Welcome to lab classes in basic histology. 33 Introducing lab class (in spanish)

The aim of these lab classes is to provide students with educational resources to acquire basic practical skills of each subject, recognize, locate and describe cell types and basic tissues.

Each of these practical sessions are structured around three main elements:
- Definition of learning objectives.
- Description of basic tissues and their cell types.
- Exercices location of cell types and basic tissues in virtual slides.

LEARNING OBJECTIVES
- Identify a simple squamous epithelial tissue.
- Identify a simple cuboidal epithelial tissue.
- Identify a simple columnar epithelial tissue.
- Identify a stratified squamous epithelial tissue.
- Identify a stratified cuboidal/columnar epithelial tissue.
- Identify a pseudostratified epithelial tissue.
- Identify transitional epithelial tissue.
- Identify goblet cells.
- Identify microvilli and cilia cells.

MAIN FEATURES OF EPITHELIAL CELLS
Epithelial tissue is formed by tightly bound cell layers that cover body surfaces (epithelial lining or cover). Also, epithelial tissue form secretory glands (epithelial glands). However, we can find a cover epithelial tissue with scattered secretory cells.

Epithelial cells show different morphologies, ranging from a flattened cell to a prismatic cell, but have a common polyhedral shape because epithelial cells are grouped into layers or masses tightly juxtaposed.

Epithelial cells nuclei also have different morphologies, ranging from a spherical or oval nucleus to elliptical nucleus. Nuclear morphology often is in agree to the cell morphology, cuboidal cells have a spherical nucleus while flat cells have a flattened nucleus.

Epithelial tissues rests on a thin layer of connective tissue called lamina propria. The main function of lamina propria is to provide physical and metabolic support to the epithelium.

Epithelial cells shows polarity. Organelles and membrane proteins are distributed in different parts of the cell. The cell membrane faced the connective tissue is called the basal pole. Between cell membrane and lamina propria, a structure called basement membrane are find. This structure binds epithelial cell membrane to the adjacent lamina propria. Opposite the basal pole there is the apical pole. Apical pole may have several cell membrane specializations such as microvilli 34 Digestive duct, Brush border (microvilli) 35 Digestive duct, Brush border (microvilli), transverse sectioned , stereocillia and cilia. Finally, cell membrane in contact with adjacent cell membranes are the side poles. Side poles shows structures related to cell junctions.

COVERING EPITHELIAL TISSUE
The covering epithelial tissue are made by cell layers that cover the outer surface or body cavities. Epithelial cover tissue is classified according to the number of layers and the morphological characteristics of the surface cells layer. A simple cell layer defines the Simple Epithelial Tissue while more than one cell layer defines the Stratified Epithelial Tissue.
Based on cell shape, simple epithelia are classified as squamous (thin cells), cubic (about as wide as thick cells) or columnar or prismatic (taller than wide cells).

SIMPLE SQUAMOUS EPITHELIAL TISSUE 1 Simple squamous epithelial tissue
The simple squamous epithelial tissue consists of a single layer of flat and tightly cells. The cells have a single flattened prominent nucleus 7 Epithelial squamous tissue surface view .
Simple epithelial tissue are found in blood 12 Muscular artery, longitudinal section and lymph vessels, which are called vascular endothelium; on the peritoneum, pleura 13 Lung, Visceral Pleura and pericardium covers, which are called the mesothelium of serous membranes; in alveoli 14 Human Lung Alveoli, H-E 1,5 um , which are called pneumocytes; and lining the loops of Henle and Bowman's capsule 31a Kidney (Cortex) H-E 1,5um of the kidney.
The main functions of simple epithelial tissue are coating and lubrication (mesothelium), and rapid exchange of substances (different metabolites, water and mineral salts in kidney, blood and lymph vessels; and gases in lungs).

SIMPLE CUBOIDAL EPITHELIAL TISSUE 2 Simple cuboidal epithelial tissue
The simple cubic epithelial tissue is formed by a highly polarized epithelial cell layer with a central spherical nucleus.
The main functions of this epithelium are absorption, secretion (thyroid gland 15 Human Thyroid, H-E 1,5 um ) and the active transport of ions (convoluted tubules and collecting ducts of the kidney 16 Kidney (Medulla) H-E 1,5um ), and forming the germinative layer of the ovary 17a Ovary Masson's trichrome 7um 1 17b Ovary Masson's trichrome 7um 2 17c Ovary Masson's trichrome 7um 3 17d Ovary Masson's trichrome 7um 4 .

SIMPLE COLUMNAR EPITHELIAL TISSUE 3 Simple columnar epithelial tissue
Simple columnar epithelial tissue is formed by taller than wide cells arranged in a layer, with nucleus in central position.
This epithelia shows scattered unicellular glands called goblet cells 4 Goblet cells 5 Goblet cells , a mucous cell involved to protect them against external agents and other substances.
The highly polarized cells can contain the brush border (small intestine 18 Small intestine (ileum) H-E 7 um and proximal tube of kidney), a structure involved in nutrients absorption or cilia (uterus 19a Uterus (secretory phase) H-E 1,5um 1 19b Uterus (secretory phase) H-E 1,5um 2 , oviduct 20 Oviduct Masson's trichrome 7um , ductus deferens 21 Ductus deferens H-E 1,5um , small bronchioles and sinuses) related by the movement of substances into the duct.

STRATIFIED SQUAMOUS EPITHELIAL TISSUE 6 Stratified squamous epithelial tissue
The stratified squamous epithelial tissue is formed by several layers of epithelial cells, also called keratinocytes. Keratinocytes shows different morphology, from cuboidal cells in the innermost layer in direct contact with the basement membrane to flat cells at the epithelial surface. Flat cells are precisely those that give the name of this epithelial type.
This type of epithelium is highly specialized in the protection and waterproofing to prevent water loss. We find this epithelial tissue in mouth, esophagus 32 Esophagus H-E 7 um , larynx, vagina 22 Vagina H-E 1,5um , anal canal and epidermis 23 Meissner corpuscle (skin) H-E 1,5um , where the epithelium is keratinized 28 Stratified squamous epithelium Masson's Trichrome 7 um .
A vision of the epithelial surface using the optical microscope shows a mosaic of tightly flat cells 7 Epithelial squamous tissue surface view .
This epithelium has a functional organization in layers, where we can differentiate 6 Stratified squamous epithelial tissue :
- Basal layer. Prismatic or high-cubic stem cell layer, resting on the basement membrane.
- Spinous layer. Polygonal flattened keratinocytes with a round nucleus. The cytoplasm shows small granules or lamellar bodies. Bundles of intermediate filaments or tonofilaments extend the cytoplasm in thorn-like processes that attach to dense plaques of desmosomes.
- Stratum granulosum. Flattened nucleated keratinocytes containing the characteristic irregular keratohyalin granules associated with tonofilaments. Lamellar bodies increase in number, releasing their contents in the extracellular space, forming a layered structure by coating the surface of keratinocytes in the upper levels of the stratum. The cover glycolipid acts as a barrier to prevent water loss.
- Stratum corneum. Anucleate keratinocytes arranged of multiple layers. Keratin intermediate filaments filled cytoplasm cell.

CUBIC OR PRISMATIC STRATIFIED EPITHELIAL TISSUE 8 Stratified cuboidal epithelial tissue 9 Stratified cuboidal epithelial tissue
Cubic or prismatic startified epithelial tissues are rare, and usually consist of two layers of cells, a basal layer in contact with the basement membrane and the apical layer in contact with lumen.
The main functions are protection developing a more robust boundary than to a simple cubic or prismatic non-stratified epithelium.
Cubic stratified epithelial tissue are located in the ducts of sweat glands, ducts of the mammary glands, ducts of the salivary glands and collector tubules in kidneys.
Prismatic stratified epithelial tissue can be found in conjunctiva membrane which limits the eyelids, which develops and protective functions of mucus secretion.

PSEUDOSTRATIFIED EPITHELIAL TISSUE 10 Pseudostratified columnar epithelial tissue
Epithelial pseudostratified tissue consist of basal cells and ciliated columnar cells resting on the basal lamina. Only the columnar cells reach the luminal surface, however.
In addition, this type of epithelium contain scattered unicellular mucus-secreting glands called goblet cells 4 Goblet cells .
The main functions of this epithelial type is the protection and transport of particles trapped in mucus secreted by goblet cells into the upper respiratory tract due to the presence of cilia (trachea 29 Monkey trachea, H-E 1,5 um and bronchi) and help spermatozoa in transit by the epididymis 25 Epididymis H-E 7um .

TRANSITIONAL EPITHELIAL TISSUE 11 Ureter, transitional epithelium
The transitional epithelium limits the urinary ducts (urinary bladder 26 Transitional epithelium (urothelium) H-E 1,5 um and ureter 27 Ureter H-E 1,5um ) and consists of two cell types. The columnar shaped-dome cells extending from basement membrane to lumen that may have two nuclei; and basal cells, strongly linked to basement membrane. The transition epithelial cells are able to stretching or contraction due to the accumulation of urine or miction.

EXERCICES
- Locate the keratinized stratified squamous epithelial tissue in the virtual slide. 28 Stratified squamous epithelium Masson's Trichrome 7 um 32 Esophagus H-E 7 um
- Locate blood vessels in virtual slides. Observe endothelial cells. 28 Stratified squamous epithelium Masson's Trichrome 7 um
- Locate non keratinized stratified epihtelial tissue in virtual slide. 22 Vagina H-E 1,5um
- Locate a mitotic cell in basal layer of the statified squamous epithelial tissue. 32 Esophagus H-E 7 um
- Locate the transitional epithelium in virtual slide. Observe and describe cell type. 27 Ureter H-E 1,5um
- Locate pseudostratified epithelial tissue in virtual slide. Observe and descibe cell type. 24 Tracheal wall, H-E 1,5 um
- Locate simple columnar epithelial tissue in virtual slide. 30a Large intestine (colon) H-E 1,5 um (1) 30b Large intestine (colon) H-E 1,5 um (2)
- Locate goblet cells in virtual slides. 24 Tracheal wall, H-E 1,5 um 32 Esophagus H-E 7 um
- Locate simple cuboidal tissue in virtual slide. 31a Kidney (Cortex) H-E 1,5um 31b Kidney (Cortex) H-E 1,5um
- Locate cilia cells in virtual slides. 25 Epididymis H-E 7um 24 Tracheal wall, H-E 1,5 um
- Locate microvilli cells in virtual slide. 18 Small intestine (ileum) H-E 7 um