Cytodifferentiation: cells within a tissue become specialized. Histogenesis: various cell types come together to form tissues. Induction: Chemical messages that affect the behavior and development of cells and tissues

Skin Development Epidermis is derived from surface ectoderm Dermis is derived from mesoderm –Somatic lateral mesoderm (limbs of body) –Dermatomes of the somites (axis of body)

Periderm: Thin layer of flattened epidermal cells that allow exchange of water, sodium, and glucose between the amniotic fluid and epidermis

Vernix Caseosa Covers fetal skin Protects skin from exposure to fluids Facilitates birth Almost like vasoline

Third month epidermis Peridermal cells –outermost layer Intermediate cells Basal cells – innermost layer

Sixth Month Epidermis Peridermal cells die Epidermis is now a barrier from the outside environment

Immigrant Epidermal Cells Melanoblasts are derived from neural crest cells Langerhans Cells are derived from bone marrow Merkel Cells derived from epidermal precursors or neural crest cells???

Dermis Somatic dermatomes (Dorsal Trunk) Neural crest cells (Head and Neck) Lateral mesoderm

Glands Derived from the epidermis Sebaceous –Buds off of follicular epidermis Sweat –Eccrine Epidermal downgrowths –Apocrine Downgrowth from stratum germinativum Begin to secrete during puberty

Hair Derived from the stratum gerinativum Lanugo –Fine hair that covers the entire body of the embryo –Replaced perinatal period by coarser hair

Nails Nail fields migrate dorsally Nail folds surround the fields The nail plate grows from the folds over the nail field

Mammary glands Specialized sweat glands Begin as downgrowths from the epidermis Develop from mammary crests

Development of the Skeletal System Mesodermal –Somites (paraxial) –Somatopleuric layer of lateral plate Neural Crest Cells – head and neck

Bone and Cartilage Intermembranous bone formation –flat bones develop in mesenchyme within preexisting membraneous sheets Endochondral bone formation –cartilage forms bone

Intramembaneous Ossification Mesenchyme surrounded by membrane osteoblasts form matrix and osteoid tissue calcium phosphate is deposited layers around vessels form Haversion systems osteoblasts and osteoclasts form spongy bone

Cartilage Mesenchyme condenses –cells become rounded chondroblasts Hyaline – at growth plate and covers end of bone, becomes bone Fibrocartilage – IVD, pubic symphysis Elastic – nose, ear, eustation tube

Intracartilaginous ossification (Endochondral) Diaphysis (body) epiphysis (ends)

Joint Development Begins about week 6 and is adult-like by week 8 Interzonal mesenchyme differntiates Fibrous joints – just ligaments Cartilagenous (hyaline or fibrous) Synovial –capsule and ligaments –space centrally –membrane that lines the capsule and surfaces

Axial Skeleton Skull vertebral column ribs Sternum Hyoid bone

Skull Neurocranium base – endochondral ossification through occipital sclerotomes Parachordal cartilage  occipital –Forms around the notocord hypophysial plate  sphenoidal –Forms around the pituitary gland trabeculae plate  ethmoid Otic capsules  3 ear bones Nasal capsules  part of ethmoid

Skull Membraneous Calvaria Shape changes can occur after birth (molding)

Skull Viscerocranium Pharyngeal arches 1 & 2 Arch 1  malleous and incus Arch 2  stapes, styloid process, and lesser cornu of the hyoid bone

Vertebral Column Sclerotomes surround the neurotube and notocord, becomes vertebra, are segmental Arteries are intersegmental loose cranially compact caudally Centrum(vertebral body) is formed by 2 adjacent sclerotomes and is therefore intersegmental notocord becomes the nucleus pulposus

Vertebral column Ossification is evident by week 8 –3 bony parts will be connected by cartilage –the arch articulates with the centrum at the neurocentral joints –disappears by 3-6 yr –Have primary and secondary ossification sites

Vertebral Column Secondary ossification centers arise after puberty until age 25 1 tip of the spinous process 2,3 tips of the transverse processes 4,5 superior and inferior rims of the vertebral body exceptions:C1, C2, C7, lumbar, sacrum, and coccyx

Ribs Arise from costal processes of thoracic vertebrae Mesenchyme in the body wall Cartilage in the embryo Ossification occurs in the fetus

Sternum Sternal bars fuse medially from 3 pieces to one Begins as cartilage even though is a flat bone xiphoid process often does not completely fuse until late in life if at all

Appendicular Skeleton Mesenchymal bone forms during the 5 th week Chondrification during week 6 primary ossification centers at weeks 7 – 12 and are epiphysis and diaphysis diaphysis will ossify first secondary ossification centers appear the last month before birth (but most form after birth)

Muscle Development Skeletal Smooth Cardiac

Skeletal Muscle Myoblasts of the trunk are derived from mesoderm of the myotomes of somites cells and nuclei of mesenchyme elongate to form myoblasts which fuse to form tubes As tubes are formed cellular organelles develop most muscles are formed before birth, the rest within the first year

Myotomes Epaxial (epimere) –innervated by dorsal ramus –extensors of the neck and vertebral column Hypaxial (hypomere) –innervated by ventral ramus

Pharyngeal arch muscles –facial, mastication, pharyngeal Ocular –precordal plate mesenchyme Tongue –occipital myotomes Limbs –myoblasts surrounding developing bone

Smooth Muscle Splanchnic mesoderm - most somatic mesoderm - vessels ectodermal mesenchyme - muscles of iris and glands myoblasts do not fuse

Cardiac Muscle Splanchnic mesenchyme do not fuse; the paired cell membranes form intercalated discs