Early Embryology & CNS Development

• Early Embryology and Development of the Central Nervous System (CNS) are being examined.
• Dr. Michele Barbeau can be reached at [email protected]; her office is MSB 489.

Early Embryology & CNS Development Objectives

• Aims to describe where fertilization and implantation occur, the fate of the inner cell mass and trophoblast during the second week, define gastrulation, and identify the 3 germ layers with 2 anatomical structures from each.
• Objectives include detailing neural tube formation, summarizing developmental events week 1-4, differentiating structural/functional divisions of the nervous system, naming adult brain parts, describing neural epithelial cell differentiation, ventricles locations, and gray/white matter distribution in the brain.

Fertilization Summary

• Fertilization includes the sperm entering the oocyte, forming male and female pronuclei, which fuse to form a single-cell zygote containing genetic material from both parents.
• Cleavage begins approximately 24-30 hours post-fertilization, leading to the formation of two cells.
• The zygote undergoes several cell divisions as it travels towards the uterus.
• After about 3-4 days, the zygote developed into a morula.

Morula and Blastocyst Formation

• Compaction occurs around day 3, where cells adhere tightly to each other, and the morula becomes more compact
• A blastocyst forms, consisting of an inner cell mass (embryoblast) and an outer layer (trophoblast), as well as the blastocyst cavity.

Passage of Zygote

• Fertilization happens in the fallopian tube and takes around 1 day to form a 2-cell zygote.
• The zygote travels to the uterus as it divides and multiplies into two cells, then eight cells, and then the morula.
• The blastocyst then undergoes implantation in the uterus.

Risk of Birth Defects

• Greatest risk of birth defects arise during the embryonic period from 3-8 weeks of gestation.
• These risks are decreased in the fetal period, which runs from 8-38 weeks of gestation.

Formation of the Bilaminar Embryo

• The inner cell mass differentiates into two layers: the epiblast and the hypoblast.
• The epiblast and hypoblast form a flat disc known as the bilaminar embryonic disc, which forms during the 2nd week.
• Amnion and amniotic sac cavity is developed by the 11th day.

Twins

• Monozygotic twins come from one zygote that splits, resulting in identical twins.
• Dizygotic twins form from two separate eggs being fertilized.

Formation Of The Tri-Laminar Embryo

• During week 3, the bilaminar embryonic disc transforms into a trilaminar embryo with three germ layers.
• The embryonic disc forms a primitive streak at 14-15 days.
• By day 16, the process of gastrulation forms the three germ layers known as the definitive endoderm, mesoderm, and ectoderm.

Formation Of The Notochord

• Formation of the notochord happens around week 3
• The notochord forms intervertebral discs but is not part of the adult CNS

Formation Of The Neural Tube

• Neurulation is the process of forming the neural tube.
• At 17 days, a neural plate appears.
• Around day 19 the surface ectoderm creates a neural fold, with neural grooves, which leads to the paraxial mesoderm.
• At 20 days, the somite becomes covered by ectoderm.
• By 22 days, the neural tube separates from the surface ectoderm.

Neural Tube Closure

• The 3rd and 4th weeks are important for the development of neurulation.
• Neural tube closure occurs so that the anterior neuropore closes around day 25, and the posterior neuropore closes around day 28.

Spinal Bifida

• Spinal Bifida arises when the caudal neural tube fails to close resulting in a permanently disabling birth defect.
• Folic acid can reduce the incidence of this, and is generally ranged in severity from Occulta to most severe meningomyelocele.

Anencephaly

• Anencephaly is where the rostral neuropore fails to close.
• Most telencephalic structures are underdeveloped, the brain stem and its functions are preserved, and patients generally don't survive.

Folding Of The Embryo

• The embryo goes through folding to become more cylindrical.

End of 4th Week

• By the end of the 4th week, both the anterior and posterior neuropores have been fully closed.

Cylindrical Body Plan

• The endoderm forms the gut lining, the ectoderm leads to the brain and epidermis and the coelom becomes the serosa.

Derivatives of Germ Layers

• The ectoderm leads to the spinal cord and skin
• The endoderm leads to the digestive tube's lining
• The mesoderm derivates into the bones, kidney, and muscle

Embryonic Layer Derivatives

• The ectorderm forms epidermis, hair, nails, glands, brain, and spinal cord.
• Sensory nerve cells, pigment cells, portions of skeleton and blood vessels form in the mesoderm.
• The endoderm derivates to form the digestive and repiratory tract.

Summary of Events

• The first week contains fertilization.
• The second week contains implantation and formation of the bilaminar embryo.
• The third week contains formation of the trilaminar embryo, and the beginning of neurulation.
• The fourth week closes the neural tube, and is responsible for the creation of a cylindrical body plan.

Anatomical Division of the Nervous System

• The Central nervous system (CNS) is composed of the brain, and the spinal cord; its cells are composed of gray and white matter.
• The Peripheral nervous system (PNS) is composed of spinal and cranial nerves, ganglia and sympathetic, and parasympathetic nerves.
• Nerve cell bodies are formed of gray matter, while axons are formed of white matter.

Functional Divison of the Nervous System

• Sensory input comes from inside and outside of the body as afferent transmissions to the CNS.
• Integrative functions process and interpret sensory input to make decisions and reactions.
• Motor input is the efferent transmission from the CNS, which responds based on a sensory input to engage effector organs like muscles and glands.

Dilation of the Neural Tube

• Forebrain is also known as the Prosencephalon.
• Midbrain is also known as the Mesencephalon.
• Hindbrain is also known as the Rhombencephalon.
• The Cephalic flexure happens at the midbrain, while the cervical flexure happens at the junction of the hindbrain and spinal cord.

Differentiation of Neuroepithelial Cells

• Neuroepithelial cells in the neural tube differentiate into various cell types.
• Sensory neurons come from the neural crest cells that are produced.
• Neuroblasts are produced in the developing brain.
• The alar plate is where the interneurons are.
• The basal plate is where the motor neurons are.
• The sulcus limitans runs down the inner brain, and also divides the motor neurons.
• The Mantle zone contains grey matter while the marginal zone contains white matter.

Origin of Adult Brain Structure Summaries

• The brain is 28 days is folded into 2 and known as the cephalic and cervical flexors.
• The prosencephalon expands and differentiates into the telencephalon and the diencephalon.
• The rhombencephalon turns into the metencephalon and myencephalon
• By 49 days, the olfactory lobe appears and turns into the cerebellum, medulla, and pons. The cervical and lumbar enlargements happen around the entrance of the upper and lower limbs.
• The cerebrum grows extensive and posterior and takes a c-shape that covers it.

Adult Ventricular System

• Lateral ventricles are in each hemisphere.
• A third ventricle in is in the diencephalon.
• A fourth ventricle above the pons and medulla and below the cerebellum
• The spinal single canal remains relatively unchanged from the neural tube
• The cerebrospinal fluid (CSF) is made in the ventricles and flows out around the brain stem.

Spinal Cord Comparisons

• In an adult spinal cord, the basal plate neurons are restricted to ventral and lateral horns while alar plate neurons form the remainder of the spinal cord, the dorsal horn.

Medulla Oblongata Comparisons

• Basal plate nuetrons form motor nuclei in the medulla
• Alar Plate neurons form sensory nuclei in the dorsal part of the medulla

Mesencephalon Comparisons

• Basal plate neurons form a few motor nuclei near the midbrain

Forebrain Comparisons

• Existance of dorsal and basal plates in embyronic diencephalon, forming hypothalamus and thalamus, respectively.