Paper - Description of a Reconstruction of the Head of a Thirty-Millimetre Embryo: Difference between revisions
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The nasal capsule is well developed, but the cribriform plate is repre- sented by a small bridge which connects the septum with the outer wall of thatside(fig.2). | The nasal capsule is well developed, but the cribriform plate is repre- sented by a small bridge which connects the septum with the outer wall of thatside(fig.2). From the outer wall of the nasal capsule there projects in an upward and forward direction the lateral nasal process of Mihalkovics, whichhasbeenregardedasevidenceofacartilaginousmaxillaryarch(palato- pterygo-quadrate) in man (figs. 1 and 3). This process partly surrounds thelacrymalduct. Attheforepartoftheunderborderofthenasalseptum one sees the two paraseptal cartilages (Jacobson), and behind them the two centresforthevomerareevident. Thesphenoidalturbinalcartilagescan be seen turning inwards at the back of the nasal cavity from the outer wallofthenasalcapsule. | ||
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<pubmed>17232851</pubmed>
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The embryo from which the model was made was kindly lent, already cut and stained, by Professor Bryce of Glasgow, and was well adapted to the purpose. I will first attempt to describe the cartilaginous neuro-cranium, and before doing so may say that itpresents many objects of very great interest, to which attention will be drawn in the proper place.
Neural Chondro-Cranium
The basilar region is completely chondrified from foramen magnum to the anterior end of the nasal capsule. The basilar plate has the usual relation to the notochord, which, as now well known, is the following. The notochord on leaving the dens passes for a short distance over the dorsal aspect of the basi-occipital cartilage, then sinks through it to lie underneath and between the cartilage and the pharyngeal mucous membrane. Then it turns upwards and forwards to enter again the basilar plate and terminate just behind the pituitary fossa.
A well-marked dorsum seler is present, and fused with the basilar mass.
The cochlear capsules have commenced to fuse with the basilar plate medially, and are now continuously chondrified with that cartilage which surrounds the semicircular canals, which, by the way, shows a well-marked fossasubarcuata. From the side of the corpus sphenoidale there passes out a large processus alaris, which by its outer end fits into a hollow in the ala temporalis (great wing), and sends back a process on the outer side of the internal carotid artery to come into contact with the cochlear capsule as noticed by Jacoby.
The ala temporalis (great wing), which, by the way, would much more suitably be named the ala zygomatica for a reason which we will see shortly, is now of characteristic form, and when seen from the front resembles the forefinger bent inwards with the superior maxillary nerve in its concavity (fig.1). The nerve actually perforates the upper part of this ala at the foramen rotundum. It is of great interest to notice that the ala orbitalis does not reach further than a very short distance into the orbital cavity; at its lower end one sees the membranous external pterygoid plate, and to the inner side of the latter the already ossified internal pterygoid plate surmounting a cartilaginous hamulus. On its outer side one sees the inferior maxillary nerve leaving the cranium, but the cartilage is neither perforated nor, for the matter of that, even grooved by either it or the middle mieninge alartery. At the inner side of its lower end this ala temporalis is hollowed out to fit on the outer end of the processus alaris. The upper end of the ala temporalis is covered in great part by the enormous Gasserian ganglion, and being small it can contribute but very litle to the basis cranii (fig.2).
Fig. 1. Part of the reconstructed head of the Bryce 30-mm. embryo as seen from the front and left side.
The ala orbitalis (lesser wing) is of relatively enormous size as compared withthealatemporalis; at its inner extremity it presents to view two limbs, of which the more posterior runs inwards to fuse with the corpus sphenoidale and complete behind the foramen opticum (fig.2). Theanteriorlimb, although well developed, has not as yet fused with the corpus sphenoidale; at its outer end the ala orbitalis forms a long tapering point, which, pro- jecting beyond the outer wall of the orbit, ends in the temporal fossa making towards but not reaching that plate of cartilage, which will be later described as the parietal plate of Spdndli, and so forming the anterior moiety of what is in many in anirnals, e.g. Echidna, Talpa, Erinaceus, Tatusia, Dasypus, Sus, Bos, and Ovis (Gaupp), the commissur aorbito-parietalis. In its outer half the anterior border of the orbital wing is prolonged into a flat plate of cartilage which passes inwards to the nasal capsule to fuse with the outer wall of the capsule. This plate,the ethmoidal plate (fig.2), forms a considerable part of the roof of the orbit at this time, and between it and the inner half of the anterior border of the ala orbitalis a spheno-ethmoidal fissure exists.
Fig. 2. View from above of the interior of the base of the neuro-cranium of the Bryce30-mm.embryo. Thecartilageiseverywhere"stippled"andcrosses are placed on the ethmoidal plate.
The nasal capsule is well developed, but the cribriform plate is repre- sented by a small bridge which connects the septum with the outer wall of thatside(fig.2). From the outer wall of the nasal capsule there projects in an upward and forward direction the lateral nasal process of Mihalkovics, whichhasbeenregardedasevidenceofacartilaginousmaxillaryarch(palato- pterygo-quadrate) in man (figs. 1 and 3). This process partly surrounds thelacrymalduct. Attheforepartoftheunderborderofthenasalseptum one sees the two paraseptal cartilages (Jacobson), and behind them the two centresforthevomerareevident. Thesphenoidalturbinalcartilagescan be seen turning inwards at the back of the nasal cavity from the outer wallofthenasalcapsule.
The crista galli is well developed, and in front of it one sees a large transversely elongated mass of connective tissue which will later become ossified to form the internal angular process of the frontal bones and the two nasal bones (figs. 1, 2, and 3).
The chondro-cranial vault is represented by two cartilages which rise up and forwards from that part of the auditory capsule which encloses the semicircularcanals. Itisoflargesize,andproceedsforwardsinthemain towards the outer pointed end of the ala orbitalis as before mentioned, butdoesnotreachit. Thisparietalplate(Spondli)istheposteriormoiety of the commissura orbito-parietalis (figs. 3 and 4).
Behind the parietal plate the auditory capsule of one side is connected withthatoftheotherbyabridgeofsomewidth,thetectumsynoticumn(fig.4).- This tectum synoticumn is of great interest, and has received considerable attentionfromLouisBolk. Inthisparticularinstanceitsendsforward in the middle line a short process which seems to correspond exactly with the ascending process of the tectum synoticum of reptiles and amphibia as figured in Hertwig's Hctndbook of Thnbryology. There is also a small backwardly projected process which, starting from the middle of the posterior edge of the tectum synoticum, projects into the membrane which closestheforamenmagnumbehind. HowmuchofthisbridgewhichI have termed belongs to the occipital and how much, if any, to the tem- poralcartilage,Iknownot. AccordingtoBolkthetectumsynoticumis formed independently of both temporal and occipital cartilages, but I must confess the appearances in his figures scarcely explain what is seen inthiscranium.
The occipital alse are separated from one another by a triangular gap which is filed in with the spino-occipital membrane (fig.4),which is tri- angular in shape, the apex of the triangle appearing to be bifid and to embrace the median posterior process of the tectum synoticum.
This is very extensive, and divided into an anterior large segment and a posterior smaller one by the tectum synoticuin; it is continued down- wardsonbothsidestoform thetemporalfosse,whicharelimiteda bove by a deepgroovecausedbytheoriginofthetemporalmuscle. Henceitruns inwards to form that part of the outer wall of the orbit generally credited tothecartilageofthegreatwingofthesphenoid. Itisperforatedbythe 3rd division of the 5th nerve and the middle meningeal artery. Pro- jected forwards from the membranous neuro-cranium is a vertical bar of connective tissue which later will be ossified to form the external angular process of the frontal bone and the malar bone; in fact, ossification has already commenced at its lower end to form the malar. This bar is perforated some distance above the mnalar bone by the inalar branch of the temporo-malar nerve (figs. 1 and 3).
Fig. 3. Profile view of neuro-cranium, visceral cranium, and upper part of neck skeleton. The cartilage is everywhere "stippled."
Visceral Cranium
MandibularArcade.-Meckel'scartilageanteriorlyendsinanupwardly projected swelling, whilst behind it ends in the malleus -cartilage which articulateswiththeincuscartilage. Thereisnosignofossificationin Meckel'scartilage.
Hyoid Arcade.-The hyoid arcade was continuous with the auditory capsule,thechordatympaniconcealingtheroot. Descendingforashort distance,thecartilagethenarchesfrontalwards,laterturningintheventral directiontoreachbutnotfusewiththebodyofthelhyoid. Thebodyof the hyoid is well formed, and in many respects resembles the bony hyoid oftheadult.
Thyroid Arcade.-This is represented by a dorsally and outwardly directed cartilaginous rod which is attached by its inner end to the body ofthehyoidbyconnectivetissue. Atitsouterenditisconnectedby connective tissue with the superior cornu of the thyroid cartilage.
The thyroid cartilage consists of two plates of cartilage separated by a fissureinthemiddleline. Theupperandlowercornuaarewellmarked and have the usual connections, the upper one being, of course, nearer the thyro-hyalthanintheadult. Inthelateralplateoftherightsideisa foramen in the position usually formed; the cricoid and arytenoids are welldeveloped(fig.3).
The Bony Cranium
This consists entirely of "covering bones," there being no sign as yet of "substitution"or primordial bone.
The frontal bones are well developed, thickest at what later will be the base of the outer angular process, as if ossification had commenced there. They are separated from one another by a wide membranous interval. Each by the lower part of its inner border is in contact with the ethmoidal plate and the ala orbitalis of the sphenoid.
The squamoso-zygoinatic presents a curious appearance, not unlike a boomerang, the posterior limb of which passes backwards (caudad) as a coveringbonetothecrusbreveoftheincuscartilage. Theanteriorlimb was ventrally on the outer side of Meckel's cartilage.
These are the only calvarial "covering" bones developed at this time (fig.3).
The Facial Bones
The mialar is of very small size, and placed between the squamoso-zygomatic and the malar process of the maxilla, below which it lies also in part. Itliesinthelowerpartoftheverticalbarofconnectivetissue; before mentioned as forming the outer wall of the orbit, and is at this time considerably below the malarbranchofthetemporo-malar nerve.
Fig. 4. View from above and behind.
The maxilla is of large size,withalitsprocessessavetheinneralveolar developed. (All the processes-nasal, palatine, malar, outer and inner alveolar-are developed as outgrowths from one common centre, and there is no warrant for the statement that the malar part, the orbito-facial, and palatine, are separate.) The nasal process has approached much more closelythenasalcapsulethanatanearlierage. Theorbitalsurfaceis stilverysmallandtriangularinshape,withapexforwards. Thegroove fortheinfraorbitalnerve isnow beingformedbytheuprisingofaprocess ofbonetotheoutersideofthenerve. Inearliertimestheinfraorbital nerveisseparatedbyawideintervalfromthemaxilla. Thereisnosign ofanypremaxillaryfisure. Thepalatineprocesshasgreatlythickened inheight. Therecanbenodoubtbutthatthewholemaxilla,exclusive ofpremaxilla,ossifiesfromonecentreonly. Cartilageisdevelopedinthe malar process in this embryo (figs. 1 and 3).
The palate bone appears as a somewhat curved plate immediately behind the maxilla, and it bears out what I some time ago stated, that the vertical plate,notthetuberosity,isthefirstparttoossify.
Thevomerisjustinitsearlieststageofossification. Itconsists,as usual at this age, of two separate halves, lying between the paraseptal cartilage and, below the nasal septum (at a later stage they will be added to by union with the ossified posterior ends of the paraseptal cartilages, 100mm. embryo).
The internal pterygoid plate has commenced to ossify in membrane, but the hamulus is cartilaginous.
Before leaving the bones it may be well to say a litle concerning the orbit.
The orbit is, as in the adult, directed forwards, and is relatively of large size. Its roof is formed partly by the cartilaginous ala-orbitalis, by the ethmoidal plate, by the frontal bone, and by unossified membrane. The floor is mainly membranous, but a small part is formed by the triangularorbitalsurfaceofthemaxilla. Theinnerwallisformedcom- pletely by the nasal capsule, whilst the outer wall is formed by connective tissue, the ala temporalis taking practically no part in its formation; in fact, the so-called ala tomporalis forms practically nothing more than the pterygoid process of the sphenoid, and as it is certainly nothing like a wing, it might well be called the ala or pars zygomatica, as it forms mainly that part of the sphenoid found in the zygomatic region (fig. 1).
The mandible is of enormous relative size; shows a ventral foramen, inner alveolar border, incisor canal, and commencing coronoid process. No condyleisasyetvisible.
The Nerves
Most of the cranial nerves are represented, but few require any special mention. Themorestrikingpointsconcerntheganglia,theGasserian being of enormous size, as well as the spheno-palatine, otic and the ganglia of the vagus and glosso-pharyngeal.
The infraorbital nerve is practically straight, running forwards from the foramen rotundum along the outer side of the palate bone over the orbital surface of the maxilla, but not grooving it except near its anterior margin.
Blood-Vessels
Both arteries and veins were modelled. The most striking feature about the arteries is the great difference in size between the intra- and the extra-cranialparts. Theintra-cranialpartsofbothcarotidsandvertebrals are relatively enormous, and are very thin-walled,- dificult, in fact, to distinguish from veins; the extra-cranial parts, on the other hand, have thickwalls,-andareofsmallsize.
The veins in the model are two large lateral intra-cranial veins which, running from before backwards, cross the parietal plate, then turn down- wards to leave the cranium at the posterior lacerated foramen, previously receivingthepetro-squamoussinus. Thesetwocranialveinsareconnected by a transverse vein under the tectuin synoticum, which, I suppose, is the precursorofthelateralsinus. Therewasanenormousbloodsinusover thenasalcapsule,butitwasnotmodelled. Itisofinteresttonotethat even at this early period the right lateral cranial vein is much larger than theleft. Bothlateralcranialveinsarecrossedbeforebeingjoinedbythe transverse vein before mentioned, by the saccus endolymphaticus, which is of very large size-as wide, in fact, as the vein itself.
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Cite this page: Hill, M.A. (2024, May 4) Embryology Paper - Description of a Reconstruction of the Head of a Thirty-Millimetre Embryo. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_Description_of_a_Reconstruction_of_the_Head_of_a_Thirty-Millimetre_Embryo
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
