Major types of adult tissues Tissues: collections of similar cells and the substances surrounding them Tissue classification: based on structure of cells, composition of non cellular extracellular matrix, and cell function Major types of adult tissues Epithelial Connective Muscle Nervous

3 major germ layers that form the embryonic disc (source of stem cells) Endoderm Inner layer Forms lining of digestive tract and derivatives Mesoderm Middle layer Forms tissues as such muscle, bone, blood vessels Ectoderm Outer layer Forms skin and neuroectoderm

Epithelial tissue

Epithelium Cellularity - Consists almost entirely of cells Covers body surfaces, lines hollow organs, and forms glands Outside surface of the body Lining of digestive, respiratory and urogenital systems Heart and blood vessels Linings of many body cavities Polarity - Has apical, basal, and lateral surfaces Rests on a basement membrane Specialized cell contacts bind adjacent cells together Avascular - no blood vessels Regenerative -Replaces lost cells by cell division

Special characteristics

Functions of epithelial tissue Protection: Epithelial cells protect underlying tissue from mechanical injury, harmful chemicals and pathogens and excessive water loss. Sensation: Sensory stimuli are detected by specialised epithelial cells. Specialised epithelial tissue containing sensory nerve endings is found in the skin, eyes, ears and nose and on the tongue. Secretion: In glands, epithelial tissue is specialised to secrete specific chemical substances such as enzymes, hormones and lubricating fluids. Absorption: Certain epithelial cells lining the small intestine absorb nutrients from the digestion of food. Excretion: Epithelial tissues in the kidney excrete waste products from the body and reabsorb needed materials from the urine. Sweat is also excreted from the body by epithelial cells in the sweat glands. Diffusion: Simple epithelium promotes the diffusion of gases, liquids and nutrients. Because they form such a thin lining, they are ideal for the diffusion of gases (e.g. walls of capillaries and lungs).

Classification of epithelia Classification of epithelial cells is based on a number of individual components including cell arrangement, cell shape, surface specializations and cell adaptations. 1. Cell arrangement Simple: single layer of cells. Each extends from basement membrane to the free surface Stratified: composed of several layers of cells. Pseudostratified: cells appear to be several layers however all are in contact with underlying matrix.

Classification by Cell shape Squamous: flat, scale-like Cuboidal: about equal in height and width Columnar: taller than wide

Simple Cuboidal Epithelium

Simple Columnar Epithelium

Transitional Epithelium

Surface specializations Microvilli finger-like extensions of plasma membrane abundant in epithelia of small intestine and kidney maximize surface area available for absorption of nutrients or across which molecules enter or leave Cilia whip-like, highly motile extensions of apical membranes move fluid or mucous over the surface of the cell confer cell motility

Epithelial cell adaptations A) SECRETORY Mucous secreting epithelial cells contain goblet cells which secrete mucous. The mucous acts: as a lubricant in the mouth and vagina. To protect the lining of the stomach from destruction by the acids present.

ii) Steroid secreting epithelial cells secrete hormones ii) Steroid secreting epithelial cells secrete hormones. Examples: adrenal glands, ovary and testis. iii) Ion pumping epithelial cells allow ions to be transported across a membrane. Examples: kidney tubules transport ions and water, stomach transports H+ ions.

Two types of glands formed by infolding of epithelium: Glandular epithelium: A gland is one or more cells that makes and secretes an aqueous fluid Two types of glands formed by infolding of epithelium: Endocrine: secrete hormones directly into the bloodstream, no contact with exterior of body; ductless; produce hormones (pituitary, thyroid, adrenals, pancreas) Exocrine: open to exterior of body via ducts (sweat, oil) Exocrine glands classified either by structure or by the method of secretion Classified by structure Unicellular: goblet cells Multicellular: sweat, oil, pituitary, adrenal

Glandular epithelia

Barrier Function Prevents diffusion of molecules between cells and therefore prevents diffusion of substances from one side of an epithelium to the other. Specializations which produce barrier functions are tight cell junctions, desmosomes and keratinization. Cell adhesions are mediated by cell junctions- form seals around cells and allow for contact and communication between cells.

Melanin formation Melanocytes are cells located in the bottom layer (the stratum basale) of the skin's epidermis, that produce melanin which is a pigment for skin& hair

Melanin formation

Albinism Albinism is an inherited condition present at birth, characterized by a lack of pigment that normally gives color to the skin, hair, and eyes. Many types of albinism exist, all of which involve lack of pigment in varying degrees. The condition, which is found in all races, may be accompanied by eye problems and may lead to skin cancer later in life. Albino lack an enzyme called tyrosinase. Tyrosinase is required for melanocytes to produce melanin from the amino acid tyrosine.