4.2. Prueba de hipótesis
4.2.5. Prueba de hipótesis específica 4
A. Development of the oropharynx
The face begins forming in the fourth week; at that time the body is about 1/
16inch long.
1. The neural plate bends ventrally, pushing the heart ventrally.
a. An oral pocket develops between the forebrain and the heart that will even- tually become the oral cavity.
B. Branchial arches
1. Externally, they appear as swellings; inter- nally, they form pharyngeal pouches. a. Resemble gill slits.
2. Externally, they will disappear by the end of the fifth week as the second grows down to contact the fifth branchial arch. a. There are five or six branchial arches.
b. They are separated by branchial clefts or grooves.
C. Branchial grooves and pharyngeal pouches 1. The clefts seen between the arches.
a. The first branchial groove deepens to become the external auditory canal leading to the middle ear.
b. The membrane at the depth of the first branchial groove becomes the tym- panic membrane.
c. The middle ear and eustachian tube develop from the corresponding first pharyngeal pouch.
d. The second pharyngeal pouch becomes the palatine tonsils.
(1) Tonsils function in the develop- ment of lymphocytes.
e. The third pharyngeal pouch be- comes the inferior parathyroids and thymus.
f. The fourth pharyngeal pouch becomes the superior parathyroid.
g. The fifth pharyngeal pouch becomes the ultimobranchial body.
(1) Ultimobranchial function is unknown. D. Vascular development
Each of the branchial arches has a right and left aortic arch vessel that leads from the heart through the arches to the face, brain, and posterior of the body.
1. The first and second branchial arches begin to develop in the fourth week and disappear in the fifth week.
2. The third arch vessels then become prominent, taking over the facial area from the first two.
3. As the fourth and fifth arch vessels arise, the fourth becomes prominent and the fifth disappears.
4. Finally, the sixth arch vessels arise and become dominant, along with the third and fourth.
5. The third arch vessels become the com- mon carotid arteries, which supply the neck, face, and brain.
6. The fourth arch vessels become the dor- sal aorta, which supplies the rest of the body.
7. The sixth arch vessels supply the lungs. E. Muscular and neural development
Muscle cells become apparent in the first arch in the fifth week.
1. They spread within the mandibular arch into each muscle’s site of origin in the sixth and seventh weeks.
2. The muscles of the first arch become the muscles of mastication.
3. By the tenth week, the muscles of the sec- ond arch have formed a thin sheet extend- ing over the face and posterior to the ear. 4. The muscles of the fourth arch form the
pharyngeal constrictor muscles.
5. By the end of the seventh week, cranial nerve (CN) V (first arch) and CN VII (sec- ond arch) have interdigitated with their respective muscle groups.
6. The CN IX interdigitates with the third arch. 7. The CN X interdigitates with the fourth
arch.
F. Cartilagenous skeletal development
1. Meckel’s cartilages appear bilaterally in the first arch.
a. These structures will later be absorbed into the forming jaw.
b. Their posterior hinge will become the malleus of the ear.
2. Reichert’s cartilage appears in the second arch.
a. The stapes, styloid process, lesser horn, and upper body of the hyoid arise from this arch.
3. The third arch forms the greater horn and lower part of the hyoid.
4. The fourth arch forms hyoid cartilage. 5. The fifth arch has no adult cartilage deriv-
ative.
6. The sixth arch forms the laryngeal carti- lage.
G. Cartilages of the face
The cartilaginous nasal capsule (ethmoid), the sphenoid, the auditory capsules, and the basioc- cipital cartilages are the first skeletal structures seen in the craniofacial area.
1. They are called the cranial base.
2. Later, these will separate to form individual bones by endochondral bone formation. H. Bones of the face
1. The protective bones of the face do not form from cartilage.
a. These include the frontal, parietal, and squamous portions of the temporal and interoccipital bones.
2. Facial bones also do not form from carti- lage.
a. They include the premaxillary, maxil- lary, zygomatic, and petrous portions of the temporal bone.
3. Maxillary bones also grow medially into the palate to support the palatine shelf tissue.
4. Mandibular bones grow laterally to the first arch cartilage and posteriorly to meet the bony body of the cartilaginous condyle. 5. Together these will replace Meckel’s carti-
lage.
6. The mandible forms from several units: a. Condylar
(1) Forms the articulation. b. Body
(1) Center of all growth. c. Angular process
(1) Responds to lateral pterygoid and masseter muscles.
d. Coronoid process
(1) Responds to temporalis muscle development.
e. Alveolar process
(1) Responds to development of the teeth.
I. Sutures of the face
Sutures are fibrous joints (articulations) in which opposing surfaces are closely joined.
1. Sutures are named for the bones they join. a. The articulations may consist of a band
of connective tissue. 2. External face
a. All sutures have a central zone or pro- liferating connective tissue cells along peripheral bony fronts.
b. All are surrounded by fibrous connec- tive tissue.
c. There are three types of sutures of the face:
(1) Simple. (2) Serrated.
(a) Interdigitating type of suture. (3) Squamosal.
(a) Beveled or overlapping. 3. Internal face
a. A synchondrosis—all have an interpos- ing band of cartilage.
b. Grow by forming new cartilage in the center of the suture.
J. Structures derived from branchial arches 1. Branchial arch I—Mandibular
a. Most of maxilla.
b. Mandible lateral to left and right Meckel’s cartilage.
c. One-half of the external ear.
(1) Three hillocks here and three from the second branchial arch.
d. Incus and malleus. e. Muscles of mastication.
g. Maxillary and mandibular branches supply structures of branchial arch I. 2. Branchial arch II—Hyoid
a. Stapes.
b. Styloid process.
c. The other three hillocks forming the external ear.
d. Hyoid bone (most).
e. Muscles of facial expression. f. Facial nerve (VII)
(1) Supplies structures of the second branchial arch.
(2) Taste to anterior two thirds of the tongue.
3. Branchial arch III
a. Forms part of the hyoid bone. b. Glossopharyngeal nerve (IX)
(1) Supplies structures of the third branchial arch.
(2) Taste to posterior one third of the tongue.
4. Branchial arches IV, V, and VI a. Larynx.
b. Pharyngeal constrictor muscles. c. Vagus nerve (X)
(1) Supplies structures of the fourth, fifth, and sixth branchial arches. K. Formation of structures
1. Week by week
a. Between the fourth and eighth weeks, all structures are formed and are recog- nizable. From that point on, they add mass.
2. Week 4
a. By the end of the third week, branchial arch I (called the mandibular arch) has divided into left and right maxillary and mandibular processes.
b. By the beginning of the fourth week, the primitive mouth (stomodeum) has formed.
c. The buccopharyngeal membrane sepa- rates the stomodeum and foregut and is located in the area of the palatine tonsils. 3. Week 5
a. The embryo is 1/
4inch long.
b. Depressions called nasal pits appear in the frontal process.
c. Nasal pits divide the frontal process into the:
(1) Medial nasal processes. (2) Lateral nasal processes.
d. Oropharyngeal membrane ruptures. e. Eyes form on side of the head.
4. Week 6
a. Globular processes appear on the medial nasal process.
(1) Will form the philtrum of the lip. b. The primary palate forms, later known
as the premaxilla.
c. The globular processes then begin to fuse with the lateral nasal processes and the maxillary processes.
(1) Takes about 2 weeks. 5. Week 7
a. The face has a more human appearance. b. Lateral growth of the brain moves eyes
to the front of the face.
c. Upper lip has fused, creating the medi- ally located philtrum.
d. The hillocks have fused to form the ears.
L. Development of vascular blood supply to the face
1. Aortic arch formation and changes. 2. A dorsal aortic arch forms corresponding
to each branchial arch.
a. The first and second aortic arch shrivel.
b. The third aortic arch becomes the com- mon carotid.
c. The fourth aortic arch becomes the dorsal aorta.
d. The dorsal aorta becomes the internal carotid, which develops a stapedial artery.
e. The sixth aortic arch forms the pul- monary circulation.
f. The ventral aorta becomes the ventral pharyngeal artery.
3. Shift from internal to external carotid artery a. The stapedial artery dwindles, and ves- sels arising from it become attached to the external carotid artery, which will supply the face.
b. The internal carotid subsequently sup- plies the brain.
M. Medial and lateral palatal processes
The palate develops from an anterior, wedge- shaped medial part and two lateral palatine processes.
1. Medial palate
a. Also called the primary palate.
b. Develops from the globular processes near the end of the sixth week.
c. Is a floor to the nasal pits. 2. Lateral palatine processes
b. Arise as left and right (L/R) palatine processes coming off the L/R maxillary processes.
c. Will form the posterior portion of the hard palate and the soft palate.
N. Palatal elevation
1. Occurs rapidly as the palatine processes flip up over the descending tongue. 2. Fusion of the hard palate begins in the
ninth week, following palatal elevation. 3. Contact between segments is initially
made behind the medial palatine segment. a. Continues in an anterior and posterior direction until the entire palate is com- plete by the end of the twelfth week. 4.3.1 Internal Facial Development
A. Muscle development
1. Muscle cells become apparent during the fifth week and spread during the sixth and seventh weeks.
a. They grow over the face and attach to developing bones.
B. Neural development
1. The CNS begins forming during the third week. 2. Cranial nerves grow from the area of the developing brain to innervate the muscles of the head.
a. CN V—innervates the muscles of masti- cation.
b. CN VII—innervates stylohyoid and stapedius muscles and the posterior belly of the digastric muscle.
c. CN IX—innervates the stylopharyngeal and upper pharyngeal constrictor muscles.
d. CN X—innervates the inferior constric- tor and laryngeal muscles.
C. Cartilaginous skeletal development 1. First branchial arch
a. Meckel’s cartilage
(1) Originally articulates with the malleus, which will become a hear- ing bone of the middle ear.
b. Eventually the TMJ becomes functional, and Meckel’s cartilage is incorporated into the mandible.
2. Second branchial arch a. Reichert’s cartilage
(1) Will form the stapes, styloid process, and the lesser horn and upper part of the body of the hyoid.
3. Third branchial arch
a. Will form the greater horn and lower part of the hyoid body.
4. Fourth branchial arch
a. Will form the thyroid cartilage. 5. Sixth branchial arch
a. Will form the laryngeal cartilage. D. Tongue
1. Muscles of the head (including the tongue) arise from blocks of mesoderm (the myo- tomes).
2. Nerves
a. CN V—sensory nerve to anterior tongue. b. CN VII—special sensory nerve to ante-
rior two thirds (body) of tongue. c. CN IX—sensory nerve to posterior
third (base) of tongue.
d. CN X—also innervates the base of the tongue.
3. Body
a. The anterior, more movable, part of the tongue develops from the first branchial arch during the fourth week.
b. Extends from the tip to a V-shaped groove called the terminal sulcus behind the circumvallate papillae. 4. Root
a. The posterior, less movable, part of the tongue develops from the second and third branchial arches.
5. Development
a. Arises from two mounds of tissue called lateral lingual swellings and the
tuberculum impar.
b. The lateral parts rapidly enlarge and merge.
c. A U-shaped sulcus develops around the anterior part of the tongue.
E. Thyroid gland
At the point of the V-shaped groove (called the
terminal sulcus) is a depression known as foramen caecum.
1. This location is the embryologic begin- ning point of the thyroid gland.
2. During development, the epithelium in this area begins to migrate into the neck. 3. The path is called the thyroglossal duct. 4. The glandular tissue that will form the
thyroid is carried into the neck with the epithelium.
5. Eventually, the pathway usually disap- pears.
6. A remnant can persist, and aberrant thy- roid tissue that can cause a cyst may be found anywhere along the path.
F. Facial clefts
1. May be unilateral, bilateral, or medial. 2. May be complete with no joining of tissue.
3. May be incomplete with joining of soft tis- sue but not bone.
4. Normally, philtrum ridges are the line of fusion.
G. Palatal clefts
1. Most cleft palates occur in combination with cleft lips.
2. They may also occur as isolated defects. 3. Palatal clefts must extend around the
medial palatal segment before they reach the midline.
H. Other defects—are involved with defects in fusion and merging.
1. Fusion
a. Epithelial adhesion followed by reor- ganization of the deep tissues to mes- enchyme.
b. After the ectoderm has fused, it is replaced by mesenchyme.
c. Sometimes globules of ectoderm per- sist and become epithelial rests, which may become cysts.
2. Merging
a. Filling in of a cleft
b. If failure occurs at the junction of the lateral nasal and maxillary processes, it causes an oblique facial cleft.
c. If failure occurs at the globular processes, it causes a midsagittal cleft in the upper lip.
4.4 General Embryology
The study of embryology allows us to see how a single cell can develop to form an entire person. The journey begins with fertilization of the egg. After fertilization, the egg (now called a zygote) begins a trip along the fallopian tube toward the uterus. As it travels it starts to divide, first into two cells, then four, then eight, and so on. Eventually it forms a solid ball called a morula (for mulberry). The morula continues to grow, and as it does, the interior becomes hollow. At this stage the aggrega- tion of cells is called a blastocyst. Along the interior of one side of the blastocyst, a mass of cells begins forming; it arches into the cavity of the blastocyst until there is a second, smaller cavity under the group of cells. The organism itself will form from this small group of cells, with the majority of the blastocyst forming the yolk sac and the yolk that will be consumed by the developing organism.
The small mass of cells is called the embryonic
disk. At first, it is only one cell layer thick; how-
ever, it soon divides into three layers. These pri-
mordial layers are significant because each will form distinct structural parts of the body.
The layer facing the smaller cavity is called ecto-
derm. Ectoderm will form nervous tissue, sensory
epithelia of the eye, ear, and nose, epidermis, hair, and nails, mammary and cutaneous glands, epithelia of sinuses, oral and nasal cavities, intra- oral glands, and tooth enamel.
The middle layer is called mesoderm. Mesoderm will form muscles, connective tissue, bone, carti- lage, blood, dentin, cementum, pulp, and the peri- odontal ligament.
The layer facing the larger cavity will form endo- derm. Endoderm will form the gastrointestinal tract epithelium.
A. Parts and organelles
1. Cell structure and function
a. Cells are composed of a nucleus, con- taining a nucleolus and the surround- ing cytoplasm, organelles, inclusions, and a surrounding cell membrane. (1) Within each cell is intracellular
material.
(2) Surrounding each cell is intercellu- lar material.
b. They are connected by cell junctions including desmosomes, tight junctions, and gap junctions.
2. Nucleus and nuclear membrane
a. A nucleus is found in all cells except mature RBCs and platelets.
(1) Usually round. (2) Usually singular.
b. Binucleate nucleus in cardiac muscle and parenchymal liver cells.
c. Multinucleate in osteoclasts and skele- tal muscle cells.
3. The nucleus contains DNA in chromo- somes, which is the location of the genetic code.
a. DNA is only visible during cell division; at other times it is dispersed and is called chromatin.
4. Nucleolus
a. Round and dense bodies in the nucleus. (1) Up to four may be found.
(2) Contain the RNA in the nucleus. (3) RNA carries instructions from the
nucleus to sites of protein synthe- sis (rough endoplasmic reticulum) in the cytoplasm.
5. Nucleus and nuclear membrane (nuclear envelope)
a. The nucleus is bound by a nuclear enve- lope with openings at the nuclear pores.
b. Its outer thickness is contiguous with the endoplasmic reticulum.
6. Cytoplasm
a. Contains structures needed for absorp- tion and creation of cell products. b. The cytosol acts as a factory, taking in
raw material and producing products. 7. Endoplasmic reticulum
a. A system of parallel membrane-bound cavities in the cytoplasm.
b. Contains newly acquired and synthe- sized protein.
c. A channel for the movement of materials. d. May be continuous with the nuclear membrane, plasma membrane, or Golgi apparatus.
e. There are two types, depending on whether ribosomes are present: (1) Rough
(a) Ribosomes are present. (b) The site of protein synthesis. (2) Smooth
(a) Ribosomes are not present. (b) The site of lipid manufacture. 8. Ribosomes
a. Small, granular bodies that are the site of protein synthesis.
b. May be found free in the cytoplasm. c. Produce proteins for cell’s own use. d. May be found attached to endoplasmic
reticulum.
e. Produce proteins for export.
f. Three types of RNA are necessary for protein synthesis: mRNA, tRNA, rRNA. 9. Golgi apparatus
a. Connected to the endoplasmic reticu- lum.
b. Composed of cisternae (flat plates) or saccules, small vesicles, and large vac- uoles.
c. Sorts, condenses, packages, and deliv- ers proteins from the endoplasmic reticulum to the cell membrane for export from the cell.
10. Lysosomes
a. Surrounded by a membrane.
b. Contain acids and digestive enzymes. c. Common in macrophages and leukocytes. d. Can digest substances both within and around the cell, including bacte- ria, parts of injured cells, or an entire cell.
11. Peroxisomes
a. Similar to lysosomes.
b. A membrane-surrounded body contain- ing various enzymes.
c. Contain catalase, which catalyzes hydrogen peroxide to water and oxygen. 12. Mitochondrion
a. Small, variable-shaped, membrane- bound organelles free in the cytoplasm. b. The powerhouse of the cell.
c. Up to 6000 or 7000 per cell. d. Has its own DNA.
e. Inner layer of their membrane is com- posed of cristae.
f. Generate ATP through the Kreb’s cycle, producing chemical energy from food. 13. Microtubules
a. Composed of the protein tubulin. b. Appear as singular, doublet, or triplet
entities.
c. Function as structural and force-generat- ing elements.
d. Centrioles are composed of micro- tubules.
(1) Centrioles replicate before mitosis begins.
14. Microfilaments
a. A thin sheath just below the plas- malemma.
b. Solid rods of actin protein.
c. Help cell move through cilia and flagella. d. Also give shape to the cell.
e. Associated with endocytosis, exocytosis. 15. Vacuole
a. Space inside the cell. b. Purpose is not known. 16. Vesicles
a. Sacs containing various substances within the cell.
b. Transport vesicles.
(1) Move proteins from endoplasmic reticulum to Golgi apparatus. c. Secretory vesicles (1) Neurotransmitters. (2) Mucus. (3) Inorganic substances. (4) Other substances. 17. Cell membrane
a. The external boundary of a cell. b. Also called the plasma membrane or
plasmalemma.
c. Composed primarily of lipid and pro- tein, with some carbohydrate.
d. Lipid bilayer is oriented with hydrophilic ends outward and hydrophobic ends inward.
(1) Functions as a selective barrier and site of transport.
(2) Has specialized areas of intercon- nection with other cells for attach- ment and communication.
e. Desmosomes—between cells.
f. Hemidesmosomes—into the basement membrane.
B. Cell division
1. Both somatic and sex cells divide.
a. In somatic cells, division (mitosis) ensures that the resulting cells contain all of the same genetic material as the original cell.
b. In sex cells, division (meiosis) ensures that the resulting cells contain half of the genetic material that the original mother and father cells contained. (1) Meiosis is also known as reduction
division.
2. Mitosis
Mitosis has four phases, plus inter- phase (a resting phase).
a. Prophase
(1) Chromatin in the nucleus thickens to become visible chromosomes. b. Metaphase
(1) The chromosomes line up on the equatorial plate.
(2) The two chromatids comprising a chromosome split at their cen- tromere.
c. Anaphase
(1) Each chromatid migrates toward the centriole at the poles of the newly forming cells.
d. Telophase
(1) Cytokinesis divides the cell into two, each containing all the genetic