HISTOLOGY: EPITHELIA AND GLANDS CONNECTIVE TISSUE PROPER CARTILAGE AND BONE
HISTOLOGY: EPITHELIA AND GLANDSCONNECTIVE TISSUE PROPER CARTILAGE AND BONE Olaleye O.O. 2B10
INTRODUCTIONThe four basic types of tissues in the body are the: •Epithelial tissue •Connective tissue •Muscular tissue •Nervous tissueThese tissue exist and function in close association with one another.
EpitheliaEpithelia are a diverse group of tissues which cover or line all bodysurfaces, cavities andTubes. They function as interfaces between different biologicalcompartments. As such, the mediate a wide range of activities suchas; •Selective diffusion •Absorption and/or secretion •Physical protection •Containment
They are closely bound to one another by a variety of membranespecialisations called cell junctions which provide physical strengthand exchange of info and metabolites.All epithelia are supported by basement membrane which separatesepithelia from underlying supporting tissues and are never penetratedby blood vessels. Thus, epithelia are dependent on the diffusion ofoxygen and metabolites from adjacent supporting tissues.
Arrangement and structure of Epithelial cellsEpithelium is named according to:• Shape• Structure• Arrangement of cells
Arrangement and structure of Epithelial cells • Squamous- thin and flat shaped cells • Cuboidal- cube shaped cells • Columnar- column shaped cells • Simple- single layer of cells • Stratified- multilayered cells • Pseudo-stratified- false stratified • Transitional- stretchable • Ciliated- cells possess cilia
Basement MemebraneThe basement membrane is athin, noncellular region thatseparates the epithelium fromthe underlying conective tissue.Can easily be seen with a lightmicroscope.
Classification of Epithelia tissuesEpithelium is traditionally classified according to threemorphological characteristics: •The number of cell layer: a single layer of epithelial cells is termed SIMPLE epithelium, whereas epithelia composed of several layers are termed STRATIFIED epithelium
•The shape of the component cells: This is based on appearance ofsections taken at right angles to the epithelial surface.NB: In stratified epithelia the shape of the outermost layer of cellsdetermines the descriptive classification. Cellular outline are oftendifficult to distinguish, but the shape of epithelial cells is usuallyreflected in the shape of their nuclei.
•The presence of surface specialisation such as cilia and keratin: an example is the epithelia surface of skin whis is classified as stratified squamous keratinized epithelium.NB: Epithelial maybe derived from ectoderm, mesoderm orendoderm origin.
Epithelium which is primarily involved in secretion is often arranged intostructures called GLANDSGlands are merely invaginations of epithelial surfaces which are formedduring embryonic development by proliferation of epithelium into theunderlying tissues
There different types of glands:Glands which maintain their continuity with the epithelial surface,discharging their secretions onto the free surface via a duct, are calledEXOCRINE glands.In some cases, the duct degenerates during development leavingisolated islands of epithelial secretory tissue deep within other tissues.The secretory products of such glands, known as ENDOCRINE orDUCTLESS glands, pass into the bloodstream; their secretions areknow as hormones.
Glands are cells or aggregations of cells whose function is secretion.Exocrine glands release the secretory product via a system of ducts thatopens upon one of the surfaces of the body which are in contact with theexternal world (skin, gastrointestinal tract etc.).
Endocrine glands release their secretory product (typically hormones)into the spaces between the secretory cells (extracellular space) fromwhich it enters the bloodstream.
Both endocrine and exocrine glands are developmentally derived fromepithelia, which form a down-growth into the underlying connective tissue.The cells forming this down-growth then develop the specialcharacteristics of the mature gland.NB: Exocrine glands maintain the connection with the surface epithelium,whereas the connection is lost by endocrine glands.
Exocrine glandsExocrine glands may be classified according to cell number,and/or the shape and branching pattern of their secretory portionsand ducts.
Unicellular Glands: consist of a single secretory cell. In mammals theonly example of unicellular exocrine glands are goblet cells, which occurin the epithelium of many mucous membranes. Goblet cells secrete theglycoprotein mucin, which by the uptake of water is converted into a slimysubstance, mucus.
Multicellular glands: The simplest form of a multicellular gland is a secretoryepithelial sheath - a surface epithelium consisting entirely of secretory cells (e.g.the epithelium lining the inner surface of the stomach, where the mucous secretionprotects the stomach wall from the acidic contents of the stomach). Othermulticellular glands have their secretory portion embedded in the connectivetissue underlying the epithelium. The secretion is either discharged directly fromthe secretory portion onto the epithelium or reaches the epithelium via a ductsystem that consists of non-secretory cells.
The secretory portion may have a variety of shapes. Secretory cellsmay formtubes in tubular glands,acini in acinar glands oralveoli in alveolar glands
Combinations exist - the pancreas is a tubulo-acinar gland, in which eachsection of the secretory system has a specialized function.The precursors of digestive enzymes are produced by the acinar cells. Tubularcells secrete the alkaline bicarbonate solution which eventually neutralizes theacidic contents of the stomach that are released into the duodenum.
Multicellular glands with an unbranched excretory duct arecalled simple.When the excretory duct is branched, it is called a compound gland.Finally, the part of the gland consisting of secretory cells is branched in abranched gland.
Secretory MechanismsThe secretory cells can release their secretory products by one of threemechanisms: • Merocrine secretion • Apocrine secretion • Holocrine secretion
Merocrine secretion: corresponds to the process ofexocytosis. Vesicles open onto the surface of the cell, and thesecretory product is discharged from the cell without anyfurther loss of cell substance.
Apocrine secretion: designates a mechanism in which part of theapical cytoplasm of the cells is lost together with the secretory product.The continuity of the plasma membrane is restored by the fusion of thebroken edges of the membrane, and the cell is able to accumulate thesecretory product anew. This mechanism is used by apocrine sweatglands, the mammary glands and the prostate.
Holocrine secretion: designates the breakdown anddischarge of the entire secretory cell. It is only seen inthe sebaceous glands of the skin.
There are two additional mechanisms by which secretory cells can release theirproducts. Lipid soluble substances may diffuse out of the secretory cell (e.g.steroid hormone-producing endocrine cells). Transporters (membrane proteins)may actively move the secretory product across the plasma membrane (e.g. theacid producing parietal cells of the gastric glands). These secretory mechanismsmay not involve any light microscopically visible specialisations of the cell.
Connetive Tissue ProperThis is the most widespread and abundant type of tissue in the humanbody. Its function is primarily to support, anchor and connect variousparts of the body. Although connective tissue exists in a number offorms, all types have three basic structural elements --cells, fibres and intercellular substance (ground substance).
Connective tissue serves as: • Packing • Holds the cells of organs together • Passes on nutrients to other tissues from the blood • And active in fighting disease-causing organisms.The cells in connective tissue are always well spaced in a thick, fluid basesubstance, or matrix, in which there may also be long, thin threads calledfibers.
The most common cell types are fibroblasts, which produce fibres and otherintercellular materials. The two most common types of fibres are: • collagen (collagenous) • and elastic.Collagen fibres are for strength while the elastic ones are for elasticity of thetissue. Both the cells and the fibres are embedded in the intercellularsubstance. The consistency of this substance is highly variable from gelatin-like to a much more rigid material.
The proportions of the cells, fibres, and intercellular substance vary, dependingon a particular nature and function of the connective tissue. For example, astrong connective tissue needs a greater proportion of the collagen fibres andfewer cells. An example would be a dense regular connective tissue, which isfound in tendons and ligaments. On the other hand, a connective tissuecomposed of mostly cells would not be very strong. An example would be anadipose (fat) connective tissue.
Connective tissue proper: encompasses all organs and body cavitiesconnecting one part with another and, equally important, separating onegroup of cells from another. This is a very large and diverse group oftissues and includes adipose tissue (fat), areolar (loose) tissue, anddense regular tissue, among others.
Specialized connective tissues: this group includes cartilage, bone,and blood. Cartilage and bone form the skeletal framework of the bodywhile blood is the vascular (transport) tissue of animals.
Connective tissue properAreolar (Loose) Connective Tissue• Is the most widespread connective tissue of the body.• It is used to attach the skin to the underlying tissue.• It also fills the spaces between various organs and thus holds them in place as well as cushions and protects them.• It also surrounds and supports the blood vessels.
• The fibres of areolar connective tissue are arranged in no particular pattern but run in all directions and form a loose network in the intercellular material. Collagen (collagenous) fibres are predominant.• They usually appear as broad pink bands.• Some elastic fibres, which appear as thin, dark fibres are also present.
• The cellular elements, such as fibroblasts, are difficult to distinguish in the areolar connective tissue. But, one type of cells - the mast cells are usually visible.• They have course, dark-staining granules in their cytoplasm.• Since the cell membrane is very delicate it frequently ruptures in slide preparation, resulting in a number of granules free in the tissue surrounding the mast cells.• The nucleus in these cells is small, oval and light-staining, and may be obscured by the dark granules.
Adipose Connective TissueThe cells of adipose (fat) tissue are characterized by a large internal fat droplet,which distends the cell so that the cytoplasm is reduced to a thin layer and thenucleus is displaced to the edge of the cell.These cells may appear singly but are more often present in groups.When they accumulate in large numbers, they become the predominant celltype and form adipose (fat) tissue.Adipose tissue, in addition to serving as a storage site for fats (lipids), also padsand protects certain organs and regions of the body. As well, it forms aninsulating layer under the skin which helps regulate body temperature.
Dense (Fibrous) Regular Connective Tissue• Dense connective tissue is characterized by:• an abundance of fibres with fewer cells, as compared to the loose connective tissue.• It is also called fibrous or collagenous connective tissue because of the abundance of collagen (collagenous) fibres.• Little intercellular substance is present.
• Furthermore, in this tissue type, the fibres are organized in a regular, parallel pattern. Hence, the name – dense regular (fibrous or collagenous) connective tissue.• In addition to the tendons, this type of tissue is also found in ligaments. Hence, the function of this tissue is to anchor skeletal muscle to bone, to attach bone to bone as well as to stabilize the bones within a joint. Fibroblasts are the only cells visible, and are arranged in rows between the fibres. These fibroblasts function to lay down or create the fibres of the tissue
Cartilage: Specialized Connective TissuesCartilage is a somewhat elastic, pliable and compact type of connectivetissue.It is characterized by three traits: • lacunae, • chondrocytes, • a rigid matrix.The matrix is a firm gel material that contains fibres and othersubstances.
There are three basic types of cartilage in the human body: • hyaline cartilage, • elastic cartilage and • fibrocartilage.
The most common type of cartilage is the hyaline cartilage.Most of the skeleton of the mammalian fetus is composed of hyaline cartilage.As the fetus ages, the cartilage is gradually replaced by more supportive bone.In the mammalian adult, hyaline cartilage is mainly restricted to: • the nose, • trachea, • Bronchi, • ends of the ribs, • and the articulating surfaces of most joints
The function of the hyaline cartilage is to provideslightly flexible support and reduce friction withinjoints. It also provides structural reinforcement.
The matrix appears as a smooth, solid, blue or pink-colouredsubstance. Fine protein fibres, are embedded in the matrix, butthey are not visible with the light microscope since they do notstain well.The large cartilage cells called chondrocytes, are trapped withinthe matrix in spaces called lacunae (singular, lacuna).Cartilage is a non-vascular tissue. As such, the chondrocytes relyon blood vessels in the tissue surrounding the cartilage for nutrientsupply and waste removal.
BONEBone is the main component of the skeleton in the adult human.Like cartilage, bone is a specialised form of dense connective tissue.Bone gives the skeleton the necessary rigidity to function as attachmentand lever for muscles and supports the body against gravity.Two types of bone can be distinguished macroscopically: • Trabecular or cancellous or spongy bone • Compact bone
Trabecular or cancellous or spongy bone consists of delicate barsand sheets of bone, trabeculae, which branch and intersect toform a sponge like network. The ends of long bones(or epiphyses) consist mainly of trabecular bone.
Compact bone does not have any spaces or hollows in the bone matrixthat are visible to the eye. Compact bone forms the thick-walled tube ofthe shaft (or diaphysis) of long bones, which surrounds the marrow cavity(or medullary cavity). A thin layer of compact bone also covers theepiphyses of long bones.
Bone is, again like cartilage, surrounded by a layer of dense connectivetissue, the periosteum.A thin layer of cell-rich connective tissue, the endosteum, lines the surface ofthe bone facing the marrow cavity.Both the periosteum and the endosteum possess osteogenic potency.Following injury, cells in these layers may differentiate into osteoblasts (boneforming cells) which become involved in the repair of damage to the bone.
Compact BoneCompact bone consists almost entirely of extracellular substance, the matrix.Osteoblasts deposit the matrix in the form of thin sheets which arecalled lamellae.Lamellae are microscopical structures.Collagen fibres within each lamella run parallel to each other.Collagen fibres which belong to adjacent lamellae run at oblique angles to eachother.Fibre density seems lower at the border between adjacent lamellae, whichgives rise to the lamellar appearance of the tissue. Bone which is composed bylamellae when viewed under the microscope is also called lamellar bone.
In the process of the deposition of the matrix, osteoblasts become encased insmall hollows within the matrix, the lacunae.Unlike chondrocytes, osteocytes have several thin processes, which extendfrom the lacunae into small channels within the bone matrix , the canaliculi.Canaliculi arising from one lacuna may anastomose with those of other lacunaeand, eventually, with larger, vessel-containing canals within the bone.Canaliculi provide the means for the osteocytes to communicate with each otherand to exchange substances by diffusion.
In mature compact bone most of the individual lamellae form concentric ringsaround larger longitudinal canals (approx. 50 µm in diameter) within the bonetissue.These canals are called Haversian canals. Haversian canals typically runparallel to the surface and along the long axis of the bone. The canals and thesurrounding lamellae (8-15) are called a Haversian system or an osteon. AHaversian canal generally contains one or two capillaries and nerve fibres.
Irregular areas of interstitial lamellae, which apparently do not belong to anyHaversian system, are found in between the Haversian systems.Immediately beneath the periosteum and endosteum a few lamella are foundwhich run parallel to the inner and outer surfaces of the bone. They arethe circumferential lamellae and endosteal lamellae.
A second system of canals, called Volkmanns canals, penetrates thebone more or less perpendicular to its surface.These canals establish connections of the Haversian canals with the innerand outer surfaces of the bone.Vessels in Volkmanns canals communicate with vessels in the Haversiancanals on the one hand and vessels in the endosteum on the other.A few communications also exist with vessels in the periosteum.
Trabecular BoneThe matrix of trabecular bone is also deposited in the form of lamellae.In mature bones, trabecular bone will also be lamellar bone.However, lamellae in trabecular bone do not form Haversian systems.Lamellae of trabecular bone are deposited on pre-existing trabeculaedepending on the local demands on bone rigidity.Osteocytes, lacunae and canaliculi in trabecular bone resemble those incompact bone.
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