Histo Final - Endocrine Glands Flashcards
This is a book about cells and tissues. Its primary objective is to build a series of visual three-dimensional images of the cells and tissues that make up the human body. This particular chapter, headed by the all- encompassing title "cells," is intended to prepare you to recognize and understand the images of cells and tissues, photographed by light and electron microscopy, that are presented in this atlas. The number and variety of cells within every person is tremendous.
Fortunately, the Herculean task of visualizing the complexity of the very cells of which we are made is greatly simplified when you realize that many cells, despite their dramatic differences in structure and function, are really more alike than not: And what is that theme? We are surrounded by chaos. Biologists are wont to refer to chaos as Entropy - the concept, described in the second law of thermodynamics, that everything tends toward disorder.
Entropy pervades our lives. Clean clothes, for example, don't happen on their own. The act of wearing and using clothes soils them. Does the pile of dirty clothes that results from a week's hard use suddenly appear in pristine form, washed, folded, and stacked in neat piles in your dresser drawers come Monday morning? You painstakingly gather the heap of soiled clothes at the end of the week, put them in the washer, dry them, press them, fold them, carry them to the dresser, sort them, and put them neatly away.
All of which takes energy. It takes energy to generate order out of chaos. In the example given above, we are dealing with several pairs of pants or skirts, blouses or shirts, a few assorted undergarments, and some socks - maybe 50 items in a busy week. A "typical" mammalian cell has about ten billion protein molecules to look after.
Since everything tends toward disorder, and numbers compound the problem exponentially, the entropic possibilities faced by a cell during the course of its daily life are bewildering. How do cells deal with Entropy? How do they generate such exquisite order in the face of potential molecular chaos? In microanatomic terms, cells accomplish order by means of beautifully bioengineered components that, at the expense of considerable amounts of energy, see that the right molecules get in the right places at the right times.
All of which is a formidable logistic problem. How do cells do it? That is, in large part, what this book is all about. A look at the microscopic anatomy of the cell can help us to understand how cells generate order from chaos and, by so doing, achieve that most precious quality - life.
When you look at a cell with the light microscope, the first thing you are likely to see is the Nucleus - alarge, round, dark-staining body that contains the genetic material. In many cells, the Nucleus appears to be suspended in a small sea of Cytoplasm , a pale-staining Matrix that often contains small, blurry objects visible only when stained.
The Cytoplasm is surrounded by an outer limiting membrane called the Cell Membrane also plasmalemma or Plasma membrane. This inability is a source of confusion to beginning students of microanatomy; if you can't see a cell's boundaries, you can't see where one cell ends and another begins. What you often see under the light microscope, then, is a gaggle of purple nuclei in a field of amorphous material. What can that tell about the organization of cells and tissues?
How does that provide any insight into the ways in which cells generate order from chaos? Enter the electron microscope. The electron microscope, which has the ability to resolve very small 2. Once you see a number of electron images of a particular cell, you will rapidly recognize that kind of cell, much as you learn to recognize a particular make of automobile among a spate of others on a busy highway.
This kind of sight-recognition not only allows you to distinguish specific cells in electron images, it allows you to mentally superimpose what you've seen in the electron microscope upon similar cells when you look at them with the light microscope.
In the following chapters, you will look at a variety of cells as seen with the light microscope and with the electron microscope at similar magnifications. By carefully comparing the light and electron images of the very same cells, you will develop a kind of "x-ray vision" that will allow you to skillfully interpret light-microscopic images that once looked like little more than a group of nuclei in a fuzzy field.
It is first helpful to look at the kinds of structures you're likely to see within cells. In order to build up a "visual vocabulary" of cell components, this overview contains two illustrations: By referring to the image of specific structures in both the drawing and the electron micrograph as they are discussed, you can develop a good mental picture that will provide a basis for recognition of these same structures as they are encountered throughout the atlas.
Starting with the outside of the cell, the first structure is the Cell Membrane. Called by a number of names, including the Plasma membrane, plasmalemma, and outer limiting membrane, the Cell Membrane is crucial to a cell's function because it is the interface between the outside world and the inside of the cell; the Cell Membrane lies between the order within the cell and the potential disorder without.
The composition of the membrane surrounding a particular cell can vary dramatically from region to region. In addition, the cell membranes surrounding different kinds of cells can be different from one another. Referring to "the Cell Membrane " as a unit can be misleading because doing so implies that the Cell Membrane is a single entity.
This erroneous notion of the homogeneity of the Cell Membrane is unfortunately reinforced by images generated by transmission electron microscopy of sectioned material. The electron images of a variety of cellular membranes look quite similar.
A "typical" Cell Membrane , for example, is shown in the electron micrograph in Figure This illustration is a relatively low magnification electron micrograph of a cell from the pancreas of the squirrel monkey. If you look at the region indicated by the arrow, you will see a pair of dark lines where two cells are adjacent.
These dark lines represent the cell membranes of two neighboring cells. At higher magnification, as shown in the inset, each Cell Membrane looks like a set of railroad tracks: This appearance led to the name the Unit Membrane , which refers to the electron image presented by the Cell Membrane and the membranes that surround the cytoplasmic organelles when viewed in cross-section by conventional transmission electron microscopy.
The uniform appearance of the membranes surrounding the cell and its organelles is misleading. Mebranes vary tremendously in structure and function. To be sure, membranes share many similarities in fundamental organization; they are bimolecular lipid leaflets that contain proteins.
But the Lipids and the proteins can vary considerably in composition, assume a variety of configurations, and perform a variety of functions. The membranes in the myelin sheaths surrounding the axons of nerves, for example, are effective electrical insulators, whereas the cell membranes of proximal tubule cells of the kidney are highly efficient ion pumps.
Looking back at the diagram Figure and electron micrograph Figure , the cell is seen to contain a number of organelles that are surrounded by membranes.
Why are so many organelles surrounded by membranes? Different parts of the cell must perform different functions, and membranes provide a superb means for compartmentalization within the cell. Membrane limited organelles may be thought of as compartments that can move about from one region of the cell to another.
In addition, each cell contains on the order of ten billion protein molecules. Many of these proteins are enzymes that catalyze biochemical reactions, which depend upon surface contact between the participants in the reaction. Membranes not only provide a tremendous amplification of surface area within the cell, they contain specific enzymes.
The specificity of molecular interactions that occur in enzymatically catalyzed biochemical reactions, then, can be greatly enhanced by the presence of membranes within cells.
The mitochondrion provides an excellent example of an Organelle that uses membranes to perform exquisite biochemical maneuvers. Often - and quite appropriately - referred to as "the power plant of the cell," the mitochondrion contains two sets of membranes: Mitochondria produce ATP , the chemical energy "currency" of the cell, in large quantities.
Cells with high energy requirements usually have many Mitochondria. Cells with very high energy requirements usually have Mitochondria that contain many Cristae. The membranes of the Cristae contain arrays of enzymes associated with oxidative phosphorylation, one of the essential phases of ATP production. Increasing the number of mitochondrial Cristae vastly amplifies the amount of membrane surface available for the enzymes involved in the process of oxidative phosphorylation.
The Endoplasmic Reticulum comes in two morphologically distinct varieties: The rough-surfaced Endoplasmic Reticulum , usually called the rough Endoplasmic Reticulum or the rough ER, consists of a series of interconnected, flattened, membrane-limited sacs called Cisternae in which the membranes are encrusted with ribosomes.
Ribosomes, which have the electron-microscopic appearance of small dense dots, are the sites of protein assembly in cells. Consequently, the rough ER, being a system of membranes and attached ribosomes, participates in the synthesis and concentration of proteins. The smooth Endoplasmic Reticulum , which lacks ribosomes, is quite different.
It is organized into a system of interconnected tubules and is associated with a variety of functions such as Glycogen metabolism, Steroid synthesis, and enzymatic detoxification of noxious substances.
Ultimately, the rough and smooth ER are physically interconnected and should be thought of as different manifestations of a common system of intercellular membranes. Electron micrograph of a thin section taken through an exocrine cell of the monkey pancreas. The Golgi Apparatus , named after a turn-of-the-century Italian anatomist who had a tremendous impact on biology, is a complex system of membrane-limited sacs and vesicles that is concerned with the modification and packaging of proteins and protein- Polysaccharide complexes.
Often working in concert with the rough ER, the Golgi Apparatus receives material elaborated by the rough ER, chemically modifies it with enzymes in the Golgi membranes, and concentrates and packages the new product within membrane-limited vesicles called Secretory Granules.
In addition, the Golgi can package proteins into membrane-limited vesicles, such as lysosomes, for use within the cell itself. Lysosomes are membrane-limited organelles that contain a broad spectrum of vicious hydrolytic enzymes capable of breaking down everything from nucleic acids to proteins to fats.
Originally called "suicide bags" because early cell biologists surmised the cell could open its lysosomes, release their contents, and rapidly dissolve itself when "its number was up," lysosomes serve a variety of essential functions.
For one thing, cells use lysosomes to dispose of worn-out organelles. In addition, specialized cells such as macrophages use lysosomes in the intracellular destruction of ingested foreign materials such as bacteria. Other cells, such as endocrine cells of the pituitary gland, use lysosomes to digest excess product synthesized by the cell that is not needed at the time. The Nucleus , which contains the genetic material, is surrounded by a double membrane continuous with the Endoplasmic Reticulum.
Consequently, the membranes surrounding the Nucleus , called the Nuclear Envelope , represent a perinuclear cisterna of the Endoplasmic Reticulum. The Nuclear Envelope is perforated by nuclear pores, small openings that permit the vital exchange of materials between Nucleus and Cytoplasm. The Nucleus contains the chromosomes - discrete units of DNA , the genetic material, complexed with protein-visible only when the cell is in the midst of Mitosis , or cell division.
At other times, the chromosomes are less condensed, and their strands are woven into an indecipherable tangle within the nucleoplasm called Chromatin. When the Chromatin is somewhat condensed, meaning that the genetic material is not unwound and thus is not available for "translation" of the genetic code into messenger RNA which later dictates the sequence of amino acids that are strung together to make protein , the Chromatin stains darkly.
This clumped, nontranscriptionally active Chromatin is called Heterochromatin. Transcriptionally active Chromatin , which takes little stain and thus looks pale, is called Euchromatin. A glance at the Nucleus , then, can determine whether a given cell is likely to be active in the Transcription of messenger RNA.