Development of Human Embryo: Stages, Formation, and Fate of Germ Layer

Development of Human Embryo: For decades, researchers could only guess at the early stages of the development of human embryos using animal studies and rare tissue samples as guides. The initial eight weeks of development after fertilisation, known as embryogenesis, are a complicated process. It’s incredible to think that humans go from a single cell to an organism with a multi-level body plan in just eight weeks.

During this stage, the circulatory, excretory, and neurologic systems all begin to develop. Fertilization, like many other complicated biological concepts, may be broken down into smaller, simpler concepts. Going from a single cell to a ball of cells to a set of tubes is the central concept of embryogenesis. Plants and mammals have different embryonic development. In mammals, embryonic development takes place in the uterus; nevertheless, the uterus’ development and role in gestation varies greatly among mammalian taxa. Read on to explore more about embryo development in human beings.

Embryology

The study of embryo development is known as Embryology. This covers the transformation of a single-cell embryo into a newborn. Embryology is the study of a foetus’ development before birth. Embryology is a crucial area of study for understanding the effects of mutation and the evolution of genetic disorders. Stem cell research is an important element of embryology. The fertilisation of an egg cell (ovum) by a sperm cell initiates embryonic development (spermatozoon). A zygote is a single diploid cell that develops from a fertilised ovum. The zygote undergoes mitotic divisions with no considerable growth (a process known as cleavage) and cellular differentiation after passing through an organisational barrier during mid-embryogenesis, resulting in the development of a multicellular embryo.

Pre-Embryonic and Embryonic Development

In humans, prenatal development, also known as antenatal development, is the process that spans the time between the formation of an embryo and the delivery of a foetus (or parturition). Cell proliferation, cell specialisation, cell interaction, and cell mobility are all dependent on the machinery.

Germinal Stage

1. It begins with fertilisation and finishes with the end of the second week of intrauterine life (IUL).
2. Fertilization, zygote transportation via the uterine tube, mitotic divisions/cleavage, implantation, and development of primordial embryonic tissues are all morphogenetic phenomena that occur during this time.
3. Fertilization happens when a sperm cell enters and merges with an egg cell successfully (ovum). It takes place in the ampulla, a part of the oviduct.
4. The sperm and egg genetic material combines to form a single cell known as a zygote, and the germinal stage of development begins.
5. The embryo takes 5 days to enter the uterine lumen. The zygote goes through cleavage divisions, which are mitotic divisions that occur without the formation of daughter cells. These cells are called blastomeres.
6. At the 8-cell stage, the embryo undergoes a peculiar process known as compaction, leaving a portion of the blastomeres facing the external side.
7. The outer trophoblast, which becomes the foetal component of the placenta, and the inner cell mass, which forms the embryo proper and extraembryonic membranes, are the two cellular lineages that result from compaction.
8. Once within the uterus, the embryo and the uterine lining recognise each other biochemically, allowing the embryo to be attached and implanted with precision.
9. At implantation, the inner cell mass reorganises into a two-layered embryo: the epithelial epiblast, which forms the embryo and amniotic membrane, and the hypoblast, which aids in yolk sac formation and embryonic axes alignment. The zygote begins to divide at this point, a process known as cleavage.

Cleavage

Fig: Cleavage

1. Cleavage is an embryological process in which a single-celled zygote is transformed into a multicellular structure called a blastula through a series of rapid mitotic divisions.
2. Holoblastic and equal cleavages occur in the human zygote. The fallopian tube is the site where cleavage happens.
3. Cleavage increases the number of cells but does not cause them to grow. It restores the cell size and nucleocytoplasmic ratio, which are both species-specific characteristics.
4. Cleavage brings about the distribution of the cytoplasm of the zygote, amongst the blastomeres.
5. It increases the mobility of the protoplasm, which facilitates morphogenetic movements necessary for cell differentiation, germ layer formation, and the formation of tissue and organs.
6. The unicellular zygote is converted into a multicellular embryo.

Fig: Stages of cleavage

Morula Formation

Fig: Morula Formation Stage of Embryo Development

1. By repeated cleavage divisions, the ovum forms a solid mass of cells( Blastomeres) called Morula (meaning mulberry, because of its appearance).
2. It is formed within the fallopian tube after fertilization.
3. Morula is made up of 16-32 cells from the inner cell mass and the surrounding cell mass that are grouped together in the centre.
4. The inner cell mass gives rise to the embryo’s tissues, while the outside cell mass gives rise to the trophoblast, which eventually develops into the placenta.
5. After fertilisation, the morula reaches the uterus about 4-6 days later, with the zona pellucida intact.
6. The morula enters the uterine cavity at a time when the endometrial lining is in a quite suitable stage to accept it.
7. Within the uterine cavity, the morula floats for about two days and implants into the endometrium on the seventh day.

Blastocyst Formation

Fig: Blastocyst Formation Stage of Embryo Development

1. The development of morula into a blastodermic vesicle or blastocoel begins with the rearranging of tiny blastomeres.
2. A blastocyst is an embryo that is made up of an exterior layer of cells called the trophoblast or trophectoderm and an inner cell mass called the embryoblast.
3. The embryonic or animal pole is the side of the blastocyst to which the inner cell mass is linked, while the abembryonic pole is the opposite side.
4.  The blastocoel and inner cell mass are encircled by the trophoblast. The embryo is formed from the inner cell mass.
5. The trophoblast’s cells help in the nourishment of the embryo. Extraembryonic membranes, chorion, and amnion, as well as a part of the placenta, are formed by them.
6. The cells of the trophoblast that are attached with the inner cell mass are called cells of Rauber.
7. The cells of the inner cell mass are differentiated into two layers: 
   A. Layered small, cuboidal cells known as the hypoblast layer
   B. Layered high columnar cells, the epiblast layer. Both the hypoblast and epiblast form a flat disc called the embryonic disc.

Implantation

Fig: Implantation of Embryo

1. Implantation is the process of attaching the blastocyst to the uterine wall. It happens seven days after fertilisation.
2. In the zone of contact between the blastocyst and the endometrium, the trophoblast produces two layers: syncytiotrophoblast and cytotrophoblast.
3. The blastocyst falls into a depression produced in the endometrium and is entirely encircled by it. To obtain nourishment, it develops villi.
4. The inner cell mass’s cells divide into two layers: hypoblast and epiblast.
5. The embryonic disc is a flat disc formed by the hypoblast and epiblast.
6. Human chorionic gonadotropin is a hormone secreted by the chorionic cells of the placenta (HCG). It has a wide range of functions:
   a. Maintains corpus luteum. 
   b. The corpus luteum stimulates the secretion of progesterone.
   c. Maintains the endometrial lining of the uterus and helps in its growth during the period of pregnancy.
7. The placenta begins to produce enough progesterone by the 16th week of pregnancy, and the corpus luteum begins to recede.

Placentation

1. The invasive syncytial trophoblast’s irregular strands are the first stage in the development of true villi, which are part of the placenta and are briefly discussed below.
2. The chorionic plate of the developing placenta is formed by the deepest embedded section of the chorionic wall.
3. The placenta assists the foetus in a variety of ways, the majority of which involve the exchange of materials carried in the mother’s and foetus’ bloodstreams. These functions are classified as follows:
   a. Nutrition
   b. Respiration
   c. Excretion
   d. Barrier action (e.g., prevention of intrusions by bacteria)
   e. Synthesis of hormones and enzymes.

Gastrulation

Fig: Gastrulation stages of embryo development

1. Gastrulation is the process of transforming a blastocyst into a gastrula with primary germ layers by cell rearrangement.
2. It involves morphogenetic movements, which are cell movements that assist the embryo in taking on a new shape and morphology.

The various stages of gastrulation are:

1. Formation of Embryonic Disc: Inner cell mass and trophoblast make up the early blastocyst. The hypoblast and epiblast cells combine to form a two-layered embryonic disc.
2. Formation of Extra Embryonic Coelom: The trophoblast cells give rise to the extraembryonic mesoderm, which is a mass of cells. Because it sits outside the embryonic disc, this mesoderm is called extraembryonic. It does not result in the formation of embryonic tissue.
3. Formation of Chorion and Amnion: The chorion and amnion, two crucial embryonic membranes, are formed at this stage. The somatopleuric extraembryonic mesoderm inside the chorion and the trophoblast outside form the chorion. Amniogenic cells inside the amnion and somatopleuric extraembryonic mesoderm outside produce the amnion. The chorion eventually becomes the placenta’s major embryonic part. Human chorionic gonadotropin (hCG), a crucial pregnancy hormone, is also produced by the chorion. The amniotic cavity, which is filled with amniotic fluid, is formed by the amnion surrounding the embryo. The amniotic fluid protects the foetus by acting as a shock absorber, regulating body temperature, and preventing desiccation.
4. Formation of Yolk Sac
   A. Inside the blastocoel, flattened cells emerging from the hypoblast spread and line. The major yolk sac is lined by endodermal cells.
   B. The yolk sac (embryonic membrane) becomes much smaller than before with the formation of the extraembryonic mesoderm and later the extraembryonic coelom, and is now known as the secondary yolk sac.
   C. The outer splanchnopleuric extraembryonic mesoderm and inner endodermal cells make up the secondary yolk sac.
   D. The yolk sac is a source of blood cells. It also functions as a shock absorber and helps prevent desiccation of the embryo.
5. Formation of Primitive Streak
   Gastrulation is the process of cells from the epiblast rearranging and migrating. The epiblast forms a primitive streak, which is a faint groove on the dorsal surface. The embryo’s head and tail ends, as well as its right and left sides, are all clearly defined by the primitive stripe.

Formation of Germ/Embryonic Layers

The epiblast cells move inward below the primitive streak and detach from the epiblast after the development of the primitive streak. Invagination is the term for this inverting movement.

1. When the cells invaginate, some of them displace the hypoblast, resulting in the formation of the endoderm. During embryonic development, the endoderm develops initially.
2. The mesoderm is formed by cells that stay between the epiblast and the newly developed endoderm.
3. Ectoderm is formed by cells that stay in the epiblast.

Fig: Formation of Germ/Embryonic Layers

Endoderm, mesoderm, and ectoderm are the three germ layers that give rise to all of the body’s tissues and organs.

The fate of Three Germ Layers: Each germ layer gives rise to specific tissues, organs, and organ systems. The germ layers have a similar fate in various animals.

Derivatives of Ectoderm: The ectoderm gives rise to the central nervous system (brain and spinal cord), peripheral nervous system, sensory epithelia of the eye, ear, and nose, epidermis and its appendages (nails and hair), mammary glands, hypophysis, subcutaneous glands, and tooth enamel, the epidermis of the skin, hair, arrector pili muscles, nails, sudoriferous (sweat) and sebaceous (oil) glands and chromatophores (pigment cells) of skin.

Derivatives of Mesoderm: The mesoderm gives rise to connective tissue, cartilage, and bone; striated and smooth muscles; the heart walls, blood and lymph vessels, and cells; the kidneys; the gonads (ovaries and testes) and genital ducts; the serous membranes lining the body cavities; the spleen; and the suprarenal (adrenal) and connective tissues including loose areolar tissue, ligaments, tendons and the dermis of the skin.

Derivatives of Endoderm: Epithelium of mouth, part of palate, tongue, tonsils, Epithelium of Eustachian tube, middle ear, Epithelium of larynx, trachea, bronchi, and lungs. Epithelium of the gallbladder, liver, pancreas, including islets of Langerhans, gastric and intestinal glands. Epithelium of thyroid, parathyroid, and thymus glands.

Neurulation

Fig: Neurulation

1. Neurulation commences the construction of central nervous system structures, while organogenesis develops the basic design for all organ systems. The ectoderm produces epithelial and neural tissue after gastrulation, and the gastrula is now known as the neurula.
2. The neural plate, which began as a thicker ectoderm plate, continues to spread, and its ends begin to fold upwards as neural folds.
3. The folding process that transforms the neural plate into the neural tube is known as neurulation, and it occurs during the fourth week.
4. They fold along a shallow neural groove in the neural plate that has evolved as a dividing median line.
5. As the folds grow in height, they will eventually meet and close together at the neural crest.
6. The embryonic disc starts off flat and spherical, but as it grows longer, it develops a wider cephalic portion and a narrow-shaped caudal end. The neural tube is formed when the cranial and caudal neuropores gradually shrink until they close entirely (by day 26).

Organogenesis

1. Organogenesis is the process of organ development that begins in the third week and lasts until birth.
2. During development, the circulatory, excretory, and nervous systems all begin to develop.
3. The mesoderm produces hematopoietic stem cells, which give rise to all blood cells.
4. At roughly 21 or 22 days, the heart becomes a functioning organ and begins to beat and pump blood.
5. The process of vasculogenesis (the formation of the cardiovascular system) begins. On day 18, cells in the splanchnopleuric mesoderm start to differentiate into angioblasts, which eventually become flattened endothelial cells.
6. The digestive system begins to grow in the third week, and the organs have correctly positioned themselves by the twelfth week.
7. The respiratory system begins with the lung bud, which appears about four weeks into development in the ventral wall of the foregut. The lung bud forms the trachea as well as two lateral growths known as bronchial buds, which increase at the start of the fifth week to produce the left and right main bronchi. Secondary (lobar) bronchi are formed by these bronchi; three on the right and two on the left (reflecting the number of lung lobes). Secondary bronchi give rise to tertiary bronchi.
8. The urogenital system starts to take shape.
9. The epidermis begins to form in the second month of development and takes on its definitive arrangement by the fourth month.

Summary

Fertilization is the first step in embryonic development. During its development, the embryo travels through several stages. A zygote is formed when an unfertilized egg is released from the ovary following fertilization. The single-celled zygote splits into the morula, a spherical mass of cells. The morula then splits to produce a blastocyst, a hollow sphere with a single layer enclosing it called the trophoblast and cells projecting from within. The blastocysts attach themselves to the uterine wall. The embryo, which is three weeks old, looks to be a little organism. The heart and blood vessels develop after five weeks. After eight weeks, the organisms begin to grow limbs and take on the appearance of a human being. Following the completion of 40 weeks, an infant is born.

Frequently Asked Questions (FAQs) on Development of Human Embryo

Q.1. What is the development of an embryo?
Ans: Embryonic development, also known as embryogenesis in developmental biology, is the growth of an animal or plant embryo. The fertilisation of an egg cell (ovum) by a sperm cell initiates embryonic development (spermatozoon). The fertilised ovum develops into a single diploid cell called a zygote.

Q.2. How many weeks is the embryonic stage?
Ans: Following conception, the zygote enters the embryonic stage, which is a period of rapid development. This period lasts from the fifth to the tenth week of pregnancy. The infant is known as an embryo at this time.

Q.3. What is the difference between embryo and fetus?
Ans: The embryonic stage is all about the development of the body’s major systems. The foetal phase, on the other hand, is more about your baby’s growth and development so that the newborn may survive in the world.

Q.4. What is gastrulation, and what are the 3 layers?
Ans: The single-layered blastula is rearranged into a trilaminar (three-layered) structure known as the gastrula during the early stages of embryonic development. The ectoderm, mesoderm, and endoderm are the three germ layers.

Q.5. How are embryos formed in humans?
Ans: Initially, the zygote forms a solid ball of cells. The blastocyst, a hollow ball of cells, is then formed. The blastocyst implants in the uterine wall and develops into an embryo that is attached to the placenta and surrounded by fluid-filled membranes.

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