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THE ENDOCRINE SYSTEM
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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. |
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The pituitary gland
of a developing frog is depicted below. |
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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. |
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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). |
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The cells of the human
thyroid and parathyroid glands are depicted in the following images. |
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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). |
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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). |
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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. |
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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) |
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