Human Anatomy Chap 3.

Tissue

 is formed by groups of similar cells, though there may be some different cells in certain types, and extracellular matrix (ECM) between cells, with the amount varying, to perform a common function.

In turn forms an organ when at least two are combined. Has four basic types.

What are the four most basic types of tissue?

Epithelial, connective, muscle, and nervous tissue.

Nervous tissue

Transmits electrical, nerve impulses, processes sensory information, and controls the functions of other tissues. Contains neurons and glial cells, which support, protect and nourish neutrons. Has no subtypes.

Muscle tissue

Allows movement with the skeleton, organ walls, and body structures by contracting force, being conductive and elastic extensible. Is made of contractile proteins, myofilaments, and cylindrical, branching, or spindle shaped cells.

Subtypes include skeletal, cardiac and smooth.

Connective tissue

Supports, strengthens, fills gaps, and binds other tissue types together. Composed from cells, protein fibers, and ground substance.

Subtypes include connective tissue proper, supporting connective tissue, and fluid connective tissue (blood, lymph).

Epithelial tissue

Thin sheets that cover all internal and external body and organ surfaces, lining body cavities and organ cavities, and forms glands. Is composed of tight packed cell within minimal extracellular matrix (ECM).

Subtypes include simple epithelium and stratified epithelium.

What are the functions of epithelial tissues?

Having many cells gives regenerative properties that allows them to act as physical protection and thicker types act as barriers. Thinner subtypes are permeable, but selective, regulating what enters and exits our body, participating in diffusion, osmosis, and absorption.

Additionally contains cells that secrete various substances depending on the situation, being used most in glands. Also has lots of nerve and sensory receptors, allowing for sensation to be detected.

How are epithelia tissues named?

They get a two-part name indicating the number of layers they have and the shape of their cells.

Columnar epithelial tissues

Cells that are taller than they are wide.

Cuboidal epithelial tissues

Cells with a square shape.

Squamous epithelial tissues

Cells that are flat, like a fried egg shape.

Stratified epithelia

Epithelial tissue that has two or more layers of cells

Simple epithelia

Epithelial tissue that only have one cell layer

Simple squamous epithelium

The thinnest and weakest of all epithelia, that consists of a single layer of flattened cells supported by a basement membrane. Viewed at a cross-section, it appears as a very thin cell and a squished nucleus in the middle (like a fried egg).

Being thin, it’s permeable. Is found in the lungs as the alveoli, the air sacs diffuse gas, the kidneys where it filters. Also makes up serous membranes, being alternatively called mesothelium, and lines the blood vessels where it’s named endothelium.

kidney tubules

structures in the kidneys that process fluid. Useful material is returned to blood while wastes are sent to the urinary bladder (and become urine).

Simple cuboidal epithelium

Ideal for absorption, moving material from filtered fluid back into blood. Is found lining kidney tubules where useful material is returned to blood and wastes are sent to the urinary bladder. Additionally good for secretion activity, being found in many exocrine glands.

Simple columnar epithelium

Consists of a single layer of columnar (column shaped) cells, occasionally goblet cells, cup shaped cells that produce secretions and mucus, are also found. Large nuclei can be found in the middle of the cells or lined up closely to the basal surface. Secretes enzymes to break down food, absorbs digested food, secretes mucus to trap dust, disposes of mucus with cilia, and also moves egg cells.

Has two different types: Non-ciliated simple columnar epithelium and ciliated simple columnar epithelium

Ciliated simple columnar epithelium

A type of simple columnar epithelium that has a cilia on the apical surface which moves mucus and microscopic particles across the apical surface. Is found in the respiratory tract as it secretes mucus to trap dust and then moves the mucus with cilia to dispose of it.

Also found in the uterine tubes, where the cilia moves egg cells from the ovaries to the uterus.

Pseudostratified Columnar Epithelium

An epithelium that appears to be more than one layer of columnar cells, but it is actually only one layer thick with varying shapes, having some cuboidal near the basal surface. In actuality, all the cells are in contact with the basement membrane. Not all the cells reach the apical surface, but this still satisfies the classification of a "simple" epithelium.

Has two types: Ciliated pseudostratified columnar epithelium and Non-Ciliated pseudostratified columnar epithelium

Non-Ciliated pseudostratified columnar epithelium

is also good for protection but not secretion (goblet cells are not present). This epithelial tissue is rare, it is only found in sections of the male urinary tract and reproductive structures

Ciliated pseudostratified columnar epithelium

has cilia, and it is good for protection (it is thicker than simple columnar) and secretion of mucus (goblet cells are present). This epithelium is found in the larger airways of the respiratory tract, including the nasal cavity, trachea, and bronchi.

Non-ciliated simple columnar epithelium

A type of simple columnar epithelium that lacks a cilia on the apical surface, having a microvilli in it’s stead which increases surface area. This epithelium is good for absorption and secretion, and lines most of our digestive tract. Secretes enzymes to break down food and is helped by the microvilli to absorb digested food

What are the characteristics shared between all types of epithelium tissues?

Cells are closely joined, they always have an apical surface, are always attached to a basement membrane, lacks blood vessels making them thin, has plenty nerves, and can regenerate due to high cell density.

Avascularity

 the absence of blood vessels.

Innervation

Presence of nerves

Apical surface

the free outer edge of epithelial tissues that faces either the inside of the body or the outside environment, being the exposed surface. Can have a cilia or microvilli.

What are the characteristics of connective tissues?

Cells are few and scattered, being mostly extracellular matrix, cannot regenerate as easily due to have less cells to divide, but has many blood vessels and nerves

stratified squamous epithelium.

The most abrasion-resistant epithelium which consists of multiple layers of cells; the younger cells (deeper, closer to the basement membrane) can be rounded in shape or cuboidal. Older cells (nearer the apical surface) flatten, taking on the classic squamous shape.

Has two versions: keratinized stratified squamous epithelium and non-keratinized stratified squamous epithelium.

Keratinized stratified squamous epithelium

A type of stratified squamous epithelium, perfect for the epidrmis, where as the older cells get pushed toward the apical surface, they fill up with a strong protein, keratin. Half-way to the apical surface, cells flatten, lose their organelles, and die, being too far from supportive connective tissue and running out of nutrients.

The dead, intact cells are held together in a thick layer by keratin which protects the living cells under and prevents water loss.

Non-Keratinized stratified squamous epithelium

A type of stratified squamous epithelium found in the body where abrasion is an issue and is found lining tracts that need protection from abrasion: mouth (oral cavity), larynx (back of the oral cavity), esophagus (tube that swallows food), anus, and vagina. Cells don’t fill with keratin, staying alive all the way to the apical surface.

Transitional Epithelium

A unique stratified epithelium that consists of several layers of randomly shaped cells; some are rounded, some are almost cuboidal. Some cells have two nuclei. Is only found lining the urinary bladder and sections of the urinary tract.

As urine is made and stored in the urinary bladder, the walls of the bladder need to able to stretch and still maintain strength.

The apical cells can stretch and flatten, but the cell-to-cell attachments (tight junctions) prevent tearing, maintaining structural integrity.

Glands

structures that specialize in secretion, producing and releasing a variety of specific substances, including hormones, enzymes, waste products, sticky fluid (mucus), lubricating fluid, and ions and water.

Small ones are mostly constructed of epithelial tissue, larger ones contain epithelia supported by connective tissue. Has two general types: endocrine glands and exocrine glands

Endocrine Glands

A type of gland that produce secretions that go directly into the interstitial fluid (between cells) and then into blood (to be distributed around the body).

They produce hormones, chemical messengers that communicate widely. Examples include testosterone, insulin, cortisol, and growth hormone.

Unicellular Exocrine glands

Glands found embedded in epithelia. They secrete directly onto the surface of the epithelia (no duct necessary). The most common are goblet cells which produce mucin, a protein that becomes mucus when mixed with water. Mucus can trap particles and lubricate the passage of larger material.

Acinar cells

Cells responsible for doing secretion and that form the blind end of the exocrine gland, the acinus.

Compound glands

Exocrine glands with several branching ducts

simple glands

Exocrine glands with a single, unbranched duct

Multicellular Exocrine glands

Consists of an acinus, made up of acinar cells, and a duct portion, made from other cells, which conducts secretions towards a surface. Larger glands have divisions, lobes, made up of multiple acini and ducts, which are divided by a septa.

Serous glands

secrete the thin, watery secretion that makes serous fluid.

Holocrine glands

A type of exocrine gland that synthesizes a lipid-rich secretion which is released via the entire cell disintegrating, causing secretion to be a mix of the secretory substance and cell fragments.The basal cells divide and replace the disintegrated cells. These glands can secrete an oily (lipid-rich) secretion, for example sebaceous glands of the skin produce the oil that protects our skin from drying out.

Apocrine glands

A type of exocrine gland that synthesizes their secretion as is released by a section of the cell’s apical portion pinching off the original cell and floating into a duct. The remaining portion repairs itself and the process repeats. These secretions have some lipid, such as mammary glands producing milk.

Merocrine glands

A type of exocrine gland that synthesizes their secretion and packages them into secretory vesicles. The contents can be easily released by exocytosis, allowing vesicles to burst from the cell’s apical surface. Typically make watery, hydrophilic secretions like salivary or sweat glands.

Exocrine glands

A type of gland that produces a wide variety of substances that are released onto the epithelium’s apical surface. This is done through several different mechanisms

Most are composed of epithelium, but can be buried deep into connective tissue. Some can be unicellular

Secretions are usually transported to the surface via a tube called a duct (unless the gland is made out of a single cell). Secretions include enzymes, sweat, mucus, waste products, and many others.

Stratified columnar epithelium

consists of several layers of cells, again with the apical cell layers taking on the columnar shape. This epithelium is not common; it is found in salivary glands, parts of the eyelid, and sections of the male urinary tract only.

Stratified cuboidal epithelium

consists of two or more layers of cells, with the older cells (near the apical surface) taking on the cuboidal shape. This epithelium is found in some glands (good for secretion), especially sweat glands.

Stratified epithelium

Epehlial tissue with multiple layers of cells where only the basal layer of cells is in contact with the basement membrane. These cells constantly divide (they are often referred to as stem cells), replacing dead cells. As cells age, they are pushed away from the basal layer up towards the apical layer. Cells on the apical surface die and fall off.

Epidermis

The epithelium that is the outer layer of our skin.