Glandular epithelium. A gland is a structure made up of one or more cells, modified
to synthesize and secrete chemical substances. Most glands consist of groups of epithelial cells.
A gland can be classified as an endocrine gland or an exocrine gland. Endocrine glands
are ductless glands that release secretions directly into the surrounding tissues and fluids.
Exocrine glands have secretions that leave through a duct. The duct opens either directly or
indirectly to the external environment. Endocrine glands. The secretions of endocrine glands are
called hormones. Hormones are released into the interstitial fluid, diffused into the bloodstream,
and delivered to targets. In other words, cells that have receptors to bind to the hormones can
be thought of as the targets. The endocrine system is part of a major regulatory system
coordinating the regulation and integration of the body responses. A few examples of endocrine glands
include the anterior pituitary, the thymus, the adrenal cortex, and the gonads. Exocrine glands.
Exocrine glands release their contents through a duct that leads to the epithelial surface.
Mucus, sweat, saliva, and breast milk, are all examples of secretions coming
from exocrine glands. These secretions are all discharged through tubular ducts.
Secretions into the lumen of the gastrointestinal tract, technically occur outside of the body and
are considered exocrine. Glandular structure. Exocrine glands are classified as either
unicellular or multicellular. The unicellular glands are scattered single cells,
such as goblet cells, found in the mucosa membranes of the small and large intestine.
The multicellular exocrine glands known as serous glands, develop from simple epithelium to form a
secretory surface that secretes directly into an inner cavity. These glands line the internal
cavities of the abdomen and chest and release their secretions directly into the cavities.
Other multicellular exocrine glands release their contents through a tubular duct. The duct is
single in a simple gland, but in a compound gland is divided into one or more branches.
In tubular glands, the ducts can be straight or coiled,
whereas tubes that form pockets are alveolar, such as the exocrine portion of the pancreas.
Combinations of tubes and pockets are known as tubulo-alveolar compound glands.
In a branched gland, a duct is connected to more than one secretory group of cells.
Methods and types of secretion. Exocrine glands can be classified by their mode of secretion
and the nature of substances released, as well as by the structure of the glands
and shape of the ducts. Merocrine secretion is the most common type of exocrine secretion.
The secretions are enclosed in vesicles that move to the apical surface of the cell, where
then the contents are released by exocytosis. For example, watery mucus containing the glycoprotein
mucin is am merocrine secretion. The eccrine glands that produce and secrete sweat, are
another example. Apocrine secretion accumulates near the apical portion of the cell that portion
of the cell and its secretory contents pinch off from the cell and are released. Apocrine sweat
glands in the axillary and genital areas release fatty secretions that local bacteria break down.
This process causes body odor. Both merocrine and apocrine glands continue to produce and secrete
their contents with little damage caused to the cell, because the nucleus and golgi regions remain
intact after secretion. In contrast, the production of halocrine secretion involves
the rupture and destruction of the entire gland cell. The cell accumulates its secretory products
and releases them only in bursts. New gland cells differentiate from the cells in the surrounding
tissue to replace those lost by the secretion. The sebaceous glands that produce the oils on
the skin and hair are halocrine glands. Glands are also named after the products they produce.
The serous gland produces watery blood plasma-like secretions rich in enzymes, such as alpha amylase,
whereas the mucous gland releases watery to viscous products rich in glycoprotein mucin.
Both serous and mucous glands are common in the salivary glands of the mouth. Mixed
exocrine glands contain both serous and mucous glands and release both types of secretions.