Reproduction + Development

Prenatal Development

Prenatal Development

The single-celled zygote forms a multicellular organism with specialized cells through the processes of:  mitosis and differentiation

By the end of the 8th week of pregnancy the embryo is called a fetus and all of the major structures are present

When the zygote is implanted in the uterine  wall a temporary organ called the placenta is formed from maternal and embryonic tissues

The developing embryo is attached to the placenta by the umbilical cord, this cord contains blood vessels

The fetus is surrounded by a membrane called the amnion which is filled with amniotic fluid

The fluid protects the fetus by giving it a stable environment and absorbing shock

zygote 🡪 multiple identical cells 🡪 differentiated cells (tissues) 🡪 organs 🡪organ systems 🡪 individual

Umbilical Cord

two arteries and a vein connects the fetus to the placenta

• There is NO exchange of blood between the embryo and the mother!

Placenta

site of diffusion

• Oxygen & nutrients diffuse from the mother’s blood vessels, across the placenta, and into the baby’s blood vessels

• Wastes diffuse from the baby’s blood vessels into the mother’s blood vessels

Human gestation

• the period between fertilization and birth

  • approximately 38-40 weeks

  • When gestation is complete the baby passes through the cervix and then through the vagina

Teratogens

• Substances that may harm the developing fetus and result in the formation of birth defects

• Include alcohol, certain drugs/medications, infections, and certain chemicals

Fertilization

the fusion of the sperm cell nucleus with the egg cell nucleus to produce a zygote (fertilized egg)

• IF fertilization takes place, it occurs in the oviduct

• The egg cell is viable for approximately 24 hours after ovulation

Sperm + ovum 🡪 zygote

Implantation

• After approximately a week, the developing embryo is implanted into the uterus

Embryo:  conception to 8 weeks

Identical twins (monozygotic)

One egg is fertilized by one sperm

Embryo splits into two during the early stages of development

Have identical genes and must be of the same sex

(Incidence:  about 3 in every 1000 births)

Fraternal twins (dizygotic)

Two eggs are ovulated and are each fertilized by a different sperm cell.

No more related than any other sibling in the family (can be of the same or different sexes)

Maternal age, genetics, use of assisted reproductive technologies are factors

varies by geography and ranges from 6 to over 20 per 1,000 deliveries

Embryonic Development

A series of cell divisions and differentiation into various tissues and organ systems

Embryo

a multicellular organism in the early stages of development

The beginning developmental processes are always the same in all animals:

1)  rapid mitotic cell divisions (cleavage)

2)  growth

3)  differentiation

Cleavage

• The cells divide rapidly with little time in between for growth

• The cells become smaller and smaller

• The cells divide until they become a solid ball called a morula

• Cells continue to divide and are pushed to the outside to form a fluid-filled ball of cells called a blastula

Gastrulation

The cells further divide until they fold in on each other called invagination

• This forms 2 cell layers

• The outside layer is called the ectoderm layer

• The inside layer is the endoderm layer

• As cells continue to divide a third cell layer forms in the middle: mesoderm

Differentiation

• the changing of unspecialized embryonic cells into the specialized cells, tissues and organs of a multicellular animal

• the ectoderm, mesoderm, and endoderm differentiates to form tissue and organs systems

• Each cell contains the same genes, but different

  genes are expressed in different cells (embryonic cells use different portions of their genetic information)

• The type of cell that forms through differentiation is controlled by genes, hormones and cell location

Germ Layers

Ectoderm: Outer layer

Endoderm: Inner Layer

Mesoderm: Middle Layer

Ectoderm

• forms nervous system including brain,

spinal cord and nerves

• forms skin, sweat glands, hair, nails

Endoderm

• forms lining of digestive tract and     associated organs (liver, pancreas)

• forms lining of respiratory tract

Mesoderm

• forms bones and muscles

• forms blood and blood vessels

• forms reproductive and excretory systems

Menstrual Cycle

a series of changes controlled by hormones that help prepare the female uterus for a possible pregnancy

The cycle is controlled by hormones of the pituitary gland in the brain and ovaries

The menstrual cycle occurs in 4 stages

Begins at puberty

Usually lasts 28 days, but can vary due to illness & other factors

Stops when a women is pregnant

Humans and primates are the only mammals to have a menstrual cycle

Stops permanently during menopause

Stage 1 - Follicle stage

Lasts 10-14 days

Ovarian hormones (estrogen and progesterone) are in low concentrations

Pituitary gland produces FSH (follicle stimulating hormone) which causes a follicle (egg cell capsule) to form

as the follicle grows it makes increased amounts of estrogen

• estrogen has a negative feedback effect on the pituitary that stops FSH secretion

• estrogen causes the uterine lining (endometrium) to grow thicker

Stage 2 – Ovulation (middle of cycle)

• increased estrogen from the follicle stimulates the pituitary gland to produce LH (luteinizing hormone)

• LH surge causes the follicle to rupture and the egg cell is sent to the oviduct=ovulation

Stage 3 – Corpus Luteum Stage

13-14 days

ruptured follicle heals inside the ovary forming the corpus luteum

corpus luteum produces progesterone

progesterone maintains the added growth of the uterine lining

If fertilization takes place:

• An embryo in the uterus will secrete a chemical called hCG  (human chorionic gonadotropin)

• hCG prevents the corpus luteum from decomposing which maintains progesterone levels

• The uterine lining is not shed

• hCG is the chemical that is detected by pregnancy tests

Stage 4 - Menstruation

• if an embryo is not present, approximately 2 weeks after ovulation the corpus luteum decomposes

• estrogen & progesterone levels decrease

• uterine lining is shed and moves out through the vagina (menstruation)

• pituitary increases the release of FSH & a new follicle matures – the cycle continues throughout the woman’s reproductive years

Male reproductive system

Testes – the male gonad;  produce sperm and testosterone

• Located outside of the body in structure called the scrotum

• Lower temperature is best for sperm production and storage

• Testes are 3°C lower than body temp

Mature sperm are stored in the epididymis

• The sperm is transported from the testes (testicles) to the vas deferens

• vas deferens:  tubes that connect to urethra

Glands that add fluid are the Cowper’s gland, seminal vesicles and prostate

• Fluid nourishes and transports sperm, protects sperm from acidity of female reproductive tract

• Sperm + seminal fluid = semen

Female reproductive system

Ovaries

Oviduct (fallopian tube)

Uterus

Cervix

Vagina

vas deferens

tubes that connect to urethra

Ovaries

the female gonad;  produce egg cells (ova) and hormones estrogen and progesterone

Oviduct

a tube that carries the egg cell (ovum) to the uterus;

****site of fertilization of the egg cell

• Cilia draws ovulated egg cell into the oviduct

Uterus

a thick, muscular, pear shaped organ (aka womb)

• A baby develops here

Cervix

narrow neck that is the opening to the uterus

Vagina

the birth canal

Meiosis

the process by which gametes (egg and sperm) are formed \n Meiosis only occurs in the gonads (ovaries and testes)

In meiosis, the chromosomes replicate once, and the cell divides twice…

Result = 4 new cells, each with ½ the number of chromosomes as the parent cell

The 4 cells are monoploid (aka haploid): have one set of chromosomes

The rest of the body cells are diploid: have the full number of chromosomes (2 sets)

The chromosomes are genetically different from the chromosomes of the parent cell

First Meiotic Division

Interphase I

Prophase I

Metaphase I

Anaphase I

Telophase I

Interphase I

chromosomes and centrioles replicate

Prophase I

• chromosomes condense

• centrioles move to opposite poles of the cell

• spindle fibers form, attach to centromeres of chromatids

• each pair of chromatids lines up with its homologous pair = synapsis

Homologous chromosomes

• chromosomes that are similar in terms of size, shape and genetic content

• In each pair, one chromosome came from the mother (maternal); one chromosome came from the father (paternal)

Synapsis

pairing of homologous chromosomes

Crossing over

exchange of segments of chromosomes during synapsis;

results in recombination of the genes

Genetic Recombination

Genetic Recombination is the result of crossing over

Because of crossing over, each gamete (egg or sperm cell) receives a unique set of genes from their parents

Chiasmata

Points at which crossing over and exchange of genetic material occur between the chromosomal strands

Metaphase 1

tetrads line up in the middle of the cell

Anaphase 1

• homologous chromosomes separate

• *** This is called disjunction

Telophase 1

• two daughter cells are formed;

• *each has half the number of chromosomes, but they are still replicated

Second Meiotic Division

• just like mitosis

• duplicated chromosomes separate - but they are actually recombined due to crossing over

Differences between Mitosis and Meiosis

Mitosis -

1. Daughter cells are genetically identical to parent cells:  same # and types of chromosomes

2. 2 diploid cells are formed

3. Occurs in growth & repair of body cells

4. Chromosomes replicate once; cell divides once

5. Homologous chromosomes do not pair; no recombination of genes

Meiosis -

1. Daughter cells are not genetically identical to parent cells:  have ½ the number of chromosomes and they have different genetic combinations

2. 4 monoploid cells are formed

3. Forms gametes (egg & sperm cells) in the gonads

4. Chromosomes replicate once; cell divides once

5. Synapsis, crossing over and genetic recombination of genes

Oogenesis

• the production of egg cells (ova) in the ovary of the female

• cytoplasm is divided unequally

• one of the daughter cells receives most of the cytoplasm – the small cells are called polar bodies

• the large egg cell formed contains yolk to nourish the developing embryo before the placenta forms

• the polar bodies disintegrate

• for each cell that undergoes meiotic division, there is only 1 ovum (egg cell) produced

Spermatogenesis

• The production of sperm in the testes

• The division of the cytoplasm is equal

• For each cell, there are 4 sperm produced