2: Histology and Embryology

Histology and Embryology

During the process of care, the dental hygienist must distinguish normal structures, variants of normal structures, and developmental abnormalities from pathology. A clear sense of developmental processes and tissue histology provides the background for competent assessment and evaluation.

Knowledge of tissue components and embryologic tissue origin supports an understanding of the physiologic changes that take place during the course of disease progression. This knowledge also provides insight into how tissue is capable of responding to a pathologic condition. This chapter contains basic general histologic information, with a focus on oral tissue components and oral and facial development. A clinician uses this knowledge to formulate a feasible dental hygiene care plan, evaluate the success of care, and make appropriate referrals to dentists and physicians when necessary.

General Histology

Cells

Smallest structures and functionally self-contained units in the body; they vary in size, shape, and surface, depending on functional specialization (Figure 2-1)

Cells possess similar common physiologic properties that permit:

The building blocks of tissues in the body are attached to each other and to noncellular surfaces by cell junctions; the structures of various types of cell junctions depend on location and function; the types of junctions are:

1. Desmosomes—cell-to-cell attachments; this type of attachment is found between ameloblasts (enamel-forming cells) and cells of the stratified squamous epithelium that lines the oral cavity

2. Tight junctions—cells attach to each other by fusion of their cell membranes; adjacent odontoblasts (dentin-forming cells) form tight junctions that prevent substances in the pulp from passing into the dentin

3. Gap junctions—contain a channel that runs between cells for communication of cell electrical impulses and passage of molecules; this type of junction is present among some odontoblasts, allowing them to coordinate their activity

4. Hemidesmosome—the attachment of a cell to a noncellular surface; the basal layer cells of stratified squamous epithelium attach to the basement membrane by hemidesmosomes; this attachment mechanism is present in the epithelial attachment to the tooth; the epithelial attachment refers to the basal lamina and hemidesmosomes that connect the junctional epithelium of the soft tissue to the tooth surface

Cells are surrounded by a cell membrane that separates them from the extracellular environment; cell membrane encloses all components of the cell:

Cell Membrane

Referred to as plasma membrane or plasmalemma; usually too thin to be seen with a light microscope; average width is approximately 7 nanometers (nm); considered selectively permeable because it controls passage of materials in and out of the cell

Trilaminar structure composed of two facing layers of lipid molecules, into which large globular proteins are inserted (Figure 2-2)

Synthesis Activities

Three types of RNA are necessary for protein synthesis

1. Messenger RNA (mRNA)—copies of short segments of deoxyribonucleic acid (DNA), the genetic code

2. tRNA—carrier of specific amino acids (building blocks of proteins)

3. Ribosomal RNA—found floating freely in the cytoplasm (polyribosomes) or attached to the ER

Protein synthesis also can occur on polyribosomes floating freely in the cytoplasm; proteins synthesized on the free polyribosomes are used in cellular metabolic processes; proteins synthesized on the ribosomes attached to the ER are transported out of the cell

Inclusions

Golgi Complex

The structure consists of stacks of closely spaced membranous sacs, in which newly formed proteins are concentrated and prepared for export out of the cell (Figure 2-4)

Responsible for secreting to the external environment a variety of proteins synthesized on the ER

Major site of membrane formation and recycling

Storage site for newly synthesized proteins

Site for packaging and transporting many cell products (e.g., polysaccharides, proteins, and lipids)

Synthesis site for lysosomes

Also involved in the production of large carbohydrate molecules and lysosomes

Mitochondria

Membranous structure bounded by inner and outer cell membranes; the membranes contain enzyme complexes in a particular array (e.g., tricarboxylic acid cycle enzymes); the inner part is formed into folds (cristae) that extend, like shelves, inside the mitochondria to provide an additional work surface area for the organelle; usually more than one mitochondria are present in a cell; the number depends on the amount of energy required by the cell

Provides the chief source of energy for the cell (“powerhouse of the cell” by oxidation of nutrients) by enzymatic breakdown of fats, amino acids, and carbohydrates; transforms the chemical energy bond of nutrients into the high-energy phosphate bonds of adenosine triphosphate (ATP)

A single cell may contain 50 to 2500 mitochondria, depending on the cell’s energy needs (Figure 2-5)

Endoplasmic Reticulum

Extensive membranous system found throughout the cytoplasm of the cell; composed of lipoprotein membranes existing in the form of connecting tubules and broad, flattened sacs (cisternae); the outer membrane may or may not be covered with ribosomes. The two types are:

Proteins are synthesized on ribosomes attached to the ER and are transported to the Golgi complex for packaging

The system contains enzymes involved in a variety of metabolic activities (e.g., lipogenesis and glycogenesis)

SER has a number of diverse roles and is found in a variety of cell types

Transport Through the Cell Membrane

Diffusion

1. Definition—continuous movement of molecules among one another in liquids or gases

2. Molecules may move across a membrane

3. Direction of diffusion of a substance is from a region of high concentration to a region of low concentration, which is the diffusion gradient

4. If equal amounts of a substance are placed at either end of a chamber such as a cell, they diffuse toward each other, and the net rate of diffusion equals zero

5. Factors that affect the diffusion rate are as follows:

6. How rapidly a substance can diffuse through the lipid matrix of the cell membrane is determined by the substance’s solubility in lipids

7. Facilitated diffusion involves the use of a carrier substance to transport a non–lipid-soluble substance across the cell membrane

8. Diffusion through pores

Osmosis

Active transport

Phagocytosis—movement of a solid particle into the cell

Pinocytosis—movement of fluid into a cell; similar to phagocytosis, except that the cell invaginates around fluid

Cell Replication

Mitosis—process of cell replication (Figure 2-6)

1. Interphase

2. Prophase

3. Metaphase

4. Anaphase

5. Telophase

Basic Tissues

At the beginning of human development, individual cells multiply and differentiate to perform specialized functions; groups of cells with similar morphologic characteristics and functions come together and form tissues

Tissue components

Tissues in the human body can be classified into four types:

Each of the four basic tissues may be further subdivided into several variations

Epithelial Tissue

Main categories

The epithelium consists exclusively of cells held together by specialized cell junctions (little intercellular material is present between cells); cells rest on an underlying connective tissue, the basement membrane

Epithelial cells (keratinocytes) form continuous sheets (tissues) and perform the following functions:

Epithelial tissue varies, depending on function—it may have surface specializations on its free surfaces

Replicates through mitosis

Surface Epithelia

The epithelium is classified according to:

Combined characteristics allow for six different types of epithelia (Figure 2-7) and locations:

Other intermediate forms of epithelium:

Other cell types found in the epithelium:

The epithelium lining the oral cavity (oral mucosa) and the skin (dermis) is an example of stratified squamous epithelium

Connective Tissue

Connective Tissue Proper

All connective tissue proper develops from the embryonic mesenchyme; contains large amounts and various types of intercellular material and few cells; highly vascular; has two main functions:

Types of connective tissue:

Types of connective tissues—differ in composition of cell products and proportions of products present:

Connective Tissue Components

Cells

Fibrous matrix

1. Matrix of connective tissue composed of some or all of the following fibers:

a. Collagen fibers—consist of three long polypeptide chains coiled in a left-handed helix to form a tropocollagen unit, which is assembled in a “quarter-stagger” model outside of the cell; fibers are highly resistant to tension and are part of the anchoring mechanism by which the connective tissue attaches the basement membrane (see Figure 2-20, B); the most abundant fibers found in connective tissue

b. Reticulin fibers—comparable with collagen fibers in their protein composition; usually found in the border areas between connective tissue and other tissues

c. Elastic fibers—consist of long fibrous proteins that differ in composition from collagen; are the branching fibers responsible for recoiling tissues when they are stretched

d. Oxytalan fibers—resemble elastic fibers in morphology and chemical composition; are believed to be immature elastic fibers

Ground substance

Types (Cartilage and Bone)

Cartilage

Cartilage and bone are sister tissues, both highly specialized forms of connective tissue, whose intercellular substances have assumed particular properties that allow them to perform support functions

Cartilage, like all types of connective tissue, has three components:

Types

1. Hyaline cartilage

2. Fibrous cartilage (fibrocartilage)

3. Elastic cartilage

Bone

A specialized vascular connective tissue composed of a mineralized organic matrix; the inorganic component of bone is hydroxyapatite:

image

Two main functions:

Characteristic of all bones

Bone morphology

1. Bone-forming cells (osteoblasts) are produced from undifferentiated mesenchymal cells of the periosteum, the endosteum, and the periodontal ligament

2. Osteoblasts become incorporated into bone during their formation; they occupy a space called lacuna (plural, lacunae)

3. Lacunae are connected to each other by means of a system of canals named canaliculi; these canals house the cytoplasmic extension of the osteocytes and provide a means for the transport of vascular components

4. Both compact and trabecular mature bones are formed in layers, or lamellae. Lamellae are found in three distinct types of arrangements present in all mature human bones (Figure 2-8):

a. Concentric, or haversian system, bone makes up the bulk of compact bone; it consists of lamellae arranged in concentric circles around a blood vessel (haversian canal) to form an osteon. An osteon, which consists of this concentrically arranged bone and haversian canal, is the basic metabolic unit of bone

b. Interstitial lamellae fill the space between the concentric circles of the haversian system bone

c. Lamellar bone is not arranged in concentric circles and is found on the surfaces of most bones. This bone is further defined by its location. When found on the outer aspects or the circumference of the bone underneath the periosteum, it is referred to as circumferential, or (sub)periosteal, bone; when found on the surfaces of trabeculae or the inner aspect of compact bone, it is referred to as (sub)endosteal bone

Bone tissue

1. Bone, like all connective tissues, has three main components:

2. Bone is formed by osteoblasts developed in one of two ways:

a. Intramembranous ossification—mesenchymal cells move closer together (condensation), differentiate into osteoblasts, and begin to deposit bone matrix; this is how the maxilla and the mandible are formed

b. Endochondral ossification—future bone is preformed in a cartilage model that is eventually resorbed and replaced by new bone formed by osteoblasts (Figure 2-9)

Structure of long bones (macroscopic)

Blood and Lymph

Vascular system

1. Develops embryonically from mesenchymal cells that come together and form delicate tubular structures composed of endothelial cells

2. Consists of the heart, blood vessels, and lymphatics

3. Functions

Lymph vessels empty into filtering organs (nodes) and generally flow toward larger lymph vessels, the thoracic duct, and the right lymphatic duct; lymph enters the venous branches of the circulatory system

Blood Vessels

Arteries—the largest of the blood vessels; walls are composed of:

Veins—usually accompany arteries but carry blood in the opposite direction

Capillaries—the simplest of the blood vessels in the structure

Blood Components

Nerve Tissue

Main functions of the nervous system

The nervous system can be classified as follows:

Afferent nerves transmit impulses (sensations) from the periphery to the CNS (sensory input); efferent nerves transmit impulses (commands) from the CNS to muscles and other organs (motor output) (Figure 2-10)

Divisions of the nervous system

Jan 1, 2015 | Posted by in Dental Hygiene | Comments Off on 2: Histology and Embryology
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