Development of endocrine glands - thyroid, parathyroid and adrenal gland Flashcards
I. Development of Adrenal glands
The adrenal (or suprarenal) glands develop from two cell types. The cells of the cortex differentiate from mesoderm of the posterior abdominal wall near the site of the developing gonad (Figure 39.2).
The adrenaline and noradrenaline secreting cells of the medulla are derived from migrating neural crest cells that formed a sympathetic ganglion nearby. These cells become surrounded by the cell mass of the cortex.
The foetal cortex produces a steroid precursor of oestrogen that is converted to oestrogen by the placenta. More mesenchymal cells surround the foetal cortex and will become the layers of the permanent cortex.
The adrenal glands are exceptionally large in the foetus because of the size of the cortex which regresses after birth. Substances secreted from the adrenal glands are involved in the maturation of other systems of the embryo, such as the lungs and reproductive organs.
II. Development of Thyroid gland
This is the first endocrine gland to develop, beginning at about 24 days between the first and second pharyngeal pouches from a proiferation of endodermal cells of the gut tube. It begins as a hollow thickening of the midline where the future tongue will develop. It eventually becomes solid and then splits into its two lobes.
As the thyroid descends into the neck it remains connected to the tongue via the thyroglossal duct with an opening on the tongue called the foramen cecum. The duct degenerates between weeks 7 and 10 and the thyroid reaches its end location anterior to the trachea by week 7.
C cells (or parafollicular cells) are derived from neural crest cells that invade the ultimobranchial body (a fifth pharyngeal pouch derivative; see Chapter 43).
III. Development of Parathyroid glands
The inferior parathyroid glands develop from epithelium (endo- derm) of the dorsal wing of the third pharyngeal pouch.
- The cells here move with the migration of the thymus gland into the neck (see Chapter 42).
- When this connection breaks down they become located on the dorsal surface of the thyroid gland.
Endoderm cells of the dorsal wing of the fourth pharyngeal arch begin to collect and differentiate to form the superior parathyroid glands (initially the superior parathyroid glands are inferior to the inferior parathyroid glands).
- These cells are associated with the developing thyroid gland and migrate with it, but for a shorter distance than the cells of the inferior parathyroid glands .
- They also rest on the dorsal surface of the thyroid, but generally more medially and posteriorly.
IV. Development of Hypothalamus
The hypothalamus begins to form in the walls of the diencephalon (see Chapter 44), with nuclei developing here that will be involved in endocrine activities and homeostasis.
