Two Flashcards
(20 cards)
Describe the formation of the hypothalamus.
The formation of the hypothalamus begins soon after the appearance of the
hypothalamic sulcus which separates the thalamus above from the hypothalamus below.
Its development occurs concurrently with the pituitary as the hypothalamic-pituitary-
endocrine axis becomes functional when the portal vascular connection to the anterior
pituitary is established around 12 weeks of age. It is around this time that the various
hypothalamic releasing factors are detected in the fetal system.
What are the two sources of the pituitary gland? What do they result in? What are some other names for them?
The pituitary gland (hypophysis) develops from two different sources of
ectoderm that give rise to a glandular portion called the anterior pituitary
(adenohypophysis) and a neural portion called the posterior pituitary
(neurohypophysis) .
Describe the development of the adenohypophysis? What are the 3 subdivisions? Describe the cellular differentiation that occurs early in gestation? What are the results?
The adenohypophysis develops from a dorsal thickening of the oral cavity
ectoderm immediately adjacent to the neural tube. As it thickens, it evaginates and
grows upward toward the developing diencephalon as Rathke’s pouch. As it grows
upward, it remains connected to the oral cavity by a stalk but over time it will degenerate
as the sphenoid bone ossifies forming the sella turcica. Rathke’s pouch will give rise to
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three subdivisions of the adenohyphophysis: the pars distalis, pars intermedia, and
pars tuberalis. Between the pars distalis and pars intermedia, remnants of the lumen of
Rathke’s pouch may persist in the adult as cystic cavities.
Early in gestastion cellular differentiation occurs in the adenohypophysis and
several hormones can be identified. Two main categories of cells begin to form as the
chromophores and chromophils develop around 5-7 weeks. The chromophils are further
subdivided into basophiclic-staining cells (thyrotophs, gonadotrophs, and corticotrophs),
which appear first, followed by the acidophilic-staining cells (somatotrophs and
mammotrophs). By 16 weeks, the anterior pituitary is fully differentiated.
How does the neurohypophysis develop? What is its subdivision? What is its function? How does it do it?
The neurohypophysis develops from a ventral thickening of the neuroectoderm
of the diencephalon and evaginates and grows downward toward Rathke’s pouch. As it
grows it abuts the pars intermedia of the adenohypophysis and forms the pars nervosa.
The neurohypophysis remains connected to the diencephalon by a stalk called the
infundibulum. The pars nervosa does not synthesize hormones but stores oxytocin and
vasopressin in axon terminals that are synthesized in cell bodies in the hypothalamic
supraoptic and paraventricular nuclei.
What is a craniopharyngioma? Where is it located? What symptoms does it cause?
Craniopharyngioma. Remnants of the stalk of Rathke’s pouch may develop into a benign
tumor most often in the sella turcica. These tumors cause symptoms by increasing local pressure
on nearby structures such as the hypothalamus and optic chiasm.
What causes hypopituitarism? What is it? What clinical symptoms are seen?
Hypopituitarism. This is a deficiency in pituitary hormone production and may result from
disorders involving the pituitary gland, hypothalamus, or surrounding structures. Clinical symptoms may include the developing genitalia, midline facial malformations including cleft lip/palate or other midline malformations, increased risk of holoprosencephaly, and sever short stature due to significant growth hormone deficiency.
Describe the development of the pineal gland? What is another name for it? What are its functions? What is brain sand? Why is it significant?
The Pineal Gland Is Derived From Neuroectoderm.
The pineal gland (epiphysis) develops from a diverticulum of the roof of the
diencephalon. Cells of this diverticulum proliferate and differentiate into pinealocytes
which produce and secrete melatonin, the hormone involved in setting our internal clock
(the circadian rhythm). The production of melatonin is stimulated by darkness and
inhibited by light. The pineal gland is large in children, but shrinks during puberty.
During adolescence the pineal plays a major role in inhibiting sexual development.
Present in the pineal after puberty are corpora arenacea (brain sand). These are
mineralized concretions that increase with age. They are radiologic hyperdense and are
used as a radiologic landmark as a means to identify the normal midline position of the
pineal.
Describe the origination, development and migration of the two different parts of the thyroid gland. What do the two different embryological sources result in? What are their functions?
The Thyroid Gland Develops From A Median Endodermal Thickening in the
Primitive Pharyngeal Floor.
The thyroid is formed from a major median pharyngeal diverticulum and two
minor lateral primordia. The median pharyngeal diverticulum develops in the floor of
the primitive pharynx just caudal to the tuberculum impar at the foramen cecum (the
depression at the base of the tongue) as a median invagination called the thyroglossal
duct that grows caudally. At the base of this duct the thyroid is first spherical in shape
but as it approaches its final site in front of the trachea at about 7 weeks it becomes
bilobed. During migration, the thyroglossal duct degenerates and in approximately 40%
of individuals the caudal part of the duct remains as the pyramidal lobe. Ectopic thyroid
tissue and thyroglossal duct cysts may persist anywhere along the migratory path between
the tongue and trachea.
Initially the thyroid primordia are solid cords but at week 10 become plates that
develop into small follicles. These consist of a simple epithelium surrounding a lumen
that at 14 weeks contains colloid, a gelatinous substance that participates in the iodination
of thyroglobulin.
The two lateral primordia of the thyroid arise from the ultimobranchial bodies
which originate from the dorsal wing of the 4th pharyngeal pouch (may also be referred
to as the 5th pharyngeal pouch) of the pharyngeal apparatus. By 8 weeks they separate
from the pharynx and migrate down each side of the neck with the superior parathyroid
primordia. As they migrate they separate from the parathyroids, their lumens become
obliterated and fuse with the dorsolateral aspects of the median thyroid primordium.
After fusion with the median thyroid the ultimobranchial bodies are dispersed among the
stroma as parafollicular cells (C cells), which surround the follicles. C cells are thought to be derived from neural crest cells that migrate into the ultimobranchial bodies before
incorporation into the thyroid. Remnants of the ultimobranchial bodies may persist
within thyroid tissue as solid cell nests that consist of clusters of epithelial cells within
the stroma demarcated by the adjacent follicles.
The parafollicular cells represent a minor component of the thyroid gland where
they comprise not more than 0.1% of the glandular mass. They have a neuroendocrine
function, being responsible for the production of the hormone calcitonin which plays a
small role in the regulation of blood calcium levels.
What causes congenital hypothyroidism? What are the clinical manifestations? Treatment?
Congenital Hypothyroidism. A condition in which the thyroid gland does not develop or there is
a defect in thyroid hormone synthesis. Infants with hypothyroidism are referred to as cretins and
have physical growth and development problems, including mental retardation. Newborns are
now routinely tested for thyroid hormone deficiency as part of newborn screening. Children with
thyroid hormone deficiency are treated with synthetic thyroxine, which enables normal growth and
development.
What causes ectopic thyroid tissue? What are the clinical symptoms?
Ectopic Thyroid Tissue. May be present just beneath the tongue (lingual thyroid) or anywhere in
the path of its descent. Thyroid tissue fails to separate from its origin and may compress the
trachea and esophagus.
Describe thyroglossal cysts.
Thyroglossal Cysts. Remnants of the thyroglossal duct are very common findings in children.
Normally the thyroglossal duct disappears but a remnant may persist as a cyst anywhere in the
path of the descent of the thyroid, usually in front of the hyoid bone. Rarely a persistent
thyroglossal duct may remain patent and form a thyroglossal fistula, which may open in the lower
neck.
Describe the development of the parathyroid glands. What is its cellularity? What are the functions of the cells?
The parathyroid glands arise as diverticula of the endoderm of the 3rd and 4th
pharyngeal pouches. They first become apparent as bilateral localized proliferations
along the dorsal wings of these pouches during week 5. The glands are referred to as
parathyroid III or parathyroid IV depending on their pouch of origin. Parathyroid III,
along with the thymus gland, forms as the 3rd pouch separates from the pharynx. At this
stage, parathyroid III lies cranial to parathyroid IV. However, differential growth and
migration of the thymus determine the final position the glands occupy after birth. The
thymus attaches to the pericardium and comes to lie largely in the thorax. The attached
parathyroid III separates from the thymus and forms the inferior parathyroid gland.
Parathyroid IV, along with the lateral primordial of the thyroid, is derived from the 4th
pharyngeal pouch. Together they separate from the 4th pouch and parathyroid IV
acquires its adult position as the superior parathyroid gland.
Variations in position are more frequent with the inferior parathyroids due their
longer migrations. Their normal position is on the posterior surface of the thyroid but
may also be found within the thyroid paranchyma. Variations in the number of glands
are less common with 95% of adults having 4 parathyroid glands, however, 2 to 8 glands
have been reported.
The primordia of the parathyroid glands develop into two types of parenchymal
cells: the chief and oxyphil cells. Arranged in sheets, the chief cells participate in
regulating the serum level of calcium by synthesizing and secreting parathyroid
hormone. The oxyphil cells, thought to be derived from the chief cells, appear late in
development and increase in number at puberty.
What causes digeorge syndrome? What anomalies may exist?
DiGeorge Syndrome. A chromosomal anomaly that results in developmental defects of
derivatives of the third and fourth pharyngeal pouches. The anomalies may consist of agenesis or
hypoplasia of the thymus and parathyroid glands, characteristic facies with downslanting palpebral
fissures with ocular and nasal anomalies, hypocalcemia, cardiovascular anomalies,
immunodeficiency, and other variable abnormalities. These malformations are probably caused by
an alteration of the migration of neural crest cells to the developing structures of the neck.
What type of tissue do the adrenal cortex and medulla derive from? Describe their development.
The Adrenal Cortex Is Derived From Mesoderm and the Adrenal Medulla Is
Derived From Neuroectodermal Cells of the Neural Crest.
The adrenal cortex develops from two waves of migration of mesodermal cells
located bilaterally between the root of the dorsal mesentery and the urogenital ridge.
The first wave of migration begins during the 6th week in which cells separate from the
posterior abdominal wall and aggregate to form a mass near the developing mesonephros.
These bilateral masses will form the temporary fetal cortex. These cells stain
eosinophilic, proliferate rapidly, and form a series of paralled columns that eventually
compose the bulk of the fetal cortex. The second wave of cells later separate from the
posterior abdominal wall and surround the fetal cortex as a thin rim of small
hyperchromatic cells. These cells are the precursors of the permanent adult cortex.
The suprarenal glands of the fetus are 10 to 20 times larger than the adult glands relative
to body weight, and are large compared to the kidneys. At the end of gestation, the fetal
cortex accounts for the bulk of the gland. This large size is due impart to its production
of estrogen precursors that the placenta uses to form estrogen. Because the placenta lacks
the enzymes needed for production of estrogen precursors, monitoring of maternal
estrogen levels during pregnancy is used clinically as an index of fetal development. In
addition, the large fetal cortex also secretes increased amounts of gluococorticoids prior
to parturition which trigger the onset of labor. After birth, cells of the fetal cortex start to
degeneration and are continuously replaced by proliferation of cells that constitute the
permanent cortex. Growth of the developing cortex is therefore centripedal (from outside
inward). The three layers of the permanent cortex (zona glomerulosa, zona fasciculata,
zona reticularis) are not recognizable until the end of the third year.
The adrenal medulla is of neuroectodermal origin. Precursor cells in the neural crest migrate from developing spinal ganglia and follow large blood vessels that penetrate the medial side of the fetal cortex. As they penetrate the cortex they become surrounded by it and are found arranged in irregularly sized nests. By 10 weeks, two sets of cell types evolve in the medulla and are responsible for the synthesis and secretion of
catecholamines (epinephrine, norepinephrine). Because of their affinity to chromium salts they are often called chromaffin cells.
What is ectopic adrenal tissue? Where is it found? What are the results?
Ectopic Adrenal Tissue. Adrenal tissue that may be cortical only, medullary only or both can be
found retroperitoneal behind the abdominal and pelvic cavities extending from the diaphragm to
the scrotum. This tissue tends to undergo hyperplasia or give rise to neoplasms that increase the
output of steroids and/or catecholamines.
What is congenital adrenal hyperplasia? What is the cause? What is the pathogenesis? What are the clinical features in females? Males? In both?
21-Hydoxylase Deficiency. An autosomal-recessive syndrome that is the primary cause of
congenital adrenal hyperplasia (CAH). In CAH there is a block in the production and manufacture
of the steroid hormones cortisol and aldosterone. Because the adrenal glands work hard trying to
make the hormones they are unable to make they wind up making too much of what they can,
androgen. In females, this usually causes masculinization (virilization) of the external genitalia,
excessive body and facial hair (hirsutism), menstrual irregularities and decreased fertility. In
males, this causes an early onset of puberty (precocious development). Deficient aldosterone
levels lead to salt wasting in which large amounts of sodium are present in urine. Signs of salt
wasting can include poor appetite, vomiting and failure to grow.
Describe the development of the pancreas as a whole?
The pancreas originates by repeated branching of cell cords that evaginate from
the endoderm of the foregut during weeks 4 and 5. It arises from two different locations.
The dorsal bud forms opposite the hepatic diverticulum, whereas the ventral bud forms
adjacent to the hepatic diverticulum. During the 6th week the ventral pancreas is carried
with the common bile duct dorsally around the duodenum to lie posterior and slightly
inferior to the dorsal pancreas. The two portions usually fuse during the 7th week. The
dorsal pancreas gives rise to the superior part of the head, entire neck, body and tail of the
adult pancreas while the ventral portion contributes to the remainder of the head and the uncinate process. The ductal systems of the two buds generally fuse to form the main
pancreatic duct of Wirsung, arising from the entire ventral pancreatic duct and the distal
part of the dorsal duct. The proximal dorsal duct may persist as the accessory duct of
Santorini.
Describe the development of the islets. How are islets arranged? Where are they more densely concentrated?
The ducts develop as solid cords of cells that push by proliferation into the
surrounding mesoderm. These ducts branch progressively and luminal spaces are
formed. Both the acinar (exocrine) and islet (endocrine) cells develop from these
primitive ducts. The cells at the terminal branches differentiate into acinar cells during
the third month of gestation. The islets develop earlier as clusters bud off from the
intralobular and interlobular ducts and surround capillaries to form discrete islets. At 16
weeks glucagon-producing α-cells and insulin-producing β-cells can be differentiated
within the islets. At first, these cell types are found at opposite poles of the islets but then
are arranged so that the α-cells are found at the periphery surrounding the centrally
placed β cells. The somatostatin-producing δ-cells and the pancreatic-polypeptide-
producing PP cells are the last to develop. These cells are much less numerous than the α
and β cells and do not display any obvious pattern of arrangement within the islets.
Approximately 1 million islets are present in the adult pancreas and constitute only 2% of
the total volume of the adult gland. Although the islets are distributed throughout the
gland the majority are found in the tail.
What causes annular pancreas? What might be the result?
Annular Pancreas. This occurs when the ventral pancreas fails to properly migrate, resulting in a
ring of pancreatic tissue encircling the duodenum. Annular pancreas may be asymptomatic or
cause partial or complete duodenal obstruction.
What is pancreatic heteropia? What is the clinical result?
Ectopic Pancreatic Tissue. Also termed pancreatic heterotopia is found throughout the gut and
its derivatives. When present, ectopic pancreatic tissue is generally asymptomatic and
therefore often discovered as an incidental finding. It is important to recognize this
phenomenon and to avoid misinterpreting this tissue as a metastatic tumor.
