The invagination of cells through the primitive streak

- Establishes the cranial-caudal, and left-right body axes

- Creates a trilaminar embryo

- Differentiates tissues into ectoderm, mesoderm, and endoderm

Notochord Formation

- Mesoderm cells invaginating at the primitive node intercalate with the newly forming endoderm. The cells grow cranially forming an elongated notochordal plate.

- The notochordal plate infolds to form a tubular structure, the notochord

- The notochord detaches from the endoderm leaving a continuous layer of endoderm

- Ectoderm overlying the notochord differentiates into the neural plate, a thickening of ectoderm cells that will become the nervous system

- Mesoderm adjacent to the notochord differentiates into:Paraxial mesoderm (closest), Intermediate mesoderm, and Lateral mesoderm (furthest from the notochord)

Middle of the 3rd week, the notochord is growing cranially and has induced the overlying ectoderm to differentiate into the neuroectoderm

- it is thicker and forms the neural plate

- marks the first step of nurulation which is the term used for formation of the nerous system

- Cells at the lateral edge of the neural plate proliferate

- This causes the lateral edges of the neural plate to elevate into the neural folds.

- The neural folds bend toward the midline and fuse together forming a hollow neural tube

- The neural tube detaches from the ectoderm, and a continuous layer of ectoderm grows to cover the tube

The neural tube is open to the amnionic cavity at both ends through the cranial and caudal neuropores

- The cranial neuropore closes around day 25

- The caudal neuropore closes around day 28

Second most common birth defect

- Severe: where the cranial vault (anencephaly) or spinal canal (spina bifida) does not close and the underlying nervous tissue is only partially formed and exposed

- Mild: spina bifida occulta. There may be no visible lesion but a defect in vertebral arch formation can be seen on radiographs

- Often thought of as the 4th embryonic germ tissue but it first originates in the ecoderm

- Forms in the elevating neural fold

- As the neural fold elevates, neural crest cells proliferate and move into the mesoderm layer forming a flattened cluster of cells in between the neural tube and overlying ectoderm

- The neural crest tissue then separates and moves to lie laterally on either side of the neural tube before migrating throughout the body

- Melanocytes in the skin and hair

- Nerve cells in the peripheral and sympathetic system

- Teeth and bones of the craniofacial skeleton

- Structures of the eye and ear

- Tissue of the thymus, pituitary, salivary and lachrymal glands

- Smooth muscle and fat of the head and neck

Bc so many structures are derived from the neural crest, teratogens have a profound effect on the developing embryo

-Paraxial: closest to the notochord

- Intermediate: further from the notochord

- Lateral: furthest from the notochord. Splits into two layers, both of which are continuous with the extraembryonic mesoderm. The space between the layers will become the intraembryonic coelom or body cavity

Paraxial mesoderm will form ____

- The parietal (or somatic) mesoderm: is continous with extraembryonic mesoderm covering the amnion. Forms the dermis of the abdominal wall and limbs, and the serosal linings of the body cavities

- The visceral or splanchnic mesoderm: is continuous with extraembryonic mesoderm covering the yolk sac. Forms the muscular layers of the gut, blood vessels, and the serosal covering on each organ

Paired blocks of paraxial mesoderm which form in a segmented manner on either side of the developing neural tube

- Form at regular intervals from the cranial to caudal

- Forms the skeleton, body wall and limbs

- Each somite segement forms an association of nerve, muscle and skin that remain linked through out development and into adult life

- They begin in the occiptal region on day 20

- By day 30 there are 38 pairs of somites

- By the end of the 5th week there are 42-44 pairs

Somite formation is so regularly formed that it can be used to accurately age the embryo during the time somites are visable

- Sclerotome: originates from the cells in the ventromedial portion of the somite (becomes vertebra and ribs)

- Dermotome: originates from the cells in the lateral portion of the somite (becomes the dermis of back and limbs)

- Myotome: originates from the cells dorsomedial and ventrolateral to the dermatome. Myotome cells migrate under the dermatome and join in a single layer (becomes the muscles of teh back and limbs and body wall)

- Begins at the beginning of the 4th week and continues through out the week

- Causes the upper layer to grow faster which bends the embryo towards the lower surface of the disk

- The head and tail folds result mainly from the elongation and growth of the nervous system in the craniocaudal direction

- The right and left lateral folds result from rapid growth of the somites and vertebra. These lateral folds will become the abdominal wall

- Folding causes the opening into the yolk sac to decrease in size forming an amnion covered stalk that will later develop inot the umbilicus

- Causes the cardiogenic region to move from a cranial position to a position caudal to the oropharangeal membrane and ventral to the foregut

- As the cranial disc folds under, a pouch (foregut)is created lined with endoderm

- The pouch ends blindly with the oropharyngeal membrane which will eventually break down to form the mouth opening

- Causes the primitive streak, which was cranial to teh cloacal membrane to becom caudal

- And endodermal lined pouch (hindgut) is created in the caudal embryo terminating with the cloacal membrane which will later break down to form the anus and reproductive and urinary tract openings

An early fetal membrane projecting into the connecting stalk

- Becomes incorporated into the embryo proper

- Comes off the newly formed hindgut and will later form part of the urinary system

- The early heart develops in the 3rd week from paired endocardial tubes in the cardiogenic region

- Late in the 3rd week, endocardial tubes develop into the primordial heart (This early heart joins with the newly formed blood vessels to create a primordial cardiovascular system)

- Blood begins to circulate and the heart beat (day 21-22) is visible by ultrasound

- The cardiovascular system is the first organ system to become functional in the embro

Vasculogenesis: the formation of new vessels by the aggregation of angioblast (vessel forming cells) into clusters called blood islands

Angiogenesis: new vessels sprout off pre-existing vessels which allows the cardiovascular system to expand as the embryo grows

Cavities appear in the blood islands. The angioblast flatten and differentiate into endotheial cells with line the new vessel

Coalesce to form the major blood vessels

- Yolk sac blood islands --> vitelline vessels

- Umbilical blood islands --> umbilical vessels

- Blood islands in the embryo --> major vessels (aorta)

The formation of blood cells

- Begins in the 3rd week and continues through out life

- Blood cells first develop from endothlial cells in blood islands of the yolk sac

Fetal and adult erythrocytes are formed from different progenitor cells

They are named after the different globin subunit in the hemoglobin molecule