Полезные материалы за все 6 курсов / Учебники, методички, pdf / INBDEBooster Head and Neck Anatomy Notes

1 The Germinal Stage

Week 1

The process of sperm and an egg fusing together within the Fallopian tube is called fertilization.

•At this point, the two originally haploid cells become a zygote, which is a diploid cell

•After this point, the diploid cell continues to multiply, forming a morula

A morula is a ball of about 8 cells, which has a similar appearance to a mulberry

•The morula then becomes a blastula, a bundle of cells containing an inner cavity known as a blastocoel

•When the blastula begins to fuse with the uterine wall (the endometrium), the process of implantation begins

•After implantation, the blastula will further develop into two predominant structures

The embryoblast is the inner cell mass which will form the majority of the embryo,

The trophoblast is the outer cell mass and will mainly become the placenta

INBDE Pro Tip:

Mnemonic: The embryoblast will form the embryo, while the trophoblast will form the ouTside of the embryo and placenTa.

Figure 1.01 The stages of egg development

Week 2

•At this point, two distinguishing layers of the egg can be seen. Recall that the embryoblast is the inner cell mass. During week two, it forms a bilaminar disc.

•The bilaminar disc contains two parts:

The epiblast will become three major germ layers and will contribute to the further development of the embryo

In contrast, the hypoblast can also be seen, but has little importance as it mostly disappears

•Also visible at this stage includes the amniotic cavity, which will encase the fetus as it develops

•The yolk sac cavity is responsible for nutrient and gas exchange for the fetus before the placenta develops to a point where it can take over this role

Head and Neck Anatomy

At the end of week two, the egg is fully Implanted within the endometrium and implantation is complete. The caudal midline can be Identified on the dorsal aspect of the embryo by locating the primitive node and primitive streak.

Figure 1.02 The bilaminar disc and associated regions

2 The embryo

Week 3

This week is crucial for the development of an embryo. Malformations may occur due to either genetic factors or environmental factors, such as drug consumption. Additionally, gastrulation takes place during the third week, where three distinct germ layers can be seen, the ectoderm, mesoderm and endoderm.

2

The three germ layers are responsible for forming different parts of the body.

•Ectoderm:

Forms epidermis, facial skeleton, nervous system

Forms teeth

-Enamel derived from surface ectoderm

-Majority of tooth derived from neural crest cells

•Mesoderm:

Forms "structural components" of the body like the skeletal system, muscular system, and connective tissue

•Endoderm:

Forms the lining of the foregut, midgut and hindgut

Head and Neck Anatomy

Week 4

Between weeks three and four, the ectoderm "pinches" down on the endoderm layer, forcing the endoderm layer to form more of an outpouching structure.

Neurulation begins, where the neural plate folds in on itself and forms the neural tube.

•The notochord is derived from mesoderm and is responsible for signalling the start of this process

o Also forms vertebral column

•The neural plate is ectodermal tissue that soon folds into the neural groove

•The neural fold is the joining of the neural plates on either side of the neural plate

•The neural groove is the inner pouch that forms as the neural plate pinches in

•The neural tube is the result of neurulation

o A tube-like structure that sits below the remaining ectodermal layer (soon to be epidermis)

•The neural crest is the former ends of the neural fold

o Neural crest cells migrate throughout the embryo and form various specialized structures

▪Ex. glia, melanocytes

3

Figure 1.05 The process of neurulation

As neurulation ends, aortic arches, pharyngeal arches, and somites begin to form cranially to caudally. Various features can now be seen:

•The buccopharyngeal membrane opens cranially

Forms stomodeum: primitive oral cavity

•Blastopore: primitive anus

Located caudally

•Otic placode: primitive ear

•Optic placode: primitive eye

2 The aortic arch and major branches

Week 4

The pharyngeal arches are visible outpouchings that are the precursors to various head and neck structures.

Head and Neck Anatomy

•There are six arches in total, but the fifth arch does not derive any structures

•The sixth arch is located deep within the embryo and is not visible

•Have distinct anatomical features:

Endodermal pouch

Mesenchymal core

-Mesoderm

-Neural crest

-Artery

-Nerve

-Muscle

-Cartilage

External ectodermal cleft

4

Figure 1.06 The pharyngeal arches and tissue types

Head and Neck Anatomy

INBDE Pro Tip:

There are various ways to remember general information for each pharyngeal arch:

•1st arch:

Many structures are associated with the mandible and anterior ear

•2nd arch:

Many structures associated with posterior ear and regions on an axis moving anteroinferiorly

-Ex. styloid process to anterior hyoid

•3rd arch:

- Inferior hyoid region, few structures

•4th arch:

The "Adam’s apple" cartilage

CN X muscles of deep pharyngeal region

•6th arch:

Most inferior cartilage before trachea

Figure 1.07 The pharyngeal arches, and their eventual derivatives

3 Other Features of Pharyngeal Arches

Week 4

Pharyngeal arches have other distinct regions that are associated with them. Pharyngeal clefts are grooves on the ectodermal side between adjacent pharyngeal arches. Any issues in cleft formation results in a brachial cyst. In contrast, pharyngeal pouches are grooves found on the endodermal side between pharyngeal arches. If pharyngeal pouches fail to develop correctly, brachial fistulas may result.

5

•Arch 1:

Placode: Trigeminal ganglion (CN V)

Cleft: External auditory meatus

Pouch: Eustachian tube and tympanic cavity

•Arch 2:

Placode: Geniculate ganglion (CN VII)

Cleft: Cervical sinus

Pouch: Palatine tonsil

•Arch 3:

Placode: Inferior sensory ganglion (CN IX)

Cleft: Cervical sinus

Pouch: Thymus and inferior parathyroids

*Note: pouch derivatives eventually migrate inferiorly by the lungs

•Arch 4:

Placode: Inferior sensory ganglion (CN X)

Cleft: Cervical sinus

Pouch: Superior parathyroids and ultimobranchial body (parafollicular cells)

*Note: pouch derivatives eventually migrate inferiorly by the lungs, but remain superior to arch 3 pouch

INBDE Pro Tip:

There are only four pharyngeal pouches, placodes, and clefts. This is because they are located directly below each pharyngeal arch. Since there are really only 5 functional arches and the last of these is not visible (and thus does not have any features below it), this makes sense.

Head and Neck Anatomy

Figure 1.08 The pharyngeal arches, clefts, and pouches

5 The Pituitary Gland

Week 4

The development of the pituitary gland has a series of steps.

•Roof of oral cavity (stomodeum) forms an evagination superiorly, known as Rathke's pouch

Forms the anterior pituitary gland

Oral ectodermal origin

•Floor of forebrain (diencephalon) evaginates inferiorly

Forms posterior pituitary gland

Neural ectodermal origin

Recall that physiologically, the anterior pituitary gland (adenohypophysis) and posterior pituitary gland (neurohypophysis) secrete hormones to organs indirectly and directly, respectively. Embryologically we can now appreciate why the endocrine system is wired this way!

6

Figure 1.09 The formation of the pituitary gland

6 The Tongue

Week 4

The tongue is said to be segmented based on its development from four different pharyngeal arches plus contributions from the occipital myotomes. Various arches and nerves contribute to different segments.

•Arch 1:

Innervation: CN V

Modality: Sensory

Region of tongue: Anterior 2/3

•Arch 2:

Innervation: CN VII

Modality: Taste

Region of tongue: Anterior 2/3

•Arch 3:

Innervation: CN IX

Modality: Sensory and taste

Region of tongue: Posterior 1/3

•Arch 4:

Innervation: CN X

Modality: Taste, both sensory and motor (palatoglossus muscle)

Region of tongue: Base

•Occipital myotomes (from somites):

Innervation: Motor (remaining muscles of tongue)

Figure 1.10 The development of the tongue, segmented into anterior 2/3, posterior 1/3, and base

7 The Thyroid Gland

Week 4

The thyroid gland is derived from tissues located at the tongue's base. Specifically, it develops from a depression known as the foramen cecum.

The thyroid gland descends caudally and ventrally to the pharynx. This is made possible via the presence of the thyroglossal duct. This duct allows for the temporary connection of the foramen cecum of the tongue and the thyroid gland.

Clinically, a lingual thyroid may be present at the base of a patient's tongue if the thyroid did not descend properly during embryological development.

Figure 1.11 The descent of the thyroid gland

8 The ear

The first and second pharyngeal arches are responsible for the formation of the ear. These two arches form six auricular hillocks per side of the embryo.

•Each pharyngeal arch contributes three auricular hillocks

The opening of the ear canal (external auditory meatus) will be located between these two groups of hillocks

•Each hillock contributes to a different region of the ear

•Clinically, if a pregnant mother takes drugs containing retinoic acid (like Accutane for acne), defects in first and second arches may occur

Microtia: small ears

Micrognathia: small mandible

9 The aortic arch and major branches

Between weeks 4 to 8

The lips are developed when numerous structures of the embryological face come together.

•Surrounding the nasal pits is the lateral nasal prominence and medial nasal prominence

Head and Neck Anatomy

•Laterally and inferiorly, the maxillary prominence can be found

•Process:

Maxillary prominences move ventrally and medially

-Forms cheeks and lateral aspects of the upper lip

-Medial nasal prominences forced medially and fuse togetherForm crest and tip of nose and

philtrum

The mandibular prominence contributes to the jaw and lower lip

•Clinically, failure of the medial nasal prominence and maxillary prominence to fuse results in a cleft lip

May occur unilaterally, bilaterally, or at the midline

Figure 1.12 Components of lip and palate development

The palate begins to develop at the sixth week, following formation of the lips.

•The primary palate is comprised of the philtrum of the lip anteriorly and forms via fusion of the two medial nasal prominences at midline

Triangular in shape when viewed inferiorly

This is where the 4 maxillary incisors will grow after birth

•The palatine shelves on either side of the embryo will also develop during week six

Move caudally and medially on both sides

8

•During week seven, the tongue moves inferiorly into the expanding oral cavity through the palatine shelves

•The palatine shelves can now move horizontally and medially to fuse at the midline

Fusion occurs in a cranial-to-caudal fashion

•The collective fusion of the palatine shelves is called the secondary palate

•The fusion of the primary and secondary palates at the midline is called the incisive foramen

•Clinically, patients with cleft palate may have missing or malformed lateral incisors

Incomplete cleft palate: failure of palatal shelves to fuse

Complete cleft palate: failure of primary palate to fuse with palatal shelves

After week 8, the embryo is now considered a fetus.

Figure 1.13 Direction of palate formation and tongue migration (coronal view)

10 DiGeorge Syndrome

Clinical relevance

DiGeorge Syndrome is a developmental disorder caused by either genetic or environmental factors. Also called third and fourth pouch syndrome.

1 Overview of the Skull

The skull is a major protective structure comprised of bone that primarily functions as a cavity for the brain and other supportive neurological components. Another major function is to support the face. There are two main components:

•The neurocranium – includes the calvaria or skullcap and encases the brain

This posterior region of the skull is formed by the paraxial mesoderm

•The viscerocranium – contains aspects of the skull not included in the calvaria, like the bones of the face

This anterior portion of the skull is formed by the neural crest cells

The skull is not a singular bone. It is made up of numerous bones through the presents of fibrous joints called sutures. Their purpose is to allow brain growth during childhood and even adolescence, until they finally fuse in adulthood.

The neurocranium

This region is formed by 8 bones in total. It has two major subdivisions, known as the roof and the base.

•The roof: functions to protect the brain superiorly and posteriorly

Include the frontal, parietal, and occipital bones

•The base: protects the brain inferiorly

Includes the frontal, ethmoid, sphenoid, temporal, parietal, and occipital bones

The viscerocranium

Contains many more bones than the neurocranium (14 total). Although this region does not protect the brain, it functions to:

•Support the soft tissues of the face

•House and protect numerous key functional areas of the skull

The sinuses, nasal and oral cavities, and even the orbits

•Include the maxilla, zygomatic, lacrimal, nasal, palatine, ethmoid (superior and middle nasal conchae), inferior nasal conchae, mandible, and vomer