Talk:Book - Human Embryology (1945) 15

CHAPTER XV INTEGUMENTARY SYSTEM[edit] the skin and its derivatives

is d “eloped jp‘° cpMermis and denrns (conura »

he ectoderm covering the surface 'of the emh ^ and the underlying mesenchyme,

of cubo.dal cells. By the 5 nsm. sLe it S'"

o. and a deeper layer, the epidt JiTpro 'er’ Tb "r* “ !=»■■- "■eV»

^ -mporary protective membrane for the laft ^ i.- former appears to be in the nature y active multiplication the cells of the ?ermina? T regarded as a germinal zone,

gra ually thickens, and, at differing rates in W'ff origin to an intermediate layer M'hich

into typical stratified squamous epithelium parts of the surface of the body, differentiates

original germinal cells, persists as the stratum ol <^eepest layer, corresponding to the

in order, from within outwards • a stratum ^ tvum. Superficial to this there gradually appear cjnnenm By the 200 mm. stage the iel^vZTrT ^ stratum lucidum and a siralmx

IS IS possibly in part due to the eruntinr^^fi^u ^ been desquamated (Fig-SppD).

pitnchium for this transitorv , hairs (see later) whence the alternative name of

/ V pun aue to the erimtinr, l • 7 — uesquamacea trig.

pjMtnchium for this transitory embrvonm * later) whence the alternative name

^baceous secretions and, as develoL^nr covering. To this cast-off layer are addf

most part {stratum disjunctum) of the yet more ectodermal cells from the oute

vermx caseosa. This persists to full term eorneum to form a whiPsh cheesy substance, tl

term covenng over most of the skm but more particulari; ft

— — - fun V ^ the joint creases. The vernix may have

c ion in protecting the underlying epidermis fron maceration by the amniotic fluid.

t an as yet undetermined stage, but probably befon ceH ^tage, the ectodermal epidermis is invaded b)

s o neural crest origin (see page 271). These cells latei eve op extensive processes, for which reason they are

45mm

60 mm

<OOll M

'm ih^ to show stages

of the cpidlr

Fig 400 — Photomicrograph of silver impregnated mclanoblast in the epithelium of a 93 mm, human foetus, X c rioo,

370

IN 1 EGUMENT\R\ S\STE\I

37 ‘

frcqucntl) called dindrilic cells and they de\clop a marked afTinitv for siher salts B% the too mm stage these argemafTin dendritic cells ha\e a wide dntnbutinn (Fig 400) m the foetal epidermis (Bo>d 1^9) In the foetuses of negroes thc> gradualK desclop melinm pii»inent Zimmermann and Cornbleet t948) which as in the adult (Rillmgles 1949) the) can transfer to the other ceils of the epidermis Because of this ahilits tosxnthetize melmm the dendritic cells are also called melanoblasts In the foetuses of white races the dcndntic cells of the epidermis do not produce osert melanin but as 1$ well known m post natal life thes possess mclanogenic possers tnd m foetal life similar cells in hair follicles (Fi^ 401; can pigment the growing inir shafts

The dermis the deeper laser of the skin has its origin from the mesench)me undcrl)ing the epidermis and is in part at least densed from the dcrmatomic portions of the somites For this reason each dermatome is sometimes called a eulis plait Some of the cells from each derma tome are bMie\ed to migrate senlnlK where, with reinforcement from the cells of the somatopleuric mcsenchsTTic the) develop into the general dermis of the bod) wall and Umbs \t about the jo mm siige fibnllae appear in the interstices of the dcrmitomic

Fir 401 — } })Oi MTucrograpb of nher impregnsird melanoblut m scalp hair follicle of a 03 nun human foetus y c 460

material and Ot a later period these filtnllae can be disiinautdted as colhgenout and clastic Itbres B) the 6 o mm stage prohrerations of the cortum form papillar) projectionj tnw the ^idermis and the supctricial part of the cniium becomes compact Fat appean in the deeper iwrlion tthich hecotnes the general subeutaneous tissue |subct>niiml Farit in deielopraent mrm na,° " nmmbrane of homogeneous malenal separating the slratum

feerf I r "1 'T ndges ts nell established m

foetal life and finger prints are tndtttdtial before birth (Cummins loan)

There ate marked regional and specific differences in llie carb deiclooment and h.stn genesis of dilTercnt parts of the skin (see Steiner, igzp 1930 for detailsl

lire nails of the f.ngets and toes make their appearance lonards the end of the third month (50-60 mm stage) as thickenings of the enidermw called the primar) nail fields These thickenings ire initialU siluiled near the tips

fingers and migrate arm eU or p:u c to their dorsal aspects This mi^a

“i?" 1 ‘fie supply of

prtraat) nail fields lag behind , I" ■“P'nent so that each comes lo “ *»Po depression bounded

\ WW'j '! nndctcnls P,„

0 liferation of the cells of the natl field in a

•" dnelop ment ol a hair and us related s baceous gtand^

372

HUMAN EMBRYOLOGY

and the outer layers of the stratum corneum, gradually gives origin to the nail. The nails grow slowly in foetal life and do not reach the tips of the digits until the end of the ninth lunar month, reaching the tips of the fingers rather earlier than the tips of the toes.

HAIR

The first hair of human embryos begins to appear m the third month as solid cylindrical epidermal downgrowths into the underlying dermis , these become club-shaped and the thickened lower part of each downgrowth becomes mvaginated by a small mesodermal papilla (Fig. 402). The central cells of the downgrowth become spindle-shaped and keratinized, fusing together to form the hair shaft. The peripheral cells become cuboidal to form the wall of the hair follicle. Growth of the hair results from continued multiplication of the epidermal cells around the papilla. The first hairs appear m the eyebrow and upper lip regions (see also page 1 1 7) ; towards the end of the third month extensively scattered fine hairs called lanugo appear. These are chiefly shed shortly before or after birth and are replaced by coarser hairs which arise from new follicles.- Dendritic cells appear m the hair roots at about the 100 mm. stage (Fig. 401; Boyd, 1950). In dark-haired individuals melanogenesis is active in the hair follicles during the second half of pregnancy.

SEBACEOUS GLANDS

Thesebaceous glands arise as epidermal buds from the cuboidal cells of the hair folhcles (Fig. 402) during the fifth month of foetal life. The buds grow into the surrounding mesoderm where theif ends become lobed. The central cells undergo a form of fatty degeneration and pass into the_hair follicle as sebum. A few sebaceous glands develop from the epidermis, independently of hair folhcles, in the anal region, nostrils and eyelids. The tarsal glands are specialized sebaceous glands and develop m a similar manner to them.

SWEAT GLANDS

The sweat or sudoripurous glands begin to develop at about 100 mm. stage as solid cylindrical epidermal downgrowths which are more compact than those of hair primordia and do not develop mesench^Tnal papillae. In later stages the downgrowths become coiled and soon develop lumina by the breaking down of their central cells They usually retain two layers of cells in their walls, an inner layer limng the lumen (the gland cells) and an outer layer which gives origin to the so-called myoepithelial cells of the glands (page 366). Specialized sweat glands are developed in certain regions, e.g., axilla, external auditory meatus and eyelids.

MAMMARY GLANDS

The first stage in the development of the mammary glands takes the form of a pair of external thickenings, the mammary ridges or lines which extend on each side of the ventral bod)

. / wall from the base of the fore-limb bud to the region medial to the

Fig. 403 — Drawing to show the position of the milk ridges

hind-limb bud. Each mammary line appears at about the 7 stage but is not easily recognized until the 1 1 mm. stage. Its cau a two-thirds normally disappears before the 20 mm. stage, but in t e intermediate portion of its cephalic one-third it thickens to form t e mammary primordium (Fig. 403). The ectoderm of the primor lurn consists of a superficial layer of flattened cells and a deeper layer o cuboidal cells. By about the 40 mm. stage the ectodermal thickening of the future gland has penetrated into the underlying mesenc yme, which has now become condensed (Fig. 4^4) • This primor luro grows slowly into the underlying dermis and gives origin to a lou •sixteen to t\\ enty-four secondary sprouts around which fat is ^

In the eighth and ninth months of intra-uterine life the initia growth and the sprouts become canalized. The secondary outgroiv become the lactiferous ducts and the tertiary sprouts from them or

INTEGUMENTARY SYSTEM

373

,ht aUeol. and small ducts of the <;la„d The ongtnal ep.lhd.al donn^ronlh as an epidermal pit into si hich the licliferous duels open At about full term w hter a non of mesoderm underli.ng the pit causes its elevuuon aboae the surface of the adjacent sUn to form the mppk If this does not occur the ducts open into pits instead of on nipples defect knoun as m\ cried or crater nipple At full term the rudimentary mammary glands arc similar in both sexes and mas occasionally show 13ns of secretory acti\ity (witch s milk) This activaiy may be due to the production of the hormone prolactin by the pituitary gland of the infant before and after birtli It JS dIno possible that this hormone is of maternal ongm and crosses the placental harrier (sec Smith 194^ for discussion) In the male the glands normalK remain rudimcntan throughout life though thes may rarely become enlarged (gymaecomasiia;

In the female at pubcrt\ and during and after pregnancy they undergo marked changes which invohe a high degree of hormonal co ordination between the glands the pituitary and the oaanes

As occasional anomalies additional nipples polythelia can be found at any point along the milk line (Fig 403) Extra mammars glands polymastia or absence of the mammarx gland amastia) are much rarer

TEETH

Teeth are restricted to tlie jawed (gnaiho stomatous) \crtebnics They ha\e no homo logues in the mxertebrates lower chordales or cvclostomes They can be regarded as nets acquisitions for seizing holding and masticntmt, food Teeth are fundamentally dent atnes of the derma! skeleton being basically similar to the placoid scales of the Euselachn

Each tooth has a basis of dentine which is ~

of mescnchymatous origin and IS covered by 'C”

enamel which is Ibrmed by specialized ecto " c* ^ 'i

dermal cells In the course of vertebrate rvolu tion the teeth have undergone many modihca tions In the Eusclachu all the teeth arc similar m form and have no firm attachment to the jaws ^Vhcn such a tooth is lost in front another one moves forward to take its place Man and most other mammals have two sets of teeth a tem porary set or milk dentition and a permanent dentition The mammals are therefore classed as diphyodonl in contrast to the other vertebrates,

"Hh indcfimte succession of teclh such smehrates ate poljphjodont In mammals the teeth

arehxedhrml) m the jaws and have undergone difTercntiation amongst themselves The homo

dom condition of the more pnmuiv e v ertebrates is replaced by a heterodont condition m w hich teeth are differentiated as incisors canines premolars and molars m the permanent dentition

Fig 404 — \ Fhoiomicrograph of a section of the developing mammary rudiment m a 44 mm female human embryo x c 115 B Photomicrograph ofa section of the developing mammary rudiment in a 160 mm female human foetus c 75 C Photomicrograph of a section of the developing mammary rudiment m a mm male human foetus X c 7^

374

HUMAN EMBRYOLOGY

DENTAL

LAVINA

MECKELS

CARTS^GE

LIP LiP FURROW

The first indication of the development of the teeth is the appearance during the 6th iveek (embryos of 1 1 mm.) of a curved continuous ectodermal thickening, the dental lamina, on the oral surface of each ja-w. The dental lamina (Fig. 405) consists of a surface layer of flattened

cells lesting on a basal layer of taller cells, which have many mitotic figures. The basal epithelial cells are separated from the mesenchyme by a basement membrane. Each lamina soon lies within the concavity of the second ectodermal thickening, the lip furrow band or labio-gmgtval lamina (Fig. 405). By the 27 mm. stage the epithelium of each dental lamina shows on each side in each jaw five symmetrically arranged budhke round, or oval, swellings which are the primordia of the enamel organs of the deciduous teeth (Fig. 405). As the ectodermal tooth bud continues to grow, it sinks fin ther into the mesenchyme, where it forms a small indented sac, the enamel organ, joined to the dental lamina by a constricted neck. At this stage the enamel organ consists on its indented, and deeper, aspect of a layer of columnar cells (inner enamel organ epithelium) which are continuous with more cuboidal cells (outer enamel organ epithelium) around the convexity of the organ. The cavity of the enamel organ contains a central core of ectodermal cells betw'een which intercellular fluid accumulates, so as to form a stellate reticulum (Fig. 406) . The intercellulai fluid is of a mucoid nature rich in albumen. Although the stellate reticulum is of ectodermal origin it is morphologically very similar to the gelatinous mesenchyme, or Wharton’s jelly, of the umbilical cor In subsequent development the indented layer of columnar cells will give oiigin to the enarae and is therefore called the ameloblastic layer Un er the orgamzing influence of the ameloblastic layer, the mesenchyme in relation with it prolifera^ and condenses to form the primordium o t e dental papilla (Fig. 407). The ameloblastic la>er is progressively mvaginated by this dental ^ and the cells of the latter in contact ivit ameloblasts become arranged into a odontoblastic layer (Figs. 408 and 409). T is ay gives origin to the dentine. The remaining ce of the mesodermal papilla differentiate into “pulp” of the tooth. The outer enamel ep thelium at the end of this stage folds between %vhich capillary vessels deve °P ^ mesenchyme. These capillanes supp y nu to the avascular enamel organ but the outer enamel epithefium is never penetrated by me ^ During the time that the tooth is developing to this stage, impoitant changes are =

in the lip furrow band. The deep cells proliferate and invade the underlying mesencn>

OUTER ENAMEL ORGAN EPITHELIUM

DENTAL LAMINA

AMELOBLASTS CINNER ENAMEL ORGAN EPITHELIUMJ

DENTAL^.

PAPILLA

Fig 4^5 Sagittal sections tHrough the hp and anterior part of the mouth showing schematicallv stages in the de\elopment of an incisor

tooth A — 20 mm embryo B :

br\'0 C — 55 rnm, embryo

J7 mm em

INTEGUMENTARY SYSTEM

375

and the band soon hollows out to become the hp sulcus between the developing lips and cheek and the gums (Figs 405 and 407) The ameloblasts Ia> dossn succeeding of calco

piobulm which later harden to form the enamel rods which arc deposited on the outer surface {Fig 408) of the dentine which is being laid down b\ the odontoblasts, enamel and derUine production thus proceeds simultaneousK Growth of the tooth takes place from the den tine enamel junction w here the oldest dentine and enamel arc in apposi non In both cases the acti\ e secrc tor> cells recede as the dentine and enamel matrices arc laid down The ameloblasts final!) disappear leaving on the surface of the enamel a thin covering (Nasmvths mem brane or dental cuticle) The odonto blasts do not disappear but persist as a regularly arrani^ed cellular lamina beneath the last formed dentine and separating it from the mesodermal papilla which has become richly V asculanzed to form the dental pulp The calcification of the tooth which begins during the sixth month of foetal life results in the formation bv — Phototmerogrsph ofa srtiion ofa developing lo\ er

full term of a well developed crown looihm a so mm human embryo x c 45

The root of the tooth is not formed

until shortly before eruption, and it becomes intimately related to the developing mandible or maxilla and attached to the bone bv specialized cement tissue which together with* the periodontal membrane is derived from the mesodermal follicular sheath The dental lamina extends backwards beyond the last deciduous tooth germ and slowlv forms the enamel organs of the permanent molars which have no deciduous pre cursors At about the 50 mm stage the dental lamina related to each deciduous tooth produces secondanly solid epithelial buds on its Imgual side (Figs 405 407 and 409) These are the enamel organs for the perma nent teeth The dental lamina persists as an epithelial tract the

gubernaculum denlts which for sometime attaches the apex of the deciduous and permanent tooth to the opitholtum of iho gum K late disappears completel) Some calcification occurs in the permanent teeth before birth For detaik see Orban (1944)

Teeth seem to be latgelj self dinerentiatiirg and Glassione ( 1 936) has groii n rat tooth germs m wtro and her Kpertmems shots that sshole or part.at tooth germs possess remarkable ponets of htstologtcal differentiation in tissue culture The presence of the internal enamel epithelium IS apparently essential for odontoblast foiTnation and one of the functions of the enamel organ

376

HUMAN EMBRYOLOGY

PERMANENT TOOTH LAMINA^

PCRflANENT TOOTH ENAMEL OOGAN

DECtOUOUS TOOTH 'LAMINA (CUQERNACULUM)

DENTINE

.AMELOBLASTIC

LAYER

ODONTOBLASTIC

LAYER

^^\OUTER ENAMEL

ORGAN EPITHELIUM

Fig

u 409 — Photomicrograph of a section of a developing lower tooth in a 180 mm numan foetus X c 28 Note bone of mandible, and inferior dental nerve

Fig 408 • — Sagittal i.ccrion through a developing incisor tooth " lowmg schematically its constituent port' in a 220 mm human embryo.

IS to determine tne gross morphological structure of the tooth (Hahn, 1941).

During the period that the teeth are developing and growing an adequate supply of calcium and phosphates is required ; in addition, vitamins A and D are essential for their proper formation. It has been shown that the diet the mother during pregnancy has a profound influence on the subsequent condition of the teet of the child (Mellanby and Goumoulos, 1946) The salts of calcium and phosphorus are essentia

for the formation of both enamel ^hd dentine, while vitamin D is required for the utilization 0 these salts. When vitamin D is ^^eficient during the development of the teeth after their eruption the surfaces of the teeth are rough instead of smooth and shiny. If there is a deficiency of vitamin A, the ameloblasts fail to differentiate properly, and as a consequence their organizing influence upon adjacent dentine is disturbed, and so dentine is formed in an atypical manner. There IS a close relationship between the structure of a tooth and its liability to caries (King, I 94 ®r

REFERENCES

Beielander, G (1941) The development and structure of the fiber system of dentin Anat Rec , 81 , 79 9 ? T>* , \ Dendritic cells in pigmented human skin J Anat, Land, 83 , iog-115

^ , D949) -^gentophil cells in foetal ectodermal epitheha 7 Anat, Land, 83 , 74

M hair follicles J Anat , Land , M, 62 human

’k (*9^9) The topographic history of the volar pads (walking pads, Tastballen) in

Wnhn w’p development of tooth germs in vitro J Anat, Land, 70 , 260 266 igf,tcd.

-The capacity of developing tooth germ elements for self differentiation when P

J Dent Res, 20 , 5-20 i- 6 6 , j-n

Mplfpnh, ° Cental Disease Committee, Med Res Cncl Sp Rep , Ser No 21 b H M S O , Lond

Mellanb>^May and Coumoulos, Helen (1946) Teeth of 5-year-old London school-children BMJ,

Embryology Kimpton, London

St#-inpr K ('innn\ Physiology of the Newborn Infant Thomas, Illinois xj , , Uber clic

’ I 9 9)' nr die Entwicklung und Differinzierungsweise der menschlichen Hau

der menschlichen Haut Zel(fr’sch , 8 , 691-72°^^ , d« 

(L93o) 11 . Die embryonale Entwicklung der Haut^biete mit fruzeitiger Mehrschicht g

Epithels Z An^ EntwGesch., 93 , 1 50-1 73 _ the Ncgrn

Zimmermann, A A , and Cornbleet, T. (1948). The development of epidermal pigmentation m fetus J Iniest Derm ,ll 383-395