The Emergence of the Ectoderm: Central Nervous System and Epidermis

Lange

BIOL 370 – Developmental Biology

Topic #12

Major derivatives of the ectoderm germ layer

Three subdivisions of the ectoderm:

• Surface• Neural Crest• Neural Tube

Major derivatives of the ectoderm germ layer (Part 1)

Know that the surface ectoderm will form an organism’s eventual

Major derivatives of the ectoderm germ layer (Part 3)

Know that the surface ectoderm will form an organism’s eventual

Gastrulation and neurulation in a chick embryo

Gastrulation and neurulation in a chick embryo (Part 1)

Neural plate - a key developmental structure that serves as the basis for the nervous system.

Notochord -a flexible rod-shaped body found in embryos of all chordates composed of mesodermal cells. In most adult organisms, the notochord remains as the nucleus pulposus of the intervertebral disc.

Gastrulation and neurulation in a chick embryo (Part 2)

Pharynx – a part of both the digestive system and also the respiratory system.

The pharynx is considered the conducting zone for both systems.

Gastrulation and neurulation in a chick embryo (Part 3)

From the gastrula stage you move into the neural stage….

Where the nervous system develops.

Gastrulation and neurulation in a chick embryo (Part 4)

Gastrulation and neurulation in a chick embryo (Part 5)

Figure 9.3 Three views of neurulation in an amphibian embryo, showing early, middle, and late neurulae in each case (Part 1)

Figure 9.4 Primary neurulation: neural tube formation in the chick embryo

For more details, see next two slides. But notice how we are focusing now exclusively on chick endoderm.

Figure 9.4 Primary neurulation: neural tube formation in the chick embryo (Part 1)

MPH (medial hinge point) – the combined Hensen's node and epiblast region that is involved in the intial bending of the neural plate during neurulation.

Figure 9.4 Primary neurulation: neural tube formation in the chick embryo (Part 2)

Anencephaly is the absence of a major portion of the brain that occurs during embryonic development. This is a cephalic disorder resulting from a neural tube defect occurring when the rostral end of the neural fails to close. This typically happebs between the 23rd and 26th day of conception.

Spina Bifida is a similar defect this time occurring at the caudal end of the neural tube. Children with this condition often times have locomotor disorders.

Figure 9.5 Neurulation in the human embryo

Figure 9.6 Expression of N- and E-cadherin adhesion proteins during neurulation in Xenopus

In the experimental protocol (B) neurulation has been altered by the injection of N-cadherin and E-cadherin. The alteration leads to the neural tube not showing the normal stages of separation from the presumptive epidermis.

Figure 9.7 Folate-binding protein in the neural folds as neural tube closure occurs

Women who are pregnant are often advised to take supplements of folic acid. The reason is due to the role that foliate binding protein exerts on neural tube closure.

Figure 9.8 Secondary neurulation in the caudal region of a 25-somite chick embryo

• In development, the rostral neural tube will begin to differentiate into more distinctive brain regions. The first differentiation is into primary vesicles (there are three of them).

• Following this vesicle development the brain continues to differentiated into the secondary vesicles (there are five of them).

Mammalian Brain Development

Figure 9.9 Early human brain development (Part 1)

Figure 9.9 Early human brain development (Part 2)

Telencephalon derived

DiencephalonDerived

Mesencephalon derived

Meten cephalon derived

Mylencephalon derived

Figure 9.10 Rhombomeres of the chick hindbrain

Rhombomeres - a transiently divided segment of the developing neural tube within the hindbrain region (a neuromere) in the area that will eventually become the rhombencephalon.

Rhombomeres appear as a series of slightly constricted swellings in the neural tube, caudal to the cephalic flexure. In human embryonic development, the rhombomeres are present by day 29.

Figure 9.11 Brain ventricle formation in zebrafish

Ventricles are a set of spaces within the brain containing cerebrospinal fluid (CSF). The ventricles are continuous with the central canal of the spinal cord. The ventricle lining consists of an epithelial membrane called ependyma, made up of ependymal cells.

The red dye is being injected to show the cavities where the cerebrospinal fluid resides.

Figure 9.11 Brain ventricle formation in zebrafish (Part 2)

The snakehead neural tube undergoes normal ventricle morphogenesis; however, the ventricles do not inflate, probably owing to impaired ion transport.

Figure 9.12 Occlusion of the neural tube allows expansion of the future brain region

A programmed and prescribed occlusion of the neural tube allows normal expansion of the brain regions.

However….

Long term, non prescribed occlusion may lead to hydrocephaly.

Note that the example here has occurred post parturition. In cases where hydrocephaly occurs earlier in development, viability of the fetus is doubtful.

Figure 9.14 Cascade of inductions initiated by the notochord in the ventral neural tube (Part 1)

Sonic Hedgehog protein concentrations (produced by the notochord) will differ depending upon distance away from the notochord (due to paracrine signaling)

Figure 9.14 Cascade of inductions initiated by the notochord in the ventral neural tube (Part 2)

In “B” we see normal development.

In “C” we see a donor notochord placed in an abnormal position near the neural tube and its guiding of a second floor plate and motor neuron region.

Shh = Sonic Hedgehog

Figure 9.15 Diagram of a motor neuron

Axonal outgrowth occurs in a neuron through the production of microtubules and F-actin at the axonal endings.

Ross Harrison discovered this basic process in 1907.

Figure 9.16 Axon growth cones

These projections form microspikes.

Figure 9.17 Myelination in the central and peripheral nervous systems

Figure 9.17 Myelination in the central and peripheral nervous systems (Part 1)

Figure 9.17 Myelination in the central and peripheral nervous systems (Part 2)

Stereotypical Human Cell Cycle

Figure 9.18 Neural stem cells in the germinal epithelium

Unlike later in life (at least for mammals), the neural stem cells display all stages of the cell cycle.

Figure 9.19 Differentiation of the walls of the neural tube

The neural tube will further differentiate into a wide array of structures that are dependent upon location.

Figure 9.20 Development of the human spinal cord

Figure 9.21 Cerebellar organization

Cerebellum - a region of the brain that plays an important role in motor control.

The cerebellum may also be involved in some cognitive functions such as attention and language, and in regulating fear and pleasure responses.

Bergmann glia - a type of glia also known as radial epithelial cells or Golgi Epithelial Cells are astrocytes in the cerebellum.

Bergmann glia express high densities of glutamate transporters that limit diffusion of the neurotransmitter glutamate during its release from synaptic terminals.

In addition to their role in early development of the cerebellum, Bergmann glia are also required for the pruning or addition of synapses.

Some studies have suggested that over pruning of dendritic spines may occur in the prodromal and early stages of schizophrenia.

Thymidine (T) is one of the four nucleosides that form the bridges with adenosine (A) in DNA. In the next experimental data, ferret cortex development is examined by using tritiated thymidine to map where the cell cycle is occurring in brain development.

Figure 9.23 Determination of cortical laminar identity in the ferret cerebrum (Part 1)

This study looks at the role of thymidine on determining cortical identity in the cerebrum in ferrets. In “A” an early pulse of thymidine is administered late in prenatal development, whereas “B” has a later pulse after parturition. The stem cells in “A” become layer 6, whereas the same cells in “B” become layers 2&3.

Figure 9.23 Determination of cortical laminar identity in the ferret cerebrum (Part 2)

The blue represents “early” neuronal precursors that have formed and been transplanted into an “old” host brain. If these cells have completed synthesis prior to transplant, they move to region 6, but if they undergo S, and G2 within the host, they will migrate to region 2&3.

Figure 9.25 Evidence of adult neural stem cells

Figure 9.27 Retention of fetal neuronal growth rate in humans

Compare the brain weights and body weights to see the significance of differential (and continued) development in human brains versus other primate brains.

Figure 9.28 Dorsal view of the human brain showing the progression of myelination (“white matter”) over the cortical surface during adolescence

What would you anticipate the effects of the increased cortical myelination be?

Figure 9.34 Retinal neurons sort out into functional layers during development

Marcello Malpighi (most work occurred in the 1660s)was an Italian physician, who identified (and named after himself) several anatomical structures. Examples include the Malpighian tubule system in the kidney and the Malpighi layer in the skin.

Figure 9.37 Layers of the human epidermis

The Malpighian layer of the skin is defined as the combination of both the stratum basale and stratum spinosum layers of the epidermis.

Copyright © 2010 Pearson Education, Inc.

The main structural features of the skin epidermis.

Melanocyte

Melanin granule

Tactile(Merkel)cellSensorynerve ending

Epidermaldendritic cell

Dermis

Dermis

Keratinocytes

Desmosomes

(b)

(a)

Stratum corneumMost superficial layer; 20–30 layers of dead cells represented only by flat membranous sacs filled with keratin. Glycolipids in extracellular space.

Stratum granulosumThree to five layers of flattened cells, organelles deteriorating; cytoplasm full of lamellated gran-ules (release lipids) and keratohyaline granules.

Stratum spinosumSeveral layers of keratinocytes unified by desmosomes. Cells contain thick bundles of intermediate filaments made of pre-keratin.

Stratum basaleDeepest epidermal layer; one row of actively mitotic stem cells; some newly formed cells become part of the more superficial layers. See occasional melanocytes and epidermal dendritic cells.

Figure 9.38 Early development of the hair follicle and hair shaft

Figure 9.38 Early development of the hair follicle and hair shaft (Part 1)

Figure 9.38 Early development of the hair follicle and hair shaft (Part 2)

Figure 9.38 Early development of the hair follicle and hair shaft (Part 3)

Figure 9.41 Facial anomalies of anhidrotic ecotodermal dysplasia, caused by mutation of an EDA gene

• Anhidrotic Ectodermal Dysplasia (also called "Christ-Siemens-Touraine Syndrome“) is a disorder resulting in the abnormal development of a variety of structures including the skin, hair, nails, teeth, and sweat glands.

• Individuals with this condition have a reduced ability to sweat (hypohidrosis) because they have fewer sweat glands than normal or their sweat glands do not function properly potentially leading to a dangerously high body temperature (hyperthermia) in certain circumstances

• Affected individuals tend to have sparse scalp and body hair (hypotrichosis). The hair is often light-coloured, brittle, and slow-growing.

• This condition is also characterized by absent teeth (hypodontia) or teeth that are malformed. The teeth that are present are frequently small and pointed.

The most common cause of Anhidrotic Ectodermal Dysplasia is due to a mutation in the EDA gene that provides instructions for ectodermal and mesodermal interaction and development.

End.