1st WEEK OF DEVELOPMENT Cleavage, Morula & Blastule Lectures – Anatomy Lectures – MBBS Lectures

Specific Learning Objectives

By the end of this session you should be able to:

• Describe Assisted reproductive technologies
• Enumerate the developmental events during first week
• Define Cleavage and describe the formation of morula.
• Describe development of conceptus in the fallopian tube & Explain it diagrammatically
• Describe the developmental events in the uterine cavity during first week
• Describe the Formation of blastocyst.
• Give an overview of first week of development

                Assisted reproductive technology (ART)

• Offspring from these conceptions show increase in prematurity (<37 weeks gestation), low birth weight (<2500gm), & infant mortality; most of these adverse outcomes are due to increased rates of multiple births.

                                                In vitro fertilization

• Some couples suffer from infertility despite normal ovulation in the female and normal sperm function in the male.
• Mostly this condition results from the pathological blockage of both uterine tubes.
• For such couples in vitro fertilization was devised which resulted in the birth of the first ‘test tube baby’ in 1978.
• Since then, millions of babies have been born as a result of IVF.

                IN VITRO FERTILIZATION (IVF) involves the following steps:

• Superovulation 
• Oocytes  are obtained directly from the ovary by surgical procedure (laparoscopy).
• Once retrieved, the oocytes are placed in a Petri dish containing appropriate tissue culture medium.
• Next, the oocytes are inseminated with capacitated spermatozoa from the father and are incubated to allow fertilization.
• The resulting zygotes are allowed to develop in the culture medium to 4-8 cell stage (or earlier) and one or two are then placed in the uterus by means of a cannula inserted through the cervix.
• Incidence of multiple pregnancy or abortion is more than normal.

• Assisted in Vivo Fertilization
  • Gamete intrafallopian transfer (GIFT)
  • Zygote intrafallopian transfer (ZIFT)
• Intracytoplasmic sperm injection (ICSI) in case of oligospermia
• Cryopreservation of embryos Early embryos from IVF can be preserved by freezing them in liquid nitrogen with cryoprotectant (e.g., glycerol)
• Surrogate (Someone who takes the place of another person) mothers (who bears the embryo & fetus & delivers it to natural mother at birth).

Contraception

SDL

Learning Resource:

• Langman’s Medical Embryology, 13^th ed Page 41

                Sequential phases of human development During FIRST WEEK:

• Development of a new human being is initiated when a zygote is formed as a result of fertilization of an ovum by a sperm.
• Three important developmental events occur during the first week:

1. Cleavage and formation of morula.

2. Formation of blastocyst.

3. Beginning of implantation.

Cleavage

• In embryology, cleavage is the division of cells in the early embryo.
• The zygote undergoes rapid cell cycles with no significant overall growth, producing a cluster of cells.
• The different cells derived from cleavage are called blastomeres and form a compact mass called the morula.
• Cleavage ends with the formation of the blastula.
• Cleavage differs from other forms of cell division in that it increases the number of cells and nuclear mass without increasing the cytoplasmic mass.
• This means that with each successive subdivision, there is roughly half the cytoplasm in each daughter cell than before that division.

ZYGOTE (not true zygote as nuclear wall is not formed)

• 0.1 mm
• Zero time of embryogenesis
• Ampulla of uterine tube
• Female Gamete contributes:
  • 23 chromosomes (including X-chromosome)
  • Plasma membrane (antigens)
  • Mitochondria
• Male gamete contributes:
  • 23 chromosomes (including sex determining chromosome; X or Y)
  • Centriole (proximal)

2-CELL STAGE (in ampulla)

§1^ST Generation

30 hrs after fertilization

4-CELL STAGE

§2nd generation

§40 hrs after fertilization

                CLEAVAGE AND FORMATION OF MORULA

• As soon as the process of fertilization is completed, the zygote undergoes rapid mitotic segmentations or divisions called cleavage;
• Conceptus continues to travel down the Fallopian tube toward the uterine cavity.
• Cleavage results in rapid increase in the number of zygote cells.
• Cells,  become smaller with each cleavage division, & are known as blastomeres.
• After about three divisions, there is tight adherence of blastomeres  with each other to form a compact ball of cells.
• Compaction is mediated by cell surface adhesion glycoproteins.

• Morula is solid mulberry-shaped ball consisting of 16-32 blastomeres, (morus = mulberry).
• The morula stage  is reached three days after fertilization
• Corona radiata cells have disappeared but the zona pellucida still surrounds the morula.
• According to their location, the blastomeres of a morula can be divided into two groups: (i) inner cell mass (embryoblast) and (ii) outer cell mass (trophoblast).

8-16 CELL STAGE (isthmus)

• 2-3 days
• 3^rd – 4^th generations
• Isthmus
• Compaction (tightly or compactly arranged clumps with maximized contacts). It helps in segregation of outer and inner cell masses which communicate extensively by gap junctions.

CONCEPTUSES WITH BLASTOMERES

MORULA

• 16-32 blastomeres
•  Intramural (uterine tube)
• 3^rd – 4^th Days

EARLY PREGNANCY FACTOR (EPF):

                Immunosuppressant proteins  produced by trophoblastic cells,  found in maternal serum within 24-48 hrs after fertilization and forms the basis of pregnancy test during1^st 10 days after fertilization.

                FORMATION OF BLASTOCYST

• Morula reaches the uterine cavity on 4th day
• Remains suspended in the secretions of the uterus for some period.
• Small fluid-filled spaces appear between the cells of the morula due to passage of fluid from uterine cavity through ZP
• Spaces become confluent to form a single larger cavity called blastocele; the conceptus is now blastocyst or blastula.
• Embryoblast becomes pushed towards one side of the blastocyst making a projection into the blastocele.
• Trophoblast maintains peripheral position and forms wall of the blastocele.
• Eccentric position of the embryoblast defines the poles of the blastocyst.
• Embryonic pole, that bears the embryoblasts, while the opposite pole is known as abembryonic pole.

BLASTULA (EARLY/ UNHATCHED)

BLASTULA (LATE/ HATCHED)

• Days 5-6 (Hatched Blastula)
• Uterine milk
• 107 cells (8-15 embryoblasts)
• Zona pellucida (degenerated)
• Rapid growth of blastula

1. Ovary
2. Fallopian tube
3. Endometrium
4. Myometrium
5. Uterine cavity

A. Spermatozoon penetrates into the oocyte (conception), day 0

B. Two-cell stage, day 1

C. Four-cell stage, day 2

D. Eight-cell stage, day 3

E. Morula (16-32 cells), day 4

F. Free blastocyst (following hatching), day 6

SUMMARY (Ovulation-1^st week-Implantation)