74.The answer is a.(Alberts, pp 1217–1218.)Open neural tube defects
(NTDs), such as anencephaly, are detected by a positive α-fetoprotein (AFP)
test and are attributed to failure of primary neurulation in which the neural
folds (plate) form the neural tube. Tubular structures form from flat sheets by
contraction of the microfilament bundles associated with the adhesion belt
junctions (zonula adherens). In the apical part of the cells, the actin filament
bundles contract, narrowing the cells at their apical ends. The position of the
zonula adherens, forming a contractile ring around the circumference of the
cell, coupled with the contractile nature of the actin microfilament bundles is
ideal for regulating morphogenetic changes. Desmosomes (answers b and e)
are involved in resisting shear forces and are not directly involved in this
process. The zonula occludens (answer c)prevents leakage between cells.
Gap junctions facilitate communication between cells (answer d).See table
in answer for question 83 for detailed summary of function of components
of the junctional complex.
75.The answer is c.(Young, p 89. Kumar, p 557. Junqueira, pp 27, 68–71.)The
transmission and freeze-fracture electron micrographs illustrate the structure
of a nexus, or gap junction. Gap junctions are composed of connexons that
traverse the intercellular gap. Gap junctions play an essential role in conduc-
tion within cardiac muscle. The action potential is transmitted from cell to cell
through the heart, providing the rhythmic contraction of the heartbeat.
Abnormalities in the spatial distribution of gap junctions or the proteins
(connexins) that compose the connexons will lead to arrhythmias and play a
role in obstructive coronary heart disease. Such mutations may also play a role
in the racial differences seen in the outcome following myocardial infarction and
cardiac disease. For example, alterations in connexin density or distribution
Epithelium Answers 163Stratified squamous Epidermis Protection, water conservation
(keratinized)
Stratified squamous Esophagus, anus, vagina Protection, lubrication, secretion
(nonkeratinized)
Pseudostratified Respiratory system (trachea Movement of material across
with cilia), male reproductive epithelial surface
system (no cilia)
Transitional Urinary system: ureter, bladder Stretch, protectionEpithelial Type Location Function