Transcript Tooth - KSUMSC

4 week
Skin
It consists of epidermis which is the superficial
epithelial tissue. It derived from surface ectoderm.
7 week
The dermis is a deeper layer composed of dense
irregularly arranged connective tissue which is
derived from mesoderm.
The embryonic skin at 4 to 5 weeks consists of a
single layer of surface ectodermal overlying the
mesenchyme.
These ectodermal cells proliferate and form a layer
of squamous epithelium ( periderm) and a basal
layer ( germinative ) .
The cells of the periderm undergo keratinization
and desquamation and are replaced by cells arising
from the basal layer.
The exfoliated peridermal cells form part of white
greasy substance ( vernix caseosa ) which covers
the fetal skin. Later, the vernix contain sebum from
sebaceous glands in the skin. This vernix protects
the developing skin from constant exposure to
amniotic fluid with its urine content. Also, it
facilitates birth because of its slipper nature.
11 weeks
Newborn
The basal layer of the epidermis becomes the stratum germinativum which
produces new cells that are displaced into the layers superficial to it.
By 11 weeks, cells from the stratum germinativum have formed an intermediate
layer. Also, it forms epidermal ridges ( 10 week ).
Replacement of peridermal cells continues until about the 21 week, after that, the
periderm disappears and the stratum corneum forms.
Proliferation of stratum germinativum
cells form epidermal ridges which
extend into the developing dermis.
This ridges begin to appear at 10 weeks
and are permanently established by the
17th week.
These epidermal ridges produce
grooves on the surface of the palm;
sloe and digits.
The pattern of development of these
epidermal ridges is determined
genetically and constitutes the basis for
examining fingerprints in criminal
investigations and medial genetics.
Dermatoglyphics:
It is the study of the patterns of the
epidermal ridges of the skin. It is used
for diagnosis of Down syndrome.
Late in the embryonic period, neural crest cells migrate into the mesenchyme of the
developing dermis and differentiate into melanoblasts. Later these cells migrate into the
dermoepidermal junction and differentiate into melanocytes. This differentiation involves
the formation of pigment granules.
The melanocytes begin producing melanin before birth. The relative content of melanin in
the melanocytes accounts for different colors of skin.
Skin is classified as thick or thin according to the thickness of the epidermis.
Thin skin covers most of the body. It contains hair follicles; arrector pilli muscles of hairs;
sebaceous glands and sweat glands.
Thick skin covers the palm and soles. It has only sweat glands.
Dermis
Most of the mesenchyme that differentiates into the
connective tissue of the dermis originates from the
somatic layer of the lateral mesoderm. Some of it is
derived from the dermatomes of the somites.
By 11 weeks, the mesenchymal cells have begun to
produce collagenous and elastic connective tissue
fibers.
The dermal ridges appear at the same time of
epidermal ridges appearance ( 10 week ) and
interdigitate with each other.
Capillary like vessels have been observed in the
dermis at the end of the 5th week. Capillary loops
develop in some of these ridges. Some capillaries
acquire muscular coats and become arterioles and
arteries.
Sensory nerve endings are formed in the other
ridges. Afferent nerve fibers are responsible about
papillary ridge formation.
By the end of the 1st trimester, the major vascular
organization of the fetal dermis is formed.
Sebaceous Glands
Most sebaceous glands develop as
buds from the sides of the developing
epithelial root sheaths of hair
follicles.
The central cells of the alveoli break
down, forming an oily secretion
( sebum ).
Sebaceous glands may develop as
buds from the epidermis, they are
called sebaceous glands independent
of hair. They are present in the glans
penis and labia minora.
Sweat Glands
Eccrine sweat glands are located in the skin
through out most of the body. They develop as
epidermal downgrowths into the underlying
mesenchyme.
As the buds elongates, its end coils to form the
primordium of the secretory part of the gland.
The peripheral cells of the secretory part of the
gland differentiate into myoepithelial cells and
secretory cells.
Myoepithelial cells are specialized smooth muscle
cells. The eccrine sweat glands begin to function
shortly after birth.
The apocrine sweat glands develop from
downgrowths of the stratum germinativum of the
epidermis that give rise to hair follicles. So, their
ducts open into the upper part of the hair follicles
superficial to the openings of the sebaceous glands.
These apocrine glands are present in the axilla,
pubic and perineal regions. They secrete during
puberty.
Hair
It begin to develop early in the fetal period
( 9th to 12 week ). It becomes recognizable at
about the 23 week. At first, it recognizes on the
eyebrows; upper lip and chin by the end of the
12th week.
A hair follicle begins as a proliferation of the
stratum germinativum of the epidermis and
extends into the underlying dermis.
The hair bud becomes club- shaped, forming
the hair bulb ( primordium of a hair root ). The
hair bulb is invaginated by a small
mesenchymal hair papilla. The epithelial cells
of the hair bulb constitute the germinal matrix
which later produces the hair.
The peripheral cells of the developing hair
follicle form the epithelial ( epidermal ) root
sheath and the surrounding mesenchymal cells
differentiate into the dermal root sheath.
As cells in the germinal matrix proliferate, they
are pushed toward the surface, where they
become keratinizing to form the hair shaft.
Lanugo is the first hairs that appear toward the
end of the 12th week and is plentiful by 17 to 20
weeks. It is fine; soft and lightly pigmented.
This hair persists over most of the body except
in the axillary and pubic regions where it is
replaced at puberty by coarser hairs.
The melanin is produced by melanocytes and is
transferred to the hair- forming cells in the
germinal matrix several weeks before birth. This
melanin accounts for different hair colors.
Arrector muscles of hairs differentiate from the
mesenchyme surrounding the hair follicle and
attached to the dermal root sheath of the hair
follicles and the papillary layer of the dermis.
These muscles are poorly developed in the hairs
of axilla and in certain parts of face. The hairs of
eyebrows and cilia forming eyelashes have no
arrector muscles.
Nails
Toenails and figernails begin to develop at the tips of the digits at about 10 weeks.
Development of the fingernails precedes the toenails by about 4 weeks.
The primordia of nails appear as thickened areas of epidermis at the tip of each digit.
The nail fields are surrounded laterally and proximally by folds of epidermis ( nail folds ).
Cells from the proximal nail fold grow over the nail field and become keratinized to form the
nail plate.
At first the nails are covered by eponychium ( superficial layer of epidermis ). This
degenerates except at the base where cuticl is formed.
The skin under the free margin of the nails is the hyponychium.
The fingernails reach the fingertips by about 32 weeks.
The toenails reach the toetips by about 36 weeks.
Nails that have not reached the tips of the digits at birth indicate prematurity.
Mammary Glands
They are modified and specialized type of sweat glands. The mammary crests
appear during the 4th week. These crests are thickened strips of ectoderm,
extending from the axillary to inguinal regions. It persists only in the pectoral
region.
Mammary buds begin to develop during 6th week as solid downgrowths of the
epidermis into the underlying mesenchyme. The mesenchyme is an inductive for
this. Each primary bud gives rise to several secondary buds that develop into
lactiferous ducts and their branches.
Canalization of these buds is induced by placental sex hormones. By full term, 15 to 20
lactiferous ducts are formed. The fibrous connective tissue and fat develop from the
surrounding mesenchyme.
During late fetal period the epidermis at the site of origin of the mammary gland becomes
depressed forming a shallow mammary pit. In newborn infants the nipple is depressed. After
birth the nipples rise from the mammary pits due to proliferation of the surrounding
connective tissue of the areola.
The smooth muscle fibers of the nipple and areola differentiate from surrounding
mesenchymal cells. Full development occurs at about 20 years.
The mammary glands develop similarly in
both sexes. Some secretion ( witch’s
milk) may be produced. These occur by
maternal hormones passing through the
placental membrane into fetal circulation.
Only the main lactiferous ducts are
formed at birth and it underdeveloped
until puberty.
In female, the breasts enlarge due to
development of the mammary glands and
the accumulation of fat associated with
them.
Growth of the duct system occurs
because of the raised levels of circulating
estrogens, prolactin, corticoids and
growth hormone.
If pregnancy occurs, the mammary
glands complete their development. The
intralobular ducts undergo rapid
development forming buds that become
alveoli.
Supernumerary Breast and
Nipples
An extra breast ( polymastia ) or nipple
( polythelia ) occurs in about 1 % of
female population and is an inheritable
condition.
They usually develops just inferior to the
normal breast.
They less commonly appear in the
axillary or abdominal regions of females.
They develop from extra mammary buds
that develop along the mammary crests.
They become obvious in pregnant
women.
Polythelia are common in males, they are
mastaken for moles.
About one third of affected persons have
2 extra nipples or breast.
Polymastia very rarely occurs in a
location other than along the course of
the mammary crest. It develops from
tissue that was displaced from these
crests.
Inverted Nipple
The nipple fail to elevated above the skin surface. It
makes breast feeding of an infant difficult. Special
exercise can be used to prepare the nipple for feeding
an infant.
Gynecomastia
It is an excessive development of the male mammary
gland. Normally, males mammary gland does not
undergo postnatal development.
It occurs in most newborn males due to stimulation of
the glandular tissue by maternal sex hormones. This
effect disappears in a few weeks.
During midpuberty about two- third of boys develop
varying degree of hyperplasia of the breasts. The
subareolar hyperplasia may persist for to few months 2
years.
A decreased ratio of testosterone to estradiol is found
in boys with gynecomastia. About 80% of males with
klinefelter syndrome have gynecomastia.
Skeletal Muscles
The muscular system develops from
mesoderm except the muscles of the iris
which develop from neuroectoderm.
The embryonic muscle cells ( myoblasts ) are
derived from mesenchyme ( embryonic
connective tissue ).
Much of the mesenchyme in the head is
derived from the neural crest cells which are
derived from the pharyngeal arches.
The original mesenchyme in the arches gives
rise to the musculature of the face and neck.
The myoblasts that form the skeletal muscles
of the trunk are derived from mesoderm in the
myotome regions of the somites.
41days
The limb muscles develop from myogenic
precursor cells ( myotomes ) in the limb buds.
7 week
Absence or variation of some muscles is
common.
Limb Muscles
The myogenic precursor cells in the limb buds originate from the ventral dermomyotome of
the somites which are epithelial in nature, in response to molecular signals from nearby
tissues. Molecular signals from the neural tube and notochord induce Pax-3 and Myf- 5 in
the somites. These cells migrate into the limb buds where they undergo
epitheliomesenchymal trasformation.
The first indication of myogenesis (muscle formation ) is elongation of the nuclei and cell
bodies of mesenchymal cells as they differentiate into myoblasts. These primordial muscle
cells fuse to form elongated; multinucleated and cylindrical myotubes.
Skeletal muscle growth during development results from the ongoing fusion of myoblasts
and myotubes. So, the striated skeletal muscle fibers are developed by fusion of cells.
Myotomes
Each typical myotome divides
into a dorsal epaxial and a
ventral hypaxial divisions.
Each developing spinal nerve
divides and sends a branch to
each division. The dorsal
primary ramus supplies the
epaxial and the ventral ramus
supplies the hypaxial.
The intercostal muscles
remain segmentally arranged
like the somites.
Most myoblasts migrate away
from the myotome and form
non- segmented muscles.
Derivatives of Hypaxial Divisions of Myotomes
The cervical myotomes form the scalene; prevertebral; geniohyoid and infrahyoid muscles.
The thoracic myotomes form the lateral and ventral flexor muscles of the vertebral column.
The lumbar myotomes form the quadratus lumborum muscle.
The sacrococcygeal myotomes form the muscles of the pelvic diaphragm and the striated
muscles of the anus and sex organs.
Derivatives of Epaxial Divisions of Myotomes
They form the extensor muscles of the neck and vertebral column.
The embryonic extensor muscles derived from the sacral and coccygeal myotomes
degenerate. Their adult derivatives are the dorsal sacrococcygeal ligaments.
Ocular Muscles
Their origin is unclear. They may be derived from mesenchymal cells near the
prechordal plate.
The mesoderm in this area give rise to 3 preotic myotomes. Myoblasts
differentiate from mesenchymal cells derived from these myotomes.
Groups of myoblasts each supplied by its own nerve ( 3rd ; 4th and the 6th ) form
the extrinsic muscles of the eye.
Cardiac Muscle
It develops from the lateral splanchnic mesoderm which gives rise to the
mesenchyme surrounding the developing heart tube.
Cardiac myoblasts differentiate from the primordial myocardium. Heart muscle is
recognizable in the 4th week. Cardiac muscle fibers arise by differentiation and
growth of single cells.
Late in the embryonic period, special bundles of muscle cells develop with few
myofibrils and larger diameters than typical cardiac muscle fibers. These atypical
cardiac muscle cells are called Purkinje fibers which form the conducting system
of the heart.
Smooth Muscles
They differentiate from splanchnic mesenchyme surrounding the endoderm of the
primordial gut and its derivatives.
The smooth muscles in the walls of many blood and lymphatic vessels arises
from somatic mesoderm.
The muscles of the iris ( sphincter and dilator pupillae ) and the myoepithelial
cells in mammary and sweat glands are derived from mesenchymal cells that
originate from ectoderm.
The first sign of differentiation of smooth muscle is the development of elongated
nuclei in spindle- shaped myoblasts.