Limb Bud Formation And Long Bone Development

University:
Western Sydney University
Course:
MEDI 2003 | Clinical Sciences 2
Academic year:

2024
Views:

327

Pages:

3
Author:

Mariam A.

Limb bud formation and long bone development         Limb bud appears on the ventro-lateral side at about day 24 (week 4) Upper limb begins about 1-2 days before the lower limb Limb bunds contain a mass of mesenchyme derived from somatic mesoderm and is covered by ectoderm Upper limb develops opposite the cervical segments and the lower limb lumbar segments Ectoderm at the distal border of the limb thickens and forms the apical ectoderminal ridge (AER) This ridge exerts an inductive influence on adjacent mesenchyme causing it to remain as a population of undifferentiated, rapidly proliferating cells, the progress zone As the limb grows, cells farther from the influence of the AER begin to differentiate into cartilage and muscle. This is also aided by FGF-8 gene. In this manner, development of the limb proceeds proximodistally. Various genes will be activated to enhance growth of the limb bud in other axis In week 5, chondrification begins where chrondrocytes differentiate within the mesenchyme and begin to secrete molecules characteristic of extracellular matrix. By the end of the 6th week, the entire limb skeleton is cartilaginous  Secretion of a certain gene leads to programmed cell death (apoptosis), which breaks down the tissue between cartilaginous digits to form fingers and toes. If this process is inhibited, it can cause a birth defect known as Syndactyl where the fingers become webbed  In week 7, ossification begins in the primary ossification region which is the diaphysis (shaft) The cartilaginous cells increase in size (hypertrophy) and eventually the matrix around the cells calcifies and the cells dies, leaving a cavity within the cartilage Blood vessels grow around the edges and convert chondrocytes to osteoblast which lays down a bone collar known as the periosteum Osteoclasts also enter through the blood vessels and help to remodel the bone creating a marrow cavity Osteoblasts and vessels continue to migrate into epiphysis (secondary centres) At birth diaphysis are completely ossified, however secondary centres will slowly ossify and a layer of articular cartilage at each end of the bone will remain Epiphyseal plate allows further growth. Once the growth of the bone has ceased (20 years) the plate ossifies and closes       Clinical correlates   Achondroplasia – this is a form of dwarfism where all bones are present but much shorter than normal. It is caused by the mutation of FGFR3 gene which causes decrease in endochondral ossification, decreased matrix production and inhibits proliferation of chondrocytes in growth plate Hyperpituitarism (gigantism) – abnormally rapid growth of infant from too much growth hormone by the pituitary gland. It caused usually by a tumour on the pituitary gland