Epithelial Tissue and Stratified Epithelium — Transcript Notes

Absorption in the Digestive System

The tract for absorbing food includes the throat, stomach, and mostly the intestines; the small intestine lining is the main site of nutrient absorption.
An adaptation in the intestinal lining involves absorptive cells that take in nutrients and simultaneously contribute mucus to the surface, helping lubricate and allow easier passage of contents.
The mucus layer works together with the absorptive surface to optimize sliding and reduce friction.
If microvilli on absorptive cells are lost (e.g., due to radiation or chemotherapy), nutrient absorption declines dramatically, leading to malabsorption and nutritional deficiencies.
The overall idea is that the lining acts like a skin covering that both absorbs nutrients and lubricates passage, which is essential for efficient digestion.

Stratified epithelium consists of multiple cell layers stacked on top of each other, not just a single layer.
The number of layers can vary from two up to many (two, 20, 50, etc.).
Cell shapes in stratified epithelia can be:
- Squamous (flat)
- Cuboidal (cube-shaped)
- Columnar (tall)
Major types (named by the shape of the surface cells):
- Stratified squamous epithelium
- Stratified cuboidal epithelium
- Stratified columnar epithelium
A fourth type exists called urothelium or transitional epithelium, but this is not covered in depth here (noted in some notes but not discussed today).
Among stratified epithelia, stratified squamous is the most widespread and is closely associated with the skin.

Keratinization refers to the accumulation of keratin, a waxy protein, in the outermost epithelial layers, which makes the surface hard and abrasion-resistant.
Keratinized stratified squamous epithelium features a thick, waxy layer of dead cells rich in keratin; it provides strong abrasion resistance.
Nonkeratinized stratified squamous epithelium lacks this keratin layer and remains softer; it is more delicate and found in mucosal areas that are not exposed to heavy abrasion.
Examples and locations:
- Keratinized: skin, especially thick skin regions like the palms of the hands and soles of the feet; these areas are thick, waxy, and highly abrasion-resistant.
- Nonkeratinized: surfaces that require moisture and flexibility but less abrasion resistance, such as the inside of the mouth and areas under the eyelid (soft skin/tissue not typically exposed to heavy abrasion).
The tongue can have a keratinized surface layer in places, leading to a hard, waxy, dead-looking layer on parts of the tongue.
The epidermis on thick skin is very keratinized, making the top layers extremely tough, while the deeper layers remain living and dividing.

The bottommost layer of stratified squamous epithelium sits on the basement membrane, which anchors the epithelium to underlying connective tissue.
The cells closest to the basement membrane are highly mitotically active and divide to replenish the tissue.
As cells are pushed upward, they move away from the nutrient source (blood vessels) in the deeper layers and progressively die as they approach the surface.
The line where cells die and lose nutrition is often described as a "killing zone" or the area where cells reach the end of their life as they migrate toward the surface.
Over time, dead cells accumulate at the surface as a layer of keratinized or nonkeratinized cells, contributing to the visible skin surface.
In thick keratinized skin, you can observe many stacked layers (approximately 30–40 layers in some regions) with a persistent keratin layer on the surface.
The topmost dead cells form a protective envelope; because the surface has little to no vasculature, damage to these layers does not bleed easily when a surface injury occurs.
The presence of a thick keratinized layer provides robust abrasion resistance; the surface is capable of withstanding friction and physical wear but is not as sensitive as thinner, nonkeratinized surfaces.

The pink tissue beneath the epithelium is connective tissue; this is the supporting layer that contains blood vessels, nerves, and other components.
Epithelial tissue (above) plus connective tissue (below) together form organs. When you have epithelial tissue on top and connective tissue beneath, you are looking at an organ.
The organ shown in the discussion is the skin, which is the body's largest organ and a key barrier and protective interface with the environment.
The phrase “epithelial tissue from here up” refers to the tissue layers that lie above the basement membrane, with the connective tissue beneath.

The roofing shingles analogy: the skin’s surface can accumulate multiple layers, much like adding another roof layer over an existing one; this can lead to a thick but potentially degraded outer surface if old layers aren’t shed or renewed.
In facial/overlying skin, the top layers are constantly renewed and shed as part of normal turnover (exfoliation).
Radiation or chemotherapy can damage the cells near the surface by reducing microvilli and absorptive capacity, similar to removing some of the “growth and renewal” capabilities, which reduces nutrient absorption and surface protection.
The palm and sole areas are typically hairless because thick keratinized skin does not support hair growth in those regions; this is a anatomical and developmental feature of skin.
There is a humorous misconception that palms have hair; in reality, thick skin on palms and soles lacks hair follicles.
The eyelid and oral mucosa are examples of nonkeratinized soft skin, which lacks the hard keratin layer and is more delicate, optimized for moisture and flexibility rather than abrasion resistance.
The mouth interior is lined with nonkeratinized stratified squamous epithelium to permit flexibility, moisture, and protection from mechanical stress without a hard keratin layer.

Key terms: absorption, mucus, microvilli, stratified squamous epithelium, stratified cuboidal epithelium, stratified columnar epithelium, urothelium (transitional epithelium, not covered here), basal membrane/basement membrane, keratin, keratinization, keratinized epithelium, nonkeratinized epithelium, exfoliation, basement membrane, connective tissue.
Major statistic: about 16 ext{%} of body weight is skin, i.e. skin mass is roughly 0.16 imes M_{ ext{body}}.
Layer depth in thick keratinized skin: approximately n \,\approx\, 30-40 layers.
Cellular dynamics: bottom layers are the most mitotically active; cells divide and move upward; the farthest layers die due to nutrition limits; the dying cells form a keratinized surface.
Practical implications: radiation/chemotherapy can damage absorptive cells (microvilli), reducing nutrient uptake; keratinized surfaces provide abrasion resistance but are less flexible than nonkeratinized surfaces.