Histogenesis is the process by which cells and tissues acquire functional specialization In humans: Cleavage  gastrulation  organogenesis  histogenesis (2 weeks) (1 week) (4 weeks) (7 months) Organogenesis: the formation of organ rudiments to establish the basic body plan. Histogenesis: differentiation of cells within the organs to form specialized tissues. Tissues are composed of cells and extracellular material that perform a specific function. Each specific tissue develops mainly from one germ layer. embryo fetus

The neural tube gives rise to the central nervous system and contributes to the peripheral nervous system Central nervous system: brain and spinal cord Peripheral nervous system: all nervous tissue outside of the skull and vertebral column. From the time of neural tube closure to birth, approximately 250,000 neurons are formed each minute. The CNS contains over 100 billion neurons when complete. How do neural cells grow and differentiate? The early neural epithelium contains a pseudostratified layer of stem cells. The basement membrane is at the outer edge and tight junctions form at the inner surface. Different cell types are formed.

Development of neuroepithelium Neuroepithelial cells differentiate to form two major types: 1. Stem cells: have a unlimited capacity for self renewal 2. Committed progenitor cells: these divide to produce 2 differentiated types: A. Neuroblasts develop into neurons B. Glioblasts form glial cells that can develop into astrocytes (glue that holds neurons together), and oligodendrocytes (form myelin around neurons). The microglia are macrophages derived from mesenchyme. Mantle: the inner area where neurons and glial cells accumulate to form gray matter. Marginal layer: area containing axons of neurons that transmit signals to other organs. They are white due to the presence of myelin = white matter. Ependymal cells form the ependymal layer that lines the cavities. May be stem cells

The spinal cord develops a dorsoventral pattern All nervous functions depend on development of complex connections between neurons. These circuits start to develop in the embryo. Alar plate: as neurons and glial cells accumulate in the mantle, they form ridges on either side of the neural tube (dorsal or afferent columns). These will develop into afferent nerves that conduct signals to the brain. Basal plate: accumulation of cells in the ventral region produces basal or efferent columns. These will develop into efferent neurons that carry signals to muscles and organs (motor neurons). The gray matter in the mantle layer is composed of cell bodies of neurons and the white matter in the marginal layer is composed of myelinated axons. The floor and roof plates are composed of glial cells. What causes this pattern?

The notochord and floor plate induce the dorsoventral pattern of neural development To examine whether ventral columns were induced by adjacent tissue, an extra notochord was grafted to the side of the developing neural plate. This created an extra floor plate and ventral column, suggesting that it induced these structures. If the notochord was removed, no ventral column or floor plate developed, consistent with the above idea. It is believed that the notochord first induces the floor plate and the floor plate then induces ventral columns. This was confirmed by grafting a floor plate, which also induced efferent nerves. What is the molecular nature of this inducer of dorsoventral patterning?

Sonic hedgehog (shh) induces the dorsoventral pattern Sonic hedgehog (shh) is a gene that is expressed in the notochord at first and later in the floor plate. Mice that lack shh fail to develop floor plates in the CNS. Shh is a secreted glycoprotein that induces a gradient that is high near the floor plate and progressively lower in dorsal regions. Shh initially induces neural plate cells to form floor plate. Other signals from the dorsal ectoderm direct the dorsal columns and the roof plate. Different levels of shh appear to specify different types of neuron differentiation.

Different concentrations of shh induce distinct types of neurons In the developing spinal cord, the floor plate produces shh and creates a concentration gradient. Motor neurons develop closest to the floor plate, type 2 interneurons are next, followed by type 1 interneurons. To see whether the shh gradient is really important, isolated cells from neural tubes were cultured in various concentrations of shh. Cells were stained with antibody specific for floor plate, motor neurons, or type 1 or 2 interneurons. Bone morphogenetic protein released by the dorsal ectoderm and roof plate has an analogous function in generating the dorsal columns

How does the brain develop? The initial stages of brain development are similar to spinal cord (neurulation and establishment of the dorsoventral pattern. The brain becomes more complex as the central canal expands to form four fluid filled ventricles. After 4 weeks, the human brain has 3 regions, the prosencephalon, mesencephalon, and rhombencephalon. By 5 weeks the prosencephalon divides into telencephalon (2 outpockets that will become the cerebral hemispheres surrounding the 1st and 2nd ventricles) and the diencephalon which forms around the 3rd ventricle. The rhombencephalon forms the metencephalon in the anterior and the myelencephalon in the posterior. The myelencephalon surrounds the 4th ventricle.

Flow chart showing brain development

Brain of a four month old fetus Telencephalon: forms the cerebral hemispheres with 1st and 2nd ventricles Diencephalon: forms the posterior pituitary gland (infundibulum), the thalamus (sleep), and hypothalamus (homeostasis) with the 3rd ventricle Mesencephalon:midbrain Metencephalon: forms the cerebellum (balance and muscle tone) and pons Myelencephalon: forms the medulla (reflexes)

Neural crest cells arise during neurulation Neural crest cells: These cells arise from both dorsal epidermis and neural plate. They migrate throughout the body. Neural crest cells form a variety of cell types including cartilage, pigment cells of skin, neurons, smooth muscle cells, and adrenal medulla. Migration staging area: the cells originate at the crests of the neural folds during neurulation. Both epidermal tissue and neural tissue contribute to this lineage. Slug: a regulatory gene that is expressed as neural crest cells start to leave the staging area. Slug appears to alter expression of cell adhesion molecules (cadherins) and it causes dissociation of desmosomes on neural crest cells.

Neural crest cells form a variety of tissues The fate of neural crest cells has been mapped by a number of techniques (radioactive tracers, transplants from pigmented species to albinos). There are 2 patterns of migration in the trunk region: Dorsolateral path: enter skin and form melanocytes Ventral path: form afferent neurons of dorsal route ganglia, sympathetic and parasympathetic ganglia, and adrenal medulla Neural crest cells help to form addition structures in the head such as bones, connective tissue, eyes, ears, and teeth. They also help to form blood vessels and connective tissue in the trunk

How do neural crest cells differentiate into many tissues? Pluripotency hypothesis: each neural crest cell has the potential to form any or all structures. Inductive signals from adjacent tissue determines their fate. Selection hypothesis: the neural crest contains a mixed population of predetermined cells. Each cell has only one possible fate and it migrates according to this fate. The real truth may lie between these two extremes. Clonal analysis: when individual neural crest cells are placed in culture, it is clear that a single cell can give rise to others that differentiate into multiple cell types (pigment cells and neurons). Premigratory cells have a wider potential than do the cells that have already started to migrate. They may become partially differentiated as they migrate.

Pluripotency of neural crest cells in vivo By injecting migrating neural crest cells with red fluorescent dye, it was possible to trace their fate. Multiple cells were injected and their fate was analyzed after several days. The structures that they formed were determined by staining with specific antibodies (ie dorsal route ganglia). Some red neural crest cells participated in dorsal route ganglia, others formed parts of sympathetic ventral route ganglia, pigment cells, or adrenal medulla. The cells appeared to be pluripotent in vivo. Premigratory cells had a greater potential to form different structures than migrating cells. Thus, differentiation appears to accompany migration.

What are the molecular signals that control differentiation of neural crest cells? Extracellular matrix (ECM): neural crest cells constantly extend filopodia to feel the ECM. Pieces of filter were placed in an embryo at the dorsolateral or ventral pathways. After the filters absorbed ECM, they were removed to a culture dish and allowed to interact with neural crest cells. Dorsolateral ECM induced melanocytes and yellow pigment cells. Ventral ECM induced neurons. No matrix allowed the cells to remain undifferentiated.

Polypeptide growth factors also stimulate Differentiation of neural crest cells 1. Steel factor: a polypeptide growth factor produced by skin cells that acts with the c-kit receptor on neural crest cells. Mutant mice that produce less steel factor appear steel gray rather than black because they have fewer melanocytes. Loss of the c-kit receptor leads to areas of skin that lack pigmentation, white spotting in animals. The same condition leads to similar symptoms in humans, called Piebaldism.

2. Endothelins: growth factors that are important in recruiting development of melanocytes and parasympathetic nerves in the gut. Mice that are deficient in endothelin-3 or its receptor, EDNRB, show unpigmented regions of skin and distention of the large intestine. The latter is due to lack of parasympathetic neurons that induce peristaltic movements. A similar condition occurs in humans due to mutation of the EDNRB receptor. Hirschsprung’s disease is characterized by irregular skin pigmentation and chronic severe constipation. 3. Transforming growth factor beta family (TGF-bs): members of the TGF-b family selectively inhibit specific types of differentiation by neural crest cells. When neural crest cells are grown in culture, they form colonies that contain cells with many patterns of differentiation. When cultured with bone morphogenetic protein-2, 50% of cells become neurons, 25% become muscle, and 25% are mixed. If the same cells are cultured in TGF-b, all clones develop into smooth muscle. The colonies of cells are very large in the presence of TGF-b, suggesting that this cytokine inhibits differentiation of other phenotypes and enhances growth. Neural crest cells are important in formation of multiple tissues and there are several important factors that induce their differentiation.

Head ectoderm is induced to form placodes Placodes: the epidermis covering the head is induced by underlying brain to form dense areas composed of columnar epithelium. Epibranchial placodes: these form along the ventral lateral region and develop into sensory ganglia of cranial nerves. Dorsolateral placodes: contribute to sensory ganglia and also form parts of the eye, ear, and nose. Ectodermal placodes and neural crest cells have common properties, such as their ability to form sense organs, neurons, and cartilage.

The otic placode forms the inner ear In humans, the otic placode appears by the third week on both sides of the rhombencephalon. The otic vesicle is induced by the underlying neural tissue in the rhombencephalon (one example of reciprocal interaction). It invaginates to form the otic pit and then pinches off to form the otic vesicle. Ganglion cells develop from its medial surface. Labyrinth:the otic vesicle expands unequally and constricts in other areas to form a complex shape. The cochlea develops to sense sound and the semicircular canals form to serve as an organ for balance and body position.

Formation of the eye involves reciprocal interactions Lens placode: the ectoderm invaginates in response to signals from the optic cup underneath. It then pinches off as a lens vesicle. Cells elongate to fill the vesicle and start to synthesize crystallins. Optic cup: forms from the neural tube by invagination. The opening (choroid fissure) closes forming a round optic cup, an extension of the brain. Optic stalk: connection to the brain that is filled with neurons to form the optic nerve. Reciprocal interaction: the lens induces the formation of the optic cup and the cup regulates formation of the lens. When the lens from a species with large eyes is transplanted, it induces an extra large optic cup and it also does not grow as large as usual.

Paradoxical arrangement of rods and cones The pigmented retina is the outer layer and the neural retina is the inner lining of the optic cup. These cells form the neurons and the rod and cone cells that detect light. It is a paradoxical arrangement because the light sensitive cells are actually in the rear of the eye behind the neurons and facing away from light! The rods and cones are covered by layers of bipolar nuclei and ganglion cells.

Nasal placodes form the olfactory epithelium In humans, the nasal placodes appear after the 4th week. They are induced by the underlying telecephalon. During the fifth week, nasal swellings appear around the placodes which now become the two nasal pits. The pits start out far apart, but the 2 maxillary swellings grow large and push the pits to the center. The medial parts of the nasal swellings fuse to form part of the upper lip. Olfactory epithelium: the original lining of the nasal pit comes to rest on the roof of the nasal cavity. It forms the epithelium that senses smell and connects to neurons in the telencephalon. The nasal cavity becomes continuous with the pharynx. What can go wrong?

Cleft lip and palate When the nasal swellings that form the upper lip and/or palate fail to fuse properly a cleft occurs (4th to 8th weeks of gestation). You can feel the fusion junction with your finger (indentation in upper lip) or tongue (fusion line on roof of mouth). Usually occurs bilaterally The cause is unknown Occurs in 1 of 700 babies. Smoking, too much vitamin A or too little folic acid in the mother may be a factor A parent with cleft has a minimum 5% chance of passing the cleft along. An autosomal dominant genetic condition causes a 50% chance of cleft palate

How does skin develop and differentiate? Epidermis: the largest derivative of ectoderm forms the outer layer of the skin. It is an epithelium and cells are connected by desmosomes and tight junctions. The epidermis consists initially of two layers: periderm is a temporary outer layer and the germanative layer lies below. The germanative layer is composed of stem cells that divide actively to produce differentiated progeny. The basal layer develops from the germanative layer (it contains stem cells in the adult). Spinous layer forms as cells are squeezed out of the basal layer. They become large and differentiate. Granular layer starts making keratin granules Cornified or horny layer is composed of dead cells that are filled with keratin It takes cells about 7 days for each cell to journey through the skin

Hair development in humans The underlying mesenchyme in skin forms a dermis, a layer of connective tissue just below the epidermis. The dermis induces a variety of epidermal structures depending on the species (feathers, hair, scales). Hair bud: hair formation begins as a small bud that that penetrates the dermis. It is induced by a group of mesenchymal cells and the hair bud then envelopes these cells to form a hair papilla. Hair follicle is the entire organ Sebaceous glands are induced to form on the side. The hair shaft is formed when the inner cells of the follicle start to differentiate and produce keratin in the form of hair. The continued production of keratin by the cells at the base of the shaft causes the hair to grow longer.

Development of mammary glands Mammary glands: these glands develop from 2 bandlike swellings in the epidermis called the mammary ridges. Depending on the species, one or more of the segments of these ridges persist on each side. In 7 week human embryos, the mammary ridge extends from the armpit to the groin. Mammary ridge: this sprouts buds that penetrate down into the mesenchyme to form the lactiferous ducts. The actual milk producing glands develop prior to the first pregnancy. The lactiferous ducts open into a small pit at the surface which is transformed into a nipple.

Mammary gland development in humans mimicks ancestral patterns In normal development of humans, only one pair of segments in the ridge survives, and the remaining precursors degenerate. In some individuals, the other segments of the mammary ridge fail to degenerate, so that accessory nipples or breasts are formed. Primates evolved from small creatures that nursed multiple offspring. An extended mammary ridge would have given human ancestors a survival advantage. Two breasts are obviously more adaptive for humans who normally have only one offspring at a time. Atavism: the occasional and abnormal persistance of a primitive adult feature in an evolved species (multiple mammary glands). It is easier to modify an older pattern of development than to develop a totally new pattern. This idea is a pervasive in developmental biology.