Tissues
• Describe the basic principle of tissue preparation for histological examination.
Tissues are first treated with a fixative (e.g. formaldehyde) to preserve them. After this,
they are dehydrated by repeatedly washing with alcohol. This allows the next step:
embedding, when paraffin wax is added in the tissue to provide structural stability.
Without this, the tissue can’t be sectioned. Sectioning is done with a microtome, which
cuts the tissue into very thin slices (5-10 μm). The last step is dyeing, which reveals cell
components and allows us to differentiate them.
• Explain the Haematoxylin-eosin (H&E) staining method.
Haematoxylin is a basic dye, it binds to DNA and RNA, staining them dark pink. Eosin is
an acidic dye, it binds cytoplasmic elements and collagen, staining them pink. Together
they provide a clear, high-contrast image of the cell.
• Describe the main characteristics of the four basic tissue types: epithelia, connective
tissue, muscle and nervous tissue.
Epithelial tissue is made up of aggregated polyhedral cells. The amount of ECM
(extracellular matrix) is small, and the functions of this tissue are lining, protection,
absorption and secretion.
Connective tissue is made up of many types of fixed or wandering cells. It has a large
amount of ECM (more than the cells themselves), and its functions are support,
protection and to connect the cells that make up organs.
Muscle tissue is made up of elongated contractile cells. The amount of ECM is medium.
The role of muscle tissue is to contract, leading to voluntary and involuntary movement.
Nerve tissue is made up of the iconic elongated cells with dendrites and axons. The ECM
comes in very small amounts, and the function of this tissue is to transmit impulses.
• Relate the principal function of epithelial tissues to their morphology.
Epithelial tissue can be classified as simple, stratified or pseudostratified. Simple
epithelia is very thin, so it is mostly used by the body for rapid diffusion, absorption and
secretion. Stratified epithelia, on the other hand, is thick, so its main function is
protection (however it also has secretory roles). Pseudostratified epithelial tissue often
has cilia and goblet cells, so it is used mucus secretion and transport. It is also not too
thin, so it serves a role in protection.
• Differentiate between the epithelial apical and basal poles, and the specialization of the
apical cell surface.
The basal poles rest on the basement membrane (BM), while the apical poles face a
space. The specialization of the apical surface reflects the specialization of the cell. This
surface can be made up of microvilli, short finger-like protrusions made of actin that
greatly increase the surface area. They make the cell better at absorption. Stereocilia are
similar, but they are longer and have a mechanosensitive function (in the ear) besides
absorbtion. Cillia are thin, hair-like projections that help with movement of particles.
Finally, keratin rests on the apical side of epithelia and is made up of dead cells. Its main
function is protection.
• Distinguish between simple, stratified, pseudostratified, squamous, cuboidal,
columnar, transitional epithelia and can give an example for each.
Simple – one layer of cells, stratified – multiple layers of cells, pseudostratified – all cells
are connected to the basal membrane, but not all reach the surface.
Squamous cells are thin and flat, cuboidal cells are roughly the same in height and width,
columnar cells are taller than they are wide and transitional epithelia has dome-like cells.
Simple squamous: lining of vessels
Simple cuboidal: covering ovary
Simple columnar: lining of intestines
Stratified squamous, non-keratinized: mouth, esophagus, larynx
Stratified squamous, keratinized: the epidermis
Stratified cuboidal: sweat glands
Stratified transitional: bladder
Stratified columnar: conjunctiva (covers the sclera and the inside of eyelids)
Pseudostratified: trachea
• Explain the role of the basement membrane in epithelia.
The basement membrane acts as a semipermeable filter for things coming into the
epithelium from its basal side.
• Describe the origin, function and appearance of mesenchymal tissue.
Mesenchymal tissue originates in the middle layer of the embryo (the mesoderm) and
produces all connective tissue. It appears as mostly ECM with bits of collagen, so it’s pink
with rare small spots of dark pink.
• Describe the cell types and their function in different connective tissues.
Fibroblasts produce and maintain the ECM. Lymphocytes have various roles in immune
defence, mainly identifying and ‘remembering’ threats, leukocytes defend against
pathogens (mainly by eating them) and plasma cells secrete antibodies used in immune
responses. Macrophages mainly recycle cells and the ECM by eating them, but also have
secretory functions. Adipocytes store fat.
• Explain the function of adipose tissue and understand the difference in white and brown
adipose tissue.
Adipose tissue has a high density of adipocytes, cells containing neutral fats. Adipose
tissue thus has a role in metabolism, but it’s also vital in thermal insulation, structural
support and protection from mechanical stress to the body.
White adipose tissue is used for long-term fat storage; white adipocytes contain a single
large drop of lipid that is removed in histological preparation. The cells appear empty, or
white. Brown adipose tissue’s main function is heat production. For this it needs a lot of
mitochondria, which are what make it appear brown. The amount of brown adipose
reduces during adolescence.
• Describe and recognize the different types of muscle tissue.
There are three types of muscle tissue. Skeletal (striated) muscle does quick voluntary
contractions and is easily recognized by the fact that its cells are fused into long muscle
fibers. Cardiac muscle performs rhythmic, involuntary contractions in the heart. Cardiac
muscle tissue can be identified by the white around the nuclei of its cells, made of
glycogen. Smooth muscle performs slow and involuntary contractions. They have long,
thin cells that aren’t fused.
• Explain the layers in the digestive tract and skin.
The skin is made up of the epidermis, dermis and subcutaneous layer (from outside to
inside). The epidermis is mostly squamous keratinized epithelium; it has 15-20 layers of
dead keratinocytes (corneocytes) that make up the stratum corneum. Beneath is the
stratum granulosum, where keratinocytes start dying and move up. Lower than this is the
stratum spinosum, where cells that moved up from the stratum basale synthesize
keratin. The stratum basale lies directly on the basement membrane and is a single layer
of cuboid cells. These cells exhibit strong mitotic activity, dividing into cells that will move
up.
The digestive tract is made up of four layers. To the lumen is the mucosa, it has an
epithelial lining that lets nutrients in (but not pathogens!), a connective layer, the lamina
propria, with capillaries and immune cells and finally a layer of smooth muscle – the
muscularis mucosae. After the mucosa is the submucosa, a dense layer of connective
tissue and epithelial tissue that makes up vessels and glands. The submucosa also has
the submucosal plexus, an enteric nervous tissue that produces involuntary movement
involved in digestion. Further out is the muscularis, which has an internal and an external
muscle sublayer. Between them is little connective and enteric nervous tissue – the
myenteric plexus. Outermost is the serosa, a thin layer with blood vessels and connective
tissue and adipose tissue. The serosa only develops in regions of the digestive tract
located in the abdomen. The higher segments have a layer of connective tissue called the
adventitia instead, which connects to the surrounding connective tissue.
Note: on the slides, the submucosal plexus is listed as autonomic. This is not true, it is
enteric.
• Relate the different layers in tissues to organ function
All layers combined determine the function of an organ. The innermost layers (to the
lumen) interact ldirectly with the environment, they filter what comes in and out. The
middle layers regulate and move secretions or cells, while the outermost layers provide
protection, support and ties to the rest of the body.