What is an Embryo?

An embryo is known as the early stage of development of a multicellular living being, which is still undergoing the process of development. It can be considered as the unhatched offspring. Embryonic development is part of the life cycle and just starts after the fertilization in organisms that reproduce sexually. An embryo can generally develop in three places, inside the mother's body, inside the egg shell, or in the egg outside mother's body.

Concept of Embryology

Embryology is a branch of science, which is mainly studied in the field of biology. It is related to the formation, growth, and development of an embryo and explains the beginning of the formation of gametes to the prenatal stage of development. It deals not only with gamete formation, but also with fertilization, zygote formation, embryo growth, development of the fetus, and ultimately leads to the birth of an offspring.

Branches of Embryology

Preformationism is the term that signifies the embryology in the early era. It means a theory that explains the development of organisms from its miniature versions. The Italian physician Marcello Malpighi is known as the preformationist because of his contribution to science. Besides Malpighi, another biologist Jan Swammerdam also worked on preformation theory. Aristotle proposed a thesis, which is currently recognized as epigenesis. Epigenesis marks the idea that elaborates the development of an organism, initiated from seed or egg in some synchronized steps. The modern embryology concept arose by Karl Ernst von Baer, which introduces him as the "Father of Embryology". Besides Karl Ernst, Malpighi is also considered the father of physiology and embryology. There are some branches of embryology known to be present, which are-

• Descriptive Embryology: It is associated with the morphological elaboration of the different embryonic stages. It describes the ontogenetic development of individuals of certain species. Embryologists who belonged to the 18th century used to work in this field.
• Comparative Embryology: It describes the comparison of the development of the embryo in different animal groups.
• Experimental Embryology: This branch includes studies that attempt to understand several basic mechanisms during the development of different animals like fertilization, cleavage, gastrulation, embryonic induction, determination, and differentiation.
• Chemical Embryology: This branch of science helps to understand biochemical, biophysical, and molecular changes at the time of embryogenesis.
• Teratology: It is concerned with the study of birth defects or deformities. The molecules that cause these malformations are called teratogens.

Embryologic categories of cells

The three major embryologic categories of cells found cumulatively are called the germ layers.

1. Ectoderm: It forms the epithelium that protects the body superficially and also enhances the production of nerve cells.
2. Endoderm: This builds up the gastrointestinal tract of the human body along with the supportive structures mandatory for the digestion process.
3. Mesoderm: It forms the connective tissues and mainly the soft tissues like muscle, fat, and bone marrow.

The stages of the Embryo development

Embryo development starts from a successful pregnancy. It happens due to the proper fusion of male gamete and female gamete. The development goes for some certain stages-

1. Fertilization
2. Cleavage
3. Gastrulation
4. Organogenesis

Fertilization

Fertilization happens when a spermatozoa (male reproductive cell) fuses with the mature, fully developed ovum or egg cell (female reproductive cell). This fusion generates fertilized single mono-nucleated cell named zygote. The cell is diploid because of the incorporation of chromosomes or genetic material from mother and father. In humans, fertilization happens in the fallopian tube of the female body, and in plants, it occurs after pollination and germination. Pollens are considered the male microgametes of seed plants. 

Cleavage

Cleavage is the repeated mitotic division of the zygote and marks the initiation of embryogenesis. Embryogenesis refers to embryonic development throughout time. Cleavage is the series of fast cellular divisions (mitosis) without any cellular growth. The types of cleavages are known for the different types of living beings. It has main two targets. The first one is the formation of the multicellular embryo and the second one is organizing the embryo into development areas. The blastocyst is the name of the spherical cell ball that comes after the morula structure of embryonic development. The blastocyst reaches the uterus through traveling and embeds itself in the cell lining. It is called implantation. The AP (anterior-posterior) axis formation in an embryo initiates at the end of the blastoderm stage. This AP marks the initiation of the next stage.

Gastrulation

It is the early phase in the embryonic development of most animals as well as humans. In this stage, a single-layered blastula undergoes a conformational change into a trilaminar shape called the gastrula. It is a crucial time for the formation of multicellular animals. At this stage, three primary germ layers are established. The basic body plan formation becomes almost complete along with the architecture of the primary body axes. During this stage, cells are arranged into some new positions, which allows them to initiate neurulation and organogenesis. The neural tube formation occurs in this stage, which later converts into the spinal cord. The brain development of an embryo starts in this stage after neural tube formation. The hindbrain and forebrain develop with folding and closing of the neural tube. Engrailed-1 or en-1 is the target of the Wnt-1 (Wingless-related integration site) signaling pathway that supervises the midbrain development. This pathway regulates cell growth, motility, and differentiation in the human as well as most animals.

Organogenesis

This states the production and development of the organs of an animal or plant. It is a series of organized, integrated phenomena that creates a proper structure in the animal and plant body. It starts after gastrulation and ends until the human or the animal is born. It helps maintain several functions like regulation of cell division, cell expansion, cell and tissue type differentiation. In chordates, limb development, limb bud structure is formed earliest. Limb bud happens due to the interaction between ectoderm and mesoderm layers. The spinal cord begins the development in the third week of the embryonic period. Mainly ectoderm germ layer builds up this spinal cord along with the "central nervous system" (CNS).

Significance of Embryology

In the field of biological science, the study of embryology is very essential. Embryology defines the study of early structures of life before birth. It is a vital branch of biological studies as it enables the understanding of the growth and development of an embryo of a certain animal or human or plant. It has a clear connection with evolutionary science. The relation between various species and how their evolution occurred, can be clear due to this concept. The study of embryology is directly connected with the study of embryogenesis. In other words, it can be said, embryonic progression is part of embryology. This part of science takes a hands-on approach in the medical field. The synchronized method of development creates a state-of-the-art in developmental science.

Context and Applications

Embryology is a huge part of biological science. It is related to evolution, cell biology, and so on. It is mainly studied in “developmental biology”. In addition to that, some other parts of biology also discuss this topic-

• Bachelors in Science in Zoology
• Masters in Science in Zoology
• Masters in Medical Science

Practice Problems

(1) Which branch of embryology deals with the chemical components?

1. 1. Comparative embryology
   2. Chemical embryology
   3. Medical embryology
   4. None of the above

Correct Answer: b. Chemical embryology. This field of embryology states the chemical ingredients for growing a healthy embryo.

(2) What is the name of the agent that causes physical deformities?

1. 1. Carcinogen
   2. Poison
   3. Teratogen
   4. Androgen

Correct Answer: c. Teratogen. This agent results in the formation of the abnormal embryo during pregnancy.

(3) Which forms the soft layer of tissues?

1. 1. Morula
   2. Blastula
   3. Zygote
   4. The endoderm layer

Correct Answer: d. The endoderm layer. This layer forms the soft tissue layers of the body like the lining of the organs like the digestive tract, liver, lungs, and pancreas.

(4) In which part of the human body does the fertilization take place?

1. 1. Uterus
   2. Fallopian tube
   3. Ovary
   4. Vagina

Correct Answer: b. Fallopian tube. The egg is released from the ovary into the fallopian tube. Male gametes reach there to impregnate it.

(5) Who is recognized as the father of embryology?

1. 1. Mendel
   2. Darwin
   3. Newton
   4. Karl Ernst von Baer

Correct Answer: d. Karl Ernst von Baer. He experimented with the modern concept of embryology and is recognized as the “Father of Embryology” for his contribution.

Common Mistakes

Students often confuse embryology with reproduction. Embryology is just a part of reproduction. The formation of embryo happens due to the reproduction, but further continuation of embryogenesis has no relation with the reproduction.

Related Concepts

The fetal development is quite related to this part. The embryonic stage remains at the first 8 weeks of development, whereas fetal development starts just after that and remains 30 weeks. So, fetal development is closely related to embryonic development. The proper embryo formation only can induce the further process of development.

• Reproductive Biology
• Evolutionary Biology
• Anatomy
• Physiology