Modes of Secretion Merocrine Holocrine

Modes of Secretion 

Merocrine 

Holocrine 

most abundant form of multicellular exocrine 

gland 

products are secreted by exocytosis (e.g., 

pancreas, sweat, and salivary glands) 

products are secreted by the rupture of gland 

cells [e.g., sebaceous (oil) glands of the skin] 

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings

Modes of Secretion 

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings 

Figure 4.5

Connective Tissue 

Found throughout the body; most abundant and 

widely distributed in primary tissues 

Connective tissue proper 

Cartilage 

Bone 

Blood 


Connective Tissue 


Figure 4.6

Functions of Connective Tissue 

Binding and support 

Protection 

Insulation 

Transportation 


Characteristics of Connective Tissue 

Connective tissues have: 

Mesenchyme as their common tissue of origin 

Varying degrees of vascularity (avascular to highly 

vascularized) 

Nonliving extracellular matrix, consisting of 

ground substance and fibers. Connective tissue can 

bear weight, withstand tension and endure abrasion 


Structural Elements of Connective Tissue 

Ground substance – unstructured material that fills 

the space between cells 

Fibers – collagen, elastic, or reticular 

Cells – fibroblasts, chondroblasts, osteoblasts, and 

hematopoietic stem cells 

Ground substance and fibers make up the 

extracellular matrix 


Ground Substance 

Unstructured material that fills the space between 

the cells and contains the fibers 

E.g. Interstitial (tissue) fluid 

E.g. Adhesion proteins – fibronectin and laminin 

E.g. Proteoglycans – glycosaminoglycans (GAGs) 

Functions as a molecular sieve through which 

nutrients diffuse between blood capillaries and 

cells 


Ground Substance: Proteoglycan Structure 

Proteoglycans consist of a protein core to 

which glycosaminoglycans are attached 

e.g. chondroitin (buy ‘em over the counter 

e.g. keratin sulfates 

e.g. hyaluronic acid 

Proteoglycans can form huge aggregates 

attached to a hyaluronic acid molecule 

GAGs intertwine and trap water forming 

a gel like substance 

The ground substance functions as a medium 

through which nutrients & dissolved substances 

can diffuse between the blood and the cells 


Figure 4.7

Fibers 

Provide support 

3 types of fibers are found in connective tissue 

matrix: 

Collagen 

Elastic 

Reticular 


Fibers: Collagen fibers 

Constructed of a fibrous protein called collagen 

Collagen molecules are secreted into the 

extracellular space where they self-assemble into 

cross-linked fibrils which bundle into thick 

collagen fibers. 

Stronger than steel 

Act as safety lines 


Fibers: Elastic Fibers 

Constructed of the protein elastin 

Pliable and can stretch and recoil 

Acts like a spring 


Fibers: Reticular fibers 

Short, fine collagen fibers that are continuous w/ 

collagen fibers 

They branch forming networks around blood 

vessels and support the soft tissue of organs 


Cells 

Each major class of connective tissue has a fundamental cell type that 

exists in immature and mature forms 

The primary blast (forming) cell types by connective tissue class are: 

Fibroblasts – connective tissue proper 

Chondroblasts – cartilage 

Osteoblasts – bone 

Hematopoietic stem cells – blood 

White blood cells, plasma cells, macrophages, and mast cells 

Note: Hematopoietic stem cells differ from the other 3 

classes in that they are not found in their tissue and do not 

form the extracellular matrix (e.g. the fluid matrix of 

plasma) of that tissue. 


Bone Marrow

Cells 

Once they synthesize the matrix, the blast cells 

assume the less active mature roles of fibrocyte, 

chondrocyte, osteocyte and cells of the blood. 

They maintain the health of the matrix 


Cells 

Other cell types in connective tissue: 

Fat cells: store nutrients 

White blood cells: (neutrophils, eosinophils, 

lymphocytes) defensive immunological cells 

Mast cells: tissue response to injury 

Macrophages: tissue response to injury 

Plasma cells: produce antibodies 


Cells: Mast cells 

A little more detail on Mast cells: 

Oval in shape and cluster along blood vessels 

Detect foreign substances (bacteria, fungi) 

Initiate local inflammatory response 

Secretory granules (mast) are abundant in the cytoplasm 

and include chemicals that mediate inflammation: 

Heparin: anticoagulent that prevents blood 

clotting 

Histamine: makes capillaries leaky 

Proteases: protein degrading enzymes 


Cells: Macrophages 

A little more detail on macrophages: 

Large, irregularly shaped cells that phagocytize 

foreign material and dead tissue 


Areolar Connective Tissue: Model 


Figure 4.8

Connective Tissue: Embryonic 

Mesenchyme – embryonic connective tissue 

Major connective tissues arise from the common 

embryonic tissue, mesenchyme, derived from the 

embryonic mesoderm 

Gel-like ground substance with fibers and starshaped 

mesenchymal cells 

Gives rise to all other connective tissues 

Found in the embryo 


Connective Tissue Proper: Loose 

Areolar connective tissue 

Gel-like matrix with all three connective tissue 

fibers 

Fibroblasts, macrophages, mast cells, and some 

white blood cells 

Wraps and cushions organs 

Widely distributed throughout the body 


Connective Tissue Proper 

Two subclasses: Loose & Dense 

Loose connective tissue: 

Areolar 

Adipose 

reticular 

Dense connective tissue: 

Dense regular 

Dense irregular 

Elastic (cartilage) 



Areolar connective tissue: 

Functions include: 

Supporting and binding of other tissues (fibers) 

Holding body fluids 

Defending against infection (WBC & macrophages) 

Storing nutrients (fat cells) 

When a body region is inflammed, the areolar tissue in 

the area soaks up excess fluid causing local swelling 

(edema) 

Areolar connective tissue is the most widely spread 

connective tissue in the body 





Adipose connective tissue: 

White Fat 

Excellent nutrient storage 

Adipocytes (adipose or fat cell) is 90% of tissue mass 

Little matrix, cells are packed tight 

Fat is triglyceride 

Cells can be very large depending on the amount of fat 

inside 

Adipose tissue is highly vascularized and has high 

metabolic activity 



Adipose connective tissue: (cont.) 

White Fat 

Accounts for 18% of total body 

weight…hopefully 

Usually accumulates in subcutaneous tissue 

Prevents heat loss from the body: found at 

kidneys, abdomen and hips (fat depots) 

Depots also found around heart, lymph nodes, 

some muscles and bone marrow 



Adipose connective tissue: (cont.) 

Brown fat: (brown adipose tissue) 

Contains mitochondria which use the lipid fuel 

to heat the bloodstream to warm the body 

Found only in babies and is located between the 

shoulder blades and anterior abdominal wall 




Figure 4.9b


Reticular connective tissue 

Loose ground substance with reticular fibers 

Reticular cells lie in a delicate fiber network 

Forms a soft internal skeleton, or stroma, that 

supports other cell types 

Found in lymph nodes, bone marrow, and the 

spleen 




Figure 4.9c

Connective Tissue Proper: Dense Regular 

Parallel collagen fibers with a few elastic fibers 

running in the “pull” direction 

White, flexible structure w/ great resistance to 

tension in a single direction 

Major cell type is fibroblasts & are found within 

the collagen fibers 

Poorly vascularized 


Connective Tissue Proper: Dense Regular 


Figure 4.9d

Connective Tissue Proper: Dense Irregular 

Same structural elements as Dense Regular 

Connective Tissue, but the bundles of collagen are 

thicker and are arranged irregularly running in 

more than one plane. 

Major cell type is fibroblasts 

Withstands tension in many directions providing 

structural strength 

Found in the dermis, submucosa of the digestive 

tract, and fibrous organ capsules 


Connective Tissue Proper: Dense Irregular 


Figure 4.9e

Modes of Secretion Merocrine Holocrine

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