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The chondrification of the human ilium.

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The Chondrification of the Human Ilium'
Department of Anatomy, The University of Alberta,
Edmonton, Alberta, Canada
This report on the early human ilium is based on the Streeter collection, Carnegie Institution of Washington, Department of Embryology, Baltimore. The
five phases in chondrogenesis, described by Streeter for the humerus ('49) and the
horizons (Streeter, '42) are used in the description of the changes observed in the
ilium. From horizon XV (7-8 mm, 30-32 days) to horizon XVIII (14-16 mm, 36-38
days) component cells of the ilium are mesoblasts. In horizon XIX (17-20 mm,
38-40 days) young cartilage cells have appeared. Both phase 1 and phase 2 cells
appear in horizon XX (21-23 mm, 4 0 4 2 days), the former characterized mainly by
the first appearance of intercellular material, the latter by their slender shape and
arrangement in tiers. Phase 3 cartilage cells, cuboidal with many vacuoles, appear in
horizon XXI (22-24 mm, 4244 days). Increasing vacuolization of the cytoplasm of
cells is noted throughout horizons XXII and XXIII until, in horizon XXIII (28-30 mm,
4648 days) total disintegration of cells, a feature of phase 5, is evident. Thus the
phases of cartilage differentiation described for the humerus are present also in the
Extensive observations on the pelvis and
lower extremity were made by Bardeen
and Lewis ('0 1) and Bardeen ('05). They
produced from the reconstructions some
very informative illustrations, but they did
not report the micro-anatomy. Since then,
no report on the micro-anatomy of the
developing ilium has appeared, except for
a brief abstract by Zawitsch ( ' 5 3 ) .
Extensive use of the work of Bardeen
and Lewis was made by Keibel and Mall
('10). Although they wrote a chapter on
the histogenesis of connective tissue and
dealt with cartilage and bone, they did not
provide any detail concerning the cells of
the developing ilium.
Some reference to the chondrification of
the ilium was made by Strayer ('43),
Haines ('47) and by Gardner and Gray
( ' 5 0 ) , while they concentrated on the hipjoint.
The present survey of cells from the
origin of the ilium in mesenchyme until
the cartilage model is well established and
primary ossification impending establishes
that in the earliest development of the definitive cartilaginous model the ilium and
the humerus are alike. The humerus was
selected by Streeter ('49) as a representative long bone when he included a chapter
on the histogenesis of cartilage and bone
in his study of human embryos in the
developmental horizon series. He felt that
the basic pattern he described underlay
other bones of an individualistic type such
as the clavicle and the vertebrae; to those
bones one may add the ilium. The ilium
was selected for further study because of
the interest in congenital dysplasia of the
hip and of the need to clarify the development of the ilium, if the problem of dysplasia is to be resolved.
The basic pattern of chondrification described by Streeter ('49) included five
phases of development. Phase one is characterized mainly by the presence of intercellular material enclosing cells emerging
from the skeletal blastema, phase two by
the slender shape of the cell, phases three
and four by their cuboidal shape, the increase in size of the cell and in vacuolization of the cytoplasm, and phase five by
the extensive disintegration of the cell.
The Streeter collection of human embryos was made available at the Carnegie
Institution of Washington, Department of
Embryology, Baltimore, Maryland, U.S.A.
through the courtesy of Dr. J. D. Ebert,
director, and the phases of cartilage differentiation (Streeter, '49), and the horizons (Streeter, '42) are adhered to throughout this report.
1This work was supported by a grant from the
Canadian Arthritis and Rheumatism Society.
The youngest embryo is 8.3 mm in
length and the oldest 30 mm. Of the 42
embryos examined, 15 are included in this
report. They had been fixed in formol,
embedded in paraffin or colloidin-paraffin,
cut in serial section in a range from 10 v
to 50 cc in transverse, sagittal, or frontal
planes, and stained by using a variety of
stains but mainly hematoxylin-eosin.
Horizon XVII, 34-36 days
Embryo no. 651 9, 10.8 mm. At the site
of the future innominate bone, a core of
condensed mesoblast cells lies within a
border of more heavily condensed cells
that is fairly distinct from the surrounding
blastema (fig. 1).
Horizon XVlII, 36-38 days
Embryo no. 144,16.0 mm. The demarcation of the components of the innominate bone has proceeded further until the
triradiate zone is well established (fig. 2).
The border of condensed cells that was
noted previously is much more distinct,
and, over the pelvic surface of the ilium,
the cells are in layers typical of perichondrium (fig. 2). Within the ilium there is
some evidence of cartilage but, on the
whole, the cells are highly differentiated
mesoblasts (fig. 3).
In embryo no. 991, 17.0 mm, remarked
upon by Streeter as being near the older
borderline of horizon XVIII, very young
cartilage cells have definitely appeared in
the ilium (fig. 4).
According to these observations, the center of chondrification for the ilium appears
in the human embryo at five weeks, and is
situated cephalad to the greater sciatic
Horizon XX, 40-42 days
The outstanding features of this horizon
are the rapid increase in the dimensions of
the ilium, and the appearance of phase one
and phase two cells. In embryo no. 8157,
20.8 mm, for example, cartilage cells
throughout the ilium have entered phase
one (fig. 5). In the ala of this embryo,
however, the elongated shape of an occasional cell and a suggestion of parallel
rows of cells suggests that phase two has
begun. In slightly older embryo, no. 4059,
21.6 mm, phase two cells are present
throughout the ala of the ilium. Their
slender shape and their arrangement in
tiers is very characteristic (fig. 6 ) .
Horizon XXII, 42-44 days
Embryo 6832, 25.8 mm. Cartilage cells
entering phase three lie in a core cephalad
to the greater sciatic notch (fig. 7).
Horizon XXIII, 46-48 days
Embryo no. 4525, 30.0 mm. The cartilage cells in the vicinity of the greater
sciatic notch are passing beyond phase
four (fig. 8), and an occasional cell is
already in phase five.
During chondrification of the ilium the
cartilage model grows as a result of proliferation of fibroblasts in the mesenchyme
surrounding the cartilage primordium and
their transformation into young cartilage
cells. This process of appositional growth
probably contributes more to the mass of
the model than does interstitial growth
(Bloom and Fawcett, '62), except perhaps
when the intercellular substance is not yet
great in amount and sufficiently malleable
(Ham and Leeson, '61) to allow the newly
emerged young cartilage cells to divide and
grow. Once, however, the intercellular substance is more in amount and less malleable and the perichondrium clearly established interstitial growth will decrease and
appositional growth will predominate.
As in the humerus (Streeter, '49), so in
the ilium, there is no evidence of migration
of cartilage cells which retain essentially
the same position they occupied when they
first appeared. Therefore, growth of the
cartilage model is the result of proliferation, not migration, of cartilage cells. Admittedly, there are two additional factors,
namely, an increase in size of each cell,
and an accumulation of intercellular matrix, but proliferation is considered to be
the most important factor in growth of the
early cartilage model.
The first cartilage cells found in the
ilium in horizon XVlII and XIX are so very
young that they are really at a stage between frank mesoblast and true cartilage.
Cells at this stage, easily recognized, are
termed precartilage, procartilage or, more
recently, protochondral cells (Bloom and ous model. Nevertheless, it is presumed
Fawcett, '62). However, having recognized that an orderly progression of phases does
that these cells were in a state of transi- exist. They can be identified, for example,
tion, and, I presume, that they defied con- in the acetabular roof. Thus it is concise description, Streeter restricted phase cluded that chondrification of the ilium is
one to those cells beginning to acquire an not unusual.
Finally, there is no evidence to support
environment of increased intercellular substance. Accordingly, phase one cells are the observation that the center of chondrifirst noted in the ilium in horizon XX, 40- fication for the ilium is remote from the
42 days, but their predecessors in horizons center of ossification (Strayer, '43). The
XIX and XVIII, 36-38 days, should not centers coincide and it is precisely in the
perichondrium over cartilage cells in phase
be ignored.
Marked change occurs in the shape of five that the first sign of impending ossifithe ilium during horizon XX, when cells in cation is seen in the vicinity of the greater
phase two become evident, and they are sciatic notch.
particularly conspicuous in the ala of the
ilium, which has increased in length. AcThe author expresses his thanks to Mr.
cordingly, with some reservation, phase
two cells are concerned with a process of Richard Grill for his excellent photography.
rapid lengthening. Probably their elonLITERATURE CITED
gated shape and their arrangement in tiers
1905 Studies of the development
is the result of rapid cell division.
of the human skeleton. Am. J. Anat., 4: 265In referring to the cells in phases three
and four, the increase in the size of each Bardeen, C. R., and W. H. Lewis 1901 Developcell, the number of cytoplasmic vacuoles,
ment of the limbs, body-wall and back in man.
Am. J. Anat., 1: 1-37.
and the amount of intercellular substance,
is of note. All three increases are recog- Bloom, W., and D. W. Fawcett 1962 Textbook
of Histology. W. B. Saunders Co., Philadelphia.
nized features of biochemical activity
London. Chap. 6, 148.
which leads to calcification of the inter- Gardner, E., and D. J. Gray 1950 Prenatal decellular matrix. Presumably, the activity is
velopment of the human hip joint. Am. J.
Anat., 87: 163-212.
increasing progressively throughout phases
three and four. As a result, the cell, shut Haines, R. W. 1947 The development of joints.
J. Anat., 81: 33-55.
off from its normal source of nutrition Ham,
A. W., and T. S. Leeson 1961 Histology.
(Ham and Leeson, '61), perishes; as one
J. B. Lippincott Co. Philadelphia. Montreal.
would expect, the deeply located cells are
Chap. 14, 271.
first to reach this phase of development. Keibel, F., and F. P. Mall 1910 Manual of
human embryology. Lippincott, Philadelphia.
Thus, phase five, recognized by the disintegration and disappearance of the affected Strayer, L. M. 1943 The embryology of the
human hip joint. Yale J. Biol. Med., 16: 13-26.
cell, begins.
Streeter, G. L. 1942 Description of age group
All the changes just discussed can be
XI, 13 to 20 somites, and age group XII, 21
somites to 29 somites. Contributions to Emobserved best in the vicinity of the greater
bryology, 30: 211-245.
sciatic notch. The earliest cartilage cells
1949 A review of the histogenesis of
appear here and are the first to reach
cartilage and bone. Contributions to Embryolphase five. The shape of the definitive
ogy, 33: 149-167.
ilium, however, makes it difficult to follow Zawisch, C. 1953 Die VerknGcherung der knorthe progression of phases of differentiation
pelig vorgebildeten platten Knochen. Acta.
Anat., 19: 384.
during the further growth of the cartilagin-
Abbreviations to plates 1 and 2
F, femoral nerve
I, innominate bone
IL, ilium
IS, ischium
0, obturator nerve
S, sciatic nerve
Photomicrographs of base of limb bud to show innominate bone (figs. 1 and 2) and of
cells of ilium (figs. 3-8).
Horizon XVII 10.8 mm. Base of limb bud medial to femoral nerve. The anlage of the
innominate bone ( I ) is just emerging from the mesoblast. 8 p. Sag. Alum cochineal.
x 75.
Horizon XVIII 16.0 mm. The triradiate zone separates ilium, ischium and pubis. 40 p.
Sag. Alum cochineal x 75.
Same embryo as figure 2. The cells of the ilium are well differentiated mesoblast cells.
x 375.
Horizon XVIII 17.0 mm. An older embryo in the same horizon as the one in figure 3.
The first evidence of chondrification. 50 p. Sag. Van Gieson x 375.
R. D. Laurenson
R. D. Laurenson
Horizon XX 20.8 mm. Cells in ala of ilium intermediate between phase one and phase
two. 20 p. Frontal. Hematoxylin-eosin. X 375.
Horizon XX. 21.6 mm. Phase two cells in ala of ilium. 15 p. Frontal. Mallory. X 375.
Horizon XXII 25.8 mm. Cells cephalad to greater sciatic notch entering phase three.
20 p. Frontal. Hematoxylin-eosin. X 225.
Horizon XXIII 30.0 mm. Cells in vicinity of greater sciatic notch i n late phase four.
20 p . Sag. Hematoxylin-eosin. X 225.
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chondrification, ilius, human
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