Organogenesis

Organogenesis is the process through which the embryo's three germ tissue layers, the ectoderm, endoderm, and mesoderm, develop into the organism's internal organs. Organs develop from germ layers by differentiation, which is the transformation of a less-specialized cell into a more-specialized cell type. As a zygote develops into a fully grown organism, this must happen several times. Embryonic stem cells express specific sets of genes during differentiation that determine their ultimate cell type. Some cells in the ectoderm, for example, will express genes specific to skin cells. These cells will then develop into epidermal cells as a result.

This article will study organogenesis meaning and organogenesis in humans in detail.

Organogenesis in Mammals

All three germinal layers are present in many organs. Glands, for example, frequently get their lining from an ectodermal or endodermal epithelium, and their connective tissue (occasionally in the form of a capsule) from mesodermal mesenchyme. The lining of the alimentary canal is also developed by parts of ectoderm and endoderm, while the connective tissue and muscular sheath of the canal are provided by mesoderm.

Human embryonic development, also known as human embryogenesis, is the process of a human embryo's development and production. It is defined by the embryo's cell division and cellular differentiation processes, which take place in the early stages of development. The growth of the human body from a single-celled zygote to an adult human being is referred to as biological development. When a sperm cell successfully enters and merges with an egg cell, this is known as fertilisation (ovum).

The sperm and egg genetic material subsequently combines to create a single cell known as a zygote, and the germinal stage of development begins. The first eight weeks of human development are referred to as embryonic development, and the embryo is referred to as a foetus at the start of the ninth week. The study of this development during the first eight weeks following fertilisation is known as human embryology. The average gestational period (pregnancy) lasts roughly nine months or 40 weeks.

Organogenesis Embryology

Fertilization occurs when the spermatozoon enters the ovum successfully and the two sets of genetic material carried by the gametes merge to form the zygote (a single diploid cell). This commonly happens in one of the fallopian tubes' ampulla. The zygote comprises the genetic material carried by both male and female gametes, which includes the 23 chromosomes from the ovum nucleus and the 23 chromosomes from the sperm nucleus. Prior to mitotic division, the 46 chromosomes undergo modifications, resulting in the development of a two-cell embryo.

When the zygote divides into two cells by mitosis, this marks the start of the cleavage process. This process of mitosis continues, with the first two cells dividing into four, eight, and so on. Each division takes anything from 12 to 24 hours to complete. The zygote is enormous in comparison to other cells, and it undergoes cleavage without expanding in size. This means that the ratio of nuclear to cytoplasmic material increases with each subdivision. The blastomeres (blastos Greek meaning sprout) are undifferentiated dividing cells that collect into a sphere encased within the ovum's glycoprotein membrane (termed the zona pellucida).

Organogenesis in Human Embryo/Organogenesis in Fetus

Organogenesis in humans is the process of organ development that begins during the third to eighth week of pregnancy and lasts till birth. In other cases, such as the lungs, full development continues after birth. Different organs have a role in the development of the body's many organ systems.

Organogenesis Products

1. Blood 

The mesoderm produces haematopoietic stem cells, which give rise to all blood cells. The yolk sac contains clusters of blood cells called blood islands where blood production occurs. Mesodermal hemangioblasts form blood islands on the umbilical vesicle, allantois, connecting stalk, and chorion outside the embryo.

Hemangioblasts create the haematopoietic stem cells that are the precursors of all types of blood cells in the centre of a blood island. Hemangioblasts differentiate into angioblasts, the progenitors to blood vessels, near the perimeter of a blood island.

2. Development of Heart and Circulatory System in Fetal Organogenesis

At roughly 22 days, the heart becomes a functioning organ and begins to beat and pump blood. The cardiogenic zone is formed by cardiac myoblasts and blood islands in the splanchnopleuric mesenchyme on both sides of the neural plate. This is a horseshoe-shaped region around the embryo's head. Two strands begin to form tubes in this location around day 19, following cell signalling, as a lumen forms within them. By day 21, these two endocardial tubes had moved towards each other and joined to form the tubular heart, a single primitive heart tube. This is made possible by the embryo folding, which forces the tubes into the thoracic cavity.

Vasculogenesis (the development of the circulatory system) has also begun at the same time as the endocardial tubes are growing. On day 18, cells in the splanchnopleuric mesoderm start to differentiate into angioblasts, which eventually become flattened endothelial cells. Angio Cysts are small vesicles that fuse together to form angioblastic cords, which are lengthy vessels. In the creation of the vascular network, these cords evolve into a ubiquitous network of plexuses. The process of angiogenesis results in the additional budding and sprouting of new vessels, which expands the network. A stage of vascular remodelling occurs after vasculogenesis and the establishment of an early vasculature.

3. Development of Digestive System in Fetal Organogenesis

The digestive system begins to grow in the third week, and the organs have correctly positioned themselves by the twelfth week.

4. Development of Respiratory System in Fetal Organogenesis

The respiratory system begins with the lung bud, which appears about four weeks into development in the ventral wall of the foregut. The trachea and two lateral growths known as bronchial buds, which increase at the start of the fifth week to create the left and right major bronchi, are formed by the lung bud. 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 gives way to tertiary bronchi.

5. Development of Urinary System During Organogenesis in Animals

The pronephros, mesonephros, and metanephros are the three kidney systems that form in the growing embryo. The metanephros is the only part of the kidney that matures into a permanent kidney. The intermediate mesoderm is the source of all three.

Pronephros

The intermediate mesoderm in the cervical region gives rise to the pronephros. It is inoperable and will disintegrate by the end of the fourth week.

Mesonephros

Intermediate mesoderm in the upper thoracic and upper lumbar segments gives rise to the mesonephros. The excretory tubules form and enter the mesonephric duct, which eventually leads to the cloaca. Females' mesonephric ducts atrophies, but males' mesonephric ducts play a role in reproductive system development.

Metanephros 

In the fifth week of development, the metanephros appears. The primitive renal pelvis, renal calyces, and renal pyramids are formed by an extension of the mesonephric duct called the ureteric bud, which penetrates metanephric tissue. In addition, the ureter is produced.

Urethra and Bladder

The urorectal septum divides the cloaca into the urogenital sinus and the anal canal during the fourth and seventh weeks of development. The bladder is formed by the upper section of the urogenital sinus, whereas the urethra is formed by the bottom part.

Integumentary System 

The ectoderm gives rise to the epidermis, the skin's outermost layer. The dermis, the deeper layer, is made up of mesenchyme. The epidermis begins to form in the second month of development and reaches its final form by the end of the fourth month. The ectoderm splits to generate the periderm, a flat layer of cells on the surface. Individual epidermal layers are formed by further division.

6. Development of Neural System During Organogenesis in Animals

The superior section of the neural tube bends ventrally as the cephalic flexure at the level of the future midbrain—the mesencephalon—late in the fourth week. The prosencephalon (future forebrain) lies above the mesencephalon, and the rhombencephalon is beneath it (future hindbrain).

As neural stem cells, cranial neural crest cells migrate to the pharyngeal arches, where they mature into neurons through the process of neurogenesis.

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Development of Physical Organ During Organogenesis in Humans 

The embryological origins of the inner ear, middle ear, and outer ear are all different.

The Inner Ear

The ectoderm on each side of the rhombencephalon thickens to generate otic placodes around 22 days into development. These placodes invade the otic pits and otic vesicles to generate otic pits and otic vesicles. The ventral and dorsal components of the otic vesicles are then formed.

Middle Ear

The first pharyngeal pouch gives rise to the tympanic cavity and eustachian tube (a cavity lined by endoderm). The tubotympanic recess, located at the cleft's distal end, widens to form the tympanic cavity. The eustachian tube is formed when the proximal part of the gap remains narrow.

Outer Ear 

The dorsal region of the first pharyngeal cleft gives rise to the external auditory meatus. The auricle of the ear is made up of six auricular hillocks, which are mesenchymal proliferations on the dorsal sides of the first and second pharyngeal arches.

Eyes 

The eyes begin to develop from the third week to the tenth week.

Limbs 

Leg development begins at the conclusion of the fourth week. Limb buds emerge on the body's ventrolateral side. They are made up of an outer layer of ectoderm and an inner layer of mesenchyme generated from the lateral plate mesoderm's parietal layer. The apical ectodermal ridge forms at the distal end of the buds, forming a zone of rapidly growing mesenchymal cells known as the progress zone. The mesenchyme gives rise to cartilage (part of which eventually forms bone) and muscle.

Organogenesis in Chick Embryo

The ectoderm, mesoderm, and endoderm cells in chick embryos eventually give rise to various tissues and organs. Skin and neural tissue are made up of ectoderm cells. The lining of the gastrointestinal and respiratory tracts is made up of endoderm cells. Among other things, mesoderm cells differentiate into the circulatory system, kidneys, and skeletal compartments. Organogenesis is the process of developing tissues and organs.

The chick's gastrulation is an important developmental process that transforms a simple multicellular embryo into a sophisticated fully functional organism. 

Despite the numerous researchers participating, issues about the mechanisms of induction and genetics involved in the cell movements that occur during gastrulation remain unanswered.

Organogenesis in Vertebrates/Organogenesis in Animals 

Vertebrate Axis Formation

The spherical shape of the ball of cells is maintained even as the germ layers form. Animal bodies, on the other hand, have axes that run lateral-medial (toward the side-toward the midline), dorsal-ventral (toward the back-toward the belly), and anterior-posterior (toward the front-toward the back). The body must develop in such a way that cells, tissues, and organs are properly organised along these axes as it grows.

Spemann and Mangold transferred dorsal cells from one embryo into the belly region of a second embryo. They discovered that the transplanted embryo now developed two notochords, one at the original cells' dorsal position and the other at the transplanted site. This suggested that the notochord and the dorsal-ventral axis were genetically programmed into the dorsal cells. Since then, scientists have discovered a slew of genes involved in axis creation. The lack of symmetry essential for organism development is caused by mutations in these genes. The Wnt signalling pathway is involved in several of these genes.

Did You Know?

Following ovulation, the endometrial lining transforms into a secretory lining in preparation for the embryo's acceptance. It thickens, and its secretory glands become lengthened, as well as becoming more vascular. The decidua is the uterine cavity (or womb's) lining, which produces a large number of big decidual cells in the enlarged interglandular tissue. The blastomeres in the blastocyst are organised into a trophoblast, which is the outer layer of the blastocyst. The trophoblast then divides into two layers: an inner cytotrophoblast and an outer syncytiotrophoblast.