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UNIT 1 - Embryogenesis

Unit 1 - Learning Objectives

A. Primitive Germ Disc Layers and Resultant Mature Systems
B. Neural Plate Formation
C. Neural Groove Formation and Mesodermal Differentiation
D. Neural Tube Formation
E. Formation of the Brain Vesicles
F. Somite Differentiation and Limb Bud Formation
G. Neural Crest Cells
H. Differentiation of the Neural Tube
I. Maturation

A.  Primitive Germ Disc Layers and Resultant Mature Systems

Upon fertilization, the resultant ovum then divides into a two celled structure called a blastomere.  The blastomere then differentiates into a group of cells called a morula, which in turn separates into a ring of cells called a trophoblast, which is attached to the inner cell mass at an area known as the animal pole.  On the twelfth day, the morula attaches itself to the uterine wall and becomes the chorion.  Its attachment to the uterine wall is at that area defined as the animal pole.  Soon after its attachment, the inner cell mass of the chorion divides into a two-layer primitive germ disc.  The outer layer is called the ectoderm, and the inner layer the endoderm.  The outer layer of the ectoderm will break off and expand to form the amniotic cavity.  The outer layer of the endoderm will break off and expand, forming the yolk sac. The yolk sac then constricts to help form the primitive body stalk and primitive gut while also supplying the germinal blood cells of the circulatory system.

Also, during this time, a third layer of the primitive disc is forming so that, by the fifteenth day, a three layer structure called the primi–tive germ disc will be formed.  The innermost layer, or the endoderm, will become the respiratory and digestive systems of the fetus.  The middle layer, or the mesoderm, becomes the skeletal, muscular, and cardiovascular systems.  The outermost layer, or the ectoderm, will become the skin and the central nervous system in the mature fetus.

Fig. 1 – Diagram illustrating early formation of allantois and differentiation of body-stalk.

Fig. 2 – Diagram showing later stage of allantoic development with commencing construction of the yolk-sac.

Fig. 3 – Diagram showing the expansion of amnion and constriction of the yolk-sac.

Fig. 4 – Diagram illustrating later stage in the development of the unibileral cord.
Henry Gray (1821–1865).  Anatomy of the Human Body. 1918.

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Neural Plate Formation

At about the eighteenth day of embryogenesis the dorsal midline of the ectoderm differentiates more rapidly than the surrounding tissues and forms a longitudinal mass called the neural plate.

Fig.– 6, 7.

Click for Printable PDF Figures 6 and 7.

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C. Neural Groove Formation and Mesodermal Differentiation

On approximately the twentieth day, the cells at the edges of the neural plate grow faster than those in the midline of the plate thus forming the neural groove.   These cells at the edges are often called the neural folds.   Also at this time, the mesodermal layer has begun to divide into somites.   Each somite will align itself with a specific spinal nerve, differentiating into the muscular, skeletal, and cardiovascular systems served by that spinal nerve.The development of each pair of somites will be completed by the thirtieth day. 

Fig. 8 –

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D. Neural Tube Formation

Around the twenty-eighth day of embryogenesis the neural groove has deepened to the extent that the rapidly proliferating neural folds approximate each other dorsally, forming a structure called the neural tube, which soon detaches itself from the overlying skin ectoderm.  The cavity inside of the tube is known as the neurocele.   The neurocele becomes the ventricular system of the brain and the central canal of the spinal cord.   At this time, the neural tube is located ventral to the remaining ectoderm and overlying neural crest cells, dorsal to the notochord, and between two rows of mesodermal somites. Note that the hindbrain area of the developing neural tube will be first to close followed by the thoriac area then the forebrain area with the lumbar, sacral and spinal cord area last to close.

Fig. 9 –

Click for Printable PDF Figures 8 and 9.

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E. Formation of the Brain Vesicles

As neural development continues, characteristic enlargements appear in the brain end of the neural tube from the twenty-eighth to the thirtieth day.   These enlargements are called primary brain vesicles.   The names of these vesicles are important terms of reference in the analysis of the adult brain.   As development continues, subdivisions occur within two of the primary vesicles forming secondary brain vesicles.

Primary		Secondary
Prosencephalon (forebrain)		Telencephalon (end brain)
Mesencephalon (midbrain)		Diencephalon (between brain)
Rhombencephalon (hindbrain)		Metencephalon (pons)
		Myelencephalon (medulla oblongata)

In the mature C.N.S., the telencephalon and the diencephalon make up the actual “brain” portion of the system, while the mesencephalon, metencephalon, and myelencephalon make up the “brain stem” portion of the C.N.S.

Also during this time, the rate of growth at the cranial end of the neural tube greatly exceeds that of the surrounding skull.  This factor causes the neural tube to bend at the level of the mesencephalon.   This mesencephalic flexure causes the prosencephalon to be aligned perpendicularly to the longitudinal axis of the brain stem and the spinal cord.   At this point ( day 30 ) the primitive germ disc becomes the embryo and is 1/3 of an inch long. Also at this time the formation of the 31 pairs of mesodermal semites has been completed.

Fig. 10 –

Fig. 11 –

From Patricia Phelps, Neurulation.
Source: http://laxmi.nuc.ucla.edu:8888/Teachers/pphelps/Published_Trays/M102_lec1/slide_20.html

Fig. 11b –

Click for Printable PDF Figure 11b.

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F. Somite Differentiation and Limb Bud Formation

Between the thirtieth and the thirty-fifth day of embryogenesis, the mesodermal somites begin to differentiate into three layers and the formation of the ventral root for each spinal nerve will occur.  The mesodermal layers are:  the sclerotome, which will make up the skeletal system; the myotome, which will make up the muscular system; and the dermatome, which will make up the deep layer of the skin called the corium.   The corium contains blood vessels, nerve fibers, connective tissue, and the lymphatic system.   The outter epidermis which originates from the ectoderm contains the sweat gland, sebaceous glands, hair and nail follicules and is non-vascular. On the thirty-fifth day, these layers will have begun to form that part of the muscular, skeletal, and circulatory system that they are responsible for so that by the thirty-eight day the formation of the limb buds will occur.

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G. Neural Crest Cells

Also between the thirty-fifth and the thirty-eighth days of embryogenesis, the cells that are located along the junction of the detached neural tube and the overlying ectoderm, neural crest cells, migrate dorsal-laterally.   These cells become segmented into clusters which will become the dorsal root ganglia (sensory neurons) of the spinal nerves and contribute to the ganglia of cranial nerves V(Trigeminal Nerve), VII(Facial Nerve), VIII(Vestibular Cochlear Nerve), IX(Glossopharyngeal Nerve), and X(Vagus Nerve). Also during this time the dorsal root for each spinal nerve will develop.

Fig. 12 –

Formation of major brain devision,The changing brain, Neuroscience.
http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=neurosci.figgrp.1466

Fig. 13 –

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H. Differentiation of the Neural Tube

At approximately the forty-second day, the primitive cells of the neural tube differentiate forming three layers.   These three layers are the ependymal, mantle, and marginal layers.
The ependymal, or germinal layer, contains neuroectodermal cells which will line the neurocele of the neural tube in the embryo and play a similar role in the adult, lining the central canal of the spinal cord and the ventricles of the brain.   Two other types of primitive cells will migrate from this layer to form the second, or mantle layer, and the third, or marginal layer.   These cells are the neuroblasts, which become the neurons of the adult nervous system and the spongioblasts, which differentiate into the supportive neuroglia of the adult.
The mantle and marginal layers in the spinal cord and the cerebral and cerebellar cortices vary in their cellular compositions regarding the neuroblast/neurons.   In the spinal cord, the mantle area contains the cell bodies of the neurons (gray matter) and the marginal layer contains the cell processes (white matter).   In contrast to this, the mantle layer of the cerebral and cerebellar cortices contain the cell processes (white matter) and the marginal layer contains the cell bodies of the neurons (gray matter).

Fig. 14 –

Fig. 14b –

Schematic diagram of transverse section through the developing spinal cord illustrating three cell layers of neural tube, The Ventral and Dorsal Roots and the Dorsal Root Ganglion, Histogenesis of the Nervous System:The Spinal Cord and Spinal Nerves, cell and developmental biology online.
From Developmental Biology Online.

Also around the forty-second day, a functional division occurs by the formation of a groove known as the sulcus limitans.   The sulcus limitans separates the neural tube into a dorsal, or alar plate, and a ventral, or basal plate.   The alar plate becomes the sensory portion of the mature nervous system and the basal plate becomes the motor portion.   Although this division is most easily seen in the spinal cord, it should be remembered that this division occurs throughout the entire length of the neuroaxis.   The clarity of this division in the mature brain is disturbed by many factors, including the faster rate of growth at the cranial end of the tube, as well as the formation of long fibrous tracts.

Fig. 15 –

Yellow: Alar Plate
Red: Basal Plate
Neuroembryology, Neuroanatomy Lab resource Appendices.
http://isc.temple.edu/neuroanatomy/lab/embryo_new/spcd/1/

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I. Maturation

As the embryo continues to develop into a fetus and eventually into a newborn child, spinal connections will continue to develop from the foundations that have already been formed.   At the sixtieth day, the embryo is one inch long, has taken on human characteristics, and is now referred to as the fetus.   By the ninetieth day, all spinal connections are complete and the sex of the child has been determined.   At six months of age all neurons are formed (approximately 100 billion).   At this time all components of the C.N.S. have developed, however, it is not until the first year of life that full dendritic growth has occurred and the second year of life that full myelinization is achieved.

Fig. 16 –

THE PROCESS OF EMBRYOGENESIS IN CHRONOLOGICAL ORDER

Day 12	Morula attaches to the uterine wall and becomes the chorion.
Day 15	Mesodermal layer is formed completing the three-layer primitive germ disc, also at this time there is development of the yolk sac, primitive body stalk, and the amniotic cavity.
Day 18	Neural plate begins to form.
Day 20	Neural groove forms and the mesoderm begins to differentiate into somites.
Day 28	The neural tube has been completed.  Between the twenty-eighth and the thirtieth days, the brain vesicles and the mesencephalic flexure form.
Day 30	The primitive germ disc, 1/3 inches long, becomes an embryo.   All mesodermal somites have formed and they begin to differentiate into three layers.Also at this time the development of the ventral root for each spinal will occur connecting the neural tube with the mesodermal somites. This will be completed by the 35 day.
Day 35	The dorsal root ganglia form from the neural crest cells. Also between the thirty-fifth and thirty-eighth days, the dorsal root of each spinal nerve will form connecting the somites with the neural tube and the mesodermal somites will begin to differentiate into the various parts of the body.
Day 38	The mesodermal somites have differentiate to the point limb buds have formed.
Day 42	The neural tube begins to differentiate and the sulcus limitans forms, in addition the ependymal, mantle, and marginal layers begin to form.
Day 60	The embryo now takes on human characteristics, is 1 inch long, and is referred to as the fetus.
Day 90	All the spinal connections have formed and the sex of the child is determined.
6 mos.	All neurons have formed.
1 Year	All dendrites have formed.
2 Year	All myelinization has occurred.

 

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