Ontogenetic changes of tissue compartmentalization and bone type distribution in the humerus of Soay sheep

Abstract

We studied ontogenetic changes of histomorphological features and bone type distribution in the humeral midshaft region of Soay sheep from three postnatal age classes (13, 25, and 33 months). Our study demonstrated a marked change of bone type distribution in the humeri with age. In the cortical midshaft region of 13-month-old individuals, periosteal fibrolamellar bone was the dominating bone type. This indicates a rapid bone growth during the first year of life, which was only interrupted by a seasonal growth arrest in the animals' first winter. In individuals from the two older age classes, periosteal lamellar-zonal bone and intermediate fibrolamellar bone had been formed at the periosteal surface, and endosteal lamellar-zonal bone at the endosteal surface. These bone types are indicative of a reduced bone growth rate. A marked reduction in radial growth was already recorded in the 25-month-old individuals. Distribution and extent of secondary bone showed a marked bilateral symmetry in the humeri of individual sheep. The presence of secondary bone was largely restricted to the anterior (cranial) and the medial cortical areas. This characteristic distribution of remodeling activity within the humeral cortex of sheep is consistent with the view that remodeling activity is largely caused by compressive stress. Our study further demonstrated the presence of a considerable cortical drift in the sheep humeri over the study period, with endosteal resorption occurring predominantly in the posterior (caudal) quadrant and formation of a prominent endosteal lamellar pocket in the anterior (cranial) and medial cortical quadrants.

Keywords: bone modeling; bone remodeling; bone resorption; cortical drift; endosteal lamellar pocket; secondary bone.

FIGURE 1

Location of sample extraction site in a left humerus

FIGURE 2

Map showing bone type distribution in a cross‐section of the humeral midshaft of individual 13 (age at death: 13 months). Green colors represent tissues of periosteal origin, brown colors, tissues of endosteal origin. Within these color families, tissue types associated with faster growth rates are represented by darker shades. Blue color indicates secondary bone. A, anterior (cranial); P, posterior (caudal); M, medial; L, lateral

FIGURE 3

Micrographs of ground sections through the midshaft of the humerus showing periosteal lamellar‐zonal bone (PLM; a,b) and endosteal lamellar‐zonal bone (ELM; c,d) viewed in normal transmitted light (a,c) and in linearly polarized light with 1λ compensator (b,d). (a,b) The PLM is characterized by lamellar bone with parallel oriented lamellae (black arrows) and spindle‐shaped osteocyte lacunae (black arrowheads). Two cement lines are visible. The upper one (white arrow) shows a scalloped outline, identifying it as a reversal line. The lower line (white arrowhead) is smooth, indicating that it represents a rest line. Periosteal surface to the bottom of the images. (c,d) The ELM exhibits mostly parallel oriented lamellae (black arrow). Note that not all lamellae are arranged in parallel, but that some meet at a sharp angle (white arrow), thereby indicating different growth phases. In the right half of the image, remnants of woven bone (asterisk) and a secondary osteon (SO) are visible. Arrowhead: reversal line. Endosteal surface to the left of the images

FIGURE 4

Micrographs of ground sections through the midshaft of the humerus showing fibrolamellar bone tissue in the pericortex (periosteal fibrolamellar bone, PFLC; a,b) and in the endocortex (endosteal fibrolamellar bone, EFLC; c,d) viewed in normal transmitted (a,c) and in linearly polarized light with 1λ compensator (b,d). (a,b) In the PFLC, primary vascular spaces lined by scaffolds of woven bone (black arrows) have initially been filled in with parallel‐fibered bone followed by the deposition of lamellar bone resulting in the formation of primary osteons (PO). Asterisks: vascular canals; arrowheads: rest lines running parallel to the bone surface. Periosteal surface to the bottom right corner of the images. (c,d) In the EFLC, a primary vascular space has been filled in with a primary osteon (PO). Asterisks: vascular canals of the primary osteon; black arrows: woven bone of the scaffold; black arrowhead: reversal line; white arrow: Volkmann’s canal. Endosteal surface to the bottom of the images

FIGURE 5

Micrographs of ground sections through the midshaft of a humerus showing lamellar bone with primary osteons (a,b), and remodeled bone with secondary osteons (c,d) viewed in normal transmitted (a,c) and in linearly polarized light with 1λ compensator (b,d). (a,b) In the intermediate fibrolamellar‐zonal bone, longitudinally oriented primary vascular canals have been filled in by lamellar bone (black arrows). White arrow: radial blood vessel canal; PO: primary osteons. Periosteal surface to the bottom of images. (c,d) Remodeled bone with secondary osteons (SO) in the central cortex. Asterisks: woven bone remnants; black arrows: reversal lines delimiting secondary osteons. In places, secondary osteons of a younger generation partly replaced that of an older generation (white double‐headed arrow)

FIGURE 6

Panel showing bone type distribution in the cross‐sectioned humeral midshafts of the studied individuals (l, left humerus; r, right humerus; A, anterior (cranial); L, lateral; M, medial). For color codes of bone types, see Figure 2

FIGURE 7

Bone type maps (a,c) and corresponding microscopic images (b,d; plain transmitted light) of the postero‐medial (caudo‐medial) (a,b) and the anterior (cranial) cortex (c,d) of the right humerus of individual 13 (age at death: 13 months). (a) In the postero‐medial (caudo‐medial) quadrant of the cortex, PFLC is the dominant bone type. A thin seam of ELM is present along the border of the medullary cavity, and secondary osteons (SO) are present medially. (b) Peripherally, the outer cortex consists of primary bone scaffolds (black arrow) whose intertrabecular spaces are yet unfilled. In the central and the outer cortex (double‐headed arrow), the primary vascular spaces show a circumferential (laminar) orientation, but in the inner cortex (double‐headed arrow with asterisk) they show a more plexiform arrangement. Arrowheads: rest line in the central cortex. (c) PFLC and SO are the dominant bone types in the anterior (cranial) cortex. The ELP is characterized by the presence of ELM and EFLC. (d) At the outer and inner rims of the humerus, the primary vascular spaces (arrowheads) are still unfilled. Larger, partially infilled resorption spaces (asterisks) are present throughout most of the cortex. The ELP is separated from the remodeled bone by a reversal line (white arrows)

FIGURE 8

Central (a), outer (b), and inner humeral cortices (c,d) of individuals 13 (a‐c) and 17 (d), ages at death 13 months, viewed at higher magnifications in transmitted light with phase contrast. (a) Remodeled bone in the central cortex of the anterior (cranial) quadrant with forming secondary osteons in different stages of infilling (asterisks) and different generations of completed secondary osteons (SO, double‐headed arrow). PO: primary osteon. (b) Remodeled bone in the outer cortex of the specimen shown in (a). White arrowheads: unfilled primary vascular spaces, asterisks: infilling secondary osteons, PO: primary osteons. (c,d) The inner boundary of the ELP is marked by white arrows. Secondary osteons (SO) of the inner cortex have been partly resorbed prior to the apposition of new endosteal bone. In places, not yet infilled primary vascular spaces (arrowheads in c) are present along the border of the ELP with the medullary cavity (MC)

FIGURE 9

Bone type maps (a,c) and corresponding microscopic images (b,d, plain transmitted light) of the postero‐medial (caudo‐medial) (a,b) and the anterior (cranial) cortex (c,d) of the right humerus of individual 15 (age at death: 25 months). (a) Bands of PLM are present in the outermost and the central cortex. PFLC and SO are the dominant bone types in this quadrant, with the latter extending from the border of the medullary cavity to the central portion of the cortex. (b) Two rest lines (arrowheads), indicating periods of growth arrest, co‐locate with PLM. The PFLC is highly vascularized, the vascular canals in the central and outer cortex (double‐headed arrow) showing a circumferential (laminar) orientation. The vascular canals in the inner cortex (double‐headed arrow with asterisks) show a more plexiform arrangement. (c) ELM is the dominant bone type in the ELP. Remodeled bone with secondary osteons (SO) occupies the inner and varying parts of the central cortex. The outer cortex consists mostly of PFLC and a small area of PLM and IFL. (d) A reversal line (white arrows) marks the border between the ELP and the remodeled bone. In the central cortex, two rest lines (arrowheads) are visible; asterisk: small resorption cavity

FIGURE 10

Higher magnification of the inner cortex of the humerus of individual 15 (age at death: 25 months) viewed in transmitted light with phase contrast (a) and in linearly polarized light (b). The original border between the ELP and the inner cortex is no longer visible due to remodeling of the former. Rest lines (white arrows) indicate intermittent growth of the ELP. Asterisks: Infilling secondary osteons; MC, medullary cavity; PO, primary osteons; SO, secondary osteons

FIGURE 11

Bone type maps (a,c) and light microscopic images (plain transmitted light, b,d) of the postero‐medial (caudo‐medial) cortex of the right humerus of individual 16 (a,b), and the antero‐medial (crania‐medial) cortex of the right humerus of individual 14 (c,d). Age at death for both individuals: 33 months. (a) The outer cortex is formed by IFL and PLM, both indicative of relatively slow bone formation. The central cortex is still dominated by PFLC. (b) Three rest lines (arrowheads) are visible in the cortex. The innermost rest line presumably indicates the growth arrest during the individual’s first winter of life. In the posterior (caudal) cortex (double‐headed arrow), the primary vascular canals show a laminar orientation. Vascular orientation in the central portion of the medial cortex is more plexiform (double‐headed arrow with asterisk). (c,d) Except for the outermost zone, the primary periosteal cortex has been replaced by secondary bone (SO). The outermost cortex is former by either avascular PLM or by IFL. In the inner cortex, at the boundary between the ELP and the remodeled cortex, large blood vessel spaces can be seen. Several rest lines (arrowheads) are visible in the outer cortex. The central primary cortex is largely remodeled, with secondary bone extending almost up to the bone surface in the anterior (cranial) cortex. The ELP is separated from the remodeled cortex by a reversal line (white arrows). In places, small areas of the primary fibrolamellar bone (large asterisk) are still present. The lumina of larger blood vessel spaces (small asterisks) have become incorporated into the inner cortex

FIGURE 12

Higher magnifications of the inner cortices of the cross‐sectioned humeri of individuals 16 (a) and 14 (b) (age at death for both individuals: 33 months) viewed in plain transmitted light (a) and in transmitted light with phase contrast (b). (a) Occurrence of multiple rest and reversal lines (white arrows) within the ELP indicates its intermittent mode of formation. PO: primary osteons. SO: secondary osteon. MC: medullary cavity. (b) Inner cortex showing signs resorption (Howship’s lacunae, black arrowheads) along the boundary with the medullary cavity (MC). White arrows point to reversal line between ELP and primary bone

FIGURE 13

Schematic illustration of ELP extension (black) and drift of the medullary cavity in posterior (caudal) direction in the humeri of individuals from the different age groups (a: age at death 13 months, b: age at death 25 months, c: age at death 33 months). A, anterior (cranial); L, lateral, M; medial, P, posterior (caudal)