Human Anatomy - Kotsan I. Y. 2009

Bones and Their Joints
Development of the Skeleton

The Skeleton is a complex of dense structures of mesenchymal origin. It consists of individual bones interconnected by connective, cartilaginous, or osseous tissue.

An internal skeleton is characteristic of all vertebrates, although some species also possess a more or less developed external skeleton alongside the internal one (such as the bony scales in fish Skin). Initially, the hard skeleton served to protect the Organism from harmful external influences (the exoskeleton of invertebrates). With The Development of the internal skeleton in vertebrates, it first acted as a structural framework to support soft Tissues. Later, especially with the transition from Water to land, individual PARTS OF THE skeleton transformed into levers driven by Muscles, enabling the skeleton to perform a locomotor function as well.

In lower chordate animals (such as amphioxus), a dorsal cord—the notochord—appears for the first time as the precursor of the internal skeleton, persisting throughout the organism's entire life. A membranous skeleton is formed around the notochord from the mesoderm. Later in the course of evolution, this connective-tissue membranous skeleton is replaced by a cartilaginous one (in cartilaginous Fishes, where cartilaginous vertebrae surround the notochord), and starting from bony fishes and extending up to mammals, by a bony skeleton. Correspondingly, human ontogeny exhibits a sequential transition through three stages of skeletal development: 1) Connective Tissue (membranous), 2) cartilaginous, and 3) osseous. Almost all bones pass through these three stages, with the exception of the BONES OF THE cranial vault, most facial bones, and the clavicles, which develop directly from connective tissue, bypassing the cartilaginous stage (primary bones). Bones developing on The basis of cartilaginous tissue are called secondary bones. The majority of bones in The Human Body undergo the cartilaginous stage of development.

During embryonic development, the skeleton is initially laid down as a long, dense strand of Cells extending from the cranial to the caudal end of the trunk, known as the dorsal cord or notochord. On either side of the notochord lie localized clusters of mesenchymal cells (primary embryonic connective tissue) that form thin membranes between the precursors of future muscles. This is the so-called membranous (or connective-tissue) skeletal stage.

As early as the 2nd month of intrauterine development, the greater part of the membranous skeleton is replaced by cartilaginous tissue, forming cartilaginous models of future bones. However, elements of the connective-tissue skeleton persist in certain regions, located primarily at the junctions of cartilaginous bone models to form primary continuous joints. This stage of skeletal development is called the cartilaginous stage. Although the cartilaginous skeleton is more robust than the membranous one, its strength is still insufficient to provide a rigid support for the intensively developing muscles.

By the 3rd month of prenatal development, The formation of the bony skeleton begins. The process of replacing Cartilage and Connective tissue with Bone tissue is very gradual and concludes only around 18–20 years of age, when the organism reaches sexual maturity.

Bone Development and growth involve processes of bone tissue formation and resorption. The deposition of bone matrix is carried out by specialized cells called osteoblasts, which proliferate rapidly, become surrounded by the bone matrix, and transform into osteocytes (mature bone cells). Bone resorption is performed by large multinucleated cells known as osteoclasts.

Bone development on a connective-tissue foundation occurs via desmal ossification: the appearance of an ossification center in the middle of individual parts of the future bone. Most bones have multiple ossification centers rather than just one.

The GROWTH AND DEVELOPMENT of bones in place of cartilage occur through so-called perichondral and endochondral ossification. The former begins externally from the periosteum, where periosteal cells (osteoblasts) produce bone tissue; the latter begins within the cartilaginous precursor of the future bone, where an ossification center emerges, leading to the resorption of cartilage and its replacement by trabeculae made of bone tissue. The growth of a bone in thickness is accompanied not only by the deposition of bone tissue from the exterior but also by resorption from the medullary cavity side, mediated by osteoclasts.

The longitudinal growth of long tubular bones occurs due to Changes in the region of the epiphyseal cartilages—that is, the cartilaginous layers or growth plates located between the diaphysis and epiphyses. These bones grow in length from both ends, being diepiphyseal. Short tubular bones (Phalanges of the fingers, metacarpals, metatarsals) grow in length through the epiphyseal cartilage at one end only, meaning they are monoepiphyseal.

In humans, many bones are laid down and develop from multiple parts that subsequently fuse to form a single monolithic bone. For instance, the hip bone develops from three major components, each of which is referred to as a bone in its own right. By 14–16 years of age, they fuse into a single continuous bone. During growth, tubular bones consist of three main parts, excluding separate ossification centers at sites of bony prominences. The diaphyses of bones begin to ossify before birth, whereas the epiphyses ossify afterward. Exceptions are the distal epiphysis of the Femur and the proximal epiphysis of the Tibia, where ossification centers appear prior to birth (their presence is used to determine fetal full-term status). The fusion of all these parts (synostosis) occurs at various times, completing by the 16th to 18th year of life. Short bones ossify similarly to the epiphyses of tubular bones. Flat bones grow desmally (in the cranium) or endochondrally (in the pelvis). After growth has ceased, bone fusion may still occur; for example, the sutures between individual bones of the cranial vault ossify, and in older individuals, this region appears as a single continuous monolithic bone. Growth in thickness occurs through the apposition (deposition) of bone tissue on the bone surfaces.

A bone is a lamellar Structure wherein growth, metabolic processes, and other Functions are regulated by the nervous and endocrine systems. In regions where a bone receives a greater and superior Blood supply, it develops more rapidly. Observations of internal bone development indicate that ossification processes proceed more quickly in areas subjected to greater mechanical pressure compared to areas with lower pressure—for example, faster in the lower limb than in the upper limb, and more rapidly in the lower vertebrae than in the upper ones. The growth rate, dimensions, and surface topography of bones depend on mechanical loads. In the 1870s, P.F. Lesgaft formulated the principle that bone growth is determined by The activity of surrounding muscles. Taking modern data into account, the functional laws of bone growth can be refined as follows:

a) mechanical loads stimulating bone growth must be rhythmic in nature;

b) activation of bone growth occurs at an optimal level of loading; insufficient or excessive loading inhibits growth;

c) the response of growing bone to mechanical loads (among other factors) is determined by individually distinct CHARACTERISTICS OF THE reaction norm to loading.

Engaging in Physical Exercise helps improve such mechanical properties of bone as resistance to fracture, bending, compression, tension, and torsion. In areas of maximal loading, the compact layer thickens, and the orientation and STRUCTURE OF THE bony trabeculae change. The spongy substance becomes coarser-meshed.

Main Latin terminology

Skeleton — skeleton

Bone — os

Periosteum — periosteum

Body of the bone (diaphysis) — diaphysis

End of the bone (epiphysis) — epiphysis

Metaphysis — metaphysis

Epiphyseal cartilage — cartilago epiphysialis

Articular surface — facies articularis

Medullary cavity — cavitas medullaris

Endosteum — endosteum

Bone Marrow — medulla ossium

Cytology/practical/86.html">Red bone marrow — medulla ossium rubra

Yellow bone marrow — medulla ossium flava

Diploe — diploe

Syndesmosis — syndesmosis

Membranes — membranae

Ligaments — ligamenta

Dentomental articulation (gomphosis) Sutures — suturae

Serrate suture — sutura serrata

Squamous suture — sutura squamosa

Plane suture — sutura plana

Synchondrosis — synchondrosis

Synostosis — synostosis

Joint — articulatio

Articular capsule — capsula articularis

Articular cavity — cavitas articularis

Synovial fluid — synovia

Simple joint — articulatio simplex

Compound joint — articulatio composita

Pivot joint — articulatio trochoidea

Hinge joint — ginglymus

Ellipsoid joint — articulatio ellipsoidea

Bicondylar joint — articulatio bicondylaris

Saddle joint — articulatio sellaris

Ball-and-socket joint — articulatio spheroidea

Cotyloid joint — articulatio cotylica

Plane joint — articulatio plana

Flexion — flexio

Extension — extensio

Abduction — abductio

Adduction — adductio

Pronation — pronatio

Supination — supinatio

Rotation — rotatio

Circumduction — circumductio