Glands Of Skin Physiology Notes

Glands Of Skin

The skin contains two types of glands, sebaceous glands, and sweat glands.

Sebaceous Glands

Sebaceous glands are simple or branched alveolar glance situated in the dermis of the skin.

Sebaceous Glands Structure

• The sebaceous glands are ovoid or spherical in shape and are situated at the side of the hair follicle.
• These glands develop from hair follicles. So, the sebaceous glands are absent over the thick skin which is devoid of hair follicles.
• Each gland is covered by a connective tissue capsule. The alveoli of the gland are lined by stratified epithelial cells.

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• The sebaceous glands open into the neck of the hair follicle through a duct.
• In some areas like the face, lips, nipple, glans penis, and labia minora the sebaceous glands open directly into the exterior.

Secretion of Sebaceous Gland – Sebum

The sebaceous glands secrete an oily substance called sebum.

Sebum is formed by the liquefaction of the alveolar cells and poured out through the ducts either via the hair follicle or directly into the exterior.

Composition of Sebum

Sebum contains:

1. Free fatty acids
2. Triglycerides
3. Squalene
4. Sterols
5. Waxes
6. Paraffin

Functions of Sebum

1. The free fatty acid content of the sebum has antibacterial and antifungal actions. Thus, it prevents the infection of the skin by bacteria or fungi
2. The lipid nature of sebum keeps the skin smooth and oily. It protects the skin from unnecessary desquamation and injury caused by dryness
3. The lipids of the sebum prevent heat loss from the body. It is particularly useful in cold climates.

Activation of Sebaceous Glands at Puberty

• Sebaceous glands are inactive till puberty. At the time of puberty, these glands are activated by sex hormones in both males and females.
• At the time of puberty particularly in males, due to the increased secretion of sex hormones especially dehydroepiandrosterone, the sebaceous glands are stimulated suddenly.
• It leads to the development of acne on the face at the time of puberty.

Acne

• Acne is the localized inflammatory condition of the skin characterized by pimples on the face, chest, and back.
• It occurs because of over activity of sebaceous glands. Acne vulgaris is a common type of acne that is developed during adolescence.
• Acne disappears within a few years when the sebaceous glands become adapted to the sex hormones.

Sweat Glands

Sweat glands are of two types:

1. Eccrine glands
2. Apocrine glands.

Eccrine Glands Distribution

The eccrine glands are distributed throughout the body. There are many eccrine glands over thick skin.

Sweat Glands Structure: The eccrine sweat gland is a tubular coiled gland.

It consists of two parts:

1. A coiled portion lying deeper in the dermis, which secretes the sweat
2. A duct portion passes through the dermis and epidermis.

• The eccrine sweat gland opens out through the sweat pore. The coiled portion is formed by a single layer of columnar or cuboidal epithelial cells, which are secretory in nature.
• The epithelial cells are interposed by the myoepithelial cells. Myoepithelial cells support the secretory epithelial cells. The duct of the eccrine gland is formed by two layers of cuboidal epithelial cells.

Secretory Activity of Eccrine Glands

• Eccrine glands function throughout life since birth. These glands secrete a clear watery sweat.
• The secretion increases during an increase in temperature and emotional conditions.
• Eccrine glands play an important role in regulating the body temperature by secreting sweat. Sweat contains water, sodium chloride, urea, and lactic acid.

Control of Eccrine Glands

• Eccrine glands are under nervous control and are supplied by sympathetic postganglionic nerve fibers, which secrete acetylcholine (sympathetic cholinergic fibers).
• Stimulation of these nerves causes the secretion of sweat. Atropine blocks the action of the sympathetic nerves on the eccrine gland.

Apocrine Glands Distribution

Apocrine glands are situated only in certain areas of the body:

1. Axilla
2. Pubis
3. Areola
4. Umbilicus
5. Structure

• The structure of apocrine glands is similar to that of eccrine glands. The coiled portion lies in the deep dermis.
• But, the duct opens into the hair follicle above the opening of the sebaceous gland.

Secretory Activity of Apocrine Glands

• Apocrine sweat glands are nonfunctional till puberty and start functioning only at the time of puberty.
• In old age, the function of these glands gradually declines. The secretion of the apocrine glands is thick and milky. At the time of secretion, it is odorless.
• When microorganisms grow in this secretion, a characteristic odor develops in the regions where apocrine glands are present.
• Secretion increases only in emotional conditions. The apocrine glands do not play any role in temperature regulation like the eccrine gland.

Control of Apocrine Glands

• The apocrine glands are innervated by sympathetic adrenergic nerve fibers.
• But, the secretory activity is not under nervous control. However, the adrenaline from the adrenal medulla causes secretion by apocrine glands.
• Glands of eyelids, glands of the external auditory meatus, and mammary glands are the modified apocrine glands.

Pheromones

• Pheromones are a group of chemical substances that are secreted by apocrine glands.
• Some scientists call this substance vomeropherins. When secreted into the environment by an organism, pheromones produce some behavioral or physiological changes in other members of the same species.
• Pheromones are mostly present in urine, vaginal fluid, and other secretions of mammals and influence the behavior and reproductive cycle in these animals.
• Pheromones are odorless chemical messengers. The details of pheromones in lower animals are well documented. However, human pheromones are not fully studied.
• Recently, it is found that the pheromones excreted in the axilla of a woman affect the menstrual cycle of her roommate or other woman living with her. These substances stimulate receptors of vomeronasal receptors.
• The vomeronasal receptors are distinct from other olfactory receptors and detect especially the odor of pheromones.
• The impulses from these receptors are transmitted to the hypothalamus, which influences the menstrual cycle via the pituitary-gonadal axis.
• This effect of pheromones on the menstrual cycle of other individuals is the caiteti dormitory effect.