CHAPTER 4 Tissues
115
Secretion in duct
Pinched-off portionof cell in the secretion
Dying cell releasessecretory products
stored in the cell
(a)
Merocrine gland
Vesicles empty their contentsinto the duct through exocytosis.
(b)
Apocrine gland
A portion of the cellcontaining secretory productsis pinched off the cell.
the duct
(c)
Holocrine gland
Entire cells are shed by the glandand become part of the secretion.
FIGURE 4.4
Exocrine Glands and Secretion Types
Exocrine glands are classified by type of secretion.
shaped in saclike structures are called
acinar
(as′i-nar) or
alveolar
(al-v ē ′ ō -lar). Tubular glands can be straight or coiled. Glands witha combination of the two are called tubuloacinar or tubuloalveolar.Exocrine glands can also be classified according to howproducts leave the cell. The most common type of secretion is
merocrine
(mer′ ō -krin)
secretion.
Merocrine secretion involvesthe release of secretory products by exocytosis (figure 4.4
a
).Merocrine secretion is used by water-producing sweat glandsand the exocrine portion of the pancreas.
Apocrine
(ap′ ō -krin)
secretion
involves the release of secretory products as pinched-off fragments of the gland cells (figure 4.4
b
). The milk-producing mammary glands release milk by a combination ofapocrine and mostly merocrine secretion.
Holocrine
(hol′ ō -krin)
secretion
involves the shedding of entire cells (figure 4.4
c
).Products accumulate in the cytoplasm of each epithelial cell, thecell ruptures and dies, and the entire cell becomes part of thesecretion. The lost cells are replaced by other cells deeper in thegland. Holocrine secretion is used by the sebaceous (oil) glandsof the skin.
ASSESS
YOUR PROGRESS
20.
Distinguish between exocrine and endocrine glands.How are multicellular exocrine glands classified onthe basis of their duct system? Their secretory portionshape?
21.
Give an example of a unicellular exocrine gland. What doesit secrete?
22.
Describe three ways in which exocrine glands releasesecretions. Give an example of each method.
4.4
Connective Tissue
LEARNING OUTCOMES
After reading this section, you should be able to
A.
List and describe the major functions of connective tissue.
B.
Identify the specialized cells found in connective tissue.
C.
Describe the three main components of the extracellularmatrix of connective tissue.
D.
Discuss the types and functions of embryonicconnective tissue.
E.
Explain how adult connective tissue is classified.
F.
Give an example of each type of connective tissue,describe its characteristic functions, and state itslocation in the body.
Connective tissue is a diverse primary tissue type that makes uppart of every organ in the body. Connective tissue differs from theother three tissue types in that it consists of cells separated fromeach other by abundant extracellular matrix. Connective tissue isdiverse in both structure and function.
Functions of Connective Tissue
Connective tissue performs the following major functions:
1.
Enclosing and separating other tissues.
Sheets of connectivetissue form capsules around organs, such as the liver and kidneys.Connective tissue also forms layers that separate tissues andorgans. For example, connective tissues separate muscles, arter-ies, veins, and nerves from one another.