Growth Changes During Prenatal Development Essay
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Life is a phenomenon based on the strictly regulated alternation of selective and nonselective transcription of DNA and RNA. Selective transcriptions are coupled with proteosynthesis, nonselective with cell division.
Growth means increase in size due to an increase of existing structural (and functional) units.
Differentiation is a process whereby a relatively simple system is changed into a more complicated one. Differentiation is accomplished by formation of new structures (morphologic differentiation) and by formation of new chemical compounds (chemical differentiation).Growth Changes During Prenatal Development Essay
Development is (related to biological systems) a process resulting from a selective, time-related switching on and off of genes.
Cell is the basic unit of life. The cell is a system of membranes, filaments and large molecules which prevent dissolved substances from mixing freely. There is no life without cells.
In different biological systems, growth and differentiation are closely related. In human biology, it is necessary to specify growth and differentiation of cells, tissues, and organs, the end result being growth and differentiation of human beings.
NORMAL PRENATAL GROWTH AND DIFFERENTIATION
Biological growth is accomplished by a preponderance of anabolic processes during which the size of the living system increases using already operating genes. Differentiation is related to the switching on of genes that were not operating previously. Organs increase in size via cellular hypertrophy and hyperplasia. Hypertrophy means an excessive increase in size; hyperplasia means an increase in the number of cells. A decrease in the size of a cell of an organ is known as atrophy.Growth Changes During Prenatal Development Essay
During embryonal and fetal development, the fertilized oocyte changes into a complete organism representing the species by which the oocyte was produced. Differentiation and growth are genetically determined. All cells of an individual (under normal conditions) comprise the same set of basic genes. Gene expression in different cells is regulated by specific stimulatory and inhibitory peptides and proteins. The developmental regulatory mechanisms of gene expression are largely unknown.
The basic mechanisms of growth are (1) an increase in cell size and number, and (2) an increase in extracellular substances. Cell number is increased mainly via mitotic divisions. Cells that undergo mitotic divisions may be classified as life-long proliferating: they divide mitotically for the whole life span of the individual, and they divide in tissue cultures. In contrast, cells that undergo terminal differentiation divide only during a certain period of life of an individual. When terminal differentiation is complete, these cells do not divide, and they do not proliferate in tissue cultures.Growth Changes During Prenatal Development Essay
The development of an individual is accomplished by a selective switching on and off of genes. The life span of every placental animal can be divided into two periods: prenatal and postnatal. The prenatal period begins with fertilization of the oocyte and ends with delivery. In humans, during the prenatal period, the age of the conceptus is calculated in one of the following ways:
Gestational age, calculated from the first day of the last menstrual period and used for clinical purposes.
“True” age (anatomic age), calculated from the date of supposed or known (e.g., in vitro) fertilization and used for anatomic purposes.Growth Changes During Prenatal Development Essay
As, under normal conditions, the fertilization occurs around day 14 of a 28-day menstrual cycle, the gestational age of a normal full-term newborn is 40 weeks, whereas the anatomic age is 38 weeks.
Trimesters of Gestation
First trimester: Embryonal period 10 weeks
Fetal period 11–16 weeks
Second trimester: 17–28 weeks
Third trimester: 29–40 ± 2 weeks
The human prenatal period is divided into following periods:
Pre-embryonal period includes fertilization, cleavage of oocyte, and formation of blastocyst. The pre-embryonal period ends at implantation, which occurs on day 7 postconception. During the pre-embryonal period, the conceptus is transported from the ovary, through the oviduct, into the uterine cavity.
Embryonal period begins with blastogenesis (i.e., formation of germ layers).Growth Changes During Prenatal Development Essay
Blastogenesis includes bilaminar and trilaminar presomite stages of embryonal development, which are present from day 7 to day 20 postconception.
Embryonal organogenesis (arbitrary) begins with formation of first somites (on day 19 or 20 postfertilization) and ends with the fusion of the eyelids, which occurs between days 56 and 60 postconception. During embryonal organogenesis (weeks 4–8 postconception), primordia of all organs are formed, and the embryo attains a specific human form. For an organism undergoing blastogenesis and organogenesis, the term embryo is used.
Fetal period. Starting with postconceptional week 9, or gestational week 11, the term fetus is used. The fetus is characterized by fused eyelids and distinct human somatic characteristics. The fetal period covers gestational weeks 11 to 26. During the fetal period, the total body weight (TBW) increases from approximately 5 to 500 g. Fetal period precedes embryonal period, during which a viable fetus is delivered.
At the beginning of the perinatal period, the eyelids separate and the eyes of the fetus reopen.
During the embryonal and fetal period, the fetus may be spontaneously aborted by premature uterine or tubal activity, or aborted by various medical procedures. The fetus is classified as aborted if its TBW is less than 500 g, and if at abortion it does not exhibit any vital signs, such as a beating heart and breathing. If the fetus with a TBW greater than 250 g has a heart beat and/or breath movements, the fetus is classified as an extremely immature newborn. The newborn is delivered (not aborted).Growth Changes During Prenatal Development Essay
Newborns are classified according to their TBW into four groups:
Group I: Extremely immature, with a TBW less than 500 g
Group II: Immature, with a very low birth weight
Group IIa: 500 to 800 g
Group IIb: 801 to 1000 g
Group III: A. TBW 1001 to 1500 g (immature)
B. TBW 1501 to 2500 g (premature)
C. TBW 2501 to 4000 g (full-term, normal; gestational age 38–42 weeks)
D. TBW 4001 g or heavier (macrosomic)
Group IV: Weight unknown
If the gestational age exceeds 42 gestational weeks, the newborn is called postmature. The newborn is small for gestational age, or intrauterine growth retarded (IUGR), if his TBW is less than minus two percentiles, related to the gestational age and standards of normal fetal growth.1 Neonatal mortality, birth weight, and gestational age are closely related.Growth Changes During Prenatal Development Essay
THE STAGING OF EMBRYONAL DEVELOPMENT
Streeter3 divided the human embryonal period into 23 developmental horizons, and his classification is the basis of the Carnegie staging system of human prenatal development.4
The Streeter term “horizon” was replaced in the newer classifications by the old term “stage” (used first by Mall5 in 1914). Each stage is characterized by defined external and internal features. Hamburger and Hamilton6 distinguished 46 stages in the chick. Staging of the development of Rana pipiens was published by Pollister7 and Moore in 1937. Table of “normal” development of a large number of animals published by Keibel and co-workers8 are almost forgotten. Jirásek9, 10 introduced the system of 10 standard comparative stages, where, in different animals, the corresponding prenatal stages were marked by the same system of numbers. Jirásek’s comparative classification (J’ staging) is based only on external developmental characteristics of quadrupled vertebrate embryos. This classification is simple and suitable for the staging of a large number of animals. The important feature distinguishing between embryonal and fetal period is the fusion of the eyelids (in most species).Growth Changes During Prenatal Development Essay
The principles of the basic body architecture in all vertebrates is similar, and the presented comparative classification is the most simple for comparing human development with other vertebrates. The Carnegie staging system (originally Streeter’s horizons3) and Jirásek’s standard J’ staging are compared in Table 1.
Table 1. Timetable and staging of human prenatal development
Anatomic age* (days) from conception
Gestational age† (weeks) of gestation
Unicellular (fertilized oocyte)
Blastomeric (16–20 blastomeres, morula)
Blastodermic (blastocyst)Growth Changes During Prenatal Development Essay
Bilaminar embryo stage (round-shaped embryonic disc)
Primary yolk sac
Secondary yolk sac
Trilaminar embryo stage (pear-shaped embryonic disc)
With primitive streak and notochordal process
With neural folds
Early somite stage (shoe-sole-shaped embryo)
Completely open neural groove
Neural tube closing, both ends open
One or both neuropores closed
Stage of limb development (C-shaped embryo)
Bud of proximal extremity
Buds of proximal and distal extremities
Proximal extremity, two segments
Proximal and distal extremities, two segments
Digital rays, foot plates
Digits, toe tubercles
Late embryonal stage (embryo with differentiated extremities including fingers and toes)
Fetus with fused eyelids
201–350 400–550 (CHL)
Third trimester fetus (newborn with open eyes)
*Age from conception; †Age from 1st day of the last menstrual period
MLL, maximal longitudinal length
CRL, crown–rump length
CHL, crown-heel length
CHARACTERISTICS OF STANDARD PRENATAL VERTEBRATE STAGES
Prenatal development comprises the following 10 principal stages:Growth Changes During Prenatal Development Essay
SCOPE OF HUMAN PRENATAL DEVELOPMENT
At fertilization, the oocyte is round, approximately 150 μm in diameter, and enclosed by zona pellucida. Zona pellucida is approximately 10 μm thick, composed of glycoproteins contributed by the oocyte during its growth, which occurs at the beginning of follicular growth.11, 12 The cleavage of the oocyte and the formation of the early blastocyst take place in the space limited by the zona pellucida. During the pre-embryonal stage (days 1–4) the conceptus is transported by the oviduct into the uterine cavity. During days 5 to 6, the blastocyst increases in size, and the zona pellucida ruptures and is rejected (hatching of oocyte). The diameter of the “free blastocyst” is approximately 200 μm. On day 7, the blastocyst implants into the edematous, compact (superficial) layer of the undecidualized functional layer of endometrium.13 The bilaminar embryonal disc, aged 7–14 days, is round and 0.2–0.7 mm in diameter. During blastogenesis, the term ectoblast, endoblast, and mesoblast may be used. Beginning with organogenesis (somite formation), these terms are replaced by ectoderm, endoderm, and mesoderm.Growth Changes During Prenatal Development Essay
The trilaminar embryonal disc, aged 14–20 days, is pear-shaped and 0.5–2.5 mm long. Organogenesis begins with the formation of somites.
During stage 6, characterized by formation of somites and closure of the neural tube, the embryo becomes delineated by anterior, lateral, and posterior amnionic folds. The embryo becomes cylindric and 2–3 mm long. After closure of neural tube, as the primordium of central nervous system grows faster than the other portions of the embryo, the embryo bends ventrally and is C-shaped. At stage 7, the head, neck, trunk, and limbs become evident. The primary palate closes (substage 7–6), and human features are conclusive.
At the end of the embryonal period, the eyelids grow and their rims finally fuse, closing the eye fissure (days 56–60 postconception).Growth Changes During Prenatal Development Essay
During the fetal period (gestational weeks 10–26, stage 9), the eyes of the fetus are closed. At the early human fetal period (gestational weeks 11–12), male and female differentiation of external genitalia occurs. The fetal weight increases from approximately 5 to 50 g. During the human midfetal period, the TBW increases from 51 g to approximately 250–300 g. During the late fetal period, the fetus weighs more than 300 g and less than 500 g (if live born), or less than 1000 g if aborted dead. Fetal increase in the weight of brain, lung, kidney, adrenal, thyroid, and placenta14 is presented in Table 2. Clinically, embryonal and fetal growth can be followed with great accuracy using ultrasound anthropometry
Prenatal and early childhood development have a critical effect on long-term health in adulthood . Early development may affect adult susceptibility to a range of non-communicable disease including cardiovascular disease  and diabetes . The aim of this study was to examine the associations between prenatal and early childhood development with cognitive, vision and hearing function in middle age. Commonly used indexes of prenatal development include birth weight and other measures of body size at birth. Measures of body size at birth represent an indirect, summative measure of influences on the developing foetus . In contrast to measures of body size at birth (an index of prenatal exposure), measures of adult leg length and height are sensitive to environmental factors and nutrition in early childhood that impact on growth .Growth Changes During Prenatal Development Essay
Birth weight and post-natal growth during early childhood and adolescence are positively associated with cognitive ability in childhood [6–17]. There is less research on whether early life development affects cognitive performance in adulthood and old age, and the findings in the existing literature are mixed. Some studies of middle aged and older adults did not detect reliable associations between early life development and cognitive function in adulthood and older age [18–20]. One possible explanation may be that early life development exerts a stronger influence on cognition in childhood than later in life. Richards and colleagues  assessed cognitive function in 3,900 participants at ages 8, 11, 15, 26 and 43 years. Birth weight was positively associated with cognitive ability at 8, 11, 15 and 26 years, though there was no association at 43 years. Other studies do support associations between both prenatal [21,22] and childhood [22,23] development and adult cognitive function. In summary, there is inconsistent evidence that pre-natal growth may affect cognitive ability in adulthood. Previous research findings are also inconsistent in relation to early life development and age-related change in cognitive function. Martyn et al  and Shenkin et al  reported no association between any anthropometric parameter at birth and estimated cognitive decline. Gale and colleagues  found associations between cognitive decline and adult head circumference, but not head circumference at birth.Growth Changes During Prenatal Development Essay
To date, most research attention has focused on the long-term cognitive outcomes of low birth weight babies and birth weight within the normal range. However, there is also evidence that unusually large babies may have poorer cognitive outcomes. Large babies are defined as being outside the normal range of distribution according to weight, head circumference or length (i.e. greater than the 90th percentile or 97th percentile, depending on the cut-point used ). Maternal diabetes and obesity are associated with higher likelihood of a large baby . Cesur and Rashad  analysed two longitudinal data sets, examining patterns of academic performance in junior school for a combined sample of 19,280 children. Children who had either low or high birth weight had poorer academic performance compared to children within the normal range of birth weight. Given increasing rates of diabetes and obesity globally [28,29] and parallel evidence of increasing numbers of babies who are large for gestational age , adverse developmental outcomes associated with overweight babies is likely to assume increased importance.Growth Changes During Prenatal Development Essay
Few studies have investigated associations between early life development and hearing and visual sensory function. Birth weight was reported to be associated with likelihood of having state registration for hearing impairment , hearing and visual acuity  and self-reported hearing problems . Sayer et al  reported that reduced growth in childhood (but not birth weight) was associated with lens opacity (but not visual acuity) and poorer hearing levels when assessed around age 67 years. Barrenas  reported that adult height was associated with hearing, and concluded that adverse early life experience may convey increased susceptibility to hearing loss in adulthood. However, not all studies support associations between prenatal and/or childhood development and adult sensory function; Ecob and colleagues  reported no association between birth weight or BMI during childhood and hearing assessed at age 45 years.Growth Changes During Prenatal Development Essay
It may be that the association between indices of early life development and adult cognition and sensory function is due to confounding; factors including socio-economic status or birth trauma may account for the association, despite attempts to control statistically for potential confounds. However, there is evidence from experimental studies with animal models that supports observational findings in humans for the influence of early life exposures on development of cognitive and sensory systems . It is plausible that there is a direct causal relationship between early development and neurosensory function in adulthood, although the mechanism is unclear. It may be that under-nutrition impacts on development of the brain and sensory organs , or there may be alterations in gene expression that affect cognitive and sensory function [39–41]. Alternatively, glucocorticoid hormones or growth factors may be modulated by early experience and impact on neurosensory development [35,42–44]. It is also possible that the effect of early development on cognitive and sensory function is via increased susceptibility to diabetes and cardiovascular disease; adverse early development is associated with increased risk of cardiovascular disease and diabetes [2,3], and cognitive decline and poor hearing and visual function are independently associated with cardiovascular disease and diabetes [45–48].Growth Changes During Prenatal Development Essay
It is unclear what the relative contribution of prenatal versus childhood development is for adult cognitive and sensory function and decline in function with age. In the present study, we examined hearing, vision and cognitive function in middle age according to birth weight (an index of prenatal development) and adult height (an index of early childhood development) in a very large sample of middle aged people, thereby increasing power for detecting small effects later in life. We also examined change in hearing, vision and cognitive function longitudinally over ~4 years as a function of birth weight and adult height. We hypothesized that i) cognitive and sensory function would be associated with birth weight, with poorer performance for low and high birth weight and better performance for larger babies within the normal range; ii) taller adults would have better cognitive and sensory function than shorter adults; iii) low and high birth weights and short adult height would be associated with a greater decline in sensory and cognitive function. Growth Changes During Prenatal Development Essay