Embryology

First week: primordial germ cells and fertilization

• Development begins with fertilization - the union between sperm and oocyte, forming the zygote
  • Sperm and oocyte are gametes - derived from primordial germ cells that are formed in the epiblast during the second week and end up in the gonads
  • Gametes undergo gametogenesis, including meiosis, to reduce their number of chromosomes and prepare for fertilization
  • Spermatogenesis - spermatogonia are transformed into spermatozoa
• Fertilization occurs in the ampullary region of the uterine tube, and there is fusion of oocyte and sperm cell
• Zygote begins dividing through mitosis, forming individual cells called blastomeres
  • Inner cell mass forms embryoblast
  • Outer cell mass forms trophoblast, later contributing to the placenta
• Implantation can now occur at the end of the first week after fertilization

Second week

• Blastocyst slowly becomes completely embedded, and the surface defect in the mucosa has healed
• Trophoblast
  • Trophoblast differentiates into cytotrophoblast (inner, actively proliferating layer) and syncytiotrophoblast (outer layer which erodes maternal tissues)
  • Lacunae develop in the syncytiotrophoblast which maternal blood can enter - the first uteroplacental circulation
• Embryoblast
  • Differentiates into the bilaminar disc (epiblast and hypoblast)
  • A small cavity appears within the epiblast, which becomes the amniotic cavity
  • Epiblast cells lining the amniotic cavity, and later give rise to the germ layers
  • Hypoblast cells - small layer surrounding the primitive yolk sac (exocoelomic cavity)
  • The yolk sac provides nutrition to the embryo until the blood supply to chorionic villi is established

Third week

• Gastrulation occurs (all three germ layers are established - ectoderm, mesoderm, endoderm)
  • Firstly, formation of primitive streak on the surface of the epiblast
  • The primitive streak has a primitive pit, surrounded by a primitive node, at the cephalic end
  • Cells from epiblast migrate into the primitive streak and differentiate into ectoderm, mesoderm and endoderm
  • Gastrulation continues from cephalad to caudad throughout the fourth week
• Notochordal plate forms - eventually serves as the basis of the axial skeleton
• Cephalic and caudal ends of the embryo are established
• Primary villi are formed in the trophoblast, allowing formation of small capillaries

Embryonic period - week 3-8

• Embryonic disc begins to lengthen and form head and tail folds, producing a curve into the 'fetal position'
• Connection with the yolk sac and placenta is maintained through the vitelline duct and umbilical cord, respectively
• Ectodermal progression
  • Forms:
    • CNS
    • PNS
    • Sensory epithelium of ear, nose, eye
    • Skin, including hair and nails
    • Pituitary, mammary and sweat glands; enamel of teeth
  • Neural plate
• Mesodermal progression
  • Forms somites, which give rise to myotome (muscle tissue), sclerotome (cartilage and bone) and dermatome
  • Also forms vascular and urogenital systems and spleen and adrenal cortex
• Endodermal progression
  • Forms:
    • Epithelial lining of GIT, respiratory tract and bladder
    • Parenchyma of thyroid, parathyroid, liver and pancreas
    • Epithelial lining of tympanic cavity and auditory tube

The gut and neural tubes

• Ectoderm forms neural tube, and endoderm forms gut tube, with mesoderm in between to hold them together - 'a tube on top of a tube'
  • Parts of mesoderm (lateral plate mesoderm) roll laterally and split into visceral (splanchnic) and parietal (somatic) layers
  • Ectoderm forms lateral body wall folds, which gradually move ventrally to enclose the body cavity, except in the region of the connecting stalk. Here, the gut tube remains attached to the yolk sac through the yolk sac (vitelline) duct.
    • Failure of the ventral body wall to close results in ventral body wall defects including gastroschisis and exstrophy of the bladder and cloaca.
  • Visceral layer is intimately connected to the gut tube, with parietal layers further laterally - the space in between is called 'the primitive body cavity' and eventually is divided into pericardial, pleural and abdominopelvic regions
• Septum transversum situates itself between thoracic cavity and stalk of yolk sac, eventually becoming the diaphragm (see separate topic)
• The gut tube is suspended dorsally from caudal to hindgut ends, which eventually becomes dorsal mesentery
• Ventral mesentery is derived from septum transversum and only exists in certain places

Fetal period (9th week until birth)

• Placenta forms with a fetal portion (derived from the chorion frondosum or villous chorion) and a maternal portion (derived from decidua basalis)
  • The space between the two portions is filled with intervillous lakes of maternal blood
  • The fetal circulation is separated from the maternal circulation by 1) a syncytial membrane and 2) endothelial cells from fetal capillaries
  • Main functions of the placenta are to exchange gases, exchange nutrient and electrolytes, transmit maternal antibodies, produce hormones, and detoxify some drugs.
• The amnion is a large sac containing amniotic fluid in which the fetus is suspended by its umbilical cord
  • It absorbs jolts, allows for fetal movements, and prevents adherences of the embryo to the amnion
  • The fetus swallows amniotic fluid, which is absorbed through its gut and cleared by the placenta
  • The fetus adds urine to the amniotic fluid, but this is mostly water
• The umbilical cord contains two umbilical arteries, one umbilical vein, and Wharton jelly (which serves as a protective cushion for the vessels.
• Twins
  • Dizygotic/fraternal - two amnions, two chorions, and two placenta; most common type
  • Monozygotic twins - two amnions, one chorion, and one placenta
  • Conjoined twins - fetuses joined, one amnion, one chorion, and one placenta

Head and neck

• Pharyngeal (previously called branchial) arches consist of bars of mesenchymal tissue separated by pharyngeal clefts
  • Form in about the fourth week
  • A core of mesenchymal tissue covered on the outside by surface ectoderm and on the inside by endodermal epithelium
  • Each arch contains its own artery (aortic arch), cranial nerve, muscle element, and cartilage bar or skeletal element
  • These arches play important roles in facial development
  • The fifth arch disappears during development and does not lead to any structures

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  Endodermal pharyngeal pouches form simultaneously along the lateral walls of the pharynx
  • Embryo has four pairs of pouches - the fifth is rudimentary
  • These pouches penetrate the surrounding mesenchyme but do not establish an open communication with the external clefts; so the whole structure is not really analogous to gills
  • Endoderm of the pharyngeal pouches gives rise to a number of endocrine glands and parts of the middle ear

• Pharyngeal clefts are transient structures that all disappear, but in some cases can persist as cervical cysts/fistulas

Vascular development

• See 'vascular system' topic
• Lymphatic system
  • Develops later than CV system
  • Six sacs - two jugular, two iliac, one retroperitoneal, and one cisterna chyli
  • Thoracic duct forms from anastomosis of the right and left thoracic ducts

Digestive system

• Largely dealt with under specific organ systems
• Epithelium of the digestive system, and parenchyma of its derivatives, form from endoderm. Associated connective tissue and peritoneal components originate in the mesoderm.

Foregut gives rise to oesophagus, trachea and lung buds, stomach, and the duodenum proximal to the entrance of the bile duct.

• • Liver, spleen and pancreas are outgrowths of foregut and develop in ventral or dorsal mesogastrium
  • Liver moves to the right due to differential growth, pulling the stomach, and the duodenum fuses to the posterior wall, creating the lesser sac behind the stomach

Midgut forms the primary intestinal loop, continuing to the junction of the proximal two-thirds of the transverse colon with the distal third.

• • The primary loop remains temporarily in open connection with the yolk sac through the vitelline duct
  • During the sixth week, this loop grows so rapidly that it protrudes into the umbilical cord
  • During the 10th week, it returns into the abdominal cavity, after rotating 270 degrees anti-clockwise (when viewed from anterior).

Hindgut gives rise to the region from the third of the transverse colon to the upper part of the anal canal.

• • The hindgut enters the posterior region of the cloaca, and the allantois enters the anterior part, before the two are divided by the urorectal septum
• Dorsal mesentery extends from the lower oesophagus to rectum as a continuous sheet
  • Toldt fascia forms between visceral peritoneum on the organ and parietal peritoneum covering the posterior body wall
• Ventral mesentery
  • Derived from mesenchyme of the septum transversum