Talk:Paper - Notochord (1900)

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NOTOCHORD

Minot CS. The study of mammalian embryology. (1900) The American Naturalist, 34(408): 913-941.

The notochord {awrda dorsalis, Wirbelsaite) is a rod of peculiar tissue, constituting the primitive axial skeleton of vertebrates. It begins immediately behind the pituitary body (hypophysis) and extends to

Fig. 3592.— Transverse Section of a Younfi: Mole's Embryo. (Alter Heape.) jSc, Ectoderm ; Md., medullary groove ; Mes., mesoderm ; Ent., entoderm. SitH of the notochord is the central line of the entoderm.

the caudal extremity . It occurs as a permanent structure in the lower types, and as a temporary one in the embryos of amphibia and amniota, including man. Comparative embryology has shown that it is a greatly modified epithelial tube, which arises as a furrow in the median dorsal, line of the entoderm, being, in position and mode of development, analogous to the ectodermal medullary canal or primitive tubular nervous system."

Development in Mammals. — The notochord appears very early in the course of development ; its differentiation from the entoderm begins at the time when the medullary groove is not fully marked out posteriori}', and isnowhere closed. The notochordal Anlage can be first detected in the entoderm just at the front of the primitive streak, as an axial band of cells, which at first in mammals is not well marked off from the mesoderm ; as themedullary groove deepens it pushes down toward the midgut until it comes into actual contact with the notochordal epithelial band (see Pig. 8593), thus dividing the mesoderm into two lateral masses; this also leads to the temporary transverse stretching of the notochordal band, which thereby loses for a while its sharp demarcation. It soon re-acquires it, and becomes considerably thicker (Fig. 3593, nch) than the adjoining entoderm, and forms, a distinct though shallow groove. Subsequently the band separates off, and the entoderm proper closes acrossunder it so that the notochordal band lies between the entoderm and the fioor of the medullary groove (or later canal), as shown in Pigs. 3598 and 3604, nch. This separation does not take place at the anterior extremity of the chorda until somewhat later, so that for a considerable period its front end remains fused with the walls of the midgut (Fig. 3598). The separation from the entoderm is effected, at least in mammals, by the entoderm proper, showing itself under the notochord toward the

Md.

Fig 3593.— Transverse section of an Embryo Mole, Stage H. (After Heape.) am.. Amnion; JIM., medullary groove ; Jtfy., myotome;; Coe coelom or body cavity ; £»»., entoderm ; Tich., notochord ;ao., aorta; i)t.«., vitelline artery; Soni., somatic mesoderm; Spl.v splanchnic mesoderm.

median line, and when the cells from one side meet thoseof the other, they unite with them and form a continuous, sheet of entoderm below the notochordal cells.

The chorda is now a narrow band of cells, starting anteriorly from the wall of the alimentary tract and run 29r

Notocliord. No4o chord.

RBFEBENCE HANDBOOK OF THE MEDICAL SCIENCES.

ning backward to the Wasto.poric canal or its equivalent, the primitive streak ; but, at the period wten the canal is open the chorda terminates in the entodermic epithelium lining the canal (Heape,' PI. xxi.. Fig. 50; compare

lined by epithelium, which is thickened on the dorsal side to form the Anlage of the notoc'hord. In transverse section the chorda appears according to the level of the section to constitute ^art of a furrow or a canal (compare also Heape,9 'loe. cit., p. 441, Figs. 40 and 41). Lieberkiihn calls this canal mesoblastic, and KOlliker follows him ; but this 'opinion 'seems tome based upon misconceptions. It 'is more reasonable 'to suppose 'tShat the csnal is really the blastoporic canal, which is 'preserved for an unusually long period. Wefenowlfllatthe blastopore first appears well fOTward, and as the primitive streak grows by concrescence of the ectental line the blastopore moves "backward, its anterior port-ion f usdii'g With '(he 'general entodermic ca'vity. There is no difBeiflty apparent in assuming that such 'fusion occuts quite late in mammals; this interpr^a'tion is confirmed by the fact that the canal ^becomes later a furrow ffiroughout its entire lengfth in

Fig. 3594.— Transverse Sections of an Embryo Chick, witn Eleven Pairs of Myotomes. (After Waldeyer.) A, Some distance behind the last myotome ; B, close behind the last myotome ; JEc, ectoderm ; >l6S., mesoderm ; Ent., entoderm ; MA., medullary groove; Ch., notochoTd; W., commencement of the Wolffian duct; MS., muscular segment or myotomes.

also Vol. II., Fig. 505, 0). The canal remains open for a time, and is called by some writers on mammalian embryology the cliorda canal {cf. infra). For a certain period the chorda continues growing tailward by accretions of cells from the walls of the blastoporic passage,

Fig. 3596. -Section of a Chicken Embryo of about Thlrty-slx Hours. (After Waldeyer.) Ec, Ectoderm ; Som., mesoderm of the somatopleure; Sp!., mesoderm of splanchnopleure ; Eret., entoderm; W., WoManduct; m, mesoderm cells; Md., medullary canal ; v, vein ; Coe., coelom ; MS., myotome ; C/i, notochord ; A.O., aorta.

front of the blastoporic canal proper, so that its cavity

fuses with that of the 'entoderm proper. After it is once formed as a band of cells the notochord

passes through various changes of form, but ultimately

becomes a cylindrical rod with tapering extremities. It attains considerable size in the embryos of most vertebrates, but in those of placental mammals is always small, particularly so in the mole (Heape"). It is probable that in mammals the notochord, when first separated from the entoderm, is a broad, flat band, as if compressed be

FiG. 3595.— Transverse Section of a Chick Embryo of the Second Day. (After Waldeyer.) Som., The somatic mesoderm, and Spl., the splanchnic mesoderm ; Be, ectoderm ; JEnt., entoderm; F.C., vein; TF., Wolffian duct ; Afd., medullary canal ; A.O., aorta ; Cb., notochord ; MS., myotome.

and after the canal is permanently obliterated the chorda may still continue its lengthening by acquisitions, at its caudal end, of additional cells from the primitive streak; such cells may, however, properly be regarded as coming from the entodermic lining of the blastopore. We can, then, distinguish thi'ee portions of the notochord : the first arising from the entoderm of the midgut ; the second from the entoderm of the blastoporic canal ; the third presumably from the entoderm of the obliterated blastopore in the primitive streak. Braun and others have sought to attribute essential importance to these differences, but, it seems to me, improperly. It is more reasonable to say that the chorda arises in the amniota, as in the lower forms, directly from the entoderm, but presents certain secondary modifications in its development.

Lieberktthn has directed attention to a special peculiarity in the early development of the notochord in mammals. There appears at first a passage — half canal, half furrow— which extends nearly the whole length of the primitive streak; it may be described as a tube running along the median line, and having anirl-egular series of openings into the entodermic cavity. The canal is

Fig. 3597.— Section through the Dorsal Region of a Chicken Embryo of FortiF-flve Hours. A, Ectoderm ; c, entoderm ; Me., medullary canal ; P.v, myotomes ; W.A, Wolffian duct ; 'p.p., pleuro-peritoneal space or coelom ; So., somatopleure ; i), v, blood-vessels ; Sp.. splanchnopleure ; op, inner edge of the area opaca , w, w, w, entoderm of the area opaca ; ao, aorta ; ch, notochord. (After Balfour and Sedgwick.)

tween the medullary canal and entoderm (cf. KSUiker, loa. cit.. Figs. 194 to 197, and loc. cit.. Fig. '94; also Heape,'" PI. XIIL, Figs. 36 to 42). The band then draws together, diminishing the transverse and increasing the vertical diameter, until it has acquired a rounded form; finally its outline becomes circular in cross section.

FHJ. 3598.— LonKitudlnal Section of the Head End of a Mole Embryo, Stage H. (After Heape.) Ec, Ectoderm ; En., entoderm ; pro.am., pro-amnion ; mb., mid-brain ; fb., fore-brain ; Bnt , entodermlc cavity ,• ht, heart ; Mes., mesoderm ; rich., notochord.

This series of changes begins near the anterior end of the chorda, and progresses both forward and backward.

The mesoderm early grows in between the entoderm and the notochord, whicli, however, for a considerable time remains close to the medullary tube (Fig. 3600). Later the mesoderm penetrates between the notochord and medulla. The layer of mesodermic eel Is immediately around the notochord, which are of the well-known anastomosing type (Fig. 3601), forms a special sheath, which at first comprises only a single layer of cells, at least in hatrachia (Gotte," p. 357, Fig. 187). This is the commencement of the so-called outer chorda sheath ; it subsequently becomes much thicker. In the lower types it is an important axial structure (Fig. 3602, «'); in most cases it is replaced by cartilage, and in all the amniota the cartilage is replaced by the osseous vertebrse, the invertebral ligaments, etc. The formation of the vertebral column involves the disappearance of the notochord as described below.

Histogenesis. — After the notochord has been formed as a rod of cells, its ceils undergo a process of histologi

day some of the central cells become vacuolated, while

the peripheral cells are still normal; at first, as in the

frog, there seems to be only one large vacuole in each

cell (Fig. 3603, B). Around the vacuole is a peripheral layer of granular protoplasm, in which the nucleus lies embedd-id, while the vacuoles themselves are filled with a perfectly clear and transparent material, which is supposed to be fluid in its natural condition. During the fourth day (chick) all the cells become vacuolated, with the exception of a single layer of flattened cells at the periphery. In the anura, it is said, there is no distinct peripheral layer of protoplasmic cells. The vacuoles go on enlarging until by the sixth day they have so much increased at the expense of the protoplasm that only a very thin layer of the latter is left at the circumference of the

cell; at one part

of which, where

there is generally

more protoplasm

than elsewhere, the

remains of a nucleus may generally be detected.

Thus the notochord

becomes transformed into a

spongy reticulum,

the meshes of which

correspond to the

vacuoles of the cells

and the sejota to

the remains of their

cell walls (Foster

and Balfour). As

GStte has pointed

out, the process is

accompanied by an

expansion of the

cells, which is the

main factor in the

widening and lengthening of the notochord, which goes

on pari passu with the growth of the surrounding tissue.

Pig. 3599.— Germinal Area of a Guinea-pig at Thirteen Days and Twenty Hours. (After Lieberkiihn.) ao.. Area opaca; ap., area pellucida ; nch., Anlageot the notochord as a canal with several irregular openings on the entodermic side. X 24 diameter.

Fig. 3600.— Section through the Eump of an Embryo Chick of the Third Day. Gh., Chorion; Am, amnion ;S(TO.,somatopleure:i),j),l), blood-vessels; Coe., coelom; Spl., splanchnopleure ; In., intestine ; oo., caudal branch of the aorta; Tfci., Wolffian duct; year., vena cardinalis; Ch.d., chorda dorsalis ; My., myotome ; Md., medullary canal.

cal differentiation unique in vertebrates. The cells at first become greatly compressed in the line of length of the chorda: and hence appear quite thin in longitudinal sections (Fig. 3603, A, ftcA.)— hardly greater in diameter than their own nuclei. Thus, in the chick, by the third

The histogenetic process is stated to be essentially similar in mammals (W. MUller, 337-338). There is the central layer of vacuolated cells and the peripheral layer of protoplasmic cells. The latter are, however, ultimately converted into vacuolated

cells. The cell walls ai'e perforate, having fine pores, that correspond probably to intercellular bridges of protop lasm. The inner .'•;chorda sheath appears early and is to be

Fig. 3601.— Mesoderm ot Chick of the Third Day, from close to the Otocyst. A, Nucleus with the chromatin loops seen in optic section, being in karyokinesis.

regarded as an anhistic basement membrane secreted by the notochordal cells. ^^^

Shape and Relations ^v.

TO Otheb Pabts. — As soon as the head bend ;

(first cerebral flexure) ap pears (Fig. 3604) the noto chord becomes corre spondingly bent.

sheath is lost. The cell walls disappear, the tissue becomes granular, and breaks up into multinucleate, irregularly reticulate masses (Pig. 3606), which are gradually resorbed (Leboucq). In mammals the resorption progresses more rapidly in the cores of the vertebrse than in the intervertebral spaces, and again more rapidly at the ends than in the centre of each vertebra; hence the

chorda persists a little longer in the centre of

the vertebra, and considerably longer in the. intervertebral spaces; in these last the final remnants of the chorda may bedetected in man even after birth. The cavity between the vertebral cartilages is a new structure, and is not the space left by the notochord, as has been some

anterior extremity lies close to Rathke's pocket (Fig. 3604, %._)— the evagination of the oral epithelium, which is destined to form the pituitary body or hypophysis cerebri. The notochord never extends farther forward than this, hence the skull and head may be divided into two parts, the prse-pltuitary and the postpituitary regions. The latter region alone contains the notochord. Romlti finds that in the chick the end of the notochord is united, at the end of the fourth and during the fifth day of incubation, with an irregular solid cord of cells, which grows out from the epithelium of the hypophysis. The cord soon disappears. Its significance is quite unknown. Romiti suggests that it may produce a strain resulting in the pulling out of the hypophyseal evagination. This notion seems to me untenable. The cranial portion of the notochord has not only the bend shown in Fig. 3604, but also follows the other curves of the head ; it takes a sinuous course besides within the base of the cranium; finally, in the region corresponding to the middle third of the sphcno-occipital cartilage, it makes a great dip ventralward. The sheath of the notochord in the cranial region is converted into the spheno-occipital cartilage ; at the dip just mentioned, however, the notochoi'd lies entirely below the cartilage, close against the wall of the pharynx (Froriep, Romiti). Writers before Froriep hart represented the chorda as having disappeared at the bottom of the dip.

Disappearance.— The disappearance of the notochord in man commences with the second month of foetal life. The first step is an alteration of the characteristic histological structure, accompanied fig. 3603.— Longitudinal Sections of the Notochord of Bombinator. (Alter by shrinking of the tissues, so that a clear SDace G<>tte.) ^, Before the appearance of the vacuoles ; B, after the appearance" ■ -' - of the vacuoles : nc?!., notochord; En., entoderm. (The cells, as Is usual

in amphibian embryos, are charged with yolk granules.)

Fig. 3603.— Transverse Section of an Advanced Embryo of a Shark, Seymnus ttcMa, through the Abdominal Region. (The dots represent nuclei.) Sp, Spinal process of the vertebra ; Ar., arachnoid space ; Md., spinal cord; n.a., neural arches of the vertebra; s., inner sheath of the notochord ; s'., outer sheath of the notochord ; Ch., notochord ; t.p., transverse process of the vertebra ; v.car., cardinal vein ; An., dorsal aorta ; mes, mesentery ; Gen., genital fold : W.d, Wolffian duct ; W., Wolffian body with tubules ; c, young cartilage ; Jlii'C, muscles developing.

proper ; antei'ior

times asserted. It appears, however, that theresorption of the chorda may leave a small space, which becomes included in the intervertebral cavity. A peculiar feature isthe frequent persistence of calcified cartilageimmediately around the notochord in ossifying vertebrae.

MOKPHOLOGY.

— The notochord was for a longtime supposed tobe exclusively characteristic of vertebrates. It is now known toexist in amphioxus, which is not a ycaf. true vertebrate, and in the tunicata. Morphologists have longbelieved that it must have somehomologue among the organs of invertebrates. The development of the notochord in the lower vertebrates indicates very plainly what must have been the general character of such an homologous invertebrate organ. In certain fishes and amphibia the notochord has been ascertained to arise as a furrow along the median dorsal line of the entoderm; the furrow deepens and then closes over to form a canal separate from the entodermic canal but the notochordal canal retains for a time its and posterior connections with the entoderm..

appears around it (see Fig. 3605). The inner chorda

"Ultimately the lumen is obliterated, the ends become detached, and so arises the solid isolated chorda. In the

higher vertebrates the course of development is similar, although several of the primitive features in the formation of the chorda are obscured. Ehlers' has pointed out that in various invertebrates there is a similar canal, the "Nebendarm" of German writers, which is derived from the entoderm and connected anteriorly and posteriorly with the entodermal cavity. It is a very plausible suggestion, which homologizes the vertebrate notochord with the invertebrate "Nebendarm." Hubrecht has sought to homologize the notochord with the proboscis of

riG. 3604.— Babbit Embryo of 6 mm.; Median Longitudinal Section of tlie Head. (After Milialkovics.) The connection between the mouth, M., and pharynx, ent., is just established; ndh., notochord; Tib., hind-brain; mb., midbrain; /ib., fore-brain; Prn.am., proamnios ; hj/., hypophysis cerebri ; Ht., heart.

Fig. 3605.-Human Embryo of about Thirty-flve Days; LonSiM^mal Sect on of the Ninth to the Eleventh Vertebrse, as numbered IX. to XI. N, Nervous system, wall of the spinal marrow ; d, meningeal layer ; Oh, notochord ; Ao, aorta.

best observations on its origin in mammals by Heape. '" For its histology see W. Mttller ; for its histogenesis see GStte;* for its anterior anatomical relations see Mihalkowics, Proriep, Rabl-Rilckhard, and Romiti; for its atrophy in mammals see Leboucq ; for its evolution see Ehlers.'

Charles Sedgwick Minot.

' Balfour : A Monograph on the Development of Ela.smobranoh Fishes, London, 1878. (Reprinted Works, i., pp. 203-520.)

^ Balfour : Comparative Embryology, vol. il,

' Ehlers, E. ; Nebendarm uud Chorda dorsalls. Nachr. Ges. Wiss., GOttingen, 1885, pp. 390-404.

• Froriep : Koptthell der Chorda dorsalis bel menschllchen Embryonen. Festschrift fiir Henle, 1882, pp. 26-40, Taf. iii.

' Gegenbauer, Carl : Ueber das Skeletgewebe der Cyclostomen (Histologie der Chorda, S. 47-49). Jena Zeitschr. Nat. Wiss., v., 1869, pp. 43-53, Tat. i.

« GBtte, Alex. : Entwiokelungsgeschichte der Unke (especially pp. 349-361), Leipzig, 1875.

' Basse, C, und Schwarct, W. : Studien zur vergleichenden Anatomie der Wirbelsaule, etc. Basse's Anat. Studien, i., p. 21.

' Hatschek, B. : Studien zur Entwickelungsgeschichte des Amphioxus. Arbelten Zool. Inst. Wien, iv.. Heft i., Taf. xiii.

"Heape, Walter: The Development of the Mole (Talpa Europea); the Formation of the Germinal Layers and Early Development of the Medullary Groove and Notochord. Q. Jour. Micr. Sci., 1883, pp. 412-452, Pis. xxvlil.-xxxi.

'" Heape, W. : The Development of the Mole. Q. Jour. Micr. Sci., xxvii., pp. 123-163.

11 Hensen : Zeitsohrift f. Anat. u. Entwickelungsges., i., p. 366.

"His, Wilhelm: Erste Anlage des Wirbelthlerleibs, 4to, Leipzig, 1868.