Modes of Secretion 1. Merocrine – secretion by exocytosis (pancreas, sweat, and salivary glands) 2. Holocrine – secretion by rupture of cells (sebaceous glands) 3. Apocrine – secretion by pinching off the top of the cell (some sweat glands)
Modes of Secretion Figure 4.5
Connective Tissue Most abundant tissue type Wide distribution Four main types 1. Connective tissue proper 2. Cartilage 3. Bone 4. Blood
Connective Tissue Figure 4.6
Functions of Connective Tissue 1. Binding and support 2. Protection 3. Insulation (adipose) 4. Transportation (blood)
Characteristics of Connective Tissue Connective tissues have: Mesenchyme as their origin Varying degrees of vascularity Nonliving extracellular matrix made of: Ground substance Fibers
Structural Elements of Connective Tissue 1. Ground substance – unstructured material that fills the space between cells 2. Fibers – collagen, elastic, or reticular 3. Cells – fibroblasts, chondroblasts, osteoblasts, and hematopoietic stem cells
Ground Substance Interstitial fluid Adhesion proteins – fibronectin and laminin Proteoglycans – glycosaminoglycans (GAGs) and proteins GAGS = keratan sulfate, hyaluronic acid, and chondroitin sulfate Molecular sieve through which nutrients diffuse between capillaries and cells
Ground Substance: Proteoglycan Structure Figure 4.7
Fibers 1. Collagen – high tensile strength 2. Elastic – allow for stretch 3. Reticular – collagenous fibers that form delicate networks
Cells Fibroblasts – connective tissue proper Chondroblasts – cartilage Osteoblasts – bone Hematopoietic stem cells Form red and white blood cells and mast cells
Connective Tissue Proper Loose – not many fibers 1. Areolar 2. Adipose 3. Reticular Dense – densely packed fibers 1. Dense regular 2. Dense irregular
Areolar Connective Tissue: Model Figure 4.8
Connective Tissue Proper: Loose 1. Areolar connective tissue Gel-like matrix All three fiber types Fibroblasts, macrophages, mast cells, and white blood cells Widely distributed In mucous membranes under epithelium Capillaries Around organs
Connective Tissue Proper: Loose Figure 4.9a
Connective Tissue Proper: Loose 2. Adipose connective tissue Gel-like matrix Sparse collagen fibers Adipocytes Reserves food stores, insulates, supports, and protects Found under skin, around kidneys, within abdomen, and in breasts Local fat deposits in highly active organs
Connective Tissue Proper: Loose Figure 4.9b
Connective Tissue Proper: Loose 3. Reticular connective tissue Loose ground substance with reticular fibers Stroma supports cells Contains WBCs, mast cells, and macrophages Found in lymph nodes, bone marrow, and the spleen
Connective Tissue Proper: Loose Figure 4.9c
Connective Tissue Proper: Dense Regular Parallel collagen fibers with a few elastic fibers Major cell type is fibroblasts Withstands unidirectional tension Found in tendons, ligaments, and aponeuroses
Figure 4.9d
Connective Tissue Proper: Dense Irregular Irregularly arranged collagen fibers with some elastic fibers Major cell type is fibroblasts Withstands tension in many directions Found in the dermis, fibrous organ capsules, and joint capsules
Figure 4.9e