of type I pneumocytes. Type II pneumocytesare responsible for the
secretion of surfactant, a primarily lipid substance that facilitates respira-
tion by reducing alveolar surface tension. The type II pneumocyte is recog-
nizable at the EM level by the presence lamellar bodiesthat contain
surfactant comprised primarily of lecithin (Dipalmitoylphosphatidyl-
choline, DPPC)as well as some cholesterol and sphingomyelin. Although
surfactant is produced beginning at about the 28th week, premature babies
born before 30 weeks are likely to suffer from respiratory distress syn-
drome (RDS)because of the absence of surfactant. There are a variety of
treatments for RDS including the administration of artificial surfactant and
ventilation for the premature infant. In cystic fibrosis, mutations in the
cystic fibrosis transmembrane conductance regulator (CFTR) result in
defective Cl-transport and increased Na+absorption. HCO 3 - transport
through the CFTR is also defective. The result is thick, more viscous (less
watery) mucus in the airways that promotes bacterial infections and
reduces the effectiveness of the mucociliary escalator.
INTEGUMENTARY SYSTEMThe epidermis of thick skin consists of five layers of cells (keratinocytes):
stratum basale(proliferative layer), stratum spinosum(characterized by
tonofibrils and associated desmosomes), stratum granulosum(characterized
by keratohyalin granules), stratum lucidum(a translucent layer notobvious
in thin skin), and stratum corneum(characterized by dead and dying cells
with compacted keratin). Specialized structures of the skin include hair fol-
licles (found only in thin skin), nails, and sweat glands and ducts. Nonker-
atinocyte epidermal cells include melanocytes (derived from neural
crest),Langerhans cells(antigen-presenting cells derived from mono-
cytes), and Merkel cells(sensory mechanoreceptors). Various sensory
receptors and extensive capillary networks are found in the underlying der-
mis. There are skin diseases with a cell biological etiology. Psoriasis is a dis-
ease characterized by dermal and epidermal infiltration of inflammatory
cells. Those cells release cytokines, which cause hyperplasia of the epider-
mis. Proliferation occurs throughout the epidermis and is no longer
restricted to the basal layer and there is a thickening of the stratum
corneum with nucleated keratinocytes present. Pemphigusis an autoim-
mune disease in which autoantibodiesare produced to the desmogleins,
members of the cadherin family. The desmosomes break apart resulting in
28 Anatomy, Histology, and Cell Biology