth pouch is located caudal to the third pharyngeal pouch and is separated from it by the third pharyngeal cleft.

Derivatives

The fourth pharyngeal pouch gives rise to several important structures:

• The superior parathyroid glands: These glands are responsible for regulating calcium levels in the blood through the secretion of parathyroid hormone.
• The ultimobranchial body: This structure eventually becomes incorporated into the thyroid gland, contributing to the formation of the parafollicular cells (also known as C cells), which produce calcitonin.

Clinical Significance

Abnormal development of the fourth pharyngeal pouch can lead to various congenital disorders. One such condition is DiGeorge syndrome, which is characterized by the absence or underdevelopment of the thymus and parathyroid glands, leading to immunodeficiency and hypocalcemia.

DiGeorge Syndrome

DiGeorge syndrome, also known as 22q11.2 deletion syndrome, is a disorder caused by the deletion of a small segment of chromosome 22. The syndrome affects the development of the fourth pharyngeal pouch, leading to defects in the structures derived from it. Patients may present with a range of symptoms, including cardiac anomalies, facial dysmorphisms, and developmental delays.

Embryological Context

The pharyngeal pouches are part of the pharyngeal apparatus, which also includes the pharyngeal arches and clefts. Each pouch is lined by endoderm and interacts with the surrounding mesoderm and ectoderm to form various head and neck structures.

Pharyngeal Apparatus

The pharyngeal apparatus is a complex structure that contributes to the formation of the face, neck, and associated structures. It consists of:

• Pharyngeal arches: These are mesodermal structures that give rise to muscles, bones, and nerves.
• Pharyngeal clefts: These are ectodermal invaginations that separate the pharyngeal arches externally.
• Pharyngeal pouches: These are endodermal outpocketings that form internally.

See Also

• Pharyngeal pouch
• Embryology
• Endocrine system
• DiGeorge syndrome

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