Medical School Embryology

Week 1- The Zygote

The first week of the developing embryo involves a complex process, and precise timing, of oocyte and sperm uniting to form a zygote. Both of these gametes are involved in many maturation stages before reaching being able to participate in fertilization, and only a small number of each have the opportunity to partake in this amazing natural process. After fertilization, the zygote goes through many stages of cellular reproduction, changing its name as it goes for descriptive purposes. Near the end of the first week, the blastocyst (a later stage of the zygote) will implant into the wall of the uterus in order to gain nutrients vital to development.
 
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Week 2- Implantation & Bilaminar Disc

In the second week of gestation the embryo continues to make its connections to the uterine wall. The chorionic villi, small protrusions from the embryo mass, use enzymes to dig into the uterus and form connections with blood vessels. The cell mass separates from the outer cell wall partially, forming two sacs (the amniotic sac and the yolk sac) held within the gestational sac/chorionic sac.
 
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Week 3- Gastrulation

The crucial stage of gastrulation begins in week three, and begins the formation of the trilaminar disc. This new arrangement of cellular placement gives the embryologic origin to the adult form. The three layers (ectoderm, mesoderm, and endoderm) will form every bone, muscle, organ, and tissue as the embryo develops.
 
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Week 4-8 – Organogenesis

This section briefly reviews an outline of the genesis of the organs. A more in-depth look will occur in the following sections. Nearly all major systems of the embryo begin during week four, explaining why this is an important stage for proper nutrition and health. However, it is also a stage early in the pregnancy when many mothers are not aware of the impregnation. Planning a pregnancy increases the chances of proper preparation, decreasing birth defects.
 
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Fetal Stage

Week 9 begins the fetal stage of development. The basic prenatal form is completing its final stages of development, including finalizing the formation of many bones. This is also when the gender of the soon-to-be baby can be determined. Not all babies are born equal, and many basic requirements are generally needed to sustain life. The age and weight of a fetus are major determinants of whether it will survive when born, and may be influenced by maternal factors (diabetes, infection, alcohol/drug use).
 
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The Placenta

The placenta is not only vital for the nutrient transport to the embryo and fetus, but may be the cause of early termination and maternal death. The placenta is made of three layers/decidua; the basalis, capsularis, and parietalis. It may cover the opening of the cervix, placenta previa, preventing birth. Alternatively, the chorionic villi of the placenta may penetrate the myometrium of the uterus in placenta accreta. The placenta acts as a barrier to many teratogens (agents of malformation), but it may be crossed by specific pharmaceuticals and pathogens.
 
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Nervous System

The nervous system (along with the cardiovascular system) is one of the few major bodily systems that begin in week three. This makes sense considering the vital importance these structures give to the developing embryo. The process of neurulation forms body folds that wrap in on themselves, forming the notocord. The neural tube will be induced and form the CNS, while the neural crest cells will migrate throughout the body and form the PNS and other structures. The neural tube will further differentiate into the separate divisions of the brain (prosencephalon, mesencephalon, and rhombencephalon) and the spinal cord. Failure to completely form in any of these stages will cause developmental abnormalities, such as anencephaly (absence of brain) and spina bifida (failure of vertebrae to close).
 
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Musculoskeletal System

The basic support structure of the adult body include the muscles and bones. The musculoskeletal system will develop mostly from mesoderm, though with small attributions to ectoderm as well. Initially, somites will form around the notocord, and will become the vertebral bodies and ribs. Bones of the skull are particularly important as they are malleable at birth due to sutures, allowing the head to "mold" to fit through the birth canal.
 
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Limb Development

The limb position of an embryo is determined by the homobox (HOX) genes. The limb buds begin to form around day 26, with the digits beginning to form via apoptosis in week 6 & 7 for the arms and legs, respectively. The myodermatomes from embryo somites form the anterior and posterior condensations, while the Zone of Polarizing Activity (along with genes, such as Sonic Hedgehog and Wnt) guide growth factors of limb development. The upper limbs progress through a 90 degree lateral turn and the lower limbs 90 degrees medially to reach their final alignment. Most limb abnormalities are due to genetic and teratogenic influences, such as the medication Thalidomide and genetic disorder Osteogenesis Imperfecta.

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Cardiovascular: Heart and Vessels

The cardiac system, unlike the majority of organ systems, begins at week 3 (not week 4). This makes sense as nutrients must be shipped to other systems for their growth to begin. The angioblastic cords form the primative heart and neural crest cells migrate to form the circuitry of the heart (EX: SA & AV nodes). The early cardiovascular system consists of several temporary venous systems. The vitelline veins sprout from the sinus venosum to the yold stalk; the left degenerates and the right becomes part of the IVC and portal vien. The cardinal viens also join the sinus venosus; the left degenerates and the right becomes the SVC. The umbilical viens reach from the placenta to the sinus venosum; the right degenerates and left becomes part of the IVC.

As the primitive heart matures, the AV septum and IV septum are formed creating the left and right atria and ventricles. The septum primum and septum segundum form the foramen ovale; defects in which can produce several important clinical presentations. Tetralogy of Fallot and persisant truncus arteriosis are two cyanosis causing defects. The pharyngeal arches are of particular important on exams. Arch 1- maxillary arteries; Arch 2- stapedial arteries; Arch 3- common carotid and proximal aspect of internal carotid; Arch 4- part of aortic arch adn right subclavian; Arch 6- proximal pulmonary artery and ductus arteriosis.

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Respiratory System

The lungs have two types of specialized pneumocytes (lung cells). Type 1 are used in gas exchange, and Type 2 secrete surfactant. The surfactant begins to be produced around week 20 and is adequate for proper lung function around week 28. Premature births may not have adequate surfactant to inflate the lungs, causing Respiratory Distress Syndrome (AKA Hyaline Membrane Disease). Corticosteroids may be given to a pregnant mother to encourage growth of the lungs and prevent this condition.

The lung buds sprout fromt the same mesodermal tube as the trachea and esophagus. Failure of the tubes to recanalize (cell apoptosis) at week 8 will form an atresia, while incomplete fusion may form tracheoesophageal fistula. This condition is often associated with other developmental abnormalities, known as VACTERL.

The lungs have 4 stages of development: Pseudoglandular (week 6-16) where lung branching develops, Canalicular (week 16-26) vascularization of lungs, Terminal Sac (week 26-birth) increase in capillaries and terminal sac surface area, and Alveolar (week 32- age 8) where terminal sacs continue to partition.

Full Playlist- Respiratory Link

Pharyngeal Arches

Knowing the Pharyngeal Arches, Grooves, and Pouches is arguably one of the highest yeild emryological aspects of the board exams. Each have specific structures, nerves, and vessels the are associated with that development, and form specific disorders and syndromes from failed development. Remember that the 5th arch is not found in humans.

1st: Arch- mandibular and maxillary parts, trigeminal nerve (CN V), incus & malleus; Pouch/Grove- tympanic cavity (ear to pharynx)
2nd: Arch- "strap" muscles, hyoid (part), facial nerve (CN VII), stapes; Pouch- tonsils
3rd: Arch- hyoid (part), glossopharyngeal nerve (CN IX); Pouch- thymus & inferior parathyroid
4th: Arch- cricothyroid and soft palate, vagus nerve (CN X); Pouch- superior parathyroid
6th: Arch- larynx (and cartilages), vagus and recurrent laryngeal nerve (CN X); Pouch- C cells

Defects may include major congenital abnormalities, such as; DiGeorge Syndrome, Tetralogy of Fellot, branchial fistula, ectopic thyroid, Treacher Collins, Pierre Robins, and even cleft tongue and palate.

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Body Cavities

The gut is formed on the rostral end (stomodeum) by ectoderm, internally by endoderm, and ends caudally (proctodeum) again in ectoderm. The mesenteries are sheets of tissue that hold the gut tube in place. As the gut tube twists, the mesenteries and nerves (vagus in particular) will rotate within the embryo. The dorsal mesentery and posterior vagus rotate to the right side of the body, and opposite for the ventral mesentery and anterior vagus. The diaphragmatic crura hold the aorta, esophagus, and IVC in place.

The most common embryologic defects in this developmental stage are congenital diaphragm/esophageal hernias, which are caused by large hiatus or unusual fusing of diaphragm. Gastroschisis is a more serious condition in which the abdominal contents form outside of the abdominal wall.

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Digestive/GIT

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Urinary/Kidney System

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Reproductive Systems

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Integument System

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Facial Development

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Special Senses (Eyes & Ears)

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