Human Histology Lecture - Lesson 1: The Cell PDF

HUMAN HISTOLOGY - LECTURE COLLEGE OF HEALTH SCIENCES LESSON 1: THE CELL JOHN EZEKIEL J. TABIOS, RMT INSTRUCTOR CELL  It is the basic morphological and functional unit of all living things. It is the smallest entity that has the capacity to perform all of life’s...

HUMAN HISTOLOGY - LECTURE COLLEGE OF HEALTH SCIENCES LESSON 1: THE CELL JOHN EZEKIEL J. TABIOS, RMT INSTRUCTOR CELL  It is the basic morphological and functional unit of all living things. It is the smallest entity that has the capacity to perform all of life’s function and is, therefore, capable, under favorable environmental conditions, of independent existence.  A human being starts as a sing cell, the fertilized ovum (zygote), which results from the union of the male gametes (sperm cell; spermatozoon) and the female gametes (ovum). Multiplication (mitosis) of the fertilized ovum and differentiation of its progenies eventually give rise to an extremely complex organism that consists of a staggering number of cells ( estimated at about 100 trillion, in adults) HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE CELL  To ensure the human being’s survival, the cell that comprise the human body form a tightly knit and highly organized society. The cell that performs the same general functions are linked together by some amount of intracellular material and/or cell-to-cell junctions to form TISSUES.  Tissues, in turn, bond together in varying proportions to form more complex functional structures called ORGANS. The organs that have interrelated functions group together to form ORGAN SYSTEMS. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE CELL MEMBRANE (PLASMA MEMBRANE; PLASMALEMMA)  The cell membrane does not merely serve as an envelope that delimits the cell from its surroundings. Other functions includes:  It protects the cell  It determines which substances can move in and out of the cell and regulates the movement of these substances to and from the cell  It provides attachment for the skeleton of the cell (cytoskeleton)  It receives and sends out stimuli  It provides binding sites and receptors for enzymes and other substances  It allows cell-to-cell recognition HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE CELL MEMBRANE (PLASMA MEMBRANE; PLASMALEMMA)  The cell membrane is approx. 8-10 nm thick and is does too thin to be distinguished under the light microscope whose revolving power is only 0.2 µm. In electron microscope, the cell membrane appears a trilaminar structure consisting of two electron-dense sheets that sandwich a thin electron-lucent layer.  The cell membrane is made up mainly of phospholipid and protein molecules, but it also contains cholesterol and polysaccharides mostly in the form of glycolipids and glycoprotein. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE PHOSPOLIPID MOLECULES IN THE CELL MEMBRANE  Each of the phospholipid molecule in the cell membrane has three parts; a head and two tails. The head is globular, polar and hydrophilic while the two tails are slender, non-polar and hydrophobic. The head is consist of glycerol that is conjugated to a nitrogenous compound by a phosphate bridge. One of the two tails consists of a straight chain saturated fatty acid while the other consists of an unsaturated fatty acid. The fatty acids of the tail are linked by covalent bonds to the glycerol of the head. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE PHOSPOLIPID MOLECULES IN THE CELL MEMBRANE  In the cell membrane, the phospholipid molecules are organized to form two closely apposed layers. In each layer, the phospholipid molecules are so arranged such that the tails occupy the inner region while the heads occupy the outer region.  The lipid bilayer formed by the phospholipid molecules is highly impermeable structure that does not allow molecules aside from water, gases, and few non-polar molecules to pass freely through it. Consequently, it acts as a barrier that regulates the entry and exit of substances to and from the cell. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE PROTEIN MOLECULES IN THE CELL MEMBRANE  Protein molecules account for about half of the mass of the cell membrane. Some, the integral proteins (integral membrane proteins, transmembrane protein), span the whole thickness and project out both surfaces of the cell membrane while others, called peripheral proteins (peripheral membrane proteins), are simply inserted into, or are loosely bound to, the outer or inner surface of the membrane.  Membrane proteins perform various functions, including the transport of certain substances across the cell membrane. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE CHOLESTEROL MOLECULES IN THE CELL MEMBRANE  Cholesterol molecules, which in the cell membrane vary in number from a few to almost as many as the phospholipid molecules, are found in the irregular spaces between the phospholipid molecules. They serve to stiffen and strengthen the cell membrane. They also make the cell membrane less permeable to water-soluble substances. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE GLYCOCALYX  In most cells, glycolipid and glycoprotein molecules project from the outer surface of the cell membrane to form a coating for the cell called glycocalyx (cell coat). It is seen in an electron micrographs as a thin layer (2-20 nm) of amorphous, electron-dense material. It is involved in cell-to-cell recognition, cell-t0- cell adhesion, and immunological response. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE SPECIALIZED JUNCTIONS FORMED BY THE CELL MEMBRANE  Specialized junctions enables the cell to adhere to one another or with the extracellular matrix, or communicate with each other. Junctions that tightly bind cells to each other or to extracellular matrix are classified into occluding and adhesive.  The only occluding junction, the zonula occludens (tight junction; closing belt), but there are several types of adhesive junctions: zonula adherens (adherens junction; adhering belt; belt desmosome; band desmosome). HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE SPECIALIZED JUNCTIONS FORMED BY THE CELL MEMBRANE  Fascia adherens, desmosome (macula adherens; spot desmosome), and hemisdesmosome.  Junctions that enable the cells to communicate with each other (communicating junctions) are exemplified by gap junctions (nexus; communicating junctions) and chemical synapses. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE CYTOPLASM  Cytoplasm refers to the homogenous substance (cytoplasmic matrix) that fills the space that is bounded externally by the cell membrane, and internally by the nuclear envelope and the various formed elements that are embedded in the cytoplasmic matrix.  The formed elements in the cytoplasm consist of three groups: organelles, inclusions, and fibrillar elements. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE CYTOPLASM  Organelles are more or less permanent structure that perform a specific functions within the cell.  Inclusions are generally temporary fixtures and are often mere accumulations of pigment, lipid, or other substances.  The fibrillar elements form the cytoskeleton or supporting framework that maintains the shape and internal organization of the cell. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE CYTOPLASMIC MATRIX (CYTOSOL)  It is viscid, translucent, and colloidal in nature. It is mainly made up of water (70% or more by volume) where a host of inorganic ions and organic molecules (proteins, lipids, carbohydrates, nucleic acids, enzymes, product of enzymatic activity, etc.) are dissolved.  It is the site of many essential biochemical processes and it provides a suitable milieu for the organelles in performing their functions. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE ORGANELLES  Includes the mitochondria, ribosomes, endoplasmic reticulum (ER), Golgi complex, lysosomes, peroxisomes, and centrosome. Except for the ribosomes and centrosome, all the cytoplasmic organelles are delimited by unit membranes.  All cells have organelles, the only noticeable exceptions are the mature RBCs and the lens fiber that comprise the lens of the eye, which have none. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE MITOCHONDRIA  These are organelles that are present in all cells, except for RBCs and lens fiber. They are often hotdog- shaped but they can alter their shape and become rod-like, filamentous, spherical, etc. They are 0.5-1.0 µm in diameter and up to 10 µm long.  Under LM, mitochondria are nit visible in H&E preparations but special stains (e.g. supravital staining using Janus green) and phase contrast microscopy demonstrate their presence.  Under EM, it is seen to consist of a wall that encloses a space or cavity (intercristal space) that is filled with amorphous substance (mitochondrial matrix). HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE MITOCHONDRIA  The mitochondrial wall is made up of two layers of unit membrane. The outer membrane/leaflet delimits the mitochondrion from the cytoplasmic matrix while the inner leaflet is infolded to form the shelf-like structures (cristae mitochondriales) that project onto the intercristal space.  Mitochondria are motile. They tend to aggregate in areas within the cell where requirement is high. It has limited span but they can replicate in a manner akin to the binary fission of bacteria. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE RIBOSOMES  These are minute organelles (about 15-30 nm in size) that can only be distinguished via high- magnification electron microscopy. They are seen under the EM as small electron-dense granules that occur singly or in small clusters called polyribosomes or polysomes.  Ribosomes and polyribosomes are present in all cells but their number and distribution vary depending on the cell type. Cells with numerous ribosomes have intensely basophilic cytoplasm due to the numerous phosphate groups in the RNA of the ribosomes.  Ribosomes are sites for protein synthesis and are thus abundant in cells that produce a lot of proteins. It is where amino acids are assembled into polypeptide HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE chains. ENDOPLASMIC  It is the most extensive membranous structure in the RETICULUM cytoplasm. It consists of a system of interconnecting tubules, vesicles, and flattened sacs (cisternae). It is not visible in routine histologic preparations but it can be appreciated with the use of special histologic techniques (e.g. with the use of fluorescent dyes) and distinguished under electron microscope.  The endoplasmic reticulum consists of two contiguous regions: the rough ER and smooth ER. The rER is “rough” because ribosomes and polyribosomes are attached to the external surface of its membranes. In contrast, the sER is “smooth” because it has no attached ribosomes. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE ENDOPLASMIC  It serves as a supporting structure for the cytoplasm, but RETICULUM more importantly, it is involved in the production of numerous substances that are to be used within, exported by, the cell.  The rER receives the proteins that are synthesized by attached ribosomes. It then processes these proteins before passing them over in the form of membrane-enclosed structures called transfer vesicles to the Golgi complex.  The sER is the site of synthesis of phospholipids, cholesterol, and other steroids. It is also involved in the transport of fatty acids and lipids.  The ER is a dynamic organelle. It is capable of remodeling, disassembly, and assembly, and it interacts with other organelles. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE GOLGI COMPLEX (GOLGI APPARATUS; GOLGI BODY)  It consists of several layers of membrane-bound, smooth-surfaced, and flattened tubes (cisternae) that are stacked on top of each other in a semicircular manner. The cisternae exhibit rounded dilatations (vesicles) on their lateral ends and their cavities are filled with fluid.  Under LM, the presence of Golgi body can be appreciated as network of solid strands in cells that have been impregnated with silver salts or osmium.  The Golgi complex have a concave surface (maturing face; trans face) and a convex surface (forming face; cis face). The maturing face is the surface of the organelle that is related to the nucleus. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE GOLGI COMPLEX (GOLGI APPARATUS; GOLGI BODY)  The Golgi complex processes, concentrates, sorts, and packages the proteins that it receives from the rER. Then, it releases the proteins into the cytoplasmic matrix in the form of membrane-wrapped structures called secretory vesicles.  Some of the proteins in the secretory vesicles are utilized within the cell (e.g., utilized as integral membrane proteins) while some are exported by the cell.  The Golgi body, like the ER, is a very dynamic organelle. On its forming surface, the membrane is continually being added by the transfer vesicles that it receives from the rER. On its maturing surface, the membrane is continually being removed as the secretory vesicles get pinched off from the organelle. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE LYSOSOMES  These are chemical-containing pouches that move about in the cytoplasmic matrix. The pouches are made up of unit membranes and the chemicals they contain are hydrolytic enzymes (hydrolases). They vary in size and shape. In general, they are spherical or ovoid bodies whose diameter ranges from 0.05 - 0.08 µm.  Lysosomes constitute an intracellular digestive enzyme that can degrade nearly all organic substances found in cells. They are the principal organelles involved in the cellular processes called heterophagy and autophagy. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE HETEROPHAGY, AUTOPHAGY, AND  Heterophagy refers to the lysosomal digestion within PHAGOCYTOSIS the cell of a particulate material (e.g., bacteria, and dead and senescent cells) that has been brought from the extracellular environment into the cell by phagocytosis.  Cell that are capable of phagocytosis like neutrophils and macrophages are termed as phagocytes.  Phagocytosis starts as the receptors on the cell surface of the phagocyte bind to the material to be ingested. The binding serves as the trigger for the cell membrane of the phagocyte to form cell protrusions (pseudopodia) at the are of binding. The membranes of the pseudopodia then fuse to completely envelope the phagocytic material. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE HETEROPHAGY, AUTOPHAGY, AND  The resulting membrane-bound structure, called PHAGOCYTOSIS phagosome or phagocytic vacuole, is then pinched off from the cell membrane and drawn into the cytoplasm.  Inside the cell, the phagosome is approached by a primary lysosome (a lysosome that has not digested anything yet) which fuses its membrane with that of the phagosome, and then, with its hydrolytic enzymes, digests the particulate material the phagosome contains. The fusion of the phagosome and the lysosome is termed as secondary lysosome or phagolysosome. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE HETEROPHAGY, AUTOPHAGY, AND  The nutrients derived from the lysosomal digestion of PHAGOCYTOSIS bacteria and other particulate material diffuse out of the phagolysosomal membrane and are recycled, undigested materials are kept enclosed with the phagolysosomal membrane and are called residual bodies.  Autophagy refers to the digestion of unneeded or senescent cell organelles. Accordingly, lysosomes are also numerous in cells with high turnover of organelles such as exocrine gland cells and neurons.  Through autophagy, lysosome plays an important role in the structural renewal of the cell, which is a continuous process. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE PEROXISOME  These are membrane-bound spherical bodies that contain chemicals. These chemicals are also enzymes which are the oxidases and catalases. The enzymes does not come from the Golgi complex but rather from the cytoplasmic matrix.  Peroxisome are present in all cells but they are particularly numerous in cells that are metabolically active such as hepatocytes (liver cells).  The oxidases are important in the process of oxidation that results in the detoxification and catabolism of various substances taken into or produced in the cell including phospholipids, fatty acids, ethanol, and formaldehyde. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE CENTROSOME (MICROTUBULE  It is a dense, spherical area in the cytoplasm – usually ORGANIZING CENTER) located near the nucleus and many times surrounded by Golgi complex. It consists of a pair of minute, short, cylindrical bodies termed centrioles that are surrounded by granular structures called centriolar satellites.  The two centrioles, which are collectively referred to as diplosome, lie perpendicular to each other. Centrosome is the site where microtubules are assembled of formed.  A centriole is seen as a tubular structure that is made up of an electron-dense wall that surrounds an electron-lucent space in the EM. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE CENTROSOME (MICROTUBULE  The centrioles are the sources of mitotic spindles ORGANIZING CENTER) that appear during mitosis. Prior to mitosis, the two centrioles replicate. They do not replicate by dividing but instead, a bud (procentriole) grows out of the lateral surface of each centriole.  At the start of mitosis (i.e., prophase stage), one centrosome migrates to one pole of the cell while the other centrosome move to the opposite pole of the cell. Mitotic spindles, which are made up of microtubules and some associated proteins, then grow out from the two sets of centrioles in both centrosomes. At the completion of mitosis, each daughter cell receives on centrosome. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE CENTROSOME (MICROTUBULE  Aside from forming the mitotic spindles, the centrioles ORGANIZING CENTER) are also the sources of cilia of ciliated cells and flagellum (tail) of the sperm cell or spermatozoon. Every cilia and flagellum is formed by growing out of a centriole. Even when already fully formed, a cilium or flagellum remains attached to its parent centriole which is often referred to as basal body. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE INCLUSIONS  Inclusions are temporary and inert structures in the cell that may or may not be membrane-bound. They vary in size, shape, and content. Some are useful and some are harmful to the body.  Some examples are fat droplets (lipid droplets), glycogen (glycogen granules), zymogen granules, pigment granules, crystals, and dust particles.  Fat or lipid droplets, are present in many cell types, but one cell type in particular, the adipose cell (adipocyte; fat cell) is specialized to store lipid. In this cell, the lipid droplets coalesce to form a single huge blob that can occupy more than 90% of the cell. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE INCLUSIONS  In routine histologic preparations, lipid is extracted by the reagents used and the areas previously occupied by lipid droplets are seen as mere round, clear areas. However, in tissues with glutaraldehyde and osmic acid, lipid droplets are preserved and appear as gray- or black-staining globules.  Glycogen, the storage form of carbohydrates, is an inclusion that is present in many cells, but is particularly abundant in liver and muscle cell. Using Periodic Acid Schiff stain, glycogen granules are colored purple or magenta. They occur in two sizes, large (alpha) and small (beta) particles that are about nm and 20-30 nm in diameter. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE INCLUSIONS  Zymogen granules, a characteristic of secretory epithelial cell. They are synonymous with secretory granules – the membrane-bound proteins coming from the Golgi complex that are destined to be released by the cells to the extracellular space by exocytosis.  Many kinds of pigments are present in cells as inclusions. They are in the form of granules (pigment granules). The more commonly occurring ones are melanin, hemosiderin, and lipochrome. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE INCLUSIONS  Melanin, the pigment that accounts for the brown to black coloration of the skin where it is synthesized by cells called melanocytes. Melanin granules are also present in the nerve cells of the substantia nigra and locus coeruleus in the brain cells and the cells of pigment epithelium of the retina (i.e., innermost histologic layer of the eye).  Hemosiderin, is a brown pigment that is the product of the lysosomal digestion of hemoglobin, the iron- containing pigment responsible for the color of RBCs. It is seen in the form of granules in cells such as those in spleen that phagocytose dying RBCs. They can easily be distinguished by other pigments by using Prussian blue stain. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE INCLUSIONS  Lipochrome granules, are membrane-bound structures that consists of coalesce residual bodies, which are undigested residues of lysosomal activity. The are yellowish-brown bodies that are common in long- lived cells such as the muscle cell in the myocardium and Sertoli cells in the testes.  Crystals, are inclusions that are present in only few cell types in the body, notably the interstitial cells (of Leydig) and Sertoli cells of the testes. They are often rod-shape but they can occur in various other shapes.  Dust particles, are numerous in the cytoplasm of phagocytes of the lungs. They are brown-black membrane-bound structures that contain exogenous materials such as dust and particulate carbon. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE MICROFILAMENTS  A microfilament is made up of F-actin, the filamentous form of actin. Actin comprises 10-15% of the total proteins in cells but only half exists as F- actin; the rest exists as G-actin (globular actin), the soluble form of protein. F-actin is formed when two strands of globular actin strands (G-actin) coil around each other, much like the fibers of a rope, to form a filament.  Microfilaments are abundant in the peripheral areas of most cells just beneath the cell membrane. They are responsible for the contraction such cells as myoepithelial cells and muscle cells that exhibit contractility to a marked degree. They are likewise involved in the locomotion of certain cells. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE INTERMEDIATE  Keratin filaments, are present only on epithelial FILAMENTS cells. They are particularly numerous in keratinocytes, the main cell type present in the epidermis. Their primary function is to protect epithelial cells from mechanical and non-mechanical stresses.  Desmin (skeleton) filaments, are characteristic of muscle cells. They are more numerous in smooth than in striated (i.e., skeletal and cardiac) muscle cell. They help maintain muscle cell architecture and structure since they connect or anchor many cytoplasmic components. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE INTERMEDIATE  Vimentin filaments, present is cells that FILAMENTS differentiate from mesenchyme such as fibroblasts and muscle cell. They are responsible for maintaining cell shape and the integrity of the cytoplasm, and stabilizing cytoskeletal interactions. They also support and anchor the cytoplasmic organelles.  Neurofilaments, characteristic of nerve cells (neurons). They are present in the cell body and all the processes of neurons. They provide internal support for the nerve cells.  Glial filaments, are present in supportive cells called glial cells (neuroglial cells). Their main function is to provide internal support for the cells. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE NUCLEUS  Often occupies the central region of the cell, is the largest structure inside the cell. It is commonly round or spherical, but it occurs in various other shapes and sometimes even forms lobes as in some WBCs  It is delimited from the cytoplasmic matrix by nuclear envelope (nuclear membrane), is filled with a homogenous substance (nuclear matrix), where, at interphase, chromatin and one or more nucleoli are embedded. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE NUCLEAR ENVELOPE (NUCLEAR MEMBRANE  The nuclear envelope can be regarded as specialized portion of rER because the perinuclear space is continuous with the cavity of the rER while the outer nuclear membrane is continuous with the membranes of the rER, and studded with ribosomes.  The nuclear envelope is perforated by circular openings (nuclear pores). In the pores, the inner and outer nuclear membranes are continuous with each other. Each nuclear pore is ringed by a nuclear pore complex.  The nuclear pore provide a channel for the exchange of substances between the cytoplasm and the nucleus. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE CHROMATIN  It refers to the chromosomes at interphase. The nucleus of nucleated human cells contains 46 chromosome (22 pairs of somatic and a pair of sex chromosome), except for the developing gametes that have already completed the first meiotic division, which contain only 23. Cells with 46 chromosomes are referred to as diploid while those with only 23 are referred to as haploid.  Each chromosome consists of one very long DNA molecule and a variety of associated proteins (nucleoproteins) which have two major types, histones and nonhistones. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE NUCLEAR MATRIX  Nuclear matrix is composed of water, proteins, (NUCLEOPLASM) metabolites, and ions. The proteins that are present in the nuclear matrix are mostly associated with DNA molecules.  In the interphase nucleus, the nuclear matrix contains a filamentous protein network called nuclear scaffold (nuclear skeleton) that is anchored to the fibrous lamina. The scaffolds has attachments for DNA loops during DNA replication. It also has links to the intermediate filaments of the cytoskeleton. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE NUCLEOLUS  It is a spherical, highly basophilic structure that is usually eccentrically located in the nucleus. It is not enveloped by unit membrane and is present only on cells that are at interphase. It disappears during prophase and reappears only during late telophase in both mitosis and meiosis.  The function of the nucleolus is to synthesize ribosomal subunits. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE MOVEMENT OF MATERIALS ACROSS  Endocytosis, refers to the transport of substances THE CELL MEMBRANE from the extracellular space into the cell. If the substance being transported is solid such as bacterium or dust particle, the process is called phagocytosis, but if it is liquid, the process is called pinocytosis.  Pinocytosis is the process that does not require the presence of receptors and the formation of pseudopodia. It simply involves the invagination of the cell membrane to enclose the fluid that needs to be ingested. If it involves the intake of large amount of liquid, it is called micropinocytosis, while pinocytic intake of minute amounts of liquid is referred to as micropinocytosis. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE MOVEMENT OF MATERIALS ACROSS  Exocytosis, refers to the process of transporting THE CELL MEMBRANE substances that are inside the cell, mostly secretory products (secretions) which are in the form of secretory vesicles from the Golgi complex, across the cell membrane and out of the cell. It involves fusion of the vesicular membrane with cell membrane followed by the release of vesicular contents to the exterior of the cell.  Exocytotic release of secretory products by cell occurs in either one of two ways: regulated secretion or constitutive secretion. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE MOVEMENT OF MATERIALS ACROSS  In regulated secretion, which occurs such cells as THE CELL MEMBRANE pancreatic acinar cells and cells of the major salivary glands, the secretory vesicles are first stored in the apical portion of the cell where they accumulate and get dehydrated. In the EM, the dehydrated secretory vesicles appear as granules. The content of the granules are released only when there is specific signal – usually in the form of a neural or hormonal stimulus.  In constitutive secretion, which occurs such cells as fibroblasts and chondroblasts, the secretory products are released from the cells as soon as they formed. Cells that secrete constitutively do not have visible secretory granules in their cytoplasm. HUMAN HISTOLOGY LECTURE BS MEDICAL LABORATORY SCIENCE  END OF TOPIC THANK YOU FOR LISTENING! R ef eren c e s: GONZALES, EDUARDO G. (2014). ESTEBAN AND GONZALES TEXTBOOK OF HISTOLOGY, (5TH ED.). QUEZON CITY: C&E PUB. MESCHER, ANTHONY L (2016). JUNQUEIRA'S BASIC HISTOLOGY: TEXT & ATLAS, (15TH ED.). NEW YORK: MCGRAW-HILL.