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Lectures 3 & 4Lectures 3 & 4

Histology: The Study of Tissues

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Tissues and Histology

• What is a Tissue?– A collection of similar cells and noncellular

substances (extracellular matrix) secreted by the cells• Tissue Level of Organization

– Epithelial – Connective– Muscle– Nervous

• Histology: Microscopic Study of Tissues

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Epithelium Characteristics

1. Consists almost entirely of cells

2. Covers body surfaces and forms glands

3. Has free and basal surface

4. Avascular5. Undergoes mitosisFig. 4.1

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Classification of Epithelium:How to Make Sense of It All?

There are 2 basic features (or criteria) for classification of epithelium:

1. Number of cell layers• Simple

– Single layer• Stratified

– More than 1 layer

Exceptions?• Pseudostratified

– Single layer; only some cells reach free surface• Transitional

– Number of cell layers decreases as it is stretched

Fig. 4.2

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Classification of Epithelium

2. Shape of cells• Squamous (=scaly)

– Cells flattened

• Cuboidal– Cells cube-shaped

• Columnar– Cells are taller than wide

Fig. 4.2

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Types of Epithelium• Name of epithelium is combination of: number of

cell layers and shape of cell (Table 4.2)

• 3 types of simple: simple squamous epithelium, simple cuboidal epithelium and simple columnar epithelium

• 3 types of stratified: stratified squamous epithelium, stratified cuboidal epithelium and stratified columnar epithelium

• Pseudostratified columnar epithelium• + Transitional epithelium: Cuboidal to columnar when

not stretched and squamous-like when stretched

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Review QuestionWhich of the following statements about epithelial tissue is

correct?

(a) All cells of pseudostratified columnar epithelial tissue have a free surface.

(b) You would predict simple squamous epithelium lining the inside of an organ that absorbs substances.

(c) Transitional epithelium has multiple layers of cells that can change shape.

(d) The epithelium of your skin is a simple squamous epithelium.

(e) Cuboidal or columnar cell shapes are optimal for diffusion.

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Glands• Secretory organs made mostly of epithelium• Form as invaginations (ingrowths) of outer layer

of epithelium in embryo

• Two basic types:1. Exocrine:– Have ducts lined with

epithelium

2. Endocrine– Have no ducts– Examples include pituitary gland, pancreas,

thyroid glandFig. 4.4

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Connective Tissue• Abundant and diverse• Consists of cells separated by extracellular

matrix• Specialized cells produce the extracellular

matrix– Suffixes

• -blasts: create the matrix• -cytes: maintain the matrix• -clasts: break the matrix down for remodeling

• Performs variety of important functions

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Connective Tissue Categories

• Embryonic or mesenchyme

• Adult– Loose

– Areolar

– Adipose

– Dense

– Cartilage

– Bone

– Blood

Table 4.6

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Loose Connective Tissue

Areolar tissue• Loose packing material of most organs and tissues• Attaches skin to underlying tissues• Contains collagen, reticular, elastic fibers and variety of

cells

Table 4.9

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Loose Connective TissueAdipose Tissue• Consists of adipocytes

– Types

• Yellow (white)

– most abundant, white at birth and yellows with age

• Brown

– found only in specific areas of body as axillae, neck and near kidneys

Table 4.9

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Dense Regular Connective TissueHas abundant collagen fibers

• Tendons: Connect muscles to bones

• Ligaments: Connect bones to bones

Table 4.10

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Dense Irregular Connective Tissue• Forms most of dermis of the skin

Table 4.10

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Elastic Connective Tissue• Ligaments in vocal folds

• In walls of elastic arteries

Table 4.10

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Review Question

In which of the following locations would you expect to find dense irregular connective tissue?

(a) Nuchal ligament(b) Dermis of skin(c) Vocal cords(d) Large arteries(e) Tendon

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Points to RememberPoints to Remember

• Two basic components to a tissue:– Similar cells specialized for a function– Non-cellular matrix secreted by the cells

• Epithelial tissue is typically a covering or lining

• Connective tissue is diverse and extremely abundant

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Questions?