s as
cell reprogramming involved in the
differentiation of human
pluripotent stem cells (hPSCs). (ICM -
inner cell mass) s All the cells in an animal body develop from one
totipotent diploid cell called a
zygote. During the
embryonic development of an
animal, the cells
differentiate into the specialised
tissues and
organs of the organism. Different groups of cells differentiate from the
germ layers. The
sponge has only one layer. Some other animals known as
diploblasts have two germ layers the
ectoderm, and the
endoderm. More advanced animals have an extra layer, the middle
mesodermal layer, and are known as
triploblastic. Triploblastic animals make up the large
clade of
Bilateria. Differentiation results in structural or functional changes to
stem cells, and
progenitor cells. The ectoderm gives rise to several different types of
epithelial tissues including the
skin, and
glands, and to the
nervous tissue. Epithelium as
mesothelium forms the lining of many organs, and
inner cavities. Epithelial cells are joined together in sheets by way of
cell junctions;
adherens junctions, and
desmosomes bind the cells together, and
hemidesmosomes bind the cells to the
basement membrane. All three types are linked to the cell cytoskeleton. There are an estimated 200 different
cell types in the human body. The estimated
cell count in a typical adult human body is around 30 trillion cells, 36 trillion in an adult male, and 28 trillion in a female.
Cell membrane The
cell membrane, or plasma membrane, is a
selectively permeable membrane as an outer boundary of the cell that encloses the cytoplasm. The membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a
lipid bilayer of
phospholipids, which are
amphiphilic (partly
hydrophobic and partly
hydrophilic). It has been best described in the
fluid mosaic model. Embedded within the cell membrane are
secretory macromolecular lipoprotein structures called
porosomes; and a number of different channels and pumps involved in
actively transporting molecules into and out of the cell.
Cell surface receptors in the membrane allow cells to detect external signaling molecules such as
hormones. Underlying, and attached to the cell membrane is the
cell cortex, the outermost part of the actin cytoskeleton.
Cytoplasm The cell membrane encloses the
cytoplasm of the cell that surrounds all of the cell's organelles. The network of filaments and microtubules of the cytoskeleton gives shape and support to the cell, and has a part in organising the cell components. The cytosol is a gel-like substance made up of water, ions, and non-essential biomolecules, and is the main site of
protein synthesis, and
degradation. The cytosol forms of the cell's volume.
Cytoskeleton The
cytoskeleton acts to organize and maintain the cell's shape; anchors organelles in place; helps during
endocytosis, and in the uptake of external materials by a cell.The cytoskeleton is composed of
microtubules,
intermediate filaments and
microfilaments. There are a great number of proteins associated with them, each controlling a cell's structure by directing, bundling, and aligning filaments. The outermost part of the cytoskeleton is the
cell cortex, or
actin cortex, a thin layer of cross-linked
actomyosins. The cytoskeleton in the animal cell also plays a part in
cytokinesis, in the formation of the
spindle apparatus during
cell division, the separation of daughter cells.
Organelles Organelles are
compartments of the cell that are specialized for carrying out one or more functions, analogous to the
organs, such as the heart, and lungs. Organelles include the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, vesicles, and vacuoles.
Membrane-less organelles include the
nucleolus in the nucleus,
centrosomes,
ribosomes,
proteasomes, and
vaults. It houses the cell's
chromosomes, and is the place where almost all
DNA replication and
RNA synthesis (
transcription) occur. The nucleus is spherical and separated from the cytoplasm by a double-membraned
nuclear envelope. A space between the membranes is called the perinuclear space. The nuclear envelope isolates and protects a cell's DNA from various molecules that could accidentally damage its structure or interfere with its processing. During processing,
DNA is
transcribed, or copied into a special
RNA, called
messenger RNA (mRNA). This mRNA is then transported out of the nucleus, where it is translated into a specific protein molecule. The
nucleolus is a specialized
biomolecular condensate within the nucleus where ribosome subunits are assembled. It is one of several types of membrane-less
nuclear bodies. In humans, the nuclear genome is divided into 46 linear chromosomes, including 22
homologous chromosome pairs and a pair of
sex chromosomes. The nucleus is a
membrane-bound organelle.
Endoplasmic reticulum The
endoplasmic reticulum (ER) is a transport network for molecules targeted for certain modifications and specific destinations, as compared to molecules that float freely in the cytoplasm. The ER has two forms: the
rough endoplasmic reticulum (RER), which has
ribosomes on its surface that secrete proteins into the ER, and the
smooth endoplasmic reticulum (SER), which lacks ribosomes.
Golgi apparatus The
Golgi apparatus processes and packages
proteins, and
lipids, that are synthesized by the cell. It is organized as a stack of plate-like structures known as
cisternae.
Mitochondria Mitochondria are self-replicating double membrane-bound organelles that occur in various numbers, shapes, and sizes in the cytoplasm of the cell. Mitochondria are descended from bacteria that formed an
endosymbiotic relationship with ancient prokaryotes. Mitochondria multiply by
binary fission and have their own DNA contained in multiple small circular chromosomes. The mitochondrial DNA (
mtDNA) is very small compared to
nuclear DNA, Mitochondria also have their own ribosomes known as
mitoribosomes.
Lysosomes A
lysosome is the most acidic compartment in the cell. It contains over 60 different
hydrolytic enzymes that need an acidic environment. They digest excess or worn-out organelles, food particles, and engulfed
viruses or
bacteria. The cell could not house these destructive enzymes if they were not contained in a membrane-bound compartment.
Peroxisomes Peroxisomes are
microbodies bounded by a single membrane. A peroxisome has no DNA or ribosomes and the proteins that it needs are encoded in the nucleus, and selectively imported from the cytosol. Some proteins enter via the endomembrane reticulum. They have enzymes that rid the cell of toxic
peroxides. The enzymatic content of the peroxisomes varies widely across the species, as it can in an individual organism.
Centrosome The
centrosome is a membrane-less organelle composed of
pericentriolar material and the two
centrioles.
Ribosomes A
ribosome is a large complex of
RNA and
protein molecules considered as a
membrane-less organelle when freely found in the cytosol, or as a
membrane-bound organelle when located in the
rough type of
endoplasmic reticulum. Most human cells have around 10,000 vaults, and in some types of
immune cell there may be up to 100,000.
Macrophages have the greatest number of vaults of any human cell. Vaults are largely overlooked because their functions are purely speculative. They may play a role in transport from the nucleus to the cytoplasm, and may serve as scaffolds for signal transduction proteins. They are present in normal tissues, and more so in secretory and excretory epithelial cells.
Animal cell types s Some types of specialised cell are localised to a particular animal group.
Vertebrates for example have specialised, structurally changed cells including
muscle cells. The cell membrane of a
skeletal muscle cell or of a
cardiac muscle cell is termed the
sarcolemma. And the cytoplasm is termed the
sarcoplasm. Skeletal muscle cells also become
multinucleated. Populations of animal groups
evolve to become distinct species, where sexual reproduction is isolated. The many
species of vertebrates for example have other unique characteristics by way of additional specialised cells. In some species of
electric fish for example modified muscle cells or nerve cells have specialised to become
electerocytes capable of creating and storing electrical energy for future release, as in stunning prey, or use in
electrolocation. These are large flat cells in the
electric eel, and
electric ray in which thousands are stacked into an
electric organ comparable to a
voltaic pile. Many animal cells are
ciliated and most cells except red blood cells have
primary cilia. Primary cilia play important roles in
chemosensation and
mechanosensation. Each cilium may be "viewed as a sensory cellular
antennae that coordinates a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation." The cilia in other cells are motile organelles, and in the
respiratory epithelium play an important role in the
movement of mucus. In the reproductive system ciliated epithelium in the
fallopian tubes move the egg from the uterus to the ovary. Motile cilia also known as flagella, drive the sperm cells.
Invertebrate planarians have ciliated excretory
flame cells. Other excretory cells also found in planarians are
solenocytes that are long and flagellated. == Plant cells ==