THE ENDOCRINE SYSTEM

THE PITUITARY GLAND

The pituitary gland begins its development as a pouch which forms in the roof of the pharynx (Rathke’s pouch). This tissue must migrate to the site of the developing hypothalamus. In humans, this migration does not always occur flawlessly. In some individuals, pituitary tissue remains in the roof of the pharynx as the pharyngeal hypophysis.

The pituitary gland of a developing frog is depicted below.

THYROID, PARATHYROID, AND THYMUS GLANDS

The initial stages of the development of the thyroid, parathyroid, and thymus glands occur around the embryonic pharyngeal arches. These tissues must subsequently migrate to reach their final destinations. In the illustration below, note that the thymus and inferior parathyroid glands develop superior to the ultimobranchial body (part of the thyroid) and superior parathyroid glands, unlike their locations in the adult.

The major portion of the thyroid gland begins its development in the floor of the pharynx near the tongue. In Amphioxus, the structure which is homologous to the thyroid gland, the endostyle, develops in the floor of the pharynx (in images below).

The cells of the human thyroid and parathyroid glands are depicted in the following images.

In humans, the thyroid must migrate from its origin in the floor of the pharynx to its destination around the larynx. As it migrates it remains connected to the tongue to through the thyroglossal duct. In some people a thyroglossal fistula persists as a remnant of this duct and abherrent thyroid tissue may be found in some individuals along the path of the thyroid from its origin in the mouth to its destination around the larynx (Sadler, p. 312).

The parathyroid glands and the thymus must also migrate from their origins to their adult positions (posterior to the thyroid gland for the parathyroid gland, behind the sternum for the thymus). In some individuals, abnormalities in this migration can result in accessory glandular tissue or tissue located in abnormal positions. Thymic cysts can produce neck masses in cervical region. Thymic neck masses in infants may be the remnants of descending thymic tissue or the only site of thymic tissue (Delbrouck, 2002; Loney, 1998; Tovi, 1978).

THE ADRENAL GLAND

The tissues of the adrenal cortex and adrenal medulla are not united into a single structure in lower vertebrates. During human development, they also begin their development as separate structures until the chromaffin cells of the medulla invade the adrenal cortex (Sadler, p. 383). The human adrenal gland is depicted in the following image.

PANCREAS

The endocrine and exocrine portions of the pancreas are separate structures in jawless fish. In jawed vertebrates, these cell types exist in the same organ. The fish embryos, the cells of the endocrine pancreas form separately in bilateral rows and later form solid islets. (Biemar, 2001)