diseases (e.g., hyperthyroidism) or drugs (e.g., alcohol and caffeine). In
addition, excess glucocorticoid induces osteoporosis. For example, in Cush-
ing’s syndrome, patients produce high levels of corticosteroids that interfere
with bone metabolism. A similar pattern may be seen during prolonged
steroid therapy. The result is increased bone resorption compared with bone
deposition. Intestinal calcium absorption is inhibited and PTH levels may be
increased.
115.The answer is c.(Sadler, pp 172, 176–179. Moore and Persaud, Developing
pp 168, 214.)The child in the vignette is suffering from DiGeorge anomaly,
which results in the absence of the thymus and parathyroid glands, which
arise from the third and fourth pairs of branchial pouches. The absence of the
thymus results in a deficiency in T lymphocyte–dependent areas of the
immune system. These areas include the deep cortex of the lymph nodes,
periarterial lymphatic sheath (PALS) of the spleen, and interfollicular areas of
the Peyer’s patches. Parathyroid hormone (PTH) stimulates the development
of osteoclasts and the formation of ruffled borders in osteoclasts. The absence
of PTH results in: i) a drastic reduction in numbers and activity of osteoclasts,
ii) reduced Ca^2 +levels in the blood, iii) denser bone, iv) spastic contractions of
muscle called tetany, and v) excessive excitability of the nervous system. The
parafollicular (C) cells arise from the ultimobranchial body that migrates into
the thyroid gland and should form normally.
116.The answer is c.(Kumar p 41. Junqueira, pp 144–145.)The transmission
electron micrograph is a high magnification view of a matrix vesicle that is
derived from the cell membrane of osteoblasts, hypertrophied chondrocytes,
ameloblasts, and odontoblasts depending on the location. After budding off
from the plasmalemma, matrix vesicles accumulate calcium and phosphate in
the form of hydroxyapatite crystals and serve as seed crystals for calcification.
Exposure of those crystals to the extracellular fluid leads to seeding of the
osteoid between the spaces in the collagen fibrils located in the matrix. Matrix
vesicular alkaline phosphatase (answers a and b)results in local increases in
the Ca^2 +/PO 42 −ratio. Adult lamellar bone contains very few matrix vesicles, sug-
gesting that mineralization in remodeling of adult bone occurs by other mech-
anisms. The three-dimensional arrangement of collagen with the presence of
holes or pores where hydroxyapatite crystals form is involved in the mineral-
ization of adult bone. Osteoprotegerin inhibits osteoclastic activity (answer d)
and osteoblasts synthesize type I collagen (answer e).
212 Anatomy, Histology, and Cell Biology