Epithelial vs. Connective Tissue: 15 Differences, Examples

Epithelial Tissue Definition

Epithelial tissue is a type of animal tissue, consisting of closely aggregated polyhedral cells connected firmly to one another in the form of cellular sheets that line the interior of hollow organs and cover the body surface.

• Cells in epithelial tissue or epithelium (epithelia; plural) are arranged in continuous sheets, in either single or multiple layers.
• The shapes and sizes of cells in the epithelium tissue range from tall columnar to cuboidal to low squamous, and often the cells’ size and morphology are dependent on their function.
• The innermost layer of cells of this tissue is connected to a basement membrane, which is a thin extracellular layer that consists of contains proteins like laminin and collagen.
• The epithelial tissue is avascular, with no blood supply. The nutrients intake and removal of waste materials occur via diffusion with the tissue underneath the epithelial tissue.
• The cells also have a nerve supply.
• Epithelial tissue is capable of continuous repair and renewal as they have a high rate of cell division where dead or injured cells are sloughed off and replaced by new ones.
• Based on their location, there are two main types of epithelial tissue; covering epithelium and glandular epithelium.
• Covering epithelium is the tissue that forms the outer covering of the skin and most internal organs like blood vessels and body cavities. It is further divided into simple and stratified epithelium on the basis of their complexity and the number of layers of cells.
• The glandular epithelium lines various glands and their ducts throughout the body, which then later proliferate to produce various hormones and enzymes. The glandular epithelium is divided into exocrine and endocrine glands on the basis of the presence of ducts.
• Epithelial tissue forms the first line of defense against pathogenic microorganisms and also protects the underlying tissue against radiation, desiccation toxins, and physical trauma.
• Besides, glandular epithelial tissue is involved in the secretion of various essential metabolites.
• Epithelial tissue of the intestinal tract is involved in absorption and that in the skin also performs the function of sensation and reception.

Connective Tissue Definition

Connective tissue is a type of animal tissue made up of cells, fibers, and gel-like substances that supports and gives structure to the body.

• Connective tissue is the most abundant tissue in the body, consisting of cells and extracellular matrix.
• The cells of the connective tissue are formed from embryonic cells called mesenchymal cells.
• All types of connective tissues consist of immature cells called blast cells that retain their capacity for cell division and secretion of the extracellular matrix.
• The cells found in different tissues include fibroblasts, macrophages, plasma cells, mast cells, adipose cells, chondrocytes, leukocytes, etc.
• The ground substance or matrix in the connective tissue may be fluid, semifluid, gelatinous, or calcified that supports cells, binds them together, stores water, and provides a medium for the exchange of substances between the blood and cells.
• The structure of the extracellular matrix determines much of the tissue’s qualities.
• For example, in cartilage, the extracellular matrix is firm but flexible, whereas the extracellular matrix of bone, by contrast, is hard and inflexible.
• Connective tissue usually is highly vascular and innervated; it has a rich blood and nerve supply, except for cartilage and tendons, which is avascular and doesn’t have nerves.
• Because of the diversity of cells and extracellular matrix and the differences in their relative proportions, connective tissues are divided into various categories. Some of these include liquid connective tissues, bones, cartilages, and adipose tissue.
• The functions of connective tissue also differ depending on the type and location of the tissue.
• Connective tissues like bones provide support and structure to the body while the cartilage helps in movements.
• Adipose tissue, present underneath the skin acts as a heat insulator and also helps in connecting the skin to the tissue underneath.
• The tendons and ligaments found throughout the body act as a connecting medium for bones, muscles, and cartilages.
• Liquid connective tissue like blood and lymph helps in the transport of nutrients and waste materials, in addition to having immune cells that protect against foreign invaders.

Key Differences (Epithelial Tissue vs Connective Tissue)

Examples of epithelial tissue

Simple columnar epithelium

• The simple columnar epithelium, like all other simple epithelium, consists of a single layer of cells which are tall column-like in shape.
• These cells are connected to a basement membrane that is made up of two layers, the basal lamina, and the reticular lamina.
• The simple columnar epithelium is present as a lining epithelium in many organs and is often changed to make it well suited for a specific function.
• Columnar epithelium in the stomach doesn’t have any surface structures. However, the free surface of the cells lining the small intestine is covered with microvilli, which increases the surface area for absorption of nutrients from the small intestine.
• The epithelium in the trachea is also ciliated and also contains goblet cells that secrete mucus. Similarly, in the uterine tubes, ova are propelled towards the uterus by ciliary action of the ciliated columnar epithelium.
• The columnar epithelium in different organs throughout the body performs different functions ranging from absorption to protection.

Endocrine glands

• Endocrine glands are the glands without ducts that secrete hormones. These glands are lined with glandular epithelium.
• The epithelium forms various parts of the glands, wherein some, it forms the parenchyma while in others it lines the ducts and surface of the gland.
• The secretions of endocrine glands, called hormones, diffuse into the bloodstream without flowing through a duct.
• These secretions have far-reaching effects because they are distributed throughout the body by the bloodstream.
• Examples of endocrine glands include pituitary gland at the base of the brain, the pineal gland in the brain, thyroid and parathyroid glands near larynx (voice box), adrenal glands superior to kidneys, pancreas near the stomach, ovaries in the pelvic cavity, testes in the scrotum, thymus in the thoracic cavity.

Examples of connective tissue

Bones

• A bone is a connective tissue where the living cells, tissues, and other components are enclosed within non-living material.
• Bone tissues together make up the skeleton system in humans and other vertebrates.
• Bone tissues in the hard structure form a honey-comb like a matrix internally composed of two different cells; osteoblasts and osteoclasts.
• Bones are mineralized tissues consisting of other types of tissues within them like the bone marrow, periosteum, endosteum, and blood vessels.
• These structures exist in different shapes and sizes with different forms of complexities fit for different functions.
• Because bone tissue is connective tissue, it is highly vascularized and is surrounded by several other connective tissues.
• It is also innervated and thus has multiple nerve supply.

Read Also: 15 differences between Bone and Cartilage

Blood

• Blood is a fluid connective tissue that continually circulates the body, allowing constant communication between tissues distant from each other.
• Blood is composed of a clear, straw-colored, watery fluid called plasma along with several different types of blood cells that are suspended.
• The constituents of plasma are about 90% water and dissolved and suspended substances like plasma proteins, inorganic salts, nutrients, waste materials, hormones, and gases.
• The blood cells include erythrocytes, leukocytes, and platelets, most of which are synthesized mainly in the red bone marrow. Some lymphocytes, however, are produced in lymphoid tissue.
• Blood is a connective tissue that is important for the transport of various materials like gases and nutrients. Besides, it also transports waste materials to organs like kidney and liver.
• Platelets in the blood contain certain cells called thrombocytes that are responsible for the clotting of blood.

References and Sources

• Waugh A and Grant A (2004). Anatomy and Physiology. Ninth Edition. Churchill Livingstone.
• Tortora GJ and Derrickson B (2017). Principles of Physiology and Anatomy. Fifteenth Edition. John Wiley & Sons, Inc.
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