cell Types Involved:
Every type of cell is involved in the Growth regulation
process in humans, except some portions of the heart muscle cells and the
brain cells in human.
Overall Process: The regulation of cell growth is mediated by the cell cycle,
which is tightly controlled by various regulators such as cyclins, cyclindependent kinases (Cdks), growth factors, and
inhibitors. This process
carried out by cellular checkpoints: At the G1/S checkpoint, the cell assesses
DNA integrity before replication. If damage is detected, the cell cycle pauses for
repair. Passing this checkpoint commits the cell to DNA replication. Failing this
checkpoint causes the cell to go to G0 phase where it is dormant until its ready
again. At the G2/M checkpoint, the cell verifies successful DNA replication and
checks for damage. Failure to pass halts the cycle for repair or triggers apoptosis
if the damage is irreparable. The metaphase-anaphase checkpoint ensures all
chromosomes are correctly attached to the spindle before allowing separation,
preventing aneuploidy. Failure to meet checkpoint criteria activates pathways to
halt the cycle or initiate cell death.
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Ligands and Receptors: The process involves various types of ligands and
receptors, including:

Steroid hormones ligands that pass through the cell membrane and
bind to intracellular receptors,

Protein hormones ligands that bind to surface receptors such as Gprotein coupled receptors (GPCRs),

Growth factors ligand that often bind to receptor tyrosine kinases
(RTKs),

Gas molecules ligands like nitric oxide that can diffuse into cells and
affect signaling pathways.

Signaling Types:
All types are involved

Autocrine signaling: A cell targets itself, releasing and
responding to its signals, as seen in cancer cells promoting their
proliferation.

Paracrine signaling: Cells communicate with nearby cells, like
growth factor release from fibroblasts stimulating surrounding cell
division.

Juxtracrine signaling: Direct contact between cells, such as
notch signaling in developmental pathways.

Endocrine signaling: Hormones travel through the bloodstream
to distant targets, like insulin regulating glucose uptake and metabolism
in various tissues.

Environmental signaling: External cues like nutrient
availability and oxygen levels directly affect cell growth and division.
Environmental Influence: Environmental factors such as nutrient availability,
oxygen levels, and physical space can significantly influence cell growth and
proliferation. For instance, the availability of glucose and other nutrients
directly impacts cellular energy metabolism and, consequently, cell growth
and division.
Signaling Transduction Pathway Example: A key signaling pathway
involved in cell growth regulation is the HGH pathway:


Ligand: Human Growth Hormone (HGH).
Receptor: Growth Hormone Receptor (GHR), a cytokine
receptor that binds HGH.

Secondary Messengers/Relay Molecules: Janus kinase 2
(JAK2) is recruited and activated by GHR shape change as Gh, leading to
the activation of Signal Transducer and Activator of Transcription (STAT)
proteins.

Responses: This pathway regulates growth, metabolism, and
has effects on various tissues-cell starts to devide, promoting postnatal
longitudinal growth and influencing lipid and carbohydrate metabolism.

Disruptions: Abnormal HGH secretion can lead to conditions
like acromegaly or Laron syndrome, impacting growth and metabolism.
Excess secretion of HGH can also made cell devision unregulated, leading
to cancer.
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