Histology It is the science which deals with

Histology : It is the science which deals with the microstructures of tissues and organs. The tissues • A basic tissue type is a group of similar cell and able to perform a common function. • Our body is composed of only four basic tissue types: Types of tissues Epithelial tissue Connective tissue Muscular tissue Nervous tissue

• In most animals, combinations of various tissues make up functional units called organs, and groups of organs that work together form systems. • For example, the human digestive (system )consists of a stomach (organ) small intestine, large intestine, and several other organs, each a composite of different tissues (epitheliumconnective-muscles-nervous).

1 -Epithelial Tissue • means covering body surface and lining body cavity. • The cells are closely aggregated with very little intercellular substance (matrix) of acid mucopolysaccharides. The cells are regular in shape. They are held tightly together by junctional complex Epithelial cells are firmly bound to the underlying CT by a thin membrane called basement membrane. Blood vessels do not penetrate the epithelium (avascular). Epithelium can be derived from all three embryonic germ layer : ectoderm ------- epidermis of the skin endoderm-------epithelium of alimentary canal mesoderm-------kidney Epithelium is in a continuous process of degeneration and regeneration. It has a high mitotic activity. It has a high renewal rate. • • • General characters of epithelial tissues :

General Functions of epithelium 1 -Protection : of the underlying CT----against injuries, bacteria and chemicals (skin –stomach). 2 -Secretory : (epithelium is specialized for producing secretion (glandular epithelium) 3 - Absorption : (small intestine-kidney) 4 -Reproduction: germinal epithelium of testis and ovary 5 -Contraction: myoepithelial cells are contractile in function. is specialized in perception of external stimuli 6 -Neuroepithelium: ex. (taste buds of tongue)).

Classification of the epithelia Classification of epithelia according to A-number of layers 1 -Simple epithelium (one layer) 2 -Stratified epithelium (more than one layer) B-Shape of cells 1 -Squamous(flat) 2 -Cuboidal (Height = width) 3 -Columnar (Hight is 2 -5 greater than width)

Based on the last two criteria we classify the epithelium: • 1 -Simple squamous epithelium: polyhedral in shape and oval nuclei , (thin to allow material transport-filteration (kidney, lung, blood vessel). -Endothelium ------in blood vessels and lymphatic -mesothelium------in peritoneal 2 -Simple cuboidal epithelium: present where ion exchange is required (kidney proximal tubule). 3 -Simple columnar epithelium i- non –ciliated columnar(Digestive tract) ii-ciliated columnar (oviducts)

4 -Sratified squamous epithelium i-non-keratinized (esophagus) ii-keratinized (skin) 5 -Pseudostratified ciliated columnar epitheliun (trachea) 6 -Transitional epithelium (urinary bladder). a- full b-empty

II-glandular epithelium Glands can be classified according to: 1 -number of cells 2 -The presence or absence of a duct system 3 -According to the mode of secretion (mechanism)

1 -number of cells • a-unicellular glands consists of one cell (goblet cell of small intestine) • b-multicellular glands composed of cluster of cells

2 -The presence or absence of a duct system : • a- Exocrine(in which secretion is released through ducts (salivary glands) • b-Endocrine glands (secretion released directly to the blood) thyroid glands. • c- Mixed glands (endocine and exocrine)---pancrease

3 -According to the mode of secretion (mechanism) • a-Merocrine glands (no loss of its cellular material) pancreas (exocytosis) • b-Holocrine glands (complete destruction of the cell)---sebaceous gland • c- Apocrine glands ( secretory product is discharged with apical parts of the cytoplasm) ----mammary glands

II-glandular epithelium Glands can be classified according to: 1 -number of cells 2 -The presence or absence of a duct system : 3 -According to the mode of secretion (mechanism) • a-unicellular glands consists of one cell (goblet cell of small intestine) • b-multicellular glands composed of cluster of cells • a- Exocrine(in which secretion is released through ducts (salivary glands) • b-Endocrine glands (secretion released directly to the blood) thyroid glands. • c- Mixed glands (endocine and exocrine)----pancrease • a-Merocrine glands (no loss of its cellular material) pancreas (exocytosis) • b-Holocrine glands (complete destruction of the cell) ---sebaceous gland • c- Apocrine glands ( secretory product is discharged with apical parts of the cytoplasm) ----mammary glands

2 -Muscular tissue ( skeletal, smooth, cardiac muscles) - Muscular tissue is composed of long cells called muscle fibers that are capable of contracting when stimulated by nerve impulses. -It has large numbers of myofibrils made of the contractile proteins actin and myosin -The sliding filament is accepted as the mechanism that accounts for muscular contraction. 16

There are three types of muscle tissue in the vertebrate body: 1 - Skeletal muscles connected to the skeleton and is concerned with body movements. 2 - Cardiac muscles forms the contractile portion of the heart. 3 -Smooth muscles does not exhibit striations. It is found as part of the walls of the viscera.