Anatomy_Concepts_Ch3 Flashcards
describe neurulation
The ectoderm in the dorsal midline thickens into a neural plate, and then starts to fold inward as a neural groove. This groove deepens until a hollow neural tube is pinched off into the body. Closure of the neural tube begins at the end of week 3 in the region that will become the neck and then proceeds both cranially and caudally. Complete closure occurs by the end of week 4. The cranial part of this neural tube becomes the brain, and the rest becomes the spinal cord.
derivatives of mesoderm
(middle to end of week 3): somites and intermediate mesoderm are segmented and form the segmented structures of the outer tube. Lateral plate mesoderm is unsegmented and is associated with the developing inner tube organs
Somites
The mesoderm closest to the notochord begins as paraxial mesoderm (near the body axis). Starting cranially and proceeding caudally, the paraxial mesoderm divides into a series of blocks called somites. The somites are visible in surface view as a row of subectodermal bulges on each side of the back. The somites are the first body segments, and about 40 pairs develop by the end of week 4.
intermediate mesoderm
This begins as a continuous strip of tissue just lateral to the paraxial mesoderm. Influenced by the segmentation of the somites, the intermediate mesoderm divides into spherical segments in a cranial-to-caudal sequence. Each segment of intermediate mesoderm attaches to a somite.
lateral plate
This, the most lateral part of the mesoderm, remains unsegmented. The lateral plate begins as one layer, but soon splits into two. A wedge of space is formed betwen these two sheets. This space is called the coelom (cavity). The two resulting divisions of the later plate are the somatic mesoderm, next to the ectoderm, and the splanchnic mesoderm (viscera), next to the endoderm. The coelom that intervenes beween the splanchnic and somatic mesoderm will become the serous cavities of the vental body cavity, namely the pertoneal, pericardial, and pleural cavities.
derivatives of ectoderm
The ectoderm becomes the brain, spinal cord, and epidermis of the skin. The early epidermis, in turn, produces the hair, fingernails, toenails, sweat glands, and oil glands of the skin. Neural crest cells, from ectoderm, give rise to the sensory nerve cells. Furthermore, much of the neural crest breaks up into a mesenchyme tissue, which wanders widely through the embryonic body. These wandering neural crest derivatives produce such varied structures as the pigment-producing cells in the skin (melanocytes) and the bones of the face.
derivatives of endoderm
The endoderm becomes the inner epithelial lining of the gut tube and its derivatives: the respiratory tubes, digestive organs, and the urinary bladder. It also gives rise to the secretory cells of the glands that develop from gut-lining epithelium: the thyroid, thymus, and parathyroid glands from the pharynx; and the liver and pancreas from the digestive track.
derivatives of mesoderm and notochord
“mesoderm’s basic parts: the notochord, the segmented portions, the somites and intermediate mesoderm, and the unsegmented somatic and splanchnic lateral plate mesoderm”
derivatives of the notochord
the notochord gives rise to an important part of the spinal column, the springy cores of the discs between the vertebrae. These spherical centers, each called a nucleus pulposus, give the vertebral column some bounce as we walk
derivatives of the segmented mesoderm
“each of the somites divides into three parts. One part is the sclerotome (““hard piece””). Its cells migrate medially, gather around the notochord and the neural tube, and produce the vertebra and rib at the associated level. The most lateral part of each somite is a dermatome (““skin piece””). Its cells migrate externally until they lie directly deep to the ectoderm, whre they form the dermis of the skin in the dorsal part of the body. The third part of each somite is the myotome (““muscle piece””), which stays behind after the sclerotome and dermatome migrate away. Each myotome grows ventrally until it extends the entire dorsal-to-ventral height of the trunk. Myotomes become the segmented trunk musculature of the body wall. Additionally, the ventral parts of myotomes grow into the limb buds and form the muscles of the limbs.<br></br>The intermediate mesoderm, lateral to each somite, forms the kidneys and the gonads. The intermediate mesoderm lies in the same relative location as the adult kidneys, outside the peritoneal cavity, or retroperitoneal.”
derivatives of the unsegmented mesoderm
the splanchnic and somatic lateral plate mesoderm are separated by the coelom body cavity. By now, the splanchnic mesoderm surrounds the endodermally derived gut tube lining. The splanchnic mesoderm gives rise to the entire wall of the digestive and respiratory tubes, except the inner epithelial lining; that is, it forms the musculature, connective tissues, and the slippery visceral serosae of the digestive and respiratory structures. Splanchnic mesoderm also gives rise to the heart and most blood vessels.<br></br>Somatic mesoderm, just external to the coelom, produces the parietal serose and the dermal layer of the skin in the ventral body region. Its cells migrate into the forming limbs and produce the bone, ligaments, and dermis of each limb.
8 weeks after fertilization
crown-to-rump ~3 cm, 2 g at end of period<br></br>head is nearly as large as the body. Nose, ears, and eyes are recognizably human. All major divisions of brain are formed. First brain waves occur in brain stem.<br></br>limbs are formed. Digits are initially webbed but separate by end of week 8. Ossification begins in long bones. Vertebrae are formed in cartilage.<br></br>heart has been pumping since week 4. Liver is large and begins to form blood cells.<br></br>all major organ systems are present in rudimentary form.
9-12 weeks (month 3)
crown-to-rump length ~6 cm at end of period<br></br>brain continues to enlarge. Cervical and lumbar enlargements are apparent in spinal cord. Retina of eye is present.<br></br>trunk and limbs elongate. Palate (roof of mouth) begins to fuse at the midline.<br></br>fetus begins to move, but mother does not feel movement.<br></br>heartbeat can be detected externally. Blood cell formation begins in bone marrow.<br></br>lungs begin to develop. Fetus inhales and exhales amniotic fluid.<br></br>Intestines move into the abdomen. Liver is prominent and producing bile. Smooth muscle is forming in the walls of hollow organs. Pancreas and thyroid have completely formed. Male and female genitalia are distinctive; sex of the fetus can be determined.
13-16 weeks (month 4)
crown-to-rump length: ~11 cm at end of period<br></br>skin development continues with differention of the dermis and subcutaneous tissue. Epidermis at tips of fingers and toes thickens to initiate nail formation. Molanocytes (pigment cells) migrate into the epidermis.<br></br>torso elongates. Bone formation begins in vertebrae. Most bones are distinct, and joint cavities are present. Hard palate is fused.<br></br>myelin begins to form around nerve cells<br></br>glands develop in the GI tract. Meconium is collecting.<br></br>kidneys attain typical structure. Primary follicles containing oocytes begin to form in the ovary (female).
17-20 weeks (month 5)
crown-to-rump length ~16 cm at end of period<br></br>hair follicles and sebaceous and sweat glands form. The body is covered with vernix coseosa (fatty secretions of sebaceous glands), and lanugo (silklike hair) covers the skin.<br></br>brown fat, a site of heat production, forms in the neck, chest, andcrown<br></br>mother can feel fetal movements (quickening)<br></br>the brain grows rapidly
