Connective Tissue

Connective tissue is characterized by large amounts of extracellular material in the spaces between the connective tissue cells. This extracellular material may be of various types and arrangements and, on this basis, several types of connective tissues are recognized: (1) connective tissue proper, (2) cartilage, (3) bone, and (4) blood. Blood is usually classified as connective tissue because about half its volume is composed of an extracellular fluid known as plasma.

Connective tissue proper includes a variety of subtypes. An example of loose connective tissue (or areolar tissue) is the dermis of the skin (see fig. 1.13). This connective tissue consists of scattered fibrous proteins, called collagen, and tissue fluid, which provides abundant space for the entry of blood and lymphatic vessels and nerve fibers. Another type of connective tissue proper, dense fibrous connective tissue, contains densely packed fibers of collagen that may be irregularly or regularly arranged. Dense irregular connective tissue (fig. 1.16) contains a meshwork of randomly oriented collagen fibers that resist forces applied from many directions. This tissue forms the tough capsules and sheaths surrounding organs. Tendons, which connect muscle to bone, and ligaments, which connect bones together at joints, are examples of dense regular connective tissue. The collagen fibers of this tissue are oriented in the same direction (fig. 1.17).

Fox: Human Physiology, I 1. The Study of Body I Text I I © The McGraw-Hill

Eighth Edition Function Companies, 2003

Epithelium-

Connective -tissue

Epithelial cord or tubule

Cells from surface epithelium grow down into underlying tissue

If exocrine gland forms

If endocrine gland forms

Connecting cells persist to form duct

Deepest cells become secretory

Capillary

Capillary

Formation Exocrine Glands

Deepest cells remain to secrete

Connecting cells disappear

Deepest cells remain to secrete into capillaries

■ Figure 1.14 The formation of exocrine and endocrine glands from epithelial membranes. Note that exocrine glands retain a duct that can carry their secretion to the surface of the epithelial membrane, whereas endocrine glands are ductless.

Duct

Secretory portion —

Secretory portion —

SimPle tubular Simple acinar ir ir

Branched Acinar
Simple branched acinar

■ Figure 1.15 The structure of exocrine glands. Exocrine glands may be simple invaginations of epithelial membranes, or they may be more complex derivatives.

Fox: Human Physiology, Eighth Edition

1. The Study of Body Function

Text

© The McGraw-Hill Companies, 2003

Chapter One

Exocrine Gland Epithelial Tissue
Collagen proteins

■ Figure 1.16 A photomicrograph of dense irregular connective tissue. Notice the tightly packed, irregularly arranged collagen proteins.

Fibroblast

Fibroblast

Labeled Ddiagram Epithelia
(b)

■ Figure 1.17 Dense regular connective tissue. (a) Labeled diagram and (b) photomicrograph of a tendon. Notice the dense regular arrangement of collagenous fibers.

Adipose tissue is a specialized type of loose connective tissue. Each adipose cell, or adipocyte, has its cytoplasm stretched around a central globule of fat (fig. 1.18). The synthesis and breakdown of fat are accomplished by enzymes within the cytoplasm of the adipocytes.

Cartilage consists of cells, called chondrocytes, surrounded by a semisolid ground substance that imparts elastic properties to the tissue. Cartilage is a type of supportive and protective tissue commonly called "gristle." It forms the precursor to many bones that develop in the fetus and persists at the articular (joint) surfaces on the bones at all movable joints in adults.

Bone is produced as concentric layers, or lamellae, of calcified material laid around blood vessels. The bone-forming cells, or osteoblasts, surrounded by their calcified products, become trapped within cavities called lacunae. The trapped cells, which are now called osteocytes, remain alive because they are

Fat Globule Connective Tissue Diagram
Nucleus of adipocyte
Adipocyte Physiology

■ Figure 1.18 Adipose tissue. Each adipocyte contains a large, central globule of fat surrounded by the cytoplasm of the adipocyte. (a) Photomicrograph and (b) illustration of adipose tissue.

nourished by "lifelines" of cytoplasm that extend from the cells to the blood vessels in canaliculi (little canals). The blood vessels lie within central canals, surrounded by concentric rings of bone lamellae with their trapped osteocytes. These units of bone structure are called haversian systems (fig. 1.19).

The dentin of a tooth (fig. 1.20) is similar in composition to bone, but the cells that form this calcified tissue are located in the pulp (composed of loose connective tissue). These cells send cytoplasmic extensions, called dentinal tubules, into the dentin. Dentin, like bone, is thus a living tissue that can be remodeled in response to stresses. The cells that form the outer enamel of a tooth, by contrast, are lost as the tooth erupts. Enamel is a highly calcified material, harder than bone or dentin, that cannot be regenerated; artificial "fillings" are therefore required to patch holes in the enamel.

Test Yourself Before You Continue

List the four primary tissues and describe the distinguishing features of each type.

Compare and contrast the three types of muscle tissue. 3. Describe the different types of epithelial membranes and state their locations in the body.

Explain why exocrine and endocrine glands are considered epithelial tissues and distinguish between these two types of glands.

Describe the different types of connective tissues and explain how they differ from one another in their content of extracellular material.

Fox: Human Physiology, Eighth Edition

1. The Study of Body Function

Text

© The McGraw-H Companies, 2003

The Study of Body Function

Canaliculi Diagram Bone
Canaliculi

■ Figure 1.19 The structure of bone. (a) A diagram of a long bone, (b) a photomicrograph showing haversian systems, and (c) a diagram of haversian systems. Within each central canal, an artery (red), vein (blue), and nerve (yellow) is illustrated.

Enamel Dentin

Pulp Cementum

Enamel Dentin

Pulp Cementum

Dentin Pulp Complex
Essentials of Human Physiology

Essentials of Human Physiology

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Responses

  • Adaldrida
    What does a duct endocrine look like?
    7 years ago
  • Richard
    Is enamel areolar connective tissue?
    6 years ago
  • massawa
    Are endocrine glands derivitives of epithelial tissue?
    6 years ago
  • ANNIKA
    Where is force applied to dense irregular connective tissue?
    6 years ago
  • biniam mewael
    Why are glands considered epithelial tissue?
    6 years ago
  • mohammed
    What is an easy way to distinguish between epithelium tissue?
    6 years ago
  • helen shoemaker
    Why are exocrine glands considered to be a type of connective?
    6 years ago
  • yohannes tewolde
    Why are exocrine glands considered a type of connective tissue?
    6 years ago
  • mario
    Where is the central canal in a photomicrograph of bone?
    6 years ago
  • Milly
    What is the function of a canaliculi in the bone?
    6 years ago
  • Kirsi K
    Which human tissue is considered connective tissue?
    6 years ago
  • Andreas
    Are exocrine glands derived from dense, fibrous regular connective tissue?
    6 years ago
  • Gaspare Sabbatini
    How does the endocrine and exocrine glands differ in structure?
    5 years ago
  • girmay simon
    How are adipose tissue and cartilage alike?
    5 years ago
  • mandy
    Why the 3 subtypes of connective tissue are classified as one?
    5 years ago
  • Maximilian Kaufmann
    What enzymes breakdown connective tissue?
    5 years ago
  • awet sheshy
    What gland is formed when the connecting cells persist to form a duct?
    1 year ago
  • Marigold
    Is blood muscle, nerve, connective, ir epithelial?
    6 months ago

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