Thyroid And Thymus: Formation And Clinical Insights

Question 1. Development of Thyroid
Answer:

The thyroid gland develops from a median endodermal thyroid diverticulum
This grows down in front of the neck from the floor of the primitive pharynx
The lower end of the diverticulum enlarges to form the gland
The rest of the diverticulum remains narrow and forms thyroglossal duct Most of it soon disappears
Remnants of it forms thyroglossal cysts or fistulas

Question 2. Development of Thymus
Answer:

The thymus develops from the endoderm of third pharyngeal pouch
After separation from the inferior parathyroid rudiment, each thymic rudiment has two parts
- Thinner cranial part
  - It forms cervical part of thymus
Broader caudal part
- Broader parts of both the sides enters the thorax and become united to each other by connective tissue
The endodermal cells of the thymus are invaded by vascular mesoderm
This mesoderm partially breaks up the thymic tissue into isolated masses
This results in its lobulated surface

Question 3. Reichert’s Cartilage
Answer:

It is cartilage of second pharyngeal arch
It is composed of two distinct cartilagenous segments joined by a faint layer of mesenchyme

Reichert’s Cartilage Derivatives:

Stapes of middle ear – formed from dorsal ends of cartilage
Lesser cornua and body of hyoid bone – formed from ventral portion of cartilage
Styloid process of temporal bone
Stylohyoid ligament – formed from cartilage lying between hyoid bone and styloid process

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