Exam 2 Review:  Chapter 6:  Modes of Ossification

intramembranous ossification - One of the two processes of embryonic bone formation:  the embryonic model for most flat bones and a few other bones is a sheet of dense fibrous connective tissue; stem cells differentiate into osteoblasts which begin laying down bone matrix in a spongy bone pattern to replace the fibrous membrane model; several such centers of ossification expand until they meet and fuse; eventually the entire structure matures to have compact bone on the outer surface and spongy bone in the interior.  [See Fig. 6.7, p. 184]

endochondral ossification - One of the two processes of embryonic bone formation:   the embryonic model for most long, short and irregular bones is a mass of dense fibrous connective tissue; stem cells differentiate into chondroblasts which begin laying down hyaline cartilage matrix in a pattern to replace the fibrous connective tissue model; then osteoclasts and blood vessels penetrate the cartilage while other stem cells follow and differentiate into osteoblasts which begin laying down bone matrix in a spongy bone pattern to replace the cartilage model; several such centers of ossification expand until they meet and fuse; eventually the entire structure matures to have compact bone on the outer surface and to have spongy bone and possibly a marrow space in the interior.  [See Figs. 6.8 & 6.9, p. 185-186]

chondroblasts - The connective tissue stem cells which synthesize and secrete the matrix of cartilage; as they mature, they occupy lacunae in the cartilage matrix and can then be referred to as chondrocytes.

chondrocytes - The mature connective tissue cells found within the lacunae of cartilage matrix; they are relatively meatbolically inactive because cartilage tissue has no direct blood supply and, therefore, these cells contribute little to the repair of injuries to cartilage.

cartilage model - The embryonic precursor to mature bone for most long, short and irregular bones which form by the process of endochondral ossification.

epiphyseal plate = metaphysis - In long bones forming by the process of endochondral ossification, the sheet of hyaline cartilage which develops between the growing bone matrix of the diaphysis (shaft) of the bone and either of the ossification centers in the epiphyses, and by its continual growth and production of new cartilage matrix in both directions away from the diaphysis, permits the developing long bone to continue to grow in length; regulatory hormonal signals change in the post-adolescent years (~18-23 years) which slow the rate of epiphyseal cartilage growth and the epiphyseal plates are eventually replaced by bone matrix fusing the diaphysis to its epiphyses; after that point no further growth in length is possible for a given long bone; growth of the epiphyseal plate is regulated by human growth hormone, hGH, and the sex hormones (estrogens and androgens).

 

epiphyseal line = metaphysis - The slightly thickened plate of spongy bone observed in the interior of long bones where an epiphyseal plate of cartilage had been present earlier in development; its formation is regulated by hormones such as human Growth Hormone (hGH) and the sex hormones (estrogens or androgens).

metaphysis - The region of a long bone between the diaphysis and the epiphysis, generally, the area within a long bone associated with the epiphyseal plate or line; composed of both compact and spongy bone.

Describe:

2. the differences between endochondral versus intramembranous ossification.

Characteristic Differences Intramembranous Ossification Endochondral Ossification
bone type(s) involved most flat bones most long, short and irregular bones
embryonic model a sheet of dense fibrous connective tissue dense fibrous connective tissue is replaced
by hyaline cartilage matrix
stem cells involved fibroblasts, osteoblasts fibroblasts, chondroblasts, osteoblasts
progression of
developing tissue types
dense fibrous connective tissue  →  bone dense fibrous connective tissue → 
hyaline cartilage  →  bone
marrow space in adult bone unlikely likely

3. how a long bone grows in length.

During the process of endochondral ossification, the sheet of hyaline cartilage, the epiphyseal plate, which develops between the growing bone matrix of the diaphysis (shaft) of the bone and either of the ossification centers in the epiphyses permits bone growth in length.  By its continual growth and production of new cartilage matrix in both directions away from the diaphysis, the epiphyseal plate permits the developing long bone to continue to grow in length.  Regulatory hormonal signals change in the post-adolescent years (~18-23 years) which slow the rate of epiphyseal cartilage growth and the epiphyseal plates are eventually replaced by bone matrix fusing the diaphysis to its epiphyses.  After that point no further growth in length is possible for a given long bone.

Sketch and Label

1. the process of Intramembranous Ossification.


2. the process of Endochondral Ossification.