Morphogenesis_Background
Morphogenesis_Background
The term morphogenesis generally refers to the processes by which order is created in the developing organism.This order is achieved as differentiated cells carefully organize into tissues, organs, organ systems, and ultimately the organism as a whole.
Questions centered on morphogenesis have aimed to uncover the mechanisms responsible for this organization.
Morphogenesis_Background Key questions
1. How are tissues formed from populations of cells? For example,
How do neural retina cells stick to other neural retina cells and not become integrated into the pigmented retina or iris cells next to them?
How are the various cell types within the retina (the three distinct layers of photoreceptors, bipolar neurons, and ganglion cells) arranged so that the retina is functional?
2. How are organs constructed from tissues? The retina of the eye forms at a precise distance behind the cornea and the lens.The retina would be useless if it developed behind a bone or in the middle of the kidney. Moreover, neurons from the retina must enter the brain to innervate the regions of the brain cortex that analyze visual information.All these connections must be precisely ordered
3. How do organs form in particular locations, and how do migrating cells reach their destinations?
Eyes develop only in the head and nowhere else. What stops an eye from forming in some other area of the body? Some cells for instance, the precursors of our pigment cells, germ cells, and blood cells must travel long distances to reach their final destinations.How are cells instructed to travel along certain routes in our embryonic bodies, and how are they told to stop once they have reached their appropriate destinations?4. How do organs and their cells grow, and how is their growth coordinated throughout development? The cells of all the tissues in the eye must grow in a coordinated fashion if one is to see. Some cells, including most neurons, do not divide after birth. In contrast, the intestine is constantly shedding cells, and new intestinal cells are regenerated each day.The mitotic rate of this tissue must be carefully regulated. If the intestine generated more cells than it sloughed off, it could produce tumorous outgrowths. If it produced fewer cells than it sloughed off, it would soon become nonfunctional. What controls the rate of mitosis in the intestine? 5. How do organs achieve polarity?
If one were to look at a cross section of the fingers, one would see a certain organized collection of tissues bone, cartilage, muscle, fat, dermis, epidermis, blood, and neurons.Looking at a cross section of the forearm, one would find the same collection of tissues. But they are arranged very differently in different parts of the arm.How is it that the same cell types can be arranged in different ways in different parts of the same structure?All these questions concern aspects of cell behavior. There are two major types of cell arrangements in the embryo:epithelial cells, which are tightly connected to one another in sheets or tubes, and mesenchymal cells, which are unconnected to one another and which operate as independent units.
Morphogenesis is brought about through a limited repertoire of variations in cellular processes within these two types of arrangements: (1) the direction and number of cell divisions; (2) cell shape changes; (3) cell movement; (4) cell growth; (5) cell death; and (6) changes in the composition of the cell membrane or secreted products.
Differential cell affinity
Many of the answers to our questions about morphogenesis involve the properties of the cell surface.
The cell surface looks pretty much the same in all cell types
We now know that each type of cell has a different set of proteins in its surfaces, and that some of these differences are responsible for forming the structure of the tissues and organs during development
Organogenesis_from gastrulation
1. Induction and compentence
Construction of organs is accomplished by one group of cells changing the behavior of an adjacent set of cells by changing their shape, mitotic rate, or fate.
This kind of interaction at close range between two or more cells or tissues of different history and properties is called proximate interaction or induction and ability to respond to a specific inductive signal is called competence.There are at least two components to every inductive interaction.
The first component is the inducer: the tissue that produces a signal (or signals) that changes the cellular behavior of the other tissue.The second component, the tissue being induced, is the responder.Epithelial-mesenchymal interactions;Epithelia are sheets or tubes of connected cells; they can originate from any germ layer.Mesenchyme refers to loosely packed, unconnected cells.
Mesenchymal cells are derived from the mesoderm or neural crest.All organs consist of an epithelium and an associated mesenchyme, so epithelial mesenchymal interactions are among the most important phenomena in nature.E.g in Lung organogenesis, tooth development ;Mechanisms of Epithelial-mesenchymal interactions ;some of the inductive molecules are soluble factors that can pass through the small pores of the filter, and that other inductive events required physical contact between the epithelial and mesenchymal cells.When cell membrane proteins on one cell surface interact with receptor proteins on adjacent cell surfaces, these events are called juxtacrine interactions.
When proteins synthesized by one cell can diffuse over small distances to induce changes in neighboring cells, the event is called a paracrine interaction, and the diffusible proteins are called paracrine factors or growth and differentiation factors (GDFs).The endocrine factors (hormones) travel through the blood to exert their effects, paracrine factors are secreted into the immediate spaces around the cell producing them;The embryo inherits a rather compact "tool kit" and uses many of the same proteins to construct the heart, the kidneys, the teeth, the eyes, and other organs.
Four major families of paracrine on the basis of their structures.These families are ;
1. fibroblast growth factor (FGF) family,
2. Hedgehog family
3. Wingless (Wnt) family,
4. Transforming growth factor beta (TGF-β ) superfamily.
Cell surface receptors and their signal transduction pathways;The paracrine factors are inducer proteins. We now turn to the molecules involved in the response to induction.These molecules include the receptors in the membrane of the responding cell, which binds the paracrine factor, and the cascade of interacting proteins that transmit a signal through a pathway from the bound receptor to the nucleus.These pathways between the cell membrane and the genome are called signal transduction pathways.Several types of signal transduction pathways have been discovered.
Each receptor spans the cell membrane and has an extracellular region, a transmembrane region, and a cytoplasmic region.When a ligand (the paracrine factor) binds its receptor in the extracellular region, the ligand induces a conformational change in the receptor's structure.
This shape change is transmitted through the membrane and changes the shape of the cytoplasmic domains.The conformational change in the cytoplasmic domains gives them enzymatic activity usually a kinase activity that can use ATP to phosphorylate proteins, including the receptor molecule itself.The active receptor can now catalyze reactions that phosphorylate other proteins, and this phosphorylation activates their latent activities in turn.Eventually, the cascade of phosphorylation activates a dormant transcription factor, which activates (or represses) a particular set of genes.
The JAK-STAT pathway;The Janus kinase (JAK) signal transducer and activator of transcription (STAT) pathway is one of the main signaling pathways in eukaryotic cells. This pathway is used during diverse growth and developmental processes in multiple tissues to control cell proliferation, differentiation, survival, and apoptosis
Eye morphogenesis
The vertebrate eye is formed through coordinated interactions between neuroepithelium surface ectoderm, and extraocular mesenchyme, which originates from two sources: neural crest and mesoderm;Following eye field formation, the neuroepithelium of the ventral forebrain evaginates, resulting in the formation of bilateral optic vesicles . The distal portion of the vesicle makes contact with the overlying surface ectoderm (lens ectoderm), which is then induced to form the lens placode.
This interaction results in invagination of the lens placode and distal optic vesicle leading to formation of a bilayered optic cup;The neural retina develops from the inner layer of the optic cup, and the retinal pigment epithelium (RPE) is derived from the outer layer.
The margin between the two layers gives rise to peripheral structures, the iris epithelium and ciliary body.The most proximal part of the optic vesicle, the optic stalk, “narrows” to become the optic fissure. The lens vesicle eventually separates from the surface ectoderm and differentiates into the mature lens.Tissue-tissue interactions, mediated by extracellular factors and intrinsic signals such as transcription factors, control differentiation of ocular tissues starting at the optic vesicle stage
Epidermis organogenesis
In mammals, epidermal development is a multistage process consisting of epidermal specification, commitment, stratification and terminal differentiation, as well as morphogenesis of its derivatives
During the whole process, distinct signaling patterns specify different developmental stages and these stage-specifically regulated signaling events ensure the correct morphogenesis of skin epidermis and its appendages.Furthermore, as in other epitheliums, every step of epidermal development is closely related to its underlying mesenchyme, the dermis.On one hand, mesenchymal signals guide the formation of epidermis and its appendages. Differences in the dermis result in the regional heterogeneities in the epidermis.On the other hand, the reciprocal mesenchymal-epithelial interactions also greatly contribute to the development of the dermis itself.
The joint development of epidermis and dermis requires a tightly controlled sequence of signaling events that involve both compartmentsThe epidermis originates from the embryonic ectoderm which also gives rise to the nervous system. The choice of ectodermal cells between epidermal and neural fates is made shortly after gastrulation, depending on the balanced effects of Wnt, FGF and BMP signaling.Without Wnt signals, the ectodermal cells respond to FGF signals which could inhibit BMP signaling activity and thus develop towards a neural fate
The surface ectoderm is a simple epithelium consisting of flat cells that express the cytokeratins K8/K18 .It should be noted that a transient protective layer of endodermis-like cells, called the periderm, forms overlaying the surface ectoderm, and it is destined to be shed off once the epidermis starts stratification.Before stratification, around E9.5 in mouse embryos for instance, K5/K14 expression replaces K8/K18 expression, which marks the event called epidermal commitment .
Then surface ectoderm becomes the embryonic epidermal basal layer that gives rise to all the structures of the future epidermisIt is believed that the sub-ectodermal mesenchyme sends signals to initiate epidermal stratification. The basal layer cells proliferate and form an intermediate cell layer under the periderm. The intermediate layer cells divide and mature into spinous cells expressing K1/K10, so the intermediate layer is then called the spinous layer
Spinous cells continue to differentiate, mature and migrate outwards to form the granular layer and the cornified layer, successively.Finally, the cornified layer cells become flattened, form cornified envelopes consisting of keratin proteins, and acquire the barrier function.
During postnatal life, dead cells of the cornified layer are constantly shed and renewed. The maintenance of epidermal homeostasis is dependent on basal layer cells.
Development of the epidermis also includes the morphogenesis of epidermal appendages such as hair follicles and sweat glands.The development of a mouse hair follicle is morphologically divided into 8 stages ;The morphogenesis of a hair follicle requires intensive communications and joint development of the epidermal and dermal compartments .A sebaceous gland (SG) usually locates at the upper part of a hair follicle and is an integral part of a philosebaceous unit that secretes sebum to lubricate the skin and keep the waterproof property of hair in mammals. In humans, sebaceous glands develop around Week 13–14 of gestation.In mice, sebaceous glands develop near the end of embryogenesis (Stage 5 of hair follicle morphogenesis) and mature after birth .
References
}RAVEN,P.H(1986) Biology QH 308.2R38, Mosby college publisher united State of America.
}GIBERT S.F (2000) Developmental BIOLOGY 6th Edition Swarthy more college Sunderland,Sinauer Associations
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