Growth and Development
Dr.Omar S.M.J.Ali PhD OrthodnticTypes of Skeletal Growth
Interstitial vs. Appositional Endochondral vs. Intramembranous RemodelingTypes of Growth Interstitial vs. Appositional
Interstitial – “inside the tissues”Growth that occurs by a combination of hyperplasia, hypertrophy, and secretion of extracellular matrix Occurs at ALL points within the tissueExamples: All soft tissuesCartilage Cranial base is formed by interstitial growthInterstitial growth
Resting cells in cartilageHYPERPLASIA proliferating cells (cell division)
HYPERTROPHY Increase in cell size
secretion
Extracellular matrix
If the extracellular matrix is NOT mineralized, interstitial growth may continue in soft tissues and cartilages.
CARTILAGE
Appositional – “addition to surfaces”Growth that occurs by a combination of hyperplasia, hypertrophy, and secretion of extracellular matrixOccurs ONLY at the SURFACE OF BONESAddition of new bone to the surface of existing boneOccurs through the activity of cells in the periosteumExamples: Bony surfaces of cranial vault Bony surfaces of maxilla and mandible Types of Growth Interstitial vs. Appositional
Appositional growth
Resting cells in periosteumHYPERPLASIA proliferating cells (cell division)
HYPERTROPHY Increase in cell size
mineralization
Extracellular matrix
When the extracellular matrix IS mineralized, bone is formed. Interstitial growth CAN NOT occur within bone.
PERIOSTEUM
BONE
secretion
Remember… Cartilage can grow interstitially, but bone can’t.Once the cartilage is transformed into bone, it continues to grow only by apposition.
Types of Growth Endochondral vs. Intramembranous
Endochondral Bone growth within cartilage Ossification centers occur within cartilage Cartilage is transformed into bone Examples Chondrocranium: ethmoid, sphenoid and basioccipital bones Epiphyseal plate cartilage of long bonesTypes of Growth Endochondral vs. Intramembranous
Intramembranous Secretion of bone matrix within and between connective tissue membranes No intermediate formation of cartilage It does NOT replace cartilage Examples Desmocranium: cranial vault (brain case), maxilla and mandibleIntramembranous bone formation The Desmocranium
Cranial vault (brain case), maxilla and mandibleImplantation
The zygote implanted in the uterine wall, the cells that will form the embryo proper divide organize themselves into a bilaminar (two-layered) disc. This disc is surrounded by an outer ring of cells, the trophoblast, which does not contribute to the new organism's tissues. After implantation, trophoblast cells multiply rapidly and invade the endometrium (uterine wall).Together, the trophoblast and endometrium form the (placenta) through which all the nutrition for the developing embryo will pass. This rich mass of tissue is filled with blood vessels, allowing rapid exchange of nutrients and waste. Another group of cells separates from the developing embryo near this time, and these cells also do not form part of the new organism. Instead, they develop into the amnion, the membrane that will surround the fetus to form the embryonic sac. This fluid-filled sac helps to cushion the fetus during later development. This phase begins during the(second week of development).
Gastrulation
During the third week, the embryo undergoes the process of gastrulation, forming a trilaminar (three layered) disc. Gastrulation establishes the three germ layers the endoderm, ectoderm, and centrally placed mesoderm all of which will give rise to the various organ systems. Mesoderm also combines with trophoblast tissue to form the umbilical cord, which transports nutrients and wastes between the fetal circulation and the placenta. Ectoderm forms the outer layer. Ectoderm forms skin, hair, sweat glands, epithelium brain and nervous system.Endoderm forms digestive, respiratory systems, liver, pancreas, gall bladder, and endocrine glands such as thyroid and parathyroid glands. The mesoderm forms body muscles, cartilage, bone, blood, reproductive system organs and kidneys
Embryogenesis
At this point, the developing human enters the actual embryonic phase, which lasts from the third week through the eighth week after conception. The organ systems differentiate at greatly varying rates during this phase. For example, the circulatory system is largely functional at the end of this period, whereas the nervous system is still engaged in massive cell division and only beginning to establish functional connections. Most embryological malformations occur during this embryonic phase. The remainder of human development, from weeks nine to thirty-eight is called the fetal period, the time during which the embryo first acquires human appearance.
The Fetal Period
The fetal period is characterized by two processes. -First is rapid growth (increase in size and cell number) -Second is continued tissue and organ differentiation (specialization of cells to perform distinct functions).Tissue and Organ Development
Early in fetal development the head dominates the body, (constituting half its length) . The face is broad and flat, eyes are still wide apart, and ears are low. The intestines temporarily protrude through the abdominal wall until the tenth week, and the external genitalia appear similar between the sexes. End 4th month,: the rest of the body has caught up to the head and the limbs have grown to give the fetus proportions more'nearly like those of a newborn.5th month :marked by the (first fetal movements- perceived by mother (quickening). The skin of the fetus secretes a lipid -rich covering substance, it also exhibits a temporary covering of fine hair
Sixth month, the fetus acquires the capacity for independent existence -because the lungs have finally matured to the point where the fetus can breathe. 7th month : nervous system develops many basic reflex responses, including the constriction of the pupils in response to light. Other reflexes controlling breathing, swallowing, and general movement can be detected much earlier, around the middle of the third month, although the effective coordination of such movements requires several more months in uter.