Bone Development & Growth

Objectives Distinguish between intramembranous and endochondral bones, and explain how such bones develop and grow Describe the effects of sunlight, nutrition, hormonal secretion, and exercise on bone development and growth

Bone Development Parts of the skeletal system begin to form during the first few weeks of prenatal development Bony structures continue to grow and develop into adulthood The process of bone development is called osteogenesis Bones form when bone tissue replaces existing connective tissue in one of two ways Intramembranous Endochondral

Intramembranous Bones Are bones that originated within sheet-like layers of connective tissue Include the bones of the skull, clavicles, sternum, mandible, maxillae and zygomatic

Endochondral Bones Are bones that begin as masses of cartilage later replaced by bone tissue Includes majority of the bones of the skeleton The process of forming endochondral bones is called endochondral ossification

Osteogenesis of Intramembranous Bones Sheets of unspecialized connective tissues appear at the site of future bones Partially differentiated connective tissue cells collect around blood vessels in these layers Connective tissue cells further differentiate into osteoblasts, which deposit spongy bone Osteoblasts become osteocytes when bony matric completely surrounds them Connective tissue on the surface of each developing structure forms a periosteum Osteoblasts on the inside of the periosteum deposit compact bone over the spongy bone

Osteogenesis of Endochondral Bones Masses of hyaline cartilage form models of future bones Cartilage tissue breaks down and periosteum develops Blood vessels and differentiating osteoblasts from the periosteum invade the disintegrating tissue Osteoblasts form spongy bone in the space occupied by cartilage Osteoblasts beneath the periosteum deposit a thin layer of compact bone Osteoblasts become osteocytes when bony matrix completely surrounds them

Long Bone Development Bone tissue begins to replace hyaline cartilage in the center of diaphysis This region is called the primary ossification center, in which bone develops from it toward the ends of the cartilaginous structure Osteoblasts from the periosteum deposit a thin layer of compact bone around the primary ossification center

Long Bone Development cont. The epiphyses of the developing bone remain cartilaginous and continue to grow A secondary ossification center appears in the epiphyses and spongy bone forms In all directions As spongy bone is deposited in the diaphysis and in the epiphysis, a band of cartilage called the epiphyseal plate remains between the two ossification centers

Endochondral ossification

Growth at the Epiphyseal Plate The cartilaginous cells of the epiphyseal plate form four layers: Zone of resting cartilage Zone of proliferating cartilage zone of hypertrophic cartilage Zone of calcified cartilage

Zone of resting cartilage Is closest to the end of the epiphysis It is composed of resting cells that do not actively participate in growth this layer anchors the epiphyseal plate to the bony tissue of the epiphysis

Zone of Proliferating Cartilage This is the second layer It includes rows of many young cells undergoing mitosis As new cells appear and as extracellular matrix forms around them, the cartilaginous plate thickens

Zone of Hypertrophic cartilage Composed of rows of older cells left behind when new cells form This layer enlarges and thickens the epiphyseal plate more The entire bone lengthens as a result At the same time, invading osteoblasts secrete calcium salts and accumulate in the extracellular matrix adjacent to the oldest cartilaginous cells The extracellular matric calcifies and the cells begin to die

Zone of Calcified Cartilage This is a thin layer It is composed of dead cells and calcified extracellular matrix In time, large multinucleated cells called osteoclasts break down the calcified matrix by secreting an acid Osteoclasts also phagocytize components of the bony matrix Afterwards, bone building osteoblasts invade the region and deposit bone tissue

Continued Growth Bones continue to develop after you are born Between early childhood and late adolescence, bone cells gradually replace the cartilage in long bones of the arms and legs When all the cartilage in the epiphyseal plate is replaced with bone, a person has reached their full height The epiphyseal plates become epiphyseal lines

Ossification Timeline Age Occurrence 3rd month of prenatal development Ossification in long bones begins 4th month of prenatal development Most primary ossification centers have appeared in the diaphysis of bones Birth to 5 years old Secondary ossification centers appear in the epiphyses 5-12 years old (females) 5-14 years old (males) Ossification rapidly spreads from the ossification centers 15-18 years old (females) 17-20 years old (males) Bones of upper limbs and scapulae completely ossified 16-21 years old (females) 18-23 years old (males) Bones of the lower limbs and hip bones completely ossified 21-23 years old (females) 23-25 years old (males) Bones of the sternum, clavicles, and vertebrae completely ossified By 23 years old (females) By 25 years old (males) Nearly all bones completely ossified

Homeostasis of Bone Tissue After the intramembranous and endochondral bones form, the actions of osteoclasts and osteoblasts continually remodel them Bone remodeling occurs throughout life as osteoclasts resorb bone tissue and osteoblasts replace the bone The processes of resorption and deposition occur on the surfaces of the endosteum and periosteum The rate of bone remodeling is not uniform, spongy bone is faster than compact bone These processes replace 10% to 20% of the skeleton each year

Factors Affecting Bone Development, Growth & Repair Nutrition Exposure to sunlight Hormonal secretions Physical exercise

Nutrition Factors Vitamin D is necessary for proper absorption of calcium in the small intestine Without enough of it, the bone matrix softens and thereby deforms the bone Vitamins A and C are also required Vitamin A is necessary for osteoblast and osteoclast activity during normal development A lack of vitamin A may retard (slow) bone development Vitamin C is required for collagen synthesis A deficiency prevents osteoblasts from producing enough collagen in the extracellular matrix of the bone tissue, and as a result bones become abnormally slender and fragile.

Hormonal Factors The pituitary gland secretes growth hormone, which stimulates division of cartilage cells in the epiphyseal plates Without this hormone pituitary dwarfism occurs If too much growth hormone is produced, then pituitary gigantism occurs The thyroid hormone thyroxine stimulates replacement of cartilage in the epiphyseal plates of long bones with bone tissue This hormone also increases cellular metabolism, including stimulating osteoblast activity The parathyroid hormone stimulates an increase in the number and activity of osteoclasts, which break down bone

Hormonal Factors Cont. Both sex hormones, testosterone for males and Estrogen for Females, promote formation of bone tissue At puberty, these hormones are abundant, causing the long bones to grow considerably Sex hormones also stimulate ossification of the epiphyseal plates and thus, stop bone lengthening at a relatively early age Estrogen has a stronger affect on the epiphyseal plates and, thus females typically reach their maximum heights sooner than males

Physical Factors Physical stress stimulate bone growth For example, when skeletal muscles contract, they pull at their attachments on bones, and the resulting stress stimulates the bone tissue to thicken and strengthen (hypertrophy) With a lack of exercise, bone tissue wastes becoming thinner and weaker (atrophy) This is why the bones of athletes are usually stronger and heavier than those of non- athletes