Embryology Flashcards
(22 cards)
Embryonic period
As soon as the egg becomes fertilized, it becomes an embryo and stays such for 9 days
Week 3 (18-19 post ovulatory days)
embryonic cells flatten into a trilaminar disc (three germ cell layers form: endoderm, mesoderm, and ectoderm)
Endoderm
inner lining of digestive and respiratory tracts, glands (including liver and pancreas)
Ectoderm
brain and nervous system, epidermis (skin, hair, nails, etc)
Week 4
paraxial mesoderm (midline axiom to body) thickens and divides into paired masses (on either side of the notocord) of mesenchymal cells called somites. Beginning of the segmentation of the spinal cord
Somites
each of these somites further separates into: dermatome, myotome, and sclerotome
Dermatome
forms the dermis
Myotome
forms the muscle of the back, thorax, and limbs
Mesoderm
muscle, bones, cartilage, circulatory system, dermis, connective tissue
Sclerotome
forms the vertebrae and ribs
Week 5
limb buds appear from the ventrolateral walls of the embryo. Limb bud consists of mesoderm (lateral somite) and ectoderm (apical ectodermal ridge)
Limb development
myotome cells migrate into the limb bud to develop into muscle and cartilage. Spinal nerves migrate with the myotome cells to innervate the muscles.
Week 6
cartilage models of the bones of the arm begin to form.
Week 9
bone formation in limbs via enchondral ossification begins
Enchondral ossification
mesenchymal cells in center of limb condense into precartilaginous aggregates and differentiate into chondroblasts (cartilaginous cells)
Chondroblasts secrete:
BMP (bone morphogenetic protein) to facilitate growth, Ihh (Indian hedgehog homolog) to induce further secretion of BMP (positive feeback)
Chondroblasts produce:
matrix to form hyaline cartilage
Steps of enchondral ossification
1) chondrocytes enlarge and begin to secrete alkaline phosphatase
2) hydroxyapatite deposits within the cartilaginous matrix form due to mineralization triggered by local secretion of alkaline phosphatase
3) cartilage cells die by apoptosis and form spaces called lacunae
4) capillaries vascularize the calcified cartilage
5) these capillaries carry both hemopoietic cells and osteoprogenitor cells into the cartilage model
6) the hemopoietic cells become the bone marrow
7) osteoblasts deposit bone matrix, replacing the cartilage
bone growth and remodeling
epiphyseal plates expand with growth hormone, bone remodeled by selective deposition and resorption of bone from opposing surfaces
remodeling of bone is due to
differential deposition and resorption of bone by osteoblasts and osteoclasts, respectively
oestoblasts are derived from
osteoprogenitor cells
osteoclasts are derived from
fused mononuclear hemopoietic progenitor cells and activated by cytokine signaling from marrow stromal cells
