Osteoclasts
Osteoclasts develop from blood
stem cells and act to break down
bone matrix, accumulating in areas
of microdamage in both cortical
and trabecular bone. Osteoclasts
form a seal over an area of damaged
bone and secrete hydrochloric
acid and enzymes that break down
collagen, releasing calcium into
the bloodstream. Measurement
of the breakdown products of
collagen can be used to monitor
bone turnover and effectiveness of
therapies for osteoporosis.
The ability of precursor cells to
develop into mature osteocytes relies
on specific signalling proteins
and other regulatory enzymes. The
key signalling protein for osteoclast
differentiation is RANKL,
which attaches to a receptor that
is widely expressed on osteoclasts — RANK. RANKL is secreted by osteoblasts,
bone marrow cells and lymphocytes, a process that increases
in the acute deficiency of oestrogen that occurs in menopause.10