Histology: Cells and Tissues

• Histology is the study of the body's tissues and how they arrange to form organs
• The word "histology" comes from the Greek words “histos” meaning "tissues” and “logos” meaning “study of"
• Histology covers the study of cells, components outside the cell, and their organization into tissues, organs, organ systems, and the human individual

The Cell

• The cell is the basic structural and functional unit in all tissues
• Cell differentiation is when cells become efficiently organized in tissues with specialized functions and usually change shape

Specialized Cells and Their Functions

• Muscle cells/contractile cells facilitate movement
• Epithelial cells form adhesions and tight junctions
• Fibroblasts, bone cells, and cartilage engage in the synthesis and secretion of components of the extracellular matrix
• Neurons and sensory cells convert physical and chemical stimuli into action potentials
• Digestive gland cells synthesize and secrete degradative enzymes
• Mucous gland cells synthesize and secrete glycoproteins
• Cells of the Adrenal Gland, Testes, and Ovary engage in the synthesis and secretion of steroids
• Kidney and salivary gland ducts cells are used for ion transport
• Macrophages and neutrophils are for intracellular digestion
• Adipocytes store fats
• Intestinal lining cells are responsible for metabolic absorption

Cell Membrane (Plasmalemma)

• It serves as a selective barrier that regulates the passage of materials into and out of the cell
• It facilitates the transport of specific molecules

Membrane Proteins

• They enable specific recognition and signaling functions
• Plays a key role in the interactions of the cell with its environment

Membrane Phospholipids

• They are amphipathic
• Have 2 nonpolar hydrophobic or water-repelling long chain fatty acids
• They have a charged polar hydrophilic or water-attracting head that bears a phosphate group

Cholesterol

• Restricts movements and modulates the fluidity of all membrane components

Glycocalyx

• It is a delicate cell surface coating
• Glycolipids are outer lipids with oligosaccharide chains
• Glycoproteins are proteins linked with sugar chains that act like receptors for cell adhesion and cell recognition

Proteins

• They are major constituents of membranes
• Integral proteins are incorporated directly within the lipid bilayer
• Peripheral proteins are bound to one of the two membrane surfaces

Ribosomes

• Assemble polypeptides from amino acids on molecules of transfer RNA in a sequence specified by mRNA
• Engage in protein synthesis

Endoplasmic Reticulum

• It assists with the synthesis of CHON (carbon, hydrogen, oxygen, and nitrogen) and lipids synthesis
• Rough Endoplasmic Reticulum synthesizes most membrane-bound proteins for secretion, plasma incorporation into the membrane, and as enzymes within lysosomes
• Smooth Endoplasmic Reticulum synthesizes lipids, metabolizes carbohydrate, detoxifies alcohols and other drugs or releases calcium ions

Golgi Apparatus

• Completes post-translational modifications of proteins produced in the rough endoplasmic reticulum
• Packages and addresses proteins to the proper destinations
• It has two distinct functional sides or faces
• Cis being the golgi receiving region, and Trans being the golgi shipping region

Lysosomes

• They are sites of intracellular digestion and turnover of cellular components
• They are membrane-limited vesicles containing about 40 different hydrolytic enzymes
• Abundant in cells that have phagocytic activity, like macrophages and neutrophils

Proteasome

• Degrades denatured or otherwise non-functional polypeptides
• Removes unneeded proteins by recognizing ubiquitin
• Denatured proteins are conjugated with Ubiquitin
• Ubiquitin is a molecule that attaches to proteins and targets them for breakdown by proteasomes

Mitochondria

• They are the site for ATP Synthesis, also known as the "powerhouse" of the cell
• Mitochondria of stressed cells releases Cytochrome C from the inner membrane
• Cytochrome C triggers a regulated series of events leading to apoptosis (cell death)

Peroxisome

• It is spherical in shape and enclosed by single membrane
• Named for their enzymes producing and degrading hydrogen peroxide
• OXIDASE oxidizes substrates by removing hydrogen atoms that are transferred to molecular oxygen to produce hydrogen peroxide
• CATALASE breaks down hydrogen peroxide to prevent damage to the cell

Cytoskeleton

• Determines the shape of cells
• Plays a role in the movements of organelles and cytoplasmic vesicles while allowing the movement of entire cells
• Composed of microtubules, microfilaments (actin filaments), and intermediate filaments

Inclusions

• Accumulates metabolites or other substances
• Have no metabolic activity, and are not enclosed by membranes
• Lipid droplets are accumulations of lipid-filling adipocytes
• Glycogen granules are aggregates of CHO polymers where glucose is stored
• Melanin is pigmented dark brown deposits/granules
• Lipofuscin is derived from lysosomal digestion
• Hemosiderin is denatured ferritin protein (dense brown in color) from phagocytosis of erythrocytes

Nucleus

• It is referred to as the captain of the cell
• Contains the molecular machinery to replicate DNA and to synthesize and process all types of RNA
• Composed of:
• Nuclear envelope, which is the largest structure within the cell
• Chromatin which is mass of DNA and its associated protein
• Nucleoli, which are specialized regions of chromatin

Cell Cycle

• Regular sequence of events that produce new cells with four distinct phases: Mitosis (PMAT), G1, S, G2
• Mitosis is the period of cell division
• Interphase includes all other stages of the cell cycle (G1, S, G2)

Cell Cycle Phases

• G1 is where cells accumulate the enzymes and nucleotides required for DNA replication
• S is the period of DNA Synthesis, marked by DNA replication, histone synthesis, and start of centrosome duplication
• G2 is the gap between DNA duplication and mitosis, a period of preparation for mitosis
• G0 is when the cell cycle put to a halt, either temporarily or permanently

Mitosis

• It is the period of cell division
• A phase of the cell cycle, routinely distinguished with the light microscope
• The parent cell divides and each of the two daughter cells receives a chromosomal set identical to that of the parent cell
• Events of mitosis are subdivided into four major stages

Meiosis

• A specialized process involving two unique, associated cell divisions
• Occurs only in cells that form sperm and egg cells
• The cells produced are haploid, having just one chromosome from each pair present in somatic cells

Mitosis versus Meiosis

• Mitosis is a cell division that produces two diploid cells
• Mitosis yields two cells that are genetically the same
• Meiosis involves two cell divisions and produces four haploid cells
• Meiosis yields haploid cells that are genetically unique

Explore histology, the study of tissues and organ formation. Learn about cell differentiation and the functions of specialized cells like muscle, epithelial, and nerve cells. Understand how cells organize into tissues and systems.