BSC 215: Development (III)

Fetal Development

-Approximately 266 days after fertilization, a fetus is considered full-term and ready to be born; scalp usually has hair, and fingers and toes have well- developed nails -If, however, a fetus is born before that time, its tissues and organs may not have developed completely

Month 4

-Body growth is rapid; lower limbs lengthen and joints are forming as skeleton continues to ossify -Fetus has nipples and some hair -By this point, kidneys are well formed, digestive glands are forming, and heartbeat can be heard with a stethoscope -Begins to develop reflexes, and will startle and turn away from loud noises and bright lights -Approximate CRL is 14 cm (5.5 in.) by end of fourth month

Month 3

-Body lengthens as its head growth slows; upper limbs grow to their birth length; ossification begins in most bones -Eyes are well developed but on lateral side of head, eyelids are fused, bridge of nose develops, and external ears are present but not prominent -Genitals are distinguishable as male or female; by end of third month, approximate crown-rump length (CRL) is 9 cm (3.5 in.)

Placentation

-Deoxygenated fetal blood leaves fetus and enter two umbilical arteries where it then flows into placenta and moves into fetal capillaries in chrionic villi, on fetal side of placental barrier -Fetal blood then picks up oxygen and nutrients and delivers waste by diffusion, but again, no blood cells move through barrier

Month 7

-Eyelids open completely -Fat is deposited in subcutaneous tissue so skin is slightly smoother, but still wrinkled and red -Fetus usually turns upside down, assuming vertex position -In males, testes begin to descend through inguinal canal and into scrotum -By end of seventh month, approximate CRL is 28 cm (11.0 in.)

Placentation

-Fetal hemoglobin has a greater affinity for oxygen than maternal hemoglobin, so it pulls oxygen away from mother's hemoglobin at placental barrier -Oxygenated blood leaves placenta and flows back to fetus through a single umbilical vein

Months 8 and 9

-Fetal neurons form networks -Blood cells form in bone marrow -Digestive and respiratory systems complete development during ninth month -Fetal skin is less wrinkled, and lanugo is shed -In males, testes complete their descent into scrotum -Approximate CRL is 30 cm (12 in.) by end of eighth month and 36 cm (14 in.) by end of ninth month

Month 6

-Fetus gains significant weight; eyebrows and eyelashes appear, and eyelids, which have been fused since third month, are partially open -Skin is wrinkled and translucent and lungs begin to produce surfactant; key to survival if infant is born prematurely -Approximate CRL is 23 cm (9.1 in.) by end of sixth month

Month 5

-Growth slows and lower limbs achieve their correct proportions relative to body -Hair grows on head, and skin is covered by a downy hair called lanugo and a white to gray to secretion composed of shed epithelial cells and sebum known as vernix caseosa -Both lanugo and vernix caseosa cover skin to protect it from amniotic fluid -Formation of brown fat helps with heat production after fetus is born -Mother can feel movements, a phenomenon referred to as quickening, as skeletal muscles begin to contract -Approximate CRL is 19 cm (7.5 in.) by end of fifth month

Fetal period

Beginning of week 9 until birth

Placentation

When embryonic period begins, blastocyst has implanted in uterus and chronic villi have penetrated uterine blood vessels to form lacunae filled with maternal blood; merge into a single blood-filled cavity called placental sinus Once placentation has begun, stratum functionalis of uterus is known as decidua -Region of endometrium that lies beneath fetus becomes decidua basalis -Region that surrounds uterine cavity forms decidua capsularis

Placentation

begins during implantation, but majority of placental growth occurs in fetal period

Endoderm

cavity of middle ear and auditory tube Internal epithelial layers of digestive, respiratory, urinary, and reproductive systems Several glands and accessory digestive organs including thyroid gland, parathyroid glands, thymus, parts of palatine tonsils, and majority of liver, gallbladder, and pancreas

Fetal circulation and cardiovascular system

change rapidly soon after birth to adjust to life without a placenta; most conspicuous anatomical changes between prenatal and postnatal state occur in cardiovascular system -Unique cardiovascular structures present during prenatal development include umbilical arteries and umbilical vein and three circulatory "shortcuts" called vascular shunts (Figure 27.11a) -Blood from umbilical vein bypasses liver via ductus venosus; connected to inferior vena cava and flows into right atrium of heart

Fetal Development

characterized by maturation of tissues and organs; fetal growth is rapid and fetal size increases dramatically -Common fetal measurements of length include crown-rump length (CRL) or crown-heel length (CHL); embryo 25 mm (1.0 in.) long can grow to be as large as 50 cm (19.7 in.; CHL) during fetal period -Weight increases as well, most obviously in last 9 weeks of development; average weight of a full-term fetus ranges from 3.2 to 3.5 kg (7 to 8 lb)

Placental barrier

composed of a basement membrane and a thin layer of syncytiotrophoblast; keeps blood supply of mother and fetus separated

Umbillical cord

connects center of placenta to fetus' umbilicus (belly button) -Cord normally contains two umbillical arteries that carry deoxygenatied fetal blood away from fetal heart to placenta, and one umbillical vein the carries oxygen and nutrients toward fetal heart -Umbillical cord also contains a soft connective tissue with a jelly-like consistency, called Wharton's jelly; insulates and protects umbillical arteries and vein; external surface is covered by amniotic membrane

End of organogenesis

end of week 8 embryo approximately 25mm(1.0in.) long will have recognizable organ systems; some already working

Ectoderm

epidermis of skin majority of nervous system and sense organs First major event of organogenesis is neurulation -Cells of ectoderm thicken and form neural plate; folds inward and deepens; edges fuse to form a neural tube, which pinches off from ectodermal layer Anterior end of neural of neural tube will become brain, and remainder will become spinal cord By end of fourth week, anterior end of neural tube develops into three enlarged areas called primary brain vesicles; forebrain, midbrain, and hindbrain

Foramen ovale

hole in interatrial septum that directly connects right and left atria; allows blood to "skip" ahead and avoid pulmonary circulation by moving directly from right to left side of heart and systemic circulation

Mesoderm

lateral side of somites -forms gonads and kidneys -spleen -adrenal cortex -most of cardiovascular system -serous membranes of body cavities -connective tissue components of limbs

Mesoderm

notochord; serves to support and organize embryo around a central axis Mesoderm on either side of notochord forms somites (blocklike structures); each has three regions: sclerotome develops into vertebrae and ribs; dermatome develops into dermis layer of skin; myotome develops into most skeletal muscles

Placentation

pre-embryonic period, developing embryo receives nutrients from uterine milk secreted from endometrial glads, and digested endometrial cells from uterine wall Embryo continues to receive nutrition from digested endometrial cells until placenta is fully formed at about week 12, during fetal period

Start of organogenesis

primary germ layers start to differentiate when undergoing cephalocaudal and transverse folding embryo approximately 1.5mm (0.6in.) long

Placentation

process of forming disc-shaped placenta; attaches to uterine wall and to embryo or fetus through umbilical cord

Organogenesis

process where three primary germ layers differentiate into organs and organ systems

Ductus arteriosus

short passage that connects pulmonary trunk to aorta; allows blood to move from pulmonary trunk directly into aorta

Placenta

temporary organ that is shed after infant is born; site of exchange of oxygen, nutrients, and waste between mother and fetus; placenta produces hormones to support pregnancy

After birth

these shunts and the umbilical vessels close, and a normal circulatory pattern begins within a year in most infants: -Flaps in interatrial septum seal and foramen ovale becomes a depression called fossa ovalis (Figure 27.11b) -Resistance and pressure changes in pulmonary trunk and aorta cause ductus arteriosus to close approximately 3 months after birth; remnant of ductus arteriosus is a permanent cord (ligamentum arteriosum) -Ductus venosus also degenerates and becomes fibrous ligamentum venosum -Umbilical arteries become medial umbilical ligaments, and umbilical vein becomes ligamentum teres, or round ligament of liver

Placenta

unique organ because it develops from both fetal (chorionic villi) and maternal (decidua basalis) structures -Chorionic villi are filled with blood vessels carrying fetal blood and surrounded by maternal blood in placental sinus; however, blood supplies do not mix

Placentation

In addition to its nutritive functions, placenta also functions as an endocrine organ; assumes production of hCG as it grows and results in subsequent reduction of syntiotrophoblast -Corpus luteum relinquishes production of progesterone and estrogens to placenta by end of third month of gestation -Placenta also produces human placental lactogen and placental prolactin, helps prepare mammary glands for milk production, and relaxin; relaxed body's muscles, joints, and ligaments, presumably to facilitate stretching during delivery of newborn

Fetal development

Maturation of tissues and organs, rapid size increase

Placentation

Blood flow between mother and fetus through placenta takes following pathway: -Oxygenated maternal blood flows to decidua basalis layer of uterus and then through maternal arteries into placental sinus -Maternal blood touches placental barrier; allows diffusion of substances, but not passage of blood cells -Once diffusion has taken place, maternal blood which is deoxygenated and carrying waste, flows into maternal veins and back to mother's cardiovascular system

Organogenesis

Primary germ layers differentiate into organs and organ systems Placenta also begins to form during this period

Gastrulation

Rearrangement and migration of cells of bilaminar embryonic disc to form trilaminar embryonic discs Results in three germ layers: ectoderm, mesoderm and endoderm

Plaentation

Since maternal and fetal blood supplies are so close, some substances are exchanged across placental barrier between fetal and maternal blood supplies -Oxygen and nutrients diffuse from maternal blood into fetus for transport to tissues -Waste diffuses from fetus to maternal blood for removal Important example of Gradients Core Principle