Unit 4 - Part A - Prenatal Development : from conception to birth

ovum

human body’s largest cell

• found in ovaries

Sperm

human body’s smallest cell

• found in testes

Zygotic Period

conception - 1 week

• conception

• cleavage

• blastulation

• implantation

• placenta

Conception

once sperm cell enter ovum all other sperm cells are rejected and partake in apoptosis; sperm cell that enters ovum loses its acrosome and the tail becomes a nucleus of the fertilized egg

• most precarious period of human development and ~50% of fertilized eggs will spontaneously abort

Acrosome

penetrating cap

Cleavage

within 12 hours the 2 nuclei merge (sperm and egg) and fertilized egg becomes a zygote w/23 pairs of chromosomes

• 1st cleavage

• 2nd cleavage

1st cleavage

zygote divides into 2 equal parts

• takes place within first 36 hours after conception

• this is where identical twins can occur

  • happens if first cell division continues to divide separately

2nd Cleavage

2 days after conception and will result in 4 cells

• day 3 - 16-36 cells and the cluster of cells is now considered a morula

  • enters uterine cavity to allow for more room to grow

Blastulation

cells form into single layer hollow sphere; called a blastocyst

• occurs by the time the morula enters the uterus

• this is where cells begin to differentiate

Implantation

blastocyst attaches itself to uterine wall

• 7-9 days after conception

• attachment takes place at embryonic end of blastocyst

  • trophoblast ‘blast’ their way into uterine wall

• day 8 - zygote forms amnion (enclosing membrane) where amniotic fluid (protective agent) will be secreted

Placenta

results from implantation process

• the interwinding of disc-like mass of tissues containing capillaries from mom and fetus

• allows for material exchange, and acts a protective barrier from possible harmful agents that may be in mom’s bloodstream

Embryonic period

2nd week-2 months

• cell differentiation

• 1st month

• 2nd month

cell differentiation

cells within embryo align themselves into 3 distinct layers

• endoderm

• mesoderm

• ectoderm

begins at end of 3rd week

gives evidence of cell plasticity (that cells are capable of taking on new functions)

rapid growth and cell division = increased susceptibility to congenital malformations

endoderm

inner layer

• forms digestive, respiratory, and glandular system

mesoderm

middle layer

• forms muscular, skeletal, circulatory and respiratory, CNS, and sensory end organs

ectoderm

outer layer

• forms PNS, skin, hair, nails

1st month

embryo is 0.5cm and ¼ oz

• has tail and minute ridges along neck that are the beginnings of mouth, heart, face and throat

2nd Month

embryo is >1cm

• see beginnings of face, neck, fingers and toes

• period of rapid brain development

• rapid growth begins and receives nourishment through placenta and umbilical cord because embryo is firmly implanted in uterine wall

Early fetal period

3-6 months

‘fetus’ period begins

3 month

fetus is ~8cm

• sexual differentiation begins

  • H-Y antigen induces medulla of gonadal tissue to develop into testes

    • mullerian suppression factor

• multiple types of growth are present

  • hyperplasia

  • hypertrophy

  • Cephalocaudal

  • proximodistal

• where first reflexes begin

Mullerian suppression factor

suppresses development of female reproductive organs in males

hyperplasia

increase in # of cells

hypertrophy

increase in cell size

Cephalocaudal

growth occurs from head to tail

proximodistal

growth occurs from inside to outside

4 month

growth is at its most rapid

• 6-8 inches, 4oz

• hans are fully shaped and appendages are moving

• cartilaginous skeleton begins conversion to true bone

• lower limbs begin to catch-up with growth of head and trunk

5th month

reaches ½ of birth length but only 10% of birth weight

• entire body is temporarily covered in lanugo

lanugo

fine soft hair present to protect skin

6 months

13inch, 1pound

• eyelids can open and close

• vernix caseosa is present

• fetus is now structurally complete, but systems need more time to become functionally mature to increase chance of survival post-natal

vernix caseosa

fatty secretion designed to protect fetal skin

• waxy in nature

Later fetal period

7-9 months

• fetus more than triples in weight

• ~7 months babies are less active in the womb because they are forming adipose tissue layer

• shed lanugo and see continual growth of hair and nails