Advances in the Canine Cranial Cruciate Ligament, 2nd edition

Histology of Cruciate Ligament Rupture 49

the loose connective tissue between collagen
fibers. It is presently unclear whether these cells
represent differing metabolic states of the same
cells or whether they are distinctly different
fibroblasts.
A histologic study of the human ACL iden-
tified three histologically different zones along
the anteromedial bundle as it coursed from
the femoral to the tibial attachment (Murray &
Spector 1999). Two of the zones, the fusiform
and ovoid, were located in the proximal one-
quarter of the bundle, while the third spheroid
zone occupied the distal three-quarters of the
fiber bundles. The fusiform cell zone was char-
acterized by a high number density of longi-
tudinally oriented cells with a fusiform-shaped
nucleus, longitudinal blood vessels, and high
crimp length. The cytoplasm of the cells in this
zone appeared to be intimately attached to the
extracellular collagen and followed the crimp
waveform of the fibers. The ovoid cell zone
was characterized by a high number density of
cells with an ovoid-shaped nucleus, longitudi-
nal vessels, and a high crimp length. Also in this
zone, the cytoplasm of the cells appeared to fol-
low the waveform of the adjacent collagen. The
spheroid cell zone was characterized by a low
density of spheroid cells, few blood vessels, and
a short crimp length. Cells were noted within
and among fascicles, as well as within lacunae.
The role of the various fibroblast phenotypes
in the maintenance of the human ACL is not
known.
The histologic structure of the cruciate liga-
ment is not homogeneous. In ACL, it has been
reported that there is a zone where the tissue
resembles fibrocartilage (Petersen & Tillmann
1999). The fibrocartilaginous zone is located
5–10 mm proximal to the tibial attachment in
the anterior portion of the ligament. Within this
zone, the cells are arranged in columns and
the cell shape is round to ovoid. Transmission
electron microscopy reveals typical features of
chondrocytes. These chondrocyte-like cells are
surrounded by a felt-like pericellular matrix,
a high content of cellular organelles, and short
processes on the cell surface. The pericellular
collagen is positive for type II collagen. An
avascular zone is located within the fibrocar-
tilage of the anterior part where the ligament
faces the anterior rim of the intercondylar
fossa. It has been proposed that the stimulus

for the development of fibrocartilage within

dense connective tissue is shearing and com-

pressive stress (Milzet al. 2005). In the ACL,

this biomechanical situation may occur when

the ligament impinges on the anterior rim of

the intercondylar fossa when the knee is fully

extended (Quasnichkaet al. 2005).

Histologic features of both the ligament cells

and the ECM appear to change with aging

(Hasegawaet al. 2013). In a study characteriz-

ing age-related changes in ACL from 80 sub-

jects (age 23–94 years), the total cell num-

ber in normal ACL decreased with aging but

increased in degenerated ACL with the forma-

tion of perivascular cell aggregates and islands

of chondrocyte-like cells. Based on immunohis-

tochemical staining, collagen I was expressed

throughout normal and degenerated ACL. Col-

lagen II and X were detected only in the

areas with chondroid metaplasia. This study

suggested that the chondrocyte-like phenotype

produces an abnormal ECM and may predis-

pose ACL to mechanical failure. Similar age-

related changes are reported in canine CrCL, as

described later in this chapter.

Histopathology of the canine cranial

cruciate ligament

Cell morphology was studied in normal cru-

ciate ligaments from disease-free stifle joints

from dogs with a high (Labrador Retriever)

and low (Greyhound) risk of CR (Smithet al.

2012). Both CrCL and caudal cruciate ligament

(CdCL) contained cells of heterogeneous mor-

phologies, where cells were arranged between

collagen bundles and frequently had cytoplas-

mic processes. Some of these processes were

long (type A cells), others were shorter, thicker

and more branched (type B cells), and some had

no processes (type C cells). Processes were fre-

quently shown to contact other cells, extend-

ing longitudinally and transversely through the

cruciate ligaments. Cells with longer processes

had fusiform nuclei, and those with no pro-

cesses had rounded nuclei and were more fre-

quent in the mid-substance of both the CrCL

and the CdCL. Cells with long processes were

more commonly noted in the cruciate ligaments

of the Greyhound. As contact between cells may