•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

MuscleNervous

• 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 epitheliaClassification 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 specializationsMicrovilli- 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 adaptationsA) SECRETORYi) 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. 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.

• 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.