Organogenesis and the Foetal Period to Birth Flashcards
Briefly describe the major developmental events of organogenesis, and of the foetal period.
During week four, folding occurs. Lateral folding establishes the 3D tube shape of the embryo and gut tube, as well as the formation of the neural tube and the heart tube. The right and left sides move ventrally, then medially, and fuse with the opposite side on the midline. Cranial folding creates the head fold, and caudal folding creates the tail fold.
From week four to eight, organogenesis occurs.
From week eight, the developing organism is called a foetus. From this stage the different organisms mature and develop. The foetal period is from the eighth week of development until birth.
The integumentary system
- Epidermis derived mostly from the ectoderm (exception of Langerhans cells from mesoderm)
- Melanocytes and Merkel cells derived from the neural crest
- Dermis derived mostly from mesoderm (exception of the dermis in the head being derived from neural crest)
- Hair follicles, sebaceous glands, sweat glands, mammary glands develop as invaginations of the epidermis, and the surrounding mesoderm contributes some elements.
The skeletal system
- Derived from mesoderm (exception of skull bones which are derived from neural crest)
- There are two types of ossification of mesenchyme (embryonic tissue): endochondrial (mesenchyme forms cartilage which ossifies to bone) and intramembranous ossification (mesenchyme directly ossifies). Intramembranous ossification occurs mostly in skull bones.
- Primary ossification centres (OC) develop in the third prenatal month in diaphysis. Most secondary OC form in epiphysis, and OC of short bones appear after birth. Epiphyseal plates close after more than ten years after birth.
The muscular system
• Derived from somites and somitomeres (mesoderm)
The nervous system
- The neural tube is formed from the ectoderm. The neural tube forms the CNS, with the cranial end forming all the CNS but the cord and the caudal end forming the cord.
- The neural crest cells become peripheral nervous system ganglia, Schwann cells melanocytes and Merkel cells of the skin, as well as other structures.
The endocrine system
- The anterior pituitary gland is derived from the ectoderm.
- The posterior pituitary is derived from the neural tube ectoderm.
- The adrenal medulla is derived from the ectoderm (neural crest).
- The adrenal cortex is derived from the mesoderm.
- The thyroid and parathyroid glands are both derived from the endoderm.
The cardiovascular system
- Derived almost entirely from the mesoderm (neural crest ectoderm contributes)
- Week four: heart tube folds, begins to contract blood
- Week five to eight: heart chambers separate by septa and valves
- Blood cells form in yolk sac, late in the third week, then liver, then bone marrow from haematopoietic stem cells from mesoderm.
The lymphatic/immune system
- Lymph vessels and lymph nodes develop from mesoderm.
- Lymphocytes from haematopoietic stem cells from mesoderm.
- Thymus in part derived from endoderm.
The digestive system
- Lateral folding of the embryo creates the gut tube. The head to tail folds of the embryo create the foregut, midgut, hindgut regions.
- Epithelium formed from endoderm.
- Smooth muscle and CT formed from mesoderm.
- Enteric nervous system formed from the neural crest (ectoderm)
- Liver, gallbladder and pancreas develop as outgrowths of the endoderm of the foregut-midgut junction.
- Peristalsis begins in the second trimester and sucking movement begins around 34 weeks.
The respiratory system
- Begins as an outgrowth of the foregut. Septum forms, separating the trachea from the oesophagus. The lung (primary bronchi) buds form at distal ends of the trachea and the bronchial buds repeatedly divide, forming a respiratory tree (branching morphogenesis).
- Epithelium from endoderm.
- Cartilage, smooth muscle and CT from mesoderm.
- Breathing movements (irregular) begin in the early foetal period, and alveoli development begins around week 28. Surfactant is not produced in significant quantities until week 35.
The urinary system
- Three sets of ‘kidneys’ develop from the mesoderm, the first two sets degenerate.
- Ureteric bud (off mesonephric duct) forms ureter to collecting ducts, and metanephros forms nephrons – branching morphogenesis.
- Kidneys are functional during foetal period, and urine is released into the amniotic fluid, but the placenta eliminates wastes.
The reproductive system
- Germ cells (primitive gametes) develop in yolk sac, then migrate to gonadal ridge of mesoderm (primordial testes/ovaries). Surrounding cells (mesoderm) become support cells and endocrine cells of testes/ovaries.
- Mesonephric ducts and paramesonephric ducts develop from mesoderm.
- The testes descend through inguinal canal in the second trimester and into the scrotum by the beginning of the ninth month.
List in order the vessels and heart chambers through which foetal blood flows from placenta, to head, and back to placenta. Give two possible routes.
- placenta
- umbilical vein
- ductus venosus
- inferior vena cava
- right atrium
- foramen ovale
- left atrium
- left ventricle
- ascending aorta
- aortic arch
- head
- jugular vein
- superior vena cava
- right atrium
- right ventricle
- aorta
- descending aorta
- umbilical arteries
- placenta
An alternative route • placenta • umbilical vein • ductus venosus • inferior vena cava • right atrium • right ventricle • pulmonary trunk • right or left pulmonary artery • Arteries, arterioles, capillaries, venules, veins in lungs • Right or left pulmonary vein • Left atrium • Left ventricle • ascending aorta • aortic arch • head • jugular vein • superior vena cava • right atrium • right ventricle • aorta • descending aorta • umbilical arteries • placenta
List the following blood vessels in order from highest to lowers % oxygen saturation of haemoglobin prenatally.
a. Arch of aorta
b. Inferior vena cava at entrance to right atrium
c. Inferior vena cava near origin from common iliac veins
d. Umbilical artery
e. Umbilical vein
e, b, a, d, c
