Histology Mentor

The fluid mosaic model depicts the plasma membrane as a bimolecular sheet of phospholipid containing proteins. Many of these proteins penetrate the full thickness of the membrane and are called trans membrane proteins. However, many others are associated with either the cytoplasmic or the external leaflet. Integral proteins require disruption of the membrane (usually with detergents) to extract them and can be either trans membrane or tightly associated with one or the other leaflet. Peripheral proteins are more easily dissociated and can be found on both surfaces but mostly on the inner surface where they serve to respond to receptor activation. The proteins can Afloat@around in the lipid. Polysaccharides forming the glycocalyx (fuzzy coat) are covalently bound to lipids or proteins and are found only on the external surface.

Freeze-fracture separates the two phospholipid bilayers and reveals the distribution of proteins in the membrane. The fracture plane preferentially follows the lipid-lipid interface of membranes and the associated proteins appear as bumps on the surface. The outer surface of the inner leaflet is called the P face and displays the greatest number of particles. The inner surface of the outer leaflet is called the E face.

Discrete droplets of extracellular fluid are wrapped in plasma membrane and taken into the cell to form a membrane limited (pinocytotic) vesicle. When particles are engulfed, the process is called phagocytosis. Endocytosis by coated vesicles occurs when receptors bind their specific ligand. This is receptor-mediated endocytosis. Clathrin is the major component of the coat and after the vesicle is formed it is returned to the inside of the plasma membrane. The smooth vesicle then takes on protons and becomes the compartment for uncoupling receptor and ligand (CURL).

The process by which the contents of membrane limited vesicles are released from the cell, the membrane becoming part of the plasma membrane.

A specialization of the plasma membranes of adjoining cells in which there is an ordered arrangement of connexins, membrane proteins, that form connexons. This permits small molecules and ions to move freely and directly from cell to cell through channels in the connexons and thus coordinate function.

Site of oxidative phosphorylation (the utilization of oxygen and formation of ATP) - they are surrounded by 2 membranes - their minimum dimension is about 0.3 microns. Mitochondrial cristae are invaginations of the inner mitochondrial membrane into the mitochondrial matrix. The matrix contains proteins, RNA (ribosomes) and DNA (circular). Mitochondria are self-replicating but not autonomous. The cytochrome system and the phosphorylating enzymes are bound to the inner membranes (elementary particles) while the other enzymes involved in the citric acid cycle are in the matrix. Many of these enzymes are formed in the cytosol and then taken up by the mitochondria. Cation binding granules are found in the mitochondrial matrix.

A system of interconnected cisternae (flattened membrane sacs) that have ribosomes attached to their outer (cytosol) surface. Ribosomes consisting of two RNA-protein particles are required for the translation of messenger RNA into a polypeptide chain. Cytoplasmic ribosomal RNA is formed in the nucleolus and the ribosomes formed from it differ from those in the mitochondria. The latter resembles bacterial ribosomes. Polysomes are several ribosomes attached to messenger RNA. When the message has a leader that codes a signal peptide the translating ribosome becomes attached to the ER. Then the new peptide passes into the cisternae of the RER and is thus segregated from the cytosol. Hemoglobin of red cells is formed by free polysomes. Aggregates of ribosomes are basophilic.

A system of membranous saccules and tubules that do not have attached ribosomes. Diverse functions are associated with these structures; steroid synthesis, lipid synthesis, glycogen synthesis, inactivation of hormones, toxin metabolism, and calcium storage in striated muscle. Cells with large amounts of this organelle are acidophilic.

Stack of flattened membranous vesicles frequently somewhat concave toward the nucleus. Vesicles bud off of the cisternae of the maturing (concave) face. The Golgi communicates with both the RER and the SER either directly or by way of transfer vesicles. These vesicles enter to form the convex or cis (also called forming or proximal) face. The concave or trans (opposite the cis face; it is also called the maturing or distal) face buds off membrane limited vesicles (secretion granules, lysosomes). Vesicles at the edges of the cisternae of all levels appear to be joining or leaving the complex. These are thought to be involved in the segregation of membrane proteins so that membrane proteins of the ER can be recycled back to the ER. The Golgi packages materials in membrane-limited vesicles and synthesizes polysaccharides, including the incorporation of sulfate. Secretion granules, lysosomes, and probably other membrane limited particles are packaged here. This involves a sorting process that depends upon newly synthesized proteins carrying markers that target them to go to specific vesicles. Mannose-phosphate targets enzymes to the lysosomes.

Long hollow cylinders about 22 nm in diameter. The wall is formed by a spiral of protein dimers, each dimer consisting of 1 alpha and 1 beta tubulin molecule. Microtubule assembly is inhibited by colchicine, that blocks mitosis at metaphase. Microtubules effect movement of a variety of cytoplasmic components as well as cilia in which they are characteristically arranged. They provide a "monorail" on which cell components are moved to (dynein) and from (kinesin) the cell center, the centrioles. Kinesin and dynein are ATP dependent "motors" that link elements to be transported to the microtubules Each centriole consists of 9 triplets of short microtubules. They function to organize the mitotic spindle apparatus that consist of microtubules.

A pair of morphologically distinctive features of the centrosome or cell center. They behave as the organizing center for microtubules and are made up of nine short segments of microtubule triplets forming a cylindrical array. The centrioles are usually oriented perpendicular to one another. They replicate before cell division, so that a pair of centrioles become located on opposite sides of the dividing nucleus where they participate in the organization of the microtubules of the spindle apparatus.

There are 3 sizes of filaments. The smaller ones (6-8 NM in diameter) also called microfilaments are mostly composed of actin. These contribute to cell movement, cytokinesis, etc. Intermediate filaments are about 10 NM in diameter and contribute to the maintenance of cell shape and intracellular organization. There is a wide variety of these often specific to the cell in which they are found. Vimentin is a major protein component of these filaments found in most cells. Glial fibrillary proteins are specific to glial cells. Myosin filaments, also called thick filaments, are about 15 NM in diameter. They interact with actin to cause cellular contraction.

Packages of acid hydrolases (digestive enzymes) form with the aid of clathrin. When the the clathrin is removed they are free to fuse with phagocytic (heterophagic) vacuoles forming secondary lysosomes. Vacuolar H⁺ ATPase is activated and the phagocytosed particles are digested. Sometimes lysosomal enzymes are secreted from the cell. Turnover of cytoplasmic components occurs when a portion of cytoplasm is enclosed in membrane (autophagic vacuole) and primary lysosomes fuse with it to form a secondary lysosome with subsequent digestion. A residual body is a cytoplasmic vacuole containing accumulated particulate, non-digestible products of metabolism, e.g. a yellowish-brown pigment called lipofuscin aging or lipochrome pigment.

Special class of membrane limited particles - containing enzymes that produce, utilize or break down hydrogen peroxide. They are formed by and often attached to SER. Catalase splits peroxide and is an important component of peroxisomes.

Features that provide a substrate for metabolism. These include cytoplasmic lipid droplets and glycogen granules. The latter do not have a membrane separating them from the cytosol but lipid droplets are now known to be surrounded by a single leaflet of a biological membrane. It contains enzymes that facilitate the deposition and mobilization of the fat stored there. Lipids stain with sudan and glycogen by the PAS method and by Best=s Carmine. Glycogen appears as black granules in electron micrographs.

Lipofuscin (aging pigment) appears to be a residual of lysosomal activity - polymerized, non-digestible lipids.

Formed from rough endoplasmic reticulum after mitosis is complete. It consists of 2 membranes separated by the perinuclear cistern and forms the nuclear pores that provide a channel through which the nucleus and cytoplasm communicate. Occasional connections between the outer nuclear membrane and the rough endoplasmic reticulum may be found. See also ribosomes on outer surface of nuclear membrane.

The deoxyribonucleic acid (DNA) of the nucleus is bound to small clumps of basic proteins (histones) and other proteins to form nucleosomes that are like beads on a string. There are about 200 base pairs associated with each nucleosome. Depending upon the state of the chromatin, it is more or less clumped by fixation. Consequently, the basophilic chromatin may be either finely granular (euchromatin) or clumped (heterochromatin), when seen in histological sections. Heterochromatin is inactive in transcription but euchromatin is available to form messenger RNA depending upon the presence of regulatory factors. When all of the chromatin is compacted or heterochromatic the nucleus is said to be pyknotic. This is the case of dying cells or cells with little or no transcription taking place.

Produced in the nucleolus, that stains lightly basophilic and is usually surrounded by condensed chromatin. DNA strands in the nucleolus code for the ribosomal RNA that is being formed.

Small molecules enter the cell by passive diffusion, facilitated diffusion or active transport (requires ATP) through the cell membrane. Large molecules and particles may enter by pinocytosis (drinking) or phagocytosis (eating) respectively. In either of the latter cases, membrane limited vacuoles are formed that fuse with lysosomes so that the large molecules can be digested within the vacuoles and small molecules, such as sugars, amino acids, and fatty acids, are transported through the membrane of the vacuole into the cytosol. These secondary lysosomes can also be called heterophagic vacuoles when they contain material brought into the cell. Autophagic vacuoles contain cytoplasm that has been sequestered within a membrane-limited vacuole, preparatory to digestion and recycling of the small molecules. Proteasomes are also involved in removing worn out or defective proteins.

Chemical bond energy of nutrients is converted to chemical bond energy of phosphates, especially ATP. Like the dollar, ATP is the energy currency for living cells. It is formed directly at certain steps in glycolysis, the cytoplasmic oxidation of glucose to pyruvate, and indirectly, by the mitochondrial oxidation of pyruvate and NADH, the latter coming from the cytoplasmic oxidations of amino acids, sugars, and fatty acids. ATP production is not the sole function of mitochondria. They are also involved in fat and steroid metabolism.

a. Intrinsic proteins

b. Membrane synthesis

c. Mitochondria

The replicating cell goes through a series of stages that vary in their length. Only one of these stages (mitosis) is recognizable morphologically, and that one is subdivided into several phases (prophase, metaphase, anaphase, and telophase). Mitosis is also the shortest of the stages so that in a rapidly dividing population mitotic figures may be rare. After mitosis (the M stage) the cell enters a period of growth and tooling up for DNA synthesis. This is the G1 or Gap 1 stage and is followed immediately by the S stage-DNA synthesis. Gap 2 (G2) is the period following DNA synthesis when the cell is preparing for mitosis.

In most organs of the body there is a large population of cells that are not in the actively replicating pool. These are said to be in the G zero stage but may be activated. This is best demonstrated when a portion of the liver is removed or the epithelium of the skin is incised. In both cases, a large number of cells begin proliferating, that is, they leave G zero and enter the cycle at G1. It is thought that proliferation of these cells is held in check until they are activated by an appropriate growth factor.

a. Pituitary trophic hormones. For example: thyroid stimulating hormone stimulates proliferation of thyroid follicular epithelium.

b. Erythropoietin, a product of the kidney, stimulates proliferation and maturation of erythrocyte-forming cells.

c. Estrogen stimulates proliferation of the uterine endometrium.

In many organs old cells are removed by a process of programmed cell death called apoptosis. Chromatin becomes highly clumped and is broken (between nucleosomes) into fragments that are multiples of 180-200 base pairs. The cell and nucleus then fragment into multiple membrane limited vesicles that are removed by macrophages without inflammation occurring.