Embryology 2-1 PDF - Prenatal Development

Embryo, day 26-30 Embryology Lecture Outline General overview of prenatal development Embryonic period phase 1 • Formation of bilaminar disk • Formation of trilaminar disk (gastrulation) Embryonic period phase 2 • Formation of neural tube • Differentiation of mesoderm • Folding of embryo • Formati...

Embryo, day 26-30 Embryology Lecture Outline General overview of prenatal development Embryonic period phase 1 • Formation of bilaminar disk • Formation of trilaminar disk (gastrulation) Embryonic period phase 2 • Formation of neural tube • Differentiation of mesoderm • Folding of embryo • Formation of pharyngeal arches Formation of Pharyngeal Arches Pharyngeal Arch Anatomy Arches have a core that’s derived from lateral and paraxial mesoderm and neural crest cells. They are covered with ectoderm on the outside and lined by endoderm on the inside. Each arch has its own cartilage, artery, and nerve. Migration of Neural Crest Cells Neural crest cells live next to the midbrain and rhombomeres. Some of them migrate to the cores of the pharyngeal arches. 1 2 3 4 Formation of Pharyngeal Arches 25-day-old embryo No arches yet
Buccopharyngeal membrane intact 35-day-old embryo Arches and pouches nicely formed Mouth now open to esophagus 26-day-old embryo showing stomatodeum and first two arches Embryo, day 26-30 Frontal prominence 1 2 3 Somites Optic placode Nasal placode Stomatodeu Maxillary m 1 Mandibular Heart 2 3 4 Somites First four arches in a 32-day-old embryo Pharyngeal arches, grooves, pouches Groove/cleft Arch Pouch Pharyngeal arches, grooves/clefts and pouches FUNCTION • Pharyngeal pouches derivatives produce tissues necessary for hearing, calcium homeostasis, and adequate immune response. • The first pharyngeal pouch develops into the middle ear cavity and the eustachian tube, which joins the tympanic cavity to the nasopharynx. • The inner surface of the eustachian tube is covered by a mucosal layer of ciliated cells, supporting cells, secretory cells, and connective tissue. • The ciliated cells in the eustachian tube allow for secretions from the middle ear cavity to enter and drain into the nasopharynx. • The primary function of the eustachian tube is to equilibrate pressures between ambient air pressure and the middle ear by permitting entry of air into the middle ear cavity. • Failure of ciliated cells leads to pathologies such as otitis media with effusions, causing conductive hearing loss. FUNCTION • The second pharyngeal pouch develops into the palatine tonsils, a secondary lymphoid organ playing a role in protecting the body from pathogens passing through the pharynx. • The third pharyngeal pouch develops into the thymus and inferior portion of the parathyroid. • The thymus is a primary lymphoid organ that supports the development and selection of T cells. • Host T-cell immunity is attributable to the development of the third pharyngeal pouch. Positive selection of T-cells takes place in the cortex of the thymus. • The medulla of the thymus is responsible for self-tolerance education in T cells. • Failure in the development of this pouch results in severe immunodeficiency against viral and fungal pathogens (Di-George Sy.). FUNCTION • The fourth pharyngeal pouch is responsible for the development of the superior region of the parathyroid and the ultimobranchial bodies. • Together, the third and fourth pharyngeal pouches play a crucial role in the homeostasis of calcium and phosphate via the function of the parathyroid gland. • The ultimobranchial cells develop into the C cells of the thyroid gland, which produce calcitonin in response to increased serum calcium levels. • The fifth and sixth pharyngeal pouches combine with the fourth pharyngeal pouch. • Cardiac neural crest cells arising from the dorsal neural tube migrate to the third and fourth pharyngeal pouches. • The cardiac cells proliferate and integrate into the parenchyma that eventually forms the aortic arches and great vessels. CLINICAL SIGNIFICANCE • Proper pharyngeal pouch development ensures appropriate tissue formation in vital processes such as hearing, appropriate immunity, and regulation of calcium homeostasis. • Pharyngeal pouch development also plays a critical role in the correct formation of pharyngeal arches. • Failure of development from any of the pharyngeal pouches has clinical consequences contributing to syndromes such as DiGeorge syndrome and branchiooto-renal syndrome, as well as additional pathologies. CLINICAL SIGNIFICANCE • Hearing loss is a common consequence of failure in the first pharyngeal pouch formation. • Failure to form the middle ear cavity lining inhibits the formation of ciliated epithelial cells. • Consequently, middle ear debris cannot be removed into the nasopharynx, resulting in recurrent otitis media infections with effusions. • Malformation of the middle ear lining surrounding the ossicles results in decreased vibration transmission from the stapes to the oval window of the inner ear. • The cochlear nerve will receive less stimulation resulting in decreased quality of hearing. • Failure of eustachian tube development causes damage to fragile inner ear structures with changing atmospheric pressure causing progressive hearing loss. CLINICAL SIGNIFICANCE • Errors in the second pharyngeal pouch formation affect palatine tonsil formation. • Failure of palatine tonsil formation results in a lack of lymphoid tissue use to stimulate an immune response at the junction in the respiratory and gastrointestinal tracts. • The thymus is an essential lymphoid organ in which T cell maturation and selection occurs, orchestrating the adaptive immune response in humans. • The thymus enhances cell-mediated immunity with the production of T-cells that can generate an adequate immune response to various viral and fungal pathogens. CLINICAL SIGNIFICANCE • The parathyroid gland secretes parathyroid hormone in response to lower plasma calcium levels. • Failure of gland development results in an inappropriate response by the body to low calcium levels. • Parathyroid hormone acts on the intestines, kidneys, and bones to increase serum calcium. Abnormally low plasma calcium levels can present as numbness or tingling of the feet, hands, or lips, muscle cramps, decreased heart rate, facial twitching, weak nails, and individuals that are more prone to fracturing bones. CLINICAL SIGNIFICANCE • Failure of the fourth pouch results in the impaired formation of the ultimobranchial bodies of the thyroid. • These cells are responsible for calcitonin secretion in response to high serum calcium levels, downregulating osteoclast function, and calcium reabsorption in the kidneys. • Failure of parafollicular cell development could result in high serum calcium levels. • Hypercalcemia affects brain function resulting in lethargy, fatigue, and confusion causes gastrointestinal upset, and can induce cardiac arrhythmias. On EKG, hypercalcemia is observable by a shortened QT interval (<300ms). CLINICAL SIGNIFICANCE • Anomalies of the branchial arches usually present as cysts, sinuses or fistulae. • Second branchial arch anomalies account for approximately 95 % of cases. Arch Important! Meckel’s cartilage indicates w here the mandible will deve lop – bu t it doe s not turn into the mandible! Nerve Muscles Skeleton V (trigeminal) Mastication muscles Mylohyoid, anterior digastric Tensors tympani and veli palatini Meckel’s cartilage (malleus, incus) 2 VII (facial) Facial expression muscles Posterior digastric Stylohyoid Stapedius Reichert’s cartilage
(stapes, styloid, lesser hyoid) 3 IX (glossopharyngeal) Stylopharyngeus Greater hyoid X (vagus) Larynx muscles Cricothyroid Levator veli palatini Constrictors of pharynx Laryngeal cartilages 1 4-6 What happens to the pouches and grooves? • • • • • 1st groove and pouch -> ear stuff Rest of grooves disappear 2nd pouch obliterated by tonsil 3rd pouch -> inferior parathyroid, thymus 4th and 5th pouches -> superior parathyroid, ultimobranchial body Head and Neck Anomalies
From Improper Groove Closure Pharyngeal sinuses and cysts Auricular sinuses and cysts PRENATAL DEVELOPMENT Embryo Phase 1 Fetus Phase 2 Cellular
Differentiation of proliferation
internal & and migration external structures 0 1 2 3 4 5 6 7 8 Fertilization We covered this in this lecture. Phase 3 Growth and maturation 40 Delivery Embryo Phase 1 Phase 2 Cellular
Differentiation of proliferation
internal & and migration external structures 0 1 2 3 4 5 6 7 8 Fertilization PRENATAL DEVELOPMENT Fetus Phase 3 Growth and maturation 40 Delivery We covered some of this in this lecture. Embryo Phase 1 Phase 2 Cellular
Differentiation of proliferation
internal & and migration external structures 0 1 2 3 4 5 6 7 8 Fertilization PRENATAL DEVELOPMENT Fetus Phase 3 Growth and maturation 40 Delivery You’ll learn more details about head and neck development (e.g., tooth development) in oral histology Embryo Phase 1 Phase 2 Cellular
Differentiation of proliferation
internal & and migration external structures 0 1 2 3 4 5 6 7 8 Fertilization PRENATAL DEVELOPMENT Fetus Phase 3 Growth and maturation 40 Delivery We’ll talk about the development of other,
non-oral organs as we go through this