"" Histomorphology of Metaphysis of Proximal Tibia in Albino Rat

Mini Review

Austin J Anat. 2014;1(2): 1010.

Histomorphology of Metaphysis of Proximal Tibia in Albino Rat

Sarah Ralte*

Department of Anatomy, North Eastern Indira Gandhi Regional Institute of Health and Medical Sciences, India

*Corresponding author: Sarah Ralte, Department of Anatomy, North Eastern Indira Gandhi Regional Institute of Health and Medical Sciences (NEIGRIHMS), Mawdiangdiang, Shillong-793018, Meghalaya, India.

Received:June 01, 2014; Accepted: July 15, 2014; Published: July 18, 2014

Abstract

The metaphysis lies at the distal end of the growth plate, between the epiphysis and diaphysis of bone and is the zone of active growth in a long bone. In this review study, the continuity of metaphysis of proximal tibia in albino rat was studied by light microscope following Hematoxylin and Eosin and Masson’s trichrome stains. The metaphysis has been described as having two distinct regions, the primary and secondary spongiosa, containing mainly osteoblasts and osteoclasts amidst abundant blood vessels. The osteoblasts are the bone forming cells whereas the osteoclasts are the chief cells mediating bone resorption. Hence, the metaphysis is the site of active bone remodelling. Before the fusion of diaphysis and epiphyses, the metaphyses are richly supplied with blood through end arteries forming hair pin bends. This is the common site of osteomyelitis in children. The bone cells are the favourite target site of action of drugs, especially the bisphosphonate class of drugs, which are potent inhibitors of excessive osteoclastic mediated bone resorption.

Keywords: Metaphysis; Primary and Secondary spongiosa; Osteoblast; Osteoclast; Bone remodelling; Tibia; Bisphosphonates

Introduction

The metaphysis is the junctional region of bone lying between the growth plate and the diaphysis. It contains slender calcified cartilage spicules and trabecular bone and is a site of active bone turnover, having large number of osteoblasts, osteoprogenitor cells and osteoclasts amidst highly vascular tissue. The metaphysis is invaded by numerous capillary loops containing osteoblastic cells, which deposit bony matrix on calcified cartilage spicules. The metaphysis is divided into two functionally distinct regions, the primary spongiosa, the area lying adjacent to the growth plate, which is rich in blood capillaries and is the site where primary spongy bone forms, which is characterized by calcified cartilage spicules covered by a thin layer of newly laid bone. The other region lying adjacent to diaphysis is the secondary spongiosa, characterized by interconnecting bony bars of trabeculae. Here, the calcified cartilage spicule is ultimately resorbed by the osteoclast, and is the site of active bone remodelling. Osteoclasts, the giant multinucleated cells, are the bone resorbing cells which resorb the mineralized bone by secreting acids and lysosomal enzymes. Increased activation of osteoclasts results in disruption of normal bone remodelling resulting in increased resorption of the bone [1]. The region of metaphysis is a common site for primary bone tumours and bone infections such as osteomyelitis. The relative predilection of osteosarcoma for the metaphyseal region of long bones in children has been attributed to the rapid bone turnover due to extensive bone remodelling during growth spurts [2]. The effects of large number of drugs have also been studied on the metaphysis of long bone [3-8]. The bisphosphonates are one of the important classes of drugs which act mainly on the bone cell, the osteoclast, and are effective inhibitors of excess osteoclastic mediated bone resorption [9]. The uses of bisphosphonates currently include the prevention and treatment of osteoporosis, osteitis deformans, bone metastasis (with or without hypercalcaemia), multiple myeloma, primary hyperparathyroidism and osteogenesis imperfecta [10- 15].

Histomorphology of Metaphysis by Light Microscope

The histomorphology of metaphysis of proximal end of tibia was studied in young albino rats by light microscope following staining with Hematoxylin and Eosin and Masson’s trichrome. The metaphysis was identified as the area lying adjacent to the lower margin of the epiphyseal growth plate and limited on the sides by the periosteum of the bone. The proximal margin of the metaphysis towards the growth plate was observed to be wider and more or less concavo-convex in comparison to its distal margin, which was narrow and irregular in outline. It was noted that the height of the lateral region was always more than the central region, giving a more or less concave appearance to the distal aspect of the metaphysis (Figure 1).

In Haematoxylin and Eosin stained sections, two distinct regions of the metaphysis were observed. The primary spongiosa, the area lying near the growth cartilage metaphyseal junction (GCMJ), was seen as a network of irregular, fine, thin, longitudinally oriented trabeculae. The trabeculae were connected to each other at places and separated by narrow marrow spaces containing haemopoeitic tissue stained deep purple. The area near the GCMJ was characterized by a scarcity of bone marrow cells when compared to tissue found further from the GCMJ. The trabeculae were composed largely of calcified cartilage spicules covered with a thin layer of bone, the amount of bone increasing as distance from the GCMJ increased. The calcified cartilage spicules were stained darkly eosinophilic while the bony trabeculae appeared light eosinophilic (Figure 1). A large number of capillaries were seen between the calcified cartilage septae. The trabecular surfaces were lined by numerous osteoblasts and occasional osteoclasts. In Masson’s trichrome stain, the bony trabeculae appeared dark blue while the central cores of calcified cartilage were stained light blue colour. At the junction of growth plate and metaphysis, most of the horizontal calcified walls of the lacunae of degenerating hypertrophied cartilage cells were partially eroded and longitudinally oriented calcified walls of lacunae were seen to be invaded by haemopoietic tissue, osteoprogenitor cells, osteoblasts and osteoclasts (Figure 2). The haemopoietic tissues were stained brownish red while the red blood cells took on a bright orange red colour. The secondary spongiosa, the region of metaphysis lying away from the GCMJ, was characterized by large trabeculae composed mainly of bone with occasional central cores of calcified cartilage. The newly formed bone appeared as bars of bony trabeculae of variable width and length. They were interconnected to each other at places and were arranged in a longitudinal direction (Figure 1 & 4). The surfaces of trabeculae were covered with osteoblast, osteoprogenitor cells and osteoclasts. Large marrow spaces containing marrow cells and red blood cells were seen between the trabeculae, the marrow spaces increasing further as the trabeculae of the secondary spongiosa extended towards the diaphyseal region. The occasional osteoprogenitor cells seen in secondary spongiosa were usually located adjacent to the bone surfaces. It was characterized by a poorly visualized cytoplasm. Its nucleus was its most prominent feature, being ovoid to spindle shaped, euchromatic and pale staining in appearance. Under Hematoxylin and Eosin stain, the osteoblasts were seen as a single layer of cells lining the surfaces of trabeculae and appeared cuboidal, polygonal or spindle-shaped cells having a basophilic cytoplasm with an oval, euchromatic nucleus with a single nucleolus were seen lying at one end while a prominent clear area was found at the other end of the cell. The osteoclasts were seen lying between the capillaries and trabeculae composed mainly of calcified cartilage cores. These cells were large, irregular, polymorphus and multi-nucleated with a varying number of closely packed nuclei. The nuclei were randomly placed in the foamy eosinophilic cytoplasm and were found to be round to ovoid, purple stained and usually a single nucleolus were seen in most cells while more than one nucleoli were seen in some cells. The osteoclasts were often seen lying in depressions or pits resorbed from the bone surfaces. The area surrounding the osteoclast closely applied to the trabecular surface was seen as a clear zone at places and fine tooth like extensions projecting from the cytoplasmic membrane were seen (Figure 3). The osteoclasts were found lying against the bone surfaces or sometimes lying freely within the narrow spaces. In Masson’s trichrome stain, the cytoplasm appeared reddish brown while the nuclei were stained brownish black. Oval to spindle shaped osteocytes were seen lying in clear spaces called lacunae embedded within the matrix of the trabeculae (Figure 4).