Book - A Text-book of Embryology 8
CHAPTER VIII. THE DEVELOPMENT OF THE CONNECTIVE TISSUES OF THE BODY AND OF THE LYMPHATIC SYSTEM
Heisler JC. A text-book of embryology for students of medicine. 3rd Edn. (1907) W.B. Saunders Co. London.
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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The Connective Tissues
The varioUHly iiKKlified forms of connective tissue distributed throughout the body, including such diversified tissues iiH thi^ l>lood and the lymph, areolar tissue, fibrous and elastic tiHHUr, adenoid tissue, tendon, cartilage, bone, and dentine, an \v«'ll an i\n* ('onnective-tissue stroma of various organs, all n^Mult from alt<»rations affecting the middle germ-layer or meioderm. As pointed out elsewhere (Chapter III.), the inner and the outer germ-layers are concerned in producing the epithelial Htruetures of the body (with the exception of the epithelium of the greater part of the genital apparatus and t»f the kidney aixl ureter), the ectoderm giving rise not only to the epithelium of the surface of the body, but also, by prnerHHeK of infolding, to such important structures as the eentrnl nervous system and the internal ear, while the entoderm dillerentiates into the epithelial parts of the respiratory and digestive systems with their associate<l glandular organs.
The proliferation of the cells of the mesoderm goes hand in hand with thedifVerentiations of the inner and outer germlayers, so that even at an early stage of development the middle germ-layer, besides having given rise to the mesothelium of the body-c^avity and to the primitive segments, C(mHtitutes a loose aggregation of cells that fill the spaces between the g<'rm-layers and spread about the deveh)ping embryonic organs. This primitive relation of the mesoderm i<^ tissue foreshadows its future office as the supporting fnunework not only of the bmly, but of the functionally active epithelial elements of the glands. Thus, the indifferent mesodermic tissue that comes to surround the notochord and the neural canal specializes into the spinal column and the brain-case ; while the parts of this tissue into which protrude the epithelial e vagi nations of the primitive alimentary canal — as, for example, the cvaginations which are the beginnings of its glandular organs, the liver and the pancreas — become intimately associated with these epithelial sacs and tubes to constitute the connective-tissue stroma and the vascular apparatus of the completed glands. All organs of the body, therefore, that have a connective-tissue constituent obtain it from the mesoderm. Owing to the varying degree of differentiation of the mesodermi(; elements in different localities there are formed tissues of widely different character. The most important factor in the production of these modifications is ihC' alteraiion of the intercellular svhstanccy as to whether it remains soft and homogeneous, whether it acquires a fibrillar or an elastic structure, or whether it becomes dense and hard, as in the case of cartilage and bone. The celh undergo comparatively little change, although, according to the kind of tissue produced, they come to be known respectively as connective-tissue cells, tendon-cells, cartilage-cells, or bone-cells.
The slightest degree of specialization results in the production of mucous tissue. In this case a reticulum is formed by the slender processes which the cells acquire, the spaces of the meshwork beinj' filled with the semifluid or semigelatinous intercellular substance.
A further alteration in the intercellular substance, whereby it acquires greater density and becomes permeated by bundles of fibers, some of which are highly elastic, results in the formation of areolar tissue. Preponderan(;e of the non-elastic fibrous element produces white fibrous tissue, while elastic tissue, such as predominates in the ligamentum nuchje, is formed if the elastic fibers are in excess. Further increase in the densitv of the intercellular material, with its accompanying conversion into bundles of non-elastic fibers having a characteristic regularity of arrangement, produces the struct
iin; of tendon. Wlion the intercellular substance gives rise to a scant amount of fihrous material and the cells become (li?*ten(le<l with oily or fatty matter, adipose tissue results.
A still greater degree of density of the intercellular subflt^incc givcffl the matrix of cartilage, the cells being enclosed in H])aceH, the lacunse, as the cartilage-cells. Partial differentiation int^» either fibrous or elastic bundles confers the r'hanicrter of eith<*r fibrous or elastic cartilage wy^n the pHKlnct.
Great condensation of the intercellular substance and its pennf.*ation with salts of lime, the cells being fixed in small «pare*^, results in the prcKlucticm of osseous tissue (see Chapter XVIII.).
Blood and lymph may be hK)ked upon as forms of connective Xx^uii in w^hieh the interc<;llular substanc^e is fluid, con^titnting the plasma, the cellular elements thus remaining free c<-II«, the blood- or the lymph-corpuscles. The development f>i br»th lymph and bl(KKl from the mesodermic elements .J4*rv*^ to l>ear out the comparisf>n.
The cadothelimn of the body is related with the connective -••^les jzenetirally as well as anatomically. Reference has ven iiaiie el?ewhere to the changes which occur in the in*^Hi»»rji:»' rt-ll^ that bound the body-cavity — the fissure
vr««>v>i Tie two layers into which the parietal plate of the iiK>!Miik«ni s?i:t> — to constitute the mesothelium of the body atvTT*-^ r*iese change? consist in the flattening of the cells
091 :*v«r ts»!*in.'.oc>^n of the characters of endothelium. ^•WtTr;;ir-\, T-u»n 'cr.^T smaller clefts are formed in the meso which may l)e the beginnings of small -r«iMr.>fwv%. r -r bUi»xl-vessels, or of bursal or articular -n xw^wr-ttc ^tfiL< of these cavities also assume the
of the serous membranes and of
Bifcr Innalt and thecal sacs may be
^ n:., loctt Slid about the origin of the
tvv^isi^ie stroma of the mem _ ^- H *nA»iiu'lmui rests, is simply a con:Min?Uifc vtf connective tissue.
The Development of the Lymphatic System
The solid elements of the lymphatic system — the " Ijrmphglands," the lympli-follicles, and the diffuse adenoid tissue — as well as the thsrmus body and the spleen, result from the specialization of mesodermic cells, while the lymph-vessels and the various Ijrmph-spaces of the economy — that is, the s(»rous sacs, joint-cavities, bursid and thecal cavities, subarachnoid and sulKlural spaces of the brain and spinal cord — are develo()ed by vacuolation or hollowing out of the mesoderm.
Definite knowledge is wanting as to many of the details of the genesis of the lymphatic system. The various lymphspaces precede the vessels and the adenoid tissue in development.
The Isrmpli-spaces result from clefts in the mesoderm, the earliest formed and most c<mspicuous space of this sort being the body-cavity or coelom. This large fissure develops, even before the differentiation of the body of the embryo, bv the coalescence of numerous small cavities that appear within the middle germ-layer. The body-cavity acquires more definite boundaries by the alteration of the mesodermic cells that border it into flattened endothelioid cells, the mesothelium of the body-ca,vity. AVhen, in the progress of development, the diaphragm and the pericardium are formed, the body-cavity is divided into the peritoneal cavity, the pleural sacs, and the pericardium. At a still later period, a diverticulum of the peritoneum protrudes, in the male fetus, through the inguinal canal into the scrotum to constitute the tunica vaginalis testis. The stomata of serous membranes are merely so many apertures of communication between the serous cavities, which are enormous lymph-spaces, and the lymphatic clefts contained within the stroma of the serous membrane, the clefts themselves being the beginnings of lymph- vessels.
The large Iymph-sacs surrounding the brain and spinal cord, the subarachnoid and subdural spaces, as well as the spaces within the capsule of Tenon and the sheath of the
w, jiihI tIm; p«*rilyiii])liati(' spaces of tlio internal ear similarly ilevdop a-^ vaeiiolati<>ns <»f the nio.s<KU*rniio ti^>ue. '1'Im> .siiiie is true of the joint-cavities, bursal sacs, Hheaths of tendons, and the small lymph-clefts ioiinil in the areiilar tissue and throughout nio>t orir»nis.
The lymphatic vessels fir>t lornie«l,a<v«)r<lin<!: toO. Schultze,
are the siilM'titaMeoii> v<*>r»(*N, whi<*h are present in a iinnian
ehihryo of 2 to .'» <'in.,' aii<l at a -onu'what later perio«] the
Jei'prr vrsH'ls apprar. From the stuthes <if Sal)in" upon
piir <'ml»rvos it appear^, liowever, that the lar<rer ves>els
preenh* th<' smalh't*. Tiii^i ol»erver found that at the jnne tion t»f th<' sulielavian an<l jugular vc-ins of eaeh si<h» a sae
<»r lymph'hoart made ilsap|M*aranee, the oritiec In'ino: guarded
l»v a valve, and from th<'s<' >aes or hearts hninches arose
wich passi'd t«»\vard th*- skin, from wich hranehes a <reneral
hulieiitaneous net\\t»rk of ve-M-lr. aroH*. From eaeh lymj)h Hie a vessi'l m'ow.s tailward, the vessel tui the left side
rt'Mehin^ \\\v a«>rta and divi<lin^ there to form two thoraeie
diU'l^, wliieli afterwani unite into a sinirh* duet. F^recpiently
ihU fi'lal condition oi* two thoracic duets is indicated in the
liinnMii adult hv a douMc condititin of the du<*t tor a greater
or h'HM extent, the diK't sometimes dividinir and reunitinir
two or three tiine>; sometime^ it i*^ «]oul)]e at its termination
ill the Mi1iclav!:in vein.
Tin* two llniraei<* «luctr* hefore fu-ioii dilate at their <-audal
ixtivmiti^, ill the re^d«»n i»f the ki^imy, to form respeetively , and a litth- fariher on miite with the
iw'i ywWlor lyinph-hearts or sa<'s, wich iiave meanwhile
.1. m\u\^A ill th- jimctitin of the s.-iati<* veins with tl ardiual
^el1^.. TIm^*' IsiHcr sMcs MiliMMpicutly lo-c all coniieeiion
,.,^1. d,. v.in^ rrniii whidi tlK-y .irrew. ()uton>wih. frmn the hmphatic system -erve f^r its
...M.-..M. nn., the vi-reni aiitl the skin. As the primary iMri- iliT Kntwii-krluntf-niMliichti- .1.- .M.u^l.vn HMlM-«»ri>:in«»f the Ly'»p f thf Kynn)h-lu*art< aiul Tlh.n. ir iMin in
lymph-sacs increase in length, but fail to correspondingly increase in calibre, they gradually become merged into the vessels.
The lymphoid or adenoid tissue is produced at a later date than the vessels. Observations upon the human lymph nodes seem to have been confined to the inguinal and lumbar nodes. The first indication of an inguinal node is seen in a 3 cm. embryo, in the shape of little aggregations of lymphoid cells that have migrated from the lymphatic cords or networks into a space hollowed out of the mesoderm. This nodule of lymphoid cells is isolated from the surrounding mesodermic elements by a fissure or space except at one point, the future hilom of the node, where strands of embryonal connective tissue connect it with the parent mesoderm. The retdcolum of the node appears later, as does also the capsule, the latter of which results from the condensation of the surrounding mesoderm.
The development of the spleen is considered with that of the alimentary system because of its relation to the evolution of the peritoneum, while the account of the development of the thymus will be found in the chapter on the respiratory system.