Histology

YES this is gonna be a little ugly for now. It's more about organizing my thoughts.

Overview

Histology is the study of bodily tissues and the cell structure within them. There are four types of tissue as we recognize it:

Before tissue can be observed, however, it must be prepared:

  1. Get a sample
  2. Fix (or stabilize) it with a fixative (like formaldehyde); bone will also be dissolved with EDTA to leave behind collagen
  3. Dehydrate it, usually with increasingly concentrated alcohol; clear the alcohol with an organic solvent
  4. Infiltrate the sample within melted paraffin, embed it in a mold, and trim
  5. Stain the sample

Usually, this stain is hematoxylin and eosin. Hematoxylin is a basic (and blue) dye that indicates acidic (basophilic) structures. It's a positively-charged dye. Eosin is acidic and pink, indicating the reverse - that is, basic (acidophilic/eosinophilic) structures - and it is negatively-charged, so it binds to positive tissue.

Most often, in these sorts of classes, a simple brightfield microscope is used. Light emitted by a condenser passes through the tissue into an objective that magnifies it to be viewed through the eyepiece. It can magnify about 1000x at best, with a resolving power of 0.2 μm. Anything smaller or closer together will be indistinguishable. Other forms of microscopy can be more precise, but expensive or niche. Phase-contrast microscopy can be used on living tissue, and confocal microscopy can even form a 3D image.

Epithelial Tissue

Epithelial tissue can be found on the surfaces of organs. It protects, senses, secretes, absorbs, and excretes. It is categorized based on two things: cell shape and arrangement.

  1. Squamous cells are sort of round and amorphous. They'll usually be found squished together.
  2. Cuboidal cells are blocky and notably found in simple arrangements around ducts, the interior of which is called a lumen. However, they can contract their interior/apical side to appear more pyramidal.
  3. Columnar cells are long and thin, but still somewhat blocky. They'll often be found with cilia on one side..
  1. Simple tissue is one cell layer thick.
  2. Stratified tissue will stack many cells atop each other.
  3. Pseudostratified tissue is exclusive to columnar cells. They're packed together so tightly in one layer that the nuclei is positioned wherever another cell's isn't. This can give the false impression that there's multiple layers of cells.

Combining these will get you the proper category of tissue type, such as simple cuboidal epithelium. The exception is transitional epithelium, which is sort of hard to categorize and occurs between layers. It is just transitional, nothing else.

Some other exceptions worth mentioning: Along the surfaces of organs is also a combination of connective tissue and simple squamous epithelium known as the serosa. Some epithelial tissue will slowly harden and form a protective surface on the apex of the tissue. This is keratin, and worth mentioning when describing the tissue's structure - though if you can see the nucleus, that's not keratin! Cap cells may also occur on the apex of bladder tissue. These expand and stretch with the rest of the organ. Epithelium within vascular tissue may be referred to as endothelium.

You will also find glands within the epithelium. They may be the size of one cell, or goblet cells, and look like holes or sponges punched into the sides of the tissue. Alternatively, multicellular glands will resemble ducts, only with a tubular extension into the rest of the tissue to open and close.

The lowermost part of a layer of epithelium will be the basement membrane, which is noncellular. It is an anchoring device, polarity inducer, boundary, tissue scaffold, and filtration barrier. This attaches to the connective tissue below. If there are two layers of epithelium, a distinction is made between the lamina rara interna and externa.

The lamina rara is about 50 nm thick; all the layers are, though this one is among the few that is bounded to that.

The lamina densa contains mostly collage type IV (3-4 nm) and laminin. Entactin keeps other proteins attached to each other. There is also fibronectin, collagen type VII (anchoring fibrils), and the proteoglycans heparan sulfate, condroitin sulfate, and perlacan.

The reticular lamina is made up of reticular fibers, or collagen type III. It is connective tissue. With the lamina densa, it forms the basal lamina.

Cell Membrane

The cell membrane is composed of a phospholipid bilayer, with polar phospholipid heads along the surfaces and non-polar fatty acid tails "inside" it. This creates a hydrophobic surface.

Things embedded in the cell membrane impact how cells interact and stick together in the epithelium and connective tissue. This is the fluid mosaic model.

Cholesterol makes the cell membrane stiff and tough. It's good, in moderation. Within this cell membrane are also proteins. Surface proteins stick to the surface, and peripheral proteins only enter the membrane partially. Integral proteins go all the way through and may be also involved in transport. Uniporters transport along a gradient one at a time, but active transport can use energy to go against such a gradient. Antiporters will sort of "equivalent exchange" two things, and symporters will sort of "teamwork" through it.

Pumps need energy to pump against the concentration gradient, but channels don't. Receptors simply detect the presence of something.

Microvilli increase surface area and are significantly smaller than cilia. This surface area allows them to transport fluids (tall) or absorb (short).

Actin is a vital microfilament with many binding proteins that it cooperates with.

Connective Tissue

Connective tissue is used for mechanical support, but also communication and transport. It is divided into loose and dense types, and it occurs beneath the epithelium. It supports, conducts, stores energy, protects, transports, and affords mechanical motion, and it develops from the mesenchyme. Blood, fat, and bone are all connective tissues. This category of tissue includes cells and a non-cellular matrix.

Connective tissue has a lot of inclusions that aren't necessarily cells, like (smallest to largest) filaments, fibrils, and fibers. The most abundant fiber here is collagen type I.

The matrix contains an extracellular ground substance (gel) and fibers. The ground substance is made up of glycoproteins, as well as the carbohydrateous proteoglycans and "GAGs" (glycosaminoglycans). GAGs form a sort of "bottle brush" structure, or like a pipe cleaner, around a protein core. These can be huge - GAGs can be as much as 50k Daltons, and they are found in skin. In turn, these brushes can line up and form a second, larger brush with hyaluronic acid.

A notable form of connective tissue is the areolar spread, or mesentery. It's incredibly loose and highlights well how it is made up of collagen type I (thick pink) and elastin (thin black) fibers. This usually attaches abdominal organs to the abdominal wall. The nuclei of fibroblasts can also be found here. These fibroblasts synthesize the collagen. When inactive, they are known as fibrocysts. A little more infrequently are super dark reticular fibers that form a sort of net or amorphous maze through the tissue, made up of collagen type III.

Collagen I tends to have a very periodic patterns exactly 67 nm wide, a triple helix of molecular collagen made up of three alpha strands about 300 nm long and 1.5 nm in diameter. It's flexible, but not stretchy, made up of glycoproteins and glycine in a repeating triplet. Hydrogen bonds between them occur at regular staggered intervals.

Other specialized cells in connective tissue include macrophages (globular) and lymphocytes (round), which are colloquially called white blood cells. Mast cells are granular and release histamines to target infection. Eosinophils specifically target parasites. Adipocytes are also known as fat cells; when in single "bubbles", they are white fat, but when multilocular, with many smaller bubbles that darken it, the fat is called brown. There's also, of course, vascular tissue with erythrocytes (red blood cells), which are about 7 μm.