QUESTIONS: From what germ layer is the inner lining of the derivatives of the first pharyngeal arches? What are the specific derivatives of the primary bronchial buds (anterior and posterior)?
Endodermal Derivatives of the 10 mm Pig Embryo (SEE IMAGE)
Position your slides such that the spinal cord appears uppermost in the field. Bear in mind that the position of the endoderm structures in the 72-hour chick embryo is essentially similar to that of the structures in thethe pig embryo although at this time there is no more midgut and anterior posterior intestinal portals. Concentrate on the structures that are appearing for the first time in the pig.
Digestive and Respiratory Systems
First pharyngeal or hyomandibular pouch and furrow: In a frontal section passing through the pharynx, the auditory vesicle and the ventralmost part of the hindbrain, observe the first pharyngeal pouch, with its thick endoderm lining and the furrow in the thin ectoderm. Note if there is a closing plate. At the anterior border of the pouch and furrow is the first visceral or mandibular arch just like in the chick embryo. Find the future Eustachian tubes, derivatives or the first pouch.
Second pharyngeal pouch or furrow: As you proceed caudally in the series, a second pharyngeal pouch is seen posterior to the second pharyngeal or hyoid arch and nearer the spinal cord.
Pharynx: This is the large cavity into which the pharyngeal pouches open. Due to the flexure of the head of the embryo the pharynx is bent and therefore divided into two parts as the slide is traced posteriorly. Trace the anterior one into the stomodaeum.
Stomodeum: Trace the stomodeum or oral aperture. Observe that as it becomes continuous to the outside laterally by a cleft, the anteriormost part of the head becomes completely separated from the rest of the body. Why is this so?
Tongue: In the first frontal sections through the pharynx, the tongue appears as an “island" in the center. The anteriorly directed tip is the tuberculum impar. Trace to see how the tongue fuses with the second and third pharyngeal arches.
Copula: This is the medial part of the second visceral arch, which becomes the future root of the tongue.
Lateral tongue swellings: These are the lateral connections of the tuberculum impar to the mandibular arch. Posteriorly as the tuberculum impar disappears, the lateral tongue swellings fuse in the midline to form the body of the tongue.The tuberculum impar will this lie between the body and the root (copula) of the tongue.
Epiglottis: This structure appears at the posterior border of the third visceral arch as a median swelling. It is considered a derivative of the third and fourth visceral arches.
Fourth pharyngeal furrow: Due to the flexure of the embryo, the third and fourth furrows fuse into a common cavity, the cervical sinus.
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Thyroid: At the level of the hyoid arch, a darkly stained body, the thyroid, appears in the floor of the pharynx. A small dorsal stub of the thyroid represents the remnants of the thyroglossal duct. Are there already vestiges of follicles in this cell mass?
Parathyroid of third pouch: The primordium of the inferior parathyroid is seen as a thickening of anterior wall of the dorsal region of the third pouch near a large blood vessel, the third aortic arch.
Thymus of third pouch: The blastemal of the thymus comes from the ventral portion of the third pouch. Tracing it ventro-medially it appears as an oval vesicle.
Arytenoid Swellings: In sections through the thymus, two prominent folds, the arytenoid swellings are formed from the floor of the pharynx.
Glottis: This opening between the arytenoid folds communicates with the larynx primordium, the dilated ventral portion of the laryngotracheal groove between the arytenoid folds.
Fourth pharyngeal pouch: a tiny lateral recess of the pharynx at the level of the glottis and arytenoid swellings.
Parathyroid of fourth pouch: This is a thick epithelial diverticulum at the dorsal portion of the fourth pouch. Is the thymus of the fourth pouch already evident in your slide? It is a thick epithelial diverticulum in the ventral portion of the pouch.
Ultimobranchial body or lateral thyroid: This is a ventral evagination of the fifth pouch.
Esophagus: As you trace the pharynx posteriorly, it soon becomes flattened and eventually becomes circular in cross-section, the esophagus.
Upper and Lower jaw: At the level of the esophagus, look at the anterior side of the same section and identify the fusion of the mandibular processes to form the lower jaw and fusion of the maxillary and median nasal processes to form the upper jaw.
Trachea: Ventral to the esophagus is a thick endodermal tube, the trachea.
Bronchial buds or lung buds: The stems of the lateral outgrowths of the trachea are the presumptive bronchi and the dilated ends are the putative lung lobes.
QUESTIONS:
- From what germ layer is the inner lining of the derivatives of the first pharyngeal arches?
- What are the specific derivatives of the primary bronchial buds (anterior and posterior)?
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