LECTURE 12 (4-8 weeks of development)

Question 1

What are the major occurrences during this period?

Answer 1

• All major external and internal structures are established
• Main organ systems have started to develop
• Embryo has a distinctly human appearance

Question 2

Exposure to what during this period may cause major birth defects?

Answer 2

TERATOGENS

[Teratogens = agents that produce or increase the incidence of major birth defects]

Question 3

What are the three phases of Human development?

Answer 3

• GROWTH = cell division + elaboration of cell products
• MORPHOGENESIS = development of shape, size or other features of a particular organ or part of the whole body
• DIFFERENTIATION = cells are organised in a precise pattern of tissues and organs capable of performing specialised functions

Question 4

What happens on Day 22-24?

Answer 4

Folding of the flat trilaminar embryonic disc into a cylindric embryo -> rapid growth of the embryo

Question 5

What happens on Day 26?

Answer 5

Folding of the ends of the embryo produces head and tail folds that result in CRANIAL and CAUDAL regions

Question 6

Describe the process of ‘Head Fold’

Answer 6

On Day 22, the neural folds in the cranial region form the PRIMORDIUM OF THE BRAIN which projects into the AMNIOTIC CAVITY -> Developing forebrain grows cranially beyond the OROPHARYNGEAL MEMBRANE and overhangs the developing heart -> Part of the ENDODERM of the UMBILICAL VESICLE is incorporated into the embryo as the FOREGUT -> SEPTUM TRANSVERSUM lies caudal to the heart where it develops into the DIAPHRAGM

Question 7

Describe the process of ‘Tail Fold’

Answer 7

Tail region projects over the CLOACAL MEMBRANE (future site of the anus) -> Endodermal layer is incorporated into the embryo as the HINDGUT (descending colon and rectum) -> Terminal part of the HINDGUT dilates to form the CLOACA (forms urinary bladder + rectum) -> UMBILICAL CORD forms from the connecting stalk

Question 8

What does the folding of the caudal end of the embryo result from?

Answer 8

Growth of:
- Distal part of the neural tube
- Primordium of the spinal cord

Question 9

What does the Ectoderm give rise to?

Answer 9

• CNS + PNS
• Sensory epithelia of the eyes, ears and nose
• Epidermis, Hair & Nails
• Mammary glands + Pituitary gland
• Subcutaneous glands + enamel of the teeth

Question 10

What does the Mesoderm give rise to?

Answer 10

• Connective tissue
• Cartilage + Bone
• Striated + Smooth muscles
• Heart, blood + lymphatic vessels
• Kidneys
• Genitalia

Question 11

What does the Endoderm give rise to?

Answer 11

• Epithelial lining of the digestive + respiratory tracts
• Connective tissue framework of tonsils, thyroid and parathyroid glands
• Thymus
• Liver
• Pancreas

Question 12

What happens on Day 24?

Answer 12

• First pharyngeal arches are visible (major part = mandible, maxillary prominence = maxilla)
• Embryo is slightly curved
• Heart pumps blood
• Rostral neuropore is closing

Question 13

What happens on Day 26?

Answer 13

• Rostral neuropore is closed
• Three pairs of PHARYNGEAL ARCHES are visible
• Forebrain produces a prominent elevation of the head
• Upper limb buds are recognisable
• Future lenses of the eyes are visible on the sides of the head

Question 14

What happens on Day 28?

Answer 14

• Pharyngeal arches + Lower limb buds are visible
• A long tail-like eminence
• Rudiments of organ systems established
• Caudal neuropore is closed

Question 15

What happens on the 5th week (Day 32)?

Answer 15

• Enlargement of head
• Rapid development of the brain and facial prominences
• Formation of the CERVICAL SINUS
• MESONEPHRIC RIDGES indicate the site of the developing mesonephric kidneys

Question 16

What happens on the 6th week?

Answer 16

• Embryos show spontaneous movements + reflex responses to touch
• Elbows, large hand plates + fingers begin to develop in the hand plates
• Development of lower limbs
• The PHARYNGEAL GROOVE between the FIRST TWO PHARYNGEAL ARCHES becomes the EXTERNAL EAR CANAL
• AURICLE forms
• Retinal pigment has formed -> eyes are now obvious

Question 17

What is Umbilical herniation and why does it happen?

Answer 17

UMBILICAL HERNIATION = In the 6th week, the intestines enter the extraembryonic coelom in the proximal part of the umbilical cord

The herniation occurs because the abdominal cavity is too small at this age to accommodate the rapidly growing intestine

Question 18

What happens on the 7th week?

Answer 18

• Limbs undergo considerable change
• Fingers and toes
• Communication between the primordial gut and umbilical vesicle is reduced
• Ossification of bones of the upper limbs

Question 19

What happens on the 8th week?

Answer 19

• Digits of hand are separated but noticeably webbed -> at end of 8th week, lengthened and completely separated
• SCALP VASCULAR PLEXUS has appeared
• Limb movements
• Primary ossification begins in the FEMORA (long bones of thigh)
• Embryo has distinct human characteristics (neck established, eyelids closing, intestines are still in proximal portion of umbilical cord)

Question 20

How is Embryonic age estimated?

Answer 20

• 3rd-4th weeks are STRAIGHT -> measurements indicate the greatest length
• 14th-18th weeks -> CROWN-RUMP LENGTH (CRL)