Somites and embryonic folding

Question 1

what are somites?

Answer 1

cells in the intraembryonic mesoderm differentiate to form pairs of cuboidal segments called somites; symmetric embryonic precursors of the vertebrate skeleton and axial muscle

they are found either side of the neural tube

each somite receives one spinal nerve from the neural tube

somite formation occurs simultaneously wiht notogenesis

Question 2

name the 3 longitudinal columns of the intraembryonic mesoderm

Answer 2

paraxial
intermediate
lateral plate

Question 3

the lateral plate of the intraembryonic mesoderm is ________ with the extra-embryonic mesoderms

Answer 3

continuous

Question 4

somite formation within the paraxial mesoderm

Answer 4

occurs at beginning of 4th week
the medial part becomes SCLEROTOME (skeleton),
the intermediate part becomes MYOTOME (muscle)
the lateral part becomes DERMOTOME (skin).

Question 5

what is the paraxial mesoderm?

Answer 5

The mesoderm at either side of the neural tube

Question 6

somite formation within the intermediate mesoderm

Answer 6

The Intermediate mesoderm becomes NEPHROTOME (forms genitourinary system)

Question 7

somite formation within the lateral mesoderm

Answer 7

Becomes either PARIETAL (wall of body cavities) or VISCERAL (wall of gut).

Question 8

somite development in the 4th week post fertilisation

Answer 8

somite formation occurs in a cranial-caudal sequence

3 occipital(form tongue musculature),

8 cervical,

12 thoracic,

5 lumbar,

5 sacral

1-5 coccygeal remain.

Question 9

caudal meaning

Answer 9

near the posterior part (back) of the body.

Question 10

development of the lateral plate mesoderm in the 4th week post fertilisation

Question 11

how is the action of muscle and skin sensation linked?

Answer 11

nerve supply of muscles and skin derived from the same somite

Question 12

what is the main event that occur in the 4th week post fertilisation ?

Answer 12

somite formation

Question 13

formation of the intraembryonic coelom

when does it occur and explain the process

Answer 13

23rd day post fertilisation (3rd week)

Clefts appear in the lateral plate mesoderm

Eventually a horse-shoe shaped cavity is formed which becomes continuous with the extraembryonic coelom over a limited area.

Intra-embryonic coelom will eventually form:

• The PERICARDIAL cavity (found in the front)
• The PLEURAL cavity (middle of either side )
• The PERITONEAL cavity (end bits of either side)

Question 14

longitudinal folding

Answer 14

at the beginning of the 4th week, the brain begins to form

At this stage the heart and diaphragm (1,2,3) lie in front of the prospective mouth (3-4) and the prospective brain (5) lies behind it.

reversal then occurs to bring about the correct order of organs

Question 15

explain reversal in more detail

Answer 15

Differential growth of the embryo causes it to fold both cranio-caudally and laterally

After reversal the amnion is only attached to the embryo via the UMBILICUS

amniotic cavity enlarges greatly

constriction is produced at the junction of the embryo and yolk sac

part of the endoderm within the yolk sac becomes part of the gut tube

Question 16

lateral folding

Answer 16

begins when neural folds converge to form the neural tube

The edges of the amnion grow downwards towards the yolk sac

this folding greatly reduces communication between intra and extra embryonic coelom

VITELLO-INTESTINAL DUCT remains as a connection between the gut tube and yolk sac

gut tube fully encloses and is surrounded by somato-pleuric/splanchno-pleuric mesoderm (both derived from lateral plate mesoderm)

Question 17

summarise what happens in the 4th week post fertilisation

Answer 17

embryonic folding boh laterally and longitudinally due to the differential growth of the different embryonic structures