Histology. Epithelial Tissue

Transcription

1 Histology Epithelial Tissue

2 Epithelial Tissue Lines internal and external body surfaces Forms glands

3 Epithelial Tissue Little extracellular matrix Attached on one side Avascular Basement membrane Apical surface

4 Epithelial Tissue Cell Junctions Desmosomes: Anchors Tight junctions Seals Gap junctions Communication

5 Classification Number of layers, cell shape

6 Classification Number of layers, cell shape Number of layers: Simple (one layer) Stratified (multiple layers)

7 Classification Number of layers, cell shape Cell shape Squamous (flat) Cuboidal (cube-shaped) Columnar (elongated)

8 Classification

9 Classification Check your understanding

10 Structure & Function Simple squamous Diffusion/filtration (thin)

11 Structure & Function

12 Structure & Function Simple cuboidal & simple columnar Secretion, absorption, movement.

13 Structure & Function

14 Structure & Function Pseudostratified columnar Secretion (mucous), movement

15 Structure & Function

16 Structure & Function Stratified Squamous Protection Can be keratinized or moist.

17 Structure & Function

18 Structure & Function Transitional Stratified & stretchable

19 Structure & Function

20 Glands Secrete substances onto a surface, into a cavity or into the blood. Exocrine vs. endocrine glands

21 Histology Epithelial Tissue 1

22 Epithelial Tissue Lines internal and external body surfaces Forms glands 2 There are four types of tissues in the human body (and we re going to march through them all in this unit): epithelial tissue, connective tissue, muscular tissue, and nervous tissue. Epithelial tissue is composed of cells that line all body surfaces both external surfaces (skin and mucous membranes) and internal surfaces (lining all body cavities, blood vessels, organs, etc.) Epithelial tissue also form glands, which secrete substances into a cavity (such as the stomach), onto a surface (such as skin, mucous membrane, serous membrane, etc.) or into the blood.

23 Epithelial Tissue Little extracellular Apical surface matrix Attached on one side Avascular Basement membrane 3 All tissues are composed of two components: the living cells themselves and nonliving extracellular matrix. The extracellular matrix is composed of structural protein fibers, minerals, and fluid. One characteristic of epithelium is that it has very little matrix. In other words, epithelium consists mostly of cells packed very closely together with very little space between the cells. This makes sense since the job of epithelium is to line the body s surfaces. Structurally, since epithelium lines surfaces, all epithelial tissues are connected to other body tissues on exactly one side. The side of the epithelium that is not attached is called the apical surface (or, simply, free surface) while the other side is attached to a basement membrane, usually made of connective tissue.

24 Epithelial Tissue Cell Junctions Desmosomes: Anchors Tight junctions Seals Gap junctions Communication 4

25 Classification Number of layers, cell shape 5 We classify most epithelium in two ways: by the number of layers of cells the tissue has and the shape of the cells. In the figures, notice that cells can be flat, squarish, or elongated. Also note that some epithelium has many layers (like the one on the left) while some has just one layer (all the rest). From left to right, these figures show vaginal epithelium, kidney tubules, the lining of the small intestine, and the lining of an alveolus in the lungs.

26 Classification Number of layers, cell shape Number of layers: Simple (one layer) Stratified (multiple layers) 6 If an epithelial tissue has just one layer of cells attached to a basement membrane, we call it simple epithelium. Simple epithelium is useful when we want to move substances across the tissue. Simple epithelium is found, for example, lining the capillary walls (we want to move gasses and nutrients), the lining of alveoli in the lungs (we want to move gasses), the lining of the small intestine (we want to move nutrients), the kidney tubules (we want to move ions and water), etc. If an epithelial tissue has more than one layer, we called it stratified epithelium (stratified means layered. ) We find stratified epithelium in places where we need to protect against abrasion and prevent the movement of substances (such as water and pathogens) across the tissue, most notably in the skin (cutaneous membrane) and mucous membranes.

27 Classification Number of layers, cell shape Cell shape Squamous (flat) Cuboidal (cube-shaped) Columnar (elongated) 7 If the cells in epithelial tissue appear flat, kind of stretched-out and spindleshaped, we call the tissue squamous epithelium. When stratified, sqamous epithelium is good for protecting against abrasion because the top layers can be scraped off without harming deeper layers. When simple, squamous epithelium is good for allowing substances to diffuse across. When epithelial cells are more-or-less square-shaped or round, we call the tissue cuboidal epithelium (don t confuse this with cubital, which means elbow. ). The larger size of cuboidal cells means they have more organelles (ER and Golgi apparatus primarily). They re found in places where we want to absorb or secrete substances. If epithelial cells are fat and elongated, we call the tissue columnar epithelium. Columnar epithelium has even bigger cells than cuboidal epithelium so you can probably imagine they are great for diffusion and secretion. A special type of columnar epithelium is a goblet cell. Goblet cells are single-celled glands that secrete mucous in various areas of the body (stomach, respiratory tract, vagina). The apical surface of columnar cells may have cilia attached (like in the respiratory tract to move mucous up and out) or have microvilli (like in the small intestine to increase cell surface area).

28 Classification 8 Quiz yourself about what is represented by each picture. In part A, the top figure shows simple epithelium and the bottom figure shows stratified epithelium. In part B, the top pair of figures shows squamous cells. The middle pair shows columnar cells. The bottom pair shows columnar cells.

29 Classification Check your understanding 9 Name these tissues. Going from left-to-right and top-to bottom: stratified squamous, simple columnar, simple cuboidal, stratified cuboidal, simple squamous (this slide is a little different because it s a surface view rather than a cross-section), simple cuboidal.

30 Structure & Function Simple squamous Diffusion/filtration (thin) 10 There are five specific types of epithelium that you should be familiar with. The things I want you to know for each type are: 1) you should be able to identify the epithelium on sight, from micrographs like the ones in this lecture, 2) you should be able to deduce a location in the body (an organ) where the tissue might be found, and 3) you should be able to explain the function of the tissue (based on its structure). Simple squamous epithelium is found lining surfaces where diffusion or filtration are important. Since the tissue has just one layer, and since that layer is so thin, simple squamous epithelium is ideal for letting substances cross the epithelial layer without processing. For example, it lines the alveoli of the lungs, where nonpolar oxygen and carbon dioxide need to cross between the blood and the atmosphere; it lines the walls of capillaries, where gasses, nutrients, hormones, etc. need to cross between the blood and the interstitial space of tissues; etc. Since squamous cells are so thin and so small, they are not good for absorption or secretion they just don t have room for enough of the organelles (Golgi apparatus, ER, ribosomes, and lysosomes) that would be involved in absorption or secretion.

31 Structure & Function Click to add an outline 11 In the lungs, we don t want to prevent the exchange of gasses between the capillaries an the atmosphere, to simple squamous epithelium is ideal!

32 Structure & Function Simple cuboidal & simple columnar Secretion, absorption, movement. 12 Simple cuboidal and simple columnar epithelium are similar because their cells are larger than squamous cells (columnar is larger than cuboidal). Since the cells are larger, they can have more Golgi apparatus, ER and ribosomes (for secretion) as well as more lysosomes (for absorption). Consequently, epithelial tissues that are involved in absorption or secretion will often have simple cuboidal or simple columnar epithelium. Some simple cuboidal epithelium consists of goblet cells, which secrete mucous to trap junk that might otherwise get into the body. Goblet cells are usually ciliated (have cilia) to move the dirty mucous into the mouth, where it is eventually swallowed. Organs that are lined with simple cuboidal or simple columnar epithelium include the respiratory passages (usually simple columnar, to secrete mucous), the intestines (usually simple columnar, to absorb nutrients), the endocrine glands (usually simple cuboidal, to secrete hormones), etc.

33 Structure & Function Click to add an outline 13 Kidney tubules absorb ions and water; the stomach lining secretes acids and enzymes to aid in digestion. These are both ideal places for simple cuboidal or simple columnar epithelium.

34 Structure & Function Pseudostratified columnar Secretion (mucous), movement 14 Pseudostratified columnar epithelium is a strange tissue. When you look at it, it almost looks stratified because the nuclei of the cells are found at all sorts of different heights. In reality, though, it s simple epithelium because it s just one layer of epithelial cells. Pseudostratified columnar epithelium almost always secretes mucous and is almost always ciliated. The mucous traps particles, which are then swept away. This type of tissue is found in the trachea (ciliated; part of the respiratory tract) and the male urethra (unciliated; part of the male reproductive system).

35 Structure & Function Click to add an outline 15 Since the nuclei are at all different levels, pseudostratified epithelium looks stratified, but it s really just one layer (simple).

36 Structure & Function Stratified Squamous Protection Can be keratinized or moist. 16 Stratified squamous epithelium is found lining many surfaces that are exposed to the outside world (both cutaneous membrane [skin] and mucous membrane). As we ll learn later, the superficial layers of stratified squamous epithelium are really non-living cells that are filled with a tough, waterproof protein called keratin. There are multiple layers to protect against abrasion rubbing against stratified squamous tissue may pull off or damage a few layers, but the underlying tissues are still safe. The cutaneous membrane (outer skin) consists of dead cells filled with keratin, so we call it keratinized. It s tough, strong, and waterproof. Mucous membranes, though, do not have much keratin (and have fewer layers). We call them moist. Recall that mucous membranes line most cavity surfaces that are inside the body, but open the outside world (oral cavity, nasal cavity, orbital cavity, vaginal cavity, anal cavity) We find stratified squamous epithelium wherever movement and abrasion is an issue the skin, the esophagus, the oral cavity, etc.

37 Structure & Function Click to add an outline 17 The esophagus takes a lot of abuse. The cells are assaulted every time we swallow or cough. Stratified squamous epithelium ensures that even if the top layers of cells flake off, underlying tissue is not hurt.

38 Structure & Function Transitional Stratified & stretchable 18 Transitional epithelium is another weird tissue. The cells in transitional tissue have the ability to change shape when the organ it s lining expands. When the organ is empty, transitional epithelium looks columnar (pseudostratified, in fact). As the organ expands, though, transitional epithelium starts to look more stretched out and squamous. Transitional epithelium lines organs that need to stretch out, such as the urinary bladder and the female uterus.

39 Structure & Function Click to add an outline 19 Transitional epithelium lining the urinary bladder changes shape as the bladder fills. There aren t many micrographs of expanded transitional tissue. Can you imagine why?

40 Glands Secrete substances onto a surface, into a cavity or into the blood. Exocrine vs. endocrine glands 20 Glands are organs that secrete a substance (usually a protein, but sometimes water, ions or lipids) out of the cell. These substances can be secreted onto a surface, into a cavity or into the blood. Endocrine glands line the capillaries of certain organs and secrete their substances (hormones; proteins, amino acid derivatives or cholesterols) into the blood stream. Exocrine glands secrete their substances (proteins, ions and water) into cavities or onto surfaces. Most exocrine glands line ducts pockets in the epithelium lined with exocrine epithelial cells. Ducts increase the surface area of the gland and result in more secretion. For example, the pores in your face are mostly sebaceous glands pockets lined with exocrine cells that secrete an oily substance called sebum.