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Introduction to Histology

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الكلية كلية الطب     القسم  التشريح والانسجة     المرحلة 1
أستاذ المادة رنا أياد غالب محمد       12/11/2016 14:29:50
The Objectives
When you have finished this course you should be able to:
1-Give the defining characteristics of each of the four major tissue types.
2-Describe the differences among the types of epithelial tissues you will study and identify them.
3-Describe the differences among the types of connective tissues and identify them.
4-Identify the three types of muscular tissue and describe the differences among them.
5-Identify the parts of neurons and supporting cells
6- State where each of these tissues is found and what its function is.
The objectives of this lecture 1:
Numerate the main functions of epithelial tissues
Classify the epithelial cells?
Define lamina propria? What is the function of its?
What is a papillae?
What is basal lamina? Numerate the layers of it?
What are the main functions of basal lamina?
How is tissues associated with each other?
Classify the junctions types?
Explain the apical cells?
Classify the epithelial tissue according to the number of layers?
Classify the epithelial tissue according to the cells shape?
What are the functions of each type? where it’s found give an example?
TISSUES
In this lecture we will provides an introduction to the concept of how cells, the fundamental building blocks of the body, are assembled into tissues and how tissues assemble into organs in order to perform complex functions necessary for the operation of the body. There are four basic tissue types in the body: 1-Epithelial tissue 2-Connective tissue 3- Muscle tissue 4- Nervous tissue. They are represented in most organs.




Epithelial Tissue
Epithelial tissues are composed of closely aggregated polyhedral cells with very little extracellular substance. These cells have strong adhesion and form cellular sheets that cover the surface of the body and line its cavities. The principal functions of epithelial tissues are: 1- Covering, lining, and protecting surfaces (eg, skin)
2-Absorption (eg, the intestines)
3- Secretion (eg, the epithelial cells of glands)
4-Contractility (eg, myoepithelial cells)
Because epithelial cells line all external and internal surfaces of the body, everything that enters or leaves the body must cross an epithelial sheet. Epithelial cells line the free external and internal surfaces of the body.
Characteristic Features of Epithelial Cells
The forms and dimensions of epithelial cells range from high columnar to cuboidal to low squamous cells. Epithelial cell nuclei have a distinctive shape, varying from spherical to elongated or elliptic. The nuclear form often corresponds roughly to the cell shape; thus, cuboidal cells have spherical nuclei, and squamous cells have flattened nuclei. The long axis of the nucleus is always parallel to the main axis of the cell.
Because the lipid-rich membranes between cells are frequently indistinguishable with the light microscope, the stained cell nucleus is a clue to the shape and number of cells. Nuclear form is also useful to determine whether the cells are arranged in layers, a primary morphologic criterion for classifying epithelia.
Most epithelia rest on connective tissue. In the case of epithelia lining the cavity of internal organs (especially in the digestive, respiratory, and urinary systems) this layer of connective tissue is often called the lamina propria. The lamina propria not only serves to support the epithelium but also provides nutrition and binds it to underlying structures. The area of contact between epithelium and lamina propria is increased by irregularities in the connective tissue surface in the form of small evaginatons called papillae. Papillae occur most frequently in epithelial tissues subject to friction, such as the covering of the skin or tongue.
Basal Laminae & Basement Membranes
All epithelial cells in contact with subjacent connective tissue have at their basal surfaces a felt-like sheet of extracellular material called the basal lamina. This structure is visible only with the electron microscope, where it appears as an electron-dense layer, 20–100 nm thick, consisting of a network of fine fibrils, the dense layer or lamina densa .In addition, basal laminae may have electron-lucent layers on one or both sides of the dense layer, called clear layers or laminae lucida. Basal laminae are found not only in epithelial tissues but also where other cell types come into contact with connective tissue. Basal laminae have many functions. Such as regulate cell proliferation and differentiation by binding and concentrating growth factors; influence cell metabolism and survival; organize the proteins in the adjacent plasma membrane (affecting signal transduction); and serve as pathways for cell migration. An extracellular basal lamina always lies at the interface of epithelial cells and connective tissue. Nutrients for epithelial cells must diffuse across the basal lamina. Nerve fibers normally penetrate this structure, but small blood capillaries (being epithelial themselves) never enter an epithelium across a basal lamina. When components of a basal lamina are resolved with the light microscope, the structure is often called a basement membrane.
Figure 1- Basement membranes

Intercellular Adhesion & Other Junctions
Several membrane-associated structures provide adhesion and communication between cells. Some are present in other tissues but all are particularly numerous and prominent in epithelia. Epithelial cells adhere strongly to neighboring cells and basal laminae, particularly in epithelia subject to friction or other mechanical forces.
Lateral surfaces of epithelial cells exhibit several specialized intercellular junctions, which serve different functions: (Fig4-4 P:76)
?? Tight or occluding junctions: the closely associated areas between two adjacent cells.
?? Adherent or anchoring junctions share the characteristic of anchoring cells through their cytoplasmic actin filaments.
?? Gap junctions are channels for communication between adjacent cells.
? Desmosome also known as a macula adherents (plural: maculae adherents) is a cell structure specialized for cell-to-cell adhesion. A type of junctional complex, they are localized spot-like adhesions randomly arranged on the lateral sides of plasma membranes.


Fig.2 Types of Junctions in Epithelial cells

Specializations of the Apical cell Surface
Cilia : hair-like appendages attached to the apical surface of cells that act as sensory structures or to produce movement.
Goblet cells: specialized cells that produce mucus to lubricate and protect the surface of an organ
Villi : finger-like projections that arise from the epithelial layer in some organs. They help to increase surface area allowing for faster and more efficient adsorption.
Microvilli: smaller projections that arise from the cell s surface that also increase surface area. Due to the bushy appearance that they sometimes produce, they are sometimes referred to as the brush border of an organ.

Epithelial tissue can be divided into two groups depending on the number of layers of which it is composes. Epithelial tissue which is only one cell thick (one layer) is known as simple epithelium. If it is two or more cells thick (more than two layers) it is known as stratified epithelium .
A- Simple Epithelium
Simple epithelium can be subdivided according to the shape and function of its cells. *Squamous (thin cells), *cuboidal (cells roughly as thick as they are wide) or *columnar (cells taller than they are wide).
1-Simple Squamous epithelium
Squamous cells have the appearance of thin, flat plates. The shape of the nucleus usually corresponds to the cell form and help to identify the type of epithelium. Squamous cells, for example, tend to have horizontal flattened, elliptical nuclei because of the thin flattened form of the cell. They form the lining of cavities such as the mouth, blood vessels, heart and lungs and make up the outer layers of the skin (Fig.1).
Fig.1 Simple Squamous epithelium
2- Simple cuboidal epithelium: As their name implies, cuboidal cells are roughly square or cuboidal in shape. Each cell has a spherical nucleus in the centre. Cuboidal epithelium is found in glands and in the
lining of the kidney tubules as well as in the ducts of the glands. They also constitute the germinal epithelium which produces the egg cells in the female ovary and the sperm cells in the male testes (Fig.2).

Fig.2.Simple cuboidal epithelium
3- Simple columnar epithelium
Cells of simple columnar epithelia are taller than they are wide. Such cells are usually highly specialized for absorption, with microvilli, and often have interspersed secretory cells or ciliated cells. Such epithelial cells always have tight and adherent junctional complexes at their apical ends, but are often loosely associated in more basolateral areas. This allows for rapid transfer of absorbed material to the space between the cells rather than transport the full length of the cells. The additional cytoplasm in columnar cells allows additional mitochondria and other organelles needed for absorption and processing. The examples shown here are from a renal collecting duct (a), the oviduct lining, with both secretory and ciliated cells (b), and the lining of the gall bladder (c) (Fig.3).



Fig.3. Simple columnar epithelium
B. Stratified Epithelium
1-Stratified squamous epithelia: have protective functions, protection against easy invasion of underlying tissue by microorganisms and protection against water loss. In the skin, protection against water loss and desiccation is particularly important and the epithelium is keratinized. As epidermal cells of the skin (a) differentiate they become filled with keratin and other substances and eventually lose their nuclei and other organelles. Epithelia lining many internal surfaces such as the esophagus (b), or covering the cornea (c) are considered nonkeratinized because the differentiating cells accumulate much less keratin and retain their nuclei (Fig.4).

Fig.4.Stratified squamous epithelia

2- Stratified cuboidal epithelium : is a type of epithelial tissue composed of multiple layers of cube-shaped cells. Only the most superficial layer is made up of cuboidal cells, and the other layers can be cells of other types. The main functions of stratified cuboidal epithelium is to protect areas such as the ducts of sweat glands, mammary glands, and salivary glands.
3-Stratified columnar epithelia; is a rare type of epithelial tissue composed of column shaped cells arranged in multiple layers.
Stratified columnar epithelia are found in the ocular conjunctiva of the eye, in parts of the pharynx and anus, the female s uterus, the male urethra and vas deferens. Also found in intralobular ducts in salivary glands. The cells function in secretion and protection.
Stratified cuboidal and stratified columnar epithelia are rare.

4- Transitional epithelium: which lines only the urinary bladder, the ureter, and the upper part of the urethra, is characterized by a superficial layer of domelike cells that are neither squamous nor columnar. These cells, sometimes called umbrella cells, are essentially protective against the hypertonic and potentially cytotoxic effects of urine. Importantly, the form of the surface cells changes according to the degree of distention of the bladder wall (Fig.5)
Fig.5. Transitional Epithelium

5- Pseudostratified Columnar Epithelium: Cells of pseudo-stratified epithelia appear to be in layers, but the basal ends of the cells are all in contact with the basement membrane, which is often very thick in these epithelia. The best example of this epithelial type is the pseudostratified ciliated columnar epithelium of the upper respiratory tract, which contains cell types which are irregularly shaped with their nuclei at different levels that give the false appearance of cellular stratification (Fig.6).

Fig.6.Pseudostratified Columnar Epithelium


المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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