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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    , a ductless gland that releases secretions directly into surrounding tissues and fluids (endo- = “inside”), or an exocrine gland    whose secretions leave through a duct that opens directly, or indirectly, to the external environment (exo- = “outside”).

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 the hormones. The endocrine system is part of a major regulatory system coordinating the regulation and integration of body responses. A few examples of endocrine glands include the anterior pituitary, thymus, adrenal cortex, and gonads.

Exocrine glands

Exocrine glands release their contents through a duct that leads to the epithelial surface. Mucous, sweat, saliva, and breast milk are all examples of secretions from exocrine glands. They are all discharged through tubular ducts. Secretions into the lumen of the gastrointestinal tract, technically outside of the body, are of the exocrine category.

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 mucous 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 compound glands is divided into one or more branches ( [link] ). In tubular glands, the ducts can be straight or coiled, whereas tubes that form pockets are alveolar (acinar), such as the exocrine portion of the pancreas. Combinations of tubes and pockets are known as tubuloalveolar (tubuloacinar) compound glands. In a branched gland, a duct is connected to more than one secretory group of cells.

Types of exocrine glands

This table shows the different types of exocrine glands: alveolar (acinar) versus tubular and those with simple ducts versus compound ducts. Each diagram shows a single layer of columnar epithelial cells with a line of cells travelling along the surface of a tissue (surface epithelium) and then dipping into a hole in the tissue. The cells travel down the right side of the hole until they reach the bottom, then curve around the bottom of the hole and then travel up the left side. Finally, the cells emerge back onto the surface of the tissue. The surface epithelial cells are those that are on the surface of the tissue; the duct cells are those that line both walls of the hole. The gland cells are those that line the bottom of the hole. The shape of the hole differs in each gland. In the simple alvelolar (acinar) gland, the duct and gland cells are bulb shaped with the gland cells being the larger end of the bulb. Simple alveolar glands are not found in adults, as these represent an early developmental stage of simple, branched glands. In simple tubular glands, the duct and gland cells are U shaped. Simple tubular glands are found in the intestinal glands. In simple branched alveolar glands, the gland cells form three bulbs at the end of the duct, similar in appearance to a clover leaf. The sebaceous (oil) glands are examples of simple branched alveolar glands. In simple coiled tubular glands, the duct and gland cells form a U, however, the bottom of the U, which is all gland cells, is curved up to the right. Merocrine sweat glands are examples of simple coiled tubular glands. In simple branched tubular glands, the duct is very short and the gland cells divide into three lobes, similar in appearance to a bird’s foot. The gastric glands of the stomach and mucous glands of the esophagus, tongue and duodenum are examples of simple branched tubular glands. Among the glands with compound ducts, compound alveolar (acinar) glands have three sets of clover leaf bulbs, for a total of six bulbs. Two of the clover leaf shaped structures extend parallel to the surface epithelium in opposite directions to each other. The third clover leaf extends down into the tissue, perpendicular to the surface. The duct is cross-shaped. The mammary glands are an example of compound alveolar glands. Compound tubular glands have a similar structure to compound alveolar glands. However, instead of three cloverleaf shaped bulbs, the compound tubular gland has three bird’s foot shaped bulbs. The duct is also cross-shaped in the compound tubular gland. The mucous glands of the mouth and the bulbourethral glands of the male reproductive system are examples of compound tubular glands, which are also found in the seminiferous tubules of the testis. Compound tubuloalveolar glands are a hybrid between the compound alveolar gland and the compound tubular gland. The two sets of bulbs that run parallel to the surface are bird-foot shaped; however, the set of bulbs that runs perpendicularly below the surface is cloverleaf shaped. The salivary glands, glands of the respiratory passages and glands of the pancreas are all compound tubuloalveolar glands.
Exocrine glands are classified by their structure.

Methods and types of secretion

Exocrine glands can be classified by their mode of secretion and the nature of the substances released, as well as by the structure of the glands and shape of ducts ( [link] ). 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 the contents are released by exocytosis. For example, watery mucous containing the glycoprotein mucin, a lubricant that offers some pathogen protection is a merocrine secretion. The eccrine glands that produce and secrete sweat are another example.

Questions & Answers

what is hypoxia
Akas Reply
I guess it's low supply the oxygen to the tissues
famuyiwa
yup
Natalie
A condition in which tissues (especially the blood) are deprived of an adequate supply of oxygen
Panthera
hanifa pia uko hapa
Panthera
where is present Glenoid Cavity ?
A- Reply
what is the muscular tissue
Md Reply
muscular tissue is a type of tissue that provide to help in cotraction to aur body.
A-
What's the difference in epithelial, connective, muscular and muscle tissue
Gifty
and it's similarities
Gifty
what is limb bone
Akshu Reply
this are bone attaching or joining to the axial bone.axial bone including skull,vertebrate and ribcage
Eliasi
how many bones make up the skull?
Matthew
22 bones
Husna
22bones
Bhanu
where is present Glenoid cavity ?
A-
how many bone in skull
Md
Explain the stages of mitosis and cell division
Bella Reply
systems of human body
Udezue Reply
define lymphatic system And give the composition of lymphatic fluid
sakshi Reply
the network of vessels through which lymphatic drains From the tissue into blood.lymph contain variety of substance like salts, glucose, proteins and fatsand water, white blood cells
Bhanu
yeah
Hassan
what is lymphatic system
Adie Reply
the network of vessels through which lymph drains from tissue into the blood
Bhanu
to describe the boundaries of four cavity
Pius Reply
homeostatic variables such as body temperature fluctuates within a normal range around the set point, or ideal, for a given homeostatic condition. for example, 98.6°F is a set point for body temperature. The response of the effector determines whether or not the homeostatic variable remains in the n
Chidinma Reply
why rbc is biconcave?
Sudhakar Reply
to carry oxygen easily
anwaar
What part of the brain controls the body temp
Ridwan
hypothalamus
JAYESH
what are epithelial tissues
Sachibu Reply
epithelial tissue that cover overall parts of the body and it's free from blood and nerves
Bhanu
Epithelial tissues are composed of cells laid out in sheets with strong cell-to-cell attachments.
Duah
Epithelial tissues perform a variety of functions that include; protection, secretion, filtration, diffusion, absorption, etc.
Duah
what control the flow of the blood ?
Donkor Reply
the pumping action of the heart
Holly
what is bony promises on the human body
Kelly Reply
what is the bony promises on human body
Kelly
what are bony prominences on human body
Kelly
support of the body
Bhanu
what are the characteristics of blood
yeboah Reply
they are red in colour
Tawoi
why blood is red in color?
Sudhakar
blood is red because it contains hemoglobin
Abena

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Source:  OpenStax, Anatomy & Physiology. OpenStax CNX. Feb 04, 2016 Download for free at http://legacy.cnx.org/content/col11496/1.8
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