Learning Objectives:
- Understand how the central nervous system beings to develop in vertebrates using the chick as an example.
- Understand how the cardiovascular system beings to develop in vertebrates using the chick as an example.
- Understand how the gut beings to develop in vertebrates using the chick as an example.
- Understand what structures arise from the endoderm, ectoderm and mesoderm in vertebrates using the chick as an example.
- Record and interpret results of an experiment.
· By 33 hours of development in the chick (or 4weeks of human development) there are two major organ systems that have obviously begun to develop; The central nervous system (CNS) and the cardiovascular (CV) system.
· We will start by looking at the development of the CNS from the ectoderm.
· Then we will discuss the development of the CV system in the chick embryo.
· We will discuss additionally structures, such as the gut, obvious in the 33-hour chick embryo along with what they will give rise to in the adult.
· Finally, we will look and interpret the results of experiments applying a variety of chemicals to the newly developed heart.
· Recall from Lab 6 that the ectoderm above the notochord rises up to form the neural plate, which folds inwards to form the neural folds.
· The neural folds fuse at the midline forming the neural tube.
· The neural tube will become the CNS (brain and spinal cord).
· Once the neural tube forms anteriorly (it is not completely closed yet, anteriorly it is still open at the anterior neuropore) it begins to swell to form three primary vesicles (Figure 7.1).
· From anterior to posterior the primary vesicles are the; prosencephalon (forebrain), metencephalon (midbrain), and the rhombencephalon (hindbrain).
· Two of the primary vesicles then subdivide. One remains undivided to form five secondary vesicles.
The prosencephalon divides to for the telencephalon and diencephalon.
· The rhombencephalon divides to form the metencephalon and myelencephalon.
· The mesencephalon remains undivided
· That is a lot of encephalons (which means brains) that start with the letter “m”. Fortunately, the order is easy to remember since they appear in alphabetical order!
· The telencephalon (at this point any area of the brain anterior to the optic vesicles) will later become the bilobed cerebrum.
· In the chick this area will contain olfactory receptors for the sense of smell. In humans it is an area for higher thinking.
· The diencephalon is complex. It evaginates later in development to form a variety of structures.
· The lateral walls of the diencephalon evaginate to form both the thalamus (a region of sensory integration) and the optic vesicles which will later invaginate to form the optic cups (Figure 7.2).
o The inner surface of the optic cups give rise to the neural retina.
o The outer surface of the optic cups give rise to the pigmented retina.
· The roof of the diencephalon evaginates to form both the epiphysis which later becomes the Pineal gland. This gland sets diurnal rhythms by secreting melatonin. The roof also forms the anterior choroid plexus; a highly vascular area that secreted cerebral spinal fluid into CNS.
· The floor of the diencephalon evaginates to form both the infundibulum (which later becomes the posterior pituitary gland (or neurohypophysis) and the hypothalamus.
· The hypothalamus sends hormones to the posterior pituitary gland including antidiuretic hormone (ADH) and oxytocin.
· The Mesencephalon (Midbrain) is an oval shaped structure that serves to process data from the eyes and the ears. The Mesencephalon appears as a primary vesicle and remains undivided as a secondary vesicle.
· The Metencephalon (formed by the subdivision of the Rhombencephalon) forms two structures in the adult brain. Dorsally it forms the Cerebellum (coordinates stimuli about body position. Ventrally the Metencephalon forms the Pons (shunts information between the Cerebellum and the Cerebrum).
· The Myelencephalon, also called the medulla oblongata, contains a series of enlargements, called neuromeres, which are associated with specific motor and sensory neurons.
· In the epidermis, in the area of the myelencephalon, Otic placodes form on either side of the head. These will later become the inner ear.
· The remainder of the neural tube will become the spinal cord.
· The heat arises from an area in the mesoderm appropriately called the cardiac mesoderm.
· When the heart first forms it appears as a tube with two swellings (Figure 7.3). Your heart once looked like this too.
· Later this tube will undergo a series of movements and will fold on itself. The 2 sides will fuse to form a heart with four chambers.
· At this point the heart consists of two chambers; an anterior ventricle and a posterior atrium.
· If you know your adult heart anatomy you will know this is upside down! After the morphogenic movements the atria will be located anteriorly, and the ventricles will move to the posterior.
· The heart will start beating at about 48 hours of development once the vascular system is complete.
· Once the heart starts beating blood, rich in nutrients from the yolk, will be carried by the Vitelline veins from the extraembryonic regions to the Sinus venosus.
· The sinus venosus later shrinks to because the sinoatrial (SA) node and acts as the pacemaker for the heart.
· Blood flows from the sinus venosus into a single, still undivided atria and then into the undivided ventricle.
· From the undivided ventricle blood enters into the Bulbus cordis.
· The Bulbus cordis later forms the base of the 2 largest arteries in the body; the aorta and the pulmonary trunk.
· Blood leaves the Bulbus cordis via paired ventral aortas and then into paired dorsal aortas.
· The gut will form from a combination of the endoderm and the mesoderm.
· The endoderm forms only epithelial tissue lining the inner portion of the gut organs.
· The mesoderm surrounding the endodermal tube forms the muscle and connective tissue layer of these organs.
· Let’s start by looking at the endoderm.
· The gut tube forms as the head folds and lateral body folds lift up the embryo and then meet in the foregut and hindgut regions and fuse at the dorsal midline
. At the anterior end of the tube the foregut forms the pharynx.
· The pharynx gives rise to the esophagus and a number of outpocketings and invaginations.
· In this region two ventral evaginations form. The anterior of these two evaginations gives rise to the thyroid gland while the posterior evaginations forms the lungs.
· A lateral evagination becomes the pharyngeal pouches that later form the estuarian (auditory) tube and middle ear cavities well as the epithelium lining the tonsils, thymus, parathyroid, and ultimobranchial body.
· The posterior region of the foregut forms the lining of the stomach.
· Between the foregut and the midgut lies the anterior intestinal portal.
· The midgut is still open and attached to the yolk sac via the vitellointestinal duct.
· The midgut will later for the small intestine.
· Between the open midgut and the hindgut lies the posterior intestinal portal.
· The hindgut will give rise to the Colon (large intestine) in the adult.
· Now let’s turn our attention to the more complex mesoderm.
· As previously mentioned, the heart will arise from the mesoderm from a region called the cardiac mesoderm.
· Located ventral to the neural tube at the midline of the embryo is a dark streak called the notochord. This structure is found in all chordates.
· The notochord acts as a primary organizer. It induces and initiates an axial organization in the organism.
· The notochord provides axial support for the embryo until the vertebra form and take over this function.
· In the adult the notochord will become the gelatinous center of the intervertebral discs called the nucleus pulposus.
· Lateral to the notochord are the somites.
· Somites appear as blocks of tissue on either side of the notochord. They appear in pairs and are often used to help determine the age of the embryo.
· The somites are subdivided into three parts; a sclerotome, a myotome and a dermatome.
· The sclerotome gives rise to the ribs and the vertebra.
· The myotome gives rise to the skeletal muscle of the back and the limbs.
· The dermatome gives rise to the dermis.
· Lateral to the somatic mesoderm lies a streak of nephrogenic mesoderm.
· The nephrogenic mesoderm gives rise to the kidneys, urogenital ducts and the gonads.
· Next to the nephrogenic mesoderm, on either side of the embryo lies the Lateral Plate Mesoderm which delaminates into two layers; The somatic and splanchnic layers. A cavity, called the coelom, forms between these two layers (Figure 7.5).
· The Somatic Lateral Plate Mesoderm associates with the epidermal ectoderm above it. Together they are referred to as the somatopleure which will later form the body wall and in the extra-embryonic region the amnion and chorion.
· The Splanchnic Lateral PlateMesoderm associated with the underlying endoderm and together they are called the splanchnopleure.
· The splanchnopleure gives rise to the gut wall (smooth muscle and connective tissue) and in the extra-embryonic region the allantois and yolk sac.
· This unique set of cells appear only in vertebrates.
· They first appear on the crest of the neural tube as it closes and soon migrate to form a wide range of cell types including melanocytes, membranous bones of the head, the adrenal medulla, sympathetic ganglia of the autonomic nervous system and smooth muscle.
· In vertebrates that have teeth (not birds) they also produce the dentin secreting cells of the teeth.