Welcome to the first instructional video of module one in the course, The Truth in Our Bones. In this video, we will learn about two things. First, we'll provide a basic overview of how the skeleton and the teeth form and grow. And second, we'll show you how we can use this growth process to estimate the age of death of an individual who was still growing. You will frequently hear us use the term subadult. Other osteoarchaeologists use the term nonadult. And what we mean by this is anyone under the age of about 18 years. We don't use the term child or juvenile for this, because those terms have a more specific meaning in many fields. For example, by one scheme, there are five age category within subadulthood. The fetal period occurring obviously before birth. Infancy occurring from birth to two years of age. Childhood from three to six years. The juvenile period from seven to 12 years of age, and adolescent from 13 to 17 years. After the age of 18, skeletal growth is minimal, being completed by your mid 20s. And you are categorized, physically at least, as an adult. The methods we use to estimate age in adults, are very different from those that we use in subadults, as you will see. There are two ways in which our bones form and grow. The first is called endochondral ossification, and is how most of your bones develop, including the long bones such as those in your arms and legs. The bones or bone sections that grow via endochondral ossification are depicted in this figure. Endochondral growth endo meaning within, and chondral meaning cartilage, is basically growth occurring within a cartilage model. In utero, this begins with a hyaline cartilage model, whose shape resembles a small version of the bone to be formed. The first bone formed is along the bone's shaft called the diaphysis. Here, bone cells deposit unmineralized bone called osteoid. This area in the diaphysis is termed the primary ossification center, and secondary ossification centers occur at the ends of the bone, and are called the epiphyses. There's a growing cartilage layer between the two areas, a point called the metaphysis, and this is where osteoid is deposited as a bone grows in length. Eventually at an age specific for each ephiphysis, osteoid production of the metaphyisis will outpace the rate of cartilage production, causing the two to fuse. So, how can we use our knowledge of endochondral growth to estimate the when someone died? We'll tell you about three major ways this works. First, did you know a newborn infant has more bony elements than an adult? A new born has around 300 bony elements. An adult usually has 206. This is because in subadults, the separate pieces of many bones have not yet fused together. Also in subadults, some bones don't form until later. Babies have cartilaginous precursors of many bones such as the kneecap called the patella. And it's not until the age of three to five years, that the kneecaps turn into bone. In fact, a four year old has more bony elements than a newborn, because those cartilaginous precursors have by then started to ossify. Have you ever noticed that it's actually quite painful to crawl on your knees? Well, not so much for an infant, without a bony patella in the way. So, by observing which bones are present in the subadult skeleton, we can get an idea of how old they must have been. Second, we can look at the state of fusion of the epiphyses. So, each epiphysis fuses at a predetermined age. Now there are some differences within a population, between different geographic groups, between boys and girls, especially once puberty hits. But in normal conditions, all humans will have an epiphysis fuse within the same one to two year period. For an example, we can look at the upper arm bone, the humerus. The head of this bone forms a part of the shoulder joint, and typically in girls, that epiphysis fuses between 15 to 17 years of age, and in boys, it's typically 16 to 18 years. So by looking at the state of fusion of different epiphyses for all of the bones, we can estimate a subadult's age with a margin of error of maybe one to two years. Finally, the third way of estimating the age of a growing subadult is by measuring the size of their bones, and comparing that to measurements of subadults from modern times of known age. Let's take an example from your upper leg bone, the femur. Now, obviously in normal circumstances, as one gets older the size of that bone increases as one gets taller. We then compare the length of that bone to modern subadults, and find the closest match. When doing this from many bones from the skeleton, we can arrive at an age of death that's accurate to within one to two years. If you'd like to watch an optional video of how this works, look under course documents, and Christine will demonstrate the process for you. Now, let's take a step back and recall that there were two types of bone growth. We've learned about endochondral bone growth, what about this other type? You may have noticed, we've not yet talked about the skull. That's because most of the bones of the skull, especially the thin bones making up the vaults and part of the face, are formed by intramembranous ossification. As the name implies, this means that bone forms within, or intra a membrane. This process does not involve cartilage. The bones or sections of bone that form via intramembranous ossification are depicted in the speaker. So, what happens? Connective tissue forms in sheets at sites where flat bones, the intramembranous bones will eventually be. Some of the cells in the connective tissue will differentiate into osteoblasts which you recall lay down unmineralized by bone called osteoid. These osteoblasts begin to lay down spicules in the middle of the connective layer. These eventually connect to each other to form a network of trabeculae, creating a type of bone called cancellous, or spongy bone. As the osteoblasts continue to lay down osteoid, eventually bone begins to accumulate along the outer and inner edges, or plates of the membrane. This bone is much more dense and flat, and it's called compact bone. So essentially, this process creates a sandwich. Two layers of compact bone forming the bread, and the spongy bone in the middle as the filling. So, what can we do with bones that form via intramembranous growth, to estimate the age and death of a subadult? We'll explain how one such method works. You're probably familiar with the soft spots on a baby's head. Well, these are real areas on the skull of a newborn or an infant. And they're called fontanelles. It's where bone has not yet formed, instead, there's a tough, fibrous material in that spot. Fontanelleles enable the bony plates of the skull to flex during birth, and also facilitate fast growth of the cranium during infancy. But just as the epiphyses have set ages during which they fuse, the fontanelles have set ages during which they close over. As seen in this figure, some of the smaller ones close over within a few weeks after birth. The larger one on the back of the cranium closes around one to three months of age, and the largest one on the front top of the cranium is the last to fully close, by 18 to 24 months. So this gives us a fourth way to estimate the age of a subadult. If we have the skull of a fetus or an infant, we can use the state of fontanelle closure to estimate its age. Finally, we talk briefly about dental growth, called odontogenesis. To understand how teeth grow, it's necessary to learn about their different parts, the different tissues they are composed of. This diagram shows the hard outer crown layer called the enamel, the inner, softer crown layer, called dentin. The root of the tooth, here surrounded by a thin layer called cementum, and the pulp cavity, which exists inside the tooth and allows connection of the tooth to the body's nerve and blood supply. A tooth first begins forming in the bone of the jaws. Undergoing four developmental stages. Your 20 baby teeth called deciduous teeth, start forming by the sixth to eighth week in the utero. And your 32 adult or permanent teeth begin forming by the 20th week in utero. Now, obviously these tiny forming tooth buds don't erupt until much later in life. But nonetheless they are there, hiding in your jaws, sometimes for many, many years. So, teeth slowly increase in size in an orderly fashion. The tooth crown develops from the bottom to the top, and then adds layers outwardly, whereas the root gradually gets longer in the opposite direction. The ordered progression of tooth formation with age has been observed on many known age individuals, and this makes it possible to take a tooth from an individual of unknown age and compare it to a standard to estimate their age. As you probably already know, teeth erupt through the bone gums at certain ages. This figure shows the average age of eruption of the baby, the deciduous teeth. And similar figures to exist for the average age of eruption of the adult permanent teeth. So, we can look at the pattern of dental eruption of a subadult, and compare that with standards of eruption for similar geographic groups, to estimate how old someone was when they died. Dental formation and eruption are less affected by environmental disruptions such as malnutrition and illness. So, actually they're our most accurate and most precise method of subadult age estimation that we have. If you want to see Anne-Marijn estimate the age of a subadult based upon the state of their dental formation and eruption, go to a video under course documents. In this video, you've had a crash course in skeletal and dental growth, and in the process learned how we have several ways listed here to use these processes to estimate the age-at-death of a subadult of unknown age from an archaeological site. In the online forum, you'll find lots of topics for conversation that we didn't have time for in this video. A big focus of these questions is on considering what factors can cause disagreement between someone's actual chronological age, and the age suggested by their bones and teeth. As we went through this video, I'll bet that you already thought of reasons why our methods, sometimes at least, may not work that well. And this is a big area of study in osteoarchaeology. As we strive to use methods that are as accurate as possible. So please join in the discussion. Coming up next, is how we estimate the age-at-death of adults for whom growth has ceased. See you there.
