Connective Tissue Ii

CONNECTIVE TISSUE II

ADIPOSE TISSUE

Marylee Kott, MD

Reading: Gardner and Hiatt Chapter 3; Klein and McKenzie, pp77-85 (Though not much here on adipose); Junqueira Chapter 6, p123

Objectives:

·  Recognize brown and white adipose tissue in the light microscope

·  Know their respective functions.

Key Words: White adipose tissue, brown adipose tissue

·  Fat cells or adipocytes

·  Found in aggregates of varying sizes, constituting adipose tissue.

·  Aggregates may only be apparent in layers or groups of cells seen microscopically.

·  Adipose tissue accounts for 20-25% of the weight of women, 15-20% of the male's weight.

·  Two separate categories of adipocytes which differ in distribution, histological appearance, and function:

1.  white adipose tissue

2.  brown adipose tissue

The mature fat cell (unilocular adipocytes) is in actuality larger in relation to other cell types.

·  Development of Fat cells:

·  Undifferentiated mesenchymal cells are transformed into lipoblasts

·  Lipoblasts accumulate fat and give rise to mature fat cells.

·  When the body mobilizes a large amount of lipid, mature unilocular fat cells return to the

·  lipoblast stage.

Undifferentiated mesenchymal cells also give rise to a variety of other cell types, including fibroblasts.

·  White Adipose Tissue (WAT: unilocular fat):

·  The predominant type of fatty tissue in humans

·  Consists of fat cells with a single large lipid inclusion.

·  Cells may occur singly or in groups.

·  White fat is ubiquitously distributed throughout the body, padding and insulating the organs.

·  Geographic deposition determined by age, sex and hormonal status.

·  White fat also functions as an energy reservoir, providing a low-weight, high calorie form of

energy (relative to glycogen), without thermogenic properties.

Photomicrograph of unilocular adipose tissue of a young mammal. Arrows show nuclei of adipocytes (fat cells) compressed against the cell membrane. Note that, although most cells are unilocular, there are several cells (asterisks) with small lipid droplets in their cytoplasm, an indication that their differentiation is not yet complete.

·  Brown Adipose Tissue (BAT: multilocular fat):

·  Uncommon in adult humans.

·  Sole purpose appears to be heat generation in mammals, in which it functions upon arousal from hibernation, survival after birth, and during cold stress.

·  The main site of non-shivering heat production under conditions of cold stress.

·  Brown fat is widely distributed in the first decade of life in the human, becoming confined with increasing age to deeper organs of the body such as the kidney.

·  Lipid accumulates in each of the fat cells as many individual droplets of varying sizes, i.e., a multilocular pattern.

·  When lipolysis is initiated, fatty acids enter the numerous large mitochondria of these cells where oxidative phosphorylation is increased, and energy is liberated as heat.

·  The mitochondria and rich vascularity contribute to its dark color.

Photomicrograph of multilocular (brown) adipose tissue (lower portion), with its characteristic cells containing central spherical nuclei and multiple lipid droplets. For comparison, the upper part of the photomicrograph shows unilocular (white) tissue.

·  BAT is not WAT

·  Unilocular fat cell (WAT) develops from a fibroblast-like precursor into a cell with multiple lipid droplets similar to those of BAT.

·  The individual droplets then coalesce into a single lipid droplet.

·  Extreme fasting can reverse this pattern, transforming the cell from unilocular to multilocular, to a narrow, fibroblast-like cell. Thus it is possible for BAT and WAT to resemble each other under certain circumstances with regard to lipid distribution. Closer examination, however, will reveal prominent differences in mitochondria, vascularity, and function.

·  However, BAT and WAT probably arise from two separate lines of precursor mesenchymal cells, both of which are capable of storing lipid in either the multilocular or unilocular form, but each of which possesses certain identifiable characteristics which set it apart from the other (see below).

WAT BAT

Location in humans Ubiquitous at all ages Important in the newborn

Composition 82% fat; 15% water; 3% protein 30% fat; 55% water; 15% protein

Blood Supply Rich capillary supply Much denser than WAT

Innervation Sympathetic Sympathetic

Function Food storage, padding Thermogenesis

Derivation WAT-specific mesenchymal cell BAT-specific cell

Morphology Usually unilocular Multilocular

80-150 nm diameter 8-60 nm diameter

Prominent mitochondria

·  Obesity

·  Hyperplasic obesity: increase in number of adipocytes (see below).

·  Hypertrophic obesity: Excessive accumulation of fat in unilocular tissue cells that become larger than usual (usual mechanism of obesity).

·  Fat accumulation in humans:

·  Human neonate is born with fat stores which have accumulated since about the 30th week of gestation.

·  Throughout a finite post parthum period, the infant continues to add fat cells.

·  A second period of hyperplastic growth occurs just prior to and during puberty, especially in females.

·  New studies have demonstrated that hyperplastic growth can also occur in adulthood, when adipocytes are filled with lipid and grow to a critical size, probably over a long period of time, and precursor cells are stimulated to differentiate.

·  Once adipocytes are formed, they remain throughout life and only a reduction of cell size is possible.

CARTILAGE

Reading: Gartner and Hiatt pp 65-88; Klein and McKenzie pp 95-110; Junquiera Chapter 7

Objectives:

·  Describe the basic histologic organization of cartilage in general.

·  Distinguish 3 types of cartilage with regard to light microscopic appearance, sites of distribution, and composition.

·  Discuss 2 ways cartilage can grow.

Key Words: Hyaline cartilage, elastic cartilage, fibrous cartilage, appositional growth and interstitial growth

·  GENERAL CHARACTERISTICS

·  Well-defined structural organization.

·  Consists of abundant amounts of intercellular material, or matrix, produced by cells that are permanently surrounded and entrapped in the matrix. It is firm and rigid, yet has some flexibility.

·  Usually surrounded by a fibrous connective tissue called the perichondrium.

·  Functions for support, growth of most bones, and articulation of bones.

·  Avascular.

·  BASIC STRUCTURE

·  Definitions:

·  Chondroblast: Immature cartilage cell, always located on the periphery (perichondrium) of the cartilage.

·  Chondrocyte: Mature cartilage cell.

·  Lacuna: A space in the cartilage occupied by a chondrocyte.

·  Capsule: Basophilic cartilage matrix immediately surrounding each lacuna.

·  Isogenic cell nest: A cluster of chondrocytes resulting from division of a mature chondrocyte into daughter cells.

·  Structure:

·  Chondrocytes lay down and maintain the surrounding matrix.

·  Nutrition to all chondrocytes is via diffusion through the matrix, limiting the size which cartilage can attain.

·  Most, but not all, cartilage is surrounded by dense irregular connective tissue, the perichondrium

·  Perichondrium serves both for protection, with an outer fibrous layer, and as a source of new cartilage, the inner chondrogenic layer.

·  The fibrous layer contains abundant Type I collagen and fibroblasts.

·  The chondrogenic layer contains Type I collagen and some fibroblasts, but also a population of stem cells called osteoprogenitor cells

·  Osteoprogenitor cells may become either chondroblasts and then a chondrocytes in cartilage or osteoblasts and then osteocytes in bone

·  With the proper stimulus, such as growth hormone, uncommitted osteoprogenitor cells in the chondrogenic layer of the perichondrium turn into chondroblasts, changing from a flat, fibroblast-like appearance into a plump, active cell that starts producing a matrix around it -- the chondrocyte.

·  CHEMICAL COMPOSITION OF MATRIX

·  Water (70%)

·  Type II collagen (40% of dry weight)

·  Glycosaminoglycans - Abundant chondroitin 4-sulfate, chondroitin 6-sulfate and keratin sulfate. GAGs are especially concentrated in the capsular areas, accounting for the strong basophilia of this region.

·  Various proteins and glycoproteins.

·  TYPES OF CARTILAGE

All cartilage shares the same basic structure; however, three types are differentiated by the types of fibers in the matrix. The various characteristics they impart determine where each type of cartilage is best located.

·  HYALINE CARTILAGE

·  Most abundant/prevalent in the body

·  Bluish-white when fresh

·  Composed of fine Type II collagen fibrils that are not resolvable under microscope, therefore under microscope appearing glassy and homogeneous.

·  A perichondrium may or may not be present.

·  Locations:

1.  Ala of nose

2.  Costal cartilage

3.  Respiratory tract

4.  Fetal skeleton

5.  Articular surfaces of synovial joints

Hyaline cartilage. Trachea.

·  ELASTIC CARTILAGE

·  Composed of elastic fibers in the matrix, in addition to Type II collagen.

The thickness of the fibers permits them to be resolved with light microscopy, therefore appearing fibrous.

·  Elastic fibers allow more flexibility, allowing bending.

·  Elastic cartilage is always associated with a perichondrium.

·  Locations:

1.  External ear

2.  Auditory tube

3.  Epiglottis

Elastic cartilage. Epiglottis. Human.

·  FIBROUS (FIBRO) CARTILAGE

·  Composed of Type I collagen in the matrix in addition to Type II collagen.

·  Type I fibers can be seen swirling though the matrix, sometimes forming bundles.

·  Found in high stress areas where strength is necessary, as at the attachments for bone and muscle.

·  A perichondrium is never associated with fibrocartilage.

·  Locations:

1. Intervertebral disks (annulus fibrosus)

2. Pubic symphysis

3. Intersections of tendons with lateral aspects of articular cartilage

4. Temporomandibular joint articular surface

Fibrocartilage. Intervertebral disc.

·  GROWTH OF CARTILAGE

·  Appositional Growth

·  Growth due to division and differentiation of chondroblasts on the outer surface.

·  Cartilage is added to the periphery.

·  Osteoprogenitor cell ® ® chondroblast ® ® chondrocyte ® ® outer matrix

·  Occurs in perichondrium

Hyaline cartilage – Fibrous perichondrium below epithelium with chondrogenic layer housing chondrogenic cells which participate in appositional growth of cartilage at the periphery.

·  Interstitial Growth

·  Growth due to division of pre-existing, medially located chondrocytes, adding matrix to the inner mass of the cartilage.

·  This process does NOT require chondroblasts.

·  Occurs in the early phases of cartilage formation, in the epiphyseal plates (growth increasing length of long bones) and within articular cartilage.

Epiphyseal plate. Zone of chondrocytes proliferation at top adding to interior mass

of cartilage – interstitial growth of cartilage.