Book - A Text-book of Embryology 12

From Embryology


Heisler JC. A text-book of embryology for students of medicine. 3rd Edn. (1907) W.B. Saunders Co. London.

   Text-book of Embryology 1907: 1 Male and Female Sexual Elements - Fertilization | 2 Ovum Segmentation - Blastodermic Vesicle | 3 Germ-layers - Primitive Streak | 4 Embryo Differentiation - Neural Canal - Somites | 5 Body-wall - Intestinal Canal - Fetal Membranes | 6 Decidual Ovum Embedding - Placenta - Umbilical Cord | 7 External Body Form | 8 Connective Tissues - Lymphatic System | 9 Face and Mouth | 10 Vascular System | 11 Digestive System | 12 Respiratory System | 13 Genito-urinary System | 14 Skin and Appendages | 15 Nervous System | 16 Sense Organs | 17 Muscular System | 18 Skeleton and Limbs

Early Draft Version of a 1907 Historic Textbook. Currently no figures included and please note this includes many typographical errors generated by the automated text conversion procedure. This notice removed when editing process completed.

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Although the nasal chambers and the pharygeal cavity contribute to the formation of the respiratory system, these parts will not be considered here, since they are described elsewhere.

Fig. los.— Scheme of the alimeiiUry canal and its accessory organs (Bonnet).

Anatomically and according to their mode of development the lungs might be looked upon as a pair of glands having a common dnct, the trachea, which latter, through the medium of its dilated proximal extremity, the larynx, opens into the pharyngeal cavity. In point of fact, these organs are developed as an ontgrowth from the entodermal alimentary canal in a manner similar to the development of the liver and the pancreas.

The first step in the development of the lungs is the outpouching of the ventral wall of the esophagus throughout its entire length. The longitudinal median groove thus formed is the pulmonary groove. It makes its appearance when the embryo has a length of 3.2 mm. (0.128 inch) or probably early in the third week. The groove is more pronounced at its lower or gastric extremity. As the groove deepens, its edges approach and finally meet and fuse ^vith each other. In this manner the groove is converted into a tube, wliich gradually separates from the esophagus, the separation beginning at the end toward the stomach and progressing toward the pharynx. The separation, however, is not complete, stopping short of the upper end of the groove, so that the tube retains communication with the pharyngeal end of the esophagus. Even l>efore the constricting oiT of this tube or pulmonary diverticulum is completed, its free end bifurcates. The pulmonary anlage consists, then, at this stage, of two short wide pouches connected by a common |>edicle with the primitive pharynx (Figs. 108 and 109), and this condition is present in the fourth week.

A very soon after the end of the first month each of the pouches undergoes division, the right one into three branches,

Fig. 109.— Transverse section to show outgrowth of pulmonary anlage from gut-tube (alter Tourneux): 1, dorsal mesentery; 2, ventral mesentery ineluding 3, mesocardlum posterius ; 4, mesocardlum antcrius; 7, esophagus;

8, diverticulum which becomes the lungs, the trachea, and the larynx;

9, heart.

epithelial cylinders, the lumina being acquired later. At first, the lining entodermal cells of the primitive tubes are tall and cylindrical, the tubes themselves having a relatively small lumen. In the fourth month the cells acquire cilia. From the anatomical standpoint, the lungs now present the characters of compound saccular glands.

From the sixth month to the end of gestation occur the changes which give to the organs their essential characteristics. Upon the dilated extremity of each terminal tube numerous little evaginations develop. These are the air-sacs, or pulmonary alveoli, the terminal tubes from which they are evaginated being the alveolar passages and the inftmdibnla. Their walls remain very thin and their lining epithelium flattens to such a degree as to closely resemble endothelium. The trachea is simply the elongated stalk of the pulmonary diverticulum. Its incomplete cartilaginous rings first appear in the eighth or ninth week.

The larsrnx is the dilated proximal extremity of the stalk of the pulmonary diverticulum specially modified to serve as an orgjin of plionation. It is first indicateil at the end of the fifth week (or, according to KalHus, in the fourth week). One of the earliest changes is the appearance of two dorsoventral ridges at the junction of the primitive tnichea with the esophagus. They are close together in front, ventral ly, but separated dorsally. They are the first indications of the true vocal cords. At this time the pharyngeal aperture of the primitive larynx is at about the level of the fourth viscenil furrow, behind the three segments of the developing tongue (p. 144), and is separated from them by the Aircnla, a horseshoe-shaped ridge which bounds the aperture in front and laterally and which represents apparently the ventral parts


Fio. 112.— Entrance to larynx in a forty- to forty-two-<luy human embryo (from KalliusK /, tuberculum impar; p, pharynjro-epiglottic fold; e, epiglottic fold : l.e, lateral part of epiglottis ; ni. cuneiform tubercle; com, cornicular tubercle.

of the third visceral arches (Fig. 71, A, 3, p. 144). A little later the furcula difTerentiates into a median eleratioii, which is the anlage of the epiglottis, and into the two lateral arytenoid ridges, each of which latter presents two little elevations^ the comicnlar and cnneiform tabercles respectively (Fig. 112). The arytenoid cartilages are thus well indicated by the sixth week. The lateral p)rtions of the furcula also produce the aryteno-epiglottidean folds.

The thsrroid cartilage develops in two lateral halves from corresponding masses of mesenchyme which chondrify from two distinct centers for each mass. It is regarde<l as representing the cartilages of the fourth and fifth branchial arches. The two alie fuse with each other ventrally as development advances. Failure of cartilaginous iniion between the two alo) constitutes the malformation, /oramr/i thyroidcum. The cricoid cartilage is regarded as being an independent cartilaginous formation in scries with the rings of the trachea. Tiie chondrification of these various elements of the larvnx begins in the eighth or ninth week.

The development of the pleurae has been descril)ed in connection with that of the ]x»ricardium and of the diaphragm (p. 175).

The Thyroid, The Parathyroid, and the Thymus Bodies

Th(\*<e organs may be considered in this connection as a matter of convenience and because of their embryological relationship to the* respiratory system, being developed, like the latter, from the epithelimn of the gut-tract.

Th(» thyroid body, an organ common to all vertebrates, genetieallv consists of two parts, a median and two lateral portions, or lateral thyroids.

The median portion originates from an evagi nation of the ventral wall of the pharynx, in the median line, posterior, caudad^ to the tuberculum imi)ar, and between the ventral extremities of the first and second visceral arches. This median diverticulum is present in the human embryo of 5 nmi. It soon pouches out on either side, assuming thereby the form of an epithelial vesicle connected by the constricted pedicle of the diverticulum with the ventral wall of the pharynx (Fig. 113, 3). From the situation of the original point of evagination behind the tuberculum impar and vcntromesial to the two halves of the posterior segment of the tongue, the orifice of the pedicle corresponds to the line of junction of the three parts of the tongue. As a consequence, when these parts

Fig. 113.— DiagTammatip representation of pharynx of human embryo seen from in front (after Tonrnenx): I, II, first and second pharyngeal pouches ; 1, tuberculum irapar; 2, course of thy mglossal duct leading from 3, median lobe of thyroid gland; •I. laryngotracheal tube: a, esophagus: 6, thymus: 7, epithelial body [parathyroid]; 8, lateral thyroid ; 9, postbranchial body [parathyroid ?].

unite, the pedicle or duct is prolonged upward and comes to open upon the surface of the tongue. The canal is known as the thyrogloBsal duct or canal of His. In the fifth week it begins to atrophy, and usually by the eighth week has become obliterated. Occasionally it persists throughout life. The foramen csecnm on the dorsum of the tongue is the vestige of the orifice of the duct. Other vestiges of the thyroglossal duct are sometimes present. For example, the lower part of the duct may persist as a short tube, the tlisrroid duct, leading upward from the median lobe to the hyoid bone; and again, according to His, isolated persistent segments of the duct constitute the little vesicles in the neighborhood of the hyoid bone which are known resjx?ctively as the accessory thyroid and the suprahyoid and prehyoid glands. According to some recent observations the lower piirt of what His calls the tliyroglossal duct gives rise ti> the pyramidal process of thn thyroid, whirh extends ujw waixl towiml the hyoiil bone, usually a litllc to the lefl of the mid-line. Tiiis impaired median anlage gives rise to tile isthmus of the adult oi^aii and abo, to a considerable jMirt at least, of eai-h lateral lobe.

The lateral thjrroida begin their development somewhat later than dues iho niodlan ]xirtion. In the embryo of 10 mm., the fourth inner visceral ftirroT or throat-noeket of each Bide pouches om to form a. vesicle {Fig. 113, S). As the vesitrle grows, its iH-dieli- W'TOmes attenuated and finally disappears. After their isolation from the thniat-jKwkets, the

Fio, lU.-Seml-dlaenmnuitlc iniutrallam (o i>how Ww iilUmnlu |>oaliliiii of (be ttifiniu, Ih^rold gUntl, *nd pcMibranoliUl txid]' on thu niTk of tlii' vlilpk {A) and the K«1f(B). niter de Mciimn: ■■(, Ihjrfiid Elnnd ; p, postbranrhlal bodjr; «. Ihymxa: r, epithelliltiody [pumlhjniW]; ;r,lrBcbBfl; A.hrort; t^, vena Jugulmrls i eo. eiRHid Ti-ln.

vesicles give out small bud-like processes after the usual manner of the development tif glantl- and gradually approach the median lobe {Fig. 1 14, B), fusing with its posterior surface. The three parts unite probably in the seventh week. In the vertebrates below mammals the lateral parts of the thvmid darated from it as the suprapericardial bodies. According to the older view of His, the lateral thyroids produce all of the lateral lobes of the adult thvroid; laler i-eBoarehes have shown that they do not, but authorities are not in harmony as to whether they produce a large part or only a small portion of the adult lateral lobes. The more recent view of His is that the adult lateral lobes develop only in part from the lateral anlages. Verdun, the most recent worker in this field, maintains that the entire thyroid body of mammals and man is developed from the median anlage and that the podbranchial bodies (Figs. 113 and 114), by which name he designates the structures referred to above as the lateral thyroids, atrophy.

After the union of the three portions of the gland, the latter consists of a network of cords of cells, the meshes of which reticulum are occupied by embryonal connective tissue. Subsequently the cords of cells become hollowed out and exhibit alternating enlargements and constrictions. By the increase of the constrictions the continuity of the cell-cords is interrupted at short intervals, and so the network is converted into numerous closed follicles lined with epithelium, the formation of follicles beginning in the eighth week. The follicles later undergo considerable increase in size on account of the secretion by their epithelial cells of a peculiar colloid material, characteristic of the thyroid body. The embryonal comiective tissue, made up necessarily of mesodermic elements, furnishes the comiective-tissue framework and the blood-vessels of the organ, while the epithelium originates in the manner indicated from the entoderm of the gut-tract.

The Parathyroid Bodies

The parathyroid bodies, usually two in number on each side, were discovered by Sandstrom in 1880. The lower pair lie upon the trachea in close relation with the thyroid body, while the upper pair lie at the level of the lower border of the cricoid cartilage, in relation with the dorsal surface of the lateral lobes of the thyroid body.

Their origin is still somewhat obscure. Apparently they are outpouchings respectively from the third and fourth visceral furrows, being composed, therefore, of entodermal epithelium. These epithelial bodies develop in a manner similar to the development of the thyroid body, but the fact that the cell-gi'oups are not broken up by the invading embryo anal connfdive timue to the «(me extent as in the ease of the thyroid rendei'a them hixlolagicaUy distinfjuhhabh from the laltrr ; moreover, it is stated by Maiirer that they never form colloid Hiibstjiiicc.

The Thymus

What remains of the thymus after the second year of life is in;i(io up chiefly of Ijrmplioid and connective tissue, (.'nilniiilrd in wliicli are c ha racl eristic little epithelial bodies, the corpascles of Hasaall.

the epithelial parts of tlic thymns, in all vertebrate aniinalij, are derived from the entodennal lining if the pharyngeal region of the p;ut-trart. In tiio lower gronps, such as reptiles, amphibians, and bony fishes, the epithelinni of all the inner visceral clefts or throat-pouches shares in the development; while in birds, only two or three clefts bike part. In mammals, however, including man, the thymus body is derived probably from but one throat-pocket, the third.

The entodennal epithelium of the third inner pouch becomes evRginated (Fig. 113) to form an epithelial sac whose connection with the pharyngeal cavity is subsequently lost. The isolated and elongateil sac soon gives out small lateral buds or processes at the distal extremity. While the original sac has from the first a cavity, the bud-like branches are solid masses of epithelinm. The liranching continues and affecl^ not only the lower or distal extremity of the thymus sac but also the proximal end, the structure now resembling an acinous gland (Fig. 115). While this growth is taking place, the epithelial muss is being invaded by lymphocytes and young conneetive tissue with developing blood-vessels. (According to some recent studies by E. T. Bell, the lymphocytes are dcrivetl from the epithelium of the originul Ihymna anlage; but this is denied byStohr.) The encroachment by these elements continues to such an extent that lymphoid tissno — including leukocytes and ervthroblasts — becomes the predominant constitnent of the thymus, the epithelial parts sutferitig rc<luction, relatively, and liecoming fiiiiilly broken up into isolated maswa which are the corptucleB of Hassall of the mature gland. The breaking down of the epithelial cords is probably res[>onsible also for the irregular cavities of the thymus. Not until after birth do the glands of the two sides of the hcKly unite to form a single vnpaiied strnctore, and the development of the thymus is not completed until tiie end of the second yi'ar of life. Having attained its full development, the organ l>egins to retrograde, and ut the time of puberty has almost disappeared. Although sometimes persistent throughout life, it is usually represented by an insigniticant vest^ ige. (It has recentlybeen said that the thymus increast^s in size and weight up to puberty, and that it is an active organ until the fortieth year, after which time it atrophies.) While the epitheli&I parts of the thymus body, represented in the fully developed oi^n by the corpuscles of Hassali, are derived from the entodennal epithelium of the third inner visceral furrow, all other parts, the Irmpboid tisBoe, conoectlTe tissue, and bloodvessels, are products of the surrounding mesodenn.

Fig. lis.— ThyiDua of ao embrro nbblt or ■liteen da;B taRer K&Ulker), magnlfled: a, canal of the tb^mus; b, npper, c, lower end of Ihe