Understanding Epithelial and Endothelial Cells - AP Biology

During embryological development, epithelial tissue develops from all three germ layers (endoderm, mesoderm, and ectoderm).

The infolding of epithelial tissue forms glands that function in the exocrine and endocrine systems. Glands secrete products that are taken up by the circulatory system for delivery.

Endothelial cells line the insides of blood vessels and lymphatic vessels and have many important functions, including but not limited to those described in the question. One additional function of endothelial cells is involvement in blood clotting.

Endothelium generally lines fully internal pathways (such as the vascular system), while epithelium generally lines pathways that are open to the external environment (such as the respiratory and digestive systems). Nerve cells are specialized for signaling, and red blood cells are specialized for oxygen transport.

Epithelial cells compose the outside of the body, namely skin and the lining of systems that connect to the outside of the body, such as the respiratory, excretory, and digestive tracts. The stomach is part of the digestive tract, as are the intestines. Knowing that the digestive tract is lined with epithelial cells allows us to eliminate the lining of the intestine from the answer choices. Similarly, the alveoli are a part of the respiratory system and nephrons are part of the excretory system.

The lining of the heart's chambers is part of the vascular lining, which is made of endothelial cells and is not exposed to the outside environment.

Light enters the eye through the pupil and is focused at the back of the eye to form an image on the retina. The retina contains rods and cones that can convert the image to nerual signals for the brain to interpret.

The iris is the muscle around the pupil that allows it to dilate or constrict. Changing the size of the pupil will alter the amount of light entering the eye.

The ciliary muscles attach the lens to the scelera (the outer white part of the eye). Contracting or relaxing these muscles will change the focal point of the lens, allowing the eye to properly focus the image on the retina. The ciliary muscles and lens do not affect the amount of light to enter the eye.

The ciliary body is adjacent to the ciliary muscles and produces aqueous humor, a liquid that fills the space between the cornea and the lens.

Epithilial tissue is usually classified in two ways: by the shape of the cells and by their organization. Cell shape can be flat and polygonal (squamous), elongated and rectangular (columnar), or short and rectangular (cuboidal). Cells can be found in a single layer (simple) or multiple layers (stratified). Pseudostratified epithelium is usually columnar in shape and consists of a single cell layer that has the appearance of multiple layers.

Sprilli is a classification of spiral-shaped bacteria.

Epithelial tissue lines the blood vessels, organs, and cavities in the human body. The primary function of epithelial tissue is to regulate secretion, absorption, and transport across surfaces. Epithelial tissue additionally plays a role in protection and detection of signals; therefore, the role of the epithelial tissue is to act as a regulatory barrier.

Epithelial tissue acts as a barrier in the human body. The functions of epithelial tissue are broad and include selective absorption, secretion, transport, and protection.

Epithelial cells can be categorized by morphology, or shape. Epithelial cells exist in the following morphologies: squamous, columnar, or cuboidal.

Epithelial cells have three distinct morphologies: squamous, cuboidal, and columnar. Squamous cells are wider than they are tall. In other words, they are flat.

Squamous cells are flat and wide. Both stratified and simple squamous epithelia will form smooth and flat surfaces that are specialized for the simple diffusion across the layer. Squamous cells are found in lung epithelia, the lining of blood vessels, and in the lining of body cavities.

Cuboidal epithelial cells are one category of epithelial cell types. These cells have equal height and width; therefore, they are cube-shaped. They possess round nuclei and are typically involved in secretion and absorption. Cuboidal epithelial cells are located in the exocrine system.

Epithelial cuboidal cells are involved in secretion and absorption in the exocrine system in the lining of glands.

Epithelial columnar cells represent one type of epithelial cell morphology. Columnar cells are taller than they are wide, have elongated nuclei, and are pillar-like. Epithelial columnar cells play a role in the digestive system and are important in sensory reception.

Epithelial columnar cells secrete digestive enzymes and absorb food in the digestive system and they also play a role in sensory reception. Epithelial columnar cells can be found in the tongue, ear, and the lining of the intestine.

Transitional epithelial cells are able to expand and contract based on the degree of tension. This transition to a different morphology depends on the cell layer in which the transitional cells are located.

Pseudostratified epithelial cells are single layers of cells—simple epithelium—that may be easily confused as a stratified epithelium. This confusion arises from the cross section, which shows nuclei located at different heights. As a result, it appears as if pseudostratified epithelial cells are multiple cell layers.

Simple epithelia are epithelial tissue that is only one cell layer thick; therefore, each cell has direct contact with the basement membrane. All epithelial cell morphologies can be found in simple epithelia.

Stratified epithelia consist of two or more layers of epithelial cells. They plan an important role in protection from chemical and mechanical forces. Stratified epithelia are specialized into keratinized and transitional layers.