Chapter 5 Glands/body membranes

Glands

• are either individual cells or multicellular organs composed predominantly of epithelial tissue.

• They secrete substances either for use elsewhere in the body or for elimination from the body.

• Glandular secretions may include mucin, ions, hormones, enzymes, or urea (a nitrogenous waste produced by the body).

Endocrine Glands

• lack ducts and secrete their products, called hormones, into the blood to be transported throughout the body.

• Hormones act as chemical messengers (or ligands) to influence cell communication

Exocrine glands

• typically oiginate from an invagination of epithelium that burrows into the underlying connective tissue.

• These glands usually maintain their connection with the epithelial surface by means of a duct, an epithelium-lined tube through which the gland secretions are discharged onto the epithelial surface.

• Examples of exocrine glands include sweat glands, mammary glands, and salivary glands.

• Exocrine glands may be unicellular (one-celled) or multicellular.

Unicellular exocrine glands

• typically do not contain a duct, and they are located close to the surface of the epithelium in which they reside.

• most common type of unicellular exocrine gland is the goblet cell, which is usually found in both simple columnar epithelium and pseudostratified ciliated columnar epithelium

multicellular exocrine glands

• contain numerous cells that work together to produce a secretion

• The gland often consists of acini (as'i-ni; acinus = grape), which are the clusters of cells that produce the secretion, and one or more smaller ducts, which merge to form a larger duct that transports the secretion to the epithelial surface.

• Multicellular exocrine glands typically are surrounded by a fibrous capsule, and extensions of the capsule called septa partition the gland into lobes.

Classification of exocrine glands

• Multicellular exocrine glands may be classified either by

  • anatomic form (i.e., by the structure and complexity of the ducts),

  • method of secretion, which may be thought of as a physiologic classification.

Classification by Anatomic Form

• Simple glands have a single, unbranched duct; compound glands have branched ducts.

• In addition, glands may be classified according to the shape of their secretory portions.

• The gland is called tubular if the secretory portion and the duct have the same diameter.

• If the secretory portion forms an expanded sac, the gland is called acinar.

• Finally, a gland with both tubules and acini is called a tubuloacinar gland.

Classification by Method of Secretion

• Exocrine glands may be classified physiologically by their method of secretion.

• The three basic types of exocrine glands in this classification are

  • merocrine glands

  • apocrine glands

  • holocrine glands

Merocrine

• mer'o-krin; meros = share) glands package their secretions into secretory vesicles and release the secretions by exocytosis

• The glandular cells remain intact and are not damaged by producing the secretion.

• Examples of merocrine glands include lacrimal (tear) glands, salivary glands; sweat glands; the exocrine glands of the pancreas, and the gastric glands of the stomach

Apocrine

• (ap'ö-krin; apo = away from, off) glands produce their secretory material when the cell's apical portion pinches off, releasing cytoplasmic content.

• Thereafter, the cell repairs itself in order to repeat its secretory activity.

• Examples include the mammary glands and ceruminous glands of the ear

Holocrine

• (höl'o-krin; holos = whole) glands are formed from cells that accumulate a product; the entire cell then disintegrates.

• Thus, a holocrine secretion is a viscous mixture of both cell fragments and the product the cell produced prior to its disintegration.

• The ruptured, dead cells are continuously replaced by other epithelial cells undergoing cellular division. T

• The oil-producing glands (sebaceous glands) in the skin are examples of holocrine glands

Organ

• tissue is a group of similar cells that perform a common function.

• Organs and body membranes also perform certain specialized functions, and they consist of two or more tissues that support their function

• An organ is a structure that is composed of two or more tissue types that work together to perform specific, complex functions. The key to organ structure is that the different tissue types must work in concert.

• For example, the stomach contains all four types of tissue. It is lined by an epithelium, has both areolar and dense connective tissue in its walls, contains three layers of smooth muscle in those walls, and possesses abundant nervous tissue. All these tissues work together to perform the functions of the stomach.

• Glands associated with epithelial tissue secrete substances for chemical digestion of ingested nutrients.

• Connective tissue houses the blood vessels and nerves that supply the stomach as well as provides shape and support.

• Smooth muscle contracts and relaxes so that contents within the stomach may be mechanically mixed and broken down.

• Nervous tissue is responsible for both regulating the contraction of muscle and stimulating secretion by glands.

Body Membranes

• formed from an epithelial layer that is bound to an underlying connective tissue.

• These membranes line body cavities or a joint surface, cover the viscera, or cover the body's external surface.

• There are four types of body membranes:

  • mucous

  • serous

  • cutaneous

  • synovial

mucous membrane

• also called a mucosa

• lines passageways and compartments that eventually open to the external environment; these include the digestive, respiratory, urinary, and reproductive tracts.

• formed by an epithelium and an underlying connective tissue called the lamina propria.

• perform absorptive, protective, or secretory functions or a combination of these functions.

serous membrane

• lines body cavities that typically do not open to the external environment and covers the external surface of many organs.

• composed of a simple squamous epithelium called mesothelium.

• produce a thin, watery serous fluid, or transudate (tran'sü-dät; trans = across, sudo = to sweat), which is derived from blood plasma.

• forms two associated layers: a parietal layer that typically lines the inside of the body cavity and a visceral layer that covers the surface of the internal organs.

• Between these two layers is a serous cavity, which is a potential space into which the serous fluid is secreted.

• The serous fluid reduces the friction between their opposing surfaces.

• Examples of serous membranes include part of the pericardium (which is associated with the heart), the pleura (associated with the lungs), and the peritoneum (associated with abdominal organs).

cutaneous membrane

• The largest body membrane is the cutaneous ((kü-ta'ne-us; cutis = skin) membrane

• also known as the skin

• covers the external surface of the body

• composed of a keratinized stratified squamous epithelium (called the epidermis) and an underlying layer of connective tissue (called the dermis).

• Its many functions include protecting internal organs and preventing water loss.

synovial membrane

• Some joints in the body are lined by a synovial (si-no've-ăl) membrane that is composed of a specialized type of connective tissue.

• The cells within this membrane secrete a synovial fluid that reduces friction among the moving bone parts and distributes nutrients to the cartilage on the articular surfaces of bone