The Tissues A tissue consists of a group of cells which are similar in structure and carry out the same function(s). There are four primary types of tissues found in the body: 1.Epithelia – An epithelium is a layer(s) of contiguous cells that covers the external and internal free surfaces of the body, e.g., surface of skin or inner surface of a small blood The epithelia carry out the functions of protection, absorption, filtration, excretion, secretion and sensation. 2.Connective Tissues – These tissues are the most widely distributed tissues in the body. They function to bind structure together, support the body, protect other organs, insulate the body from impact and heat loss and (as blood) transport materials throughout the body. 3.Muscle Tissues – Permit the movements of the body and its parts (skeletal muscle), move blood throughout the body (cardiac muscle) and regulate the activities of internal organs (smooth muscle). 4.Nerve Tissue – Regulate and coordinate the activities of the entire body.

Remove skin cell from adult Collect unfertilized oocyte Collect DNA from oocyte Fuse skin cell with oocyte Embryo forms stem cells (arrow) Pancreatic beta cells blood cells cardiac muscle cells liver cells neurons Formation of Stem Cells to Produce Various Tissues

The Epithelial Tissues Classification of Epithelia by Cell Shape: There are four categories of epithelia based on cell shape: Squamous – Cells are flat or shield-like. Cuboidal – The cells have the same height as width. They appear box-like. The nucleus is spherical and found in the center of the cell. Columnar – The cells are taller than wide. The nucleus is oval and tends to be found near the bottom of the cell. The apical surface of columnar epithelial cells often shows the presence of microvilli and cilia. Transitional epithelium – The appearance of transitional epithelial cells will vary with distension of the urinary organ which they line.

Classification of Epithelia by Number of Layers or Strata of Cells 1. Simple – An epithelium that is one cell thick. 2. Stratified epithelium - the identity of the epithelium is based on the appearance of the uppermost cells. 3. Pseudostratified - a columnar epithelium that appears stratified but is not. The misleading appearance of the epithelium is due to the crowding of adjacent cells

Characteristics of Epithelia 1. Epithelial cells exhibit polarity, i.e., they have a top (apical end) and a bottom (basal end). 2. All epithelia have a basement membrane. This structure is not a cell membrane or membranous layer. It consists of two components: a. Basal lamina - a filamentous sheet attached to the basal surface of the epithelial cell. It is a product of the epithelial cell, itself. b. Reticular lamina - located under the basal lamina of most basement membranes, consisting of a condensed ground substance mixed with collagenous fibers. The basement membrane supports the epithelium and functions as a semi permeable filter separating the epithelium from the underlying tissues. 3. Epithelia are avascular, i.e., epithelia lack a direct blood supply. The cells of an epithelium must receive nutrients and oxygen from and eliminate wastes to capillaries across the basement membrane. Transport of these materials is achieved by diffusion.

4. Epithelia are capable of considerable regeneration. Surface cells are constantly lost due to abrasion, microbial activity, toxic substances or extremes of temperature. 5. The free or apical surface of the epithelial cell often shows specializations in structure: a. Nonmotile processes such as microvilli, serve to increase the surface area of the cell and, therefore, facilitate transport of materials into and out of the cells. Sensory hairs are receptor processes found in certain sensory epithelia concerned with taste (taste buds), smell (olfactory epithelium). b. Motile Processes like the cilia are short extentions of the cell membrane. Cilia can be seen on the free surfaces of epithelia lining the respiratory tract and oviducts tubes in mammals. A flagellum has the same internal structure as a cilium but is much longer and is found on a sperm cell.

6. A variety of intercellular junctions can be seen between the cells of an epithelium. 1. Desmosomes are dense regions of attachment between epithelial cells. 2. "tight junction" seems to prevent materials from the intestinal lumen from leaking into the intercellular spaces of the epithelium. 3. A gap junction represents a very small (20A) continuity between the cytoplasms of adjacent cells. They appear to represent sites of cell to cell communication.

Simple Squamous Epithelium Simple squamous epithelium consists of a single layer of flat, shield-like cells. In a cross sectional view, the cells bulge due to the presence of the nucleus This tissue may have a protective function or perform secretory functions. The image shows this tissue lining the alveoli of the lungs. It also lines the blood vessels (endothelium) and the chambers of the heart (endocardium). It also lines the body cavities as the mesothelium of the peritoneum.

Simple Columnar Epithelium Simple columnar epithelium consists of cells which are taller than wide (*). This tissue lines the alimentary canal from the stomach to the rectum.A number of mucous producing cells can be seen in this view. *

Simple Cuboidal Epithelium Simple cuboidal epithelium consists of short cube, prism or trapezoid-shaped cells (*). The nuclei are large, spherical and centrally located in the cells. This tissue often has a secretory function. Found in many glands both exocrine and endocrine, for example, lining thyroid follicles and ducts of sweat glands. They are especially common in kidney tissue. *

Stratified Squamous Epithelium (non-keratinized) The stratified squamous epithelium that lines interior spaces of the body does not form keratin or a dead surface layer. In a vertical cross section, the deepest layer rests on a basement membrane and is cuboidal in shape This epithelium is located lining the mouth, esophagus and portions of the pharynx and larynx

Stratified Squamous (keratinized) Stratified squamous consists of several layers of generally flat cells (*) which rest on a supporting layer of connective tissue. In skin the cells of the stratified squamous produce a tough fibrous protein called keratin which waterproofs the skin. On the top of the epithelium the cells die (arrow) forming the stratum corneum. *

Pseudostratified Ciliated Columnar As its name implies, pseudostratified columnar appears stratified but is not. Each cell in this tissue touches the basement membrane. This tissue is often ciliated (arrow) and is richly endowed with mucous producing cells. Pseudostratified epithelium rest on a supporting connective tissue called the lamina propria (lp). The lamina propria consists mainly of areolar connective tissue. lp

Transitional Epithelium Transitional epithelium is located exclusively in the urinary system. It is found lining the pelvis of the kidney, the ureter, urinary bladder and a portion of the urethra. This epithelium rests on a lamina propria of areolar tissue. It is capable of great distension. The cells of transitional epithelium appear balloon-like when the bladder is empty of urine.