What is an Embryo?
Let’s know what an embryo is! The early developmental stage of an animal, while it is in the egg or within the uterus of the mother, is known as the embryo. In humans, the term embryo is applied to the unborn child until the end of the seventh week; from the eighth week, till the unborn child is known as a fetus.
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In the organisms that generally reproduce sexually, the union of an ovum with a sperm results in the formation of a zygote, or fertilized egg, which undergoes a series of divisions known as cleavages as it passes down the fallopian tube. After several cleavages have taken place, the cells form a hollow ball which is known as a blastula. In most mammals the blastula attaches itself to the uterine lining, thus stimulating the formation of a placenta, which is responsible for transferring nutrients from the mother to the growing embryo. In lower animals, the yolk nourishes the embryo.
Embryonic Stage in Animals
During the development of an animal embryo, its cells divide, grow, and migrate in specific patterns to make a more and more elaborate body (plant cells perform differential expansion instead of migration). To function properly, that body needs well-defined axes (such as head vs. tail). Embryos need a specific collection of many-celled organs and other structures, positioned in the right spots along the axes and connected up with one another in the right ways. How are all of these complex processes accomplished and coordinated? They occur via four essential stages in early animal development which we will discuss below:
Fertilization: The process of a single sperm cell combining with a single egg cell to form a zygote is known as fertilization.
Cleavage: The rapid, multiple rounds of mitotic cell division where the overall size of the embryo does not increase is known as cleavage. The developing embryo is known as a blastula following completion of cleavage.
Gastrulation: The dramatic rearrangement (movement) of cells in the blastula to create the embryonic tissue layers is known as gastrulation. These tissue layers will go on to produce the tissues as well as organs of the adult animal.
Organogenesis: The process of organ and tissue formation via cell division and differentiation is known as organogenesis.
The last two stages, gastrulation, and organogenesis, together contribute to morphogenesis: the biological processes that result in an organism’s shape and body organization.
Embryos of Different Animals
Embryos of many different kinds of animals: mammals, birds, reptiles as well as fishes look very identical and it is often difficult to tell them apart. It is known that many traits of one type of animal appear in the embryo of another type of animal. For example, fish embryos, as well as human embryos both, have gill slits. In fish they develop into gills, but in humans, they disappear before birth.
This basically shows that the animals are similar and that they develop similarly, implying that they are related, have common ancestors, and that they started out the same, gradually evolving different traits, but that the basic plan for a creature's beginning remains indistinguishable.
Embryonic Formation Stage in Humans
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These stages will help you understand the embryo meaning. The embryo is about the size of a pea at the age of four weeks. A primitive heart is beating, the head of the baby is defined with rudimentary eyes as well as ears, and tiny bumps represent arms and legs. The embryo also contains a primitive nervous system, and the head generally starts to enlarge. A cartilage skeleton has appeared, and the muscles of the child have taken shape.
By the end of eight weeks, the embryo starts looking like a human. Facial features are evident, and most of the organs are well developed by the end of eight weeks. From this point onward, the development consists chiefly of growth and maturation. The embryo is about 1.5 inches in length. Henceforth it is known as a fetus.
Nourishment of the embryo, and then the nourishment of the fetus, is accomplished through the placenta. The maternal, as well as embryonic blood supplies, meet at this organ, but the blood does not mix. Instead, diffusion is responsible for the passage of gases, nutrients, and waste products across the membranous barriers.
The placenta is also an endocrine gland because it secretes estrogen and progesterone to continue to inhibit follicle development and maintain the integrity of the endometrium. As the embryo becomes a fetus, it moves away from the placenta, and a length of tissue known as the umbilical cord becomes its source of attachment to the maternal blood supply.
Types of Embryo
Based on the number of megaspores, we can classify the embryo sacs into three types: monosporic, bisporic, and tetrasporic.
What are the 4 Stages of Embryonic Development?
Germinal stage. 1.1 Fertilization. 1.2 Cleavage. 1.3 Blastulation. 1.4 Implantation. 1.5 Embryonic disc.
Gastrulation.
Neurulation.
Development of organs and organ systems.
FAQs on Embryo in Humans and Animals
1. What is an embryo, and how does it differ from a zygote and a fetus?
An embryo is the early stage of development of a multicellular organism. In humans, it begins after the zygote implants in the uterine wall and lasts until the end of the eighth week of gestation. It differs from the other stages in the following ways:
- A zygote is the single-celled organism that results from the fusion of a sperm and an egg during fertilisation. It is the very first stage before it begins to divide.
- A fetus is the stage that follows the embryonic stage. In humans, this period starts from the beginning of the ninth week after fertilisation and continues until birth, and is primarily characterised by the growth and maturation of already formed organs.
2. What are the four main stages of embryonic development in animals?
The four essential stages of early embryonic development that lead to the formation of a complex organism are:
- Fertilisation: The initial stage involving the fusion of a sperm and an egg to form a single-celled zygote.
- Cleavage: A series of rapid mitotic divisions of the zygote. This process forms a hollow ball of cells called a blastula, without increasing the overall size of the embryo.
- Gastrulation: A dramatic rearrangement and migration of cells in the blastula to create the three primary germ layers: ectoderm, mesoderm, and endoderm.
- Organogenesis: The final stage where the three germ layers differentiate and develop into the specific tissues and organs of the animal's body.
3. How is an embryo nourished inside the mother's uterus?
The embryo is nourished through the placenta, a specialised organ that connects the developing embryo to the uterine wall. The mother's and embryo's blood supplies come into close contact at the placenta, but they do not mix. Nutrients, oxygen, and essential antibodies pass from the mother to the embryo via diffusion across this barrier. Simultaneously, waste products like carbon dioxide diffuse from the embryo's blood back to the mother's to be eliminated.
4. Why do embryos of different animals, such as fish and humans, look so similar in their early stages?
The striking similarity between the early embryos of different vertebrates, such as the presence of gill slits and tails in both fish and human embryos, is strong evidence of a common ancestry. This concept, known as comparative embryology, suggests that diverse species share ancient developmental genes and pathways. While these features develop into functional gills in fish, they are modified or disappear in humans as development proceeds, highlighting how evolution builds upon a conserved foundational body plan from a common ancestor.
5. What is the fundamental importance of gastrulation during embryonic development?
Gastrulation is a critical phase because it establishes the three-dimensional blueprint for the entire body. Its main significance is the transformation of the simple, single-layered blastula into a multi-layered structure (gastrula) with three primary germ layers:
- Ectoderm (outer layer): Forms the nervous system, skin, and hair.
- Mesoderm (middle layer): Forms muscle, bone, the circulatory system, and other internal organs.
- Endoderm (inner layer): Forms the lining of the digestive tract and respiratory system.
Without the formation of these layers during gastrulation, cell differentiation and the subsequent formation of all tissues and organs could not occur.
6. How does the process of cleavage in a developing embryo differ from normal cell division (mitosis)?
While cleavage is a form of mitosis, it has a key difference from normal cell division. In typical mitosis, cells undergo a growth phase (interphase) to increase in size before dividing. In cleavage, this growth phase is skipped, and the cell division cycles are extremely rapid. As a result, the daughter cells, called blastomeres, become progressively smaller with each division, and the overall size of the embryo does not increase. The purpose of cleavage is to rapidly multiply the number of cells, not to grow the organism in size at this stage.
7. What is the role of the placenta beyond just providing nutrition to the embryo?
Besides its primary function in nutrient and waste exchange, the placenta also acts as a vital endocrine organ. It produces several crucial hormones to regulate and maintain the pregnancy:
- Human Chorionic Gonadotropin (hCG): This hormone maintains the corpus luteum, which produces progesterone to sustain the pregnancy in its early stages.
- Progesterone and Estrogen: Later in the pregnancy, the placenta takes over the production of these hormones, which are essential for maintaining the uterine lining (endometrium) and preparing the mother's body for birth.
