ZOOLOGY STUDY GUIDE - Ye. O. Nevedomska - 2013

LECTURE 5. THE TISSUE LEVEL OF ORGANIZATION OF LIVING MATTER. ANIMAL TISSUES

1. Characteristics of Animal Tissues

1.1. Epithelial Tissue

Epithelial Tissue, or epithelium (from the Greek epi — upon, and thele — nipple), is a tissue consisting of closely packed Cells that cover the outer surface of the body, line Body Cavities and Internal Organs, and form most glands. There is almost no intercellular substance in epithelial Tissues.

Epithelial tissues vary in Structure, which is closely related to their Functions. The following types of epithelial tissues are distinguished:

✵ Simple Epithelium (Fig. 7): cells are tightly packed, forming a single layer. Functions: regulates METABOLISM, such as gas exchange, excretion of metabolic waste, and absorption of nutrients in the intestine. Based on Cell shape, simple epithelium can be squamous (thoracic and abdominal cavities, walls of pulmonary alveoli), cuboidal (excretory ducts of glands, renal tubules), or columnar (gastrointestinal tract).

✵ Stratified Epithelium (Fig. 7): cells are tightly packed, forming multiple layers. Functions: barrier and protective — it shields deeper tissues. Based on cell shape, it is classified into squamous, cuboidal, and columnar. Stratified squamous epithelium lines the mucous membranes of the Oral Cavity and Pharynx, and forms the outer layer of the Skin, the epidermis. Skin derivatives include Hair (thermal insulation function) and Nails (supportive and protective function).

✵ Ciliated epithelium: cells have cilia on their free surface capable of beating in a specific direction, which helps remove dust from the Airways. Function: protective — shields against foreign particles and microorganisms. Location in the body: lines the Upper Respiratory Tract.

✵ Glandular (secretory) epithelium consists of cells adapted for the synthesis, accumulation, and secretion of substances. Depending on their Structural and functional characteristics, Glandular Epithelium is divided into exocrine glands and Endocrine glands. Exocrine glands have ducts through which secretions are discharged into organs or onto the skin surface (Salivary Glands, Glands of the gastrointestinal tract, sebaceous, sweat, and Mammary Glands).

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Fig. 7. Types of epithelium:

1 — simple squamous; 2 — simple cuboidal; 3 — simple columnar; 4 — simple ciliated;

5 — pseudostratified columnar; 6 — pseudostratified ciliated; 7-8 — stratified epithelium.

Endocrine glands lack ducts and release their secretions (Hormones) directly into the bloodstream (Pituitary Gland, Pineal Gland, Thyroid Gland, Thymus). There are also mixed glands, certain parts of which function as exocrine glands, while others function as endocrine glands (Pancreas, Gonads).

Epithelial tissues lack Blood Vessels and receive nutrients through narrow intercellular spaces. However, as a barrier tissue, epithelial tissue is richly supplied with nerve endings.

Despite differences in Structure and function, epithelial tissues are characterized by a high capacity for renewal — regeneration (from the Latin regeneratio — rebirth, restoration) and an extremely low content of intercellular substance.