embryological and fetal development pdf

Embryological And Fetal Development Pdf

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Al-Daraji, H.

While you might think of child development as something that begins during infancy, the prenatal period is also considered an important part of the developmental process. Prenatal development is a time of remarkable change that helps set the stage for future psychological development. The brain develops over the course of the prenatal period, but it will continue to go through more changes during the early years of childhood.

Human Fetal Growth and Development

To review the recent data about eyelid morphogenesis, and outline a timeline for eyelid development from the very early stages during embryonic life till final maturation of the eyelid late in fetal life. The authors extensively review major studies detailing human embryologic and fetal eyelid morphogenesis. These studies span almost a century and include some more recent cadaver studies.

Numerous studies in the murine model have helped to better understand the molecular signals that govern eyelid embryogenesis. The authors summarize the current findings in molecular biology, and highlight the most significant studies in mice regarding the multiple and interacting signaling pathways involved in regulating normal eyelid morphogenesis. Eyelid morphogenesis involves a succession of subtle yet strictly regulated morphogenetic episodes of tissue folding, proliferation, contraction, and even migration, which may occur simultaneously or in succession.

Understanding the extraordinary process of building eyelid tissue in embryonic life, and deciphering its underlying signaling machinery has far reaching clinical implications beyond understanding the developmental abnormalities involving the eyelids, and may pave the way for achieving scar-reducing therapies in adult mammalian wounds, or control the spread of malignancies. Prenatal development in humans is divided into 2 periods: an embryonic period and a fetal period.

The embryonic period begins with fertilization and ends 8 weeks later, and the fetal period extends from that point onwards until delivery. Since , the staging of human embryos in the first 8 weeks has been divided into 23 Carnegie stages, with each stage representing an arbitrary point along the timeline of development. But some newer more comprehensive cadaver studies are now available, and dozens of recent articles document the molecular basis of eyelid development.

To accommodate old and new studies, to avoid confusion with nomenclature alien to Ophthalmologists, and for the sake of standardization, throughout the current manuscript the age mentioned in weeks is the postfertilization age with no reference to gestational age. All the measurements in mm refer to the crown-rump length, and any reference to Carnegie staging is disregarded.

For the sake of brevity, the authors limit the current discussion to eyelid morphogenesis alone ignoring ocular embryogenesis or congenital eyelid defects. These topics have already been studied extensively, and pertinent literature can be sought elsewhere. To understand eyelid development, a basic appreciation of embryonal development, particularly integumentary system embryogenesis, is important. In early embryonic life, the 3 primary germ layers are the ectoderm on the outside, the endoderm on the inside, and the mesoderm in between.

Generally speaking, the mesoderm later gives rise to a loosely woven tissue called the mesenchyme among other components in the developing embryo. The surface ectoderm is initially a single cell layer thick. It later proliferates forming a new outer transient layer of simple squamous epithelium called the periderm cells, and an underlying proliferating layer called the basal layer, which is separated from the underlying dermis by a basement membrane.

On the other hand, the most superficial layer, or the periderm cell layer, is of transient nature and is gradually shed into the amniotic fluid until it completely disappears by the 21st week.

Eyelid morphogenesis is a dynamic process involving complex interactions between the epidermis and dermis. Both types of interactions are integral for normal eyelid formation. Derivatives of embryonic tissue in the eyelid 9 — 11 , 13 — Eyelid development has been divided into 3, 13 4, 17 or even 5 distinct phases, 14 namely eyelid formation, fusion, development, separation, and maturation of eyelid structures.

The authors prefer to classify it in a more simple chronological fashion into embryonic and fetal stages because several events of those aforementioned phases can simultaneously coexist. Ocular and eyelid development occurs in a step-wise fashion, and any misstep will be followed by failure of subsequent steps. Both folds are covered by epithelium on their anterior and posterior surfaces, with 2 layers of epithelium on the anterior surface and a single layer of epithelium on the posterior surface.

In an attempt to fill the gap, the flattened periderm cells undergo a morphogenetic change into rounded or cuboidal cells which proliferate and start to migrate centripetally toward each other from the rim of both eyelids.

This begins the remarkable process of eyelid fusion just anterior to the horizontal equator of the globe Fig. This process of eyelid fusion involves 2 coordinated yet distinct processes: epithelial cell migration and proliferation of the epithelium at the migrating edge Fig. The process of eyelid fusion. A , At around week 8, the flattened periderm cells arrowhead undergo a morphogenetic and proliferative change into rounded or cuboidal periderm cells.

B , The leading edge of these proliferating cells helps make contact with the advancing edge of the opposing eyelid periderm cells until a connection is established. C , When a connection is established between both sides, these periderm cells flatten again and form a continuous sheet ultimately covering the cornea.

Controversy exists over where eyelid fusion actually begins. Although some studies have persistently demonstrated that the upper and lower eyelids first meet at the lateral end of the palpebral fissure with fusion progressing medially in a zipper-like fashion, 15 , 18 — 20 others have shown that fusion begins in the temporal and nasal canthi simultaneously and progresses toward the center of the eye in a purse-string fashion. Concomitant with this whole process, the reflected edge of the epithelium along the advancing eyelid margins also extends a less distinct covering of periderm cells along the conjunctival surface.

Only the periderm and epidermal layers are involved in eyelid fusion Fig. Some signaling molecules like fibroblast growth factor 10 Fgf10 , instrumental for coordinating mammalian epithelial eyelid fusion as well as corneal development, are expressed in the eyelids only in mesenchymal cells. The development of eyelid structures begins in the 9th week immediately following eyelid fusion. Similar to other muscles innervated by the facial nerve, the orbicularis oculi muscle develops from the mesenchyme of the second pharyngeal arch and later migrates into the eyelids.

Maturation of the eyelids during the fetal period. The development of eyelid structures begins in the 9th week immediately following eyelid fusion with mesenchymal cell condensations and an occasional ingrowth of surface epithelium into the underlying mesenchyme, which together contribute to the formation of some eyelid structures.

The first to appear is the orbital part of the orbicularis oculi muscle. The eyelid is now clearly divided into separate layers.

Meibomian gland branching is first observed and the tarsal plate has lengthened significantly. The orbicularis oculi muscle looks more fully developed.

Nearly mature eyelash follicles about to pierce the eyelid margin are also evident. The eyelid has taken its nearly fully developed appearance. Meibomian glands increase in length and are present in two-thirds of the length of the tarsal plate. The eyelids are fully separated by now but the eyes are still visibly closed. E , Full term. Final appearance of the eyelids at birth, which is not dissimilar from the adult counterpart.

These differentiate into connecting ductules and secretory holocrine sebaceous acini, while the central epithelial cylinder of the meibomian anlage forms a central canal that subsequently develops into the central duct.

The orbital part of the orbicularis oculi muscle is well developed. The levator muscle and aponeurosis, which develop from the same mesenchymal complex as the superior rectus and superior oblique muscles and tendons, starts to appear separately by a method of differential growth. The developing lower canaliculus cuts the caruncle off from the remaining eyelid tissue.

Byun et al. This may explain why Doxanas and Anderson 22 reported separation at 24 weeks. Sevel correctly points out that eyelid separation is a continuous process and not a short-term event, and even goes further to claim that several tissue strands normally persist between both eyelids until term. Histologic sections reveal that the eyelids start to separate nasally and anteriorly then extend temporally and posteriorly by gradual sloughing of periderm cells into the amniotic fluid.

The corneal epithelium, which was continuous with the surface ectoderm prior to eyelid fusion and was 4 layers thick, undergoes an initial reduction to 2 layers. But by the time the eyelids separate, the cornea has almost fully matured developing 5 layers of epithelium. Until recently eyelid separation has lacked any reasonable explanation. It was mistakenly attributed to keratinization of the intermediate cell layer, which would presumably soften and help disengage the cells in the junctional region.

It also appears to smooth out the proliferating periderm cells, eliminating any useless clumps and paving the way for a regular periderm sheet. It could be deduced that apoptosis plays a secondary but earlier role, and keratinization remains the ultimate and final process in eyelid separation. Opening of the puncta onto the eyelid margin is also observed around the 28th week after the eyelids separate. The onset of dynamic eyelid movements blinking has rarely been scrutinized in detail in the literature.

However, Petrikovsky et al. The actual onset of blinking may be much earlier, but the eyelids may be too small for dynamic movements to be detected by ultrasound. For 2 decades, the mouse has been the favorite model for the study of mammalian organogenesis, because it has been possible to modify the mouse germ line with the addition, replacement, modification, and even deletion of genes through homologous recombination of embryonic stem cells.

Modification of the germ line and creation of mouse mutants with techniques, such as null mutations, microdeletions, point mutations, gene insertions, or chromosomal rearrangement, have dramatically deepened our understanding of eyelid embryonic development.

A key to the development of tissues and organs during embryogenesis in multicellular organisms is the synchronized interaction of various cells that would make up the entire body. In the developing embryo, cell-to-cell contact is established in a paracrine short distance , endocrine long distance , or direct cell-to-cell juxtacrine signaling manner.

This complex system of communication is regulated throughout the body by 11 major signaling pathways defined by the signal transducer or ligand involved. At the molecular level, regulation of eyelid development requires a bidirectional mesenchyme—periderm interaction, which has been extensively studied in the mouse and is surprisingly complex, requiring 7 of those 11 signaling pathways WNT, Sonic hedgehog, transforming growth factor, fibroblast growth factor, epidermal growth factor, Jun N-terminal kinase, and notch pathways , not less than 22 genes, and 2 modes of communication juxtacrine and paracrine.

Some of the cytokines and growth factors expressed by those 7 signaling pathways may be differentially or solely expressed in the leading edge cells or in the inner edge cells during eyelid fusion, in the mesenchyme, or in the cornea, and some may even be over-expressed in the upper eyelid more than the lower eyelid.

In general, the unique processes taking place during human eyelid morphogenesis share common features throughout the mammalian family tree, particularly in viviparous mammals, 17 , 37 and in mice in particular this process has been studied in detail in more than 70 research articles in the past 25 years.

Despite their similarities, embryologic mimicry between mouse and human eyelid is not absolute. Mouse eyelid formation, which starts at embryonic day 11 and fuses between embryonic days 15 and 16, does not start to reopen except at postnatal day 12 to 14, and by postnatal day 16 eyelid separation is complete.

This is in stark contrast to humans where the entire process is completed in utero. As of October , the Mouse Genome Informatics lists gene defects associated with this phenotype up from in , and the number is likely to rise in the future with complete or partial knockout of new genes. Although a systematic appraisal of congenital eyelid anomalies is beyond the scope of the current review, several valid clinical points can be inferred from studying the process of eyelid morphogenesis, principally the Yin and Yang of eyelid fusion and separation.

In mammals, eyelid fusion serves several important functions; it is crucial for differentiation of eyelid appendages, and also imparts a protective function on the developing components of the eye, especially the cornea. The periderm cells, which are closely tied to the development of the eyelids, are also concomitantly involved in temporary fusion of the digits. It has been suggested recently that the process of adult wound healing, particularly skin healing that involves a synchronized series of tissue remodeling and tissue movements, is remarkably similar to the embryonic process of eyelid fusion.

This could be an evolutionary response to protect adults against microbial invasion if the skin barrier is broken, whereas the sealed environment of the embryo does not require such protection against infection. Embryonic eyelids fuse by first advancing filopdia and lamellipodia, which reach out and interdigitate with the opposite eyelid followed by closure of the gap in a sweeping purse-string like manner by forming a homogenous cable of contractile actin.

In adults, on the other hand, the skin heals by filopodial and lamellipodial crawling of cells only without involvement of the actin cytoskeleton. In conclusion, the authors have summarized the recent developments in eyelid morphogenesis, a temporally precise and morphogenetically intricate process that requires the cells to fold, proliferate, migrate, fuse, and later separate in a synchronized and timely manner.

The authors have no financial or conflicts of interest to disclose. Patrick Yeatts served as guest editor for this paper. National Center for Biotechnology Information , U. Ophthalmic Plastic and Reconstructive Surgery. Ophthal Plast Reconstr Surg. Published online Nov 9. Hatem A.

Journal of Histology & Histopathology

To review the recent data about eyelid morphogenesis, and outline a timeline for eyelid development from the very early stages during embryonic life till final maturation of the eyelid late in fetal life. The authors extensively review major studies detailing human embryologic and fetal eyelid morphogenesis. These studies span almost a century and include some more recent cadaver studies. Numerous studies in the murine model have helped to better understand the molecular signals that govern eyelid embryogenesis. The authors summarize the current findings in molecular biology, and highlight the most significant studies in mice regarding the multiple and interacting signaling pathways involved in regulating normal eyelid morphogenesis. Eyelid morphogenesis involves a succession of subtle yet strictly regulated morphogenetic episodes of tissue folding, proliferation, contraction, and even migration, which may occur simultaneously or in succession. Understanding the extraordinary process of building eyelid tissue in embryonic life, and deciphering its underlying signaling machinery has far reaching clinical implications beyond understanding the developmental abnormalities involving the eyelids, and may pave the way for achieving scar-reducing therapies in adult mammalian wounds, or control the spread of malignancies.

Prenatal development , also called antenatal development , in humans, the process encompassing the period from the formation of an embryo , through the development of a fetus , to birth or parturition. The human body , like that of most animals, develops from a single cell produced by the union of a male and a female gamete or sex cell. This union marks the beginning of the prenatal period, which in humans encompasses three distinct stages: 1 the pre-embryonic stage, the first two weeks of development, which is a period of cell division and initial differentiation cell maturation , 2 the embryonic period, or period of organogenesis , which lasts from the third to the eighth week of development, and 3 the fetal period , which is characterized by the maturation of tissues and organs and rapid growth of the body. The prenatal period ends with parturition and is followed by a long postnatal period. Only at about age 25 years are the last progressive changes completed. Much of the embryonic developmental machinery the cellular apparatus used in human development is similar to that used by other vertebrates as well as some invertebrates. The machinery is essential for four processes: cell proliferation, cell specialization, cell interaction, and cell movement.

Pregnancy, embryo-fetal development and nutrition: physiology around fetal programming. J Histol Histopathol. The purpose of this brief narrative review is to highlight the role of nutrition during the gestation period. We focused on the possible effects of imbalance of some nutrients in normal course of pregnancy and embryonic development. We strongly believe that the understanding of these events can be a valuable tool in order to prevent the onset of disorders and diseases in postnatal life. Keywords : Infant feeding, malnutrition, nutritional support, nutritional surveillance, pregnancy. In mice, the nutritional stress, during the pre-implantation phase, is often responsible of blastocyst death and pregnancy block [ 4 ].

Stages of Prenatal Development

The start of pregnancy is actually the first day of your last menstrual period. This is called the gestational age, or menstrual age. Your healthcare provider will ask you about this date and will use it to figure out how far along you are in your pregnancy. Each month, your body goes through a reproductive cycle that can end in one of two ways. You will either have a menstrual period or become pregnant.

У сотрудников лаборатории систем безопасности была единственная обязанность - поддерживать ТРАНСТЕКСТ в чистоте, следить, чтобы в него не проникли вирусы. Он знал, что пятнадцатичасовой прогон может означать только одно: зараженный файл попал в компьютер и выводит из строя программу.

Pre-embryonic and embryonic development

Она повернулась. Неужели АНБ прослушивает мои телефонные разговоры. Стратмор виновато улыбнулся. - Сегодня утром Дэвид рассказал мне о ваших планах. Он сказал, что ты будешь очень расстроена, если поездку придется отложить.

 - Не думаю, что это ключ. Фонтейн глубоко вздохнул. Его темные глаза выжидающе смотрели на Сьюзан. - Мисс Флетчер, как вы полагаете, если это не ключ, то почему Танкадо обязательно хотел его отдать. Если он знал, что мы его ликвидируем, то естественно было бы ожидать, что он накажет нас, допустив исчезновение кольца. В разговор вмешался новый участник. - Д-директор.

 Похоже, что-то стряслось, - сказала Сьюзан.  - Наверное, увидел включенный монитор. - Черт возьми! - выругался коммандер.  - Вчера вечером я специально позвонил дежурному лаборатории систем безопасности и попросил его сегодня не выходить на работу. Сьюзан это не удивило.

 Но… офицер ничего не сказал о… - Разумеется. Я не сказал ему про спутницу.  - Взмахом руки Клушар величественно отверг вопрос Беккера.

В его голосе слышалось беспокойство.  - Быть может, Хейл был прав, говоря, что система резервного питания подает недостаточное количество фреона. - А как же автоматическое отключение. Стратмор задумался.

Fetal Development: Stages of Growth

 Я ищу одного человека. - Знать ничего не знаю.

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3 Comments

  1. Stacey M.

    This book addresses the complexities of growth and maturation, structural and functional developments as well as differentiations of the human fetus.

    23.04.2021 at 18:17 Reply
  2. Gracia A.

    Throughout this chapter, we will express embryonic and fetal ages in terms of weeks from fertilization, commonly called conception.

    29.04.2021 at 12:14 Reply
  3. Leon J.

    Management skills and entrepreneurship assignment pdf john dies at the end download pdf

    02.05.2021 at 16:38 Reply

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