1. DEVELOPMENT OF THE AXIAL
SKELETONvertebra, ribs, sternum.
DEVELOPMENT OF SKIN AND SKIN
APPENDAGES

2. DEVELOPMENT OF THE AXIAL
SKELETON
E) DEVELOPMENT OF THE VERTEBRAL COLUMN
G) DEVELOPMENT OF THE SKULL

3. DEVELOPMENT OF THE VERTEBRAL
COLUMN
the vertebral column developes from the sclerotomes

4. somites differentiate into
 sclerotome - axial
skeleton
 myotome - skeletal
muscles
 dermatome - dermis
the intermediate
mesoderm - nephrotomes
gives rise to the urinary
system (pro-, meso-, and
metanephros)
Nephrogenic cord???
the lateral mesoderm is originally solid but divides later into two layers:
 a somatic (parietal) layer
adjacent to the ectoderm
 a splanchnic (visceral)
layer adjacent to the
endoderm
(they border the intraembryonic
coelom)

5. cells of both sclerotomes move ventrally and surround the notochord 4th week
preserving the their original segmental arrangement; by their fusion, the sclerotomal
anlage of the body of future vertebra arises

6. at the first, the sclerotomal anlages are in register with the segmental myotomes or
muscle blocks of the somites
later, each body anlage differentiates in the cranial and caudal halves, which differs
from each other in a number of properties – the caudal half appears to be densely
packed than the cranial half
the next step of vertebral body development consists in fusion caudal
densely packed half of the cranial sclerotome with the loose cranial half of
the caudal sclerotome.

7. a result of this process is that myotomes alternate to vertebral bodies and the
muscles developing from myotomes can act on the joins to flex the vertebral column
the notochord degenerates except the cranial loose parts of sclerotomes, from them
intervertebral discs develop; it gives rise to the central nucleus pulposus
the last step development of the vertabrae is formation of
 neural arches - they unite in midline – the arcus vertebrae. THEY GROW
AROUND THE NEURAL TUBE!
 costal processes (mesenchymal) – they give rise to ribs in the thoracic
vertebrae
(in the cervical region – they fuse with with transversal process – processus
costotransversarius, in the lumbar vertebrae are transformed in the processus

8. Development of sternum
The sternum develops
independently of the ribs from
two bands of cartilage running
granicaudally, sternal bars,
mesenchymal bandswhich
move towards each othe as
the body cavity closes and
finally fuse

9. up to the sixth week the bodies and porocesses are composed of mesenchyme, but
after this centres of cartilage formation begin to appear, the centres extend
and are propagated in the processes
in the second half of gestation the cartilage undergoes transformation into the
bone (ossification) – andra halvan av graviditeten!!
DEVELOPMENT OF THE SKULL
the bone elements of the skull can be classified in two major groups
 the neurocranium
 the viscerocranium
each of which may in turn be divided into subgroups
chondrocranium (base of the skull) and dermatocranium (vault of
the skull)
cartilaginous viscerocranium and dermal viscerocranium

10. Development of the chondrocranium
it develops from the mesenchyme lying beneath the primary brain vesicles
during the sixth week
paired cartilaginous
condensations occur in the
mesenchyme
trabeculae cranii (ethmoid bone)
alae orbitales (lesser wing of sphenoid)
hypophysial cartilages (Sphenoid bone)
ala temporales
Otic capsules(Mastoid + sqaumos port.)
parachordal cartilages + (4 occipital
segements)
(around cranial end of notochord)
= base of occipital bone
capsules for sense
organs

11. cartilages then fuse each other and later with capsules for sensory organs
so that basal plate arises
cartilages of the plate then differentiate as follows:

12. Development of the dermatocranium
it develops from the mesenchyme that covers the the brain vesicles
five ossification centres form in it: two frontal, two parietal and one
occipital, centres extend towards each other and finally they meet in
suture lines

13. the frontal plates fuse after birth to form the frontal bone, two parietal
plates give rise to the pariatal bones and the occipital plate fuses to
elements of the chondrocranium to become the definitive occipital bone
fontanelles = are
membrane covered
openings in the forming
bony vault at the time of
birth
there are six in total
 the anterior fontanelle –
it closes in the 3rd year
 the posteriorfontanelle
– it closes in the sixth month
 the sphenoid fontanelle
(paired)
 the mastoid fontanelle
(paired)

14. Development of the cartilaginous viscerocranium
it is formed by cartilages of the branchial apparatus

15. Development of the dermal viscerocranium
it develops from the mesenchyme of the branchial arches
nasal-, lacrimal-, zygomatic bones, maxilla, palatine bones, vomer

16. DEVELOPMENT OF SKIN
AND
SKIN APPENDAGES

17. Epidermis
initially, a single layer of ectodermal cells covers the embryo
starting from the 2nd month, the ectodermal cells divide and form a superficial protective layer
of flattened cells, the periderm or epitrichium
at the end of 4th month, the epidermis acquires its definitive arrangement and 4 layers are
distinguished: basal, spinous, granular and horny layer

18. Cells that have been exfoliated during fetal life form part of the vernix caseosa, a white,
cheese-like, protective substance that covers the fetal skin
During the early fetal period, melanoblasts migrate from the neural crest to the
dermoepidermal junction, where they differentiate into melanocytes

19. Dermis
the dermis is derived from the mesenchyme underlying the
surface ectoderm
the mesenchyme arises from 2 sources:
 from the somatic layer of lateral mesoderm (most of the
mesenchyme),
 from the dermomyotome regions of the somites (in lesser extent)
By 11 weeks, the mesenchymal cells have begun to produce
collagenous and elastic connective tissue fibers
As the epidermal ridges form, the dermis project upward into
the epidermis and forms dermal papillae.

20. Eccrine sweat glands develop as solid epidermal downgrowths that extend
into the underlying dermis
As buds elongate, their ends become coiled, forming the primordia of future
secretory portions of glands

23. Development of hairs
begin to develop early in the fetal period, but they do not become readily visible
until about twentieth week
first recognizable hairs occor on the eyebrows, upper lip, and chin
A hair follicle begins as a solid downgrowth of the stratum
germinativun of the epidermis and extends into the underlying
dermis
the deepest part of the hair bud soon becomes club-shaped, forming a hair bulb.
the epithelial cells of the hair bulb constitute the germinal matrix- it gives rise to hair
The hair bulb is then invaginated by a small mesenchymal hair papilla
the peripheral cells of the developing hair follicle form the epithelial root sheath
the surrounding mesenchymal cells differentiate into the dermal (connective tissue) root
sheath
The first hairs are called lanugo, are fine and colourless
these hairs are replaced during the perinatal period by coarser hairs, called vellus
that persist over most of the body, except in the axillary and pubic regions
Hairs of these reagions are replaced during puberty

26. apocrine sweat glands (axilla, pubic region, anal region, areolae) develop from the
hair follicle as sebaceous glands

27. Development of mammary gland

28. The mammary glands develop during the sixth week as a solid
downgrowth of the epidermis that extend into the underlying
mesenchyme
these downgrowths occur along the mammary ridges, two
thickened strips of ectoderm that extend from the axillary to
the inguinal regions
in human embryos, these epithelial ridges occur during the
fourth week, but except the pectoral area rapidly disappear.
each primary mammary bud soon gives rise to several
secondary buds that develop into lactiferous ducts and
their branches. The fibrous connective tissue and fat develop
from the surrounding mesenchyme

29. Nails
Toenails and fingernails begin to develop at the distal ends of
the digits at about 10 weeks, development of fingernails
precedes that of the toenails
The nails first appear as thickened areas of the developing
epidermis on the dorsal aspect of each digit
These nail fields are surrounded laterally and proximally by
folds - nail folds
Cells from the proximal nail fold grow over the nail field and
become keratinised to form the nail, or nail plate
At first, superficial layers of epidermis called the eponychium
cover the developing nail. This later degenerates, except at
the base of the nail, where it persists.