Embryology Prelims Part 3

Ectoderm, Mesoderm and Endoderm

primary germ layers.

Ectoderm

____ forms epidermis of the skin, epithelium of the oral and nasal cavities, and the nervous system and sense organs.

Mesoderm forms

_____ muscle and connective tissue, including bone, and components of the circulatory, urinary and genital systems.

Endoderm

____ forms mucosal epithelium and glands of respiratory and digestive systems.

Gastrulation, primitive gut formation, gastrula

The morphogenic process that gives rise to three germ layers:

ectoderm, mesoderm, and endoderm. (In some species,

evidence of_____ can be seen [___Gr.= little stomach])

blastocyst

Gastrulation includes the following sequence, beginning with

a _____:

thickened embryonic disc

A ________ becomes evident at the

blastocyst surface, due to cell proliferation of the inner cell

mass cells. Trophoblast cells overlaying the inner cell mass

degenerate in domestic mammals (in the mouse and human,

trophoblast cells overlaying the inner cell mass separate and,

instead of degenerating, become amnionic wall.)

delaminate,

➢From the inner cell mass, cells proliferate, break loose (_____), and migrate to form a new cell layer inside the trophoblast layer.

hypoblast,

The new layer of cells, called the ____, will form a yolk sac.

epiblast

The remaining inner cell mass may be called the _____.

primitive streak

On the epiblast surface, a ______forms as differential cell growth generates a pair of ridges separated by a depression.

primitive streak

The ______ defines the longitudinal axis of the embryo and indicates the start of germ layer formation.

coelom/celom, body cavities

Deep to the primitive streak, a space (_____) becomes evident between the hypoblast layer and epiblast. Subsequently, the coelom is filled by mesoderm that undergoes cavitation and gives rise to _____.

Epiblast cells

_____proliferate along primitive streak margins and migrate through the streak into the coelom. The migrating cells form endoderm & mesoderm layers.

endoderm

Initial migrating cells join the hypoblast layer, forming embryonic ____ (hypoblast cells constitutes yolk sac endoderm).

mesoderm

The majority of migrating cells enter the coelom as primary mesenchyme and become _____.

laterally and cranially

The primary mesenchyme migrates ______ (but not along the midline region directly cranial to the primitive streak where notochord will form).

paraxial, intermediate, and lateral mesodermal regions

Mesoderm divides into: ______ .

coelom (horse shoe-shaped)

➢Within the lateral mesoderm, cavitation re-establishes a ______.

somatic mesoderm, splanchnic mesoderm

➢The mesoderm splits into two layers bordering the coelom— _____ is attached to the ectoderm and ______ is joined to endoderm.

ectoderm, skin epidermis & nervous system

The remaining epiblast becomes _____ which forms _______

mesenchyme and epithelioid.

Mesoderm can exist in two morphologic forms:

Mesenchyme

_____ features aggregates of stellate cells withinan abundant extracellular matrix composed of fluid and macromolecules (polymers).

Epithelioid

______ refers to organized cells having distinct apical and basal surfaces; the latter commonly rests on a basal lamina produced by epithelioid secretion.

mesenchyme, epithelioid, primary mesenchyme.

Mesoderm can transform from a _____ to _____ and vice versa: The mesoderm that streams through the primitive streak is ____.

temporarily epithelioid

Somatic, splanchnic, and somite mesoderm can be ______.

secondary mesenchyme

The temporary epithelioid transforms to a _____ which ultimately forms muscle and connective tissue (including cartilage, bone, ligaments, tendons, dermis, fascia, and adipose tissue).

mesenchyme

Thus, the term “______” refers to the morphologic appearance of embryonic tissue.

ectomesenchyme, neural crest ectoderm

Although most mesenchyme is mesoderm, the other germ layers can also form mesenchyme, e.g., ______ from ______.

notochord

The _____ is a rod-shaped aggregate of cells located between ectoderm and endoderm anterior to the primitive streak of the embryo.

1. formation of the head process,

2. development of the nervous system, and

3. formation of somites

The notochord is important because it induces:

vertebral column and the base of the cranium.

The notochord marks the future location of the

nucleus pulposus

The ultimate fate of the notochord is to become _____ of intervertebral discs.

primitive node

The notochord develops from the _____ located at the cranial end of the primitive streak. From the node, mesoderm-forming cells proliferate and migrate forward into the future head region where they become the rod-shaped notochord.

teratogens

Each organ system has a critical period during development when it is most sensitive to external agents _____ that produce birth defects.

Neurulation

_____refers to notochord- induced transformation of ectoderm into nervous tissue.

third week

The process begins during the _____ in the region of the future brain and then progresses caudally into the region of the future spinal cord.

neuroectoderm

Ectodermal cells overlaying the notochord become tall columnar ______; they form a thickened area designated the neural plate. The other ectodermal epithelium is flattened.

neural plate

Ectodermal cells overlaying the notochord become tall columnar (neuroectoderm); they form a thickened area designated the _____. The other ectodermal epithelium is flattened.

neural groove

A ______ is formed as edges of the neural plate become raised on each side of a midline depression. (Apical ends of individual neuroectodermal cells constrict.)

neural tube

A _____ is then formed as the neural groove undergoes midline merger of its dorsal edges. The tube separates from non-neural ectoderm which unites dorsal to it.

neural crest

Bilaterally, where the neural groove is joined to non-neural ectoderm, cells detach as the neural groove closes; the cells proliferate and assume a position dorsolateral to the neural tube—forming______.

Neural tube

______ becomes the central nervous system, i.e., the brain and spinal cord.

Neural crest, pigment cells, neurons and glial cells, adrenal medulla cells

_____ cells are remarkable for the range of structures they form. Some cells migrate dorsally and become ______ in skin. Other cells migrate ventrally and become _____ of the peripheral nervous system, or ______.

meninges, bone, fascia, and teeth.

In the head, neural crest forms mesenchyme (ectomesenchyme) which becomes :

Mesoderm blocks

_____ located just lateral to the notochord, which induced somite development.

pair of somites, every vertebra

A _______ develops for _______, plus a half dozen somite pairs in the head.

craniocaudal order.

Number of somites in an embryo is indicative of age, individual somites develop chronologically, in ______.

Mesoderm, paraxial mesoderm

_____, designated ______, accumulates on each side of the notochord

rostral to caudal

Progressing from _______over time, transverse fissures divide the paraxial mesoderm into blocks

somite, 90°

Each block becomes a _____ (epithelioid cells within a ____ block re-orient___, from transverse to the notochord to longitudinal)

Head (occipital) somites

________develop from proliferation of local mesenchyme lateral to the cranial end of the notochord

somitomeres, muscles of the jaw, face, pharynx, & larynx.

Rostral to the notochord, mesenchyme forms less- developed somites, called ______; these migrate into into pharyngeal arches and form _____, _____, ______, ______

Sclerotome. Dermatome. Myotome

•Each somite differentiates into :

Sclerotome

_____(ventromedial region) gives rise to vertebrae, ribs, and endochondral bones at the base of the skull.

Dermatome

_____ (lateral region) gives rise to the dermis of skin

Myotome

______ (intermediate region) gives rise to skeletal muscles of the body