Embryology

Question 1

Embryonic period

Answer 1

As soon as the egg becomes fertilized, it becomes an embryo and stays such for 9 days

Question 2

Week 3 (18-19 post ovulatory days)

Answer 2

embryonic cells flatten into a trilaminar disc (three germ cell layers form: endoderm, mesoderm, and ectoderm)

Question 3

Endoderm

Answer 3

inner lining of digestive and respiratory tracts, glands (including liver and pancreas)

Question 4

Ectoderm

Answer 4

brain and nervous system, epidermis (skin, hair, nails, etc)

Question 5

Week 4

Answer 5

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

Question 6

Somites

Answer 6

each of these somites further separates into: dermatome, myotome, and sclerotome

Question 7

Dermatome

Answer 7

forms the dermis

Question 8

Myotome

Answer 8

forms the muscle of the back, thorax, and limbs

Question 9

Mesoderm

Answer 9

muscle, bones, cartilage, circulatory system, dermis, connective tissue

Question 10

Sclerotome

Answer 10

forms the vertebrae and ribs

Question 11

Week 5

Answer 11

limb buds appear from the ventrolateral walls of the embryo. Limb bud consists of mesoderm (lateral somite) and ectoderm (apical ectodermal ridge)

Question 12

Limb development

Answer 12

myotome cells migrate into the limb bud to develop into muscle and cartilage. Spinal nerves migrate with the myotome cells to innervate the muscles.

Question 13

Week 6

Answer 13

cartilage models of the bones of the arm begin to form.

Question 14

Week 9

Answer 14

bone formation in limbs via enchondral ossification begins

Question 15

Enchondral ossification

Answer 15

mesenchymal cells in center of limb condense into precartilaginous aggregates and differentiate into chondroblasts (cartilaginous cells)

Question 16

Chondroblasts secrete:

Answer 16

BMP (bone morphogenetic protein) to facilitate growth, Ihh (Indian hedgehog homolog) to induce further secretion of BMP (positive feeback)

Question 17

Chondroblasts produce:

Answer 17

matrix to form hyaline cartilage

Question 18

Steps of enchondral ossification

Answer 18

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

Question 19

bone growth and remodeling

Answer 19

epiphyseal plates expand with growth hormone, bone remodeled by selective deposition and resorption of bone from opposing surfaces

Question 20

remodeling of bone is due to

Answer 20

differential deposition and resorption of bone by osteoblasts and osteoclasts, respectively

Question 21

oestoblasts are derived from

Answer 21

osteoprogenitor cells

Question 22

osteoclasts are derived from

Answer 22

fused mononuclear hemopoietic progenitor cells and activated by cytokine signaling from marrow stromal cells