Unit 1

Question 1

What does the endoderm proliferate into?

Answer 1

Epithelial lining of digestive tract

Lining of the respiratory tract

Question 2

What does the mesoderm proliferate into?

Answer 2

CT (including tissues with dermis layer)
Skeletal tissue (bone and cartilage)
Circulatory system (heart, blood)
Lymphatics 
Urinary system
Reproductive organs
Dentin
Pulp
Cementum
PDL
Muscle tissue (including cardiac heart)

Question 3

What are the significances of neural crest cells?

Answer 3

During closure of the neural tube, 2 peripheral steps of neuroectoderm separate from the tube (still covered in ectoderm) = neural crest cells

NCC migrate from crests of the neural folds and then join the mesoderm to form the mesenchyme

Question 4

What do NCC contribute to?

Answer 4

Tissues of the face:
Cartilage 
Bones
Muscles
Teeth but not enamel
Ligaments

Question 5

What does the ectoderm differentiate into?

Answer 5

Nerve tissue
Epithelium (including oral)
Tooth enamel
Hair follicle
Finger nails
Cutaneous glands

Question 6

What is the buccopharyngeal membrane?

Answer 6

Also known as the oropharyngeal membrane
A temporary membrane consisting of ectoderm overlying endoderm
Separates the stomodeum from the primative pharynx

Question 7

Which branchial arches contribute to the formation of the face and oral cavity?

Answer 7

1st-mandibular arch
2nd-hyoid arch
3rd
4th
Pharyngeal pouches

Question 8

What is the first phase of embryonic development?

Answer 8

Fertilization:

• first 2 weeks
• zygote - fertilization from union of 2 gametes
• cellular proliferation and migration, some differentiation

Proliferation

• weeks 1 and 2
• implantation of enlargment of blastocyst

Question 9

What is the second phase of embryonic development?

Answer 9

Morphogenesis

• embryonic period - takes shape of embryo
• differentiation of major external and internal structures - organ systems, heart, face and oral structures all begin formation
• defects - teratogen: agent/factor causing physical defect in embryo

Question 10

What is the third phase of embryonic development?

Answer 10

Growth and maturation

• 8 weeks to birth
• increase in size and weight, maturation of organs development of human characteristics

Question 11

What are the different phases of embryonic development?

Answer 11

Fertilization
Proliferation
Morphogenesis
Growth and Maturation

Question 12

Describe the first week of embryonic development

Answer 12

Day 1: fertilization, fertilized egg grows and becomes the Zygote (cell division)

Day 4: cell increase to 16-32, same size (free floating), now called a Morula (solid ball of cells multiplying)

Day 5: Morula increases in size and hollows out, now becoming blastocyst. Accumulation of cells at one end of the embryonic pole. This mass of cells will form the embryo which eventually will become the fetus. Smaller peripheral cells are called the trophoblast layer )figure 3.4 implantation of blastocyst).

Day 6: Attachment to uterine wall occurs at area of the embryonic pole. Implantation begins. Implantation ensures development of the placenta of which is critical for fetal/maternal circulation.

Day 7: Further attachment and embedment, uterine tissue will eventually surround the embryo.
o Mass of cells (embryonic begin to form two cell layers = embryonic disc.
 Epiblast (future ectoderm)
 Hypoblast (future endoderm)
o A second cavity forms between Epiblast and peripheral cells; this will become the Amniotic Cavity (figure 3.6 and 3.7)
o The second cavity adjacent to the hypoblast will become the future Yolk Sac.

Question 13

What happens in the second week of embryonic development?

Answer 13

the embryonic disc forms from the epiblast and hypoblast layers

Question 14

What is the primitive streak and when does it form?

Answer 14

Week 3
o Rod shaped thickening on Epiblast layer of disc
o Epiblast cells proliferate and move downward (downward growth) between hypoblast and epiblast layers forming a middle layer (proliferation between the 2 layers allows the formation of a third layer = mesoderm)
o Now 3 layers exist in all part of the embryo (embryonic disc initially) except at the oropharyngeal and cloacal membranes
o 3 distinct germ layers = a structure called the trilaminar Disc.
o These germ layers are now referred to an endogerm (org. epiblast), ectogerm (org. hypoblast) and mesoderm and will be coded in yellow, blue and red, respectively

Question 15

What is the primitive streak and how is it related to the formation of the mesoderm layer?

Answer 15

 A. It is a rod shaped thickened on the epiblast layer that grown downward to the hypoblast allows for the formation of a third layer. It is related to the mesoderm because it helps to form that third layer also known as the mesoderm.

Question 16

What are the three germ layers of the trilaminar disc?

Answer 16

Ectoderm
Endoderm
Mesoderm

Question 17

What happens in the fourth week of embryonic development?

Answer 17

o Formation of the CNS is initiated
o Derived from a thickened central band of ectoderm or neuroectoderm. This structure is called a neural plate.
o The neural plate folds and becomes the neural groove, which eventually closes to become the neural tube. This process is called neurulation (proceeds in zipper like process from cephalic and caudal).
• During closure of the neural tube, 2 peripheral strips of neuroectoderm separate from the tube. These are still covered in ectoderm. These are called the neural crest cells
• Neural Crest Cells* eventually migrate to other regions of the body and differentiate into many different types of cells
o For our purposes, these cells contribute to tissues of the face, such as cartilage, bones, muscles, teeth (not enamel) and ligaments
o Side Notes:
 Neural crest cells are from ectoderm during CNS formation (neurulation). It remains in mesoderm and is responsible for what tissues form in the mesoderm
 Cell layers also called ectomeschyme (sp?)

Question 18

Describe the caudocephalic foldings

Answer 18

• This process begins approx. 3 to 4 weeks in utero (iu)
• The flat embryonic disc becomes a curved tubular structure/embryo, folding in 2 planes
• Ectoderm grows faster than endoderm and these faster growing regions “bulge” forming arches in the head and neck region
• The cephalic end (head end) evolves in advance of the caudal end (recall that differential growth* occurs throughout the life of embryo)
• Sequence: Folding’s of the disc
o Initially the structure is a flat disc
o By the end of the 4th week it is a tubular structure
o Ectoderm is outside and endoderm becomes closed inside
o The amniotic cavity is enlarged to surround the embryo

Question 19

Describe the development of the oral face region

Answer 19

• In the 4th week of development, when the neural tube is forming, the developing embryo is folding and the result is complete enclosure of the yolk sac which becomes elongated (again with folding) and become the foregut.
• The primitive mouth, the stomodeum, is separated from the foregut by a membrane known as the oropharyngeal membrane (also called the buccopharyngeal membrane).
o This membrane consist of only 2 germ layers.
Endoderm and ectoderm
• At the 5th week this membrane (oropharyngeal membrane) will rupture, opening the oral cavity into the foregut or future pharynx
o Note*: the lining of the oral cavity is ectodermal origin, while the lining of the pharynx is of endodermal origin.
o An additional ectodermal pouch (invagination), in the area of the developing oral cavity called rathke’s pouch forms and eventually will give rise to the anterior lobe of the pituitary gland (The posterior lobe of the gland is derived from the brain)

Question 20

What is ectodermal dysplasia?

Answer 20

o Lack of development of the ectodermal tissue
o A group of condition in which there is abnormal development of the skin, hair, nails, teeth or sweat glands
o Varies in severity
o Dental *hypodontia

Question 21

What does the forebrain develop into??

Answer 21

• Approx 3-4 weeks in utero (IU)
• During the folding of the embryo, a large buldge begins to form in the area above the stomodeum. This bulge is called the forebrain and it is the development of the frontonasal process, which will give rise to the following facial structure
o Forms: forehead, nose, philtrum, middle of upper lip, anterior 1/3 of hard palate and alveolar process

Question 22

Describe the mandibular branchial arch

Answer 22

 Consequences of the development of the face; it will form the mandible, most of the maxilla and the mid and lower parts of the face (max process, which give rise to upper cheeks, sides of upper lip and most of the hard palate)
 This arch also contributes to the formation of the tongue, specifically the ant. 2/3’s of the tongue

Question 23

Describe the hyoid branchial arch

Answer 23

 Form the muscles of the facial expression, some of the hyoid muscles, hyoid bone, parts of the tongue and other head and neck structures

Question 24

Describe the progress of development of the branchial arches

Answer 24

o The arches get progressively smaller ant to post, and arches III, IV and V are actually divided before the reach the midline because of the heart bulged
 These arches will eventually be covered by developing II arch and will become internal structure
 By week 7 the neck appears as a smooth external structure
o The arches are divided by “slits” or pharyngeal pouches (also numbered). Note*: arches bulged outward and pouches protrude inward
 Pharyngeal pouches: do give rise to internal structures; such as, the external auditory meatus develops from the membrane in the 1st branchial groove, which is derived from the first pouch

Question 25

What structures does the nose develop from?

Answer 25

Ethmoid and vomer

Question 26

Describe olfactory pits

Answer 26

during week 5, depressions of the future nose openings develop from the lower frontal processess
o Olfactory pits are areas of specialized thickened ectoderm found at locations of developing organs of special sense; and include eye, ears and nose.
o Olfactory pits will develop into olfactory epithelium
o As the pits deepen the surrounding tissue grow and develop into 2 medial nasal processes and 2 lateral nasal processes
 The lateral processes form the sides of the nose
 The medial processes form the bridge of the nose and the central band or philtrum of the lip
o At week 6 the nasal pits deepen and form an opening into the primitive oral cavity
o Inwardly from the medial processes, an ingrowth forms the nasal septum (this is referred to as the intermaxillary (located where incisive suture is located and medial to canines) segment or globular process (figure 4.7))
 This structure (ingrowth) also gives rise to the primitive or primary palate, which will form the incisor region and the alveolar arch of the maxillary jaw and palate anterior to the incisive foramen (premaxillla)
• The max arches are continuing to grow forward and will eventually merge with the medial nasal processes

Question 27

Describe the formation of structures from the first branchial arch

Answer 27

• On the mand arch, enlargements appear on the post ends of the upper surface, which grow upward and medially at the sides of the stomodeum. These are called the maxillary processes
o The max processes will give rise to the upper cheeks, sides of upper lip and most of the palate (figure 4.5 and 4.6)
o The growth of the max arch is rapid, while the growth of the lower face slows down.
o As the max processes proliferate in growth, they slowly move toward the midline soon to join up with the medial nasal processes forming the upper lip area.
 As this is occurring the in the nasal cavity 2 palatal processes of the max processes are forming and moving to the midline of the future palate area

Question 28

What is differential growth?

Answer 28

refers to the fact that some parts grow at different rates than others, resulting in marked progressive changes in size and in position of structures

Question 29

Describe facial fusion

Answer 29

• Initially, grooves of various depths separate the facial processes from each other (ex: max and mand processes). For normal development, speed and amount of forward growth of the facial processes need to be correlated, and then the grooves must be eliminated
o Externally, grooves are eliminated by proliferation of the mesenchyme underneath the ectoderm lining the deepest part of the groove. If the grooves are not eliminated a gab or a cleft may result.
o This may present unilaterally or bilaterally and would be evident in the embryo by about the 6-7th week
• At the corners of the mouth, evidence of embryonic merging may show as Fordyce’s granules, extending laterally along the inside of the cheek from either corner of the mouth
o These are entrapped sebaceous glands from the epidermis
o Failure to fill in the groove with underlying mesenchyme in the commissure area (the junction of the max and mand processes) may result in Macrostomia (large mouth opening)
 This is an anomaly affecting the oral region
• Once all the fusion and merging occurs in this area of the max arches, the only opening that remain will be the nares and the oral cavity
• Also, consider mand processes through the merging process. 2 larger bulges on either side are joined in the middle. The center are must undergo a lot of mesodermal (mesenchymal) proliferation during this time. If it does not proliferate enough, a cleft in the chin may result. This may appear as a dimple or a cleft, depending on the severity (another possible anomaly)
• These variations/anomalies have all been the result of insufficient mesodermal growth in a developing structure (from the same arch)

Question 30

What are fordyce’s granules

Answer 30

evidence of embryonic merging

• extend laterally along th einside of the cheek from either corner of the mouth
• these are entrapped sebaceous glands from the epidermis

Question 31

Describe palatal fusion

Answer 31

• Occurs when 2 separate structures join. The actual fusion of tissues is created through a glue-like substance produced by the epithelial cells of the edges of each fusing process
• Once the 2 surfaces join, eventually the “seams” disappear as the central layer of cells is broken down. This process by which the palatal processes join (figure 5.1)
• Globulomaxillary Cyst: forms between the primary and secondary palate at line of fusion

Question 32

Where does the remaining palatal structure develop from?

Answer 32

from the surfaces of the maxillary process inside the primitive oral cavity

o As the max processes proliferate bilaterally inside the oral cavity space, two shelf-like processes grow, initially, downward toward the tongue
o Eventually, due to a growth spurt of the mand, the tongue drops to the floor of the mouth and the vertical max (palatal) processes flip up to a horizontal position
 from this position, they grow medially toward each other
 these palatine processes meet and fuse with (a) each other, (b) with the lower border of the nasal septum and (c) they first meet in the anterior region and fuse with the primary palate which has been growing inward (primary palate) and posteriorly from the structure called the intermaxillary segment
 palatal fusion at the midline of the palate then occurs in a zipper like fashion, front to back. Initially, the fusion is simply in soft tissue and is eventually replaced with underlying bone

Question 33

How does palatal clefting occur?

Answer 33

• Clefting can occur at any stage of the fusion process. The extent of the cleft will depend on when the lack of fusion took place.
• If early (7th week) the entire palate may remain open, if clefting occurs later (14th week) failure of the fusion process will result in clefting of the uvula
• Clefting is the most common congenital anomaly, resulting in malformation of the Orofacial region. It created functional problems with eating, speech, respiratory, etc. Etiology may be part of a syndrome, genetic predisposition or other (ex: nutritional, drugs (street/prescription))
• Severity ranges from entire lip and palate to lip only. However, clefts are treatable. But it is a lengthy process and involves may integrated care individuals and treatment, including surgeries. Care requires a multidisplinary team to manage the individual from the time of birth often up to early adult life

Question 34

Describe the development of the tongue

Answer 34

• Beginning at 4 weeks, within the oral cavity, swelling from branchial arches I, II and II begin to form the tongue. From arch I, 3 swelling will form the ant 2/3 of the tongue, these are 2 lateral lingual swellings and the tuberculum impar
• A swelling from arch II and III, the copula aspect of these branchial arches forms the post 1/3 or base of the tongue.
• The swellings that develop into the tongue grow upward and ant resulting in a recognizable form (a tongue) at around 8 weeks

Question 35

Describe the development of the thyroid gland

Answer 35

• the thyroid gland originates and proliferates of epith at the foramen cecum (between branchial arches I and II, at the middle of the developing tongue).
• Descending as the tissue proliferates, the resulting gland remains attached to the tongue by a thyroglossal duct which eventually disappears