Welcome to lab classes in basic histology. 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 the loose connective tissue.
- Identify regular dense connective tissue.
- Identify irregular dense connective tissue.
- Identify collagen fibers, reticular fibers and elastic fibers.
- Identify fibroblasts.
- Identify the tissue, macrophages, mast cells and plasma cells.

MAIN FEATURES OF CONNECTIVE TISSUE
The connective tissue provides the supportive and connecting framework for all the others tissue of the body. The connective tissue is formed by cells and the extracellular matrix (ECM). The ECM represents a combination of collagens, noncollagenous glycoproteins, and proteoglicans (ground substance) surrounding the cells of connective tissue. The cells of connective tissue have important roles in the storage of metabolites, immune and inflamatory responses, and tissue repair after injury.
Unlike epithelial cells, wich are almost free of intercellular material, connective tissue cells are widely separated by components of the ECM. In addition, epithelial cells lack direct blood and lymphatic supply, whereas connective tissue is directly supplied by blood and lymphatic vessels and nerves.
Connective tissue can be classified into major groups: embryonic connective tissue, adult connective tissue, and special connective tissue.
The embryonic connective tissue is a loose tissue formed during early embryonic development. This type of connective tissue, found primarily in the umbilical cord, consists predominantly of a hydophilic ECM and therefore has a jelly-like consistency. Because of this consistency, it is also called mucoid connective tissue or Wharton’s jelly.
The adult connective tissue has considerable structural diversity because the proportion of cells to fibers and ground substance varies from tissue to tissue. This variable cell-to-ECM ratio is the basis for the subclassification of adult connective tissue into two types of connective tissue, the loose connective tissue and the dense connective tissue. In addition, reticular and elastic fibers predominate in irregular connective tissue.
The special connective tissue comprises types of connective tissue with special properties not observed in the embryonic or adult connective tissue proper. There are four types of special connective tissue: adipose tissue, cartilage tissue, bone tissue and blood tissue.

LOOSE (OR AREOLAR) CONNECTIVE TISSUE Loose connective tissue
Loose connective tissue contains more cells than collagen fibers and is generally found in the mucosa and submucosa of various organs and surround blood vessels, nerves, and muscles.

DENSE CONNECTIVE TISSUE Loose versus dense connective tissue
Dense connective tissue contains more collagen fibers than cells. When the collagen fibers are preferinitially oriented -as in tendons, ligaments, and the cornea- the tissue is called dense regular connective tissue Regular dense connective tissue . When the collagen fibers are randomly oriented -as in the dermis of the skin- this tissue is called dense irregular connective tissue Irregular dense connective tissue .

CELL COMPONENTS OF CONNECTIVE TISSUE Connective cells
Four major cell components of connective tissue are the fibroblasts, the macrophage, the mast cell, and the plasma cell.

FIBROBLASTS Fibroblast Fibroblast
Under light microscopy, the fibroblasts appears as spindle-shaped cell with an elliptical nucleus. The cytoplasm is very thin and generally not resolved by light microscopy. Under electron microscopy, the fibroblast shows the typical feature of a protein-secreting cell: a well-developed rough endoplasmic reticulum and Golgi apparatus. The fibroblast synthesizes and continuously secretes mature proteoglicans and glycoproteins and the precursor molecules of various types of collagens and elastin.

PLASMA CELLS Plasma cells Fibroblasts and plasma cells
The plasma cell, wich derives from the differentiation of B lymphocytes (also called B cells), synthesizes and secretes a single class of immunoglobulin. At the light microscopic level, most of the cytoplasm of plasma cells is basophilic because of the large amount of ribosomes associated with the endoplasmic reticulum. The plasma cell nucleus, in contrast to the fibroblast nucleus, has a characteristic cartwheel configuration created by the particular distribution of heterochromatin.

MAST CELL Mast cell Mast cell
Like macrophages, mast cell originate in the bone marrow from precursor cells lacking cytoplasm granules. When mast cell precursor migrate into the connective tissue, they proliferate and accumulate cytoplasmic granules. Mast cells and basophils circulating in blood derive from the same progenitor in the bone marrow.
The mast cell is the source of vasoactive mediators contained in cytoplasmic granules. These granules contain histamine, heparin, and chemotactic mediators to attract monocytes, neutrophils, and eosinophils circulating in blood to the site of mast cell activation.

MACROPHAGE Macrophage Macrophage Macrophage
Macrophages have phagocytic properties and derive from monocytes, cells formed in the bone marrow. Monocytes circulate in blood and migrate into the connective tissue, where they differentiate into macrophages. Macrophages have specific names in certain organs; for example, they are called Kupfer cells in the liver, osteoclasts in bone, and microglial cells in the central nervous system. They contain abundant lysosomes required for the breakdown of phagocytic materials. Active macrophages have numerous phagocytic vesicles (or phagosomes) for the transient storage of ingested materials. The nucleus has an irregular outline.

EXTRACELLULAR MATRIX Connective fibers

COLLAGEN Collagen fibers
Collagens are, in most animals, the most abundant protein in the ECM. In fact, collagens is the most abundant protein in the human body and accounts for 90% of bone matrix protein. Collagens are presents in the ECM as fibrilar proteins and give structural support to resident cells. Collagen is exocytosed in precursor form (protocollagen), which is then cleaved by procollagen proteases to allow extracellular assembly.
Distribution of a collagen:
- Type I collagen: Present in bone, tendon, dentin, and skin as banded fibers with a transverse periodicity of 64 nm. This type of collagen provides tensile strength.
- Type II collagen: Observed in hyaline and elastic cartilage as fibrils thinner than type I collagen.
- Type III collagen: Present in the reticular lamina of basement membranes, as a component of reticular fibers.
- Type IV collagen: Present in the basal lamina. This type of collagen does not form bundles.
- Type V collagen: Observed in amnion and chorion in the fetus and in muscle and tendon sheets. This type of collagen does not form banded fibrils.

RETICULAR FIBERS Reticular fibers
Reticular fibers or reticulin is a histological term used to describe a type of fiber in connective tissue composed of type III collagen.
Reticular connective tissue contains fibers, which form the stroma of organs of the lymphoid-immune system (lymph nodes and spleen), the hematopoietic bone marrow, and the liver. This type of connective tissue provides a delicate meshwork to allow passage of cells and fluids.

ELASTIC CONNECTIVE TISSUE Elastic fibers Elastic fibers
Elastin is a protein in connective tissue allowed in many tissues in the body to resume their shape after stretching or contracting.
Elastic connective tissue contains irregularly arranged elastic fibers in ligaments of the vertebral column or concentrically arranged in sheets or laminae in the wall of the aorta. This type of connective tissue provides elasticity.

EXERCICES
- Locate the loose connective tissue and the dense connective tissue in the virtual slide. Dense vs. loose connective tissue H-E 1,5 um
- Locate the regular dense connective tissue in the virtual slide. Dense regular connective tissue Masson's trichrome 7 um
- What kind of connective tissue shows the virtual slide? (23))
- FInd the connective cells (fibriblasts, plasma cells, mast cells and macrophages) in the virtual slide. Cells from connective tissue H-E 1,5 um
- What kind of fibers shows the virtual slide? Elastic fibers of the connective tissue Orcein 10 um
- What kind of fibers shows the virtual slide? Reticular fibers of the connective tissue Gordon-Sweet 15 um