Summary Mays chapter 1 - The nature of bones and teeth.docx

**Bioarchaeology: Osteology** **[Mays chapter 1: The nature of bones and teeth\ \ Bones]** **206 bones** in the adult human body (plus semasoids in the tendons of hands and feet) Divided into **three classes** by basic shape:\ **long bones**, found in limbs: hollow tube closed at both ends\ **fla...

**Bioarchaeology: Osteology** **[Mays chapter 1: The nature of bones and teeth\ \ Bones]** **206 bones** in the adult human body (plus semasoids in the tendons of hands and feet) Divided into **three classes** by basic shape:\ **long bones**, found in limbs: hollow tube closed at both ends\ **flat bones**: broad bony plates\ **irregular bones**, e.g. vertebrae: fit into neither category **Composition** Bone is a composite material: **64% mineral, 26% organic material, 10% water.** Majority of organic part is **long collagen (protein) fibers**. This part gives bone a slight 'give' which gives it its strength (reduces its brittleness). This organic part degrades after death, which is why archaeological bone is so brittle. Mineral part of bone consists of **small mineral crystals embedded in matrix of collagen fibers**. This bone mineral is primarily hydroxyapatite, a form of **calcium phosphate**. The mineral part gives bone its rigidity. **Structural build-up** Two different types of bone tissue: cortical bone and trabecular bone. **Cortical bone** is solid and dense and forms the outer layer of bones: thickest in the shafts of the long bones, forms a thin layer around other bone parts. **Trabecular bone** has a honeycomb structure and is less dense than cortical bone. Located within the ends of long bones and in the interior of the flat and irregular bones. In living bone, most of the outer surface of bone is surrounded by a thin membrane: **periosteum**. The internal walls of the central cavity of the long bones (where marrow is located) are lined with another membrane, the **endosteum**. This also lines the network of tiny cavities in trabecular bone. **Functions of the skeleton** Bones: -provide a general **framework to support the body**\ -bear ridges for **attachment of muscles and tendons**\ -have surfaces which form **joints**\ -**protect vital organs**\ -provide a **store of fats**\ -provide a **store of blood-forming marrow**\ -act as a **bank of mineral salts** **Form reflects functions: long bones (example)** -**tubular form** of long bones **maximises strength and minimises weight**\ -**medullary cavity** within shaft provides **space for blood-forming cells (marrow) and fat storage**\ -the **metaphyses** (flared 'neck' portion of the shafts) support the **epiphyses** (rounded tips of long bones), which bear the joint surfaces through which the long bones are connected to their neighbours\ -**The epiphyses consist almost entirely of trabecular bone.** This is less rigid than cortical bone and thus better **withstands and dissipates mechanical forces** working on joints\ -The epiphyses also form a **site of red blood cell production in juveniles**\ -The **large surface area of the epiphyses** facilitates the **exchange of minerals to and from bone\ **-The walls of the **diaphysis** (shaft) is formed of cortical bone for strength and rigidity **Bone is a living tissue.** It is permeated with nerves and blood vessels and is continuously being formed and broken down. It can repair itself, and it **adapts its form to the strains put on it: Wolff's Law.** An increase in mechanical forces due to weight bearing or muscular contraction results in bones that are thicker and stronger, especially in the orientation in which the loading predominantly occurs. **Microscopic structure** When bone is formed, it is initially laid down as **woven or primary bone**. This is a temporary tissue that is gradually replaced by **mature or lamellar bone**. Woven bone is coarser and more porous than lamellar bone: they can be distinguished with the naked eye. Lamellar bone is composed of **microscopic layers called lamellae** which are 4-12 microns thick. Both trabecular and cortical bone have a lamellar structure, but these differ in the way they are organised. Cortical bone is permeated by innumerable **interconnected channels** forming the so-called **Haversian system**, which provide its **blood supply.** The bony lamellae are arranged concentrically around the Haversian channels, normally 4-20 lamellae around each channel: such a unit of bone organisation is called an **osteon.\ **The Haversian channels run **parallel to the long axis** of the bone. They interconnect through transverse channels called **Volkmann's canals.** Between the osteons there are some lamellae called the **interstitial lamellae**, and encircling the entire outer and inner surface of the bone are the **circumferential lamellae.** Trabecular bone does not have a Haversian system. It receives its blood supply from blood vessels meandering through its honeycomb structure. **Bone cells** Three main types of bone cells: **osteoblasts, osteocytes** and **osteoclasts**. **Osteoblasts** are responsible for the **formation of new bone.** They produce the organic matrix, the osteoid, which is then mineralised to form bone. They are often concentrated on bone surfaces. **Osteocytes** are involved with the **maintenance of bone as a living tissue.** They are located in lacunae within the bone tissue, and they receive nutrients via tiny channels called the **canaliculi**, which connect them with other osteocyte lacunae within the same or neighbouring lamellae. **Osteoclasts** are responsible for the **resorption (removal) of bone tissue.** They do this through **acidic dissolution of the mineral component** and **enzymatic degradation of the organic component of bone tissue.** They are located in shallow depressions in bone surfaces called Howship's lacunae. **Bone growth (ossification)** Bones form from an embryonic tissue called **mesenchyme** (also in adults). There are two main types of ossification: **endochondral ossification** and **intramembranous ossification**. Most bones are formed through endochondral ossification, but the clavicle, mandible and most bones of the skull are formed through intramembranous ossification. **Process of** **endochondral ossification**: - Bone develops from a cartilaginous (cartilage) model. Initially, mesenchymal cells (stem cells) differentiate into **chondrocytes**, which are cells that form **cartilage**. This happens within the perichondrium membrane, which later forms the periosteum (the thin membrane around the outer surface of the bone). Cartilage mainly consists of collagen fibres, proteoglycans, and water. - The chondrocytes grow, enlarge, and die, leaving spaces in the cartilage. - Blood vessels invade the area, conveying osteoblasts and osteoclasts, and the osteoblasts use the cartilage as a framework to lay down bone matrix. Initially, a thin sleeve of bone is formed around the center of the cartilage model. Subsequently calcium is deposited upon the cartilage within this sleeve. This calcified cartilage is then converted to bone. - This process continues as the bone grows in length and width, forming compact bone on the outside and spongy bone inside. The initial center of bone formation is called the **primary ossification center.** As the cartilage cells grow outward from this center, ossification of the calcified organic matrix proceeds behind them. Together with the elongation of the periosteum this results in the longitudinal growth of the bone. - The interior of the shaft is initially filled with bone, but as growth continues it is hollowed out by osteoclasts to form the medullary cavity. - During the first few years of post-natal life, secondary centers of ossification called the epiphyses appear at long-bone ends. They are united with the shaft via cartilages: the epiphysial growth plates. When growth is complete, the growth plate also ossifies, fusing the epiphysis with the shaft. This is how we can tell juvenile (long) bones and adult (long) bones apart. - The growth in width of a long-bone works differently. In the diaphysis (shaft), bone is deposited on the outer surface beneath the periosteum (outer bone membrane), and resorbed from the internal surface under the endosteum (inner bone membrane). In a growing bone, the rate of bone resorption from the endosteal surface is exceeded somewhat by the rate of deposition beneath the periosteum. Thus the bone increases in overall width, but the thickness of the walls of the diaphysis also increases during growth. - Since the metaphysis (neck parts of the long-bones) are flared, their width needs to be reduced during longitudinal growth as metaphysis (neck) becomes diaphysis (shaft). To achieve this, bone is deposited on the internal surface of the cortex (cortical bone) and removed from the surface of the periosteum (outer bone membrane). **Process of intramembranous ossification**: - **Mesenchymal (stem) cells** differentiate into **osteoblasts** (bone-forming cells) in a fibrous connective tissue membrane. This membrane will eventually form the periosteum. - Osteoblasts secrete organic bone matrix (**osteoid**), which then hardens as minerals, primarily calcium, are deposited. - The osteoblasts become trapped in the matrix and differentiate into **osteocytes** (mature bone cells), which maintain bone tissue. - The bone expands outward as more osteoblasts form, eventually forming a trabecular (spongy) network of bone that later gets filled in by more compact bone. Ossification of the bony skeleton begins in about the **sixth week of prenatal life**, and skeletal growth is **complete by about 25 years.** **[Teeth]** Human teeth are classified into **four types** according to their shape and function: **Incisors**: chisel-like teeth for cutting food\ **Canines**: more conical in shape, for puncturing and tearing things\ **Premolars** and **molars**: broad, flattened surfaces, for crushing and grinding Humans develop two sets of teeth: deciduous (milk) teeth and permanent (adult) teeth. **Deciduous dentition:** 20 teeth in total, four incisors, two canines and four molars per jaw **Adult dentition:** 32 teeth in total, four incisors, two canines, four premolars and six molars per jaw **Tooth structure** Each tooth consists of a **crown** projecting above the gum and one or more tapering **roots** which occupy sockets in the jaw called the **alveoli**. The junction between the crown and the root is called the **neck** or **cervix**. Teeth are attached to the surrounding tissue through the **periodontal ligament**, a group of specialized connective tissue fibres that anchor the tooth to its surrounding alveolar bone within the jaw. This periodontal ligament also contains blood vessels and nerves.\ At the tip of the root of a tooth there is a small opening called the **apical foramen.** This serves as the passageway for nerves, blood vessels, and other connective tissues to enter and exit the tooth\'s pulp chamber from the surrounding bone and tissues. Teeth consist of hard tissues and soft tissue. The three hard tissues are **enamel**, **cementum** and **dentine**. These three enclose the **dental pulp**, the soft tissue located in the pulp cavity and in the root canal inside the tooth, which includes **nerves and blood vessels**. The three hard tissues of the tooth lack a blood supply and are not continually turned over like bone is. For this reason, they cannot reshape or repair themselves in response to damage, disease or mechanical forces. **Enamel** is almost entirely composed of **inorganic matter**, whose chemical composition is **similar to bone mineral** (hydroxyapatite). It is arranged in thin rods or prisms. Enamel **lacks a cell structure** and unlike other skeletal hard tissues is **not a living tissue.** However, it does contain some organic matter, importantly the **sexually dimorphic amelogenin peptides** that allow us to use enamel to sex an individual using the proteomic approach. **Dentine**: 75% inorganic material (mainly hydroxyapatite)\ 20% organic compound, mainly collagen\ 5% is water\ **Like enamel, dentine lacks cells, except** on its inner surface where cells called **odontoblasts** line the pulp cavity inside the tooth. Dentine is penetrated by **microscopic tubes** that run from the pulp cavity to its outer margins. **Males typically have more dentine than females**, because Y chromosomes have genes responsible for both enamel and dentine development, while X chromosomes only have genes involved in the production of enamel. As a result, male teeth are typically 3-7% larger than female teeth, making teeth **useful for sex estimation.** While in adults most teeth show some degree of sexual dimorphism, the canines are normally the most dimorphic, followed by the molars. In children who still have their deciduous teeth, the first and second molars are the most sexually dimorphic. **Cementum**: 65% inorganic material\ 23% organic material, mainly collagen\ 12% water Cementum **coats the tooth roots.** Like dentine, some of it shows a cell structure. **Tooth formation** All three hard tissues are formed but the creation of an organic matrix followed by the mineralisation of that matrix. Enamel formation has a third phase: after the creation and mineralisation of the organic matrix, there is a maturation phase in which the enamel loses most of its organic content. The organic matrix of dentine is formed by odontoblasts, while that of enamel is formed by ameloblasts. Cementum continues to be formed throughout life, so it increases in thickness with age.