Scope of Forensic Anthropology: Bones and their Origin

1-Is it a bone?

The first and foremost thing that a forensic anthropologist needs to confirm is whether the given substance is a bone. It has been found in numerous cases that bones have often been confused with ivory, stones or celluloid material. To jump into further investigation a forensic anthropologist should first carry out scrutinization concerning bone’s origin. These may include:

1. Visual examination of surface for the presence of parallel striations, exfoliation, chipping
2. Long bones have a hollow shaft
3. Visual examination for shape 
4. Examination under magnifications

2-Bone origin: Human or Animal

After confirming whether the suspected specimen is bone, the next question that arises is whether the bone is that of a human or an animal. The specimen procured, if present, in its whole form can give substantial evidences of its origin, but not always. Human bones have been found to resemble many animals in terms of their shape and size. The case becomes more complicated in case when the bone specimens are not of human adult. It has been found that bones of a child, as they are unfused, do not match with that of human adult. Instead, they are quite similar to that of animals. Thus, determination of bone’s origin is the next big challenge that forensic anthropologists face.

Human and non-human bones exhibit a wide range of dissimilarities when examined carefully. These differences are due to:

1. Variability in functions and role that a bone plays depending upon the species of organism
2. Evolution is another significant factor. Human bones have undergone various changes over the period of time as humans have evolved through the ages
3. Mode of living-terrestrial, aquatic, aerial etc.
4. Mode of nutrition
5. Species type: as different species have different shape, size and number of bones

The features that are used to distinguish human and non-human bones can be broadly studied under three categories:

1. Gross anatomical features: they include shape, size and number of bones
2. Macroscopic features: visible characteristics of bone
3. Microscopic features: microscopic examination of cross-sections and histological differences

Gross anatomical features

1-Cranium

Picture 1: Human skull vs Pig skull vs Horse skull.

Picture 2: Human (biped) skeleton vs Dog (quadruped) skeleton.

2-Dentition

Picture 3,4: Human vs Dog dentition

3-Vertebral column

Picture 5: Human vs Non-human spine.

4-Scapula and Clavicle

Picture 6,7: Dog scapula vs Human scapula.

5-Thorax

6-Forelimbs

7-Pelvis

Picture 8,9: Dog Pelvis vs Human Pelvis.

8-Hindlimbs

9-Thumbs and foot

Macroscopic features

Microscopic features

Determining whether a bone is human or non-human is often coupled with microscopic examination to improve the accuracy of results. Before studying the microscopic features of a bone, we must know about the histology of bone tissue i.e. bone structure at microscopic level.

1-Composition

Bones are basically composed of two components- bone tissue and extracellular matrix. The extracellular matrix of bone is composed of water (15%), collagen fibers (30%), mineral salts in crystalline forms (55%) which include Ca₃(PO₄)₂ and Ca(OH)₂. These salts combine to form hydroxyapatite [Ca₁₀(PO₄)₆(OH)₂]. These crystals lateral combine with others salts and ions like that of Mg²⁺, K⁺, (SO₄)^2-. Collagen fibers present in the matrix form a mesh network between which all the salts and minerals are deposited and crystallize, a process called calcification. The strength of bone is determined by the intensity of calcification and its flexibility is defined by the collagen fibers present in there.

The bone tissue comprises four types of cells:

1. Osteogenic cells: These are mesenchymal stem cells that differentiate into osteoblasts. They are the only type of bone cells capable of cell division. They are found in the inner lining of haversian canals, endosteum, and inner region of periosteum.
2. Osteoblasts: These are the bone cells that secrete extracellular matrix called osteioids comprising collagen.
3. Osteocytes: Osteoblasts surrounded by extracellular matrix are called osteocytes. They are responsible for exchange of nutrients and metabolic activities of the bone.
4. Osteoclasts: Osteoclasts are differentiated macrophages. They are present in the inner portion of endosteum. Osteoclasts have a fibrous border facing bone surface. They release lysozymes that dissolve compact and spongy bone tissue during normal resorption process occurring during bone repair.

2-Compact Bone Tissue

The compact bone tissue as the name suggests is hard and strongest tissue of the bone. It is generally present on those areas which experience considerable amount of pressure e.g. diaphysis of long bones. It can be histologically described as follows:

1. Composed of repeating structural units called osteons, that occur in cylindrical forms, present parallel to each other to the vertical axis of bone
2. Each osteon is composed of a network of lamellae present in the form of concentric rings with a hollow canal at the center called haversian canal, through which blood vessels, nerves and lymphatic vessels pass
3. Lamellae are nothing but calcified extracellular matrix
4. Between the concentric rings of lamellae are empty spaces called lacunae. These lacunae contain osteocytes and have radiating canals in all directions called canaliculi filled with extracellular fluid
5. Canaliculi of different lacunae are interconnected and also connected with the haversian canal that allows exchange of substances
6. The areas between osteons are called interstitial lamellae which are nothing but old and worn out osteons
7. The outer membrane lining the bone is called periosteum under which are the outer circumferential lamellae adhered to periosteum by Sharpe’s fibers
8. Periosteum has transverse canals called Volkmann’s canals through which blood vessels, nerves and lymphatic vessels from the periosteum connect with those of haversian canal
9. A network of lamellae lines the medullary cavity of the bone, they are the inner circumferential lamellae

Picture 10: Arrangement of osteons in compact bone tissue.
Picture Reference : Source

3-Spongy Bone Tissue

Spongy bone tissue is a relatively softer tissue as compared to compact tissue. It has lower density and is much more porous. It is always present inside a protective covering of compact tissue as in the epiphyses of long bones. It houses bone marrow tissue. It can be described as follows:

1. Unlike the compact bone, lamellae in spongy bone tissue are randomly arranged in form of trabeculae
2. Between the trabeculae are large spaces filled with bone marrow, these spaces have a rich supply of blood vessels
3. Each trabecula contains concentric rings of lamellae, spaces called lacunae that enclose osteocytes.  These lacunae have radiating canals called canaliculi
4. Trabeculae of spongy bones are arranged along the line of stress to absorb and transfer stress without breaking

Picture 11: Enlarged section of spongy bone tissue.

4-Microscopic differences in bone histology (Humans vs Non-humans)

1. Osteons 

1. Nutrient foramina 

*foraminal index is the ratio of the distance between the superior end of the bone and the nutrient foramina multiplied by 100 to the total length of the bone.

References 

1-www.e-pgpathshala.com, Paper no.7 (forensic anthropology), Module 9.

2-Principles of Anatomy and Physiology, Gerard Tortora and Bryan Derrickson.

3-Petra Urbanova, Vladimir Novotny, Distinguishing Between Human and Non-Human Bones: Histometric Method For Forensic Anthropology, Anthropologie, XLIII/1, pp. 77-85, 2005.

4-Beckett S. et al., Differentiating human versus non-human bone by exploring the nutrient foramen: implications for forensic anthropology, Int J Leg Med, DOI: 10.1007/s00414-017-1662-y, 2017, Springer.