Biological membrane

HUMAN MICROMORPHOLOGY

Histology · Cytology · Embryology

Part I

Cytology. Embryology. Tissues

Микроморфология человека

Гистология · Цитология · Эмбриология

Часть I

Цитология. Эмбриология. Ткани

Учебное пособие для студентов медицинских вузов

Харьков ХНМУ 2011

Рецензенты – В.Д. Садчиков, д-р мед. наук, зав. каф. патологической анатомии Харьковской медицинской академии последипломного образования,

Ю.П. Костиленко, д-р мед. наук, проф., зав. каф. анатомии человека Полтавской медицинской стоматологической академии.

Авторы – С.Ю. Масловский, И.И. Шеститко, А.Ю. Степаненко, Е.В. Мирошниченко, В.Д. Карамышев.

Микроморфология человека. Гистология, цитология и эмбриология. Часть I. Цитология. Эмбриология. Ткани: Учеб. пособие для студентов мед. вузов / С.Ю. Масловский, И.И. Шеститко, А.Ю.Степаненко и др. – Харьков: ХНМУ, 2011. – 120 с.

ISBN 978-966-2094-19-0

ISBN 978-966-2094-20-6

Рассмотрены строение клетки, клеточная теория, ранние этапы эмбриогенеза человека и строение тканей. Материал соответствует модулю «Основы цитологии и общей эмбриологии. Ткани».

Human micromorphology. Histology, Cytology and Embryology: Part I. Cytology. Embryology. Tissues. Manual for medical students / S.Yu. Maslovsky, I.I. Shestitko, O.Yu. Stepanenko et al. – Kharkov: KhNMU, 2011. – 120 p.

The cell structure, cell theory, early stages of human embryogenesis and tissue organisation are considered. The material corresponds to the meaning module "Basic Cytology and General Embryology. Tissues".

Cytology

Definitions.Cytology is the science that deals with the detailed structure and functions of the cell. The cell is the smallest structural and functional unit of the human organism. So the cell is the elementary level of organization of any multicellular organism. A more detailed definition of the cell is the following one.

The cell is a self-governing, self-regulating and self-regenerating biological system, which possesses all features of the whole organism, such as metabolism, growth, irritability, movement, and others.

All cells have similar principles of organisation. Any cell has three main parts:

1) the cell's membrane, which separates the cell from the external environment;

2) the cytoplasm – this is the inner medium of the cell;

3) the nucleus, which contains chromosomes and nucleolus.

Biological membrane

The structural and functional unit of the cell is the biological membrane. Any biologicalmembrane consists of two layers of phospholipid molecules. Each phospholipid molecule has an outer head, which possesses hydrophilic properties, and a thin tail, which possesses hydrophobic properties.

The protein molecules are embedded within the thickness of the lipid bilayer. Some of them project on one of its surfaces. Some proteins occupy the entire thickness of the membrane and may project out of the both surfaces. Such proteins are called integral proteins. Therefore the proteins are situated by the mosaic principle and do not create entire layer.

The current interpretation of how the cell membrane is organised, is referred to as the modified fluid-mosaic model.

In addition, on the external surface of the cell membrane carbohydrate molecules are attached to proteins, thus forming glycoproteins, or to lipid molecules, thus forming glycolipids. These glycoprotein and glycolipid molecules constitute a layer which is called the cell coat or glycocalix.The glycocalix has specific functions in cell metabolism, cell recognition and may serve as cell receptor.

The proteins of the cell membrane perform many different functions. They are: pumps, channels, receptors, and enzymes. Pumps serve to transport certain ions, such as Na+ (sodium) actively across membranes. Ions get attached to the protein on one surface and move with the protein to the other surface. It is active and selected transport. The pumps also transport metabolic precursors of macromolecules, such as aminoacids and sugar. This is active and selective transport. Channels allow for the passage of small ions and molecules across the cell membrane in either direction, that is passive diffusion. Receptor proteins allow for the recognition of some substances (such as hormones or enzymes, for example). Some proteins can act as enzymes.

Functions of the cell membrane. The membrane supports the shape of the cell. It controls the passage of every substance into or out of the cell by phagocytosis and pinocytosis.

Larger molecules entering and leaving the cell may cause local configurational change in the cell membrane, which is called vesicular transport. This is active transport.

There are two types of vesicular transport: endocytosis and exocytosis. Endocytosis is vesicular transport of entering the cell. Endocytosis may be phagocytosis or pinocytosis. Exocytosis is vesicular transport of leaving the cell.

Cell membrane forms a sensory surface of the cell. This function is most developed in the nerve cells.

Cell membrane forms intercellular contacts (junctions) between adjacent cells.

At last it forms various projections from the cell surface.

Intercellular junctions. Desmosomes. This is the most common type of junction between the adjoining cells. At the site the desmosome the cell membrane is thickened because of the presence of a dense layer of protein on its inner surface. Between two thickened membranes there is a small gap. The region of the gap is rich in glycoprotein, which looks like a thin filaments. The thickened areas of the two membranes are held together by loop-shaped fibrils.

Zonulae adherens. This is similar to the desmosome, but the gap is devoid of the filaments.

Zonulae occludens.Two layers of glycocalyx are fused here. These junctions act as a barrier, that prevents the movement of molecules into the intercellular spaces. Zonulae occludens are also called tight junctions.

Gap junctions. The cell membranes lie very close to each other. The gap between two cell membranes is reduced. There are bead-like protein molecules in form of hexagon in this site. A minute canaliculus passing through each “bead” allows free passage of substances from one cell to another.

Cell membrane projections. Cilia are hair-like projections of the cell membrane, which contain a central pair of tubules, surrounded by nine pairs of tubules. All of them are linked with the basal body. Basal bodies are derived from centrioles. The proteins of the cilia are tubulin and dynein, which is ATP-active. The action of cilia helps in the movement of ova through the uterine tube, and of spermatozoa through the male genital tract.

Flagella have the same basic structure as cilia.

In the human body the best example of a flagellum is the tail of a spermatozoon.

Microvilli are finger-like projections from the cell surface. They contain numerous microfilaments and enzymes. The microvilli of some cells look like a brush border, for example, in digestion tract. They greatly increase the surface area of the cell. Modified microvilli are called stereocilia.

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