LECTURE 12 (4-8 weeks of development) Flashcards
What are the major occurrences during this period?
- All major external and internal structures are established
- Main organ systems have started to develop
- Embryo has a distinctly human appearance
Exposure to what during this period may cause major birth defects?
TERATOGENS
[Teratogens = agents that produce or increase the incidence of major birth defects]
What are the three phases of Human development?
- 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
What happens on Day 22-24?
Folding of the flat trilaminar embryonic disc into a cylindric embryo -> rapid growth of the embryo
What happens on Day 26?
Folding of the ends of the embryo produces head and tail folds that result in CRANIAL and CAUDAL regions
Describe the process of ‘Head Fold’
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
Describe the process of ‘Tail Fold’
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
What does the folding of the caudal end of the embryo result from?
Growth of:
- Distal part of the neural tube
- Primordium of the spinal cord
What does the Ectoderm give rise to?
- CNS + PNS
- Sensory epithelia of the eyes, ears and nose
- Epidermis, Hair & Nails
- Mammary glands + Pituitary gland
- Subcutaneous glands + enamel of the teeth
What does the Mesoderm give rise to?
- Connective tissue
- Cartilage + Bone
- Striated + Smooth muscles
- Heart, blood + lymphatic vessels
- Kidneys
- Genitalia
What does the Endoderm give rise to?
- Epithelial lining of the digestive + respiratory tracts
- Connective tissue framework of tonsils, thyroid and parathyroid glands
- Thymus
- Liver
- Pancreas
What happens on Day 24?
- First pharyngeal arches are visible (major part = mandible, maxillary prominence = maxilla)
- Embryo is slightly curved
- Heart pumps blood
- Rostral neuropore is closing
What happens on Day 26?
- 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
What happens on Day 28?
- Pharyngeal arches + Lower limb buds are visible
- A long tail-like eminence
- Rudiments of organ systems established
- Caudal neuropore is closed
What happens on the 5th week (Day 32)?
- 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
