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Lebanese UniversityLebanese University Faculty of Sciences
Faculty of Sciences
B1100
Animal Histology
Department of Life and Earth Sciences
Fall Semester
2021/2022
©opyright Reserved
, 1
INTRODUCTION
Histology and concept of tissues
Histology is the science that studies the microscopic structure of tissues. Tissues are
sets of differentiated cells that are localized in one place and that contribute to the
same biological function. Tissues are made up of cells and of all the elements filling
the intercellular or extracellular medium.
Classification of tissues
There are four types of tissues:
- Covering (lining or surface) epithelial tissues and glandular epithelial tissues;
- Non-specialized connective tissues and specialized connective tissues:
adipose tissue, cartilage tissue, bone tissue and blood tissue;
- Skeletal striated muscle tissue, striated cardiac muscle tissue and smooth
muscle tissue;
- Nervous tissue of the central and peripheral nervous system.
Epithelium Covering epithelium
Glandular epithelium
Loose connective tissue
Dense connective tissue
Reticular tissue
Elastic tissue
Connective Adipose tissue
Cartilage tissue
Bone tissue
Blood tissue
Skeletal striated muscle tissue
Muscle Striated cardiac muscle tissue
Smooth muscle tissue
Nervous Nervous tissue of the central nervous system
Nervous tissue of the peripheral nervous system
, 2
Histogenesis
The tissues differentiate during embryonic development from the three
primitive embryonic germ layers:
1- Ectoblast: gives part of the epithelial tissues (the epidermis) and the nervous
tissue.
2- Mesoblast: it is at the origin of the non-specialized connective tissues, the
cartilage and the bone, the wall of the heart and vessels, the smooth and striated
muscles.
3- Endoblast: gives rise to the covering epithelium lining the intestine, the
respiratory system and the urogenital tract.
Cross section of the 3 embryonic germ layers
An organ may contain several varieties of tissues that may have a different
embryological origin.
, 3
Chapter 1
EPITHELIAL TISSUES
Epithelial tissues are composed of cells that are tightly packed together with
very narrow intercellular spaces, at the limit of visibility under the light microscope.
Thus, an epithelium is formed of a set of polarized cells, closely juxtaposed, integral
with each other, ensuring their cohesion by different adhesion junctions and involved
in one or more common physiological functions; it rests on a basal membrane that
separates it from a connective tissue.
The epithelia of the body derive from the three embryological germ layers: the
ectoderm, the endoderm and the mesoderm. They differentiate morphologically and
functionally into two main types:
• Covering (surface or lining) epithelia that cover other tissues,
• Glandular epithelia (with secretory function) that are specifically involved in
the elaboration of a secretory product.
First Part:
COVERING EPITHELIA
The covering epithelia are tissues formed by one or many layers of epithelial
cells closely juxtaposed, with sometimes the presence of non-epithelial cells. These
tissues rest on an underlying connective tissue through a basal lamina. They are
innervated and non-vascularized (the blood vessels of the underlying connective
tissue do not cross the basal lamina), and they have a great capacity for regeneration.
Lumen
Apical pole
Epithelium
Basal pole
Connective tissue
Blood vessel
Organization of the covering epithelium
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