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Evidence - embryology

Page history last edited by Charles Forstbauer 14 years, 1 month ago

Totaled 3/29 Mr F

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Totaled 3/20/10 Mr F

Darwin's evidence for evolution: Embryology

Embryology- A science which is about the development of an embryo from the fertilization of the ovum to the fetus stage. After cleavage, the dividing cells, or morula, becomes a hollow ball, or blastula, which develops a hole or pore at one end.

I. The "facts of embryology"

A. There is usually a difference in form between embryo and adult.

B. Serial structures (e.g., segments, limbs) are usually identical in the embryo, but specialized and diverged in the adult.

C. Different species in the same Class (e.g., mammals) often have very similar embryos, even if the adult forms are quite different.

D. Embryonic structure is unrelated to "conditions of existence", unless the embryo is active (e.g., feeding)

E. Embryos are sometimes more highly "organized" than adults (e.g., some parasitic forms).

II. The "principles" that explain these facts

A. Adult variations supervene at a rather late stage

B. Variations in adult features are inherited (expressed by progeny) at correspondingly late stages

C. These "principles" are generally but not universally true (an exception was the tumbler pigeon) when one looks at empirical evidence from domesticated varieties

III. How do these principles explain the facts?

A. Breeders select on adult characters, regardless of the juvenile characters

B. Given this, and the two principles, the young of a new variety and the parental stock will tend to be more similar than the adults

C. Darwin extrapolates this to larger groups: For example, the forelimbs might be legs in an ancestral species, but would be modified as flippers, arms, wings, etc. at a late stage in development; but the pattern in the embryonic stage would remain similar if not unchanged.

D. In some cases, variation may occur at an early developmental stage, and new basic patterns might thus be produced (e.g., the tumbler pigeon); these types of exceptions to the "principles of embryology" obviously do not affect the power of descent to explain similarities in embryonic patterns among related species.

Here is a video explaning the meaning of embryology through evolution.

Heckle did take some artistic licence to make his point. The argument late in the video that talks about the earliest stages of embryo development kind of misses the point. There are big differences in blastulas. 1st: recognise that the body plan for most animals is tube-in-tube (mouth to anus is a tube) Which end of this tube forms first is a distingushing charateristice. (in humans the anus forms first) 2nd: all the embryos follow a common form factor (Heckles drawing) at some point demonstrating an evolutionary convergence. 3rd: the closer the common ancestor the closer the embryos. Human/chimp are alot more similar than human/ chicken and the further you go back the greater the difference.

http://embryology.med.unsw.edu.au/wwwhuman/Stages/Images/Cst800.jpg

^Theses are the stages of embryology.

This is a picture of 3 stages in the embyotic process. This picture depicts a lizard, a tortoise, a pig and a human in equal stages. It is easy to see how they all start out looking relatively similar.

*Other examples of embrylogists are Ernst Haeckel, Joseph Needham, and William Harvey.*

In the 18th century, scientists believed that human embryology was based on the theory of preformation. Preformation was the idea that male semen actually contained a premade embryo or mini-infant in it, that was enlarged when developing. But 2,000 years before this theory was invented, Artistotle thought up epigenesis. This theory stated that the actual form of a human or animal was created gradually from a formless egg. Later, biologists realized that embryos "took shape in a series of progressive steps". Therefore, epigenesis displaced preformation.

Stages of Human Embryology:

Fertilization

The first two weeks of the human development are called the preembryonic period. This period begins with the fertilization. Fertilization is the beginning of the pregnancy and can be considered as the beginning of a new life.
In order to have successful fertilization, several events have to be take their course (e.g. gametogenesis). Female gamete (oocyte) is produced during the menstrual cycle and expelled during the ovulation. During each ovarian cycle, only one follicle with an oocyte reaches full maturity. At the 14th day in an average 28-day cycle this follicle bulges on the surface of the ovary. Immediately before the ovulation, the oocyte and some surrounding cells (cumulus oophorus) detach from the interior of the follicle. Also, shortly before the ovulation the fimbriae of the oviduct start covering the surface of the ovary. During ovulation, follicle bursts and the oocyte is expelled into the uterine tube.

Secondary oocyte

1. Corona radiata
2. Spindle
3. Zona pellucida
4. Sperm

The expelled secondary oocyte is surrounded by the zona pellucida and several layers of the follicular cells arranged as the corrona radiata.
Male gametes are produced during the spermatogenesis and stored in the epididymis. Upon ejaculation into the female genital tract, the spermatozoa are not capable of fertilizing the oocyte. They must undergo a capacitation period that lasts approximately 7 hours, during which the glycoprotein coat and seminal proteins are removed from the surface of the sperm acrosome by the action of the substances secreted by uterus or uterine tubes. When capacitated spermatozoa come into contact with the corrona radiata surrounding the secondary oocyte, they undergo the acrosomal reaction. This process includes release of the acrosomal vesicles content that helps the sperm digest its way to the oocyte plasma membrane in order to fuse with it.

Mature ovum

1. Perivitelline space
2. Zona pellucida
3. Polar bodies
4. Sperm

Fertilization, the process by which the male and female gametes fuse, marks the beginning of the pregnancy. It lasts 24 hours and occurs in the ampullary region of the uterine tube. The first event is the scattering of the corrona radiata cells by the released contents of the acrosomal vesicle (hyaluronidase), tubal mucosa enzymes and sperm tail movements. Penetration of the zona pellucida is enabled by the action of other enzymes released from the acrosome (acrosin and neuraminidase). When the first sperm passes through the zona pellucida, cortical and zona reaction make it impermeable to other sperms. This mechanism ensures that each oocyte is fertilized by only one sperm.

Fusion of the pronuclei

1. Fusion of pronuclei
2. Perivitelline space
3. Zona pellucida
4. Polar bodies

When the sperm enters the oocyte, it leaves its plasma membrane behind. After the sperm entry, the secondary oocyte finishes its second meiotic division, forming an ovum and a second polar body. The nucleus of the mature oocyte is known as the female pronucleus. Male pronucleus is formed by the enlarging of the nucleus in the head of the sperm. During the growth of the pronuclei they replicate their DNA. At this stage, male and female pronuclei are indistinguishable. Fertilization ends with the fusion of female and male pronucleus and formation of the zygote. Within 24-48 hours after fertilization, early pregnancy factor (EPF) can be detected in the maternal serum.

Zygote

1. Zygote
2. Perivitelline space
3. Zona pellucida
4. Polar bodies

Cleveage:

Cleveage

After the fertilization, the zygote undergoes a series of rapid divisions called cleavage. The zygote first divides into two cells known as blastomeres (30 hours after fertilization). Three days after the fertilization, a rapid increase in the number of the cells results in the formation of a solid ball of 12-16 cells, the morula. These repeated mitotic divisions happen during the zygote's passage through the uterine tube toward the uterus. Four days after fertilization, when the morula enters the uterus, fluid-filed spaces between the blastomeres appear and fuse into a central cavity called the blastocoele. At this stage of the development, the conceptus is called a blastocyst.
Two-cell stage Four-cell stage	1. Zona pellucida 2. Blastmere
	Morulla

Blastocyst later loses the zona pellucida through the process called blastocyst hatching. The blastocyst consists of inner cell mass (embryoblast) and outer cell mass (trophoblast). Embryoblast gives rise to the embryo and a part of the amnion. The trophoblast cells form most of the extraembryonic membranes, i.e., the bulk of the placenta. During the 6th day after fertilization, the blastocyst attaches to the endometrial epithelium with it's embryonic pole. This triggers the differentiation of the trophoblast into an inner cytotrophoblast and an outer syncytiotrophoblast. By the end of the first week, the blastocyst is superficially implanted. At about seven days a flattened layer of cuboidal cells, called hypoblast (primitive endoderm), appears on the surface of the embryoblast.

Blastocyst

1. Outer cell mass or trophoblast
2. Blastocyst cavity
3. Inner cell mass or embryoblast

Implantation:

Implantation

The blastocyst usually implants on the posterior uterine wall. The implantation begins at the end of the first week and is completed by the end of the second week. During the 6th day, the blastocyst attaches to the endometrial epithelium. During the second week, the trophoblast begins to differentiate into the syncytiotrophoblast and the cytotrophoblast. Penetration of the blastocyst into the mucosa result from proteolitic enzymes produced by the syncytiotrophoblast. At the beginning of the second week the blastocyst is embedded in the endometrial stroma. The endometrial cells around the early conceptus enlarge and accumulate glycogen and lipids. These cellular changes, together with the vascular and glandular alterations in the endometrium, are called the decidual reaction.

Implantation site

Blastocyst during implantation

1. Endometrium
2. Uterine wall
3. Implanted blastocyst
4. Uterine tube

1. Blood vessel
2. Endometrial stroma
3. Syncytiotrophoblast
4. Cytotrophoblast
5. Surface epithelium
6. Epiblast
7. Aminotic cavity
8. Hypoblast

Three different regions of the decidua are identified according to the implantation site. The decidua basalis is the portion of the endometrium that underlies the implantation site. The decidua basalis forms a compact layer, called the decidual (basal) plate. The decidua capsularis is a thin portion of the endometrium that overlies the conceptus. The decidua parietalis (vera) includes the remaining endometrium of the uterus and the cervix.

Implantation

1. Decidua capsularis
2. Uterine wall
3. Uterine cavity
4. Placenta
5. Decidua parietalis
6. Decidua basalis
7. Chorion leave
8. Embryo
9. Connecting stalk
10. Yolk sac
11. Chorion frondosum
12. Amnion
13. Chorionic cavity
14. Amniotic cavity

Finally:

The period from the 3rd to 8th week of the development is known as the embryonic period. The embryonic period is very important because this is the time when all internal and external structures develop in the embryo. In the period from the 4th to the 8th week, each of the three germ layers gives rise to different tissues and organs. As the result of the organogenesis, the shape of an embryo greatly changes. During this critical period, the exposure of an embryo to certain agents may cause major congenital malformations. The period from the beginning of the 3rd month to the birth is known as the fetal period. The main characteristic of the fetus in this period is the rapid growth of the body and the maturation of the tissues. The fetal growth can be monitored with ultrasound.

External appearance - 4th week

1. Lens placode
2. Pharyngeal arches
3. Otic placode
4. Heart bulge
5. Limb ridge
6. Somites

During the 4th week of the development, the age of the embryo is expressed in the number of somites.
The length of the embryo between the 4th and the 8th developmental weeks is indicated as the crown-rump length (CRL) and is expressed in millimeters. During fetal development (9th - 40th week) the length of the conceptus is indicated as CRL or as the crown-heel length (CHL) expressed in centimeters. External appearance, weight or biparietal diameter are also used as a measures during the gestation.
Growth in length is most intensive during the 3rd to the 5th month, while the increase in weight is most evident during the last two months of gestation. During the fetal life, head growth rate slows down. During the 5th month, the mother can recognize fetal movements. The birth occurs 266 days or 38 weeks after the fertilization.

External appearance - 5th week