CHAPTER 20 — CELL COMMUNITIES

Tissue

A tissue is a group of cells, along with their extracellular matrix, that work together to perform a specific function.

Relationship between cells, tissues, and organs

Cells form tissues, and multiple tissues combine to form organs, each contributing specialized functions.

Four major tissue types in animals

• Epithelial tissue

• Connective tissue

• Nervous tissue

• Muscle tissue

Extracellular matrix (ECM)

The extracellular matrix is a network of proteins and polysaccharides surrounding cells that provides structural and biochemical support.

Collagen

Collagen is a structural protein in the ECM that provides tensile strength and forms fibrils and fibers.

Procollagen

Procollagen is the precursor form of collagen that is secreted and then cleaved extracellularly to form active collagen.

Fibronectin

Fibronectin is an ECM protein that links collagen fibers to cells via integrins.

Glycosaminoglycans (GAGs)

GAGs are negatively charged polysaccharides that جذب water, forming a hydrated gel that resists compression.

Proteoglycans

Proteoglycans are large complexes of GAGs attached to proteins that provide structural support and compressional strength.

Integrins

Integrins are transmembrane receptors that connect cells to the extracellular matrix and transmit survival signals.

Anoikis

Anoikis is a form of apoptosis that occurs when cells lose attachment to the extracellular matrix.

Basal lamina / basement membrane

A specialized ECM layer beneath epithelial cells composed of collagen IV and laminins that anchors cells to connective tissue.

Laminin

Laminin is an ECM protein in the basal lamina that helps organize and anchor epithelial cells.

ECM

The extracellular matrix (ECM) is a dynamic, three-dimensional network of proteins and polysaccharides that provides structural support, biochemical signaling, and mechanical cues to cells, regulating tissue organization and cell behavior.

Epithelial tissue

Epithelial tissue consists of tightly packed, polarized cells that cover surfaces and line organs.

Apical surface

The apical surface faces the external environment or lumen.

Basal surface

The basal surface attaches to the basement membrane.

Polarization of epithelial cells

Epithelial cells have distinct apical and basal regions with different structures and functions.

Tight junction

Tight junctions seal gaps between cells using claudins and occludins, preventing leakage and maintaining polarity.

Claudin

Claudin is a protein that forms the backbone of tight junction strands.

Occludin

Occludin is a protein that helps stabilize tight junctions and regulate permeability.

Adherens junction

Adherens junctions connect cells through cadherins linked to actin filaments, forming a belt-like structure.

Cadherin

Cadherins are calcium-dependent adhesion proteins that connect cells to each other.

Desmosome

Desmosomes are strong cell junctions linking intermediate filaments via cadherins for mechanical strength.

Desmoglein / Desmocollin

Specialized cadherins in desmosomes that connect intermediate filaments between cells.

Hemidesmosome

Hemidesmosomes anchor epithelial cells to the ECM using integrins and intermediate filaments.

Gap junction

Gap junctions allow direct communication between cells by permitting ions and small molecules (<1 kDa) to pass.

Connexin

Connexin is the protein that forms gap junction channels.

Connexon

A connexon is a hexameric complex of connexins that forms a channel between cells.

Stem cell

A stem cell is a cell capable of self-renewal and differentiation into specialized cell types.

Precursor cell

A precursor cell is a partially differentiated cell that can still divide and give rise to specialized cells.

Terminally differentiated cell

A fully specialized cell that has exited the cell cycle and no longer divides.

Adult stem cell

Adult stem cells are multipotent cells that generate a limited range of cell types.

Embryonic stem cell

Embryonic stem cells are pluripotent cells capable of forming nearly all cell types.

Totipotent

Cells capable of generating all cell types, including extraembryonic tissues.

Pluripotent

Cells capable of generating most cell types.

Multipotent

Cells capable of generating a limited range of cell types.

Induced pluripotent stem cells (iPS)

Adult cells reprogrammed to behave like embryonic stem cells by introducing specific transcription factors.

Intestinal stem cell system

Stem cells in crypts produce precursor cells that differentiate into goblet and absorptive cells along villi.

Wnt signaling

A signaling pathway that promotes stem cell proliferation in intestinal crypts.

Cancer

Cancer is uncontrolled cell proliferation that invades beyond normal tissue boundaries.

Benign tumor

A non-invasive growth that remains localized.

Malignant tumor

A tumor that invades surrounding tissues and can spread to other parts of the body.

Metastasis

The spread of cancer cells from the primary tumor to distant locations.

Oncogene

A mutated or overexpressed gene that promotes cell proliferation.

Tumor suppressor gene

A gene that normally inhibits cell division or promotes apoptosis; loss leads to cancer.

Ras

A proto-oncogene that becomes constitutively active when mutated, promoting cell division.

p53

A tumor suppressor that halts the cell cycle or induces apoptosis in response to DNA damage.

Genetic instability

Increased mutation rate that accelerates cancer progression.

Cancer stem cell

A cell within a tumor capable of self-renewal and generating the tumor mass.

Epithelial tissue

Epithelial tissue consists of tightly packed, polarized cells that form sheets covering body surfaces or lining organs. These cells have an apical surface facing the environment or lumen and a basal surface attached to the basal lamina. Functions include protection, absorption, secretion, and transport.

Connective tissue

Connective tissue is composed of cells embedded within an extracellular matrix of proteins (like collagen) and polysaccharides. It provides structural support, mechanical strength, and connects different tissues. Examples include bone, tendons, ligaments, and blood.

Nervous tissue

Nervous tissue consists of neurons and supporting cells (such as Schwann cells) that specialize in transmitting electrical signals for communication throughout the body.

Muscle tissue

Muscle tissue is composed of specialized cells capable of contraction, allowing movement of the body or internal organs. It includes skeletal, cardiac, and smooth muscle types.