LBBBIO7 LE 1 | Topic 2: Connective Tissues

Loose and Dense CT

Main classifications of connective tissue

Embryonic CT

- resembles the mesenchyme or mucous connective tissue; this is loose and irregular connective tissue.

- The fibroblasts are numerous, and fine collagen fibers are found between them, some coming in close contact with fibroblasts.

- Embryonic connective tissue is vascular. Capillaries lined with endothelium and filled with red blood cells are visible in the ground substance.

Loose CT

- It is characterized by a loose, irregular arrangement of connective tissue fibers and abundant ground substance.

- more prevalent than dense CT

Collagen fibers, fibroblasts, adipose cells, mast cells, and macrophages

give an example of common cell types that can be found in loose CT

Areolar tissue

- a type of loose connective tissue characterized by a loosely arranged network of collagen and elastic fibers, abundant ground substance, and diverse cell types, including fibroblasts and immune cells.

- Its "airy" appearance, from which it gets its name, allows for the diffusion of nutrients and waste to surrounding avascular tissues.

- This tissue provides support, protection, and cushioning, binding tissues together while remaining flexible and absorbing shock.

- It is widely distributed throughout the body, found under the skin, around blood vessels, nerves, and organs, and in mucous membranes.

Dense CT

- contains thicker and more densely packed collagen fibers, exhibiting a regular and parallel arrangement, with fewer cell types and less ground substance.

- The collagen fibers in dense irregular connective tissue exhibit a random and irregular orientation

- Fibroblasts are also the most abundant cells, which are located between the dense collagen bundles.

dense regular CT

- contains thicker and more densely packed collagen fibers, exhibiting a regular and parallel arrangement, with fewer cell types and less ground substance.

- has collagen fibers bundled in parallel, providing high tensile strength in one direction and is found in tendons and ligaments.

dense irregular CT

- has randomly interwoven collagen fibers, offering resistance to forces from multiple directions and is found in the skin's dermis, organ capsules, and joint capsules.

tendon

- a flexible but inelastic cord of strong fibrous collagen tissue attaching a muscle to a bone

adipose tissue

- specialized connective tissue that stores energy as fat, insulates the body, and provides mechanical protection.

- It functions as an active endocrine organ, releasing hormones like leptin and influencing insulin sensitivity, immunity, and temperature regulation.

white adipose tissue

- Occur singly or in groups

- Store fat (lipid) as single large droplet primarily as triglycerides

- Appear as empty cells because lipid is dissolved during tissue preparation

- Distributed throughout body, serves as insulation, and forms fat pads for organ protection

- Exhibit numerous receptors for different hormones that influence accumulation and release of lipid

- Secrete hormone leptin to increase lipid metabolism and to inhibit appetite

brown adipose tissue

- Cells smaller than white adipose cells; store lipid as multiple droplets

- Best developed in hibernating animals

- In newborns or animals emerging from hibernation, generates body heat

- Norepinephrine from sympathetic nervous system promotes hydrolysis of lipids

fibroblasts and fibrocytes

two most common cell types in the connective tissue

macrophages or histiocytes

phagocytic cells and are most numerous in loose connective tissue. They are difficult to distinguish from fibroblasts, unless they are performing phagocytic activity and contain ingested material in their cytoplasm.

mast cells

- usually closely associated with blood vessels, these cells are widely distributed in the connective tissue of the skin and in the digestive and respiratory organs

- spherical cells filled with fine, regular dark-staining and basophilic granules

plasma cells

- arise from the lymphocytes that migrate into the connective tissue. These cells are found in great abundance in loose connective tissue and lymphatic tissue of the respiratory and digestive tracts.

leukocytes, neutrophils, and eosinophils

- three types of cells that migrate into the connective tissue from the blood vessels. Their main function is to defend the organism against bacterial invasion or foreign matter.

collagen, elastic, and reticular

3 types of connective tissue fibers

collagen fibers

Tough, thick, fibrous proteins that do not branch. They are the most abundant fibers and are found in almost all connective tissue of all organs

Type I

These collagen fibers are found in the dermis of skin, tendons, ligaments, and bone. They are very strong and offer great resistance to tensile stresses.

Type II

- These collagen fibers are present in hyaline cartilage and elastic cartilage. The fibers provide resistance to pressure.

Type III

- These are the thin, branching reticular fibers that form the delicate supporting meshwork in such organs as the lymph nodes, spleen, and bone marrow.

Type IV

- These collagen fibers are present in the basal lamina of the basement membrane, to which the basal regions of the cells attach

reticular fibers

- these fibers consist mainly of type III collagen, and are thin and form a delicate netlike framework in the liver, lymph nodes, spleen, hemopoietic organs, and other locations where blood and lymph are filtered

- support capillaries, nerves, and muscle cells. These fibers become visible only when the tissue or organ is stained with silver stain

elastic fibers

- thin, small, branching fibers that allow stretch. They have less tensile strength than collagen fibers and are composed of microfibrils and the protein elastin

- In the walls of the aorta and pulmonary trunk, the presence of these fibers allows for stretching and recoiling of these vessels during powerful blood ejections from the heart ventricles.

large lymphocyte

- a spherical cell that has a great amount of cytoplasm present.The dense-staining nuclei of all lymphocytes have condensed chromatin but no nucleol

plasma cell

- exhibits a smaller, eccentrically placed nucleus with condensed, coarse chromatin clumps distributed peripherally in a characteristic radial (cartwheel) pattern and one central mass. A prominent, clear area in the cytoplasm is adjacent to the nucleus

fibroblast

- an elongated cell with cytoplasmic projections, an ovoid nucleus with sparse chromatin, and one or two nucleoli

fibrocyte

- a more mature, smaller spindle-shaped cell without cytoplasmic projections; the nucleus is similar but smaller than that in the fibroblast

eosinophil

- a large blood cell with a bilobed nucleus and large, cytoplasmic granules that fill the cytoplasm

neutrophil

- a large blood cell, characterized by a multilobed nucleus and a lack of stained granules in the cytoplasm

small lymphocyte

- a spherical cell that has a small amount of cytoplasm in comparison to the large lymphocyte. The dense-staining nuclei of all lymphocytes have condensed chromatin but no nucleoli.

ground substance

- Consists of extracellular matrix, a semifluid gel with high water content.

- Contains polysaccharide chains of glycosaminoglycans, proteoglycans, and adhesive glycoproteins

- Hyaluronic acid is the main glycosaminoglycan

- Other glycosaminoglycans from proteoglycan aggregates, which attract water

- Facilitates diffusion between cells and blood vessels

- Barrier to spread of pathogens

cartilage

- a special form of connective tissue that also develops from the mesenchyme

- Similar to the connective tissue, it consists of cells and extracellular matrix composed of connective tissue fibers and ground substance

- exhibits tensile strength, provides firm structural support for soft tissues, allows flexibility without distortion, and is resilient to compression

hyaline, elastic, and fibrocartilage

3 main types of cartilage in the body

hyaline cartilage

- the most common type of cartilage

- In embryos, it serves as a skeletal model for most bones

elastic cartilage

- is similar in appearance to hyaline cartilage, except for the presence of numerous branching elastic fibers within its matrix

- highly flexible and occurs in the external ear, walls of the auditory tube, epiglottis, and larynx.

fibrocartilage

- is characterized by large amounts of irregular and dense bundles of coarse collagen fibers in its matrix.

- In contrast to hyaline and elastic cartilage, it consists of alternating layers of cartilage matrix and thick dense layers of type I collagen fibers. The collagen fibers normally orient themselves into the direction of functional stress.

perichondrium

Most of the hyaline and elastic cartilage is surrounded by a peripheral layer of vascularized, dense irregular connective tissue called ______________

cartilage matrix

- Produced and maintained by chondrocytes and chondroblasts

- Contains large proteoglycan aggregates and is highly hydrated

- Allows diffusion and is semirigid shock absorber

- Adhesive glycoprotein chondronectin binds cells and fibrils to surrounding matrix

ossification

- the natural process of bone formation or the conversion of soft tissue into bone

- This process involves specialized cells called osteoblasts that deposit bone tissue into connective tissue or a cartilaginous framework

endochondral ossification

- hyaline cartilage model calcifies and cells die

- Mesenchyme cells in periosteum differentiate into osteoprogenitor cells and form osteoblasts

- Osteoblasts synthesize osteoid matrix, which calcifies and traps osteoblasts in lacunae as osteocytes

- Osteocytes establish cell-to-cell communication via canaliculi

- Primary ossification center forms in diaphysis and secondary center of ossification in epiphysis

- Epiphyseal plate between diaphysis and epiphysis allows for growth in bone length

articular cartilage

In endochondral ossification, all cartilage is replaced except the ___________________

epiphysis

the expanded, usually rounded, end of a long bone that articulates with other bones at a joint and is a site for red blood cell production,

diaphysis

the hollow, tubular shaft in the middle of the bone containing the medullary cavity filled with yellow bone marrow, responsible for bone growth and remodeling

intramembranous ossification

- Bone develops directly from osteoblasts that produce the osteoid matrix

- Initially form spongy bone that consists of trabeculae

- Mandible, maxilla, clavicle, and flat skull bones are formed by this process

- Fontanelles in newborn skull represent areas where intramembranous ossification is occurring

decalcification

- the softening of bones due to the removal of calcium ions, and can be performed as a histological technique to study bones and extract DNA

- This process also occurs naturally during bone development and growth, and when uninhibited, can cause diseases such as osteomalacia

compact bone

dense, hard, outer layer of bones, providing strength and protection against compressive forces

spongy bone

lighter, less dense, and porous, found inside bones, especially at the ends, where it offers flexibility, shock absorption, and houses red bone marrow for blood cell production

osteons, trabeculae

The key differences in the structures of compact and spongy bones: compact bone is organized into dense ________, while spongy bone consists of a network of interconnected struts called _______.

compact bone

Structure: Dense, with a microscopic structural unit called on osteon (or Haversian system)

Location: Forms the outer cortex of all bones

Function: Provides strength, protection, and can withstand high stress

Characteristics: Smooth, heavy, and strong

Color: White

spongy/cancellous bone

Structure: Porous, with an internal network of interconnected rods and plates called trabeculae

Location: Found in the interior of bones, especially at the ends (epiphyses) of long bones and in flat and short bones

Function: Reduces bone weight, provides flexibility and support, and contains bone marrow for blood cell formation

Characteristics: Lightweight, less dense, and porous, resembling a sponge but still firm

Color: Often pinkish

bone marrow

- the soft, spongy, fatty tissue found in the hollow centers of bones that acts as the body's "blood cell factory"

- It contains hematopoietic stem cells, which develop into red blood cells (carrying oxygen), white blood cells (fighting infection), and platelets (helping with blood clotting).

red and yellow bone marrow

two main types of bone marrow

red bone marrow

produces blood cells

yellow bone marrow

stores fat

periosteum

Outer layer, fibrous layer of bone

endosteum

- Inner layer of bone, condensed CT

- Contain bone and blood cell precursors

osteoprogenitor cells

- undifferentiated, pluripotential stem cells derived from the connective tissue mesenchyme.

- These cells are located on the inner layer of connective tissue periosteum and in the single layer of internal endosteum that lines the marrow cavities, osteons (Haversian system), and perforating canals in the bone

osteoblasts

- present on the surfaces of bone.

- They synthesize, secrete, and deposit osteoid, the organic components of new bone matrix.

osteocytes

- the mature form of osteoblasts and are the principal cells of the bone; smaller than osteoblasts

- lie in the cave-like lacunae and are very close to a blood vessel. In contrast to cartilage, only one osteocyte is found in each lacuna.

osteoclasts

- large, multinucleated cells found along bone surfaces where resorption (removal of bone), remodeling, and repair of bone take place.

- They do not belong to the osteoprogenitor cell line, Instead, the osteoclasts originate from the fusion of blood or hemopoietic progenitor cells that belong to the mononuclear macrophage-monocyte cell line of the bone marrow.

Organic components/osteoid

Inorganic components

components of bone matrix

organic components/osteoid

- Constitute approximately 50% of bone volume; 25% bone weight

- Major component is 90-95% Type I Collagen

- Ground substances: proteins, carbohydrates, lipids, proteoglycans

inorganic components

- Bone minerals make up 50% bone volume and 75% bone weight

- Consists mainly of calcium and phosphate (hydroxyapatites)

- With some bicarbonates, citrate, magnesium, potassium, and trace metals

blood

- a unique form of connective tissue that consists of three major cell types: erythrocytes (red blood cells), leukocytes (white blood cells), and platelets (thrombocytes).

- transport gases, nutrients, waste products, hormones, antibodies, various chemicals, ions, and other substances in the plasma to and from different cells in the body

hemopoiesis

Blood cells have a limited life span, and, as a result, they are continuously replaced in the body by a process called __________-

pluripotential hemopoietic stem cell

stem cell that can produce all blood cell types

pluripotential myeloid stem cells and pluripotential lymphoid stem cells

Pluripotential stem cells, in turn, produce two descendants that form ________________

myeloid stem cells

_________ develop in red bone marrow and give rise to erythrocytes, eosinophils, neutrophils, basophils, monocytes, and megakaryocytes

B lymphocytes

Some lymphoid cells remain in the bone marrow, proliferate, mature, and become _____________

T lymphocytes

Other undifferentiated lymphoid cells migrate to the thymus gland, where they proliferate and differentiate into immunocompetent _____________

yolk sac, liver, spleen, and lymph nodes

sites of hemopoiesis in an embryo

limited to red bone marrow (skull, sternum, ribs, vertebrae, pelvis)

sites of hemopoiesis in an adult

RBCs/erythryocytes

- Most numerous cells in blood

- nonnucleated cells that remain in the blood

- Contains hemoglobin with iron molecules in cytoplasm

- Biconcave shape increases surface area to carry respiratory gases

- Life span is about 120 days, after which cells are phagocytosed in spleen, liver, and bone marrow

carbaminohemoglobin

RBCs carry oxygen as oxyhemoglobin and carbon dioxide as __________

WBCs/leukocytes

Nucleated, larger than RBCs

granulocytes

___________ contain cytoplasmic granules; they are neutrophils, eosinophils, and basophils

agranulocytes

____________ are without cytoplasmic granules; they are monocytes and lymphocytes

neutrophils

- Cytoplasm appears clear under microscope

- Nucleus contains several lobes connected by thin chromatin strands

- Have a short life span in blood or connective tissue, from hours to days

- Are very active phagocytes that are attracted to foreign material by chemotactic factors

- Destroy phagocytosed (ingested) material with lysosomal enzymes

- Constitute about 60 to 70% of blood leukocyte

eosinophils

- Cytoplasm filled with large pink granules

- Nucleus typically bilobed

- Have a short life span, in blood or connective tissue

- Are phagocytic with affinity for antigen-antibody complexes

- Release chemical that neutralizes histamine and other mediators of inflammatory reactions

- Increase during parasitic infestation to destroy helminthic parasites

- Constitute about 2 to 4% of the blood leukocytes

basophils

- Cytoplasm contains dark blue or brown granules

- Have a short life span

- Granules contain histamine and heparin

- Exposure to allergens release histamine that causes intense inflammatory response in severe allergic reactions

- Constitute less than 1% of blood leukocytes

lymphocytes

- No granules in cytoplasm and vary in size from small to large

- Dense-staining nucleus surrounded by a narrow cytoplasmic rim

- Life span is from days to months

- Essential in immunologic defense of organism

- Plasma cells release antibodies to counteract or destroy invading organisms

- Constitute about 20 to 30% of blood leukocytes

b lymphocytes/cells

Function: create antibodies, which are proteins that identify and tag foreign invaders, such as bacteria, viruses, and toxins.

Mechanism: Antibodies bind to antigens (markers on invaders), marking them for destruction or preventing them from causing infection.

Immunity type: responsible for humoral immunity

Maturation: mature in the bone marrow

t lymphocytes/cells

Function: act as direct fighters, destroying infected cells and tumor cells, and also regulate the immune system by sending signals to activate other parts of the immune system

Mechanism: They directly attack foreign invaders and infected or cancerous cells.

Immunity type: are the key components of cell-mediated immunity

Maturation: migrate to the thymus for maturation

monocytes

- Largest agranular leukocyte characterized primarily by horseshoe-shaped nucleus

- Live in connective tissue for months where they become powerful phagocytes

- Are part of the mononuclear phagocyte system

- Constitute about 3 to 8% of blood leukocytes

platelets/thrombocytes

- Are fragments of bone marrow megakaryocytes and not blood cells

- Function in blood vessels to promote blood clotting when blood vessel wall is damaged

- In damaged vessels form plug; increase plug size through adhesive glycoproteins and fibrin

- Fibrin traps platelets and blood cells, and forms blood clot

- Cause clot retraction and removal through enzymatic action

- contains thicker and more densely packed collagen fibers, exhibiting a regular and parallel arrangement, with fewer cell types and less ground substance.