embrayonic dovolopment and regeneration
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Answer:
Tissues can grow by cell proliferation, cell enlargement or accretion.
Often tissues grow by cell proliferation.
Cells such as differentiated muscle or nerve cells growth by increasing their size.
Neurons extend axons and dendrites while muscle cells increase in mass and recruitment of satellite cells.
Accretionary growth, as in cartilage and bone, involves a cell secretion to increase the volume of the extracellular spaces.
Many adult tissues will only divide when induced by injury or other stimuli.
Some tissues such as the hematopoietic tissues and the epithelia are continually renewed throughout the vertebrate's lifetime.
Cell proliferation: controlled by an intrinsic program & external signals.
The eukaryotic cell duplicates via the cell cycle: M-phase (mitosis), G1 (pre-synthetic interphase), S phase (DNA synthesis), G2 (pre-mitotic interphase).
Growth factors and other signaling proteins are essential for cells to progress through the cell cycle.
Cells must receive such signals not only to divide (see mitotic domains) but for survival & in the absence of growth factors, cell undergo apoptosis.
The cyclins are proteins that control the cell cycle at specific check points.
Cyclins form complexes with and activate cyclin-dependent kinases that then phosphorylates proteins to direct phase-specific events.
Cyclin concentrations oscillate.
Growth of mammals is dependent on growth hormones.
Human embryo increases length from 150 um at implantation to 50 cm at birth.
During first 8 weeks, there is little increase in size but the basic form is laid down.
The greatest rate of growth is at 4 months after implantation.
The head is more than 1/3 of entire length at 9 weeks of gestation, ~1/4 length at birth and ~1/8 length in the adult.
Different parts of the body grow at different rates.
Male and female humans grow at different rates, primarily due to earlier growth spurt in females.
Maternal environment controls fetal growth.
Embryonic growth depend upon growth factors and new born mice lacking insulin-like growth factor 2 (IGF-2) are only 60% of normal birth size.
Growth hormone (GH) is secreted throughout fetal life and secretion by the pituitary begins during the first year of life to control growth.
Growth-hormone releasing hormone (promotes GH synthesis and secretion) and somatostatin (inhibits GH production and release) are both made in the hypothalamus.
GH induces both IGF-1 and IGF-2 which are largely responsible for both embryonic and post-natal growth.
Growth of long bones occurs in the growth plates.
Developing organs can have their own intrinsic growth programs.
Demonstrated by limb transplants in salamanders, limb size is genetically determined.
The long bones (humerus, femur, radius & ulna) are first laid down as cartilage then become ossified.
Early growth (cell proliferation & matrix secretion) follows a well-defined pattern.
In endochondrial ossification, the cartilage is replaced by bone, starting in the centres (diaphyses) and spreading outward and at secondary centres (epiphyses) at the ends of the bones.
The growth plates are internal regions of the bone, near but not at each end, that are columns of cartilage cells arranged in several distinct layers.