DEVELOPMENT OF THE AXIALSKELETONvertebra, ribs, sternum.DEVELOPMENT OF SKIN AND SKINAPPENDAGES
DEVELOPMENT OF THE AXIAL SKELETONE) DEVELOPMENT OF THE VERTEBRAL COLUMNG) DEVELOPMENT OF THE SKULL
DEVELOPMENT OF THE VERTEBRAL COLUMNthe vertebral column developes from the sclerotomes
somites differentiate into sclerotome - axial skeleton myotome - skeletal muscles dermatome - dermisthe intermediatemesoderm - nephrotomesgives rise to the urinarysystem (pro-, meso-, andmetanephros)Nephrogenic cord???the lateral mesoderm is originally solid but divides later into two layers: a somatic (parietal) layeradjacent to the ectoderm a splanchnic (visceral)layer adjacent to theendoderm (they border the intraembryonic coelom)
cells of both sclerotomes move ventrally and surround the notochord 4th weekpreserving the their original segmental arrangement; by their fusion, the sclerotomalanlage of the body of future vertebra arises
at the first, the sclerotomal anlages are in register with the segmental myotomes ormuscle blocks of the somiteslater, each body anlage differentiates in the cranial and caudal halves, which differsfrom each other in a number of properties – the caudal half appears to be denselypacked than the cranial halfthe next step of vertebral body development consists in fusion caudaldensely packed half of the cranial sclerotome with the loose cranial half ofthe caudal sclerotome.
a result of this process is that myotomes alternate to vertebral bodies and themuscles 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
Development of sternumThe sternum developsindependently of the ribs fromtwo bands of cartilage runninggranicaudally, sternal bars,mesenchymal bandswhichmove towards each othe asthe body cavity closes andfinally fuse
up to the sixth week the bodies and porocesses are composed of mesenchyme, butafter this centres of cartilage formation begin to appear, the centres extendand are propagated in the processesin the second half of gestation the cartilage undergoes transformation into thebone (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
Development of the chondrocraniumit develops from the mesenchyme lying beneath the primary brain vesiclesduring the sixth weekpaired cartilaginouscondensations occur in themesenchymetrabeculae cranii (ethmoid bone)alae orbitales (lesser wing of sphenoid)hypophysial cartilages (Sphenoid bone)ala temporalesOtic capsules(Mastoid + sqaumos port.)parachordal cartilages + (4 occipitalsegements)(around cranial end of notochord)= base of occipital bonecapsules for senseorgans
cartilages then fuse each other and later with capsules for sensory organsso that basal plate arises cartilages of the plate then differentiate as follows:
Development of the dermatocraniumit develops from the mesenchyme that covers the the brain vesiclesfive ossification centres form in it: two frontal, two parietal and oneoccipital, centres extend towards each other and finally they meet insuture lines
the frontal plates fuse after birth to form the frontal bone, two parietalplates give rise to the pariatal bones and the occipital plate fuses toelements 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)
Development of the cartilaginous viscerocraniumit is formed by cartilages of the branchial apparatus
Development of the dermal viscerocraniumit develops from the mesenchyme of the branchial archesnasal-, lacrimal-, zygomatic bones, maxilla, palatine bones, vomer
Epidermisinitially, a single layer of ectodermal cells covers the embryostarting from the 2nd month, the ectodermal cells divide and form a superficial protective layerof flattened cells, the periderm or epitrichiumat the end of 4th month, the epidermis acquires its definitive arrangement and 4 layers aredistinguished: basal, spinous, granular and horny layer
Cells that have been exfoliated during fetal life form part of the vernix caseosa, a white,cheese-like, protective substance that covers the fetal skinDuring the early fetal period, melanoblasts migrate from the neural crest to thedermoepidermal junction, where they differentiate into melanocytes
Dermisthe dermis is derived from the mesenchyme underlying the surface ectodermthe 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 producecollagenous and elastic connective tissue fibersAs the epidermal ridges form, the dermis project upward intothe epidermis and forms dermal papillae.
Eccrine sweat glands develop as solid epidermal downgrowths that extendinto the underlying dermisAs buds elongate, their ends become coiled, forming the primordia of futuresecretory portions of glands
Development of hairsbegin to develop early in the fetal period, but they do not become readily visibleuntil about twentieth weekfirst recognizable hairs occor on the eyebrows, upper lip, and chinA hair follicle begins as a solid downgrowth of the stratumgerminativun of the epidermis and extends into the underlyingdermisthe 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 hairThe hair bulb is then invaginated by a small mesenchymal hair papillathe peripheral cells of the developing hair follicle form the epithelial root sheaththe surrounding mesenchymal cells differentiate into the dermal (connective tissue) rootsheathThe first hairs are called lanugo, are fine and colourlessthese hairs are replaced during the perinatal period by coarser hairs, called vellusthat persist over most of the body, except in the axillary and pubic regionsHairs of these reagions are replaced during puberty
apocrine sweat glands (axilla, pubic region, anal region, areolae) develop from thehair follicle as sebaceous glands
The mammary glands develop during the sixth week as a solid downgrowth of the epidermis that extend into the underlying mesenchymethese downgrowths occur along the mammary ridges, two thickened strips of ectoderm that extend from the axillary to the inguinal regionsin 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
NailsToenails and fingernails begin to develop at the distal ends of the digits at about 10 weeks, development of fingernails precedes that of the toenailsThe nails first appear as thickened areas of the developing epidermis on the dorsal aspect of each digitThese nail fields are surrounded laterally and proximally by folds - nail foldsCells from the proximal nail fold grow over the nail field and become keratinised to form the nail, or nail plateAt 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.