Section I- chapter 5



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Independent Study 9

The Developing Human



Section I- Chapter 5

  1. There are three phases of embryonic development- The first phase is growth, which involves cell division and the elaboration of cell products. The second phase is morphogenesis which is the development of shape, size, and other features of a particular organ or part or the whole body. The third phase is differentiation. Completion of differentiation results in the organization of cells in a precise pattern of tissues and organs that are capable of performing specialized functions.

  1. Development at the 6th week of pregnancy: the baby is the size of a pea. Curved spine, C shape. Senses, cheeks and jaws are forming. Arms and legs are forming buds to become arms and legs. Essential organs such as kidneys, lungs, and liver are developing. Heartbeat is running from 80 to 150 times per minute. You may be able to notice dark spots on the face that denote where nostrils and eyes will go.

  2. Development at the 7th week of pregnancy: baby is now about 10,000 times bigger than at conception. The kidneys are getting ready to function and the excretory system is beginning to work in waste management. Lips and tooth buds are forming. The majority of the development in this week is in the head. Brain cells are beginning to form, as well as the mouth. The eyes and ears are becoming noticeable. The limbs begin to sprout. The liver will soon begin to produce red blood cells. Bone marrow is now fully developed.

Section III Chapter 18

  1. The eyes are derived from four sources- 1.The neuroectoderm of the forebrain differentiates into the retina, the posterior layers of the iris, and the optic nerve. 2. The surface ectoderm of the head, forms the lens of the eye and the corneal epithelium.3. The mesoderm between the neuroectoderm and the surface ectoderm gives rise to the fibrous and vascular coats of the eye.4. Neural crest cells migrate into the mesenchyme and differentiate into the choroid, sclera, and corneal endothelium.

  2. The first evidence of eye development is at 22 days when optic grooves appear in the neural folds at the cranial end of the embryo.

  3. Common anomalies of the eye are: Coloboma- results when the optic fissure fails to close completely, leaving a gap in eye structures.

Coloboma of the iris - is a defect in the inferior sector of the iris or a notch in the pupillary margin, giving the pupil a keyhole appearance.

Detachment of the Retina - occurs when the inner and outer layers of the optic cup fail to fuse during the fetal period to form the retina and obliterate the intraretinal space.

Cyclopia (very rare anomaly) - the eyes are partially or completely fused, forming a single median eye enclosed in a single orbit.

Microphthalmia- congenital microphthalmia is a heterogeneous group of eye defects. The eye may be very small and associated with other ocular defects or it may be a normalappearing rudimentary eye.

Anophthalmia (unilateral or bilateral) - denotes absence of the eye, which is rare. The eyelids form, but no eyeball develops.

Section 5- Chapter 19


  1. The integumentary system consists of the skin and its appendages: sweat glands, nails, hairs, sebaceous glands, arrector muscles of hairs (arrector pili muscles), mammary glands, and teeth.

  2. The skin consists of two layers, the epidermis and the dermis.

  3.  The anomalies associated with the integumentary system are:

-Ichthyosis- is a general term that is applied to a group of skin disorders resulting from excessive keratinization (formation of horny layers of skin). The skin is characterized by dryness and scaling, which may involve the entire body surface.

-A harlequin fetus results from a rare keratinizing disorder that is inherited as an autosomal recessive trait with a mutation on the ABCA12 gene. The skin is markedly thickened, ridged, and cracked. Most babies die within the first week of life.

-A collodion infant is covered by a thick, taut membrane that resembles collodion (a protective film) or parchment. The membranous skin cracks with the first respiratory efforts and begins to fall off in large sheets.

-Lamellar ichthyosis is an autosomal recessive disorder. A baby with this condition may at first appear to be a collodion baby; however, the scaling persists. Growth of hair may be curtailed and development of sweat glands is often impeded. Affected infants usually suffer severely in hot weather because of their inability to sweat.

-Ectrodactyly-ectodermal dysplasia-clefting syndrome is a congenital skin condition that is inherited as an autosomal dominant trait. Associated with hypopigmentation of skin and hair, scanty hair and eyebrows, absence of eyelashes, nail dystrophy, hypodontia and microdontia, ectrodactyly, and cleft lip and palate.

-In generalized albinism, an autosomal recessive trait, the skin, hairs, and retina lack pigment; however, the iris usually shows some pigmentation.

-Nevus flammeus denotes a flat, pink or red, flame-like blotch that often appears on the posterior surface of the neck.

-A port-wine stain hemangioma is a larger and darker angioma than a nevus flammeus and is nearly always anterior or lateral on the face and/or neck.

-Supernumerary nipples and breasts. An extra breast (polymastia) or nipple (polythelia) occurs in approximately 1% of the female population; it is an inheritable condition.

-Pili Torti- In this familial disorder, the hairs are twisted and bent. Other ectodermal defects (like distorted nails) may be associated with this condition. Pili torti is usually first recognized at 2 to 3 years of age.

-Hypertrichosis- Excessive hairiness results from the development of supernumerary hair follicles, or from the persistence of lanugo hairs that normally disappear during the perinatal period.

-Aplastic anonychia- Congenital absence of nails at birth is extremely rare. Anonychia results from failure of nail fields to form or from failure of the proximal nail folds to form nail plates.



-Congenital alopecia (hair loss) may be caused by failure of hair follicles to develop, or it may result from follicles producing poor-quality hairs.

Section VII Chapter 20 and 21

  1. Harmful substances such as drugs or radiation that invade the womb and result in birth defects are called teratogens.

  2. Gap juncture is an organized collection of protein channels in cell membranes that allows ions and small molecules to pass between adjacent cells. Gap junctions are a means for cells to directly communicate with one another. 

Section VIII

  1. Down Syndrome or Trisomy 21 occurs when there is a presence of three chromosome copies in a given chromosome pair. Meaning it’s a gamete with 24 instead of 23 chromosomes and subsequently a zygote with 47 chromosomes. Will cause mental deficiency; brachycephaly, flat nasal bridge; upward slant to palpebral fissures; protruding tongue; transverse palmar flexion crease; clinodactyly of fifth digit; congenital heart defects; gastrointestinal tract abnormalities

  2. Turner syndrome, a condition that affects only females, results when one of the X chromosomes is missing or partially missing. Turner syndrome can cause a variety of medical and developmental problems, including short height, failure of the ovaries to develop and heart defects.

  3. The most critical period in development is when cell division, cell differentiation, and morphogenesis are at their peak. The critical period for brain development is from 3 to 16 weeks. Tooth development continues long after birth. The skeletal system also has a prolonged critical period of development extending into childhood.



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