Developmental stages of spermatogenesis http://www.embryology.ch/anglais/cgametogen/spermato03.html

QuizQuiz 12

In the course of spermatogenesis the germ cells move towards the lumen as they mature. The following developmental stages are thereby passed through:

Quiz 13

Quiz 14

A-spermatogonium B-spermatogonium Primary spermatocyte (= spermatocyte order I) Secondary spermatocyte (= spermatocyte order II) Spermatid Sperm cell (= spermatozoon)

The spermatogenesis can be subdivided into two successive sections: The first comprises the cells from the spermatogonium up to and including the

secondary spermatocyte and is termed spermatocytogenesis. The second one comprises the differentiation/maturation of the sperm cell, starting

with the spermatid phase and is termed spermiogenesis (or spermiohistogenesis). The temporal course of spermatogenesis

The approximate 64 day cycle of the spermatogenesis can be subdivided into four phases that last differing lengths of time:

Mitosis of the spermatogonia

16 days Up to the primary spermatocytes

First meiosis 24 days For the division of the primary spermatocytes to form secondary spermatocytes

Second meiosis A few hours

For engendering the spermatids

Spermiogenesis 24 days Up to the completed sperm cells

Total ~64 days

Fig. 12 - The spermatogenesis generations Legend

Fig. 12The stem cell population of the germinal cells lies on the basal lamina of the convoluted seminiferous tubules. These are Type A spermatogonia. These cells undergo mitosis: one of the daughter cells renew the stock of type A spermatogonia, the other becomes a type B spermatogonia.These divide and their daughter cells migrate towards the lumen. In roughly 64 days they differentiate themselves thereby into sperm cells up to the outer surface of the epithelium (one should note that in these cellular divisions, the separation of the cytoplasm is not complete. Whole networks of connected cells arise. So, for example in the last generation, the spermatids, far more cells are bound to each other than as shown here).

SpermatocytogenesisAmong the spermatogonia (all in all, over 1 billion in both testicles) that form the basal layer of the germinal epithelium, several types can be distinguished: certain type A cells are seen as spermatogonia that divide mitotically and reproduce themselves (homonymous division), whereby the spermatogonia population is maintained.The beginning of spermatogenesis is introduced through the so-called heteronymous division, in which the daughter cells (second group of type A cells) remain bound together by thin bridges of cytoplasm. Through the preservation of these cytoplasmic connections, spermatogonia are inducted into the spermatogenesis process.After a further mitotic division type B spermatogonia are engendered that also divide themselves mitotically into primary spermatocytes (I). The freshly created primary spermatocytes (I) now enter into the first meiosis. They then go immediately into the S phase (that is, into the preleptotene meiosis), double their internal DNA,

CommentaryIn the heteronymous division the cytoplasmic division is not completed; the daughter cells stay bound together through thin cytoplasmic bridges. Also in the subsequent meiosis the cytoplasmic division is

leave the basal compartment and reach the special milieu of the luminal compartment. Following the S phase, these cells attain the complex stage of the prophase of the meiosis and become thereby noticeably visible with a light microscope.

This prophase, which lasts 24 days, can be divided into five sections: Leptotene Zygotene Pachytene Diplotene Diakinesis

incomplete, so that from one spermatogonium a network of daughter cells arises that doubles in size in each generation. The forming of such networks assures that all of the processes in each generation occur in step with each other.

In the prophase in every germ cell a new combination of maternal and paternal genetic material occurs. After the long prophase follow the metaphase, anaphase and telophase that take much less time. One primary spermatocyte yields two secondary spermatocytes.

Commentaryabout meiosis

The secondary spermatocytes go directly into the second meiosis, out of which the spermatids emerge. Since in the secondary spermatocytes neither DNA reduplication nor a recombination of the genetic material occurs, the second meiosis can take place quickly. It lasts only around five hours and for that reason secondary spermatocytes are rather seldom seen in a histological section. Through the division of the chromatids of a secondary spermatocyte, two haploid spermatids arise that contain only half the original DNA content.

Besides the sperm cells the spermatids are the smallest cells of the germinal epithelium. In a process lasting several weeks (so-called spermiogenesis or spermiohistogenesis) they are transformed into sperm cells with the active assistance of the Sertoli's cells.Development of the germ cells in the ovary http://www.embryology.ch/anglais/cgametogen/oogenese01.html#entwicklung

Following the immigration of the primordial germ cells into the gonadal ridge, they proliferate, are enveloped by coelomic epithelial cells, and form germinal cords that , though, keep their connection with the coelom epithelium. Now a cortical zone (cortex ovarii) and a medulla can be distinguished, whereby it should be mentioned that in females the germinal cords never penetrate into the medullary zone. In the genital primordium the following processes then take place:

A wave of proliferation begins that lasts from the 15th week to the 7th month: primary germ cells arise in the cortical zone via mitosis of oogonia clones, bound together in cellular bridges, that happens in rapid succession. The cell bridges are necessary for a synchronous onset of the subsequent meiosis.

With the onset of the meiosis (earliest onset in the prophase in the 12th week) the designation of the germ cells changes. They are now called primary oocytes.The primary oocytes become arrested in the diplotene stage of prophase I (the prophase of the first meiotic division). Shortly before birth, all the fetal oocytes in the female ovary have attained this stage. The meiotic resting phase that then begins is called the dictyotene and it lasts till puberty, during which each month (and in each month thereafter until menopause) a pair of primary oocytes complete the first meiosis. Only a few oocytes (secondary oocytes plus one polar body), though, reach the second meiosis and the subsequent ovulation. The remaining oocytes that mature each month become atretic.The primary oocytes that remain in the ovaries can stay in the dictyotene stage up to menopause, in the extreme case, without ever maturing during a menstrual cycle.

More infoStages of the first meiotic prophase of the oocyte.

While the oogonia transform into primary oocytes, they become restructured so that at the end of prophase I (the time of the dictyotene) each one gets enveloped by a single layer of flat, follicular epithelial cells (descendents of the coelomic epithelium). (oocyte + follicular epithelium = primordial follicle).

From birth there are thus two different structures to be distinguished that, at least conceptually, do not develop further synchronously:

On the one hand, the female germ cell that at birth is called the primary oocyte, and which can develop further only during (and after) puberty (hormonal cycle is necessary).

On the other hand, the follicular epithelium that can develop further from the primordial follicle via

several follicle stages while oocytes remain in their primary state.

The developmental sequence of the female germ cells is as follows: Primordial germ cell - oogonium - primary oocyte - primary oocyte in the

dictyotene BirthThe continuation of the development / maturation of the oocyte begins again only a few days before ovulation (see fertilization module).

The developmental sequence of a follicle goes through various follicle stages: Primordial follicle - primary follicle - secondary follicle - tertiary follicle

(graafian follicle) Since a follicle can die at any moment in its development (= atresia), not all reach the tertiary follicle stage.

 Tabular comparison of spermatogenesis and oogenesis

A tabular comparison of spermatogenesis and oogenesis furnishes evidence for major differences in the timing of production, number and size of gametes.

Spermatogenesis OogenesisNumber of gametes

Principle: continuous production. Although from puberty to old age sperm cells are constantly being engendered, the production is subject to extreme fluctuations regarding both quantity and quality.

Principle: Using up the oocytes generated before birth. Continual decrease of the oocytes, beginning with the fetal period. Exhaustion of the supply at menopause.

Meiotic outputFour functioning, small (head 4 m), motile spermatozoids at the end of the meiosis

One large, immotile oocyte (diameter 120 m) and three shriveled polar bodies are left at the end of the meiosis

Fetal periodNo meiotic divisions Entering into meiosis (arrested in the dictyotene

stage)No germ cell production Production of the entire supply of germ cells

Stages of Embryo Development by T Timothy Smith, PhD http://www.sharedjourney.com/articles/dev.html

Zygote: Fertilized egg (about 20 hours after insemination)

Zygote: Fertilized egg (about 20 hours after insemination)

2 cell embryo (about 20 hours after insemination)

4 cell embryo (about 48 hours after insemination)

8 cell embryo (about 72 hours after insemination)

Morula: 16-32 cell embryo (about 96 hours after insemination)

Blastocyst stage embryo (about 115 hours after insemination)

Development during the 2nd embryonic week - bilaminar germ disk (2nd week) - http://www.embryology.ch/anglais/hdisqueembry/diderm01.html

QuizQuiz 01

 The development of the bilaminar germ disk 3 and the establishment of a feto-maternal blood circulation system were given in detail in the placenta and implantation modules.

In the bilaminar primordium of the embryo (hypoblast or primary endoderm and epiblast) one recognizes in the epithelium of the epiblast a fluid-filled space, the first primordium of the amniotic cavity 5 . Ventrally, the roof of the still incompletely uncovered primary umbilical vesicle 5 (previously the blastocyst cavity) is formed by the hypoblast.

Schematically, amniotic cavity and primary umbilical vesicle together form two hemispheres with two layers (epi- and hypoblast) lying close to one another, thus representing the first embryonic primordium. However, only the epiblast is responsible for forming the embryo. The hypoblast develops into a part of the extraembryonic appendages.

Fig. 1 - Bilaminar embryonic disk,12 days; stage 5

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Extraembryonic mesoblastAmniotic cavityPrimary umbilical vesicle

Differentiation of the blastocyst

Figure 2–6 : A 10-day-old conceptus implanted in the uterine wall. The fetal membranes, amnion, yolksac, and chorion (syncytiotrophoblast and cytotrophoblast) have formed. The forming placenta surroundsthe conceptus.

7.2 The trilaminar germ disk (3rd week)Introduction

The bilaminar germ disk differentiates itself further into a trilaminar embryo, in that the cells flow in over the primitive streak between the two already existing germinal layers and so form the third embryonic germinal layer (mesoblast/derm). This phenomenon is also termed epithelio-mesenchymal transition (gastrulation in lower vertebrates) 6-7 . During this period the embryo experiences profound alterations. Afterwards, one speaks of the dorsally lying ectoblast/derm (and no longer of an epiblast/derm), from intermediate mesoblast/derm, as well as from ventrally lying endoblast/derm, which replaces the hypoblast. In order to have a better overview, the developments of the third week should be divided into several phases. One must keep in mind, though, that these do not always follow each other - they can just as easily take place concurrently.

Formation of the primitive streak QuizQuiz 05

Beginning with the third week ectoblast cells migrate through the primitive streak and form the mesoblast. At this stage the embryonic disk is oval -shaped and the ectoblast is bathed in amniotic fluid.

Viewed dorsally the wide end represents the rostral (front) region. The narrower end forms the caudal region. At this point, one can speak of right and left halves of the embryo if one lays a longitudinal (rostro-caudal) axis through the embryo.

Fig. 2 - Dorsal view of the embryonic disk Legend

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Primitive groovePrimitive pitPrimitive nodeOropharyngeal membraneCardial plateSectional edge of amniotic membraneMesodermEndodermFuture cloacal membrane1+2+3 primitive streak

Fig. 2Embryonic disk viewed dorsally. The red arrows show schematically the migration directions of the epiblast cells to their points of final destination.

From the 17th day 6 a thickening of the embryonic disk begins around the median line along the rostro-caudal axis. This median structure (primitive streak) lengthens until it occupies roughly half the embryo 7 . The primitive streak arises thanks to the proliferation and migration of epiblast cells in the direction of the embryonic disk's median line. After the 19th day 7 , the primitive streak grows through the addition of cells at its caudal end. At the anterior end, a groove forms in the ectoblast (primitive groove). The cranial region is strengthened by the epiblast cells and so forms the primitive pit 8 with the primitive node 7 (Hensen's node in birds). The head of the embryo will form at the extremity of the embryonic disk near the primitive pit.

Brushing up on the nomenclature

In the bilaminar stage: The dorsal germinal layer = epiblast. The ventral germinal layer = hypoblast.

In the trilaminar stage, the nature of the germinal layers changes as soon as the mesoblast has taken up its position.

The dorsal germinal layer becomes the ectoblast/-derm. The middle germinal layer (3rd germinal layer) becomes the mesoblast/-derm. The ventral germinal layer receives the name endoblast/-derm.

Summary of cell fates: The epiblast cells form the ectoblast, the mesoblast (or chorda-mesoblast) and the

intraembryonic endoblast. The hypoblast cells give rise to the extraembryonic endoblast (the umbilical vesicle

and the allantois).

Figure 2–7 Cross-sectionalview of the embryonic disk(center), fetal membranes

Figure 2–8 Longitudinalsection through a 14-day old embryo, depicting the fetal membranes (amnion, yolk sac, and Allantois) and sites of Ectoderm/endoderm contact

Genesis of the germ layers QuizQuiz 10

From the 17th day 6 the primitive streak forms an entry location for the epiblast cells to invade the underlying mesoblast and to proliferate there.Histological methods have shown that during their migration along the primitive groove, the epiblast cells form pseudopodia. They lose contact with each other thereby. This phenomenon of the inflow of cells to form the third embryonic layer is termed epithelio-mesenchymal transition 6-7 (gastrulation in the lower vertebrates) .

Fig. 3 - Primitive streak seen dorsally Legend

Figure 2–9 Schematic cross section of theembryonic disk at bilaminar (above)and trilaminar (below) stages.Enlarged view of the center of Fig. 2–7

Figure 2–10 Dorsal surface view of the embryonic disk with the amnion cover removed. The prechordal plate demarcates the future mouth. The body stalk will form the umbilical cord connecting the embryo to the placenta.

Fig. 3Oval-shaped embryonic disk (dorsal view).The red arrows show schematically the migration directions of the epiblast cells to their points of final destination.

Depending on their origin and the moment of their flowing in, the epiblast cells migrate away from the primitive streak in various directions.

The first cells that enter through the node and the primitive groove replace the hypoblast layer and form the definitive endoblast (origin of the future intestine and its derivates). At the same time, due to the migration of cells through the primitive node in the cranial direction, two further structures are formed:

the prechordal plate 6 , which is located cranial to the primitive node

the notochordal process 8 the development of which will be treated in the next section.

Fig. 4 - Transverse section at the level of the primitive groove

Legend

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6

Primitive grooveEpiblastExtraembryonic mesoblastDefinitive endoblastInvading epiblastic cells forming the intraembryonic mesoblastHypoblast

Fig. 4Transverse section at the level of the primitive groove with the immigration of epiblast cells, which form the future mesoblast, as well as the endoblast, which replaces the hypoblast.

The largest proportion of these immigrated cells form a third germinal layer, the intraembryonic mesoblast 6 . The mesoblast cells wander in all directions: laterally, cranially and caudally. This middle germinal layer lies between the definitive endoblast and epiblast. Exceptions are the cloacal membrane as well as the pharyngeal membrane, where the ectoderm and endoderm lie directly opposite each other 6 .

More infoThe migration of the epiblast cells with the subsequent formation

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Primitive groovePrimitive pitPrimitive nodeOropharyngeal membraneCardiogenic plateSectional edge of amniotic membraneMesoderm EndodermFuture cloacal membrane1+2+3 primitive streak

Cranial to the prechordal plate, mesenchyma cells of the embryonic disk will form the pericardium as well as the septum transversum 9 . At the caudal extremity the cloacal membrane forms the primordium of the future opening of the uro-genital tract and the rectum.

of the mesoblast and endoblast is a portion of embryogenesis that is summarized under the concept of the epithelio-mesenchymal transition.

The epiblast is a precursor of three cell layers in the trilaminar embryo: Ectoblast/-derm Mesoblast/-derm Definitive endoblast/-derm

More infoThe cellular adhesion molecules (or CAM):Starting with gastrulation the epiblast cells secrete hyaluronic acid, which gets deposited in the intercellular spaces between the epiblast and the hypoblast. This molecule is able to bind a large amount of water to itself (up to 1000 times its own weight), is often associated with cell migration, and plays an anti-aggregational role for the mesoblast cells. The presence of hyaluronic acid, however, does not suffice to explain the migration of the epiblast cells away from the primitive streak.In vertebrate embryos it is assumed that the migration also depends on the presence of fibronectins that are found on the basal lamina under the epiblast. Fibronectin is an extracellular glycoprotein (10).

The integrins:The integrins belong to the 4 families of the Cell Adhesion Molecules (CAM): integrin, cadherin, selectin, and the large family of the immunoglobulins. They are responsible for the recognition and binding of two cells with each other or a cell and components of the extracellular matrix. The kind of the connection can be homophil (two identical molecules) or heterophil (two differing molecules bound to each other). Being transmembranal glycoproteins, integrins insure the cohesion (aggregation) and influence the migration of cells. They are also responsible for the organization of the cells in the tissue and the tissue in the organs. The integrins arrange contacts with the collagen filaments in the basal membrane and the extracellular matrix. Laminin is a ligand of the basal membrane. Fibronectin, on the other hand, is a ligand of the intercellular substance of e.g. the mesoblast and plays a role in cell migration.epithelio- mesenchymal transition / Gastrulation : Migration of the epiblast cells.The migration of the epiblast cells and the resulting formation of the mesoblast and endoblast is an embryogenetic process that is called the epithelio-mesemchymal transition (6-7) (=gastrulation in lower vertebrates). In the beginning the epiblast cells form an epithelial tissue with cells that are closely bound together. One finds therein characteristic molecules that serve for recognition as well as cohesion (e.g. e-cadherin). The breaking away and migration of the epiblast cells require that they lose contact and recognition possibilities among each other. When the cells get to the mesoblast layer they lose the ability of secreting these molecules. Although the ability for excretion disappears with the migration into the mesoblast, it returns when the cells organize themselves into somites.

The induction of the mesoblast stands under the control of genes that code for growth factors like, for example, the TGF- (Transforming Growth Factor-), the activin and the FGF (Fibroblast Growth Factor). These growth factors represent several molecule families that have a signaling effect and so bring about the induction of the phenotypic transformation of the mesoblast. It has been shown under culture conditions, for example, that TGF is able to induce the transformation of ectoblast into mesoblast in frogs.Genesis of the notochord Quiz

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QuizQuiz 15

Around the 19th day cells that invade into the primitive node region and migrate along the median line cranially form the chordal process 7 (also known as the axial process). One can compare this phenomenon with the sliding of a finger into a glove. Thanks to the transparency of the ectoblast this cell migration can be observed in the embryos of experimental animals.The chordal process grows longer through proliferation of the primitive node cells at its front end up to the prechordal plate 6 . At the same time the primitive streak recedes back into the caudal region 6 .

Fig. 5 - Primitive streak seen dorsally LegendFig. 5The red arrow shows schematically the migration direction of the epiblast cells coming from the primitive node to form the notochordal process and later the notochord.

Fig. 5 a - The chordal process at roughly the 19th day (Stage 7)

Fig. 5 b - The chordal process at roughly the 19th day (Stage 7)

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Chordal processEmbryonic endoblast

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Amniotic cavityBody stalkExtraembryonic mesoblastAllantois

Fig. 5aSchematic diagram showing the genesis of the chordal process at the 19th day through the invagination of epiblast cells coming from the primitive node.

Fig. 5bCut along C.

Fig. 5 c - The chordal process atroughly the 21rst day (Stage 7)

Fig. 5 d - The chordal process at roughly the 21rst day (Stage 7)

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Chordal processPrimitive nodeEmbryonic endoblastAmniotic cavity

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Body stalkExtraembryonic mesoblastAllantois

Fig. 5cSchematic diagram showing the genesis of the axial canal through invagination of epiblast cells that migrate into the region of the primitive node (on roughly the 21rst day).

Fig. 5dCut along C

On the 23th day the chordal process consists of chordal mesoderm and a central axial canal 7 . At this point in time, the chordal process fuses with the endoderm lying below it. While

the chordal process and endoderm are merged, for a short time (approximately one day), the amniotic cavity communicates with the umbilical vesicle, via the so-called neurenteric canal 8 . The chordal tissue that thereby goes over into the endoderm becomes thus the chordal plate 8-9 .

Fig. 6a - The chordal process at roughly the 23rd day (Stage 8)

Fig. 6b - The chordal process at roughly the 23rd day (Stage 8)

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Fused chordal processPrechordal platePharyngeal membraneEmbryonic endoblastAmniotic cavityNeural groove

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Canalis neurentericusIntraembryonic mesoblastCloacal membraneUmbilical vesicle Allantois

Fig. 6aThe notochord has separated itself from the endoblast. At the caudal end of the notochord, in the region of the primitive node, thanks to the neurenteric canal, a temporary communication takes place between the umbilical vesicle and the amniotic cavity (arrow).

Fig. 6bCut along C

On the 25th day 9 the chordal plate cuts itself off from the entoderm, which then fuses again, and forms a complete cord: the notochord. This is in the middle of the mesoderm, between the ectodern and endoderm, and plays a role in the induction of the neuroectoblast that lies over it. Moreover, the notochord plays a role in the genesis of the vertebral body and becomes the nucleus pulposus in the intervertebral disk.

Fig. 7a - The chordal process at roughly the 25th day (Stage 9)

Fig. 7b - The chordal process at roughly the 25th day (Stage 9)

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Chordal processEmbryonic endodermAmniotic cavityNeural grooveBody stalkIntraembryonic mesoblastPrechordal plate

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Pharyngeal membraneCloacal membraneAortae Umbilical veinsCardiogenic plateAllantois

Fig. 7aAt around the 20th day the chordal tissue bonds with the endoderm and thus forms the chordal plate. Between the 22nd and 24th day it cuts itself off from the endoderm (the endoderm fuses again) in order to form a complete cord: the notochord. This finds itself in the middle of the mesoderm, between the ectoderm and endoderm.Fig. 7bCut along C

Fig. 7c - The chordal process at roughly the 28th day (Stage 10)

Fig. 7d - The chordal process at roughly the 28th day (Stage 10)

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Chordal processEmbryonic endoblastAmniotic cavityNeural tubeBody stalkIntraembryonic mesoblastPrechordal plate

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Pharyngeal membraneCloacal membraneAortaeUmbilical veinsCardiogenic plateAllantois

Fig. 7cThe endoderm, under the notochord, comes together again. At this point the neural groove has partly fused into becoming a neural tube.Fig. 7dCut along C

Summary: the notochord determines the longitudinal axis of the embryo. It defines the future situation of the vertebral body and induces the ectoblast in its differentiation to become the neural plate.prechordal plate: In the literature there is disagreement in relation to the delimitation and fate of the prechordal plate. This concept is used in order to describe the following:

A collection of mesoblast cells, cranial to the notochord, that are descendents of the endoblast (Noden 1988)

A thickening of the endoblast, cranial to the notochord (Gray 1995) The pharyngeal membrane (Carlson 1994, Moore 1998) A compact mass of mesoblast cells that enter via the primitive pit and come to lie

cranially from it (Larsen 1996)

These contradictory definitions can probably be explained by the experimental transplantation technology that nowadays is so severely limited.

In accordance with Noden (1988), we advocate the idea that the prechordal plate is a thickening

More infoIn the literature there is some disagreement related to the delimitation and fate of the prechordal plate.

of the mesoblast that is located cranial to the chordal process. It consists of the first endoblast cells that migrate into the area from the primitive node. A small portion of the mesoblast cells stay between the cranial part of the chordal process and the prechordal plate and thus become prechordal mesenchyma. The prechordal plate and the mesoblast cells surrounding them appear to play an organizer role for the cervical region. The external muscles of the eyes derive from this structure (2-3). Location of the epiblast cell target and the development of the primitive streak

QuizQuiz 03

Depending on their place of origin and the timing of the invagination, the epiblast cells migrate away from the primitive streak in three main directions (1).It is assumed that the extraembryonic mesoblast comes from ectoblastic cells passing through the posterior segment of the primitive streak. The intraembryonic mesoblast, on the other hand, arises from cells migrating through the middle and cranial segment up to the 4th week.The ectoblast cells that converge to the primitive node engender the paraxial mesoblast, the notochord, the prechordal plate, the endoblast and the medial part of the somites (in animal experiments as determined with the help of marked cells).

At around the 19th day 9 the primitive streak extends over half the whole length of the embryo, but recedes with advancing gastrulation and is shifted caudally. In the 4th week its length is approximately only 15% of that of the whole embryo. The primitive streak is confined to a region termed the caudal eminence. On the 29th day 11 it disappears completely. Remainders of it can lead to a sacro-coccygeal teratoma.

QuizQuiz 06

QuizQuiz 07

Fig. 8 - Dorsal view of the primitive streak at around the 17th day

Fig. 9 - Dorsal view of the primitive streak at around the 19th day

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Primitive streakPrechordal platePrimitive node

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Neural plateCloacal membraneChordal process

Fig. 8, Fig. 9Schematic diagrams: dorsal view of the embryonic disk during the 3rd week. One sees the growth of the primitive streak and the genesis of the chordal process.

Fig. 10 - Dorsal view of the primitivestreak at around the 23rd day

Fig. 11 - Dorsal view of the primitive streak at around the 25th day

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Primitive streakPrimitive nodeNeural tube

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Cloacal membranePrechordal plateChordal process

Fig. 10, Fig. 11Schematic diagrams: dorsal view of the embryonic disk during the 4th week. To be seen are the back-formation of the primitive streak and the growth of the chordal process.

The primitive streak is confined to a region termed the caudal eminence and disappears at stage 11 (29 days).

The bilaminar membranes QuizQuiz 12

Two regions exist where the ectoderm and endoderm lie directly on each other without any mesoderm cells lying between them. These two rounded regions are found along the median line. One lies cranially to the prechordal plate, the other caudally to the primitive streak

During the course of the 3rd and 4th weeks, the ectoderm in these regions bonds tightly with the endoderm lying below it in order to form a bilaminar membrane that is called a pharyngeal membrane (membrana oropharyngea) cranially 10 and cloacal membrane (membrana cloacalis) 6 caudally. Both of these membranes will be resorbed during the subsequent development. There the openings for the stomodeum 11 and of the urogenital tract (cloacal membrane) 19 are located.

Fig. 12 - Dorsal view of the embryo3rd week Legend

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Primitive groovePrimitive pitPrimitive node

Fig. 12Schematic diagram of the dorsal view of an embryo during the mesoblast emigration (3rd week). To be seen are the two bilaminar regions at the cranial and caudal extremities.

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Oropharyngeal membraneCardiogenic plateSectional edge of amniotic membraneMesodermEndodermFuture cloacal membrane1+2+3 = primitive streak

Evolution of the mesoblast

The extraembryonic mesoblast 5 is formed already during the 2nd week. It is involved in the construction of the placenta and the other appendage organs.

The intraembryonic mesoblast is the third germinal layer. It arises in the 3rd week via the immigration of cells at the primitive streak. Out of this develop the various tissues and organs of the embryo.

In the beginning, the cells of the mesoblast (mesodermal cells) build a thin, widely meshed layer on both sides of the median line, between the ectoderm and the endoderm.While the notochord is forming - it grows to the same extent that the primitive streak recedes - the intraembryonic mesoderm cells multiply on both sides of the median line and so form 3 structures in the shape of longitudinal columns.This process begins at the cranial pole and continues up to the 4th week in the caudal direction. In the following section we shall see which specific structures arise from these three formations.

ReviewDifferentiation of the blastocyst and the blastodermic layer.

Fig. 13 - The mesoderm after the completed mesoblast immigration(roughly the 25th day)

Legend

The red arrows show schematically the migration directions of the epiblast cells to their points of final destination.

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Paraxial mesodermIntermediate mesodermLateral plate mesodermChordal processAmnionIntraembryonic coelomEndoblastEctoblastSomatopleural (mesoderm and ectoderm)Splanchnopleural (mesoderm and endoderm)Neural grooveNeural plate

Fig. 13Schematic 3D-diagram of an embryo at around the 25th day. The red part shows the intraembryonic mesoderm, which consists of three parts. The paraxial mesoderm forms on both sides of the median line A bit more laterally one recognizes the intermediate mesoderm (2nd pouch) that gets thinner laterally into the lateral plate mesoderm. This last divides in order to participate in the formation of the somatopleural and splanchnopleural mesoderm which enwrap the intraembryonic coelom.

The intraembryonic mesoderm differentiates itself into three subdivisions on both sides of the primitive streak as it recedes:

Paraxial mesoderm Intermediate mesoderm Lateral plate mesoderm

The paraxial mesoblast and the differentiation of the somites

The paraxial mesoblast comes from the epiblast cells that migrated into the region of the primitive node or the cranial portion of the primitive streak. It forms a pair of cylinder-shaped epithelially-organized mesenchyma segments that are in the immediate vicinity of the neural tube and the notochord. After the beginning of the 3rd week, these cylinders become segmented from the cranial to the caudal end into so-called somitomeres (process of metamerization). Originally, each consists of a pseudostratified epithelium that is arranged around a central cavity, the somitocoel.

More infoThe metameres are based on the division of the original embryo into several segments.

Except for the somitomeres (1 to 7) that form no somites,

Fig. 14 - Evolution of the mesoblast

Legend

but are involved in the formation of the pharyngeal arch mesoblast, the others form somites in the cranio-caudal direction. After the 25th day 3-4 somites per day are formed thereby 9 . In humans 42-44 somite pairs

9 - 13 are formed along the neural tube. These range from the cranial region up to the embryo’s tail. Several caudal somites disappear again, which is why only 35-37 somite pairs can be counted in the end.The number of the somites that are found is used to determine the embryo's age in this developmental stage.

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Paraxial mesodermIntermediate mesodermLateral plate mesodermNeural grooveEctoblastEndoblast

Fig. 14Transversal section at the 25th day. To be seen is the differentiation of the mesoblast into the paraxial, intermediate and lateral plate mesoderm

Fig. 15 - Genesis of the intraembryonic coelom, at

roughly the 23rd day

Fig. 16 - Appearance of the somitomeres, at roughly the

25th day

Legend

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Paraxial mesoblastIntermediate mesoblastLateral plate mesoblastChordal processSectional edge of the amnionIntraembryonic coelom

78910

EndoblastEctoblastSomatopleure with ectoblastSplanchnopleure with endoblast

Fig. 15Transversal section through a 23-day-old embryo. One can recognize the first space of the future intraembryonic coelom.

Fig. 16Transverse section with a dorsal view at around the 25th day. The cylindrical cell collections of the paraxial mesoderm segment form the somitomeres (arrows). The intermediate mesoblast is

involved in the urogenital development.

More infoThe somites are embryonic transitional organs that are formed through the segmentation of the paraxial mesenchyma. They organize themselves without cell differentiation (primary organs). They are responsible for the segmental organization of the embryo and contribute to its restructuring.They contain the precursor cells for the axial skeleton (sclerotome), the striated musculature of the neck, the trunk and the extremities (myotome), as well as those of the subcutaneous tissue and skin (dermatome). The somites are the prerequisites for metamerism. The segmental (metamere) partitioning of the spine, the neural tube, the trunk wall and the thorax (ribs) depends on the ordered arrangement of the somites.

QuizQuiz 09

The intermediate mesoblast

The intermediate mesoblast 10 is found between the paraxial mesoblast and the lateral plate mesoblast. This longitudinal, dorsally lying crest is called the urogenital crest and serves as the origin of the kidney and gonads. It forms cell masses in the neck and upper breast regions that exhibit a metamerism: the nephrotomes. In the more caudally lying regions it remains unsegmented and forms the so-called nephrogenic cord.

The lateral mesoblast

The lateral plate mesoderm is composed of two thick layers that surround a cavity, the intraembryonic coelom (the coelom represents the future serous cavity of the trunk: peritoneal, pleural and pericardiac cavities). The somatopleure, which is close to the ectoderm, is involved in the formation of the lateral and ventral walls of the embryo. The splanchnopleure, which lies on the endoblast, takes part in the formation of the wall of the digestive tube.

QuizQuiz 08

QuizQuiz 13

Fig. 17 - Lateral plate mesoderm

Fig. 18 - Lateral plate mesoderm

Legend

at roughly the 23rd day at roughly the 25th day

123456

Lateral plate mesodermIntermediate mesodermParaxial mesodermNeural grooveCoelom vacuolesIntraembryonic coelom

789101112

SomitesNotochordSplanchnopleure with endodermSomatopleure with ectodermAmniotic cavityUmbilical vesicle

Fig. 17After the 23rd day coelomic vacuoles form in the lateral plate.

Fig. 18On the 25th day the intraembryonic coelom divides the lateral plate into the splanchnopleural and somatopleural mesoderm.

The intraembryonic coelom

The intraembryonic coelom first appears in the lateral plate mesoderm in the form of several isolated vacuoles. During the lateral unfolding of the embryo in the 4th week these vacuoles fuse and form a U-shaped cavity: the intraembryonic coelom. In the beginning a connection exists between the intra- and extraembryonic coeloms.With the progress of the unfolding, though, the merging of the ectoblast layers along the medial line has the effect that the intraembryonic coelom is separated from the extraembryonic one and remains enclosed in the lateral mesoblast.

Fig. 19 - Mesoderm before the vacuoles form on roughly the

20th day

Legend

1234

Paraxial mesodermIntermediate mesodermLateral plate mesodermChordal process

Fig. 19Transverse section through a 20-day-old embryo. The separation into the paraxial, intermediate mesoderm and the lateral plate is shown.

Fig. 20 - Coelomic vacuoles at Fig. 21 - Coelomic cavity at Legend

roughlythe 23rd day

roughly the 25th day

1234567

Lateral plate mesodermIntermediate mesodermParaxial mesodermNeural grooveCoelomic vacuolesIntraembryonic coelomExtraembryonic coelom

89101112

Extraembryonic mesoblastNotochord Splanchnopleure with endodermSomatopleure with ectodermDorsal aorta (paired)

Fig. 20After the 23rd day coelomic vacuoles form in the lateral plate.

Fig. 21On the 25th day the intraembryonic coelom of the lateral plate divides the mesoderm into somatopleural und splanchnopleural mesoderm.

Induction of the neural plate - neurulation QuizQuiz 04

With the primary neurulation begins the genesis of the nervous system. The notochord exercises an inductive effect on the ectoblast that lies above it. This causes the ectoblast cells to transform themselves into neuroectoblast cells. This was already known at the beginning of the 20th century and has now been proven to be true.On the 19th day the neural plate 7 appears. It represents the first step in the genesis of the nervous system. The neural plate is identifiable as the medio-sagittal thickening of the ectoblast rostral to the primitive streak. At the cranial end the neural plate is wider and consists of the region where the brain will arise. At the caudal end it is narrower and gives rise to the spinal cord. Approximately 50% of the ectoblast becomes the neural plate and the remainder forms the epidermis.The neural plate develops in step with the genesis of the notochord, i.e. under the inducing influence of the axial mesoderm lying below it (prechordal plate and cranial portion of the chordal plate). The induction process is very complex, but has its origin in the secretion of inducing factors by axial mesoderm cells. These factors diffuse in the direction of the ectoderm cells that lie above them where they activate genes that are responsible for the differentiation of epithelium that has come from the ectoderm into several rows of prismatic epithelium: the neuroectoblast. In the course of the 3rd week the edges of the neural plate rise up and become neural folds 9 , enclosing the neural groove 8 .

QuizQuiz 16

Fig. 22 - Neural plate: 19 – 23rd day Fig. 23 - Neural plate at roughly the 25th day

Legend

1234

Neural platePrimitive streakPrimitive nodesNeural groove

567

SomitesCut section of the amnionNeural folds

Fig. 22With the appearance of the neural plate on the 19th day, the development of the future nervous system begins.

Fig. 23The cranial end the neural plate is wider and encloses the region where the brain will arise. The caudal end is narrower; there the spinal cord will form.

The neural folds approach each other after the 25th day and merge to form the neural tube (delimitating the future central canal, which is coated with ependymal cells). The closure of the neural tube begins in the cervical area (in the middle of the embryo) and extends from there in both the cranial and caudal directions.The anterior neuropore (cranial) closes itself on the 29th day 11 . The posterior neuropore (caudal) closes a day later 12 . The top of the anterior neuropore corresponds to the terminal lamina of the adult brain and the posterior neuropore to the terminal filum at the end of the spinal cord.

If the posterior neuropore does not close, a spina bifida occurs. If, on the other hand, the closure of the anterior neuropore fails to take place, an anencephaly results.While the neural tube is in the process of closing, cells on the lateral side of the neural plate detach themselves and form the neural crest.

Fig. 24 - The neural tube at roughly the 28th day

Fig. 25 - The neural tube at roughly the 29th day

Legend

1234

Neural tubeNeural foldNeural grooveSomites

5678

Neural crestProtrusion of the pericardiumCranial neuroporeCaudal neuropore

Fig. 24In the course of the 3rd week the edges of the neural plate rise and form two folds that bound the neural groove.

Fig. 25The closure of the neural tube begins in the cervical area (in the middle of the embryo) and spreads from there in the cranial and caudal directions.

The neural crest cells 9 form, so to speak, a 4th embryonic germinal layer. This contains a partial segmentation that contributes to the formation of the peripheral nervous system (neurons and glia cells of the sympathetic, parasympathetic and sensory nervous systems).

The neural crest cells are distinguished by a great migrating ability and phenotypic heterogeneity, since numerous and various differentiated cell types will arise from them. Not only do the nerve and glia cells mentioned above arise from the neural crest, the epidermal pigment cells (melanocytes), the calcitonin cells of the thyroid gland, the cells of the adrenal medulla and some components of the skeletal and connective tissue in the head area are also generated from them.

Fig. 26 - The forming neural crest (neural plate stage) Legend

AB123

Neural plate stageNeural groove stageEpiblastNeural grooveNeural crest

Fig. 26The beginning of neurulation in the cervical region. The neural groove forms. The cells of the future neural crest are in orange. The arrows show the direction of the lateral folding.

Fig. 27 - Migrating neural crest cells (neural groove stage)

Fig. 28 - Neural crest after a completed detachment

(neural tube stage)

Legend

123

EpiblastNeural foldMigrating neural crest cells

456

NeuroepitheliumCentral canal Neural tube

Fig. 27, Fig. 28On the neural plate the formation of the neural groove 9 follows and afterwards the neural tube 11 .Masses of cells detach themselves from the lateral side of the neural plate and form the neural crest. As soon as the neural crest cells leave the neuroepithelium they lose their cohesion (8 - 9).

NB: Note that in the neural tube formation stage the neuralepithelium is multilayered which, in this diagram, for the sake of simplicity, could not be shown.

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Figure 2–2 Schematized synopsis of salient features of general embryology.

TABLE 2–1 Chronology of events during the embryonic periodCarnegie Stage

Postconception Age

Craniofacial Features

6891011

12

13

141516

17

18

19

2022

23

Foetus

14 days17 days20 days21 days24 days

26 days

28 days

32 days33 days37 days

41 days

44 days

47-48 days

50-51 days54 days

56-57 days

Fetus 60 days

Primitive streak appears; Oropharyngeal membrane formsNeural plate formsCranial neural folds elevate; otic placode appearsNeural crest migration commences; fusion of neural folds; otic pit formsFrontonasal prominence swells; first arch forms; wide stomodeum; optic vesicles form;

anterior neuropore closes; olfactory placode appearsSecond arch forms; maxillary prominences appear; lens placodes commence; posterior

neuropore closes; adenohypophysial pouch appearsThird arch forms; dental lamina appears; fourth arch forms; oropharyngeal membrane

rupturesOtic and lens vesicles present; lateral nasal prominences appearMedial nasal prominences appear; nasal pits formwidely separated, face laterallyNasal pits face ventrally; upper lip forms on lateral aspect of stomodeum; lower lip fuses

in midline; retinal pigment forms; nasolacrimal groove appears, demarcating nose; neurohypophysial evagination

Contact between medial nasal and maxillary prominences, separating nasal pit from stomodeum; upper lip continuity first established; vomeronasal organ appears

Primary palate anlagen project posteriorly into stomodeum; distinct tip of nose develops; eyelid folds form; retinal pigment; nasal pits move medially; nasal alae and septum present;mylohyoid, geniohyoid and genioglossus muscles form

Nasal fin disintegrates; (failure of disintegration predisposes to cleft lip); the rima oris of the mouth diminishes in width; mandibular ossification commences

The lidless eyes migrate medially; nasal pits approach each other; ear hillocks fuseThe eyelids thicken and encroach upon the eyes; the auricle forms and projects; the

nostrils are in definitive positionEyes are still wide apart but eyelid closure commences; nose tip elevates; face assumes

a human fetal appearance; head elevates off the thorax; mouth opens; palatal shelves elevate; maxillary ossification commences

Palatal shelves fuse; deciduous tooth buds form; embryo now termed a fetus

Pre-fertilization week 1 Ovarian follicle development, menstrual phase Pre-fertilization week 2 Ovarian follicle development complete, proliferative phase Week 1 Blastocyst formation, implantation begins, secretory phase Week 2 Embryonic disc formation, maternal blood supply begins Week 3 Primitive streak formation, neural plate is evident Week 4 Otic pit, limb bud, and heart begins to beat Week 5 Primitive mouth, eye formation Week 6 Otic and nasal cavities, lip and nose, digital rays formed Week 7 Eyelid formation, genital tubercle Week 8 Upper limbs longer and jointed, distinct fingers, sexual differentiation Week 9 Fetal period begins, further sexual development Week 10 Face has human profile, chin growth

The development of the human embryoThe time table of human development, taken from a standard human embryology text (often used in medical school anatomy classes), Langman, Medical Embryology, Williams and Wilkins Press: While some of this is technical, there is still information of value here.

--------------------------------------------------------------------------------*day 1 fertilization process - sperm enters egg, conception occurs. *day 2 two cell stage (30 hours), 4 cell stage (40 hours)- if a cell falls off the cluster, it can develop into another embryo plus placenta, i.e, identical twin) A magnified high quality human blastocyst The developing fetus itself is the area marked as "ICM" (inner cell mass)The blastocyst cavity in the center is marked as "C". The trophectoderm cells that will form the placenta surround the cavity - one is markedwith a "T" *day 3 morula (solid ball of cells) *day 4 early blastocyst (ball of cell forms a small central cavity - cells are now committed to becoming either placental cells or embryo cells, though they can't be recognized as such) *day 6-7 implantation starts. Group of cells at one pole of the cyst lining is now recognizable as the embryonic precursor tissue. *day 8 two-layer disc of embryonic tissue *days 8-12 conceptus gradually covered by maternal tissue, maternal placenta bed vessels expand, placental primitive vessels start to form *day 13 uteroplacental circulation begins *day 14 embryonic disc forms "primitive streak," from which a third layer of embryonic cells emerge Menstruation is now due. *day 15 three-layer embryonic disc complete *day 17 notochord forms (axial scaffold later replaced by spinal column) *day 19 top layer of disc (ectoderm) forms a central flat strip (neural plate) that is the nervous system precursor *day 20 middle layer of disc (mesoderm) starts partitioning into segments called somites, which will later become ribs, muscles, etc. *days 20-23 neural plate rolls into a tube and sinks below the surface of the ectoderm. This tube will form the spinal cord and brain eventually. Menstruation now 1 week late *days 21-22 heart tube forms (yes, a simple tube). Other vessels and blood cells also form at this time. Menstruation now 2 weeks late - 28 days from conception *days 28-31 arm, leg, and tail buds form. These look like flippers. Endoderm layer gives rise to lung, liver buds. Menstruation now 3 weeks late *week 5 kidney buds form. Eye and lens buds form in association. Heart tube folds and forms the four chambers. Heart now beating. Menstruation now 4 weeks late *week 6 "indifferent" gonads form, having no male or female characteristics at this time. External genitalia also indistinguishable. Hand and foot plates start to separate the fingers and toes. Menstruation now 6 weeks late *week 8 lung buds continue to branch. Gonads now distinguishable as ovary or testis. Eyes, nostrils, mouth identifiable externally. Primitive uterus and fallopian tubes form. *** By the end of the 8th week, all major organs have their basic forms. The rest of development is required to make the organs functional. Most major anatomic birth defects are caused by damage during the 3rd to 8th week of gestation. This is why it is so important, when using herbs to terminate pregnancy, if they should fail, to follow up with a clinical abortion. As you can see, in the first few weeks of pregnancy are vital to the survival of the developing child, if any of these processes are disrupted, as by herbs or other substances, the risk of birth defects is pronounced, from these few cells comes all the groundwork for the future person, the foundation for the major organs, brain and spinal column are laid at this time.