Endochondral ossification

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Endochondral ossification_table_infobox_0

Endochondral ossificationEndochondral ossification_header_cell_0_0_0

Endochondral ossification is one of the two essential processes during fetal development of the mammalian skeletal system by which bone tissue is created. Endochondral ossification_sentence_0

Unlike intramembranous ossification, which is the other process by which bone tissue is created, cartilage is present during endochondral ossification. Endochondral ossification_sentence_1

Endochondral ossification is also an essential process during the rudimentary formation of long bones, the growth of the length of long bones, and the natural healing of bone fractures. Endochondral ossification_sentence_2

Growth of the cartilage model Endochondral ossification_section_0

The cartilage model will grow in length by continuous cell division of chondrocytes, which is accompanied by further secretion of extracellular matrix. Endochondral ossification_sentence_3

This is called interstitial growth. Endochondral ossification_sentence_4

The process of appositional growth occurs when the cartilage model also grows in thickness due to the addition of more extracellular matrix on the peripheral cartilage surface, which is accompanied by new chondroblasts that develop from the perichondrium. Endochondral ossification_sentence_5

Primary center of ossification Endochondral ossification_section_1

The first site of ossification occurs in the primary center of ossification, which is in the middle of diaphysis (shaft). Endochondral ossification_sentence_6

Then: Endochondral ossification_sentence_7

Endochondral ossification_ordered_list_0

  1. Formation of periosteumEndochondral ossification_item_0_0
    • The perichondrium becomes the periosteum. The periosteum contains a layer of undifferentiated cells (osteoprogenitor cells) which later become osteoblasts.Endochondral ossification_item_0_1
  2. Formation of bone collarEndochondral ossification_item_0_2
    • The osteoblasts secrete osteoid against the shaft of the cartilage model (Appositional Growth). This serves as support for the new bone.Endochondral ossification_item_0_3
  3. Calcification of matrixEndochondral ossification_item_0_4
    • Chondrocytes in the primary center of ossification begin to grow (hypertrophy). They stop secreting collagen and other proteoglycans and begin secreting alkaline phosphatase, an enzyme essential for mineral deposition. Then calcification of the matrix occurs and osteoprogenitor cells that entered the cavity via the periosteal bud, use the calcified matrix as a scaffold and begin to secrete osteoid, which forms the bone trabecula. Osteoclasts, formed from macrophages, break down spongy bone to form the medullary (bone marrow) cavity.Endochondral ossification_item_0_5

Secondary center of ossification Endochondral ossification_section_2

About the time of birth in mammals, a secondary ossification center appears in each end (epiphysis) of long bones. Endochondral ossification_sentence_8

Periosteal buds carry mesenchyme and blood vessels in and the process is similar to that occurring in a primary ossification center. Endochondral ossification_sentence_9

The cartilage between the primary and secondary ossification centers is called the epiphyseal plate, and it continues to form new cartilage, which is replaced by bone, a process that results in an increase in length of the bone. Endochondral ossification_sentence_10

Growth continues until the individual is about 20 years old or until the cartilage in the plate is replaced by bone. Endochondral ossification_sentence_11

The point of union of the primary and secondary ossification centers is called the epiphyseal line. Endochondral ossification_sentence_12

Appositional bone growth Endochondral ossification_section_3

The growth in diameter of bones around the diaphysis occurs by deposition of bone beneath the periosteum. Endochondral ossification_sentence_13

Osteoclasts in the interior cavity continue to resorb bone until its ultimate thickness is achieved, at which point the rate of formation on the outside and degradation from the inside is constant. Endochondral ossification_sentence_14

Histology Endochondral ossification_section_4

During endochondral ossification, five distinct zones can be seen at the light-microscope level. Endochondral ossification_sentence_15

Endochondral ossification_table_general_1

NameEndochondral ossification_header_cell_1_0_0 DefinitionEndochondral ossification_header_cell_1_0_1
Zone of resting cartilageEndochondral ossification_cell_1_1_0 This zone contains normal, resting hyaline cartilage.Endochondral ossification_cell_1_1_1
Zone of proliferation / cell columnsEndochondral ossification_cell_1_2_0 In this zone, chondrocytes undergo rapid mitosis, forming distinctive looking stacks.Endochondral ossification_cell_1_2_1
Zone of maturation / hypertrophyEndochondral ossification_cell_1_3_0 In this zone, the chondrocytes undergo hypertrophy (become enlarged). Chondrocytes contain large amounts of glycogen and begin to secrete alkaline phosphatase.Endochondral ossification_cell_1_3_1
Zone of calcificationEndochondral ossification_cell_1_4_0 In this zone, chondrocytes are either dying or dead, leaving cavities that will later become invaded by bone-forming cells. Chondrocytes here die when they can no longer receive nutrients or eliminate wastes via diffusion. This is because the calcified matrix is much less hydrated than hyaline cartilage.Endochondral ossification_cell_1_4_1
Zone of ossificationEndochondral ossification_cell_1_5_0 Osteoprogenitor cells invade the area and differentiate into osteoblasts, which elaborate matrix that becomes calcified on the surface of calcified cartilage. This is followed by resorption of the calcified cartilage/calcified bone complex.Endochondral ossification_cell_1_5_1

Fracture healing Endochondral ossification_section_5

During fracture healing, cartilage is often formed and is called callus. Endochondral ossification_sentence_16

This cartilage ultimately develops into new bone tissue through the process of endochondral ossification. Endochondral ossification_sentence_17

Recently it has been shown that biomimetic bone like apatite inhibits formation of bone through endochondral ossification pathway via hyperstimulation of extracellular calcium sensing receptor (CaSR). Endochondral ossification_sentence_18

Additional images Endochondral ossification_section_6

Endochondral ossification_unordered_list_1

  • Endochondral ossification_item_1_6
  • Endochondral ossification_item_1_7

See also Endochondral ossification_section_7

Endochondral ossification_unordered_list_2

Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Endochondral ossification.