2

Fig. 4922 hours after the fertilization a mitotic spindle has formed (more info).The nucleic membranes of the two pronuclei dissolve. This is the visible sign that the zygote has been created.

Fig. 5023 hours after fertilization: the spindle apparatus in the anaphase; the chromosomes are pushed apart. (More info with videos 620 kB or 940 kB)

The zygote - by definition the first cell of the embryo undergoes only an incomplete cell cycle. With the penetration of the sperm cell, the genetic information of the two parents are not instantly brought together in order that they are then duplicated together and distributed in the subsequent mitosis. In reality, the paternal and also the maternal genetic information are first duplicated in two independent pronuclei and also there condensed again into chromosomes so they can align themselves along the equator of the mitotic spindle.

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The formation of the zygote - Embryology

Sexual differentiation, in human embryology, the process by which the male and female sexual organs develop from neutral embryonic structures. The normal human fetus of either sex has the potential to develop either male or female organs, depending on genetic and hormonal influences.

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sex: Differentiation of the sexes

Differentiation between the sexes exists, therefore, as the primary difference represented by the distinction between eggs and sperm, by differences represented by nature of the reproductive glands and their associated structures, and lastly by differences, if any, between individuals possessing the male and female reproductive

In humans, each egg contains 23 chromosomes, of which 22 are autosomes and 1 is a female sex chromosome (the X chromosome). Each sperm also contains 23 chromosomes: 22 autosomes and either one female sex chromosome or one male sex chromosome (the Y chromosome). An egg that has been fertilized has a full complement of 46 chromosomes, of which two are sex chromosomes. Therefore, genetic sex of the individual is determined at the time of fertilization; fertilized eggs containing an XY sex chromosome complement are genetic males, whereas those containing an XX sex chromosome complement are genetic females.

Every fetus contains structures that are capable of developing into either male or female genitalia, and, regardless of the complement of sex chromosomes, all developing embryos become feminized unless masculinizing influences come into play at key times during gestation. In males, several testis-determining genes on the Y chromosome direct the sexually undifferentiated (indeterminate) embryonic gonads to develop as testes. The X chromosome also participates in the differentiating process, because two X chromosomes are necessary for the development of normal ovaries.

Two precursor organs exist in the fetus: the Wolffian duct, which differentiates into the structures of the male genital tract, and the Mllerian duct, the source of the female reproductive organs. During the third month of fetal development, the Sertoli cells of the testes of XY fetuses begin to secrete a substance called Mllerian inhibiting hormone. This causes the Mllerian ducts to atrophy instead of develop into the oviducts (fallopian tubes) and uterus. In addition, the Wolffian ducts are stimulated by testosterone to eventually develop into the spermatic ducts (ductus deferens), ejaculatory ducts, and seminal vesicles. If the fetal gonads do not secrete testosterone at the proper time, the genitalia develop in the female direction regardless of whether testes or ovaries are present. In normal female fetuses, no androgenic effects occur; the ovaries develop along with the Mllerian ducts, while the Wolffian duct system deteriorates. Sexual differentiation is completed at puberty, at which time the reproductive system in both women and men is mature.

In such a complex system there are many opportunities for aberrant development. The causes of disorders of sexual differentiation, while not fully understood, have been greatly elucidated by advances in chromosomal analysis, the identification of isolated genetic defects in steroid hormone synthesis, and the understanding of abnormalities in steroid hormone receptors.

For more information about the embryological and anatomical aspects of the gonads and genitalia, see human reproductive system. For descriptions of chromosomes and the genes that they carry, see human genetics.

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Sexual differentiation | embryology | Britannica.com

The embryonic time comprises 56 days, i.e., 8 weeks from the moment of fertilization. This time span is divided into 23 Carnegie stages and the stage classification is based solely on morphologic features. Carnegie stages are thus neither directly dependent on the chronological age nor on the size of the embryo. This can be illustrated by two examples: The closure of the rostral neuropore occurs by definition in stage 11 and that of the caudal neuropore in stage 12. Further, between the 25th and 32nd days of the pregnancy, the stages are determined according to the number of the somites 9-13 that have been engendered. The individual stages thus differ in how long they last .During the embryonic period most of the organ systems are established and this with an enormous rapidity. Cell divisions, movement and differentiation are the basic processes taking place during this phase. It is thus hardly surprising that this pregnancy phase is very vulnerable and that deformities are produced most often during this time. The type of deformity depends on the embryonic developmental stage.

Fig. 1Segment A represents the embryonic period in which the embryo is especially sensitive with respect to deformities. Within the first eight weeks, the incidence of deformities (blue curve), that lead to miscarriages, decreases from more than 10% to 1% during the fetal period (B). The frequency of neural tube defects decreases from 2.5% to 0.1% (green curve) by the end of the embryonic period. (2)

According to estimates, over 90% of the 4500 designated structures of the adult body are already established - and can be distinguished - during the embryonic period (1). During the fetal period the organs that formed during the embryonic period grow and differentiate (organogenesis).

Figure 2 shows the various temporal phases during a pregnancy. A rough classification is made by assigning trimesters (trimenon). The LMP (Last Menstruation Period) is not the real beginning of the pregnancy but serves as a point of reference for determining the date of ovulation and thus the moment of fertilization. Normally this occurs 14 days after the beginning of menstruation, but can vary a lot temporally. From the time of the last period, one estimates 40 weeks after the last menstruation in order to determine the approximate date of birth (the second and third grid marks represent the lunar month [of 28 days] or 4 weeks). On average, though, the duration of an actual pregnancy amounts to 266 days or 38 weeks (fourth grid). The embryonic period (A) lasts 8 weeks and the fetal period (B) from the 9th week to the birth.

Fig. 2The schematic diagram shows the various time periods during the entire pregnancy. LMP = Last Menstruation Period. The embryonic period (A) lasts 8 weeks and the fetal period (B) from the 9th week to the birth, i.e., 30 weeks.

In obstetrics the pregnancy weeks (PW) are normally reckoned from the date of the Last Menstrual Period (LMP). This is a point in time that many women can easily remember. Computed this way, the pregnancy lasts 40 weeks and the embryonic period - accordingly - 10 weeks. Caution is advisable, though, when wishing to calculate the moment of ovulation - and thus fertilization, closely connected with it - because the moment of ovulation can vary and depends on many factors (conditioned by the environment and psychological aspects). In embryology the temporal indices (i.e., the PW), therefore, always refer to the moment of fertilization even though in practical midwifery the time following the LMP is still used for computations.

After the 8th week, the fetus takes on typical human features, even though at the end of the first trimenon, the head is still relatively large in appearance. The eyes shift to the front and the ears and nasal saddle are formed. The eyelids are also clearly recognizable now. On the body, fine lanugo hairs are formed, which at the time of birth are replaced by terminal hairs. The physiologic umbilical hernia that arises in the embryonic period 15-20 has mostly disappeared. In the second trimenon the mother feels the first movements of the child. In the last trimenon the subcutaneous fatty tissue is formed and stretches the still wrinkled skin of the fetus. The skin becomes covered more and more with vernix caseosa. This is a whitish, greasy substance und consists of flaked off epithelial cells and sebaceous gland secretions. In neonatology this vernix caseosa is an important criterion for judging the maturity of the child. If the birth occurs post-term, it disappears again.

Fig. 3 - Fetus at 8 weeks

Legend

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Developmental stages - Embryology

Embryo, the early developmental stage of an animal while it is in the egg or within the uterus of the mother. In humans the term is applied to the unborn child until the end of the seventh week following conception; from the eighth week the unborn child is called a fetus.

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morphology: Embryology

Development typically begins in animals with the cleavage, or division, of the fertilized egg (zygote) to form a hollow ball of cells called the blastula; the blastula then develops into a hollow cuplike body of two layers of cells, the gastrula, from which the embryo ultimately is formed. At

A brief treatment of embryonic development follows. For full treatment, see morphology: Embryology.

In organisms that reproduce sexually, the union of an ovum with a sperm results in a zygote, or fertilized egg, which undergoes a series of divisions called cleavages as it passes down the fallopian tube. After several cleavages have taken place, the cells form a hollow ball called a blastula. In most mammals the blastula attaches itself to the uterine lining, thus stimulating the formation of a placenta, which will transfer nutrients from the mother to the growing embryo. In lower animals the embryo is nourished by the yolk.

By the process of gastrulation, the embryo differentiates into three types of tissue: the ectoderm, producing the skin and nervous system; the mesoderm, from which develop connective tissues, the circulatory system, muscles, and bones; and the endoderm, which forms the digestive system, lungs, and urinary system. Mesodermal cells migrate from the surface of the embryo to fill the space between the other two tissues through an elongated depression known as the primitive streak. As the embryo develops, the cell layers fold over so that the endoderm forms a long tube surrounded by mesoderm, with an ectodermal layer around the whole.

Nutrients pass from the placenta through the umbilical cord, and the amnion, a fluid-filled membrane, surrounds and protects the embryo. The division of the body into head and trunk becomes apparent, and the brain, spinal cord, and internal organs begin to develop. All of these changes are completed early in embryonic development, by about the fourth week, in humans.

Between the head and the heart, a series of branchial arches, cartilaginous structures that support the gills of fishes and larval amphibians, begin to form. In higher vertebrates these structures form part of the jaw and ear. Limb buds also appear, and by the end of the embryonic stage, the embryo is distinguishable as a representative of its species.

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Embryo | human and animal | Britannica.com

ABOUT THE 3D ATLAS OF HUMAN EMBRYOLOGY

The 3D Atlas of Human Embryology comprises 14 user-friendly and interactive 3D-PDFsof all organ systems in real human embryos between stage 7 and 23 (15 till 60 days of development), and additional stacks of digital images of the original histological sectionsand annotated digital label files.

The atlas was created by students and embryologists of the Department of Anatomy, Embryology & Physiologyof the Academic Medical Center (AMC) in Amsterdam, the Netherlands and it is made freely available to the scientific community to facilitate veracious embryology education and research.

Click here to access the Science publication,hereto learn more about the project, hereto access the atlas content or feel free to donate.

Stage 13embryo(28-32days)

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Embryology questions (practice) | Cells |Khan Academy

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Embryology questions (practice) | Cells | Khan Academy

We cannot pursue the mind into embryology: we can only trace how, after birth, it begins to grow.

See further the same author's article "Embryology" in the Ency.

This comparison I have been able to make from a study of the embryology of Isotoma.

Is there any fallacy in speaking of the Embryology of the New Life?

But that belongs to the domain of Embryology, which is a separate science.

He is not only a master of zoology, but has a good command of botany and embryology.

The same holds good with the evidence from Embryology and from Palontology.

Until then embryology had been regarded as a purely descriptive science.

The embryology of Blatta germanica and Doryphora decimlineata.

The whole of embryology, in every department, is made up of examples of the same law.

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Embryology Synonyms, Embryology Antonyms | Thesaurus.com

A human begins life as a fertilized ovum. This single cell gives rise to the millions of cells that form the human body. In the first few days following fertilization, the developing embryo consists of a ball of cells. This implants on the wall of the uterus and begins to grow further, supported by nutrients and blood from the mother.

As the developing embryo grows in the first few weeks, there is increasing complexity from differentiation of the cells into specialized tissues to form specific organs. This differentiation is directed by genetic factors inherited via the chromosomes from both mother and father. Most organs are formed between 5 and 8 weeks of life. After that, there is continued growth and development to the time of delivery of the baby, which typically occurs following 38 to 42 weeks of gestation in utero.

Embryology Simplified

The three major embryologic categories of cells, called the germ cell layers, are:

Ectoderm: forms the epithelium that covers the body, and gives rise to cells in the nervous system

Endoderm: forms the gastrointestinal tract and associated structures involved in digestion

Mesoderm: forms the connective tissues and "soft" tissues such as bone, muscle, and fat

After birth, some cells within the body continue to proliferate, while others do not and remain or are lost in the aging process. Aging results from the inability of cells to maintain themselves or replace themselves.

The following discussion will introduce you to the types of cells and tissues that constitute the human body. Examples of the major cell types, along with the organs they compose, will be demonstrated with histologic sections.

The genes that direct cellular proliferation and development in embryologic life are "turned off" or suppressed once appropriate growth has been achieved. However, when some of these genes are "turned on" inappropriately because of mutations or alterations (oncogenes), or when the genes that suppress growth (tumor suppressor genes) become faulty later in life, then the result can be neoplasia.

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Human embryogenesis (article) | Embryology |Khan Academy

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Human embryogenesis (article) | Embryology | Khan Academy

Embryology can be defined as the study of an embryo from fertilization through its development to the fetus stage. Our trained team of embryologists give careful attention to nurturing embryos as they develop so that healthy embryos (with a high chance of implantation) can be transferred back into the uterus, ultimately giving you the best chance at a healthy pregnancy. While some women may opt to have their embryos transferred fresh (after retrieval and in vitro culturing), our embryologists are also able to culture several embryos for later transfer with Frozen Embryo Transfers, using the best in cryopreservation to keep embryos for an optimal transfer time.

In addition, at New Hope Fertility Center, our definition of embryology extends past just taking care of an embryo from the point of fertilization. HRSS andICSI allow our embryology team to choose healthy sperm to fertilize your egg, and our advanced methods for assisted hatching and in vitro maturationgive your embryos the highest chance for healthy development.

Last, but certainly not least, we are proud to be one of a handful of centers who have EmbryoScope technology. Using EmbryoScopes, we are able to get time-lapsed information and monitoring of your embryos development.

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Embryology | Embryo Development | New Hope Fertility Center