early+embryology

SELF STUDY: EARLY EMBRYOLOGY Dr. Leon J. Martino

Embryology, in simple words, is the study of life before birth. It explains how structures in the body come to be what they are and explains their interrelationships. It holds the keys to where development may go wrong, resulting in anatomical defects, malfunction and biochemical abnormalities; all under the umbrella term congenital errors. This is an area of medical specialty, but even to non-medical persons, it can explain many common phenomena that we encounter in this field. Fertilization is the meeting of male and female germ cells. The single cell thus formed divides to form a mass which is embedded in the wall of the mother’s uterus, a process called implantation. The second week of development leads to the formation of a bilaminar (two-layered embryo). The third week of development is characterized by the formation of the three-layered embryo (gastrulation). During week’s three to eight, the embryonic period, the flat three-layer disc folds in a complex manner and forms a tube. This is followed by the development of the organs and systems of the body. Within these few weeks the embryo takes on the human form. At the end of this stage the organs systems are not necessarily in a fully functional state. The events of life before birth take place in the mother’s reproductive system. We therefore begin with a review of the female reproductive system and the formation of germ cells. Fetal development is a complex occurrence with multiple events occurring concurrently. For simplicity, in this synopsis we will study each of those events independently. Realize however that these are snapshots of isolated events among many simultaneously activities.

Readings: Langman’s Medical Embryology, 12th edition, by TW Sadler Chapters 3, 4, 5 & 6

A self-study of early embryology: fertilization, implantation, gastrulation and the embryonic period

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Female Reproductive Anatomy

The female reproductive organs consist of the vagina, the uterus, the fallopian tube and the ovary. The ovary (1) is the egg-producing organ. The uterine tube (also called the oviduct or fallopian tube) carries the egg to the uterus. The end of the tube facing the ovary has a number of fimbriae (singular: fimbria). The fimbriae ‘sweep’ the surface of the ovary and pick up the oӧcyte. The next part is called infundibulum (2), followed by a dilated portion, the ampulla (3). Next to it is a narrow portion, the isthmus (4). The last part is within the thick wall of the uterus, and is called the intramural part (5). In the uterus, the great thickness of the wall is largely due to muscle. The lining is formed by epithelium and supporting connective tissue. This lining is the endometrium (metros = uterus). The embryo, when it reaches the uterus, is embedded in the endometrium.

Anatomically, the top of the uterus is the fundus, followed by the body and the narrow cervix (= ‘neck’). The vagina is the birth passage with a muscular wall. Germ cells (“Gametes”). The male and female germ cells are quite distinctive. In biological terminology we use the common term ‘gamete’ for both male and female germ cells. The male gamete is a spermatozoon, “sperm” for short. The female gamete is an ovum. It is often called the ‘egg’. It is one of the largest cells in the body with a diameter of over 100 µm. Henceforth we avoid the term germ cell and use the term gamete. Gametes have 23 single chromosomes, as opposed to 23 pairs in other cells. The reduction in the number is due to meiotic cell division (meiosis).

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Meiosis Meiosis is a process involving two divisions. The first division (Meiosis I) has two features of importance. 1. Replicated homologous chromosomes (maternal and paternal chromosomes of the same pair) are arranged next to each other as if in an embrace. Their corresponding arms cross at some points. At the points of crossing, they exchange equivalent parts of their lengths – that is, they exchange genetic material. This process is called crossing over. When they separate, each of the homologous chromosomes has a mixture of paternal and maternal genetic material. The exact lengths and therefore the proportion of material exchanged differs from pair to pair, and even for the same pair in different divisions. This means that each chromosome, after crossing over, is a unique mix of maternal and paternal genes. 2. The chromatids do not separate during meiosis I. Instead, replicated chromosomes move to opposite ends of the cell, one member of each pair moving to one end, the other member to the other end.

At the end of meiosis I, there are two daughter cells, each with a haploid number (1N) of replicated chromosomes In meiosis II, the two cells divide. The chromatids of the single replicated chromosomes separate. This may sound like mitosis, but remember that it is a single replicated chromosome that splits, and that the single chromosome has a mixture of genetic material from the two homologous chromosomes. . At the end of meiosis I I, there are tour daughter cells, each with a single haploid (1N) chromosome To summarize: Meiosis comprises two divisions. The end products are four haploid cells. The unpaired chromosomes differ from each other in the mixture of parental genetic material.

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In the male, some cells are ‘set aside’ in the testis for this purpose of division. These are called spermatogonia. After puberty, these cells divide by mitosis, some of the daughter cells undergo meiosis while others stay as spermatogonia to maintain the ‘pool’. This process continues throughout life. Thus, a male can produce sperms indefinitely. In contrast, the cells set aside in the female form a fixed pool. They begin meiotic division even before female is born. But this division is not completed– a female is born with a fixed number of such cells, in a state of suspended meiosis. After puberty, approximately every month a few of these cells (oӧcytes) mature and proceed with meiosis. Normally only one of these is released for fertilization by a sperm. This cell completes the second stage of meiosis after the entry of the sperm. Therefore we say that it is an oocyte that is released for fertilization.

An oӧcyte has a non-cellular covering in addition to the cell membrane. This covering is the zona pellucida (the ‘clear zone’, so called because of its translucent pink appearance under the microscope. Outside the zona pellucida is a layer of ‘supporting cells’. One layer of these supporting cells surrounds the oӧcyte even as it is released. This layer appears like a radiating crown in a section and is called the corona radiata. During each stage of meiosis two cells are produced. However, one of these receives almost all cytoplasm, is large and capable of being fertilized. The other small cell remains within the zona pellucida and is called the polar body. There can be two or three polar bodies. Understand the relationship of the oӧcyte with the zona pellucida and the corona radiate.

Note : This discussion is limited to understanding the oӧcyte, its haploid nature and fertilization.

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Fertilization – the beginning Sperms are deposited deep into the vagina just below the opening of the uterus. Though sperms are motile, their speed is far from adequate to carry them to the oöcyte in time. The oöcyte has a ‘long’ journey along the length of the fallopian tube. Other factors operate in transporting sperms to the oöcyte, probably movements in the smooth muscle wall of the uterus.

The nucleus and the scanty cytoplasm of the sperm are all in the ‘head’ of the sperm. At the very tip is a tiny structure called acrosome. (akron = highest point, soma = body). The acrosome (shown as a green line in the adjacent image) has enzymes which allow it to penetrate the zona pellucida. In the female reproductive tract, the acrosome undergoes changes which allow the acrosomal enzymes to be exposed. This is called capacitation. Fertilization takes place most commonly in the dilated part of the uterine tube, the ampulla. When a sperm comes in contact with the zona pellucida, the acrosomal enzymes act on it and facilitate entry of the sperm.

The cell membranes of the sperm and the oöcyte fuse and the sperm enters the oöcyte. The zona pellucida undergoes molecular changes which prevent the entry of any other sperm. The fertilized oöcyte immediately completes meiosis II and the second polar body is released.

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The fertilized cell (also called the zygote) now has two haploid nuclei. These are called the male and female pronuclei. The pronuclei enlarge as the DNA within them is replicated. The nuclear envelopes disappear and the 23 pairs of replicated chromosomes (diploid or 2N) are lined up for separation. Depending upon the sex chromosome present in the sperm, the sex of the individual is determined at fertilization. The oöcyte can only have an X chromosome, the sperm can have either X or Y.

The embryo becomes multicellular. The zygote now undergoes the first series of mitotic divisions increasing the number of cells. These divisions, called cleavage, are preceded by DNA replication, but the cytoplasm does not increase. Remember: the oöcyte has an enormous amount if cytoplasm. Thus, gradually the amount of cytoplasm in the zygote is distributed among the cells. The ‘nucleo-cytoplasmic ratio’ comes down to ‘normal’. The cells, which become smaller with each cleavage division, are known as blastomeres. After a number of cleavages the blastomeres maximize their contact forming a compact clump of loose cells.

All this while the embryo is being propelled through the uterine tube towards the uterus. After the 16th cell stage, at approximately 3 days, the cell mass resembles a mulberry and is called morula. The zona pellucida is still intact when it enters the uterus. This is important, because it prevents the attachment (implantation) of the embryo to the wall of the uterine tube.

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Blastocyst formation (Days 8-12)

The cells of the morula are initially a loose clump. They gradually get compacted, and after a while show two distinct regions, an inner cell mass and an outer cell mass, a process known as compaction. The inner cell mass eventually develops into the embryo proper. The outer mass is called the trophoblast (“nutrition forming”). It is somewhat like a shell around the inner cell mass. It gives rise to the placenta which is the link between the embryo/fetus and the mother.

About the time the embryo enters the uterine cavity, fluid seeps in through the zona pellucida into the intercellular spaces of the inner cell mass forming a cavity called the blastocele within the embryo. At this stage the embryo is called the blastocyst, consisting of the compact inner cell mass, the embryoblast , the outer cell mass, the trophoblast, and the blastocyst cavity.

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Implantation and Formation of the Placenta (for the purposes of this narrative the changes in the embryoblast will not be discussed until later in the discussion)

The wall of the uterus has an outer layer of muscle and an inner layer, the endometrium which is comprised of connective tissue and epithelium. The endometrium has a rich vascular supply. Implantation begins at the end of the first week and is complete in the second week.

The cells of the trophoblast flatten and form the epithelial wall of the blastocyst. The zona pellucida degenerates, allowing implantation to begin. The trophoblast has an invasive property. When it comes in contact with the endometrium, it burrows into the endometrium by loosening the epithelium and destroying the connective tissue. Gradually the embryo will sink deeper into the endometrium until finally it is covered by the uterine epithelium, completing this process called implantation.

As the blastocyst embeds in the endometrial stroma, the trophoblast differentiates into two layers: an inner layer of mononucleated cells, the cytotrophoblast and an outer multinucleated zone without distinct boundaries, the syncytiotrophoblast. These cells are collectively termed the syncytium (a multinucleated mass of cytoplasm that is not separated into cells).

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As the blastocyst penetrates the endometrium, maternal capillaries within the endometrium enlarge and are become filled with maternal blood forming sinusoids. The blastocyst is more deeply embedded in the endometrium and the penetration defect in the surface epithelium is closed by a fibrin coagulum (plug). At the embryonic pole vacuoles appear in the synctium. When these vacuoles fuse they form large lacunae, called trophoblastic lacunae.

The cells of the syncytiotrophoblast penetrate deeper into the endometrial stroma and erode the endothelial lining of the maternal sinusoids. The trophoblastic lacunae become continuous with the material sinusoids and maternal blood enters the synctium’s lacunae system. The trophoblast is now characterized by villous sinusoids.

Further differentiation of the placental villi will be covered later in this synopsis.

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Changes in the inner cell mass (embryoblast). Recall, the cells of the embryonic mass are initially a loose clump, called a blastocyst. During the second week of development, they gradually get compacted, and after a while show two distinct regions, an inner cell mass and an outer cell mass.

The inner cell mass eventually develops into the embryo proper, thus is termed the embryoblast.

Formation of the Bilaminar Embryoblast

By the end of the second week the embryonic development, cells of the inner cell mass or embryoblast differentiate into two layers – a layer of tall columnar cells, the epiblast and one of smaller cuboidal cells, the hypoblast. The embryo is now a bi-laminar disc (bi = two, lamina = plate or layer) composed of the epiblast above and the hypoblast below. Concurrently, a small cavity appears in the embryoblast. This is the amniotic cavity. The cells lining this cavity, facing the cytotrophoblast are called amnioblasts. This complex is termed the amnion.

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At the time the bilaminar disc is forming, cells from the hypoblast migrate and form a thin layer of cells that expands towards the cytotrophoblast. This is the exocoelomic membrane, known as Heuser’s membrane. This layer forms the lining of the exocoelomic cavity which will later become the primitive yolk sac.

At the end of the second week the embryo is a bi-laminar disc, surrounded by above and below by cavities. The junction between the cavities is compressed. This is the embryo proper (epiblast and hypoblast). Think of this arrangement as two soap bubbles touching each other. The junction between the bubbles is flat, the bilaminar embryoblast. Above (top bubble) is the anmion and amniotic cavity; below (lower bubble) is the primitive yolk sac.

Heuser’s membrane continues to expand. It approaches the inner surface of the cytotrophoblast, enlarging the exocoelomic cavity, which now called the primitive (primary) yolk sac. The amniotic cavity is situated above around the cells of the epiblast and is lined by the surrounding amnioblasts.

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Concurrent with the formation of the primitive yolk sac, some of the migrating hypoblast cells trans-differentiate into mesenchymal cells that fill the space between Heuser's membrane and the cytotrophoblast, forming the extraembryonic mesoderm.

As development progresses, small lacunae begin to form within the extraembryonic mesoderm. These lacunae expand and separate Heuser’s membrane from the cytotrophoblast. This cavity enlargers and forms the extraembryonic cavity or coelom. It divides the extraembryonic mesoderm into two layers: extraembryonic splanchnic mesoderm, which lies adjacent to Heuser's membrane around the outside of the primitive yolk sac, and extraembryonic somatic mesoderm, which lies adjacent to the cytotrophoblast layer of the embryo.

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With time the yolk sac shrinks away from the cytotrophoblast and giving rise to what is termed the secondary (definitive) yolk sac, lined by hypoblast cells and extraembryonic splanchnic mesoderm. As the amnion increases in size, it begins to expand and form a cleft between the amniotic cavity and the yolk sac. This space is the intra-embryonic cavity, as it will eventually be the inside of the embryo. The extra-embryonic cavity and the intra-embryonic cavity are continuous at this stage and freely communicate.

The extra-embryonic cavity will later become the chorionic cavity. The extraembryonic somatic mesoderm lining the inside of the cytotrophoblast becomes the chorionic plate. Condensations of the extraembryonic mesoderm attach the developing embryo to the chorionic plate and ultimately the placenta. With the development of blood vessels this becomes the umbilical stalk.

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Development of the placenta villi: The following is an overview of aspects of the basic fetal subunit of the placenta - the placental villi. In early placentation, the villi proceeds through a similar initial program of development. In later placentation, villi morphologically differentiate into a range of villi in accordance with functional changes reflecting their specialization. The major initial contribution is from the trophoblast shell that surrounds the fetus and later by the development of extraembryonic mesoderm and blood vessel differentiation.

Finally, at about the end of the third week, cytotrophoblastic cells penetrate progressively into overlying syncytium until they reach the maternal endometrium. There they establish a thin outer cytotrophoblastic shell which attaches to the maternal endometrial tissue. Spiral arteries from the material vessels form connections between the maternal capillaries in the endometrium and the intervillous spaces of the syncytium. The intervillous space is lined by a thin layer of syncytiotrophoblast cells and the cytotrophoblastic cells of the villi. The outer shell of cytotrophoblast becomes the placenta barrier.

The cells of the cytotrophoblast proliferate locally and penetrate into the syncytiotrophoblast, forming cellular columns surrounded by the syncytium. These cellular columns with the syncytical (syncytiotrophoblast) covering are known as the primary villi of the placenta.

During further placenta development extraembryonic mesodermal from the chorionic plate penetrate the core of the primary villi and grow towards the decidua of the endometrium (secondary villi). Some mesodermal cells in the core of the villus differentiate into blood vessels and cells, forming the villous capillary system known as tertiary or definitive placenta villi.

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Gastrulation: Formation of Ectoderm, Mesoderm and Endoderm

The primitive streak The most characteristic event during the third week is Gastrulation. Gastrulation begins with the formation of the primitive streak on the surface of the epiblast. Looking down at the epiblast, the primitive streak is seen as a faint midline ridge on the epiblast side of the embryo. At one end the streak has a knot-like swelling, the primitive node. This establishes the axes of the embryo. The part of the embryo beyond the node will be the cephalic (head end) of the embryo. The other end, near the narrow tip of the streak is the caudal (tail end) of the embryo. The primitive streak and node are temporary structures that play vital role in establishing the laterality (left and right sidedness) of the embryo. Fibroblast growth factor 8 (FGF8), secreted by the primitive node establishes the expression of Nodal, a secretory protein that belongs to the Transforming Growth Factor (TGF-beta) superfamily. Nodal initiates and maintains the primitive streak. Once the streak is formed the nodal protein accumulates on the left side of the embryo and up regulates a number of genes (Lefty and PITX1) which are responsible for the establishment of left sidedness of the embryo. Importantly, the neurotransmitter serotonin (5HT) plays a role in the signaling cascade that establishes laterality. 5HT is concentrated on the left side and is upstream from FGF8. 5HT is broken down by its metabolizing enzyme monoamine oxidase MAO) on the right side.

Formation of the germ layers The head end of the embryo begins to differentiate first. The sequence of development is often described a cephalocaudal. (From head to tail. kephale = head, cauda = tail).

The cells of the epiblast proliferate and migrate towards the primitive streak. Upon arrival at the region of the primitive streak they enlarge, detach from the epiblast and slip beneath the primitive streak. This inward movement of the modified epiblast cells is known as invagination. Migration and specification of the cells of the epiblast is controlled by fibroblast growth factor 8 (FGF8).

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Once the cells have invaginated into the primitive streak, they sink deeper inside where they displace the hypoblast laterally, giving rise to what will become the endoderm. The endoderm gives rise to the epithelium of the digestive system and respiratory system.

With the formation of the endoderm, more cells proliferate and pour between the epiblast and the newly formed endoderm giving rise to the intra-embryonic mesoderm. The intra-embryonic mesoderm gives rise to mesenchyme (connective tissue). Cells remaining in the epiblast then form the ectoderm. Ectoderm will give rise to epidermis and related structures as well as portions of the brain and nervous system.

Expansion of the Mesoderm Mesodermal cells pouring into the primitive streak spread out laterally and towards the head end in a definite pattern. Migrating laterally and cranially the mesoderm fills the “gap” between the ectoderm (epiblast) and endoderm (hypoblast).

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Formation of the Notochordal Process

Prechordal plate Prenotochordal cells are a group of specialized cells that migrate through the primitive node located at the cranial end of the primitive streak, and give rise to the prechordal plate and notochordal process. The cells migrating most anteriorly form the prechordal plate, whereas the ones migrating most posteriorly form the notochordal process. Prechordal plate cells participate in the formation of the endodermal layer of the mouth. In this area the ectoderm is attached directly to the endoderm without intervening mesoderm. This area is known as the oropharyngeal membrane, and it will become the mouth.

Notochordal process Additional prenotochordal cells invaginating into the primitive node move forward cranially in the midline behind the prechordal. This layer forms a tubular mass between the epiblast and hypoblast known as the notochordal process. The prenotochordal cells of the notochordal process become intercalated in the developing hypoblast (future endoderm): (see below).This will become the future notochord.

As the intra-embryonic mesoderm spreads out from the primitive streak, the whole embryo increases in size and the primitive streak becomes relatively smaller. When the process of gastrulation is complete the primitive streak disappears.

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Formation of the Notochord The notochord is vital link in the chain of events. In vertebrates, it exists as a continuous column only during embryonic life. Parts of it persist as normal structures in the vertebral column. Embryologically the notochord is of great importance. After the primitive streak disappears, it is the definitive ‘axis’ of the body. Moreover, it ‘directs’ a strip of ectoderm to develop into the neural plate, the precursor of the nervous system.

The notochordal process becomes intercalated within the developing endoderm. As more of the hypoblast is replaced by endodermal cells, the tubular notochordal process fuses with the endodermal layer beneath it and is morphologically modified from a tube shape to a flattened plate shape, the notochordal plate.

The notochordal plate then detaches from the endoderm and rolls into a solid mass of cells, which migrates into the intraembryonic mesoderm. This mass of cells is now the “definitive” notochord.

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Neurulation (Formation of the neural tube) Neurulation is the process of conversion of the neural plate to form the neural tube.

A band of ectoderm across the midline becomes thickened to form a plate called the neural plate. Since it forms nervous tissue and it is a part of ectoderm, it is called neuroectoderm. As mentioned prior, the notochord ‘induces’ the ectoderm in the formation and subsequent modification of the neural plate.

By the third week the lateral edges of the neural plate become elevated to form neural folds and the depressed mid-region becomes the neural groove.

Gradually the neural folds approach each other turning the neural groove into a deeper tube-like depression.

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Soon, the groove closes as the neural folds come together to form the primitive neural tube. The edges of the neural folds fuse. The fusion begins in the middle of the embryo and proceeds towards both ends. As closure progresses there are cranial and caudal neuropores.

The neural tube ‘sinks’ in the surrounding mesoderm, losing its contact with the ectoderm on the surface. The surface ectoderm becomes continuous across the midline when the neural tube closes. The neural tube forms the brain and the spinal cord

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Neural Crest The main derivatives of the neural crest are the ganglia of peripheral nervous system and sensory nerve fibers. The neural crest also gives rise to a number of non-neural structures. These will be referred to as we come across them.

Neural crest cells are specified at the border of the neural plate. During neurulation, the borders the neural folds converge at the dorsal midline to form the neural tube. The lips of the groove separate out as a long column on either side of the tube. These form the neural crest, which also sinks deeper along with the tube.

Subsequently, neural crest cells from the roof plate of the neural tube undergo an epithelial to mesenchymal transition, delaminating from the neuroepithelium.

These cell clusters detach from the neurotube and migrate through the developing embryo where they differentiate into varied cell types.

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The spread of the mesoderm. Mesodermal cells spread out laterally and towards the head end in a definite pattern. They follow arc-like courses between the ectoderm and the endoderm. When the spread is complete the mesoderm is seen mainly as three columns along the length of the embryo.

Cells from the head end of the streak keep close to the midline, spreading in ‘narrow’ arcs. In the final picture, this column of mesoderm is close to the midline or the axis. This column is the paraxial (“para + axial”) mesoderm. Cells from the middle part of the streak occupy a position just lateral to the paraxial mesoderm. They form a column called the intermediate mesoderm. Cells from the tail end (caudal part) of the streak spread out in broad arcs and finally form a plate close to the lateral boundaries of the embryo. This part of the mesoderm is called the lateral plate mesoderm. Some cells from the region near the head end are special – they reside at the cranial tip of the lateral plate and indeed form a horseshoe-like band at the head end of the embryo (heart field). These cells form the mesodermal precursor of the heart. This will be further discussed with the development of the heart.

The spread of the mesoderm to the head end, however, leaves a small area where ectoderm and endoderm are in contact. This is called the oropharyngeal membrane. At the tail end there is a similar small area where ectoderm and endoderm are in contact with no mesoderm in between. This is called the cloacal membrane.

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Divisions of the mesoderm. The three divisions are shown as above are completely separate from each other with gaps in between. This is for the sake of clarity in the picture! However, the fates of these divisions are quite specific.

At the beginning of the third week, the column of paraxial mesoderm begins to form segmental masses called somites. Somites appear as two columns of mesodermal masses adjacent to the notochord. The formation of paraxial mesoderm into somites begins at the head end and progresses caudally. The sequence of development is cephalocaudal (‘head to tail’). As more and more somites appear towards the tail end, the cephalic somites begin develop into various structures. These segments (“slices” or “blocks”) are the basis of the structure of the vertebrae and dermis, connective tissue & musculature of the body wall and limbs. The intermediate mesoderm is mainly involved in the formation of the urinary and reproductive systems.

The lateral plate mesoderm will split into two parts. One layer of mesoderm follows the ectoderm (somatic mesoderm) and the other (splanchnic mesoderm) migrates towards the endoderm. During further development the somatic mesoderm will give rise to body wall connective tissue structures, while the splanchnic mesoderm will give rise to the mesenteries of the gut tube.

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Folding of the embryo The flat trilaminar embryonic disk becomes more a cylindrical embryo due to the longitudinal and transverse folding that occurs as a result of embryonic growth. The folding occur simultaneously and are not separate sequential events. The longitudinal fold or cephalic-caudal fold creates a cranial and caudal region of the embryo. This is caused by the rapidly developing neural tube. Essentially the head and tail of the embryo fold towards each other.

Transverse fold

At three weeks the embryo is a slightly curved flattened disk. The lateral plate mesoderm has separated into somatic (following the ectoderm) and splanchnic (around the yolk sac) mesoderm. The space or cavity between the two layers is called the intraembryonic coelomic cavity. .

The transverse fold (called flexion) produces right and left lateral folds. Flexion, a process of curving, transforms the embryo into a sort of "tube". The left and right sides of the embryo curve and migrate toward the mid-line.

The intraembryonic coelom communicates freely with the extra-embryonic coelom.

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The combination of somatic mesoderm and adjacent ectoderm, (termed the somatopleure) along with the overlying amniotic membranes and cavity, folds toward the midline, rolling the edges of the embryonic disk ventrally to form a cylindrical embryo.

As the sides come together, they pinch off the yolk sac drawing the splanchnopleure (the combination of the splanchnic mesoderm and the adjacent endoderm) into the intraembryonic coelom.

As lateral and ventral body walls form, part of the yolk sac is incorporated into the embryo as the midgut; simultaneously, the connection of the midgut with the yolk sac is reduced to a yolk stalk or vitelline duct (the connection between the yolk sac and the primary intestinal loop of the midgut).

With complete closure the intra-embryonic coelom or cavity is closed off. The mid-gut is incorporated into the intra-embryonic coelom surrounded by splanchnic mesoderm. The outer periphery of the intra-embryonic cavity is lined by somatic mesoderm.

The amniotic cavity enlarges and obliterates the extra-embryonic coelom.

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SUMMARY

Do not try to memorize this list! Understand the explanations from the discussions above.

Concepts and facts fundamental to study of the embryology of the human structure:

Haploid gametes unite: beginning of a new human being.

Each gamete has a unique mixture of genetic material from two parents.

Fertilization most commonly takes place in the ampulla of the uterine tube.

Zona pellucida prevents the entry of more than one sperm. It also prevents implantation as long as it is intact.

The early embryonic cell mass specializes into two groups: the inner cell mass ‐ embryo proper,

and the outer cell mass ‐ trophoblast (significant contribution to the interface between mother and fetus).

Trophoblastic tissue is invasive. This property is responsible for implantation

The inner cell mass (embryo proper) divides into two layers (bilaminar embryo), the epiblast above and the hypoblast below.

The epiblast layer of the bilaminar embryo gives rise to the trilaminar embryo. The

hypoblast, for the most part disappears. The primitive streak, a thickening of the epiblast, is the site where epiblast cells migrate to form

the three layers of the trilaminar embryo.

The trilaminar embryo is comprised of three basic layers of the embryonic body: ectoderm, mesoderm and endoderm.

The notochord, which arises from the tip of the streak, defines the axis and symmetry of the embryo.

It also induces the formation of the neural tube.

The neural tube is an ectodermal structure.

The mesodermal layer has three subdivisions – paraxial, intermediate and lateral plate.

Paraxial mesoderm forms ‘blocks’ stretching from the head end to the tail end of the embryo. These blocks, called somites, establish the structural pattern of the body wall (dermis, connective tissue and muscle).

Lateral plate mesoderm splits into somatic and splanchnic mesoderm Somatic mesoderm forms body wall connective tissue structures, while splanchnic mesoderm will give

rise to the mesenteries of the gut tube. The flat trilaminar embryo undergoes complex changes in form, with the head and tail ends (celphalic‐

caudal fold) and the sides (transverse fold) folding up making it a three‐dimensional tubular structure.

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